Review
actuators
Piezoelectric Transformers: An Historical Review
Review
Alfredo Vazquez Carazo
Piezoelectric Transformers: An Historical Review
Micromechatronics, Inc., 200 Innovation Blvd. Suite 155, State College, PA 16803, USA; avc@mmech.com;
Tel.: +1-814-861-5688;
Fax: +1-814-861-1418
Alfredo
Vazquez Carazo
Micromechatronics,
Inc.,Uchino
200 Innovation Blvd. Suite 155, State College, PA 16803, USA; avc@mmech.com;
Academic
Editor: Kenji
Tel.:
+1-814-861-5688;
Fax:
+1-814-861-1418
Received:
15 November
2015;
Accepted: 20 April 2016; Published: date
Academic Editor: Kenji Uchino
Abstract:
Piezoelectric
transformers
(PTs)
solid-state
Received: 15
November 2015;
Accepted: 20
Aprilare
2016;
Published:devices
26 Aprilthat
2016transform electrical energy
into electrical energy by means of a mechanical vibration. These devices are manufactured using
Abstract: Piezoelectric transformers (PTs) are solid-state devices that transform electrical energy
piezoelectric materials that are driven at resonance. With appropriate design and circuitry, it is
into electrical energy by means of a mechanical vibration. These devices are manufactured using
possible to step up and step down the voltages between the input and output sections of the
piezoelectric materials that are driven at resonance. With appropriate design and circuitry, it is
piezoelectric transformer, without making use of magnetic materials and obtaining excellent
possible to step up and step down the voltages between the input and output sections of the
conversion efficiencies. The initial concept of a piezoelectric ceramic transformer was proposed by
piezoelectric transformer, without making use of magnetic materials and obtaining excellent
Charles A. Rosen in 1954. Since then, the evolution of piezoelectric transformers through history has
conversion efficiencies. The initial concept of a piezoelectric ceramic transformer was proposed
been linked to the relevant work of some excellent researchers as well as to the evolution in
by Charles A. Rosen in 1954. Since then, the evolution of piezoelectric transformers through history
materials, manufacturing processes, and driving circuit techniques. This paper summarizes the
has been linked to the relevant work of some excellent researchers as well as to the evolution in
historical evolution of the technology.
materials, manufacturing processes, and driving circuit techniques. This paper summarizes the
historical evolution
of the technology.
Keywords:
piezoelectric
transformer; piezoelectric-based power supply; non-magnetic
transformers
Keywords: piezoelectric transformer; piezoelectric-based power supply; non-magnetic transformers
1. Historical Introduction
1. Historical Introduction
1.1.
Using Salt
Salt Rochelle
Rochelle Single
Single Crystal
Crystal
1.1. 1920s–1930s:
1920s–1930s: Early
Early Studies
Studies on
on Piezoelectric
Piezoelectric Transformers
Transformers Using
The
first studies
studies on
on electrical
electrical to
to electrical
electrical conversion
conversion using
using piezoelectric
piezoelectric materials
materials took
The first
took place
place in
in
the
late
1920s
and
early
1930s.
Alexander
McLean
Nicolson
has
the
honor
of
being
the
first
researcher
the late 1920s and early 1930s. Alexander McLean Nicolson has the honor of being the first researcher
to
investigate the
idea of
of aa piezoelectric
piezoelectric transformer.
transformer. Nicolson
Nicolson considered
considered two
two single
single crystal
crystal blocks
blocks
to investigate
the idea
preloaded against
against each
each other
other with
with an
an external
external frame
frame (Figure
(Figure 1).
1).
preloaded
Piezoelectric
crystals
Representative sketch
sketch of the work of Alexander Mclean Nicolson on piezoelectric crystal
Figure 1. Representative
transformers [1].
One of the crystal elements was driven with an input electric signal generating a vibration in
www.mdpi.com/journal/actuators
the crystal. The second crystal element was used to convert the vibration back to an electric energy.
Actuators 2016, 5, 12; doi:10.3390/act5020012
Actuators 2016, 5, 12; doi:10.3390/act5020012
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Actuators 2016, 5, 12
2 of 22
Actuators
2016,
5, 12crystal elements was driven with an input electric signal generating a vibration in
2 ofthe
22
One
of the
crystal. The second crystal element was used to convert the vibration back to an electric energy. The
ratio of input to output electric energy was measured to evaluate the energy conversion. The work of
The ratio of input to output electric energy was measured to evaluate the energy conversion. The work
Nicolson on piezoelectric transformers, disclosed in multiple patents [1–3], was limited to the use of
of Nicolson on piezoelectric transformers, disclosed in multiple patents [1–3], was limited to the use of
Salt Rochelle single crystals, the only extensively available material at the time. The use of this
Salt Rochelle single crystals, the only extensively available material at the time. The use of this material
material led to obvious limitations in performance, design and applicability as compared to the later
led to obvious limitations in performance, design and applicability as compared to the later developed
developed piezoceramic materials.
piezoceramic materials.
1.2. 1940s
Work at
at General
General Electric,
Electric, Syracuse,
Syracuse, NY,
NY, with
with BaTiO
BaTiO3
1.2.
1940s to
to Early
Early 1960s:
1960s: Charles
Charles A.
A. Rosen’s
Rosen’s Work
3
Ferroelectric Ceramics
Ceramics
Ferroelectric
Prior to
Prior
to about
about 1940,
1940, only
only two
two types
types of
of ferroelectrics
ferroelectrics were
were known,
known, Salt
Salt Rochelle
Rochelle and
and some
some closely
closely
related tartrates,
tartrates, and
and potassium
potassium dihydrogen
dihydrogen phosphate
phosphate and
and its
its isomorphs
isomorphs [4].
[4]. That
changed in
in 1942
1942
related
That changed
with
the
announcement
by
Wainer
and
Salomon
[5]
of
barium
titanate
(BaTiO
3
)
as
a
new
ferroelectric
with the announcement by Wainer and Salomon [5] of barium titanate (BaTiO3 ) as a new ferroelectric
ceramic. This
associated with:
the electrical
electrical poling
poling process
process
ceramic.
This discovery
discovery came
came to
to be
be associated
with: (i)
(i) the
the discovery
discovery of
of the
in polycrystaline
polycrystaline ceramics
Roberts [6,7];
(ii) the
the development
development of
of the
the systematics
systematics of
of the
the
in
ceramics by
by Gary
Gary and
and Roberts
[6,7]; (ii)
piezoelectric
effect
in
polarized
ceramics
by
Mason
[8];
and
(iii)
the
extraction
of
the
first
values
for
piezoelectric effect in polarized ceramics by Mason [8]; and (iii) the extraction of the first values for the
the piezoelectric
coefficients
of BaTiO
3 by Hans Jaffe in 1948 [9].
piezoelectric
coefficients
of BaTiO
3 by Hans Jaffe in 1948 [9].
Ferroelectric
ceramics
were
quickly
significant advantages
advantages for
many
Ferroelectric ceramics were quickly recognized
recognized to
to have
have significant
for many
electromechanical
applications,
including
ease
of
fabrication
in
desired
shapes,
ability
to
operate
electromechanical applications, including ease of fabrication in desired shapes, ability to operate
under high
highhumidity,
humidity,
control
of response
through
selection
of polarization
directions,
a high
under
control
of response
through
selection
of polarization
directions,
a high dielectric
dielectric
constant,
high
electromechanical
coupling,
and
potential
low
cost.
Furthermore,
the
constant, high electromechanical coupling, and potential low cost. Furthermore, the physical properties
physical
properties
the
ceramics
could be
the use and
of additive
materials
and
of
the ceramics
couldofbe
altered
selectively
byaltered
the useselectively
of additiveby
materials
by variations
in firing
by variations
in to
firing
procedures
so as to produce
theparticular
characteristics
desiredDuring
for particular
procedures
so as
produce
the characteristics
desired for
applications.
the late
applications.
During
late BaTiO
1940s 3and
through
the 1950s, available,
BaTiO3 became
commercially
available,
1940s
and through
thethe
1950s,
became
commercially
triggering
the proliferation
of
triggering theapplications.
proliferation of piezoelectric applications.
piezoelectric
During this
this period,
period,Charles
CharlesAbraham
AbrahamRosen
Rosen(Figure
(Figure
a young
researcher
joined
During
2),2),
a young
researcher
thatthat
hadhad
justjust
joined
the
the
team
of
the
Electronics
Laboratory
at
General
Electric
Company
in
Syracuse,
NY,
USA,
started
team of the Electronics Laboratory at General Electric Company in Syracuse, NY, USA, started working
working
on his research
doctoral on
research
on piezoelectric
transformers
newly BaTiO
available
BaTiO3 and
on
his doctoral
piezoelectric
transformers
using the using
newlythe
available
3 and applying
applying
the
theoretical
work
developed
by
Mason.
The
impact
of
Rosen’s
work
in
piezoelectric
the theoretical work developed by Mason. The impact of Rosen’s work in piezoelectric transformer
transformerhas
technology
has been tremendous.
work
has been
extensively
technology
been tremendous.
Although hisAlthough
work has his
been
extensively
referenced,
hisreferenced,
figure has
his
figure
has
been
rarely
remembered.
We
want
to
use
some
lines
of
this
paper
as
a
tribute
to his
been rarely remembered. We want to use some lines of this paper as a tribute to his relevancy
in
relevancy
in
this
field.
this field.
Figure
December 2002
2002 [10].
[10].
Figure 2.
2. Charles
Charles Abraham
Abraham Rosen,
Rosen, 77 December
December 1917–8
1917–8 December
Born in Canada, Charles Rosen [10,11] received a B.S. degree in Electrical Engineering at Cooper
Born in Canada, Charles Rosen [10,11] received a B.S. degree in Electrical Engineering at Cooper
Union, NY, USA, in 1940. He returned to Canada where he spent the period of World War II
Union, NY, USA, in 1940. He returned to Canada where he spent the period of World War II supervising
supervising a group testing Canadian fighter planes before they were sent to battle in Britain. After
a group testing Canadian fighter planes before they were sent to battle in Britain. After the war, in 1950,
the war, in 1950, he received his M. Eng. in Communications at McGill University in Montréal (QC,
he received his M. Eng. in Communications at McGill University in Montréal (QC, Canada). Shortly
Canada). Shortly after, he joined the General Electric Company (G.E.) team in Syracuse, NY, USA.
after, he joined the General Electric Company (G.E.) team in Syracuse, NY, USA. While at G.E., Rosen
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While at
at G.E.,
G.E., Rosen
Rosen worked
worked on
on solid-state
solid-state devices
devices and
and coauthored
coauthored an
an influential
influential textbook
textbook on
on
While
transistors
[12].
By
that
time,
Rosen
entered
in
the
doctoral
program
at
Syracuse
University
and
transistors
[12]. By that
time,and
Rosen
entered an
in influential
the doctoral
program
at Syracuse[12].
University
and
worked
on solid-state
devices
coauthored
textbook
on transistors
By that time,
developed an
an extended
extended work
work that
that established
established the
the foundation
foundation on
on piezoelectric
piezoelectric ceramic
ceramic transformer
transformer
developed
Rosen entered in the doctoral program at Syracuse University and developed an extended work that
technology. His
His research
research work on
on this
this area
area is
is gathered
gathered in
in several
several U.S.
U.S. Patents [13–15]
[13–15] and
and in
in his
his
technology.
established the
foundationwork
on piezoelectric
ceramic
transformer
technology.Patents
His research work
on this
doctoral
work
published
in
1956
[16,17].
doctoral
work published
1956
[16,17].[13–15] and in his doctoral work published in 1956 [16,17].
area is gathered
in severalinU.S.
Patents
Unlike the
the earlier
earlier attempts
attempts by
by Nicolson
Nicolson to make
make piezoelectric
piezoelectric transformers
transformers using two
two pieces of
of
Unlike
Unlike the earlier attempts by Nicolson to
to make piezoelectric
transformers using
using two pieces
pieces of
piezoelectric
solid
crystals
bonded
together
and
clamped
with
an
external
structure,
Rosen's
piezoelectric solid
solidcrystals
crystals
bonded
together
and clamped
with
an external
Rosen's
piezoelectric
bonded
together
and clamped
with an
external
structure,structure,
Rosen's approach
approach involved
involved the
the use
use of
of aa single
single rectangular
rectangular block
block of
of BaTiO
BaTiO33 polycrystalline
polycrystalline ceramic
ceramic comprising
comprising
approach
involved the use of a single rectangular block of BaTiO3 polycrystalline ceramic comprising two
two distinct
distinct polarization
polarization zones (Figure
(Figure 3).
3). Rosen
Rosen created the
the two
two zones
zones by
by applying
applying separated
separated
two
distinct
polarization zones zones
(Figure 3). Rosen
created created
the two zones
by applying
separated electrodes
electrodes
and
different
polarizing
voltages
to
the
halves
of
the
block.
Half
of
the
block
was
polarized
electrodes
andpolarizing
different polarizing
voltages
to the
of theHalf
block.
of thewas
block
was polarized
and different
voltages to
the halves
ofhalves
the block.
of Half
the block
polarized
in the
in
the
longitudinal
direction
and
the
other
half
in
the
thickness
direction.
The
total
length
of the
the
in
the longitudinal
direction
thehalf
other
halfthickness
in the thickness
direction.
The
totaloflength
of
longitudinal
direction
and theand
other
in the
direction.
The total
length
the ceramic
ceramic block
block determines
determines the
the resonant frequency
frequency of
of the
the transformer,
transformer, while
while the ratio
ratio of
of the
the input
input and
and
ceramic
block determines
the resonantresonant
frequency of the transformer,
while the ratiothe
of the input
and output
output
sections
determines
the
voltage
conversion.
The
concept
was
great
but
the
results
were
poor,
output
determines
the voltage
conversion.
The concept
was
great
but thewere
results
were
poor,
sectionssections
determines
the voltage
conversion.
The concept
was great
but
the results
poor,
because
because it
it was
was very
very difficult
difficult to
to make
make aa bipolarized
bipolarized block
block that
that would
would not
not break
break into
into pieces
pieces when
when the
the
because
it was very difficult to make a bipolarized block that would not break into pieces when the alternating
alternating
current
was
applied.
Rosen
needed
just
one
unit
to
prove
his
concept,
but
manufacturers
alternating
was
applied.
Rosen
justtoone
unithis
to concept,
prove hisbut
concept,
but manufacturers
current wascurrent
applied.
Rosen
needed
justneeded
one unit
prove
manufacturers
had to be
had to
to be
be able
able to
to produce
produce them
them reliably.
reliably.
had
able to produce them reliably.
Figure 3.
3. Rectangular
Rectangular piezoelectric
piezoelectric transformer
transformer presented
presented by
by Rosen
Rosen in
in 1954
1954 [13].
[13].
Figure
Rectangular
piezoelectric
The rectangular piezoelectric
piezoelectric transformer of
of Figure 33 is
is traditionally known
known as the
the “Rosen-type”
The
The rectangular
rectangular piezoelectric transformer
transformer of Figure
Figure 3 is traditionally
traditionally known as
as the“Rosen-type”
“Rosen-type”
transformer.
Rosen
presented
several
other
PT
configurations
including
a
cylindrically-segmented
transformer.
other PT
PT configurations
configurations including
including aa cylindrically-segmented
cylindrically-segmented
transformer. Rosen
Rosen presented
presented several
several other
PT, a radially
radially polarized
polarized disc
disc PT,
PT, and
and aa ring-type
ring-type PT
PT with
with double
double polarization.
polarization. This is
is summarized in
in
PT,
PT, aa radially
polarized disc
PT, and
a ring-type PT
with double
polarization. This
This is summarized
summarized in
Figure
4.
However,
none
of
these
other
embodiments
have
gained
the
attention
of
the
rectangularFigure
4. However,
However,none
noneofofthese
theseother
other
embodiments
have
gained
attention
of rectangular-type
the rectangularFigure 4.
embodiments
have
gained
thethe
attention
of the
type PT.
PT. In
In 1957,
1957, Charles
Charles Rosen
Rosen moved
moved to
to California
California and
and joined
joined the
the Stanford
Stanford Research
Research Institute
Institute where
where
type
PT. In 1957, Charles Rosen moved to California and joined the Stanford Research Institute where
he
he
switched
his
research
into
artificial
intelligence
[10]
and
became
a
relevant
figure
in
that
field.
he
switched
his
research
into
artificial
intelligence
[10]
and
became
a
relevant
figure
in
that
field.
switched his research into artificial intelligence [10] and became a relevant figure in that field. Rosen
Rosen passed
passed away
away in
in California
California in
in 2002, at
at the age
age of
of 85
85 [10,11].
[10,11].
Rosen
passed
away in California
in 2002, at2002,
the agethe
of 85 [10,11].
Figure 4. Other piezoelectric transformer embodiments presented by Rosen in 1954 [13].
Figure 4.
4. Other
Other piezoelectric
piezoelectric transformer
transformer embodiments
embodiments presented
presented by
by Rosen
Rosen in
in 1954
1954 [13].
[13].
Figure
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The work of Rosen was continued by Stephen W. Tehon at the University of Illinois (Urbana, IL,
The work of Rosen was continued by Stephen W. Tehon at the University of Illinois (Urbana, IL,
USA). Tehon, also a member of the Electronic Laboratory at General Electric Co. (Syracuse, NY, USA),
USA). Tehon, also a member of the Electronic Laboratory at General Electric Co. (Syracuse, NY, USA),
extended his work to new ferroelectric materials and magnetostrictive materials. His doctoral thesis
extended his work to new ferroelectric materials and magnetostrictive materials. His doctoral thesis
was published in 1958 [18]. From 1960 to 1962, the team of General Electric at Syracuse continued the
was published in 1958 [18]. From 1960 to 1962, the team of General Electric at Syracuse continued the
work of Rosen and Tehon on ceramic power transformers through a research contract granted by the
work of Rosen and Tehon on ceramic power transformers through a research contract granted by the
U.S. Navy [19].
U.S. Navy [19].
In 1959, H. W. Katz, also a member of the Electronic Laboratory at General Electric Co., edited
In 1959, H. W. Katz, also a member of the Electronic Laboratory at General Electric Co., edited
the book “Solid State Magnetic and Dielectric Devices” [20], with the collaboration of Rosen (by then
the book “Solid State Magnetic and Dielectric Devices” [20], with the collaboration of Rosen (by then
at Stanford Research Institute, Menlo Park, CA, USA) and Tehon, among others. This is the first
at Stanford Research Institute, Menlo Park, CA, USA) and Tehon, among others. This is the first
published book discussing piezoelectric transformer technology in detail and is still a key reference
published book discussing piezoelectric transformer technology in detail and is still a key reference to
to any researcher in this field.
any researcher in this field.
1.3.
1.3. Late
Late 1950s
1950s to
to Late
Late 1960s:
1960s: Clevite
Clevite Corporation—Introduction
Corporation—Introduction of
of PZT
PZT to
to PT
PT Technology
Technology
Most
of
the
work
of
the
General
Electric
team
on
piezoelectric
transformers
during
the 1950s–
Most of the work of the General Electric team on piezoelectric transformers during
the 1950s–1960s
1960s
was based
on Barium
Titanate,
the commercially
only commercially
available
ferroelectric
material
the
was based
on Barium
Titanate,
the only
available
ferroelectric
material
at theattime.
time.
This
situation
changed
when,
in
1954,
Bernard
Jaffe
recognized
the
importance
of
the
This situation changed when, in 1954, Bernard Jaffe recognized the importance of the morphotropic
morphotropic
phase
boundary
in
the
Lead
Zirconate
Lead
Titanate
(PZT)
family
of
ceramics
[21–23].
phase boundary in the Lead Zirconate Lead Titanate (PZT) family of ceramics [21–23]. Over the decade
Over
decade
of the
B. Jaffe
was
the head
of the
center ofdivision
the piezoelectric
of thethe
1960s,
B. Jaffe
was1960s,
the head
of the
research
center
of research
the piezoelectric
at the thendivision
Clevite
at
the
then
Clevite
Corporation,
Bedford,
OH,
USA,
which
later
became
known
Vernitron
Corporation, Bedford, OH, USA, which later became known as Vernitron Corporation,as
and
today is
Corporation,
and today
is known
as Morgan
Ceramics.
B. Jaffe’s
studies
were
followed
by more
known as Morgan
Ceramics.
B. Jaffe’s
studies were
followed
by more
detailed
studies
by Hans
Jaffe
detailed
studies
Hans Jaffe
(not R.
a relative
of B.
Jaffe),
William. R.(all
Cooke,
Jr., Don
A. Berlincourt
(not a relative
of by
B. Jaffe),
William.
Cooke, Jr.,
Don
A. Berlincourt
at Clevite
Corporation)
and
(all
at Clevite
Corporation)
and others,
whichfamily
led toof the
of theOver
entire
others,
which led
to the evolution
of the entire
PZTevolution
piezoceramics.
thefamily
decadeofofPZT
the
piezoceramics.
Over
the
decade
of
the
1960s,
the
group
led
by
B.
Jaffe,
at
Clevite
Corporation,
were
1960s, the group led by B. Jaffe, at Clevite Corporation, were unquestionably world leaders in this
unquestionably
world leaders in this subject area [24–26].
subject area [24–26].
Briefly
after
publication
of Rosen
work,
H. Jaffe
D. and
A. Berlincourt,
on behalf of
Clevite
Briefly afterthe
the
publication
of Rosen
work,
H.and
Jaffe
D. A. Berlincourt,
onthe
behalf
of
Companies,
filled for the filled
patentfor
“Piezoelectric
Ceramic Resonator”
forResonator”
use in voltage
the Clevite Companies,
the patent “Piezoelectric
Ceramic
for transformation
use in voltage
[27].
Strictly speaking,
this speaking,
patent was
the field toofthe
ceramic
but the
transformation
[27]. Strictly
thisaddressed
patent wastoaddressed
field ofresonators,
ceramic resonators,
configurations
presented
in
this
patent
have
also
been
used
for
piezoelectric
transformers.
The
but the configurations presented in this patent have also been used for piezoelectric transformers.
topology
presented
by H.
and and
Berlincourt
consisted
of a of
disc
or plate
polarized
onlyonly
in the
The topology
presented
byJaffe
H. Jaffe
Berlincourt
consisted
a disc
or plate
polarized
in
thickness
direction
and
with
two
separated
electrodes
representing
the
input
from
the
output
the thickness direction and with two separated electrodes representing the input from the output as
as
illustrated
illustrated in
in Figure
Figure 5.
5.
Figure 5. Unipoled
Unipoled piezoelectric
piezoelectric transformers
transformers by Jaffe and Berlincourt [27].
Figure
This
topologyhas
has
been
extensively
evaluated
by other
many
other researchers,
only for
This topology
been
extensively
evaluated
by many
researchers,
not only fornot
piezoelectric
piezoelectric
but more for
extensively
for ceramic
filters and resonators.
the most
transformerstransformers
but more extensively
ceramic filters
and resonators.
Perhaps thePerhaps
most appealing
appealing
characteristic
of
this
topology
is
the
single
polarization
required
for
the
full
ceramic,
so the
characteristic of this topology is the single polarization required for the full ceramic, so the electrodes
electrodes
shape
can beby
obtained
partially
the full electroded
C. Munk
shape can be
obtained
partiallybyetching
theetching
full electroded
ceramic. Inceramic.
1965, E. In
C.1965,
MunkE.presented
presented
the
equivalent
circuit
development
for
the
radial
modes
of
the
unipoled
disc-type
the equivalent circuit development for the radial modes of the unipoled disc-type configuration [28].
configuration [28].
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In 1972,
1972,Berlincourt
Berlincourtpatented
patentedone
oneofofthe
thefirst
firstspecific
specific
applications
piezoelectric
transformers
In
applications
forfor
piezoelectric
transformers
as
as starter
ballast
gaseous
discharge
lamps
[29].
In this
patent,
Berlincourt
suggested
a variety
starter
andand
ballast
forfor
gaseous
discharge
lamps
[29].
In this
patent,
Berlincourt
suggested
a variety
of
of configurations
for piezoelectric
transformers
Figurethe6.different
Amongconfigurations,
the different
configurations
for piezoelectric
transformers
as shownasin shown
Figure 6.inAmong
configurations,
of special
relevancy configuration
is the rectangular
having
two sections
with
of
special relevancy
is the rectangular
havingconfiguration
two sections with
thickness
polarization
thickness
(Figureof6a).
The6b,c
configurations
Figure
6b,c correspond
to the
same
(Figure
6a).polarization
The configurations
Figure
correspond toofthe
same topologies
illustrated
above
in
topologies
Figure
5 asbyintroduced
in Berlincourt.
the 1960 patent by H. Jaffe and Berlincourt.
Figure
5 as illustrated
introducedabove
in the in
1960
patent
H. Jaffe and
Figure 6.
6. Configurations
Configurations of
of piezoelectric
piezoelectric transformers
transformers suggested
suggested by
by Berlincourt
Berlincourt [29].
[29].
Figure
1.4. Late
Late 1960s–1980s:
1960s–1980s: Period
Period of
of Proliferation
Proliferation
1.4.
Until the
the mid-1960s,
mid-1960s, piezoelectric
piezoelectric transformers
transformers were
were not
not practical
practical due
due to
to the
the lack
lack of
of materials
materials
Until
able
to
operate
with
low
mechanical
losses
at
high
amplitudes.
This
situation
improved
withwith
the
able to operate with low mechanical losses at high amplitudes. This situation improved
introduction
of
hard-doped
piezoelectric
ceramic
compositions,
such
as
PZT8,
showing
high
the introduction of hard-doped piezoelectric ceramic compositions, such as PZT8, showing high
mechanical and
and dielectric
dielectric Q
Q factors
factors at
at high
high amplitudes.
amplitudes.
mechanical
Several
U.S.
and
Japanese
companies,
like
Motorola, Inc.
Inc. (Chicago,
(Chicago, IL,
Several U.S. and Japanese companies, like Motorola,
IL, USA)
USA) [30–32],
[30–32], RCA
RCA Corp.
Corp.
(Wilmington
,
DE
,
USA)
[33,34],
Denki
Onkyo
Co.
Ltd.
(Tokyo,
Japan)
[35–41],
and
Matsushita
Electric
(Wilmington, DE, USA) [33,34], Denki Onkyo Co. Ltd. (Tokyo, Japan) [35–41], and Matsushita
Industrial
Co. Ltd. Co.
(Osaka,
[42],Japan)
focused
their
attention
toattention
PTs for generating
high voltage
Electric
Industrial
Ltd.Japan)
(Osaka,
[42],
focused
their
to PTs forthe
generating
the
required
for
the
cathode-ray
tubes
in
black
and
white
television
receivers
(see
Figure
7b
forFigure
a typical
high voltage required for the cathode-ray tubes in black and white television receivers (see
7b
example).
Several
attempts
were
also
reported
for
using
piezoelectric
transformers
as
igniter
in
gasfor a typical example). Several attempts were also reported for using piezoelectric transformers as
based stoves
(Matsushita
Industrial
Ltd. [43]),
small
engine
applications
(Briggs &
igniter
in gas-based
stoves Electric
(Matsushita
ElectricCo.
Industrial
Co.inLtd.
[43]),
in small
engine applications
Straton
(Milwaukee,
WI)
[44],
Figure
7a)
and
in
automobiles
(Nippon
Soken,
Nishio,
Japan)
In
(Briggs & Straton (Milwaukee, WI) [44], Figure 7a) and in automobiles (Nippon Soken, [45]).
Nishio,
the 1980s,
(Berlin,
Germany)
[46],
Figure 7c,
and
General
Electric
(Schenectady,
NY, USA)
Japan)
[45]).Siemens
In the 1980s,
Siemens
(Berlin,
Germany)
[46],
Figure
7c, and
General
Electric (Schenectady,
[47]
were
among
some
companies
working
on
the
application
of
PTs
for
triggering
power
switch
NY, USA) [47] were among some companies working on the application of PTs for triggering power
gates
such
as
triacs,
thyristors,
Mosfets,
etc.,
with
galvanic
decoupling.
switch gates such as triacs, thyristors, Mosfets, etc., with galvanic decoupling.
None of
of these
success
because
trial
production
revealed
the
None
these PT
PTapplications
applicationsreached
reachedcommercial
commercial
success
because
trial
production
revealed
piezoelectric
transformer
to
be
fragile
and
the
overall
piezoelectric-solution
to
be
more
expensive
the piezoelectric transformer to be fragile and the overall piezoelectric-solution to be more expensive
than the
the electromagnetic
ofof
the
technical
problems
were
related
to:
than
electromagnetic coil
coil ititwas
wasmeant
meanttotoreplace.
replace.Some
Some
the
technical
problems
were
related
immature
materials
fabrication
technology,
lack
of
mechanical
reliability,
inadequate
mounting,
and
to: immature materials fabrication technology, lack of mechanical reliability, inadequate mounting,
bulky
driving
circuits.
and
bulky
driving
circuits.
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(a)
(a)
(b)
(b)
(c)
(c)
Figure
Some
representative
applications
considered
the
1970s
Figure
7.
Some
representativeapplications
applicationsconsidered
considered for
for piezoelectric
piezoelectric
transformers
during
the
1970s
Figure
7. 7.
Some
representative
for
piezoelectrictransformers
transformersduring
during
the
1970s
and
1980s.
(a)
Gate
drivers
for
solid
state
switches;
(b)
High
voltage
power
supply
for
TV
cathodeand
1980s.
(a)
Gate
drivers
for
solid
state
switches;
(b)
High
voltage
power
supply
for
TV
cathodeand 1980s. (a) Gate drivers for solid state switches; (b) High voltage power supply for TV cathode-ray
ray
tube;
(c)
in
engine
ignition.
ray(c)
tube;
(c) Application
Application
in small
small
engine
ignition.
tube;
Application
in small
engine
ignition.
120
120
Millionsof
ofunits
units
Millions
JP850
850
JP
US150
150
US
JP175
175
JP
1986-90
1986-90
US25
25
US
US75
75
US
US15
15
US
Numberof
ofPatents
PatentsFilled
Filled
Number
1.5.
1.5. 1990s–2000s:
1990s–2000s: Commercial
Commercial Introduction
Introduction of
of PTs
PTs for
for CCFL
CCFL Backlighting
Backlighting
1.5. 1990s–2000s: Commercial Introduction of PTs for CCFL Backlighting
In
In the
the late
late 1980s,
1980s, several
several Japanese
Japanese companies,
companies, including
including NEC
NEC Corp.
Corp. (Kawasaki,
(Kawasaki, Japan),
Japan), Tokin
Tokin
In the
late 1980s,
several
Japanese
companies,
including
NEC Corp.
(Kawasaki,
Japan),
Tokin Corp.
Corp.
(Sendai,
Japan),
Tamura
Corp.
(Tokyo,
Japan),
Matsushita
Electric
Industrial
Co.
Corp. (Sendai, Japan), Tamura Corp. (Tokyo, Japan), Matsushita Electric Industrial Co. Ltd.,
Ltd., Nihon
Nihon
(Sendai,
Japan),
Tamura
Corp.
(Tokyo,
Japan),
Electric Industrial
Nihon
Cement
Cement
Kabushiki
Kaisha
(Tokyo,
Japan)
and
others,
the
PTs,
taking
advantage
Cement
Kabushiki
Kaisha
(Tokyo,
Japan)
andMatsushita
others, reconsidered
reconsidered
the use
use of
ofCo.
PTs,Ltd.,
taking
advantage
Kabushiki
Kaisha
(Tokyo,
Japan)
and
others,
reconsidered
the
use
of
PTs,
taking
advantage
of
of improvements
improvements in
in novel
novel piezoelectric
piezoelectric materials,
materials, more
more reliable
reliable manufacturing
manufacturing technology,
technology, including
including of
improvements
in novel
piezoelectric
materials,
reliable
manufacturing
including
multilayer
processes,
new
on
integrated
circuits,
and
solutions.
main
multilayer co-firing
co-firing
processes,
new concepts
concepts
on more
integrated
circuits,
and housing
housingtechnology,
solutions. The
The
main
multilayer
co-firing
processes,
new
concepts
on
integrated
circuits,
and
housing
solutions.
The
main
application
was
to
generate
high
voltage
for
backlighting
the
small
cold
cathode
fluorescent
lamps
application was to generate high voltage for backlighting the small cold cathode fluorescent lamps
(CCFLs)
used
in
liquid
crystal
displays
(LCDs)
of
the
increasingly
popular
portable
cell
phones
and
application
was
to
generate
high
voltage
for
backlighting
the
small
cold
cathode
fluorescent
lamps
(CCFLs) used in liquid crystal displays (LCDs) of the increasingly popular portable cell phones and
computers.
In
this
the
transformer
enabled
thinner,
lighter,
more
(CCFLs)
used in
crystal displays
(LCDs) of the
increasingly
popular
portable
cell and
phones
and
computers.
In liquid
this application,
application,
the piezoelectric
piezoelectric
transformer
enabled
thinner,
lighter,
and
more
efficient
modules,
thus
the
of
screen
thickness
and
computers.
In this backlighting
application, the
piezoelectric
transformer
enabled
lighter,
more efficient
efficient CCFL
CCFL
backlighting
modules,
thus allowing
allowing
the reduction
reductionthinner,
of LCD
LCD
screenand
thickness
and
improving
the
life
portable
devices.
Due
small
size,
cost
the
improving
the battery
battery
life of
of
portable
devices.
Due to
to its
its
smallscreen
size, the
the
cost of
of and
the piezoelectric
piezoelectric
CCFL
backlighting
modules,
thus
allowing
the reduction
of LCD
thickness
improving the
transformers
was
relatively
low
and
competitive
for
this
application,
rapidly
enabling
high-volume
transformers
was relatively
lowDue
andto
competitive
for this
rapidly enabling
high-volume
battery
life of portable
devices.
its small size,
theapplication,
cost of the piezoelectric
transformers
was
production.
production.
relatively
low and competitive for this application, rapidly enabling high-volume production.
During
this
period,
many
technical
publications
Duringthis
thisperiod,
period,many
many technical
technical publications
publications and
and
patents
(Figure
8a)
were
issued
forfor
During
and patents
patents (Figure
(Figure8a)
8a)were
wereissued
issuedfor
modifications
of
Rosen’s
initial
concept,
including
variations
of
the
design,
driving
circuitry,
modifications
of
Rosen’s
initial
concept,
including
variations
of
the
design,
driving
circuitry,
modifications of Rosen’s initial concept, including variations of the design, driving circuitry, mounting
mounting
solutions,
and
materials to
enhance the
of
devices
[48–55].
During
mounting
solutions,
and novel
novel
the performance
performance
of these
these[48–55].
devices During
[48–55]. the
During
solutions,
and
novel materials
tomaterials
enhance to
theenhance
performance
of these devices
1990s,
the
1990s,
CCFL
technology
enjoyed
a
significant
commercial
boom
for
compact,
portable
devices.
the
1990s,
CCFL
technology
enjoyed
a
significant
commercial
boom
for
compact,
portable
devices.
CCFL technology enjoyed a significant commercial boom for compact, portable devices. By early the
By
that 25%–30%
By early
early the
the 2000s,
2000s, it
it was
was estimated
25%–30% of
of the
the produced
produced CCFL
CCFL backlighting
backlighting circuits
circuits were
2000s,
it was
estimated
thatestimated
25%–30%that
of the
produced
CCFL
backlighting
circuits were
madewere
using
made
made using
using piezoelectric
piezoelectric transformer
transformer technology
technology (Figure
(Figure 8b).
8b).
piezoelectric transformer technology (Figure 8b).
100
100
80
80
CCFL
CCFL Inverters
Inverters
60
60
40
40
Piezoelectric
Piezoelectric Transformers
Transformers
20
20
0
0
1991-95
1991-95
(a)
(a)
1996-2002
1996-2002
1994
1994
1995
1995
1996
1996
1997
1997
1998
1998
1999
1999
2000
2000
(b)
(b)
Figure
8. Evolution
PT
technology
during
1980s to
early 2000s
based on
author sources.
Figure
Evolutionofof
ofPT
PTtechnology
technology during
during late
late
sources.
Figure
8. 8.Evolution
late 1980s
1980s to
to early
early2000s
2000sbased
basedononauthor
author
sources.
(a)
Number
of
filled
patents
in
U.S.
and
Japan;
(b)
Production
of
piezoelectric
transformers
(a) Number of filled patents in U.S. and Japan; (b) Production of piezoelectric transformers for
for cold
cold
(a) Number of filled patents in U.S. and Japan; (b) Production of piezoelectric transformers for cold
cathode
cathode fluorescent
fluorescent lamps
lamps (CCFL)
(CCFL) applications
applications compared
compared to
to total
total CCFL
CCFL inverter
inverter production.
production.
cathode fluorescent lamps (CCFL) applications compared to total CCFL inverter production.
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Companies like Toshiba (Tokyo, Japan), NEC, Hitachi (Tokyo, Japan), Panasonic (Osaka, Japan)
Companies like Toshiba (Tokyo, Japan), NEC, Hitachi (Tokyo, Japan), Panasonic (Osaka, Japan)
and Apple (Cupertino, CA, USA) adopted PT-based backlighting in their laptops. Figure 9 shows a
and Apple (Cupertino, CA, USA) adopted PT-based backlighting in their laptops. Figure 9 shows a
commercial piezoelectric inverters for CCFLs integrated in the Apple MacBook Pro 15″ and 17″ LCD
commercial piezoelectric inverters for CCFLs integrated in the Apple MacBook Pro 15” and 17” LCD
screen computers during the mid-2000s.
screen computers during the mid-2000s.
(a)
(b)
Figure 9.
9. Piezoelectric
Piezoelectric inverter
inverter used
used in
in Apple
Apple Powerbook
Powerbook 15”
15” and
and 17”
17” computer
computer for
for LCD
LCD backligting
backligting
Figure
(courtesy
of
Micromechatronics,
Inc.,
State
College,
PA,
USA).
(a)
5W
piezoelectric
transformer;
(courtesy of Micromechatronics, Inc., State College, PA, USA). (a) 5W piezoelectric transformer;
(b)Assembled
Assembledpiezoelectric
piezoelectricinverter.
inverter.
(b)
In addition to the improvements in the manufacturing process of PTs, developments in the
In addition to the improvements in the manufacturing process of PTs, developments in the driving
driving techniques for PTs were critical for their commercial success. Two relevant examples are the
techniques for PTs were critical for their commercial success. Two relevant examples are the use of
use of resonant converter topologies for driving the PTs and the use of integrated control circuits to
resonant converter topologies for driving the PTs and the use of integrated control circuits to provide
provide stable operation to the PT under different operation conditions.
stable operation to the PT under different operation conditions.
Resonant converters were originally considered in the early 1990s as an extension of the general
Resonant converters were originally considered in the early 1990s as an extension of the general
approach undertaken in power electronics to use high frequency switching circuits to minimize the
approach undertaken in power electronics to use high frequency switching circuits to minimize the
size of DC-DC and AC-DC converters. Numerous efforts were undertaken in this area mainly with
size of DC-DC and AC-DC converters. Numerous efforts were undertaken in this area mainly with
magnetic components [56]. In general, to drive a PT, either a sine wave or a square wave voltage can
magnetic components [56]. In general, to drive a PT, either a sine wave or a square wave voltage can
be used. A sine wave voltage is preferred for minimizing the circulating energy through the shunt
be used. A sine wave voltage is preferred for minimizing the circulating energy through the shunt
input capacitance characteristic of the input of the PT. However, generating a sine wave requires
input capacitance characteristic of the input of the PT. However, generating a sine wave requires
more reactive components in the converter than generating a square wave. With resonant converters,
more reactive components in the converter than generating a square wave. With resonant converters,
the sine input waveform is generated by using a square wave voltage in combination with an
the sine input waveform is generated by using a square wave voltage in combination with an inductive
inductive element in series with the transformer. The control of the ON/OFF of the switches of the
element in series with the transformer. The control of the ON/OFF of the switches of the converter
converter is selected to minimize the switching losses. This technique is the so-called Zero Voltage
is selected to minimize the switching losses. This technique is the so-called Zero Voltage Switching,
Switching, ZVS [57]. The combination of the ZVS technique and the PT can reduce the capacitive
ZVS [57]. The combination of the ZVS technique and the PT can reduce the capacitive turn-on loss due
turn-on loss due to the input capacitance of the PT, and achieve high efficiency.
to the input capacitance of the PT, and achieve high efficiency.
There are two main strategies to achieve ZVS in a PT-driven topology. First, and most common,
There are two main strategies to achieve ZVS in a PT-driven topology. First, and most common,
the PT should present an inductive characteristic at the operating frequency. In order to achieve this,
the PT should present an inductive characteristic at the operating frequency. In order to achieve this,
normally an inductor is connected in series with the PT. In this type of strategy, the PT can be driven
normally an inductor is connected in series with the PT. In this type of strategy, the PT can be driven
at a fixed duty cycle of 0.5. A second approach to achieve ZVS is to use the PT without an external
at a fixed duty cycle of 0.5. A second approach to achieve ZVS is to use the PT without an external
inductor (so-called “inductor-less” driving). In this case, a variable duty-cycle is required for the
inductor (so-called “inductor-less” driving). In this case, a variable duty-cycle is required for the
control signal to the Mosfets to achieve ZVS.
control signal to the Mosfets to achieve ZVS.
Figure 10 shows some of the standard topologies [54] used to drive PTs using a series inductor,
Figure 10 shows some of the standard topologies [54] used to drive PTs using a series inductor,
including push-pull [53], half-bridge and Class-E [57–59]. Inductor-less techniques is discussed in
including push-pull [53], half-bridge and Class-E [57–59]. Inductor-less techniques is discussed in
Section 1.6.
Section 1.6.
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Vcc
Q1
L2
L1
Vcc
L
L2
L1
L
Q1
PT
Vcc
Q1
L
PT
Vcc
Q1
Q2
L
Vcc
PT
Q
Q
Q2
(a)
(a)
PT
PT
Q2
Q2
PT
Vcc
(b)
(b)
(c)
(c)
Figure 10.
10. Commonly
topologies: (a)
Figure
Commonly used
used piezoelectric
piezoelectric transformer
transformer circuit
circuit topologies:
(a) push-pull;
push-pull; (b)
(b) half-bridge;
half-bridge;
Figure
10. Commonly used piezoelectric transformer circuit topologies: (a) push-pull; (b) half-bridge;
(c)
Cass-E.
(c) Cass-E.
(c) Cass-E.
In order to ensure the ZVS condition and to maintain the stability of the converter under load
In order to ensure the ZVS condition and to maintain the stability of the converter under load
ordervoltage
to ensure
the ZVS condition
maintain the stability
the converter
load
and In
input
fluctuation,
strategies and
for to
controlling
PTs wereofdeveloped
andunder
integrated
and input voltage fluctuation, strategies for controlling the PTs were developed and integrated within
and
input
fluctuation,
strategies
controlling
thefrequency
PTs wereoscillators
developednear
andthe
integrated
within
ICs.voltage
The first
control circuits
werefor
designed
as fixed
resonant
ICs. The first control circuits were designed as fixed frequency oscillators near the resonant frequency.
within
ICs.
The
first
control
circuits
were
designed
as
fixed
frequency
oscillators
near
the
resonant
frequency. However, due to the high mechanical quality factor (Q), of the PT, changes in the resonant
However, due to the high mechanical quality factor (Q), of the PT, changes in the resonant frequency
frequency.
to the
high mechanical
quality
(Q), oforthe
PT, changes
the resonant
frequency However,
associateddue
with
aging,
temperature,
load factor
variation,
input
voltage,incould
not be
associated with aging, temperature, load variation, or input voltage, could not be compensated for,
frequency
associated
with aging,
temperature,
load variation,
compensated
for, negatively
affecting
the performance
of the PT. or input voltage, could not be
negatively affecting the performance of the PT.
compensated
for, negatively
performance
of the
PT.
Progressively,
differentaffecting
control the
schemes
have been
developed
to ensure the stability of the
Progressively, different control schemes have been developed to ensure the stability of the output
Progressively,
control
have the
been
developed
to ensure
stability
of the
output
voltage or different
current, as
well schemes
as maximize
circuit
efficiency
under the
different
operation
voltage or current, as well as maximize the circuit efficiency under different operation conditions.
output
voltage
or
current,
as
well
as
maximize
the
circuit
efficiency
under
different
operation
conditions. Since PTs are frequency-dependent devices, a way to control the operation of the PT is to
Since PTs are frequency-dependent devices, a way to control the operation of the PT is to vary the
conditions.
Since PTsaccording
are frequency-dependent
devices,orato
way
to control
operation of the PT is to
vary the frequency
to output load changes
input
voltagethe
variations.
frequency according to output load changes or to input voltage variations.
vary the
according
to output load
changes
or to
voltage
variations.
Onefrequency
of the first
control techniques
introduced
was
theinput
so-called
“frequency
control,” sometimes
One of the first control techniques introduced was the so-called “frequency control,” sometimes
One
of
the
first
control
techniques
introduced
was
the
so-called
“frequency
control,”
sometimes
referred as PFM (pulse frequency modulation). Frequency control requires one or several
feedback
referred as PFM (pulse frequency modulation). Frequency control requires one or several feedback
referred
PFMThese
(pulsefeedback
frequencyloops
modulation).
one voltage
or several
control as
loops.
provide Frequency
informationcontrol
aboutrequires
the output
(orfeedback
current)
control loops. These feedback loops provide information about the output voltage (or current)
control
loops.
These
loops provide
information
the output
(or current)
variations
so they
canfeedback
be appropriately
regulated.
Different about
companies
such asvoltage
Rohm (Tokyo,
Japan
variations so they can be appropriately regulated. Different companies such as Rohm (Tokyo,
variations
so
they
can
be
appropriately
regulated.
Different
companies
such
as
Rohm
(Tokyo,
Japan
(BA9785 [58], BA9802), Sanyo (Tokyo, Japan) (LA5663V), Texas Instrument (Dallas, TX, USA)
Japan (BA9785
[58], BA9802),
Sanyo
(Tokyo, Japan)
(LA5663V),
Texas Instrument
(Dallas,TX,
TX, USA)
(BA9785
[58],
BA9802),
Sanyo
(Tokyo,
(LA5663V),
Texas
(Dallas,
(UCC3975
[59]),
O2 Micro
(Santa
Clara,Japan)
CA, USA)
(OZ9913),
andInstrument
Maxim (Sunnyvale,
CA,USA)
USA)
(UCC3975 [59]),
[59]), O2
O2 Micro
Micro (Santa
(Santa Clara, CA,
CA, USA) (OZ9913),
(OZ9913), and Maxim
Maxim (Sunnyvale,
(Sunnyvale,CA,
CA,USA)
USA)
(UCC3975
(MAX8785), among
others, have Clara,
developed USA)
ICs integrating aand
combination
of these strategies
to
(MAX8785), among
others,
have
developed
ICsICs
integrating
a combination
of these
strategies
to provide
(MAX8785),
among
others,
have
developed
integrating
a
combination
of
these
strategies
to
provide full control of the PTs. There are several possibilities that have been proposed regarding the
full control
the PTs.
areThere
several
that have been
proposed
parameters
provide
fullof
control
of There
the PTs.
arepossibilities
several
possibilities
thatincluding:
have
been regarding
proposed the
regarding
the
parameters
to be measured
to determine
the frequency
control,
to
be
measured
to
determine
the
frequency
control,
including:
parameters to be measured to determine the frequency control, including:
(i) Frequency control by measuring the output voltage.
(i) Frequencycontrol
control by measuring
measuring the output
output voltage.
(i)
(ii) Frequency
Frequency controlby
by measuringthe
the outputvoltage.
current (Figure 11a).
(ii)
Frequency
control
by
measuring
the
output
current
(Figure
11a).
(ii)
Frequency
control
by
measuring
the
output
current
(Figure
11a).
(iii) Frequency control by measuring the phase difference
between
input voltage and current.
(iii)
Frequency
control
by
measuring
the
phase
difference
between
input voltage
current.
(iii)
between
input
(vi) Frequency
Frequencycontrol
controlby
bymeasuring
measuringthe
thephase
input difference
to output phase
difference
(Figureand
11b).
(iv) Frequency
Frequencycontrol
control by
by measuring
measuring the
the input
input to
to output
output phase
phase difference
difference (Figure 11b).
(vi)
VC
+
+-
VC
ERROR
AMPLIFIER
ERROR
AMPLIFIER
(a)
(a)
REF
REF
ILOAD
ILOAD
VOLTAGE
CONTROLLED
VOLTAGE
OSCILLATOR
CONTROLLED
OSCILLATOR
PT
PT
DC INPUT
VOLTAGE
DC INPUT
VOLTAGE
PWM
RESONANT
CONTROL
DRIVER
PWM
RESONANT
CIRCUIT
CONTROL
DRIVER
CIRCUIT
INTEGRATING
CIRCUIT
INTEGRATING
CIRCUIT
VCO
VCO
PT
PT
Current
Detection
Current
Circuit
Detection
Circuit
ILOAD
ILOAD
DC INPUT
VOLTAGE
DC INPUT
VOLTAGE
RESONANT
POWER
RESONANT
STAGE
POWER
STAGE
PHASE
DIFFERENCE
PHASE
DETECTION
DIFFERENCE
CIRCUIT
DETECTION
CIRCUIT
(b)
(b)
Figure 11. Different tracking control schemes for piezoelectric transformers: (a) based on measuring
Figure
11.
for
transformers:
the output
current; tracking
(b)
basedcontrol
on the schemes
input
to output
phase difference.
Figure
11.Different
Different
tracking
control
schemes
forpiezoelectric
piezoelectric
transformers:(a)
(a)based
basedononmeasuring
measuring
the
output
current;
(b)
based
on
the
input
to
output
phase
difference.
the output current; (b) based on the input to output phase difference.
In some applications, the use of a single frequency loop cannot ensure the complete control of
In
the use
of ainput
singlevoltage
frequency
loop
ensure
complete
control the
of
the PT some
underapplications,
large variations
of the
and/or
thecannot
output
load. the
When
this happens,
the PT under large variations of the input voltage and/or the output load. When this happens, the
Actuators 2016, 5, 12
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Actuators
2016, 5, applications,
12
In some
9 of 22
the use of a single frequency loop cannot ensure the complete control
of the PT under large variations of the input voltage and/or the output load. When this happens,
frequency
shift
required
in the
transformer
toto
adjust
the
frequency
shift
required
in the
transformer
adjustthe
theoutput
outputconditions
conditionsmay
maybe
beso
so large
large that
that the
the
efficiency
of
the
PT
will
be
significantly
affected
or/and
the
correct
transformation
ratio
to
achieve
efficiency of the PT will be significantly affected or/and the correct transformation ratio to achieve the
the expected
values
is possible.
not possible.
In order
to overcome
limitations,
a second
voltage
feedback
expected
values
is not
In order
to overcome
thesethese
limitations,
a second
voltage
feedback
loop
loop
control
is
sometimes
considered
to
extend
the
regulation
range
of
the
converter.
control is sometimes considered to extend the regulation range of the converter.
The voltage
voltage feedback
feedback has
been implemented
implemented in
basic ways.
ways. One
use
The
has been
in two
two basic
One proposed
proposed option
option is
is to
to use
pulse
modulation
technique
to
control
the
driving
circuit
signal
of
the
transistor
gates
(PWM
pulse modulation technique to control the driving circuit signal of the transistor gates (PWM technique).
technique).
The secondincludes
alternative
includes
the input
control
the input
DC voltage
by circuit
using acontrolled
chopper
The
second alternative
the control
of the
DCofvoltage
by using
a chopper
circuit
controlled
at
much
lower
frequency
than
the
PT
(for
instance,
100–1000
Hz),
which
switches
at much lower frequency than the PT (for instance, 100–1000 Hz), which switches the input voltage to
the input voltage to the switching transistor. This sometimes referred as “dimming technique”.
the switching transistor. This sometimes referred as “dimming technique”.
By the late 2000s and early 2010s, the CCFL backlighting technology used for LCD started to be
By the late 2000s and early 2010s, the CCFL backlighting technology used for LCD started to
replaced by light-emitting diodes (LEDs). This resulted in a production decline of high voltage
be replaced by light-emitting diodes (LEDs). This resulted in a production decline of high voltage
piezoelectric transformers and eventually stopped the high-volume production of these components
piezoelectric transformers and eventually stopped the high-volume production of these components
by most of the leading suppliers. This also impacted the production of IC and many of the specifically
by most of the leading suppliers. This also impacted the production of IC and many of the specifically
designed ICs for the CCFL business are currently discontinued.
designed ICs for the CCFL business are currently discontinued.
1.6. Beyond
1.6.
Beyond the
the 2000s:
2000s: Power
Power Piezoelectric
Piezoelectric Transformers
Transformers
The evolution
evolution in
the know-how
know-how of
PT technology
technology during
first decade
decade of
of 2000s
2000s
The
in the
of PT
during the
the 1990s
1990s and
and first
inspired
new
developments
beyond
the
CCFL
backlighting
in
areas
such
as
fluorescent
ballasts,
ACinspired new developments beyond the CCFL backlighting in areas such as fluorescent ballasts, AC-DC
DC
battery
adapters
and,
more
recently,
for
AC-DC
LED
drivers.
Compared
to
backlighting
inverters,
battery adapters and, more recently, for AC-DC LED drivers. Compared to backlighting inverters,
where the
the typical
typical power
power level
level is
is 55 W
W and
and the
the output
output voltage
voltage is
is several
several hundred
hundred volts
volts (step
(step up),
up), most
most
where
DC-DC
or
AC-DC
applications
require
several
tenths
to
hundreds
of
watts
with
output
voltage
levels
DC-DC or AC-DC applications require several tenths to hundreds of watts with output voltage levels
of just
just aa few
few volts
volts (step-down
(step-down conversion).
conversion). The
not suitable
suitable for
for those
those levels
levels of
of
of
The Rosen
Rosen PT
PT topology
topology is
is not
power conversion
conversion since
since its
its design
design is
is prominent
prominent in
in high
high voltage
voltage generation.
generation. Thus,
power
Thus, new
new types
types of
of PTs
PTs
designs
and
revisions
of
some
previously
suggested
were
considered
by
researcher
interested
in
designs and revisions of some previously suggested were considered by researcher interested in power
power conversion
applications.
The effort
was mainly
bycompanies:
two companies:
Electronics
inU.S.
the
conversion
applications.
The effort
was mainly
led byled
two
Face Face
Electronics
in the
U.S.
and
NEC
in
Japan.
Other
groups
in
Europe
and
Korea,
with
alternative
proposals,
followed
these
and NEC in Japan. Other groups in Europe and Korea, with alternative proposals, followed these
tendencies toward
toward power
power applications.
applications.
tendencies
Strictly speaking,
was undertaken
undertaken in
Strictly
speaking, the
the first
first published
published research
research effort
effort on
on power
power PTs
PTs was
in 1966
1966 by
by
Othmar
M.
Stuetzer
[60]
of
Sandia
Laboratory
(Albuquerque,
NM,
USA).
Stuetzer
analyzed
the
Othmar M. Stuetzer [60] of Sandia Laboratory (Albuquerque, NM, USA). Stuetzer analyzed the power
power capabilities
of a PT consisting
of two
thin piezoelectric
discstobonded
opposite
sides wall
of a
capabilities
of a PT consisting
of two thin
piezoelectric
discs bonded
oppositetosides
of a metal
metal
wall andin operating
in the
thickness
In spiteand
of mainly
this earlier
and mainly
and
operating
the thickness
mode
(Figure mode
12). In (Figure
spite of 12).
this earlier
theoretical
study,
theoretical
study,
no
other
work
on
high-power
PTs
is
mentioned
until
the
1990s.
no other work on high-power PTs is mentioned until the 1990s.
Figure 12. Thickness
Thickness mode
mode transformer suggested by Othmar M. Stuetzer, Sandia Labs, in 1966 [60].
Early in
inthe
the1990s
1990s
intense
work
re-initiated
in Japan
byon
NEC
on the
same ofconcept
of
Early
intense
work
waswas
re-initiated
in Japan
by NEC
the same
concept
thickness
thickness
mode
PTs
(Figure
13)
and
its
use
for
power
applications.
Some
of
the
proposed
thickness
mode PTs (Figure 13) and its use for power applications. Some of the proposed thickness mode designs
mode designs
consisted
of 1plates
to 2 mm-thick
with input
and output
multilayer
sections
and
consisted
of 1 to
2 mm-thick
with inputplates
and output
multilayer
sections
and operating
at very
operating
at very such
high as
frequencies,
such asT.1 Inoue,
MHz or
T. Y.
Inoue,
O.among
Ohnishi,
Y. Sasaki,
among
high
frequencies,
1 MHz or higher.
O.higher.
Ohnishi,
Sasaki,
others
at NEC,
hold
others atofNEC,
hold several
of the patents
for[61,62]
thickness
mode PT [61,62] designs.
several
the patents
for thickness
mode PT
designs.
One of the main operational challenges of using a thickness mode is the large number of high
order modes for the longitudinal or width extensional vibrations resulting in spurious modes near
the thickness mode resonance frequency. The reason is related to the fact that PZT ceramics have high
electromechanical coupling factor kt necessary for thickness operation, but also have large coupling
factor k31 for longitudinal and width vibrations. The k31 large coupling factor results in a large number
of high-order harmonics that couple with the thickness mode, leading to poor operational
characteristics. To suppress the spurious resonances, based on the unstiffened piezoelectric effect,
ceramic materials with large anisotropy were considered, in particular the PbTiO3 ceramic material,
Actuators
12
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where2016,
kt is5,larger
than 50% and k31 is less than 5% [63].
Actuators 2016, 5, 12
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electromechanical coupling factor kt necessary for thickness operation, but also have large coupling
factor k31 for longitudinal and width vibrations. The k31 large coupling factor results in a large number
of high-order harmonics that couple with the thickness mode, leading to poor operational
characteristics. To suppress the spurious resonances, based on the unstiffened piezoelectric effect,
ceramic materials with large anisotropy were considered, in particular the PbTiO3 ceramic material,
where kt is larger than 50% and k31 is less than 5% [63].
Figure
piezoelectrictransformer
transformerby
byNEC,
NEC,1992
1992
[61,62].
Figure13.
13.Thickness
Thickness mode
mode piezoelectric
[61,62].
Simultaneously
with the development
in PTmode
designs
for power
applications,
One
of the main operational
challengesand
of improvement
using a thickness
is the
large number
of high
intensive
also
by NEC, resulted
in publications
circuit strategies
the PTs
on power
order
modeswork,
for the
longitudinal
or width
extensional on
vibrations
resultingtoindrive
spurious
modes
near the
applications
[64–67].
During
this period,
activities
initiated
at Virginia
Electronichave
Center
thickness
mode
resonance
frequency.
The
reason were
is related
to the
fact thatPower
PZT ceramics
high
(VPEC)
of
Virginia
Tech
University,
led
by
Fred
Lee,
on
converters
using
piezoelectric
transformers
electromechanical coupling factor kt necessary for thickness operation, but also have large coupling
[68–70].
activities were
in width
part thevibrations.
result of theThe
collaboration
NEC.in
In aJuly
factor
k31 These
for longitudinal
and
k31 large between
couplingVPEC
factorand
results
large
1993, T. Zaitsu, a member of NEC team, joined as a visiting researcher the Virginia Power Electronic
number of high-order harmonics that couple with the thickness mode, leading to poor operational
team at Virginia Tech [65]. He stayed for one year, until August 1994, and returned to Japan where
characteristics. To suppress the spurious resonances, based on the unstiffened piezoelectric effect,
he continued his work at NEC [71,72] and published his doctoral dissertation in 1997 [73].
ceramic materials Figure
with large
anisotropy
were considered,
in particular
the PbTiO
3 ceramic material,
13. Thickness
mode piezoelectric
by NEC, 1992that
[61,62].
The evaluation of thickness
expansion
mode PTs transformer
by NEC demonstrated
these
transformers
where kt is larger than 50% and k31 is less than 5% [63].
can deliver higher net output power due to their higher driving frequency. However, it was found
the
and
improvement
inPT
PTdesigns
designs
forpower
power
applications,
Simultaneously
with
thedevelopment
development
improvement
in
for
applications,
thatSimultaneously
their
efficiency with
was
low,
in
the high 80%
range,
and their integration
in DC-DC
converters
was
intensive
work,
also
by
NEC,
resulted
in
publications
on
circuit
strategies
to
drive
the
PTs
intensive
work,
also
by
NEC,
resulted
in
publications
on
circuit
strategies
to
drive
the
PTs
on
power
difficult due to limitations relating to efficient driving circuits (especially field-effect transistor, FETs) on
power
applications
[64–67].
During
this
period,
were
initiated
at Virginia
Power Electronic
applications
[64–67].
During
this
period,
activities
wereNEC
initiated
at Virginia
Power
Center
for
such
frequency
and
large
output
power.
In activities
1997,
changed
its focus
to a Electronic
different
type
of
(VPEC)
of
Virginia
Tech
University,
led
by
Fred
Lee,
on
converters
using
piezoelectric
transformers
Center
(VPEC)
of
Virginia
Tech
University,
led
by
Fred
Lee,
on
converters
using
piezoelectric
piezoelectric transformer, the longitudinal vibration mode transformer shown in Figure 14 [71].
[68–70]. These
activities
were
in part the
result
thethe
collaboration
between
VPEC andbetween
NEC. In July
transformers
[68–70].
These
activities
were
in of
part
result of the
collaboration
VPEC
1993,
T. Zaitsu,
member
of NEC
team, joined
as a visiting
researcher
the Virginia
Power Electronic
and
NEC.
In July a1993,
T. Zaitsu,
a member
of NEC
team, joined
as a visiting
researcher
the Virginia
teamElectronic
at Virginiateam
Techat[65].
He stayed
for one
until
1994,
returned
Japan
where to
Power
Virginia
Tech [65].
He year,
stayed
for August
one year,
untiland
August
1994,toand
returned
he
continued
his
work
at
NEC
[71,72]
and
published
his
doctoral
dissertation
in
1997
[73].
Japan where he continued his work at NEC [71,72] and published his doctoral dissertation in 1997 [73].
The
evaluationofofthickness
thicknessexpansion
expansion mode
mode PTs
transformers
The
evaluation
PTs by
by NEC
NECdemonstrated
demonstratedthat
thatthese
these
transformers
can
deliver
higher
net
output
power
due
to
their
higher
driving
frequency.
However,
it
was
found
can deliver higher net output power due to their higher driving frequency. However, it was
found
that
that their efficiency was low, in the high 80% range, and their integration in DC-DC converters was
their efficiency was low, in the high 80% range, and their integration in DC-DC converters was difficult
difficult due to limitations relating to efficient driving circuits (especially field-effect transistor, FETs)
due to limitations relating to efficient driving circuits (especially field-effect transistor, FETs) for such
for such frequency and large output power. In 1997, NEC changed its focus to a different type of
frequency and large output power. In 1997, NEC changed its focus to a different type of piezoelectric
piezoelectric transformer, the longitudinal vibration mode transformer shown in Figure 14 [71].
transformer, the longitudinal vibration mode transformer shown in Figure 14 [71].
Figure 14. Piezoelectric transformer using longitudinal and transverse mode vibration [72].
Figure 14. Piezoelectric transformer using longitudinal and transverse mode vibration [72].
Figure 14. Piezoelectric transformer using longitudinal and transverse mode vibration [72].
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The longitudinal
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2016, 5, 12
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transformer.
11 of 22
transformer provided better efficiency than the thickness mode
The longitudinal transformer provided better efficiency than the thickness mode transformer. In
In addition, the multilayer construction of both the input and output allowed better design flexibility
The longitudinal
transformer
provided
efficiency
than theallowed
thickness
mode
transformer.
In
addition,
the multilayer
construction
of bothbetter
the input
and output
better
design
flexibility
The longitudinal
transformer
provided better efficiency than the thickness mode transformer. In
foraddition,
AC-DC
adapter
integration
[72–77].
multilayer
construction
for AC-DCthe
adapter
integration
[72–77].of both the input and output allowed better design flexibility
addition, the
multilayer
construction
of bothNorfolk,
the input
and
output
allowed
better
designofflexibility
the
mid-1990s,
Face[72–77].
Electronics,
VA,
USA,
invented
a new
concept
power PT,
forToward
AC-DC
adapter
integration
Toward
the mid-1990s,
Face Electronics, Norfolk, VA, USA, invented a new concept of power
® (registered
for AC-DC
adapter
integration
[72–77].
Transoner
name
by
Face
Electronics,
Figure
15),
based
on
a
laminated
construction
[78].
® (registered
Toward the
mid-1990s,name
Face by
Electronics,
Norfolk,Figure
VA, USA,
invented
new concept
of power
PT, Transoner
Face Electronics,
15), based
on aa laminated
construction
Toward
the
mid-1990s,
Face
Electronics,
Norfolk,
VA,
USA,
invented
a
new
concept
of
power
®
The
radial
operation
and
a symmetric
design
was
included
in aa second
second
patent
PT,Transoner
Transoner
(registered
name
by Face
Electronics,
Figure
15),
based
on a laminated
construction
[78].
The Transoner
radial
operation
and
a symmetric
design
was
included
in
patent
by by
® (registered name by Face Electronics, Figure 15), based on a laminated construction
PT,
Transoner
A. A.
V.
Carazo
[79].
The
power
density,
design
flexibility,
manufacturing
simplicity
and
toughness
[78].
Transoner
radial
operation
a symmetric
was included
in a second
patent by
V.The
Carazo
[79]. The
power
density,and
design
flexibility,design
manufacturing
simplicity
and toughness
of
[78].Transoner
The Transoner
radial operation
and a becoming
symmetric the
design
was included
in applications
a second patent
by
of the
the
have
resulted
in
thisdesign
design
reference
power
many
A.
V.
Carazo [79].
The
powerin
density,
design
flexibility,
simplicity
and toughness
of
Transoner
have
resulted
this
becoming
themanufacturing
reference
forfor
power
applications
in in
many
A. V. Carazo
[79]. The power density, design flexibility, manufacturing simplicity and toughness of
research
publications.
the Transoner
have resulted in this design becoming the reference for power applications in many
research
publications.
the Transoner have resulted in this design becoming the reference for power applications in many
research publications.
research publications.
(a)
(b)
(c)
(a)
(b)
(c)
(a)
(b)
(c) (a) (a)
Figure
Different
configurations
of Transoner
radial-type
piezoelectric
transformer:
squareFigure
15.15.
Different
configurations
of Transoner
radial-type
piezoelectric
transformer:
square-shape
Figure Transoner;
15. Different(b)configurations
of Transoner
piezoelectric
(a)
squareshape
disc-shape
(c) radial-type
commercial
sample of transformer:
non-isolated
Transoner
Transoner;
(b)Different
disc-shape
Transoner;Transoner;
(c)
commercial
sample ofpiezoelectric
non-isolated
Transoner piezoelectric
Figure 15.
configurations
of Transoner
radial-type
transformer:
(a) squareshape Transoner;
(b) disc-shape
Transoner; (c) commercial sample of non-isolated Transoner
piezoelectric
transformer
[78,79].
transformer
[78,79]. (b) disc-shape Transoner; (c) commercial sample of non-isolated Transoner
shape Transoner;
piezoelectric transformer [78,79].
piezoelectric transformer [78,79].
In the late 1990s and early 2000s, the VPEC at Virginia Tech began a characterization study of
In
thethe
late
early
the
at Virginia
Virginia
Tech
began
characterization
study
In
late1990s
1990sand
and
early2000s,
2000s,
the VPEC
VPEC
at
Tech
began
aacharacterization
study
of of
Transoner
piezoelectric
transformer,
funded
in part
by
a grant
from
Virginia’s
Center for Innovative
In the
late 1990s and
early 2000s,
the VPEC
at Virginia
Tech
began
a characterization
study of
Transoner
piezoelectric
transformer,
funded
in
part
a
grant
from
Virginia’s
Center
for
Innovative
Transoner
piezoelectric
transformer,
funded
in
by
a
grant
from
Virginia’s
Center
for
Innovative
Technology (CIT), and also by Face Electronics [80,81]. The objective of this collaboration led to
Transoner(CIT),
piezoelectric
transformer,
funded in part
by a grant
from
Virginia’s
Center
for Innovative
Technology
also
The objective
objective
this
collaboration
led
Technology
(CIT),and
also
byFace
FaceElectronics
Electronics
[80,81].
The
ofofthis
collaboration
to to
intensive
research
inand
the
areaby
of
fluorescent
ballast[80,81].
using piezoelectric
transformers.
Perhaps theled
most
Technology (CIT), and also by Face Electronics [80,81]. The objective of this collaboration led to
intensive
research
area
fluorescent
ballast
using
transformers.
intensive
researchin
in
the
ofoffluorescent
using
piezoelectric
Perhaps
the
mostthe
relevant
outcome
ofthe
thisarea
research
was theballast
development
ofpiezoelectric
a new transformers.
inductor-less
conceptPerhaps
to drive
intensive research in the area of fluorescent ballast using piezoelectric transformers. Perhaps the most
relevant
outcome
of this
research
waswas
thethe
development
of input
aofnew
inductor-less
concept
to to
drive
most
relevant
outcome
of this
research
development
a new
inductor-less
concept
drive
piezoelectric
transformers
(Figure
16),
which
eliminated
the
series
inductor used
in previous
relevant outcome of this research was the development of a new inductor-less concept to drive
piezoelectric
transformers
(Figure
16),
which
eliminated
the
input
series
inductor
used
in
previous
piezoelectric
transformers
(Figure 16), which eliminated the input series inductor used in previous
driving approached
[82–84].
piezoelectric transformers (Figure 16), which eliminated the input series inductor used in previous
driving
approached[82–84].
[82–84].
driving
approached
driving approached [82–84].
Figure 16. Inductor-less Piezoelectric Transformer-based Linear Ballast with Power Factor Correction
Figure
Inductor-less
Piezoelectric
Transformer-based
Linear
Ballast
Factor
Correction
Figure
16.
Inductor-less
Piezoelectric
Linear
Ballastwith
withPower
Power
Factor
Correction
using
a16.
Transoner
radial
piezoelectricTransformer-based
transformer (Courtesy
of Micromechatronics).
Figure 16. Inductor-less Piezoelectric Transformer-based Linear Ballast with Power Factor Correction
using
a
Transoner
radial
piezoelectric
transformer
(Courtesy
of
Micromechatronics).
using a Transoner radial piezoelectric transformer (Courtesy of Micromechatronics).
using a Transoner radial piezoelectric transformer (Courtesy of Micromechatronics).
Also, late in the 1990s, shortly after the invention of Transoner, NEC switched the research on
Also,
late
in
the
1990s,type
shortly
the invention
of approach
Transoner,asNEC
theFace’s
research
on
power
PTs
to
athe
different
of after
PT, using
a similar
the switched
Transoner
radial
Also,
late
inin
1990s,
of Transoner,
Transoner, NEC
NEC
switchedthe
theresearch
research
Also,
late
the
1990s,shortly
shortlyafter
afterthe
the invention
invention of
switched
onon
power
PTs
to
a
different
type
of
PT,
using
a
similar
approach
as
the
Transoner
Face’s
radial
piezoelectric transformer. The NEC approach used a square geometry with an internal
power
PTsPTs
to a to
different
type of
PT, of
using
similara approach
as the Transoner
Face’s radial
piezoelectric
power
a different
type
PT,ausing
similar approach
as the Transoner
Face’s
radial
piezoelectric
transformer.
The
NEC approach used
a square
geometry
with an internal radial
electrode
[85,86],
operated in
a contour-extensional
vibration
mode
(Figure 17).
transformer.
The
NEC approach
used approach
a square geometry
with geometry
an internalwith
radial
[85,86],
piezoelectric
transformer.
The NEC
used a square
an electrode
internal radial
electrode [85,86], operated in a contour-extensional vibration mode (Figure 17).
electrode
operated in a contour-extensional
vibration
operated
in a[85,86],
contour-extensional
vibration mode (Figure
17).mode (Figure 17).
Figure 17. Contour-mode piezoelectric transformer [86].
Figure 17. Contour-mode piezoelectric transformer [86].
Figure17.
17.Contour-mode
Contour-mode piezoelectric
piezoelectric transformer
Figure
transformer[86].
[86].
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The use of Transoner PTs and the concept of inductor-less driving techniques was followed by
several other
research groups, including Ben-Gurion University (Beersheba, Israel) of
the Negev,
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Beer-Sheva, Israel [87–89].
Several other research groups have investigated other structures for step-down conversion. The
The use of Transoner PTs and the concept of inductor-less driving techniques was followed
unipoledbyPT,
originally
developed
byincluding
Berlincourt
in lateUniversity
1960s, is one
of the Israel)
topologies
that have also
several
other research
groups,
Ben-Gurion
(Beersheba,
of the Negev,
been considered
byIsrael
several
researchers for step-down applications.
Beer-Sheva,
[87–89].
Several
other Spain,
researchFerroperm,
groups haveDenmark,
investigatedthe
other
structures for
step-down de
conversion.
In Europe,
Alcatel
Universidad
Politécnica
Madrid, Spain,
The
unipoled
PT,
originally
developed
by
Berlincourt
in
late
1960s,
is
one
of
the
topologies
that
have for the
and the Universidad de Oviedo, Spain, collaborated in the ESPRIT IV project “TRAMST”
also been considered by several researchers for step-down applications.
development of a piezoelectric AC-DC converter for mobile phone battery chargers. The PT design
In Europe, Alcatel Spain, Ferroperm, Denmark, the Universidad Politécnica de Madrid, Spain,
was based
on
a ring-shaped
in thickness
mode,“TRAMST”
where theforprimary
and
and the
Universidad deelement
Oviedo, operating
Spain, collaborated
in the vibrational
ESPRIT IV project
the
secondary
sections are
an isolation
[90–100].
development
of aseparated
piezoelectricby
AC-DC
converterlayer
for mobile
phone battery chargers. The PT design
was
based
on
a
ring-shaped
element
operating
in
thickness
vibrational
where
the primary
andInfineon,
In the mid-2000s, Face Electronics, Fraunhofer Institute, led by mode,
Matthias
Radecker,
and
secondary
sectionsresearch
are separated
by an isolation
layer [90–100].of piezoelectric transformers, including
collaborated
in several
projects
on the application
In the mid-2000s, Face Electronics, Fraunhofer Institute, led by Matthias Radecker, and Infineon,
the development of a control IC [101–105]. One of the outcomes of this work was the development of
collaborated in several research projects on the application of piezoelectric transformers, including
a duty cycle
control strategy
implemented
byOne
a mathematical
thatwas
setthe
thedevelopment
duty cycle to meet
the development
of a control
IC [101–105].
of the outcomesfunction
of this work
ZVS depending
on the
frequency
[106]. Figure
shows afunction
40 W that
isolated
piezoelectric
of a duty cycle
control
strategy implemented
by a 18
mathematical
set the radial
duty cycle
to
meetdesigned
ZVS depending
on the
frequency [106].
Figure battery
18 showscharger
a 40 W isolated
radial piezoelectric
transformer
for a 24
V piezoelectric
AC-DC
for universal
input applications
transformer
designed
for
a
24
V
piezoelectric
AC-DC
battery
charger
for
universal
input
applications
(85–265 V), which was the result of this development collaboration.
(85–265 V), which was the result of this development collaboration.
Figure 18. Isolated piezoelectric radial transformer for use in AC-DC converter for universal input
Figure 18.
Isolated piezoelectric radial transformer for use in AC-DC converter for universal input
voltage operation (85–265 W) and 24V, 40 W output load (Courtesy of Micromechatronics, Inc.).
voltage operation (85–265 W) and 24V, 40 W output load (Courtesy of Micromechatronics, Inc.).
Late in the 2000s and the early 2010s, work was done at the University of Sheffield, UK,
LatebyinE.the
2000s and
the early
2010s,
work
was donetoatinductor-less
the University
of Sheffield,
L. Horsley
on Transoner
radial
PT and
its application
converters
[107–109].England, by
a research
line on
transformers
been established
at the Department
E. L. HorsleyRecently,
on Transoner
radial
PTpiezoelectric
and its application
tohas
inductor-less
converters
[107–109].
of
Electrical
Engineering
of
the
Technical
University
of
Denmark,
led
by
Michael
A.
E.
Andersen.
Recently, a research line on piezoelectric transformers has been established at the Department
The main focus of this research work is on inductor-less and bidirectional piezoelectric
of Electrical Engineering of the Technical University of Denmark, led by Michael A. E. Andersen. The
transformer-based converters [110–114].
main focus ofCurrently,
this research
work is onproduction
inductor-less
andof
bidirectional
piezoelectric
transformer-based
the development,
and sale
Transoner radial
transformers
is done at
converters
[110–114].
Micromechatronics, Inc., State College, Pennsylvania, USA, under the same technical team that started
the development
of the technology
at Face. and sale of Transoner radial transformers is done at
Currently,
the development,
production
Figure
19
illustrates
a
summary
of the
most relevant power
that has
Micromechatronics, Inc., State College,
Pennsylvania,
USA,transformers
under theconfigurations
same technical
team that
been considered through the state of the art.
started the development of the technology at Face.
Figure 19 illustrates a summary of the most relevant power transformers configurations that has
been considered through the state of the art.
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75 W
Step-up
Low Power
Step-down
High Power
40W/cm3
97%
30 W
20W/cm3
95%
20 W
5W/cm3
20W/cm3
85-87%
5W
1954 Rosen
Longitudinal
20W/cm3
89%
15W/cm3
92%
20W/cm3
89-92%
1992 Thickness 1996 Longitud.
Mode PT
Mode PT
1996 Radial
Mode PT
1997 Contour
Mode PT
1997 Contour
Mode PT
1997 Thickness
Mode PT
Figure
The
stateofofthe
theart
artofofpower
powerpiezoelectric
piezoelectric transformers
transformers (PTs)
Micromechatronics,
Inc.).
Figure
19.19.
The
state
(PTs)technology
technology(courtesy
(courtesyofof
Micromechatronics,
Inc.).
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2. Space, Defense and Security Applications of Piezoelectric Transformers
2. Space,
Defense
and NASA
Security
Applications
of Piezoelectric
Transformers
During
the 2000s,
granted
several Small
Business Innovation
Research (SBIR) projects to
integrate
piezoelectric
transformers
into
space
applications
[115].
During the 2000s, NASA granted several Small Business Innovation Research (SBIR) projects to
integrate piezoelectric transformers into space applications [115].
2.1. High Voltage Piezoelectric-Based Power Supply to Drive the Main Payload of Satellite Communication
Systems:
Traveling
Wave Tube Power Supply to Drive the Main Payload of Satellite Communication
2.1.
High The
Voltage
Piezoelectric-Based
Systems: The Traveling Wave Tube
This NASA SBIR-founded research program, led by Dr. Carazo, developed a piezoelectric-based
This NASA
research
program,
led bypower
Dr. Carazo,
developed
electronic
powerSBIR-founded
conditioner (EPC)
module
to supply
to traveling
wavea piezoelectric-based
tubes (TWTs) used
electronic
power
conditioner
(EPC)
module
to
supply
power
to
traveling
wave
tubes
(TWTs)
for space-earth long-distance wireless communication [116–118]. The developed design
eliminated
used
for
space-earth
long-distance
wireless
communication
[116–118].
The
developed
design
conventional electromagnetic transformers with a compact and highly efficient piezoelectric
eliminated
conventional
with
a compact
and highly
efficient
piezoelectric
transformer
driver. The electromagnetic
developed EPCtransformers
unit consisted
of several
modules
providing
different
levels
transformer
driver.
The
developed
EPC
unit
consisted
of
several
modules
providing
different
levels
of high voltage and power to the different circuits of a TWT. Figure 20 illustrates the concept of the
of
high (Figure
voltage 20a)
and and
power
circuits
of a TWT.used
Figure
20 illustrates
project
onetoofthe
thedifferent
developed
EPC modules
to drive
a 4 kV/10the
W concept
cathode of
of the
the
project
TWT. (Figure 20a) and one of the developed EPC modules used to drive a 4 kV/10 W cathode of
the TWT.
SWITCHING
REGULATOR FOR
BUS VOLTAGE
FEEDBACK
LOOP
PROTECTION
CIRCUIT
OUTPUT
MULTIPLIER
SWITCHING
HALF-BRIDGE
INVERTER
(a)
SPACE FOR THE
PIEZOELECTRIC
TRANSFORMER
(b)
Figure 20.
20. (a)
(a) Modular
Modular concept
concept using
using Transoner
Transoner technology
technology applied
applied to
to aa traveling
traveling wave
wave tube
tube (TWT);
(TWT).
Figure
(b)
Detail
of
a
4
kV/10
W
cathode
power
supply
using
piezoelectric
transformer.
(b) Detail of a 4 kV/10 W cathode power supply using piezoelectric transformer.
2.2. Development of New Integrated Ignition Systems for Small Satellite Thruster by Using Piezoelectric
2.2. Development of New Integrated Ignition Systems for Small Satellite Thruster by Using Piezoelectric
TransformerTechnology
Technology
Transformer
This NASA
NASA SBIR-founded
SBIR-founded research
research program
program developed
developed aa new
new discharge
discharge initiation
initiation (DI)
(DI) system
system
This
using
a
piezoelectric
transformer
to
initiate
the
discharge
of
Pulsed
Plasma
Thrusters
used
in
small
using a piezoelectric transformer to initiate the discharge of Pulsed Plasma Thrusters used in small
satellites
using
solid-state
spark-plugs
[119–122].
The
program
took
as
a
reference
the
design
satellites using solid-state spark-plugs [119–122]. The program took as a reference the design
specifications of
of the
(EO-1)
launched
in
specifications
the PPT
PPT development
developmentfor
forthe
theEarth
EarthOrbit
Orbitexperimental
experimentalsatellite
satellite
(EO-1)
launched
November
2000.
The
complete
prototype
of
the
system
(called
IGNIT-SONER)
was
tested
in
May
in November 2000. The complete prototype of the system (called IGNIT-SONER) was tested in May
−6 to
−7 torr)
´10
6 to
2005 at
at NASA
vacuum
conditions
(10(10
over
2005
NASA Glenn
GlennResearch
ResearchCenter
Centerunder
underhigh
high
vacuum
conditions
10´7for
torr)
for50,000
over
accumulated
cycles.
Moreover,
the
unit
was
tested
for
over
500,000
accumulated
cycles
under
50,000 accumulated cycles. Moreover, the unit was tested for over 500,000 accumulated cycles under
atmospheric lab
lab conditions
conditions under
under maximum
maximum firing
firing rates
rates of
of 55 Hz.
Hz. The
The developed
developed transformer
transformer is
is an
an
atmospheric
input to
to output
output 33 kV-isolated
kV-isolated PT,
PT, and
and able
able to
to step
step up
up the
the input
input voltage
voltage to
to over
over 2.5
2.5 kV.
kV. The
The physics
physics
input
associated
with
semiconductor-type
spark
plugs
used
for
ignition
under
vacuum
conditions
led to
to
associated with semiconductor-type spark plugs used for ignition under vacuum conditions led
the
development
of
a
piezoelectric-to-capacitive
discharge
topology.
This
topology
allows
the
storing
the development of a piezoelectric-to-capacitive discharge topology. This topology allows the storing
the high
high voltage
voltage energy
energy delivery
delivery by
by the
the piezoelectric
piezoelectric transformer
transformer in
in aa capacitor,
capacitor, and
and then
then suddenly
suddenly
the
releasing
it
to
the
spark-plug.
This
sudden
release
makes
it
possible
to
achieve
very
high
releasing it to the spark-plug. This sudden release makes it possible to achieve very high currentcurrent
levels
levels
over
A formicroseconds,
few microseconds,
as required
to generate
the spark.
Figure
21a shows
of
of
overof100
A 100
for few
as required
to generate
the spark.
Figure
21a shows
one one
of the
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IGNIT-SONER
developed
prototypes.
Figure
21b shows
the vacuum
chamber
at NASA
Glenn where
the IGNIT-SONER
developed
prototypes.
Figure
21b shows
the vacuum
chamber
at NASA
Glenn
the IGNIT-SONER developed prototypes. Figure 21b shows the vacuum chamber at NASA Glenn
the
prototype
was
tested.
where the prototype was tested.
where the prototype was tested.
TEFLON
PROPELLANT
TEFLON
BAR
PROPELLANT
BAR
PPT
ELECTRODES
PPT
ELECTRODES
D.I. SPARK
D.I.PLUG
SPARK
PLUG
(a)
(a)
COAXIAL
CABLE
COAXIAL
CABLE
(b)
(b)
−3
Figure 21.
21. 15kV
kV initiationspark
spark
generated
with
IGNIT-SONER
prototype
system
Figure
generated
with
thethe
IGNIT-SONER
prototype
system
underunder
1.10´31.10
torr
−3
Figure 21. 15
15 kVinitiation
initiation spark
generated
with
the
IGNIT-SONER
prototype
system
under
1.10
torr
vacuum
conditions.
(a)
Prototype
of
the
IGNIT-SONER,
(b)
Vacuum
chamber
with
a
Pulsed
vacuum
conditions.
(a)
Prototype
of
the
IGNIT-SONER;
(b)
Vacuum
chamber
with
a
Pulsed
Plasma
torr vacuum conditions. (a) Prototype of the IGNIT-SONER, (b) Vacuum chamber with a Pulsed
Plasma Thruster
during
tests atGlenn
NASA Glenn Research
Thruster
during the
teststhe
at NASA
Center. Center.
Plasma Thruster
during
the
tests at NASAResearch
Glenn Research
Center.
2.3. High Voltage Power Supplies for Compact Neutron Generators
2.3. High Voltage Power Supplies for Compact Neutron
Neutron Generators
Generators
This project developed a very high voltage, (1 MV) and high power, (40 W) piezoelectric-driven
This project developed a very high voltage, (1 MV) and high power, (40 W) piezoelectric-driven
power supply to drive compact neutron generators. The power supply is based on a modular
power supply
The power
power supply
supply is based on a modular
supply to
to drive
drive compact
compact neutron
neutron generators.
generators. The
stackable system which combines piezoelectric transformers and Cockroft-Walton voltage multiplier
stackable system which combines piezoelectric transformers and Cockroft-Walton voltage multiplier
circuits [123,124]. Figure 22 illustrates a 200 kV stage using the high voltage/high power piezoelectric
circuits [123,124]. Figure
Figure 22
22 illustrates
illustrates aa 200
200 kV
kV stage
stage using
using the
the high
high voltage/high
voltage/high power piezoelectric
transformer driving a multi-stage Cockroft-Walton circuit.
transformer driving aa multi-stage
multi-stage Cockroft-Walton
Cockroft-Waltoncircuit.
circuit.
Figure 22. Connection of two modules able to generate 200 kV.
Figure
to generate
generate 200
200 kV.
kV.
Figure 22.
22. Connection
Connection of
of two
two modules
modules able
able to
3. Other Applications of Piezoelectric Transformers
3. Other Applications of Piezoelectric Transformers
3. Other Applications of Piezoelectric Transformers
3.1. High Voltage Non-Resonant Piezoelectric Transformer for Monitoring High Voltage Networks
3.1. High
High Voltage
Voltage Non-Resonant
Non-Resonant Piezoelectric
Piezoelectric Transformer
Transformer for
for Monitoring
MonitoringHigh
HighVoltage
VoltageNetworks
Networks
3.1.
One of the less usual fields of applications was proposed in the late 1990s by the University
One of
of the
the less
less usual
usual fields
fields of
of applications
applications was
was proposed
proposed in
in the
the late
late 1990s
1990s by
by the University
University
One
Politecnica
de Catalunya,
Spain, by
A. V. Carazo in
his doctoral thesis
research
[125]. the
In this work, a
Politecnica
de
Catalunya,
Spain,
by
A.
V.
Carazo
in
his
doctoral
thesis
research
[125].
In
this
work,
a
Politecnica
depiezoelectric
Catalunya, Spain,
by A. V.
Carazo in his
thesis
[125].measurement
In this
work,
non-resonant
transformer
construction
wasdoctoral
evaluated
as aresearch
high voltage
non-resonant
piezoelectric
transformer
construction
was evaluated
as
voltage
measurement
atransducer
non-resonant
piezoelectric
transformer
construction
as aa high
highinside
voltage
for high
voltage networks.
Figure
23a showswas
oneevaluated
of the prototypes
of ameasurement
high voltage
transducer
for
high
voltage
networks.
Figure
23a
shows
one
of
the
prototypes
inside
of
a
high voltage
transducer
for
high
voltage
networks.
Figure
23a
shows
one
of
the
prototypes
inside
of
a
high
voltage
testing chamber. The figure shows the external epoxy isolating housing inside of which
there
is a
testing chamber.
chamber. The
figure
shows
the
external epoxy
epoxy isolating
isolating housing
inside
of
which
there is
is aa
testing
The
figure
shows
the
external
housing
inside
of
which
there
large piezoelectric actuating unit connected to a high voltage, and a small sensor unit connected that
large
piezoelectric
actuating
unit
connected
to
a
high
voltage,
and
a
small
sensor
unit
connected
that
large
piezoelectric
actuating
unit connected
to a high
voltage, and
small sensor
unit connected
provides
low voltage.
Both units
are mechanically
connected
but aelectrically
isolated,
as shownthat
the
provides low
low voltage.
voltage. Both
Both units
units are
are mechanically
mechanically connected
connected but
but electrically
electrically isolated,
isolated, as
as shown
shown the
the
provides
sketch of Figure 23b. The unit was designed for 36 kV.
sketch of
of Figure
Figure 23b.
23b. The
The unit
unit was
was designed
designed for
for 36
36 kV.
kV.
sketch
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Vout
Vout
Vin
Vin
(a)
(a)
(b)
(b)
Figure23.
23.Non-resonant
Non-resonantpiezoelectric
piezoelectrictransformer
transformerfor
formeasuring
measuringhigh
highvoltages
voltagesininpower
powerdistribution
distribution
Figure
Figure
23.
Non-resonant
piezoelectric
transformer
for
measuring
high
voltages
in
power
networks.(a)
(a)Piezoelectric
Piezoelectricmeasuring
measuringtransformer
transformerinside
insideofofaahigh
highvoltage
voltagetesting
testingchamber;
chamber,distribution
(b)Sketch
Sketch
networks.
(b)
networks.
(a)
Piezoelectric
measuring
transformer
inside
of
a
high
voltage
testing
chamber, (b) Sketch
illustrating
the
principle
of
operation
of
the
high
voltage
measuring
transformer.
illustrating the principle of operation of the high voltage measuring transformer.
illustrating the principle of operation of the high voltage measuring transformer.
3.2.Piezoelectric
PiezoelectricTransformers
Transformersfor
forIsolation
IsolationFeedback
Feedbackand
andGate
GateDriver
DriverApplications
Applications
3.2.
3.2. Piezoelectric Transformers for Isolation Feedback and Gate Driver Applications
Therehave
havebeen
beenseveral
severalpublications
publicationsrecently
recentlyon
onthe
theuse
useofofpiezoelectric
piezoelectrictransformers
transformerstotoisolate
isolate
There
Therecircuits
have been
severalas
publications
recently
on
the
useof
of Mosfet
piezoelectric
transformers
to isolate
feedback
[126–128],
well
as
for
driving
the
gate
transistors
while
providing
feedback circuits [126–128], as well as for driving the gate of Mosfet transistors while providing
feedback This
circuits
[126–128],
as well
as for driving
the gate
of Mosfet
transistors
while providing
isolation.
topic
wasinitially
initially
considered
inthe
the1980s
1980s
[46,47]
andthere
there
nowrenewed
renewed
interest
isolation.
This topic
was
considered
in
[46,47]
and
isisnow
interest
isolation.
This topic
was initiallyPiezoelectric
considered in
the 1980s [46,47]
andan
there
is now isolation
renewed interest
with
innovative
technologies.
transformers
provide
excellent
means,
with innovative technologies. Piezoelectric transformers provide an excellent isolation means,
with
innovative
technologies.
Piezoelectric
transformers
provide
an
excellent
isolation
means,
simultaneously providing
providing power
power and
andsignal
signaltransfer
transfertotodrive
drivethe
thegate
gateofoftransistors.
transistors. Figure
Figure
24
simultaneously
24
simultaneously
providing
power
and
signal
transfer
to
drive
the
gate
of
transistors.
Figure
24
exemplifies
the
use
of
PTs
for
control
of
the
gate
of
a
Mosfet.
In
this
case,
the
control
gate
signal
exemplifies the use of PTs for control of the gate of a Mosfet. In this case, the control gate signalis
exemplifies to
the useresonance
of PTs for control of required
the gate of a Mosfet. In
this case,
the control
gate signal
is
ismodulated
modulated tothe
the resonancefrequency
frequency requiredforfordriving
drivingthe
thePT.
PT.InInthe
theoutput
outputofofthe
thePT,
PT,a
modulated
to
the
resonance
frequency
required
for
driving
the
PT.
In
the
output
of
the
PT,
a
may
consist
of aofrectifier
circuit,
converts
the signal
backback
into the
ademodulator
demodulatorcircuit,
circuit,which
which
may
consist
a rectifier
circuit,
converts
the signal
intoinput
the
demodulator
circuit, which may consist of a rectifier circuit, converts the signal back into the input
control
signal.
input
control
signal.
control signal.
OSCILLATOR
OSCILLATOR
GATE
DRIVING
GATE
SIGNAL
DRIVING
MODULATOR
DEMODULATOR
MODULATOR
DEMODULATOR
SIGNAL
Figure 24. Piezoelectric-based isolated circuit for feedback or gate driver applications.
Figure 24. Piezoelectric-based isolated circuit for feedback or gate driver applications.
Figure 24. Piezoelectric-based isolated circuit for feedback or gate driver applications.
4. Conclusions
4.4. Conclusions
Conclusions
During the mid-1990s and through the first decade of 2000s, piezoelectric transformers spread
During
the
and through
first decade
of 2000s,
transformers
spread
During
themid-1990s
mid-1990s
throughthe
the
2000s,piezoelectric
piezoelectric
transformers
spread
widely
to a diverse
range of and
applications.
Thefirst
maindecade
area ofofapplication
was high voltage
piezoelectric
widely
to
a
diverse
range
of
applications.
The
main
area
of
application
was
high
voltage
piezoelectric
widely tofor
a diverse
range of applications.
The main
area of application
highapplications
voltage piezoelectric
inverters
CCFL backlighting
used in laptop
computers
LCD panels.was
Other
included
inverters
for
CCFL backlighting
backlightingused
used
in
laptop
computers
LCD
panels.
Other applications
inverters
for
CCFL
in
laptop
computers
LCD
panels.
Other
applications
included
ozone generators for medical, sanitary and beauty related applications and air cleaners. The
nonincluded
ozone generators
for medical,
sanitary
andrelated
beautyapplications
related applications
and air cleaners.
ozone
generators
for
medical,
sanitary
and
beauty
and
air
cleaners.
The
nonmagnetic characteristic of PTs, their compact size, their ability to generate high voltages, and their
The
non-magnetic
characteristic
of
PTs,
their
compact
size,
their
ability
to
generate
high
voltages,
magnetic
characteristic
of PTs,
their match
compact
their ability
to generate
highthis
voltages,
and
high
power
density was
a perfect
forsize,
portable
applications.
During
period,
it their
was
and
their
high density
power density
was
a perfect
match
for
portable
applications.During
Duringthis
this period,
period, ititwas
high
power
was
a
perfect
match
for
portable
applications.
estimated that the total sales per year exceeded 25–30 million with the main production locatedwas
in
estimated
thethe
total
sales
perper
yearyear
exceeded
25–30
million
with the
main
locatedlocated
in Japan.
estimatedthat
that
total
sales
exceeded
25–30
million
with
the production
main production
in
Japan.
Toward
the
end
of
the
2000s,
the
LCD
backlighting
technology
transitioned
from
the
use
of
CCFLs
Japan.
Toward the end of the 2000s, the LCD backlighting technology transitioned from the use of
to LEDs.
This has
in a2000s,
progressiveLCD
decline
in sales oftechnology
high voltage
piezoelectric
transformers.
Toward
the resulted
end of
backlighting
transitioned
from
the use of
CCFLs
to LEDs.
This
hasthe
resulted the
in a progressive
decline in sales of
high voltage
piezoelectric
Most
of
the
manufacturers
of
PTs
for
CCFL
backlighting
have
now
discontinued
their
production.
CCFLs to LEDs.
This
hasmanufacturers
resulted in a of
progressive
decline
in sales of
high
voltage
piezoelectric
transformers.
Most
of the
PTs for CCFL
backlighting
have
now
discontinued
their
Some
production
stillofremains
for some ionizing
products
using
single layer-type
PTs.
transformers.
Most
the
manufacturers
of
PTs
for
CCFL
backlighting
have
now
discontinued
their
production. Some production still remains for some ionizing products using single layer-type PTs.
Significant
research
has been
done
on the
applicability
of PTs
for other
commercial
uses, such
as
production.
Some
production
still
remains
for
some
ionizing
products
using
single
layer-type
PTs.
Significant research has been done on the applicability of PTs for other commercial uses, such as
powerSignificant
convertersresearch
for AC-DC
and DC-DC
applications.
For these
the required
PTsas
has been
on applications.
the
applicability
PTs applications,
forapplications,
other commercial
uses,PTs
such
power converters
for AC-DC
and done
DC-DC
For of
these
the required
are
are
typically
the
step-down
type,
isolated,
and
with
higher
power
requirements.
The
cost
and
power
converters
for
AC-DC
and
DC-DC
applications.
For
these
applications,
the
required
PTs
are
typically the step-down type, isolated, and with higher power requirements. The cost and limitation
limitation
of
readily
available
products
are
one
of the
mainpower
challenges
in extending
the
applicability
of
typically
the
step-down
type,
isolated,
and
with
higher
requirements.
The
cost
and
limitation
of readily available products are one of the main challenges in extending the applicability of power
power
transformers.
of readily
available products are one of the main challenges in extending the applicability of power
transformers.
transformers.
Actuators 2016, 5, 12
17 of 22
Currently, the main area of interest of piezoelectric transformers is in medical, defense, security
and sensitive measurement equipment, where the PT can offer a differential value added compared to
magnetic transformers, even at a higher cost.
Conflicts of Interest: The authors declare no conflict of interest.
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