Study of low voltage high current single phase controlled rectifier with
a bi-directional IGBT switch on the primary side of the transformer
Seshanna Panthala
Faculty of Engineering, Assumption University
Bangkok, Thailand
Abstract
An IGBT bi-directional switch is applied in a circuit to realize a single-phase
controlled rectifier without using any scr or triac. A micro controller based gate drive
circuit is used to control the on-off instants of the switch to produce one pulse in each
half cycle of a.c. Three modes of switch control are described which result in unity,
leading or lagging fundamental power factor on supply side.
Keywords: IGBT bi-directional switch, switch control, harmonic spectrum.
(b) to-back connected SCRs on primary side of
the transformer and then rectify the secondary
voltage with power diodes. Such schemes are
shown in Fig.1 using single-phase supply. In
practice, the scheme shown in Fig.1(b) is used
as the current commutation takes place on low
current side. The principles of operation of
such schemes are well understood and applied
widely in industrial power supplies and also
discussed in textbooks on power electronics
(Sen 1988). The input fundamental power
factor will always be lagging unless special
gate control is adopted using forced
communication.
1. Introduction
Many industrial applications need lowvoltage, high-current d.c for their operation.
Examples of such industrial applications are
electroplating, extraction of metals by
electrolysis etc. Most often voltage/current is to
be controlled. Since standard electric supply
available is a.c, controlled rectifiers are used to
obtain the variable d.c from a.c source.
Normally a transformer is used to step down
the a.c to the required level and also provide
isolation. There are two options to obtain the
controlled d.c voltage: (a) use silicon controlled
rectifiers on secondary side to rectify a.c and
SCR1
SCR2
SC R 1
L
D1
a.c
L
a.c
RL
RL
SC R 2
D2
(a)
(b)
Fig.1. Conventional low-voltage controlled rectification
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In this paper a novel controlled rectifier
is studied using a true bi-directional switch on
the primary side of the transformer as shown in
Fig.2. On the secondary side power diodes are
used for full wave rectification. Large smoothing inductor is used to make the load current
constant.
G
(a)
E1
E2
C1 C2
G2
G1
(b)
Fig.2. Controlled rectifier using a bidirectional switch and diodes
A true bi-directional switch is that switch
which can carry current in either direction and
can be made on or off at any time with a
control signal.
Such a true bi-directional
switch implemented on a silicon die as a single
integrated package is not available in the
market to date (to the author’s knowledge).
However, such a bi-directional switch can be
realized with the available components such as
IGBTs (or MOSFETS) and diodes in the forms
depicted in Fig.3. For the switch configuration
shown in Fig 3(a), we have two diodes and the
IGBT conduct during on state whereas for the
switch configuration shown in Fig. 3(b) we
need two floating d.c supplies for the two gate
drives as collectors are made common. The
configuration shown in Fig. 3(c) with common
emitters needs one floating d.c supply for gate
drive. It is to be noted that anti parallel diodes
are required across the IGBTs for the switch
configurations shown in Fig.3(b) and Fig.3(c).
Hence if the IGBTs used have internal in built
diodes there is no need for external diode
connection. Otherwise external diodes are to be
connected across the IGBTs. For the switch
configuration shown in Fig.3(a) an IGBT with
or without an in built diode can be used.
However for the study presented in this paper
the switch configuration shown in Fig.3(c) is
used and such modules are available (Dynex
2003; International Rectifier, nd.).
E1 E 2
C1
C2
G2
G1
(c)
Fig.3. Three possible bi-directional switch
configurations
2. Working Principles of the
Controlled Rectifier
The bi-directional switch of the
controlled rectifier (see Fig. 2) can be
controlled in different modes to produce one
pulse in each half cycle of a.c. The control
signal for the bi-directional switch is to be
synchronized to the a.c line. In each half cycle
of the a.c , the switch is made on and off only
once thus operating the switch in single pulse
mode in this study. The bi-directional switch
can be controlled in three different modes and
produce a single pulse in each half cycle. They
are: (i) the switch is made on and off such that
the conduction period is symmetrical about the
peak of the a.c in each half cycle called
symmetrical angle control (ii) the switch is
made on at the zero crossing of a.c in each half
cycle but made off after an angle β(<180o)
called extinction angle control, and (iii) the
switch is made on after a delay angle of α after
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the zero crossing of a.c, but made off at the
next zero crossing in each half cycle called
delay angle control.
2.1 Symmetric Angle Control
The signal waveforms at different points
in the circuit are shown in Fig. 4(a) under ideal
conditions of operation for symmetric angle
control of the switch called mode 1. The
average load voltage is given by:
Vod.c =
1 π −α
Vom sinθdθ
π ∫α
=
2
Vom cos α
π
for α varying from 0 - 90o . Assuming a large
smoothing inductor in series with the load, the
load current can be regarded as constant. In
such a case the a.c supply current will be
square pulses as shown in Fig. 4(a) neglecting
the magnetizing current of the primary and the
charging current of the R-C circuit across
primary. The Fourier series of this type of
current is given by:
Fig. 4(a)
4
4
Is cos α sin ω t Is cos 3α sin 3ω t
π
3π
4
+
Is cos5α sin5ω t + ...
5π
Is =
2.2 Extinction Angle Control
In the second mode called extinction
angle control, the switch is made on at the
beginning of each half cycle and kept on for an
angle β and then switched off. The waveforms
are shown in Fig. 4(b). The average load
voltage in this case is given by:
1 β
VLav = ∫ Vom sin θ .dθ = Vπm (1 − cos β )
π 0
The Fourier series of the supply current is
given by:
4
4
3β
β
is = Is sin sin(ωt + ϕ 1) +
Is sin
sin(3ωt + ϕ 3) + ...
2
3π
2
π
π nβ
when ϕn = −
2 2
It is to be noted that the fundamental
component of the supply current leads the
supply voltage. That means this rectifier acts
like a capacitive load and can compensate for
the lagging volt-amperes drawn by other loads
and provides a means for power factor
correction. This is an interesting feature of this
mode of operation and merits consideration for
The fundamental component of the a.c
supply current is in phase with the supply
voltage - thus making the displacement factor
(also called fundamental power factor) unity
and there is no fundamental reactive power
transport to and from the load. This feature is
in contrast with the controlled rectifier schemes
shown in Fig. 1(b) where in the power factor is
always lagging. However, there is reactive
power transport due to harmonic components
of the current. It can be seen from equation (2)
that the 3rd harmonic component in the supply
current can be eliminated entirely by making
α=30o that is the conduction period becomes
120o. This fact has been verified during
experimentation (see Fig. 8). If α is fixed at 30o
then the d.c load current can only be controlled
by the variation of a.c supply voltage.
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controlled rectifiers using SCRs. The ideal
signal waveforms are shown in Fig. 4(c). In
this case the average load voltage is given by:
application in industry. The displacement
power factor is given by:
DPF = sin
β
2
VLav =
rd
Again the 3 harmonic component in the
supply current can be eliminated by making the
extinction angle β =120o. This fact has been
verified during experimentation. See Fig. 8(b)
1
π
Vom sin θ .dθ = Vπm (1 + cos α )
π ∫α
The Fourier series of the supply current is
given by:
is =
4
π
Is cos
α
2
sin(ωt − ϕ 1) +
when ϕn =
nα
2
4
3α
sin(3ωt − ϕ 3) + ...
Is cos
3π
2
Vs (V)
Vm
π
0
3π
2π
V g (V)
ωt
α
ωt
V p(V)
ωt
V sw (V)
ωt
is(A)
IS
ωt
V L(V)
ωt
iL(A)
IL
iD(A)
ωt
IL
0
IL /2
π
2π
3π
ωt
Fig 4 (c)
It can be seen that the fundamental
component of the supply current lags the
supply voltage thus drawing reactive
power(lagging) at fundamental frequency and
this is in addition to the reactive volt-amps
drawn by the harmonic components of the
current. The displacement power factor is
given by:
α
DPF = cos
2
From the industrial application point of
view, operation of the controlled rectifier in
modes 1 and 2 will be of interest as the
fundamental power factor is unity and leading
respectively. Implementation of such schemes
Fig 4 (b)
Fig.4. Signal waveforms under ideal conditions
(a) Symmetric angle control
(b) Extinction angle control
(c) Delay angle control
2.3 Delay angle control
In the third mode called delay angle
control, the switch is made on after an angle α
from the start of each half cycle but switched
off at the end of each half cycle. This mode is
similar to the control of the conventional
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8bit micro controller (P89C51RD2HBP) with
its PCA and associated module 0 is used to
develop the gate control signal for the switch.
The zero crossing pulses derived from the a.c
supply will synchronize the gate signal whereas
the d.c control voltage to the ADC will be used
to control the gate drive pulse width. The gate
drive signal from the micro controller is
optically isolated using an optocoupler 4N33.
The 15 volt d.c supply used to drive the gates is
referenced to the floating emitter potential.
is easy using the switch configurations shown
in Fig. 3(c) compared to the earlier schemes
using SCRs with forced commutation.
It is interesting to analyze what happens
during the blanking period when the bidirectional switch is in off condition, thus
making the a.c supply current zero and
consequently the primary current also goes to
zero. However, the load current on secondary
side continues to flow due to the large inductance
in series with the load. The primary cannot
have equivalent balancing ampere turns during
the off period of the switch which forces the
secondary ampere turns also to be zero. In order
to produce zero ampere turns on the secondary
side and at the same time keep the load current
flowing and constant, the load current splits
equally into the two halves of the secondary as
shown in Fig. 5. Both diodes conduct equally
and simultaneously. This phenomenon takes
place naturally twice in each cycle of the a.c
wave. This fact has been verified during
experimentation. See Fig. 7(a), 7(b), 7(c).
L
3.2 Experimental Results
The waveforms of voltage and currents
obtained at different points in the test setup for
the three modes of operation described above
are presented in Fig. 7 under steady state
condition. The d.c load current was kept at 5
amperes level while the primary supply voltage
was at 110 Vrms. The waveforms obtained
experimentally agree closely with the ideal
waveforms obtained experimentally agree
closely with the ideal waveforms shown in
Fig. 4. It can be seen from the experimental
waveforms that both diodes on the secondary
side conduct the load current equally when the
bi-directional switch on primary side goes to
off state periodically as shown in Fig. 5 Also
the supply current waveforms and their
harmonic contents are shown in Fig. 8 for the
switch conduction period equal to 120o. It can
be seen that the 3rd harmonics is clearly
minimized as predicted in theory given in
sections 2.1 and 2.2
iL /2
is = 0
L
R
L
iL
N
iL /2
Fig. 5. Load current flow paths when the
switch is in off condition
3.3 Some practical considerations
3. Experimental Study
The R-C snubber circuit across the
primary of the transformer is required in order
to: (a) provide a path for the magnetizing
current of the transformer when the primary
side switch goes to off state assuming the
secondary load side is open circuited, and (b)
absorb the magnetic energy stored in the
leakage inductance of the primary even with
secondary load side has a closed path when the
switch goes from on to off state repeatedly.
Failure to provide this R-C circuit will lead to
the damage of the switch components due to
large induced voltages. However the design of
3.1 Description of the experimental set up
The basic circuit arrangement of the
controlled rectifier studied this paper is shown
in Fig.6. The two gates have been joined, thus
needing only one gate control drive signal. In
this case only that IGBT of the switch will
conduct depending on the a.c supply polarity.
But the gate drive circuit has to charge both the
gate capacitances. Alternatively it is possible to
drive the gates selectively if the polarity
information of the a.c wave is available. Philips
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switching type power supplies is the voltsecond balance in each half cycle. Hence it is
important to ensure that the volt-seconds
applied to the primary in each half cycle be
same. Otherwise core saturation in one
direction will occur over a period of time
leading to the failure of the power supply.
Precise switching instants in each half cycle
can be implemented with micro controllers
with errors in the range of microseconds and
the problem does not appear to be serious now
if the IGBTs are matched.
these R-C components is difficult as the
applied voltage to the primary is not a regular
sinusoid-in fact only part of the a.c sine wave is
applied in each half cycle. The reactance of the
capacitor XC is made equal to the magnetizing
reactance Xm of the transformer referred to
primary. This is only a guideline.
The series resistance R is there to limit
the initial charging current of the capacitor at
each switch on. This initial capacitor current is
to be limited to be well within the rating of the
bi-directional switch elements.
Another problem that is always present in
5V
8
ADC
VC
b
i
t
m
I
c
r
o
c
o
n
t
r
o
l
l
e
r
Vs
iD
is
+5 V +15 V
R
Isolated
gate
driver
VG
Vs w
C
iL
Vp
L
RL
- VL +
Step-dow n
Transformer4:1
Zerocrossing
pulses (100Hz)
Fig. 6. Basic controlled rectifier experimental set up
Fig. 7(b). Extinction angle control
Fig. 7(a). Symmetrical angle control
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similar performance. Selected harmonic
component in supply current can be eliminated
by adjusting the conduction angle of the
switch. Multi-pulse operation can eliminate
more than one unwanted harmonic and is under
investigation.
Fig. 7(c). Delay angle control
Fig. 8(b). Supply current and harmonics
5. References
Dubey, G.K.S.R.Doradla; et al. 1996.
Thyristorised
Power
Controllers”,8th
reprint, New Age International (P) Ltd.,
Publ., New Delhi, India.
Dynex. 2003. IGBT Bi-Directional Switch
Module. Dynex Semiconductor, Lincoln,
U K.
International Rectifier. (no date). Gate Drive
Characteristics and Requirements of
HEXFETRS. Application Note 937, Section
10.
Fig. 8(a). Supply current and harmonics
4. Conclusions
IGBT based bi-directional switch is
applied in a single phase controlled rectifier
circuit. The ease with which the switch can be
controlled is demonstrated. The rectifier can be
operated with unity or leading fundamental
power factor without any forced commutation
circuit elements using symmetrical or
extinction angle Control respectively whereas
as conventional scr based controlled rectifiers
need forced commutation elements to produce
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