High-Frequency Link Inverter Based on
Multiple-Carrier PWM
Philip T. Krein, Xin Geng,
Robert Balog
University of Illinois
March 2002
Outline
• The PWM cycloconverter.
• Dual-carrier PWM to build waveforms for HF links.
• Properties of dual-carrier signals for gate drives and other
purposes.
• Experimental results.
• Conclusions.
The PWM Cycloconverter
• High-frequency (HF) link inverters can be constructed as:
– A cascade of a high-frequency dc-dc converter and inverter.
– A “square-wave cycloconverter,” in which a high-frequency
square-wave inverter provides the input to a cycloconverter.
High
Frequency
Inverter
Inverter
Load
(a) DC/DC converter type
High
Frequency
Inverter
CycloConverter
(b) Cycloconverter type
Load
The PWM Cycloconverter
• The dc-dc converter alternative has multiple power
conversion stages.
• The cycloconverter would seem to have complicated
operation, since it is treated as a nonlinear phase control
problem.
• The complexity has been one factor limiting use.
• Now consider a conventional PWM inverter. A two-level
inverter has a single input (Vin) and can produce an output
of Vin.
• There should be some way to work from an input of Vin
and generate exactly the same output waveform.
The PWM Cycloconverter
• This is the PWM cycloconverter: use as input a simple
square wave input at high frequency, then control the
switches to produce an output that is exactly a
conventional two-level PWM waveform.
• The PWM cycloconverter is not a new concept.
• What is new is that conventional PWM can be extended to
cycloconverter operation with a multiple-carrier PWM
process.
Dual-Carrier PWM
• Consider the use of two separate PWM waveforms,
modulated with a desired low-frequency waveform m(t).
• This could be separate rising and falling ramps, triangles
with phase shifts, or the like.
• Call these Carrier 1 and Carrier 2.
• Now modulate both Carrier 1 and Carrier 2 with the signal
m(t), to give PWM 1 and PWM 2.
• The sum PWM 1 + PWM 2 has low-frequency content
2m(t).
• The difference PWM 1 – PWM 2 has no low-frequency
content.
Dual-Carrier PWM
• Is the result trivial? Not if we use time multiplexing to
make sure the final waveform retains switching behavior.
• Several choices of combinations are available.
Decommutator
C1(t)
Comparator
gate drive sequence
P1
+/carrier
modulating
function
o
0/180 Phase
Shifter 1
C2(t)
0/180o Phase
Shifter 2
0/180o Phase
Shifter 3
HF link
voltage input
M1(t)
M2(t)
Comparator
P2
Sw itching
Control
cycloconverter
voltage output
Dual-Carrier PWM
• There are systematic ways to develop specific desirable
properties in the final output waveform.
• Example:
– Create two carriers from a single ramp just by blanking
every other pulse.
– Modulate both with m(t), then subtract.
– The result is a three-level high-frequency link signal.
• Another example:
– Split a triangle into separate rising and falling ramps.
– Modulate, respectively, with m(t) and –m(t).
– This yields a two-level signal “PWM” signal with no lowfrequency content.
Dual-Carrier PWM
C lo ck (squ a rew av e )
T r iang le
F irs t ca rr ie r
S econd ca rr ie r
C om p a re f irs t
to s ign a lm ( t)
C om p a re second
to s ign a l m
- ( t)
Dual-Carrier PWM
• The PWM sum has 50% duty, but retains the information.
C om p a re f irs t
to s ign a lm ( t)
C om p a re seco n d
to s ign a l m
- ( t)
U se o n e com p a ra to r to
se t , th e o th e r to re se t .
T h is is w
t o -ca r r ie r PW M ( t ) .
T h e o r ig in a l c lo ck
Dual-Carrier PWM
•
•
•
•
What about the desired two-level PWM output?
Use the square wave clock as the input to a cycloconverter.
Use the sum waveform PWM(t) as the gate control.
This “convolution” process of clock and PWM(t) recovers
the desired two-level PWM output.
U se o n e com p a ra to r to
se t , th e o th e r to re se t .
T h is is w
t o -c a r r ie r PW M ( t ) .
T h e o r ig in a l c lo ck
C lo ck co n v o lv ed w ith PW M ( t ) .
T h is is co n v en t io n a l PW M ,w ith
e a ch tu rn -o f f adv an c ed re la t iv e
to th e c lo ck .
Dual-Carrier PWM
• The PWM waveform is this example is also always phaseadvanced with respect to the original square wave.
• List the combinations for two-carrier PWM.
TABLE 1. COMBINATION CONDITONS (WITH DECOMMUTATOR) AND RESULTING TWO-CARRIER PWM SEQUENCES
Carrier
Type
Phase
Shifter 1
Phase
Shifter 2
Phase
Shifter 3
Combining
Method
Gate Drive
Signal Type
Output PWM Equivalent
1
Triangle
0
0
180º
Add
2-Level
Ramp PWM at double fswitch
2
Triangle
0
180º
0
Add
2-Level
Ramp PWM at double fswitch
3
Triangle
0
0
0
Subtract
3-Level
Triangle PWM
4
Triangle
180º
0
180º
Add
2-Level
Ramp PWM at double fswitch
5
Triangle
180º
180º
0
Add
2-Level
Ramp PWM at double fswitch
6
Triangle
180º
0
0
Subtract
3-Level
Triangle PWM
7
Ramp
0
0
180º
Add
2-Level
Triangle PWM
8
Ramp
0
180º
0
Add
2-Level
Triangle PWM
9
Ramp
0
0
0
Subtract
3-Level
Ramp PWM at double fswitch
Dual-Carrier PWM
• Triangle-based carrier sets with output delay and advance,
respectively.
comparing C1(t)
and M1(t)
P1(t)
comparing C2(t)
and M2(t)
P2(t)
gate drive
sequence
HF link
voltage
output
voltage
(a)
(b)
Dual-Carrier PWM
• Three-level PWM examples for HF links.
comparing C1(t)
and M1(t)
P1(t)
comparing C2(t)
and M2(t)
P2(t)
gate drive
sequence
HF link
voltage
output
voltage
(c)
(d)
Properties in the Two-Carrier Case
• We can select among several properties:
– By using carriers that alternately control turn-on and turnoff, gate waveforms with 50% duty can be generated.
– The combined signal can have pure advance or delay.
– The resulting PWM output can be generated with an
effectively doubled switching frequency.
Properties in the Two-Carrier Case
1. The two-carrier process allows conventional PWM
modulators, combined with some simple logic, to generate
waveforms for PWM cycloconverters.
2. With use of both advanced and delayed gating
waveforms, natural commutation can be supported, in a
manner equivalent to conventional sine wave SCR
cycloconverters.
3. The cases that yield 50% duty ratio gating signals are
especially valuable for transformer gate drives.
Experimental Results
• The two-carrier technique has been used to build a simple
“naturally commutated PWM cycloconverter.”
• This represents a high-frequency link inverter that makes
use of conventional PWM to provide control – with no
intermediate dc-dc converter.
• SCRs are used – only the leading edge of the combined
two-carrier signal is needed for the gate drives. (IGBTs
could have been used instead.)
Experimental Results
• NCC square-wave cycloconverter (three-phase).
Inverter
S1
S3
Cycloconverter
HF-TR
Q1
Q3
LC Filter
Load
E
S2
S4
Q2
Q4
Experimental Results
• Test circuit, single-phase output.
Cycloconverter
Q1
HF-TR
Q5
Q8
Gate Pulse for Q1
25
60mH
Square
waveform
Q3
current
sensor
Load
Gate Drive
Gate Drive
Q6
Q4
Q2
Q7
Gate Drive
Gate Drive
SGN(i)
+
- LM311
Ramp
m(t)
+
-1
-m(t)
PWM1
LM311
+
-
PWM2
LM311
Mono- Qd
stable
1/2 74LS221
Qa
Gate Drive
Gate Pulse for Q3
Gate Pulse for Q4
Gate Pulse for Q5
Gate Pulse
Qd
Mono- Qa
stable
1/2 74LS221
15  s
Gate Pulse for Q2
Gate Drive
15 s
Gate Pulse
Gate Pulse for Q6
Gate Drive
Gate Pulse for Q7
Gate Drive
Gate Pulse for Q8
Experimental Results
• The
modulation
process.
• Only the
leading
edges are
needed.
Experimental Results
• The crossover
behavior from
delayed gating
to advanced
gating.
Experimental Results
• Devices here
switch at 3750
Hz.
• Output is twolevel PWM at
7500 Hz.
Conclusion
• A multiple-carrier method can be used to generate PWM
gating signals with a variety of properties.
• Two-carrier signals chosen to cancel the baseband signal
m(t) support high-frequency link inverters.
• These inverters, PWM cycloconverters, eliminate a power
stage but produce conventional PWM output waveforms.
• The control complexity is similar to familiar PWM, but
with multiple signal paths.
Conclusion
• The multiple-signal output can be tailored for useful
properties, such as gate drives with 50% duty ratio under
all modulating signals, and outputs that provide an
effective doubling of the switching frequency.