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7264 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 31, NO. 10, OCTOBER 2016
Performance Comparison of Three-Step and Six-Step
PWM in Average-Current-Controlled Three-Phase
Six-Switch Boost PFC Rectifier
Laszlo Huber, Senior Member, IEEE, Misha Kumar, and Milan M. Jovanovi
´
c, Fellow, IEEE
Abstract—In this paper, a three-step PWM method for the
average-current-controlled three-phase six-switch boost PFC rec-
tifier is proposed. It is shown that the three-step PWM compared
to the conventional six-step PWM exhibits a lower total harmonic
distortion of the input currents and higher power factor. However,
the three-step PWM, unlikethesix-stepPWM,isadversely affected
by the duty-cycle limitations and has unbalanced conduction losses
of the upper and lower switches of the three-phase rectifier bridge.
The average-current control of the three-step and six-step PWM
is illustrated with MATLAB/Simulink simulation waveforms and
experimentally verified on a 3 kW prototype.
Index Terms—Average-current control, discontinuous space-
vector modulation (SVM), duty-cycle limitation, low-pass filtering
and sampling, power factor(PF),segment detection, six-step PWM,
three-phase six-switch boost PFC rectifier, three-step PWM, total
harmonic distortion (THD), zero-sequence signal (ZSS) injection.
I. INTRODUCTION
T
ODAY, the active three-phase power factor correction
(PFC) rectifiers need to meet very challenging perfor-
mance requirements. In the majority of applications, the in-
put current of the active three-phase PFC rectifiers is required to
have a total harmonic distortion (THD) less than 5% and a power
factor (PF) greater than 0.99 [1]. One of the most cost-effective
topologies that can meet these requirements is the three-phase
six-switch boost PFC rectifier [2], which is usually implemented
without a neutral-point connection.
A number of control methods that can achieve a high quality
of input currents in the three-phase six-switch boost PFC rec-
tifier are available [3], [4]. Generally, approaches using direct
control of input current versus, for example, direct power con-
trol, result in a better quality of the input currents [5]. Today, the
control circuit is usually implemented with digital technology.
One direct current control method, well suited for the digital
implementation, is the average current control [6], [7].
In a three-wire three-phase applications because the sum of
the phase currents is zero, the control can be implemented by
having only two out of the three current controllers actively
shaping the current at a given time. The desired current in the
inactive-controller phase is obtained by the sum of currents of
Manuscript received August 10, 2015; accepted October 29, 2015. Date of
publication December 8, 2015; date of current version May 20, 2016. Recom-
mended for publication by Associate Editor T. Shimizu.
The authors are with the PowerElectronics Laboratory, Delta Products Corpo-
ration, Research Triangle Park, NC 27709 USA (e-mail: lhuber@deltartp.com;
misha.kumar@delta-corp.com; milan@deltartp.com).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TPEL.2015.2506554
the actively controlled phases. One implementation of this con-
trol method is based on dividing the line cycle of the input phase
voltages into six 60° segments [six-step pulse width modulation
(PWM)] as shown in Fig. 1(a) [8], [9]. In each 60° segment, the
controller in the phase with the highest absolute value voltage is
disabled, i.e., switches in the corresponding leg are turned
OFF,
which results in reduced switching losses.
In this paper, another implementation of this control method
is proposed. It is based on dividing the line cycle of the input
phase voltages into three 120° segments (three-step PWM) as
shown in Fig. 2. In each 120° segment, the controller in the phase
with the most positive (or most negative) phase voltage is dis-
abled, i.e., the switches in the corresponding leg are turned
OFF.
The three-step PWM is equivalent to the discontinuous space-
vector modulation with unbalanced conduction losses between
the upper and lower switches [10].
A detailed performance comparison of the three-step and six-
step PWM in the average-current controlled three-phase six-
switch boost PFC rectifier is also provided in this paper. It is
shown that the three-step PWM compared to the six-step PWM
exhibits lower THD of input currents and higher PF. The oper-
ation of the three-step and six-step PWM is illustrated with the
MATLAB/Simulink simulation waveforms and experimentally
verified on a 3-kW prototype.
II. A
VERAGE CURRENT CONTROL WITH THREE-STEP AND
SIX-STEP PWM
In the six-step PWM, a line cycle of input phase voltages
is divided into six 60° segments such that within a 60° seg-
ment none of the three-phase voltages changes sign, as shown
in Fig. 1(a). In each 60° segment, the controller in the phase with
the highest absolute value voltage is disabled, i.e., switches in
the corresponding leg are turned
OFF. For example, in segment
I, the controller in phase “a” is disabled, i.e., switches S
ap
and
S
an
are turned OFF, as shown in Fig. 1(b). The simplified circuit
diagram of the three-phase six-switch boost PFC rectifier with
six-step PWM in segment I is shown in Fig. 1(c). Since in seg-
ment I, the leg in phase “a” is disabled and the phase current i
a
is positive, rectifier input aR is connected to the positive output
rail (through diode D
ap
). By sinusoidal modulation of switches
in legs “b” and “c,” desired sinusoidal average phase-to-phase
voltages v
aRbR
and v
aRcR
can be generated between the rectifier
inputs, respectively. As the sum of the phase–phase voltages at
the rectifier inputs must be zero, a desired voltage v
bRcR
is au-
tomatically generated. To generate sinusoidal average phase-to-
phase voltages between rectifier inputs, the current controllers
0885-8993 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications
standards/publications/rights/index.html for more information.
HUBER et al.: PERFORMANCE COMPARISON OF THREE-STEP AND SIX-STEP PWM 7265
Fig. 1. Six-step PWM: (a) 60º segments, (b) circuit diagram in segment I, (c) simplified circuit diagram in segment I.
Fig. 2. Three-step PWM with 120º segments referenced to: (a) positive enve-
lope, (b) negative envelope of input phase voltages.
are designed to control phase–phase currents. The output of
the phase–phase-current controllers determines the phase–phase
duty cycles d
ab
, d
bc
, and d
ca
so that in steady-state operation
v
aRbR
= d
ab
V
o
, v
bRcR
= d
bc
V
o
, and v
cRaR
= d
ca
V
o
. Switch
duty cycles d
ap
, d
bp
and d
cp
are obtained from phase–phase
duty cycles so that d
ab
= d
ap
− d
bp
, d
bc
= d
bp
− d
cp
, and
d
ca
= d
cp
− d
ap
. For example, in segment I, d
ap
=1and, there-
fore, d
bp
=1− d
ab
and d
cp
=1+d
ca
. It should be noted that
d
ap
=1means that the rectifier input aR is connected to the
positive output rail due to the conduction of diode D
ap
.The
steady-state duty cycle of all switches is summarized in Table I,
whereas, the variation of duty cycle d
ap
during the whole line cy-
cle is shownin Fig. 3 as an example. As can be seen in Fig. 3, duty
cycle d
ap
exhibits abrupt changes at 60°-segment transitions,
TABLE I
S
TEADY-STATE DUTY-CYCLE OF SWITCHES FOR SIX-STEP PWM
60° Segment d
ap
d
an
d
bp
d
bn
d
cp
d
cn
I101 − d
ab
d
ab
1+d
ca
−d
ca
II −d
ca
1+d
ca
d
bc
1 − d
bc
01
III 1+d
ab
−d
ab
101 − d
bc
d
bc
IV 0 1 −d
ab
1+d
ab
d
ca
1 − d
ca
V 1 − d
ca
d
ca
1+d
bc
−d
bc
10
VI d
ab
1 − d
ab
01−d
bc
1+d
bc
Fig. 3. Duty cycle of upper switch in phase “a” for six-step PWM at 120 V
rms
input voltage.
which induces input-current transients at the segment transi-
tions. These transients, which can be seenas notchesand glitches
in the input current waveforms and can be regarded as segment-
transition noise, may cause false segment detection in the
six-step PWM.
In the proposed three-step PWM, a line cycle of input phase
voltages is divided into three 120° segments such that within
a 120° segment one-phase voltage is always greater or smaller
than the other two phase voltages, as shown in Figs. 2(a) and
(b), respectively. Accordingly, the three-step PWM is either
referenced to the positive or to the negative envelope of the
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