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IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 1, JANUARY 2008
229
Unity-Power-Factor Operation of Three-Phase
AC–DC Soft Switched Converter Based On Boost
Active Clamp Topology in Modular Approach
H. M. Suryawanshi, Member, IEEE, M. R. Ramteke, K. L. Thakre, and V. B. Borghate
Abstract—In this paper, a three-phase ac–dc converter using
three single-phase pulse width modulated active clamped,
zero-voltage-switched boost converter in modular approach is
presented. The active clamp technique is used for zero-voltageswitching of the main and auxiliary switches. The operating
modes, analysis, and design considerations for the proposed
converter are explained. To evaluate the performance of the proposed converter, finally simulation and experimental results for a
500-V, 1.5-kW prototype converter are presented. The proposed
converter operates at almost unity power factor with reduced
output filter size. The output voltage is regulated without affecting
zero-voltage-switching, even under unbalanced three-phase input
voltages.
Index Terms—Active clamp, boost converter, three-phase ac–dc
converter, unity power factor.
I. INTRODUCTION
U
SAGE of power electronic (PE) converters are ever increasing in the processing of electrical energy in industrial
applications such as adjustable speed drives (ASD), SMPSs,
UPSs, etc. [1]. Therefore, the converters with high power factor
are increasingly required in industries. In high-power range,
mainly a three-phase system is employed. Most of the PE systems which get connected to ac utility mains use diode rectifiers at the input. The nonlinear nature of diode rectifiers cause
significant line current harmonic generation; thus, they degrade
power quality, increase losses, failure of some crucial medical
equipment, and so on. Therefore, stringent international harmonic standards are imposed, and hence power factor correction
(PFC) circuits are incorporated in PE systems. Earlier, to reduce
rectifier-generated harmonics, expensive and bulky filter inductors and capacitors were installed, but they effectively eliminate only certain harmonics [2]. The active power line conditioners (APLCs) used for harmonic reduction are generally hard
switched; hence, the components are subjected to high-voltage
stresses which increases further with increase in the switching
frequency. Also, hard switching results in low efficiency, large
EMI, etc., as discussed in [3].
Manuscript received January 25, 2007; revised May 23, 2007. Recommended
for publication by Associate Editor P. M. Barbosa.
The authors are with the Department of Electrical Engineering,
Visvesvaraya National Institute of Technology, Nagpur-440011, India
(e-mail: [email protected]; [email protected]; klthakre@
vnitnagpur.ac.in; [email protected]).
Digital Object Identifier 10.1109/TPEL.2007.911842
The soft-switched resonant converters are also reported in
[4]–[6]. However, some of their characteristics such as large
conduction losses, high component stresses, load limitation, and
high cost restrict the practical use of these converters as discussed in [7]. Such converters are usually operated in variablefrequency mode, and thus components are required to be designed at the lowest operating frequency. Also, resonant tank
circuits are required to be designed at a much higher kVA/kW
rating.
The active clamp technique is one of the most attractive zerovoltage-switching (ZVS) topologies [8], [9]. Medium and high
power ac–dc converters usually make use of continuous conduction mode (CCM) boost topology as it gives near to unity power
factor (UPF) at the AC input [10].
Industries are focusing on extension of existing and wellproven single-phase technology for the development of highpower converters. A modular approach provides a convenient
paralleling of modules, thus facilitating power expandability.
Modules being identical, reserve inventory requirement, manufacturing cost, and time are also reduced. It would also reduce
a problem like arduous heat dissipation and expensive components of high rating which may occur in single high power design [11], [12].
Modular approach is also presented in [13] and [14]. The
proposed converter has higher efficiency and lower THD in
input current than [13]. In [14], there exist interactions between
different modules which causes boost inductor current of the
same module to be different during the off interval of the main
switches. In the proposed converter, no such interaction takes
place between modules.
The active clamp technique is well known for ZVS operation
in various converters, but its application in three-phase, high
input power factor ac–dc converters in modular approach is not
yet reported in the literature.
In this paper, a three-phase ac–dc converter with input powerfactor almost unity and soft-switching topology in modular approach is presented. An identical single-phase boost type active clamped ac–dc converter is connected in each line of a
three-phase AC source. Outputs of all the three converter modules are connected in parallel to raise the power level. Each
single-phase module is operated from 115-V, 50-Hz input to
give regulated output of 500 V with a switching frequency of
50 KHz. The power rating of each module is 500 W. The circuit
configuration, operation, analysis, and design considerations of
the proposed converter are presented. Finally, simulation and experimental results obtained with the prototype are presented to
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IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 1, JANUARY 2008
Fig. 2. Single-phase module of the proposed converter.
Fig. 1. Three-phase modular converter system.
demonstrate the performance and design considerations. Similar to a three-phase active rectifier, the output voltage has a
dominant ripple of six times the line frequency. Thus, a smaller
output filter capacitor is required. It also works under unbalanced input ac voltage maintaining almost unity power factor
at the input. As a separate PFC-PWM controller is provided for
each module, the independent control is facilitated even under
unbalanced input voltages. The PFC-PWM controllers are not
very costly; therefore, increase in the cost is marginal. Reliability in term of availability of output power to some extent in
case of failure of a module or single phasing is also achieved.
The proposed converter maintains an input power-factor almost
unity, regulated output voltage, and ZVS for all the switches
even under variable load conditions. It has only two switches in
each module, and has lesser components and lesser complexity
compared to [3], [5], and [15]. The proposed converter has applications such as drives, power conditioning stages of UPSs,
etc.
IC (UC3854) and fed to Driver IC (UC3706). Drive IC provides
complementary gate drive pulses with sufficient dead band. A
PCB-mounted miniature current LEM is used for sensing the
boost inductor current. When boost inductor current exceeds
the set limit, drive pulses are disabled, hence the converter is
protected. Proposed converter uses average current mode control. In average current mode control, boost inductor current is
continuously monitored and controlled to follow the reference
signal proportional to ac line voltage. Thus, input current is sinusoidal. To regulate output voltage, a multiplier circuit controls
the amplitude of the sinusoidal current reference signal in accordance with the voltage error signal generated using the output
voltage and rectified input ac voltage. When the load decreases,
the output voltage increases. To maintain constant load voltage,
the control circuit senses the load voltage and the pulse width
is automatically reduced in the switching cycle and the output
voltage is regulated and maintained almost constant. The control circuit varies the duty ratio in switching cycles over the input
supply voltage cycle, as the instantaneous input supply voltage
is varying over the cycle.
B. Operating Principle
II. CIRCUIT DESCRIPTION AND OPERATING PRINCIPLE
A. Circuit Description
Fig. 1 shows simplified block-diagram of a three-phase ac–dc
converter in a modular system. Fig. 2 shows circuit diagram
of the proposed single-phase module. The proposed converter
and
folconsists of a small line filter comprising of
lowed by single-phase line rectifier
and a very small
. Unlike the conventional
high-frequency bypass capacitor
and the
boost converter, in addition to the boost inductor
high-frequency (HF) rectifier output diode ; the resonant inductor
in series and resonant capacitor
in parallel are
. The auxiliary switch
with
connected to the main switch
is connected between
series connected clamping capacitor
and the cathode of the . The small capacthe drain of the
itor
is used as a high-frequency bypass filter at the output
of each module. Both the switches are driven in a complemenis used at the
tary manner. The single output filter capacitor
output of the proposed three-phase converter whose size get reduced drastically owing to the fact that, like the three-phase active rectifier, the dominant ripple frequency is six times the input
source frequency.
By sensing boost inductor current, output dc and input ac voltages, gating pulses are generated accordingly using PFC-PWM
To simplify the analysis and operating modes of the circuit,
the following assumptions are made.
• The semiconductor devices, inductors, and capacitors are
ideal.
• The output filter capacitor
is large enough to maintain
constant output voltage .
is constant over one
• The rectified output voltage
switching cycle as switching frequency is very high compared to ac input frequency.
is much larger than
; and
• The boost inductor
is much larger than capacitor .
clamping capacitor
• The energy stored in
is larger than energy stored in .
The circuit behavior during one switching cycle can be explained in seven modes with the help of key waveforms in
Fig. 3 and operating states of the converter in Fig. 4 under
steady state. The sequential circuit states are described below.
: Fig. 4(a) shows the operating state of the
Mode 1
is already in
circuit during this interval. The auxiliary switch
starts conan off-state. The body diode of the main switch
ducting, and the output diode continues conduction. During
continues to decrease
this mode, the boost inductor current
also starts decreasing. The negand the output diode current
continues to decrease. This
ative resonant inductor current
mode ends when it decreases to zero, and the body diode of the
SURYAWANSHI et al.: UNITY-POWER-FACTOR OPERATION OF THREE-PHASE AC–DC SOFT SWITCHED CONVERTER
231
: As shown in Fig. 4(d), the current, which
Mode 4
was flowing through the main switch, is diverted to resonant
. The voltage across the main switch
starts
capacitor
increasing from zero, and when it reaches , this mode ends.
During this mode
(4)
(4a)
Fig. 3. Key waveforms of the proposed converter.
main switch
ceases to conduct. To turn-on the main switch
with ZVS, a gate pulse must be applied during this interval.
During this mode
(1)
(1a)
(1b)
(1c)
Mode 2
: Fig. 4(b) shows the operating state of the
turns on
circuit during this interval. At , the main switch
and starts carrying the positive resonant inductor current
.
is achieved. The boost inductor current
Thus, ZVS of switch
continues to decrease. This mode ends when output diode
becomes zero. During this mode
current
where
and
.
As soon as the main switch voltage reaches approximately
starts conducting. The body
equal to , the output diode
diode of the auxiliary switch
starts conducting only after
. Since the interval
the main switch voltage reaches
between the start of conduction of the diode and the start of
conduction of the body diode of auxiliary switch is very small
compared to switching cycle, this is not treated as a separate
continues until the body
mode. Actually, current through
starts conducting.
diode of the auxiliary switch
: Fig. 3(e) shows the operating state of
Mode 5
the circuit during this interval. The boost inductor current
starts decreasing. The output diode
continues conducting,
starts increasing. If clamping capacitor
and its current
is large, then clamping capacitor voltage
is almost constant,
and inductor current
decreases linearly, otherwise varies resonantly. This mode ends when the resonant inductor current
becomes zero. Thus, the body diode of the auxiliary switch
ceases to conduct. During this interval, a gate pulse must be applied to auxiliary switch in order to achieve ZVS. During this
mode
(5)
(5a)
(2)
(5b)
(2a)
(5c)
(5d)
(2b)
Mode 3
: During this mode, the main switch continues to conduct as shown in Fig. 4(c). A load is supplied by the
output filter capacitor . Input power is stored in boost inductor
and resonant inductor . Therefore, the boost inductor curstarts increasing. The resonant inductor current
conrent
is
tinues to increase. This mode ends when the main switch
turned off. During this mode
where
and
Mode 6
: As shown in Fig. 4(f), the auxiliary switch
starts conducting. Thus, ZVS of switch
is achieved. The
boost inductor current
continues to decrease. This mode
ends when the auxiliary switch is turned off. During this mode
(3)
(6a)
(3a)
where
is the duty cycle of the main switch, and
duty cycle for the interval
.
is the
(6)
(6b)
(6c)
(6d)
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IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 1, JANUARY 2008
Fig. 4. Operating modes of the proposed converter.
Mode 7
: Fig. 4(g) shows the operating state of the
concircuit during this interval. The boost inductor current
continues to decrease due
tinues to decrease. The voltage
. When
becomes zero, the
to negative inductor current
starts conducting and this
body diode of the main switch
and the resonant capacmode ends. The resonant inductor
form a resonance circuit. To achieve ZVS of the main
itor
switch
,
must reach zero at the end of this mode. This
SURYAWANSHI et al.: UNITY-POWER-FACTOR OPERATION OF THREE-PHASE AC–DC SOFT SWITCHED CONVERTER
requirement compels that stored energy in the resonant inductor
must be greater than resonant capacitor :
233
TABLE I
COMPONENTS VALUES
(7)
(7a)
(7b)
and
where
.
III. DESIGN CONSIDERATION
Neglecting transition intervals between two switches and
based on voltage-second balance of the boost inductor , from
(1)–(3) and (5) and (6); we get
(8)
Therefore, output voltage
is given by
completely
Thus
(16)
(9)
If
and
, we get
(10)
If peak-to-peak ripple current of the boost inductor
Neglecting
interval
and , the required value of
By choosing a suitable value of
can be found out.
plays an important role. The main
The clamp capacitor
. If
switch voltage is clamped to the voltage level
is small,
will be relatively large. Thus, voltage stresses on
the switches will increase, and hence design of
is crucial.
is approximately
The duration required to reach to
can be
one half of the off period of the main switch, thus
expressed as
(11)
Therefore, from (10) and (11), we get
; thus, the time required for discharging
is given by
(17)
where
(12)
is the switching frequency.
will be maximum at full load with minimum rectified line
. Also, peak-to-peak ripple voltage of clamp
output voltage
capacitor
can be expressed as
(13)
will be maximum for
the value calculated by (13) at
value of boost inductor
.
should be greater than
; therefore, minimum
(18)
is
(14)
As per discussion in Mode 7, energy stored in the resonant inmust be greater than the energy stored in the resonant
ductor
to achieve ZVS for the main switch
. Thus
capacitor
(15)
discharges
At the end of Mode 7, the resonant capacitor
to zero in the quarter cycle of the resonant frequency
Peak-to-peak ripple voltage of clamp capacitor is generally kept
of the maximum capacitor dc voltage
below 20%
calculated using (17) above. If
, using (17)
can be obtained as in
and (18), the value of clamp capacitor
[10]
(19)
The value of the output filter capacitor
is dependent on
allowable peak-to-peak value of output voltage ripple
and maximum load current
and input line frequency
.
has to limit the six times the line frequency component
The
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IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 1, JANUARY 2008
Fig. 5. Simulation results at full load. (a) Clamp capacitor voltage (V ),
(b) boost inductor current (i ), and (c) resonant inductor current (i ).
Fig. 6. Simulated input currents (i
at full load.
;i ;i
) and input R-phase voltage (v )
Fig. 8. Experimental input R-phase voltage (v ) and input currents
(i ; i ; i ) at (a) full load (b) 50% of full load (c) 25% of full load, and
(d) full load with unbalanced input voltage (v = 115 V, v = 126:5 V, and
v = 103:5 V),(scales: 50 V/div., 2 A/div., 5 ms/div.).
of the rectifier output voltage. For the given
can be obtained [5], [15] as
Fig. 7. Experimental main switch voltage (v ) and main switch current
(i ), and auxiliary switch voltage (v ) and auxiliary switch current (i ) at
full load (scales: 200 V/div., 5 A/div., 1 s/div. (zoom view)).
and
,
(20)
SURYAWANSHI et al.: UNITY-POWER-FACTOR OPERATION OF THREE-PHASE AC–DC SOFT SWITCHED CONVERTER
235
Fig. 10. Experimental main diode current (i ) and resonant inductor current
(i ) at full load (scales: 5 A/div., 5 s/div.).
Fig. 9. Experimental performance characteristics of the proposed converter (a)
efficiency, (b) power factor, and (c) % THD.
IV. SIMULATION AND EXPERIMENTAL RESULTS
The converter shown in Figs. 1 and 2 is designed as discussed
in Section III and simulated. The converter behavior is studied
at different load conditions as well under unbalanced input voltages. The components’ values (shown in Table I) and other specifications of the single-phase module of the proposed converter
are as follows.
Specifications:
: 115 V , 50 Hz.
Input voltage
: 500 V.
Output voltage
: 500 W.
Output power
Total output power
W.
: 50 kHz.
Switching frequency
For inductors, Litz wires NELC175/40SPSN which has 175
strands of 40 AWG is used. This reduces the effective resistance
,
of inductor as well as its skin effects. For boost inductor
, N87 ferrite core (Siemens make) and for resonant
,
, N87 ferrite core (Siemens make) is
inductor
used.
,
Fig. 5 shows the simulated clamping capacitor voltage
, and resonant inductor current
boost inductor current
at full load, respectively. Fig. 5(b) of boost inductor current
shows that the converter operates in continuous conduction
is
mode (CCM). The maximum resonant inductor current
. The voltage
the same as that of maximum boost inductor
is maximum at full load,
across the clamping capacitor
which is 23.5 V in this case. Fig. 6 shows the simulated threeat
phase input currents with input R-phase input voltage
full load. It is observed that ac input line currents follow their sinusoidal input voltages; thus, input power-factor is almost unity.
Fig. 11. Output voltage under step change in load. (a) From full load to half
load. (b) From half load to full load. (scales: 100 V/div., 5 ms/div).
The experimental switch voltages and currents at full load are
shown in Fig. 7, and it is evident that both the switches operate
with ZVS. Fig. 8 shows the experimental three-phase input current with R-phase voltage under variable load conditions from
full load to 25% of full load with balanced three-phase input
voltages. From Fig. 8(a)–(c), it is evident that the proposed converter operates at almost unity power factor over a wide range
of load variation, and input current decreases with decrease in
load. Fig. 8(d) shows the experimental input currents under uninput voltages (
V,
balanced Rated voltage
V, and
V) at full load. It is seen that the
proposed converter maintains unity power factor even under unbalanced input voltages. Under unbalanced input voltages, neutral current will have triple harmonics. The performance characteristics of the proposed converter under variable load condition
are given in Fig. 9. It is observed that the efficiency varies from
94.9% to 92.5%, power factor varies from 0.999 to 0.997, and
total harmonic distortion (THD) varies from 1.1% to 2.7% from
full load to 25% of full load. In a hard-switched converter, efficiency is nearly about 90% [16]. The proposed converter has
a higher efficiency compared to [13] and [16]. Fig. 10 shows
and main diode current
.A
resonant inductor current
SiC diode is used as an output diode . Therefore, it has no reverse recovery characteristics [17]. However, if a fast recovery
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IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 1, JANUARY 2008
Si diode is used, its reverse recovery current is controlled by resconnected in series with main switch
.
onant inductor
For the given output voltage, reverse recovery current will decrease with increase in . Fig. 11 shows the variation in output
voltage under step-change in load condition. It is observed that
the converter maintains almost constant load voltage.
V. CONCLUSION
High power quality three-phase ac–dc converters are being
widely used in the industries. In this paper, a three-phase ac–dc
converter using three single-phase modules adopting active
clamped boost topology has been presented. The operating
modes, analysis of the circuit, and design considerations are
explained. The simulation and experimental results on laboratory prototype (500 V, 1.5 kW) are presented. The experimental
results are in good agreement with simulation results. The
proposed converter has smaller output filter capacitor and lesser
component count as compared to other topologies. It operates
at almost unity power factor, low THD, and high efficiency. In
addition, it maintains unity power factor and regulated output
voltage with ZVS over the wide range of the load even-with
unbalanced input voltages.
[13] M. L. Heldwein, A. Ferrari de Souza, and I. Barbi, “A simple control
strategy applied to three-phase rectifier units for telecommunication
applications using single-phase rectifier modules,” in Proc. IEEE PESC
’99, pp. 795–800.
[14] G. Spiazzi and F. C. Lee, “Implementation of single-phase boost
power-factor correction circuits in three-phase applications,” IEEE
Trans. Ind. Electron., vol. 44, no. 3, pp. 365–371, Jun. 1997.
[15] M. A. Chaudhari and H. M. Suryawanshi, “High power factor operation
of three-phase ac-to-dc resonant converter,” IEE Proc. Elect. Power
Appl., vol. 153, pp. 873–882, Nov./Dec. 2006.
[16] H. Bodur and A. F. Bakan, “A new ZVT-PWM dc-dc converter,” IEEE
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[17] M. M. Jovanovic and Y. Jang, “State-of-the-art, single-phase, active
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overview,” IEEE Trans. Ind. Electron., vol. 52, no. 3, pp. 701–708,
Jun. 2005.
H. M. Suryawanshi (M’06) received the B.E. degree
from Shivaji University, Kolhapur, India in 1988, the
M.E. degree from IISc., Bangalore, India in 1994, and
the Ph.D. degree from Nagpur University, Nagpur,
India, in 1998, all in electrical engineering.
He is currently working as an Assistant Professor
in the Department of Electrical Engineering, Visvesvaraya National Institute of Technology, Nagpur.
His research interests include the field of power
electronics, emphasizing developmental work in the
area of resonant converters, power factor correctors,
active power filters, FACTs devices, and electric drives.
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M. R. Ramteke received the M.Tech. degree in electronics engineering from Nagpur University, Nagpur,
India, in 1994. He is currently pursuing the Ph.D. degree at Nagpur University.
He is currently working as an Assistant Professor
in the Department of Electrical Engineering, Visvesvaraya National Institute of Technology. (VNIT),
Nagpur. His research interests includes resonant
converters and power quality.
K. L. Thakre received the B.E., M.E., and Ph.D.
degrees in electrical engineering from Nagpur
University, Nagpur, India, in 1971, 1973, and 2001,
respectively.
He is currently working as a Professor in the
Department of Electrical Engineering, Visvesvaraya
National Institute of Technology, Nagpur, India.
His research interests include the fields of power
electronics; AI techniques for control, protection,
and monitoring of power systems; and power system
operation in deregulated environment.
V. B. Borghate received the B.E. and M.Tech. and
Ph.D. degrees in electrical engineering from Nagpur
University, Nagpur, India, in 1982, 1984, and 2007,
respectively.
Currently, he is working as an Assistant Professor
in the Department of Electrical Engineering, Visvesvaraya National Institute of Technology, Nagpur,
India.