This paper proposes an actively clamped twoswitch flyback converter. Compared to the conventional twoswitch flyback converter, the proposed twoswitch flyback converter operates with a wide duty cycle range. By using an activeclamp circuit, the proposed converter achieves zerovoltage switching for all of the power switches. Zerocurrent switching of an output diode is also achieved. Thus, compared with the conventional converter, the proposed converter realizes a higher efficiency with an extended duty cycle. The performance of the proposed converter is verified by the experimental results with use of a 1.0 kW prototype circuit.
I. INTRODUCTION
High switching frequency pulsewidth modulated DCDC converters have been widely used for switchmode power supplies
[1]

[8]
. Among these converters, the flyback converter is most popularly used because of its simple power circuit structure
[9]
,
[10]
. However, the conventional flyback converter is limited by high switch voltage stress
[11]
,
[12]
. The twoswitch flyback converter shown in
Fig. 1
uses an additional switch and two clamping diodes to overcome the drawback of the conventional flyback converter
[13]
. The two switches
S_{1}
and
S_{2}
are turned on and off simultaneously. The two clamping diodes
D_{C1}
and
D_{C2}
clamp the voltage across
S_{1}
and
S_{2}
by the input voltage
V_{in}
. The energy stored in the transformer
T
is recycled to the input side through the clamping diodes
D_{C1}
and
D_{C2}
[14]
. However, the conventional twoswitch flyback converter operates under a hardswitching condition
[15]
,
[16]
. The energy stored in the leakage inductor
L_{lk}
causes voltage spikes when
S_{1}
and
S_{2}
are turned off. The voltage spikes increase the switching losses and consequently decrease the power efficiency. Moreover, the duty cycle of the conventional twoswitch flyback converter is limited to 0.5 because the demagnetization of the transformer should be guaranteed
[17]
. The narrow duty cycle range limits the practical use of the twoswitch flyback converter.
Circuit diagram of the conventional twoswitch flyback converter.
To address these problems, this paper proposes an actively clamped twoswitch flyback converter.
Fig. 2
shows the circuit diagram of the proposed converter. The proposed converter has auxiliary switches
S_{3}
and
S_{4}
and one clamping capacitor
C_{c}
. By using an activeclamp circuit, the proposed converter extends the duty cycle of the converter. With the help of the clamping capacitor voltage
V_{c}
, the transformer can be demagnetized for the duty cycle from 0 to 1. Furthermore, zerovoltage switching (ZVS) of all of the power switches is achieved. Zerocurrent switching (ZCS) of an output diode is also achieved. Given that the proposed converter operates under softswitching conditions, this converter can better improve the power efficiency compared with the conventional converter. The operation principle and converter features are described with simulation verifications. The performance of the proposed converter is verified by the experimental results with the use of a 1.0 kW prototype circuit. Compared with the conventional converter, the proposed converter improves the efficiency by 1.5 % at the rated output power.
Circuit diagram of the activeclamped twoswitch flyback converter.
II. PROPOSED CONVERTER
 A. Operation Principle
Fig. 2
shows the circuit diagram of the proposed converter. The primaryside circuit consists of power switches (
S_{1}
,
S_{2}
,
S_{3}
,
S_{4}
), a clamping capacitor (
C_{c}
), and a transformer (
T
). Power switches have body diodes (
D_{1}
,
D_{2}
,
D_{3}
,
D_{4}
) and output capacitors (
C_{1}
,
C_{2}
,
C_{3}
,
C_{4}
). The transformer
T
has a magnetizing inductor (
L_{m}
) and leakage inductor (
L_{lk}
) with the turns ratio of 1:
N
, where
N
=
N_{s}
/
N_{p}
. The secondaryside circuit consists of an output diode (
D_{o}
) and an output capacitor (
C_{o}
).
V_{in}
is the input voltage.
V_{c}
is the clamping capacitor voltage.
V_{o}
is the output voltage.
Fig. 3
shows the operation modes of the proposed converter during one switching period
T_{s}
. The converter has five operation modes during
T_{s}
.
Fig. 4
shows the switching waveforms of the proposed converter during
T_{s}
.
Fig. 4
(a) shows the switch voltages
V_{S1}
,
V_{S2}
,
V_{S3}
, and
V_{S4}
and switch currents
i_{S1}
,
i_{S2}
,
i_{S3}
, and
i_{S4}
.
Fig. 4
(b) shows the output diode voltage
V_{Do}
, diode current
i_{Do}
, and primary current
i_{p}
. When
S_{1}
and
S_{4}
are turned on,
S_{2}
and
S_{3}
are turned off. When
S_{2}
and
S_{3}
are turned on,
S_{1}
and
S_{4}
are turned off. Power switches operate complementarily with a short dead time
T_{d}
. The duty cycle
D
is based on the ontime of
S_{1}
and
S_{4}
. Then, the duty cycle of
S_{2}
and
S_{3}
is 1 –
D
. Before
t
=
t_{0}
,
S_{2}
and
S_{3}
have been turned off. The voltages
V_{S1}
and
V_{S4}
have been zero when the primary current
i_{p}
flows through
D_{1}
and
D_{4}
.
Operation modes of the proposed converter during T_{s}.
Switching waveforms of the proposed converter during T_{s}: (a) switch voltages V_{S1} ,V_{S2}, V_{S3}, and V_{S4} and switch currents i_{S1}, i_{S2}, i_{S3}, and i_{S4} and (b) output diode voltage V_{Do}, diode current i_{Do}, and primary current i_{p}.
Mode 1 [t_{0}, t_{1}]:
At
t
=
t_{0}
,
S_{1}
and
S_{4}
are turned on at zero voltage.
L_{m}
and
L_{lk}
stores energy from
V_{in}
. The magnetizing inductor current
i_{Lm}
increases linearly as follows:
Mode 2 [t_{1}, t_{2}]:
At
t
=
t_{1}
,
S_{1}
and
S_{4}
are turned off. The primary current
i_{p}
charges
C_{1}
and
C_{4}
and discharges
C_{2}
and
C_{3}
.
V_{S1}
increases from zero to
V_{in}
.
V_{S4}
increases from zero to
V_{in}
+
V_{c}
.
V_{S3}
decreases from
V_{in}
+
V_{c}
to zero.
V_{S2}
decreases from
V_{in}
to zero. Given that the switch output capacitor
C_{s}
(=
C_{1}
=
C_{2}
=
C_{3}
=
C_{4}
) is very small, the time interval during this mode is considered negligible compared with
T_{s}
.
i_{Lm}
is considered to be constant. The leakage inductor
L_{lk}
starts discharging its energy by the primary current
i_{p}
.
D_{2}
and
D_{3}
conduct the primary current
i_{p}
.
Mode 3 [t_{2}, t_{3}]:
At
t
=
t_{2}
,
S_{2}
and
S_{3}
are turned on at zero voltage.
i_{Lm}
decreases linearly as follows:
When the output diode
D_{o}
is turned on, the energy stored in
L_{m}
is transferred to the output. A seriesresonance between
L_{lk}
and
C_{c}
occurs. As the energy stored in
L_{lk}
is fully discharged by the seriesresonance, the output voltage
V_{o}
at the secondary side is reflected to the primary side. The primary current
i_{p}
flows as follows:
Z_{r}
is the impedance of the seriesresonant circuit.
ω_{r}
is the angular resonant frequency as follows:
Mode 4 [t_{3}, t_{4}]:
At
t
=
t_{3}
, the seriesresonance is finished when the output diode current
i_{Do}
is zero.
D_{o}
is turned off at zero current. ZCS of
D_{o}
is achieved. The leakage inductor
L_{lk}
has no energy in this mode.
Mode 5 [t_{4}, t_{5}]:
At
t
=
t_{4}
,
S_{2}
and
S_{3}
are turned off. The primary current
i_{p}
charges
C_{2}
and
C_{3}
and discharges
C_{1}
and
C_{4}
.
V_{S1}
decreases from
V_{in}
to zero.
V_{S4}
decreases from
V_{in}
+
V_{c}
to zero.
V_{S3}
increases from zero to
V_{in}
+
V_{c}
.
V_{S2}
increases from zero to
V_{in}
. The leakage inductor
L_{lk}
starts charging its energy by the primary current
i_{p}
.
D_{1}
and
D_{4}
conduct the primary current
i_{p}
. The next switching cycle repeats when
S_{1}
and
S_{4}
are turned on at zero voltage.
 B. Converter Features
By the voltsecond balance law on
L_{m}
during
T_{s}
, the following relation between
V_{in}
and
V_{c}
is obtained as follows:
From (6), the clamping capacitor voltage
V_{c}
is obtained as follows:
By the voltsecond balance law on the secondary winding of
T
during
T_{s}
, the following relation between
V_{in}
and
V_{o}
is obtained:
Fig. 5
shows the graph between the normalized voltage gain and the duty cycle
D
. The duty cycle ranges from 0 to 1. As shown in (7), the clamping capacitor voltage
V_{c}
is changed by the duty cycle
D
. This clamping capacitor voltage affects the transformer in the form of demagnetizing voltage when
S_{2}
and
S_{3}
are turned off. The proposed converter has a wider duty cycle compared with that of the conventional twoswitch flyback converter.
Graph between the normalized voltage gain and the duty cycle D.
At
t
=
t_{0}
, to achieve ZVS of
S_{1}
and
S_{4}
, the energy stored in
L_{m}
is larger than the energy stored in
C_{s}
. The following condition should be satisfied to achieve ZVS of
S_{1}
and
S_{4}
:
At
t
=
t_{2}
, to achieve ZVS of
S_{2}
and
S_{3}
, the energy stored in
L_{m}
is larger than the energy stored in
C_{s}
. The following condition should be satisfied to achieve ZVS of
S_{2}
and
S_{3}
:
At
t
=
t_{3}
, to achieve ZCS of
D_{o}
, the diode current
i_{Do}
becomes zero before
D_{o}
is turned off. The time interval from
t_{3}
to
t_{4}
should be ensured to achieve ZCS of
D_{o}
. This time duration can be changed by the angular resonant frequency
ω_{r}
. The critical condition is
i_{p}
(
T_{s}
) =
i_{Lm}
(
T_{s}
). Then, the angular resonant frequency must satisfy the following condition: as
where the critical angular resonant frequency
ω_{rc}
= 2
πf_{rc}
is decided by
where
R_{o,min}
is the minimum output resistance.
III. SIMULATION VERIFICATIONS
Fig. 6
shows the simulation results of the proposed converter when
V_{in}
is 350 V,
V_{o}
is 200 V, and
D
is 0.4.
Fig. 6
(a) shows the switch voltages
V_{S1}
and
V_{S2}
and switch currents
i_{S1}
and
i_{S2}
for a 1.0 kW output power.
Fig. 6
(b) shows the switch voltages
V_{S3}
and
V_{S4}
and switch currents
i_{S3}
and
i_{S4}
for a 1.0 kW output power. Switch currents are negative before the power switches are turned on. Switch currents flow through the body diodes of the power switches before the power switches are turned on. Thus, ZVS of power switches is achieved.
Fig. 7
shows the simulation results of the proposed converter when
V_{in}
is 350 V,
V_{o}
is 450 V, and
D
is 0.6.
Fig. 7
(a) shows the switch voltages
V_{S1}
and
V_{S2}
and switch currents
i_{S1}
and
i_{S2}
for a 1.0 kW output power.
Fig. 7
(b) shows the switch voltages
V_{S3}
and
V_{S4}
and switch currents
i_{S3}
and
i_{S4}
for a 1.0 kW output power. As shown in
Fig. 7
, ZVS of power switches is achieved. Moreover, the duty cycle of the converter can be extended to 0.6. The proposed converter operates with a wide duty cycle and reduced switching losses.
Fig. 8
shows the simulated waveforms of the diode voltage
V_{Do}
, diode current
i_{Do}
, and primary current
i_{p}
when
V_{in}
is 350 V,
V_{o}
is 200 V, and
D
is 0.4 for a 1.0 kW output power. For the seriesresonance between
L_{lk}
and
C_{c}
,
L_{lk}
= 7.0 μH and
C_{c}
= 1.0 μF are selected. The resonant frequency
f_{r}
(=
ω_{r}
/2
π
) is decided as
f_{r}
= 60.1 kHz. Before the output diode is turned off, the diode current becomes zero. ZCS of an output diode is achieved, which reduces the switching power losses of the converter.
Simulation results for D = 0.4: (a) switch voltages V_{S1} and V_{S2} and switch currents i_{S1} and i_{S2} and (b) switch voltages V_{S3} and V_{S4} and switch currents i_{S3} and i_{S4}.
Simulation results for D = 0.6: (a) switch voltages V_{S1} and V_{S2} and switch currents i_{S1} and i_{S2} and (b) switch voltages V_{S3} and V_{S4} and switch currents i_{S3} and i_{S4}.
Simulated waveforms of the diode voltage V_{Do}, diode current i_{Do}, and primary current i_{p} for D = 0.4.
IV. EXPERIMENTAL RESULTS
A 1.0 kW prototype circuit has been developed to verify the operation principles and performance of the proposed converter.
Table I
shows the electrical specification of the proposed converter.
Table ІІ
shows the parameters of the power circuit components.
ELECTRICAL SPECIFICATION OF THE PROPOSED CONVERTER
ELECTRICAL SPECIFICATION OF THE PROPOSED CONVERTER
PARAMETERS OF THE POWER CIRCUIT COMPONENTS
PARAMETERS OF THE POWER CIRCUIT COMPONENTS
Fig. 9
shows the experimental results of the proposed converter for an openloop test. When
D
is 0.4,
V_{o}
is 200 V for
V_{in}
= 350 V.
Fig. 9
(a) shows the switch voltages
V_{S1}
and
V_{S2}
and switch currents
i_{S1}
and
i_{S2}
for a 1.0 kW output power.
Fig. 9
(b) shows the switch voltages
V_{S3}
and
V_{S4}
and switch currents
i_{S3}
and
i_{S4}
for a 1.0 kW output power. ZVS of power switches is achieved, which reduces the switching losses at the primary side.
Fig. 10
shows the experimental results of the proposed converter for an openloop test. When
D
is 0.6,
V_{o}
is 450 V for
V_{in}
= 350 V.
Fig. 10
(a) shows the switch voltages
V_{S1}
and
V_{S2}
and switch currents
i_{S1}
and
i_{S2}
for a 1.0 kW output power.
Fig. 10
(b) shows the switch voltages
V_{S3}
and
V_{S4}
and switch currents
i_{S3}
and
i_{S4}
for a 1.0 kW output power. The proposed converter can operate when the duty cycle is over 0.5.
Fig. 11
shows the experimental waveforms of the diode voltage
V_{Do}
, diode current
i_{Do}
, and primary current
i_{p}
when
V_{o}
is 200 V with
D
= 0.4 for a 1.0 kW output power. The resonant frequency
f_{r}
is around
f_{r}
= 60 kHz, which is above the switching frequency
f_{r}
= 50 kHz. ZCS of output diode is also achieved, which reduces the switching power losses at the secondary side.
Fig. 12
shows the experimental waveforms of the proposed converter for a closedloop test. This figure also shows the output voltage
V_{o}
and output current io when the output power is changed abruptly. The output voltage
V_{o}
is regulated when the output power changes from 0.5 kW to 1.0 kW.
Fig. 13
shows the measured power efficiency curves of the different power levels. The conventional twoswitch flyback converter achieves the efficiency of 93.0 % for a 1.0 kW output power. On the contrary, the proposed converter realizes the efficiency of 94.5 % for a 1.0 kW output power. The proposed converter improves the converter efficiency by 1.5 %. The main factor for the efficiency improvement is the reduced switching losses. Given that the proposed converter is developed for high input voltage applications, the switching losses are more dominant than the conduction losses. The input voltage of the proposed converter is 350 V. For such a high input voltage, the switching losses are more significant than the conduction losses. This significance is because the average current of the switching devices is reduced as the input voltage of the converter is increased. The duty cycle range is also extended from 0 to 1 for the practical use of the proposed converter for a wide input voltage range.
Experimental results for D = 0.4: (a) switch voltages V_{S1} and V_{S2} and switch currents i_{S1} and i_{S2} and (b) switch voltages V_{S3} and V_{S4} and switch currents i_{S3} and i_{S4}.
Experimental results for D = 0.6: (a) switch voltages V_{S1} and V_{S2} and switch currents i_{S1} and i_{S2} and (b) switch voltages V_{S3} and V_{S4} and switch currents i_{S3} and i_{S4}.
Experimental waveforms of the diode voltage V_{Do}, diode current i_{Do}, and primary current i_{p} for D = 0.4.
Experimental waveforms of the output voltage V_{o} and output current i_{o} when the output power is changed abruptly.
Measured power efficiency curves of the different power levels.
V. CONCLUSION
This paper has proposed an actively clamped twoswitch flyback converter. Operation principle and converter features of the proposed converter are described. The duty cycle range is extended by using an activeclamp circuit. ZVS of all power switches is achieved. ZCS of an output diode is also achieved. The proposed converter reduces switching power losses with an extended duty cycle range. Simulation verifications and experimental results are presented to verify the performance of the proposed converter. The proposed converter realizes the efficiency of 94.5 % for a 1.0 kW output power. This converter improves power efficiency by 1.5 % for a 1.0 kW output power compared with the conventional converter. The proposed converter is suitable for a highefficiency isolated power supplies for a wide input voltage range.
Acknowledgements
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MISP) (20100028509).
BIO
MinKwon Yang was born in 1987 in Jeonju, South Korea, in 1987. He received his B.S. in Electronic Engineering from Chonbuk National University, Jeonju, South Korea, in 2012. He is currently working toward his Ph.D. in Electronic Engineering at Chonbuk Nation University. His current research interest is designing of switching power converters including the circuit design and control.
WooYoung Choi was born in 1979 in Gwangju, South Korea. He received his B.S. in Electrical Engineering from Chonnam National University, Gwangju, South Korea, in 2004; and his Ph.D. in Electronic and Electrical Engineering from the Pohang University of Science and Technology, Pohang, South Korea, in 2009. Since 2010, he has been with the Division of Electronic Engineering at Chonbuk National University, Jeonju, South Korea, where is currently working as an Associate Professor. His research interest is power electronics system design for highefficiency switching power converters.
Lin B. R.
,
Chiang H. K.
,
Wang S. L.
2014
“Interleaved ZVS DC/DC converter with balanced input capacitor voltages for highvoltage applications,”
Journal of Power Electronics
14
(4)
661 
670
DOI : 10.6113/JPE.2014.14.4.661
Chen Z.
,
Zhou Q.
,
Xu J.
,
Zhou X.
2014
“Asymmetrical pulsewidthmodulated fullbridge secondary dual resonance DCDC converter,”
Journal of Power Electronics
14
(6)
1224 
1232
DOI : 10.6113/JPE.2014.14.6.1224
Zhang J.
,
Wang S.
,
Wang Z.
,
Tian L.
2014
“Design and realization of a digital PV simulator with a pushpull forward circuit,”
Journal of Power Electronics,
14
(3)
444 
457
DOI : 10.6113/JPE.2014.14.3.444
Lin B. R.
,
Nian Y. B.
2014
“Analysis and implementation of a new threelevel converter,”
Journal of Power Electronics
14
(3)
478 
487
DOI : 10.6113/JPE.2014.14.3.478
Jeong D. K.
,
Ryu M. H.
,
Kim H. G.
,
Kim H. J.
2014
“Optimized design of bidirectional dual active bridge converter for lowvoltage battery charter,”
Journal of Power Electronics
14
(3)
468 
477
DOI : 10.6113/JPE.2014.14.3.468
Baei M.
,
Narimani M.
,
Moschopoulos G.
2014
“A new ZVSPWM fullbridge boost converter,”
Journal of Power Electronics
14
(2)
237 
248
DOI : 10.6113/JPE.2014.14.2.237
Park C. H.
,
Cho S. H.
,
Jang J. H.
,
Pidaparthy S. K.
,
Ahn T. Y.
2014
“Average current mode control for LLC series resonant DCtoDC converters,”
Journal of Power Electronics
14
(1)
40 
47
DOI : 10.6113/JPE.2014.14.1.40
Lin B. R.
2013
“Analysis, design and implementation of a soft switching DC/DC converter,”
Journal of Power Electronics
13
(1)
20 
30
DOI : 10.6113/JPE.2013.13.1.20
Hu W.
,
Zhang F.
,
Xu Y.
,
Chen X.
2014
“Output voltage ripple analysis and design considerations of intrinsic safety flyback converter based on energy transmission modes,”
Journal of Power Electronics
14
(5)
908 
917
DOI : 10.6113/JPE.2014.14.5.908
Kim D. H.
,
Park J. H.
2013
“High efficiency stepdown flyback converter using coaxial cable coupledinductor,”
Journal of Power Electronics
13
(2)
214 
222
DOI : 10.6113/JPE.2013.13.2.214
Kim J. K.
,
Moon G. W.
2015
“Derivation, analysis, comparison of nonisolated singleswitch high stepup converters with low voltage stress,”
IEEE Trans. Power Electron.
30
(3)
1336 
1344
DOI : 10.1109/TPEL.2014.2316324
Vartak Q.
,
Abramovitz A.
,
Smedley K. M.
2014
“Analysis and design of energy regenerative snubber for transformer isolated converters,”
IEEE Trans. Power Electron.
29
(11)
6030 
6040
DOI : 10.1109/TPEL.2014.2301194
Kim M. G.
,
Jung Y. S.
2009
“A novel softswitching twoswitch flyback converter with a wide operating range and regenerative clamping,”
Journal of Power Electronics
9
(5)
772 
780
Yazdani M. R.
,
Rahmani S.
“A new zerocurrent transition twoswitch flyback converter,”
in Proc. PEDSTC
2014
390 
395
MurthyBellur D.
,
Kazimierczuk M. K.
2011
“Zerocurrent transition twoswitch flyback pulsewidth modulated DCDC converter,”
IET Power Electron.
4
(3)
288 
295
DOI : 10.1049/ietpel.2009.0253
inaba C. Y.
,
Konishi Y.
,
Nakaoka M.
2004
“Highfrequency flybacktype softswitching PWM DCDC power converter with energy recovery transformer and auxiliary passive lossless snubbers,”
IET Electric Power Applications
151
(1)
32 
37
DOI : 10.1049/ipepa:20031060
Zhao J.
,
Dai F.
“Softswitching twoswitch flyback converter with wide range,”
in Proc. ICIEA
2008
250 
254