A Novel Type High-Efficiency High-FrequencyLinked Full-Bridge DC-DC Converter Operating
under Secondary-Side Series Resonant Principle
for High-Power PV Generation
Daisuke Tsukiyama, Yasuhiko Fukuda,
and Shuji Miyake
Dispersed Power System Division
Daihen Corporation
Osaka, Japan
d-tsukiyama@daihen.co.jp
(1)
Saad Mekhilef, (2)Soon-Kurl Kwon,
and (1), (2)Mutsuo Nakaoka
(1)
(2)
University of Malaya, Malaysia
Kyungnam University, Republic of Korea
Abstract— This paper is mainly concerned with the development
of a new state-of-the-art prototype, high-efficiency, phase-shift,
soft-switching, pulse-modulated, full-bridge DC-DC power
converter with a high-frequency power transformer, which is
designed for utility-grid-tied photovoltaic (PV) power inverters.
The proposed high-frequency transformer (HFTR) link DC-DC
converter topology is based on a new conceptual secondary-side
series resonant principle and its inherent nature. All the active
power switches on the HFTR primary side can achieve lossless
capacitive snubber-based zero-voltage switching (ZVS) with the
aid of transformer parasitic inductances. Moreover, passive
power switches on its secondary side can also perform ZVS and
zero-current switching (ZCS) transitions for input voltage and
load variations. First, the operation principle of the newly
proposed DC-DC converter and some of its noteworthy features
are described in this paper on the basis of the results of analysis
by simulation. Then, the experimental setup of the DC-DC
converter with an output of 5 kW treated here is demonstrated
and its experimental results are discussed from a practical
viewpoint. Finally, some comparative evaluations between
simulation and experimental data are discussed and considered,
together with future works.
Keywords-component; DC-DC power converter, full-bridge
topology, high-frequency transformer link, secondary-side LC
resonant principle, primary-side ZVS, secondary-side ZCZVS,
photovoltaic generation system
I.
INTRODUCTION
A variety of collaborative developments on renewable,
sustainable energy conversion devices technologies and
advanced power electronics are urgently required toward the
concrete realization of a global zero-carbon society. Among
the studies related to energy electronics, the effective
utilization of clean PV generating power and energy with the
aid of recently developed energy storage devices such as
batteries and/or capacitors have attracted special interest in the
fields of power electronic distributed power supply system
applications and DC-feeding smart grids. With this
background, the development of next-generation highefficiency, high-power, high-performance solar converters is
Fig. 1. Overview of utility-grid-tied photovoltaic (PV) system with
high-frequency isolation and circuit diagram of the new
phase-shift, soft-switching, pulse-modulated, full-bridge DCDC converter, which is based on the secondary-side series
LC resonant principle.
practically needed, which include high-frequency-switching,
pulse-modulated DC-DC converters and utility-interactive,
sinewave-modulated inverters based on digital control
schemes.
In particular, front-end, high-power, high-efficiency, highfrequency-switching boost DC-DC power converters such as
non-isolated and isolated circuit topologies have been
considered and discussed for use with solar power converters
in order to improve their efficiency and control performances
including the noise issue. We have proposed several circuit
topologies for high-frequency-switching DC-DC high-power
converter circuits operating under the conditions of soft
commutation schemes for high-power industrial applications
[1-3]. We have also developed a utility-grid-tied three-phase
sinewave inverter operating with a high-efficiency control
strategy [4]. To further decrease the power consumption, an
advanced high-efficiency and high-power soft-switching DCDC converter topology with a high-frequency transformer link
is proposed as a new power-electronics technology, which is
based on the secondary-side LC resonant circuit principle for
the front-end solar converter in PV generation systems.
Also, studies on a variety of DC-DC converters with a highfrequency transformer have been carried out with the aim of
increasing their efficiency and power density [5-15]. These
converters include series-resonant, soft-switching DC-DC
converters with a high-frequency transformer [14-15]. In these
studies, a DC-DC converter utilizes the leakage inductance of
the transformer for energy storage and/or soft commutation.
However, although leakage inductance is useful for realizing a
soft-switching DC-DC converter, a large leakage inductance
of more than 2 μH or the presence of parasitic capacitance of
more than 1 μF decreases the magnetic coupling coefficient of
the transformer. This leads to not only a reduction in the
efficiency of the high-frequency transformer but also an
increase in the surge voltage applied to the high-frequency
full-bridge inverter circuit included in this DC-DC converter
with a high-frequency transformer. In addition, it is extremely
difficult to use some series-resonant DC-DC converters that
utilize the leakage inductance of a transformer in high-voltage
and high-power applications with power above 1 kW owing to
the reduced power transmission efficiency from the primary
side to the secondary side.
In this paper, we present a high-power, improvedefficiency, phase-shift, soft-switching, pulse-modulated fullbridge DC-DC converter with a high-frequency transformer
stage and a front-end boost converter cascade stage, which
includes a soft-switching full-bridge diode rectifier operating
on the basis of the resonant operating principle and the inherent
nature of the secondary-side LC series resonant circuit. This
new DC-DC power converter, which is suitable for solar
converters, can achieve not only a soft-switching transition
based on ZVS on the primary side but also ZVS and ZCS
commutation for the full-bridge diode rectifier on the
secondary side. The operating principle of this DC-DC
converter in a periodic steady state is described by considering
switching-mode equivalent circuits and the results of analysis
by simulation, along with its inherent notable features
compared with those of conventional converters. The simulated
operating voltage and current waveforms are comparatively
illustrated with experimental waveforms. The actual efficiency
versus output power characteristics and power loss analysis are
demonstrated experimentally. The practical effectiveness of the
proposed converter for PV generation systems is confirmed and
verified by means of 5 kW setup implementation and
simulation analysis.
II.
NOVEL TYPE SOFT-SWITCHING DC-DC CONVERTER
A. Circuit Description
Figure 1 shows the proposed high-frequency transformer
link, phase-shift, soft-switching, pulse-modulated, full-bridge
DC-DC converter with a front-end boost converter cascade
stage that incorporates a soft-switching full-bridge rectifier
operating with the secondary-side LC resonant principle. The
LC resonant components are located across the output diode
rectifier. This high-frequency isolated DC-DC converter
designed for PV generation systems can operate under the ZVS
condition of the primary-side active switches S1 - S4 in the Hbridge arms with the aid of a lossless capacitor, a leakage
inductor Ll and the magnetizing inductance Lm of a twowinding high-frequency transformer with a ferrite core.
On the other hand, the passive switches in the secondaryside full-bridge rectifier can also operate under the principle of
ZVS and ZCS transitions due to the secondary-side LC
resonant circuit property.
B. Principle of Converter Operation
Figure 2 illustrates the relevant operating voltage and
current waveforms during a complete switching period for the
gate-driving pulse sequences. The switching operation modes
of the soft-switching full-bridge DC-DC converter with a
high-frequency transformer are divided into six operations
modes from mode 1 to mode 6 in accordance with the
operational timing points t0 to t6. As can be seen in Fig. 3, the
operation principle is described using equivalent circuits
corresponding to each operating mode.
•
Mode 1 (t0 - t1):
During the active state, the corresponding set of
primary switches and secondary rectifier diodes (S2, S3
and D2, D3) conduct simultaneously so that the
secondary voltage and current have the same polarity.
The output power is delivered by applying a DC source
to the load. A positive voltage with magnitude Vin is
applied to the high-frequency transformer. Both VS1 and
VS4, the voltages across S1 and S4, respectively, are also
equal to the source voltage Vin.
•
Mode 2 (t1 - t2):
The turn-off signal is applied to S3 at time t1. After
S3 is turned off, VS1 begins to decrease gradually
because the capacitor parallel with S1, previously
charged to the source voltage Vin, discharges linearly to
zero. At the same time, the capacitor parallel with S3,
previously discharged to zero, charges linearly toward
a positive voltage. Therefore, the low-side switch S3 is
able to be turned off with ZVS due to current flowing
into the capacitor parallel with S3. On the other hand,
the diode currents (iD2, iD3) form a sinusoidal wave that
approaches zero so that not only D2 but also D3 can be
easily turned off with extremely soft recovery during
this period.
•
Mode 3 (t2 - t3):
On the other hand, the current through the secondaryside diodes (D1 and D4) starts to flow after VD1 and VD2
become equal to zero at the same time when both VS2
and VS3, the voltages across S2 and S3, respectively,
reach the source voltage Vin. At time t4, the primary
current still flows through the diode parallel with S1
and S4 (DS1 and DS4) while the turn-on signal is applied
to S1.
•
Mode 5 (t4 - t5):
The current through diodes DS1 and DS4 decreases
gradually toward zero as the secondary-side current
through D1 and D4 increases. At time t5, both S1 and S4
are simultaneously turned on with ZCS.
•
Fig. 2.
Operating voltage and current waveforms of soft-switching,
high-frequency transformer link, phase-shift, full-bridge DCDC converter based on the secondary-side LC resonant
principle.
After the capacitor voltage VS1 reaches zero, the diode
parallel with S1 (DS1) starts to conduct and clamps the
voltage on the high-side switch S1 at zero.
Consequently, the voltage across S3 is clamped at the
source voltage Vin. On the other hand, the turn-off gate
pulse signal is applied to S2 at time t2. After S2 is turned
off, VS4 begins to decrease gradually because the
capacitor parallel with S4, previously charged to the
source voltage Vin, discharges linearly to zero. At the
same time, the capacitor parallel with S2, previously
discharged to zero, charges linearly toward a positive
voltage. Therefore, the high-side switch S2 is turned off
with almost ZVS due to current flowing into the
capacitor parallel with S3. Although all primary
switches (S1 - S4) are turned off, the primary current is
maintained in the same direction during this state.
•
Mode 6 (t5 - t6):
The sinusoidal resonant current flows through the
primary-side active switches S1 and S4 and the winding
in the primary side of the high-frequency transformer.
The current through the secondary-side diodes D1 and
D4 and the secondary-side winding of the transformer
also forms a sinusoidal resonant wave. The current
through the primary side lags behind the voltage, while
the phase difference between the voltage and current
through the secondary side is close to zero, because the
resonant capacitor located between the transformer and
the full-bridge diode rectifier acts to form a leading
current. This behavior is effective for improving the
converter efficiency. In other words, there is no phase
difference between the voltage and current on the
secondary side, so that diodes in this side are able to be
turned off without generating a reverse recovery
current. In addition, all the diodes parallel with the
primary-side active switches utilize the lagging current
during freewheeling states and can be turned off with
zero reverse recovery current at time t5.
C. Unique Features
The specific advantages of the proposed DC-DC converter
in Fig. 1 are as follows.
(1) Higher conversion efficiency (98%, 380 V DC, 3 kW
output) among high-power isolated converters.
(2) Utilization of the high-efficiency small-sized
transformer with less leakage inductance and parasitic
capacitance.
(3) Minimum reverse recovery current switching loss in the
diode full bridge stage.
(4) Primary-side ZVS transition of all the active switches.
(5) Secondary-side ZVS and ZCS transition of all the
passive switches.
(6) Flexible design with high voltage conversion ratio.
Mode 4 (t3 - t4):
(7) Ground leakage current protection from PV panel to
power conversion circuit.
Similarly to in Mode 3, current begins to flow in
diode DS4 and clamps the voltage on the bottom switch
S4 at zero after the capacitor voltage VS1 reaches zero.
(8) Easy to connect a front-end boost converter that can
operate MPPT with high efficiency.
III.
CONVERTER SIMULATIONS
Table 1. Design specifications and circuit parameters
Real-time-based computer simulations were carried out
using PSIM version 9.0 to clarify the operation of the converter
under constant loading conditions. Figure 4 shows the
waveforms of the gate-driving pulse signals (a) (b), the output
rectifier (c), the MOSFET primary switch (d), the transformer
secondary current (e) and the voltage across the resonant
capacitor connected in series with the transformer in the
secondary side (VCr) and the inductor current, which composes
the output filter connected to the load (iLout) (f) in the case of 5
kW output. As shown in Fig. 4 (c), diode D1 is turned off with
ZVS and turned on with ZCS. As shown in Fig. 4 (d), switch S1
is turned off after iS1 reaches zero due to the capacitor parallel
with it. The current flowing through the transformer forms the
sinusoidal wave due to the series LC resonant circuit, as
depicted in Fig. 4 (e). Figure 4 (f) indicates the energy
commutation between the resonant capacitor (1/2CV2) and the
resonant inductor (1/2Li2) connected to the output diode
rectifier.
IV.
EXPERIMENTAL RESULTS AND DISCUSSION
A 380-V-input, DC 5 kW, soft-switching, phase-shift, pulse
modulated, full-bridge DC-DC converter with a soft-switching,
full-bridge diode rectifier operating on the basis of the
secondary-side LC resonant topology was built and tested in
an experiment. The design specifications and circuit
parameters of the converter are listed in Table 1. The
measured voltage and current waveforms of this prototype
converter are illustrated in Fig. 7. Comparing Fig. 7 with Fig.
4 (c), it can be seen that these observed voltage and current
waveforms are in good agreement with the simulated
waveforms.
Item
Symbol
Value
Unit
DC input voltage
(from boost converter)
Vin
380
- 388
V
DC output voltage
Vout
380
V
Maximum input power
Pin
5.5
kW
Switching frequency
fsw
30.6
kHz
Switching period
Ts
16.33
μs
Phase shift time
Tφ
0.95
μs
Dead time
Td
1.6
μs
Leakage inductance of
high-frequency transformer
Ll
< 1.3
μH
Magnetizing inductance of
high-frequency transformer
Lm
640
μH
Magnetic coupling coefficient of
high-frequency transformer
k
0.998
-
Inductance of output filter
(series resonant inductor)
Lout
20
μH
Capacitance of capacitors
parallel with transistors
CS1
- CS4
5
nF
Capacitance of
input/output capacitors
Cin,
Cout
50
μF
Capacitance of
series resonant capacitor
Cr
0.88
μF
Turn ratio of
high-frequency transformer
N1 : N2
1:1
-
Item
Symbol
Product type
Primary-side
MOSFET switches
S1
- S4
FCA76N60N
Secondary-side
rectifier diodes
D1
- D4
RHRP3060
Core material of
high-frequency transformer
-
ferrite
(PQ107)
The measured efficiency versus output power characteristic
under a constant output voltage and various specifications is
(a)
S
W
1
S
W
3
S
W
2
S
W
4
1
0
.
0
.
0
.
0
.
8
6
4
S2
2
0
(b)
1
0
.
8
0
.
6
0
.
4
0
.
2
(C)
S3
S4
S2
S1
S4
0
0
.
1
9
9
9
4
ID1*20
0
. 1
9
9
9
5
0
. 1
9
9
9
6
T
i
m
e
(
s
)
0
.
1
9
9
9
7
0
.
1
9
9
9
8
0
.
1
9
9
9
9
V_D1
400
VD1 (V)
200
iD1×20 (A)
VG_S2 (10 V/Div)
0
(d)
600
I_S1*20
V_S1
400
VS1 (V)
iS1×20 (A)
200
0
-200
(e)
VD1
Itrans_pri
(100 V/Div)
20
itr (A)
0
-20
(f)
150
100
50
0
-50
-100
I_Lout*5
0.19994
Vcap
VS2 (250 V/Div)
ID1
(5 A/Div)
iLout×5 (A)
Vcap (V)
0.19995
0.19996
Time (s)(s)
Time
0.19997
0.19998
0.19999
Fig. 4. Simulated results of (a) gate driving signals S1 and S3, (b)
gate driving signals S2 and S4, (c) voltage and current of
D1, (d) voltage and current of S1, (e) resonant current of
high-frequency transformer and (f) inductor current and
capacitor voltage of LC resonant circuit under the
conditions of 380 V and 5.5 kW.
Fig. 7.
Experimental waveforms of gate-source voltage of S2
(VG_S2, 10 V/div), drain-source voltage of S2 (VS2, 250
V/div), current of diode D1 (ID1, 5 A/div) and voltage of
anode and cathode (VD1, 100 V/div).
(a)
Efficiency (%)
of fsw = 43.3 kHz became shorter than that of fsw = 30.6 kHz
while the dead time was fixed at 1.6 μs in both conditions.
Without wiring
for measurement
98.5
98
97.5
97
96.5
With wiring
for measurement
96
95.5
95
Vin = 380 V
fsw = 30.6 kHz
Lout = 20 μH
94.5
94
93.5
93
0
1
2
3
4
5
6
7
Output Power (kW)
(b)
Efficiency (%)
fsw = 43.3 kHz
98.5
98
97.5
97
96.5
fsw = 30.6 kHz
96
Vin = 380 V
Lout = 20 μH
95.5
95
0
1
2
3
4
5
6
Output Power (kW)
Fig. 8. Measured efficiencies of the trial soft-switching, fullbridge DC-DC power converter with secondary-side
series LC resonant rectifier as a function of the output
power.
illustrated in Fig. 8. As shown in Fig. 8 (a), the efficiency
reaches a maximum value of 98% at an output power of 3-4
kW, as indicated by white diamonds. Above this power range,
the efficiency slightly decreases, mainly because the total
conduction loss of the converter increases with the increasing
load current. However, 97.8% efficiency can still be obtained
at 5.2 kW output. Efficiency of above 97% was measured for
outputs of 1-5.2 kW. Figure 8 also depicts the efficiency as
black circles. Under the measurement conditions, the terminals
of these components were connected directly to the measuring
instruments by lengthening cables to examine the losses of
each component composing the soft-switching DC-DC
converter with the high-frequency transformer. The maximum
decrease in efficiency of 0.3% was measured at an output
power of over 3 kW because of the increased conduction loss.
Fig. 8 (b) shows that the peak of converter efficiency shifts by
changing switching frequency. In the condition of fsw = 43.3
kHz, the efficiency reaches a maximum value of 98.1% at an
output power of 1.5 kW, as indicated by black circles.
However, the decrease in efficiency of 0.4% was measured at
an output power above 5 kW, mainly because the PWM width
By connecting the input and output terminals of each power
circuit component composing the phase-shift, soft-switching,
pulse-modulated, full-bridge DC-DC converter with a highfrequency transformer to the measuring instrument, the input
and/or output power of each component can be measured
directly so that the losses of these components can also be
investigated. The power loss of the components of the
converter was analyzed, the result of which is illustrated in Fig.
9. As can be seen in Fig. 9, the sum of these losses is increased
with increasing output power. Under the experimental
condition of Vin = 380 V, the total loss of the soft-switching,
full-bridge DC-DC converter with a secondary-side resonant
circuit is increased from 30.7 W to 144 W (approximately a
4.7-fold increase) with increasing output power from 0.5 kW
to 5.5 kW (11-fold growth). In the MOSFET full-bridge
circuit, the loss is increased from 24.5 W to 81 W (3.3-fold
growth). Similarly, 8.8-fold (from 4.1 W to 36 W) and 12.9fold (from 2.1 W to 27 W) increases in the power loss were
calculated for the HF transformer and the rectifier with the
output filter, respectively.
Figure 10 shows the loss ratio of the elements composing
the converter. The primary-side MOSFET switches that
comprise the full-bridge circuit in the soft-switching DC-DC
converter account for 80% of the total power consumption at
output powers of 1.1 kW or less and about 60% of the
consumption at 1.7 kW or more. In addition, the MOSFET
switches also account for 56.3% of the total power loss at a
full load. These results indicate that most of the total converter
loss is due to the full-bridge circuit, which consists of
MOSFET switches. The loss ratio of the HF transformer
decreases from about 30% to 25% with increasing output
power above 1.6 kW. In contrast, the loss ratio of the rectifier
with the filter grows from 7.5% to 18.8% due to increasing
conduction loss. Comparing Fig. 8 (a) and Fig. 10, the results
indicate that the effect of the increasing loss of the rectifier
with the filter on the efficiency degradation of the converter
becomes more significant as the output power increases.
V.
CONCLUSION
In this paper, a new high-power, high-efficiency, phase-shift,
soft-switching-transition, pulse-modulated DC-DC power
converter with a high-frequency transformer for PV generation
systems has been presented, which is based on the secondaryside LC series resonant circuit principle.
The operation principle of this new DC-DC converter in a
steady state has been schematically described using the results
of analysis by simulation and confirmed experimentally. This
new conceptual high-frequency link is achieved by highefficiency power conversion processing based on the
secondary-side LC resonant principle. The key operating
waveforms obtained by simulation and experimentally have
been discussed and evaluated in detail. The actual efficiency
versus output power characteristics have also been
demonstrated and discussed.
(3) A feasibility study of this DC-DC converter for utilitygrid-tied PV generation systems.
Loss (W)
160
140
120
21.0
80
MOSFET switches
2.1
4.1
2.4
4.7
24.5
28.2
0.5
1.1
3.2
12.8
36.0
12.9
31.0
REFERENCES
24.0
7.7
3.2
40
20
(4) The integration of a multiphase DC-DC converter
system and its practical evaluation for a power
electronic building block.
27.0
HF-Transformer with
resonant capacitor
100
60
Rectifier + Output filter
Vin = 380 V
fsw = 30.6 kHz
Lout = 20 µH
[1]
19.0
15.3
73.0
81.0
59.0
26.7
31.3
1.7
2.1
41.4
0
3.0
4.1
5.1
5.6
Output Power (kW)
Fig. 9. Results of power loss analysis of full-bridge inverter
circuit consisting of MOSFET primary switches, highfrequency transformer and diode rectifier with output
filter.
Loss (p. u.)
[2]
[3]
Rectifier + Output filter
1
[4]
0.9
0.8
0.7
0.6
[5]
0.5
0.4
0.3
0.2
0.1
HF-Transformer with
resonant capacitor
Vin = 380 V
fsw = 30.6 kHz
Lout = 20 µH
[6]
MOSFET switches
0
[7]
0.5
1.1
1.7
2.1
3.0
4.1
5.1
5.6
Output Power (kW)
[8]
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converter components for full-bridge inverter circuit
adopting power MOSFETs as primary-side switches,
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[9]
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