High Frequency Transformer - Assisted Passive Soft Switching
PWM DC-DC Converter with Energy Recovery for Compact
Auxiliary Power Supply in Rolling Stock Transportation
Claudio Y. Inaba, Yoshihiro Konishi, and Mutsuo Nakaoka
Division of Electrical Systems Engineering
The Graduate School of Science and Engineering, Yamaguchi University
2-16-1 Tokiwadai, Ube City, Yamaguchi, 755-8611, Japan
Abstract — In this paper, a two - switch high frequency flyback
type zero voltage soft switching PWM DC-DC converter using
IGBTs is proposed. Effective applications for this power
converter can be found in auxiliary power supplies of rolling
stock transportation and electric vehicles. This power converter
is basically composed of two active power switches and a flyback
high frequency transformer. In addition to these, two passive
lossless snubbers with power regeneration loops for energy
recovery, consisting of a three winding auxiliary high frequency
transformer, auxiliary capacitors and diodes are introduced to
achieve zero voltage soft switching from light to full load
conditions. Furthermore, this power converter has some
advantages such as low cost circuit configuration, simple control
scheme and high efficiency.
Its operating principle is described and to determine circuit
parameters, some practical design considerations are discussed.
The effectiveness of the proposed power converter is evaluated
and compared with the hard switching PWM DC-DC converter
from an experimental point of view.
Input filter
circuit
Series connected DC-DC
converter circuits
Co
DC power supply
(1500V, 750V, 600V)
Ro
Vo
Index Terms: Passive snubbers, Flyback type DC-DC converter,
Current regeneration, Soft switching
I.
Fig. 1. Series connected DC-DC converters for
high input DC power supply
INTRODUCTION
In recent years, industrial demands for DC power supplies
for electric vehicles and automobiles, new energy interface
utilization stand-by power sources, UPS for telecommunication
network systems and industrial energy power plants are
becoming higher and higher from a power conditioning system
enhancement point of view. In particular, for auxiliary power
supply applications in rolling stock transportation systems,
high frequency insulated DC-DC power converters have been
developed and applied. Since comparatively high DC voltages
like 1500V, 750V and 600V are provided from the main power
supply of the rolling stock transportation system, active power
semiconductor devices with high maximum voltage rates have
to be utilized in the power converter. However, the switching
response becomes lower as the maximum voltage rate
increases, making high frequency switching and downsizing of
the power converter difficult to be realized. To solve this
problem, series connected DC-DC power converters are
employed as illustrated in Fig. 1 and active power
semiconductor devices with lower voltage rate and fast
switching response can be utilized.
Under these technological backgrounds, a variety of high
frequency PWM DC-DC power converter topologies have been
proposed for increasing their power density and actual
efficiency. However, in traditional hard switching PWM DCDC power converters, high frequency switching power losses
of power semiconductor devices and modules utilized become
larger as well as dv/dt and di/dt related electromagnetic noise
levels.
In order to overcome these problems, a two-switch flyback
type passive soft switching PWM DC-DC power converter
with a current regeneration function for energy recovery is
originally proposed by the authors. Due to its simple circuit
configuration with minimum components and since only
passive components are utilized in the auxiliary snubber to
achieve zero voltage soft switching, the proposed PWM DCDC power converter is able to be controlled by a single PWM
signal, establishing a low-cost circuit configuration and simple
control scheme for high power applications such as rolling
stock transportations, new energy interfaced distributed power
supplies and power conditioners for electric vehicles. The
0-7803-7883-0/03/$17.00 © 2003 IEEE
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operating principle, steady state operating characteristics and
some practical parameters design considerations are described
and analyzed from a theoretical point of view. To verify the
effectiveness of the proposed soft switching PWM DC-DC
power converter, a 1kW, 25kHz breadboard setup using IGBTs
is implemented. Moreover, its operating performances are also
compared with the hard switching PWM power converter.
II. CIRCUIT CONFIGURATION AND ITS OPERATING PRINCIPLE
A. Circuit Arrangement
In Fig. 2, the hard switching flyback type DC-DC converter
circuit using two switches is depicted and in Fig. 3, the circuit
of the proposed soft switching power converter using passive
snubbers with energy recovery function is illustrated. The
proposed PWM DC-DC power converter can also operate as a
forward type one. However, an additional diode and inductor
have to be connected in the output side, increasing cost, and
physical volumetric space.
The transformer primary side of the PWM DC-DC power
converter is composed of two active power switches (S1, S2)
and two clamp diodes (D3, D4) which suppress voltages across
S1 and S2 to the input DC voltage Vs. The passive snubber
circuits with current regeneration for energy recovery are
D1
S1
D4
L1
Vs
Tm
i1
S2
D3
Do
L2
i2
Ro
Co
Vo
D2
composed of snubber diodes (Ds1, Ds3), snubber capacitors (Cs1,
Cs2), auxiliary diodes (Ds2, Ds4) and a three winding high
frequency transformer Ta where resonant leakage inductors Ls1
and Ls2 are included. The high frequency flyback transformer
Tm is represented by its turns ratio m and leakage inductors L1
and L2.
B. Operating Principle and Analysis
In Fig. 4, typical voltage and current operating waveforms
of the proposed circuit topology are illustrated and each
operation stage is represented in Fig. 5. Its operating principle
under steady - state condition is described with the following
assumptions.
(i) All the active and passive power switches and
components are ideal.
(ii) Primary and secondary winding of the auxiliary
transformer Ta are identical, so the turns ratio in relation to the
tertiary winding and their resonant leakage inductances are
respectively represented by n1 = n2 = n and Ls1 = Ls2 = Ls.
(iii) Capacitors Cs1 and Cs2 are identical, Cs1 = Cs2 = Cs.
The steady - state operation of this circuit is described as
follows:
(a) Mode 0 (t0 ~ t1): At time t0, according to the duty factor
D (= ton/T) of the DC-DC power converter treated here, S1 and
S2 are turned on simultaneously under the condition of zero
current since and leakage inductances of the main and auxiliary
high frequency transformers are in their current path. A voltage
nVs is reflected across the primary and secondary windings of
Ta. As a result, resonance based on Ls and Cs starts partially.
For energy recovering, current ils flows through the current
regeneration loop composed of Ls1 (Ls2), Ds2 (Ds4), Cs1 (Cs2) and
Vs and the snubber capacitor voltage vcs is discharged toward
zero. The circuit state equations for this circuit operation mode
are expressed below:
Vs
Switch
S1 or S2
Fig. 2. Conventional hard switching flyback type DC-DC converter
circuit using two switches
vs1
S1
D1
D4
Ds2
Ds1
Vs
Snubber
capacitor
Cs1 or Cs2
Cs1 vcs
n1:n2:1
Ta
vls
L1
D3
i1
L2
i2
v2
Co Ro
S2
ils
ics1
vls1
ils1
-nVs
Vo
il2
il1
Flyback
transformer
Tm
m:1
Ds3
vcs1
Auxiliary nV
s
transformer
n1, Ls1 or n2, Ls2
Do
Ls2
Ds4
Vs
Vs
Tm
v1
Ls1
is1
vs1
t0
D2
t1 t2
ton ( PWM signal )
t3 t4
t5
T
Cs2
Mode
5
0 1
2
3
4
5
Fig. 4. Typical theoretical waveforms of the proposed converter
Fig. 3. Proposed high frequency soft switching flyback type DC-DC
converter circuit
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Cs1 vcs
S1
S1
Ds2
n1:n2:1
Vs
Ta
Vs
Ls1
L1
Tm
L2
i2
i1
Vs
Co
Ro
n1:n2:1
Ta
Vs
Vs
Vo
Ls1
Ls2
ils
Ds4
S2
Cs2
L1
n1:n2:1
i1
Co
Ro
Vs
Vo
n1Vs
Ta
Vs
Tm
i1
m:1
Ds3
Co
Ro
Vo
m:1
S2
ils
Mode 1 ( t1 ~ t2 )
Cs1 vcs
Mode 2 ( t2 ~ t3 )
D4
n1:n2:1
Vs
Tm
S2
Mode 0 ( to ~ t1 )
Ds1
L1
Ls2
m:1
Ds4
S1
Ds2
Ds1
Do
Ta
L1
n1:n2:1
Tm
Vs
i1
Co
Ro
Ta
L1
i1
i2
Vo
Do
Co
Tm
Ro
L2
i2
Vo
m:1
m:1
Ds3
L2
Tm
Do
Co
Ro
Vo
m:1
D3
Cs2
Mode 4 ( t4 ~ t5 )
Mode 3 ( t3 ~ t4 )
Mode 5 ( t5 ~ to )
Fig. 5. Equivalent circuits for each commutation stage
Vs = − Ls
dils
+ vcs + nVs ,
dt
dvcs
1
= − ils
dt
Cs
determine the turns ratio of the auxiliary high frequency
transformer Ta is rearranged as:
(1)
Assuming that vcs = Vs0 and ils = 0 at time t0, the equations
for the snubber capacitor voltage and regeneration current are
represented as follows:
ils =
Vs 0 − (1 − n ) Vs
{
Zs
sin ω s t
}
where Zs = Ls C s is the characteristic impedance and ωs =
Ls C s is the resonant angular frequency.
From (2), it is noted that the snubber capacitor Cs is fully
discharged at ωst = π and the snubber capacitor voltage vcs
becomes less than zero at this time, so from vcs < 0, the
condition of 1 − Vs 0 2Vs < n is obtained.
On the other hand, the current through switches S1 and S2 is
derived from the flyback transformer secondary side current i2
and regeneration current ils. Therefore, the maximum di/dt of
the active power switches at turn on can be represented as
follows:
nV − n(1 − n )Vs Vs + mVo
di
= 2 s0
+
dt
Ls
L1 + m 2 L2
(4)
where Vs0 is the initial voltage across the snubber capacitor
Cs at t0.
(c) Mode 2 (t2 ~ t3): In this mode, the regeneration current
ils reaches zero and energy is stored into the primary side of the
flyback type high frequency transformer Tm.
(2)
vcs = Vs 0 − (1 − n ) Vs cos ω s t + (1 − n ) Vs
1
1 − V s 0 2V s < n < 1
(3)
(b) Mode 1 (t1 ~ t2): When Cs is fully discharged, Ds1 and
Ds3 turn on. In this mode, the regeneration current ils for energy
recovery is not allowed to flow continuously. So, the additional
condition of n < 1 should be considered and the conditions to
(d) Mode 3 (t3 ~ t4): According to the duty factor D (=
ton/T), S1 and S2 turn off simultaneously under zero voltage soft
switching. Cs starts to charge and voltage across the active
power switches increases linearly with a certain slope. Voltage
across Cs is charged to Vs0.
(e) Mode 4 (t4 ~ t5): Ds1 and Ds3 turn off and energy stored
into leakage inductances of the main and auxiliary high
frequency transformers is released to Vs through clamp diodes
D3 and D4. Current i2 starts to flow through the secondary side
of flyback transformer.
(f) Mode 5 (t5 ~ t0): Energy stored into the primary side of
the flyback transformer is discharged through the secondary
side and current i2 flows through L2, output diode Do and
output capacitor Co as well as power is supplied to the load Ro
by a DC voltage Vo.
III. DESIGN CONSIDERATIONS OF PASSIVE SNUBBERS
Considering the turns ratio of the auxiliary high frequency
transformer as n1 = n2 = n (turns ratio of primary and secondary
winding in relation to the tertiary winding); setting n according
to (4) is indispensable in order to achieve a soft switching
mode transition which is independent of output power.
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The circuit parameters of passive snubbers are designed for
a DC-DC power converter with a DC source voltage Vs =
300V. The output voltage Vo and switching frequency f are
100V and 25kHz, respectively. In addition, the practical
conditions indicated below must be met in order to select
optimum parameters.
vs1
(a) Full discharging interval ts of snubber capacitors Cs1 and
Cs2, in this case, is to be designed at 3% ~ 5% of one switching
period T ( = 1/f ). So, ts is given by,
ts = π
Ls C s , 0.03 < ts. f < 0.05
(5)
is1
(b) Maximum dv/dt during turn off of active power
switches is 1000V/µs.
(c) Maximum di/dt during turn on of active power switches
is 50A/µs.
Each circuit parameter, which meets conditions (a) ~ (c)
mentioned above, is designed by the following methods:
(a)
(i) Cs1 = Cs2 = Cs is set to 0.015µF and from (4), n is set to
0.67.
vs2
(ii) From (5), Ls1 = Ls2 = Ls is determined, so 9.7µH < Ls <
27µH.
(iii) To satisfy condition (b), the allowable maximum
switch current ismax when S1 and S2 turn off is 15A since dv/dt =
is/Cs and to satisfy condition (c), maximum di/dt is determined
from (3).
IV. EXPERIMENTAL EVALUATIONS AND DISCUSSIONS
is2
The operating principle and steady state characteristics of
the proposed soft switching PWM DC-DC power converter are
verified by a 1kW (Vo = 100V) and 25kHz breadboard setup
and IGBTs are implemented. The main design specifications
and circuit parameters of this power converter are as follows:
Vs = 300V, Cs1 = Cs2 = Cs = 0.015µF
S1/D3, S2/D4: CM75DY-12H, Vces = 600V, Ic = 75A
(b)
Ds1, Ds2, Ds3, Ds4: 30JL2C41, VRRM = 600V, IF = 30A
Fig. 6. Voltage and current waveforms of active power switches S1 and
S2 under Pout=1kW (vs1, vs2: 100V/div; is1, is2: 10A/div; time: 10µs/div),
(a) Switch S1, (b) Switch S2
Tm: L1 = 19.5µH (magnetizing inductance Lm = 2.63mH), L2
= 11.8µH, m = 1.285
Ta: Ls1 = Ls2 = Ls = 16µH, n1 = n2 = n = 0.67
From Fig. 8, it is noted that the snubber capacitor voltage vcs1 is
discharged toward zero before S1 turns off and the regeneration
current only flows during the turn on switching mode transition
interval. It can be also observed that for heavy load conditions,
the maximum voltage across Cs1 is higher than the input
voltage Vs since parasitic inductances exist through DC bus
line.
Experimental voltage and current waveforms of active
power switches S1 and S2 are respectively illustrated in Fig. 6
under the specifications of Vs=300V, Vo=100V, Pout=1kW.
From these results, it is verified that both active power switches
turn off under zero voltage condition since the snubber
capacitors are fully discharged and turn on under zero current
condition due to the leakage inductances of flyback transformer
and auxiliary three winding high frequency transformer. In Fig.
7, the turn on and turn off waveforms of switch S1 are
illustrated. Here, the di/dt and dv/dt at turn on and turn off are
respectively 25A/µs and 700V/µs when Pout = 1kW.
In Fig. 9, voltage and current waveforms of active power
switches S1 and S2 under light load conditions of Vs=300V,
Vo=100V and Pout=150W are also depicted. From these results,
it is verified that zero voltage soft switching is also achieved
under light load conditions.
In Fig. 8, voltage across snubber capacitor Cs1 and the
regeneration current ils1 for energy recovery are illustrated.
In Fig. 10, total measured actual efficiency in relation to the
output power Pout is represented for the conventional two -
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vs1
vs1
vs1
is1
is1
is1
Fig. 7. Turn on and turn off switching waveforms (Pout = 1kW)
(vs1: 100V/div; is1: 10A/div; time: 0.2µs/div)
(a)
vcs1
vs2
ils1
is2
Fig. 8. Voltage across snubber capacitor Cs1 and regeneration current ils1
(Pout = 1kW), (vcs1: 100V/div; ils1: 5A/div; time: 10µs/div)
switch flyback type PWM DC-DC power converter and the
proposed soft switching PWM DC-DC power converter using
passive lossless snubbers with energy recovery function. From
these results, it is verified that the maximum efficiency
obtained from the zero voltage soft switching PWM DC-DC
power converter is 93.3% and for output power higher than
550W, total efficiency increases approximately 1.3% to 3% in
relation to the conventional hard switching converter.
(b)
Fig. 9. Voltage and current waveforms of active power switches S1 and
S2 under Pout=150W (vs1, vs2: 100V/div; is1, is2: 10A/div; time: 10µs/div)
Efficiency [%]
90
V. CONCLUSIONS
A two – switch high frequency flyback type zero voltage
soft switching PWM DC-DC power converter using IGBTs for
auxiliary power supply applications in rolling stock
transportation systems has been presented in this paper.
It was proved by theoretical analysis and experimental
evaluations that the proposed soft switching DC-DC power
converter circuit could efficiently work with high performances
in comparing with the conventional two switch flyback type
hard switching PWM DC-DC power converter. The operating
principle has been illustrated as well as the steady-state
analysis and a practical design of circuit parameters on the
lossless passive snubber with a current regeneration loop for
energy recovery was also discussed. From experimental results,
it could be verified that the actual efficiency of the power
converter increases when the passive lossless snubber circuit
80
Soft Switching
Hard Switching
70
0
0.5
1
1.5
Output power Pout[kW]
Fig. 10. Measured actual efficiency vs. output power under conditions
of Vs=300V and Vo=100V
using a auxiliary three winding high frequency transformer is
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implemented. Finally, the following features could be verified
in the proposed converter circuit.
(i) Simple control scheme by a single PWM signal since the
active power switches operate simultaneously and passive
snubbers with energy recovery are implemented.
[4]
[5]
(ii) Soft switching operation from light to full load
conditions could be achieved.
[6]
(iii) The proposed power converter is suitable for high
power applications such as auxiliary power supplies for rolling
stock transportation systems.
[7]
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[8]
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