2004 35th Annual IEEE Power Electronics Specialisls Conference
Aachen. Germany, 2004
Bi-directional Fonvard-Flyback DC-DC Converter
Fanghua Zhang ,Lan Xiao, Yangguang Yan
Aero-Power Sci-tech Center, Nanjing University of Aeronautics &
Astronautics, Nanjing,China.
Email: pubboxOl@163.com
Abstract-Bi-directional DC-DC Converter is widely used in
more and more areas. The paper proposed a navel type of
bidirectional DC-DC converter topologies-Forward-Flyback
bi-directional DC-DC converters. The converter has the
following merits: 1) The spike on the switches is much smaller
than the current-fed type converter, the energy that cause the
spike is much smaller than that in the current-fed converter; 2)
The current of one side ofthe bi-directional DC-DC converter is
continuous, the current ripple is small; 3) There is no the
start-up problem in the Forward-Flyback bidirectional
DC-DC converter; 4) It is easy to realize soft switching; and 5)
The hybrid structure offorward and flybackconverter makes it
suit for high power situation. The paper analyzed the steady
state operation principles in detail. The experimental results
verify the analysis. Based on the principle of active clamp
forward-flyback bi-directional DC-DC converter, a family of
bi-directional DC-DC converter is proposed.
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INTRODUCTION
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The Bi-directional DC-DC Converters (BDC hereafter)
with galvanic isolation are being increasingly needed for
applications such as battery charge/discharge systems [I1,
unintermptible power supplies (UPSS)~'-~~,
hybrid electric
vehiclesl'l, aero power systems151,and etc.
Most of the existing BDCs fall into the generic topologies
illustrated in fig. 1 (awe). Fig. 1 (a) shows the type of the
BDCs that impress the voltage at the input and output
terminals1". The leakage inductance is used for storing and
transferring all the power, which puts severe burden on
manufacturability and performance of the transformer. Fig. 1
(b) is characterized by a current-fed high-frequency (HF)
inverterhectifier on one side of the HF transformer Tr, and a
voltage-fed HF rectifiedinverter on the other side".*'. The
current-fed converter (isolated boost) presents some severe
performance limitations, i.e. lack of self-starting capability
and h i e transient voltage ['-'I. Fig. 1 (c) is the flyback type
BDCs ,which cannot be used in high power situations.
11.
THEMECHANISM OF THE VOLTAGE SPIKE IN
CURRENT-FED CONVERTER
High voltage spike on the switches is an inherent problem
in current-fed converter. Fig. 2(a) is an example of fig. I (b)
with full bridge topologies on both sides. The mechanism of
the spike in voltage-fed and current-fed converter is distinct.
Fig. 2 (b) and (c) is the equivalent circuit that gives
prominence to the voltage spike problem. Fig. 2(b) is the
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Fig. I . Generic topologies of BDCs
equivalent circuit in the voltage-fed converter. When Sbuct
turns off, the resonant cell composed ofthe leakage
inductance LIkand the output capacitance of Sbvcrcauses the
voltage spike on Shuck. The resonance is passive and the
resonant energy that causes the spike is limited, so the spike
is small and can be easily restrained. But the spike on the
current-fed converter is quite different. The boost inductor is
large enough to be regarded as a constant current source
The current in the diode
cannot jump to IL immediately
because of the leakage inductance Lr when Sh., turns off.
Before the current in Dbuckreaches IL,IL-IDbu& surges into the
output capacitance of Sbmr,,and causes the high voltage spike
on Sbmr,.At the same time lDDbuFk
increases with the slope of
( VrbaaqDS)-Vo)/Llk.The process is active and the energy that
causes the spike is tremendous, so the transient voltage on
S b n is very high, and it is difficult to restrain. Fig. 2(c)
shows its equivalent circuit. The current-fed converter has
the model shown in fig.2 (c), while the forward converter
and the flyback converter have the same model shown in fig.
2 (b). To avoid the high voltage spike in current-fed
converters, we propose a novel forward-flyback hybrid BDC.
4058
2004 35th Annual IEEE Power Eiecrronics Specialists Conference
Aachen.
Germany, 2004
powering the down stream load converters. In this mode of
operation only the switches S, and S2 are gated and the body
diodes of S3 and S4 provide battery side rectification. The
references [IO-1 I ] introduced the two transformers active
clamp forward-flyback converter, and the operation
principles were described. So we do not rewrite them in this
paper.
C. Operation principles
C)
Fig. 2. The equivalent models ofvoltage-fed convener and current-fed
wnvener
111.
STEADY STATE OPERATION PRINCIPLES OF
FORWARD-FLYBACK
BDC
A. Proposed Power topology
The proposed forward-flyback bi-directional DC-DC
converter with active clamp is shown in fig. 3. Where V,,
and Vba,represent the bus voltage and the battery voltage
respectively. On the high voltage side the windings Npl and
Np2 are in series (series sides, SS hereafter), and on the low
voltage side the windings N,, and Ns2are parallel (parallel
sides, PS hereafter). NpI, N,,, nl=NpI/N,I,represent the
winding turns and the turns ratio ofthe forward transformer
TI; Np2,Nd, n2=Np2/Na,are the winding turns and the turns
ratio of the flyback transformer T2. The turns ratio n, is
suooosed to be eaual to n, in this oaoer. C“,
” . is the active
clamp capacitor; Llk is the total leakage inductance of TI and
T2: Cl-CI are the equivalent output capacitance of the
switches SI-S4. The converter has two modes of operation:
charging mode and backup mode.
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of backup mode
On failure of the dc bus, reversal of power flow occurs
resulting from a switchover to the battery. Now, the battery
supplies the load power at the dc bus voltage. In backup
mode, the switches S2,S, and S4 are gated and the body diode
of SI provide rectification at the load side.
a) The feasible time of S4 switches
Suppose that the gated signal of S3 is complementary
rigidly with the gated signal of S2 and S4,there are two
intervals during the operation.
Period 1: S I is in the on state, the energy stores in the
flyback transformer T2, at the same time TI will demagnetize
through S2 by the voltage across the capacitor Ccl.
Period 2: S4 turns off and SI turns on simultaneously, the
flyback energy stored in Tt transfers via DSIto the dc mains
together with the forward energy obtained directly from the
battery.
But in fact, the gated signal of S, and SI cannot be
complementary exactly, so an effective span to switch
securely is necessary. Fig. 4 (a) shows the feasible time of the
switches. The gated signal of S2 and S, is complementary
with some dead time to avoid short circuit on the PS. The
positions “a” “c” are the possible time S4turns on, and the
positions “d” “f’are the possible time S4 turns off.
During the off duty of Sa,the primary of T2 can be
considered a current source, and during the off duty of S,, the
primary of TI can be considered a current source (an inductor
with different current initials) too. Ifthe gated signals of S3
and SAhave dead time. it means that two current sources with
different initials in se& mis camot be accepted in
circuitry. SO “a” and “f’are the only feasible positions that S I
switches (fig, 4(b)),
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Fig. 3. The active clamp forward-flybackbi-directional DC-DC convener
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B. Operation principles of charging mode
In the charging mode the energy from the dc bus charges
the battery over a specified input voltage range while
4059
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(b)
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Fig. 4. The feasible time of the switches
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Aocken. Germany, 2004
2004 351h A n n u l IEEE Power Elecrronics Specialisls Conference
b) The operation principles of backup mode
Fig. 5 shows the equivalent circuit and the key waveforms
of backup mode. There are six intervals during the backup
mode.
Period 1: [t&] At b,S3 is in the “on”state, and Sq turns off
with Co snubber. Lp2(the inductor of T2primary) acts as the
filter inductor. The magnetizing current of TI, i,,, increases
to the first quadrant, and the magnetizing current of 7;,im2,
decreases at the same time. im2 is in the first quadrant all the
operation cycle. The flyback energy stored in T2transfers via
D,I to the dc mains together with the forward energy obtained
directly from the battery.
Period 2: [tl-t3] At tl, So turns on, im2 increases, the
magnetic energy stores in Tz in this period. i,, increases with
the same slope as in the period I . The current through the
leakage inductance Llt decreases to zero at t2, and then the
rectifier DSIturns off. So there is no load current flow
through S,. At t3, S3turns off with zero current switching.
Period 3: [t3-t5]S3turns off with ZCS at t3, and then the
magnetizing current of TI discharges the capacitor CI. The
voltage across C2decreases to zero at to, then the magnetizing
current conducts through Da, S2 can be gated at t5 with zero
voltage.
Period 4: [t&] The voltage across NPI,VCcI-nVb,,
demagnetizing the forward transformer TI, and the
magnetizing curve comes from the first quadrant into the
third quadrant. The magnetizing current of T2 increases with
the same slope as in period 3, the magnetic energy stores in
T2.
Period 5: [t&] At b, S2 turns off with the snubber of the
capacitor C2.The magnetizing current of TI discharges CI
and charges C2, and the voltage across N,,
increases to
nVb, ( V , ) at t,. So the voltage across the switch S3 is zero,
and S3 can be turned on with zero voltage after t,. (In fact,
there is no load current delivers in S3,so S3turns on with zero
current too). At t8, S3turns on with ZCZVS.
Period 6: [t&] So turns off with snubber at tg (b), and the
next switching cycle begins.
D. Basic relationship offorward-flyback BDC
-
In backup mode, the duty ratio is defined as D = s , * S,
(fig. 4 (b)). The steady state relationship between output and
input voltage can be derived from volt-seconds equation on
the flyback transformer T2.
The voltage across CCIcan be obtained from volt-seconds
equation on the forward transformer TI.
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(2)
E. A family afforward-flyback BDCs
We detailed the active clamp forward-flyback BDCs as an
example in this paper, but in fact there is a family of the
derivative hybrid forward-flyback BDCs for the various
clamp and reset circuits of forward and flyback converters
such as the active clamp, the LCD reset, the ZVT reset, and
the two transistors forward converters [ 121. So the topologies
of forward-flyback BDCs are very abundance.
IV.
EXPERIMENTAL
RESULTS
A prototype was built to evaluate the performance of the
proposed active clamp forward-flyback BDC, the
specifications of the prototype are:
Churgingmode: V~=120-180V; P,h,.=25OW;
Vb,=ISV;
f,=50kHz;
Backup mode: Vbat=12-18V; Pbackup=150W;
Vb.,=150V, f,=50kHz;
Fig.6 (a) and (b) is the experimental waveforms of the
charging mode when Vb,=kSOV, Ib.,=1.15A,Vb,=15V,
Ib,=IOSA. The two channels of fig. 6 (a) are VPI and VII,
the two channels of fig. 6 (b) are V, and Vdr2, from which
we can see that the switches S i and S2 are all operating at
Zero-Voltage-Switching.
Fig. 6 (c) are the experimental waveforms of the backup
mode with the active clamp technique in the flyback branches.
The circuit operates at Vba,=I5V, Ib.,=8.3A, Vb,=lSOV,
lb,=0.76A. The four channels of fig. 6 (c) is V,, I,?, V,
and Vdd, From the experimental waveforms we can see that
SI operates with zero-current-switching and So operates with
zero-voltage-switching.
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Fig. 5 . Key waveforms of charging and backup modes
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time: 5.Ou.‘@d
2004 35th Annual lEEE Power Electronics Specialists Conference
Aachen, Germany, 2004
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REFERENCES
[ I ] Edith Navarro, Philippe Perol, Enrique J Dede, et al. A new eficiecy
low mass bidirectional battery discharge-charge regulator far low
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[2] K. Venkatesan, Current mode controlled bi-directional flyback
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[4] Bojrup M, Karlsson P, Alakula M, et al. A dual purpose battery charger
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[6] Gang Chen, Dehong Xu, Yim-Shu Lee, “A novel fully
zero-voltage-switching phase-shift bi-directional DC-DC converter“,
APEC 2001,pp.974-979
[7] Kunrong Wang, Fred C. Lee, Jason Lai, Operation principle of
bi-directional full-bridge X i D C converter with unified soft-switching
scheme and soft-staning capability, APEC 2000, pp.l I I-I 18
[8] Manu Jain, Daniele M, Praveen K. Jain “A bidirectional DC-DC
converter topology for low power application”, IEEE Transactions on
Power Electronics, 2OOO,vo1.15, “0.4, pp.595-606
[9] Victor Yakushev, Valery Meleshin, Simon Fraidlin, “Full-bridge
isolated current fed converter with active clamp”, Proc. of APEC 1999,
pp.560-566
[IO] I. Cohen, D.Hills Pulse width modulated DC/DC convener with
reduced ripple current stress and zero voltage switching capability. U.S.
patent 5,291,382
[I I] Yonghan Kang, Byungcho Choi, Wonseok Lim. Analysis and design
offorward-flyback wnverteremploying two transfoners. PESC 2001,
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[I21 Rudy Severns. The history of the forward wnverter. Switching Power
Magazine. July, 2000 pp: 20-22
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CONCLUSIONS
The paper proposes a novel family of bi-directional
DC-DC converter ----forward-flyback bi-directional DC-DC
converters. The proposed converter can be the following
merits: I)The voltage spike on the switches is much smaller
than the current-fed type converters; 2) The current on one
side of the BDC is continuous, the current ripple is small; 3)
There is no start-up problem in the forward -flyback BDC; 4)
It is easy to realize soft switching; and 5) The hybrid structure
of forward and flyback converter makes it suitable for high
power applications. The steady state operation principles are
detailed, and then verified by the experimental results. There
is a family of bi-directional DC-DC converters based on the
research of the active clamp forward-flyback BDC.
406 1