02-04-2012, 11:24 AM
A Three-Phase Zero-Voltage and Zero-CurrentSwitching DC–DC Converter for Fuel Cell Applications
A Three-Phase Zero-Voltage and Zero-Current.pdf (Size: 1.16 MB / Downloads: 53)
Abstract
In spite of having many advantages, such as low
switch voltage and easy implementation, the voltage-fed dc–dc
converter has been suffering from problems associated with large
transformer leakage inductance due to high transformer turn ratio
when it is applied to low-voltage, high-current step-up application
such as fuel cells. This paper proposes a new three-phase voltagefed
dc–dc converter, which is suitable for low-voltage, high-current
applications. The transformer turn ratio is reduced to half owing
to Δ-Y connection. The zero-voltage and zero-current switching
(ZVZCS) for all switches are achieved over wide load range without
affecting effective duty cycle. A clamp circuit not only clamps
the surge voltage but also reduces the circulation current flowing in
the high-current side, resulting in significantly reduced conduction
losses. The duty cycle loss can also be compensated by operation
of the clamp switch. Experimental waveforms from a 1.5 kW prototype
are provided.
Index Terms—Fuel cells, high power dc–dc converter, threephase
dc–dc converter, ZVZCS.
I. INTRODUCTION
SINCE a dc voltage generated from fuel cells is usually
low and unregulated, it should be boosted and regulated
by a dc–dc converter and converted to an ac voltage by a dc–
ac inverter. High-frequency transformers are usually involved
in the dc–dc converter for boost as well as galvanic isolation
and safety purpose. The single-phase dc–dc converter based
on the push–pull [2] or full-bridge [3]–[7] topology has been
used as an isolated boost dc–dc converter for less than several
kilowatt power levels. For higher power level, the single-phase
converter could suffer from severe current stresses of the power
components.
The three-phase dc–dc converter has been proposed
[8]–[12] as an alternative for high-power application. The threephase
dc–dc converter has several advantages over the singlephase
dc–dc converter: (1) easy MOSFETs selection due to
reduced current rating; (2) reduction of the input and output
Manuscript received October 29, 2008; revised February 8, 2009 and April
28, 2009. Current version published February 12, 2010. This work was supported
in part by Seoul National University of Technology and by KESRI.
Recommended for publication by Associate Editor U. K. Madawala.
H. Kim is with the POSCON Co., Ltd., Pohang, Gyeongbuk 790 719, Korea.
C. Yoon is with the Advanced Drive Technology Co., Ltd., Gunpo-si,
Gyeonggi-do, Korea.
S. Choi is with the Department of Control and Instrumentation Engineering,
Seoul National University of Technology, Seoul 139-743, Korea (e-mail:
schoi[at]snut.ac.kr).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TPEL.2009.2026418
filters’ volume due to increased effective switching frequency
by a factor of three compared to single-phase dc–dc converter;
and (3) reduction in transformer size due to better transformer
utilization. The three-phase isolated boost dc–dc converter can
be classified to dual active bridge (DAB) converters [8], currentfed
converters [10], [11], and voltage-fed converters [12].
The DAB can achieve ZVS on both high- and low-side
switches and has no inductors involved in the power circuit.
However, the DAB has many active switches and high ripple
currents. Also, the VA rating of the transformer is comparably
large, and manufacturing of the high-frequency transformer
with large leakage inductance is a challenging issue.
The current-fed converter, in general, exhibits lower transformer
turns ratio, smaller input current ripple, lower switch
current rating, and lower diode voltage rating. However, higher
switch voltage rating of the current-fed converter implying
larger Rds(ON) of MOSFET switches is a major disadvantage
since switch conduction loss at the primary side is actually a
dominant factor in determining overall efficiency of the dc–
dc converter for low-voltage high-current application such as
fuel cells. A clamping or snubber circuit is usually required for
the current-fed converter to limit the transient voltage caused
by transformer leakage inductance. The current-fed converter is
also lack of self-starting capability and, therefore, it necessitates
an additional start-up circuitry. The three-phase current-fed dc–
dc converter proposed for step-up applications [10] has only
three active switches, but the active switches are hard switched
and the passive clamping circuit on the high-current side may
cause large amount of losses. The three-phase current-fed dc–dc
converter with an active clamping circuit [11] not only clamps
the surge voltage but also offers ZVS on the active switches.
However, this scheme suffers from the high ripple current imposed
on the clamp capacitor located at the high-current side.
The voltage-fed dc–dc converter has also been used in fuel
cell applications. An important advantage of the voltage-fed
type is lower switch voltage rating since the switch voltage
is fixed to input voltage, and therefore MOSFETs with lower
Rds(ON) can be selected. This is critically beneficial in the fuel
cell application where more than 50% of the power loss is lost
as a switch conduction loss at the low-voltage side. Also, the
voltage-fed converter does not have a self-start problem unlike
the current-fed converter. However, the voltage-fed converter
suffers from a high transformer turns ratio, which causes large
leakage inductance resulting in large duty cycle loss, increased
switch current rating, and increased surge voltage on the rectifier
diode. The three-phase voltage-fed dc–dc converter, so-called
0885-8993/$26.00 © 2010 IEEE
Authorized licensed use limited to: Sri Manakula Vinayagar Engineering College. Downloaded on March 23,2010 at 07:14:21 EDT from IEEE Xplore. Restrictions apply.
392 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 25, NO. 2, FEBRUARY 2010
Fig. 1. Proposed three-phase dc–dc converter.
V6 converter, proposed for step-up applications, [12] significantly
mitigates the problem associated with high transformer
turn ratio of the voltage-fed type by utilizing the open Δ-Y type
transformer connection, which reduces the required turn ratio to
half. Also, the size of the input filter capacitor to reduce the input
current ripple is reduced, since the effective switching frequency
is increased by three times due to the interleaved operation.
In this paper, a three-phase voltage-fed dc–dc converter for
isolated boost application such as fuel cells is proposed. The
turn ratio of the high-frequency transformer is reduced to half by
employing theΔ-Y connection.Aclamp circuit that is located at
low-current, high-voltage side not only clamps the surge voltage
but significantly reduces the circulating current flowing through
high-current side, resulting in reduced switch conduction losses
and transformer copper losses. Further, with the help of the
clamp circuit zero-voltage and zero-current switching (ZVZCS)
for all switches over wide load range is achieved. The duty cycle
loss can also be compensated by the clamp switch. The operating
principles and features of the proposed converter are illustrated
and experimental results on a 1.5kWprototype are also provided
to validate the proposed concept.
II. OPERATING PRINCIPLES
As shown in Fig. 1, the proposed three-phase voltage-fed dc–
dc converter includes sixMOSFET switches at low-voltage side
and a three-phase diode bridge, an LC filter, and a clamp circuit
consisting of a MOSFET switch and a capacitor at high-voltage
side. The three-phase transformer could be configured in Y-Y,
Δ-Δ, Δ-Y, or Y-Δ as shown in Table I. Among them, the Δ-Y
configuration is shown to be the best choice in two aspects.
First, the Δ-Y transformer requires the smallest turn ratio
for step-up application, and in fact the required turn ratio is
half that of Y-Y or Δ-Δ transformers [12]. The reduction of
turn ratio significantly mitigates problems associated with large
leakage inductance, which are large duty cycle loss, increased
switch current rating, and surge voltage on the rectifier diode.
This is a big advantage of the three-phase dc–dc converter over
the single-phase dc–dc converter based on the push–pull or fullbridge
type and makes the voltage-fed dc–dc converter viable
for high gain step-up application. Second, the Δ-Y configuration
is also shown to have the smallest transformer kVA rating
and switch current rating. The transformer winding voltage and
current waveforms of each transformer configuration are shown
in Table I for a specific gating signal. In Δ-Y transformer configuration,
three switches conduct at one time while in Y-Y
or Δ-Δ transformer configurations only two switches conduct.
This reduces the transformer winding voltage and current and in
turn reduces the switch and diode current. The kVA rating of the
Δ-Y transformer is 81.6% and 86.7% of those of Δ-Δ and Y-Y
transformers, respectively. The rms switch and diode currents
of the Δ-Y configuration are 69.3% of those of the Δ-Δ and
Y-Y configurations. Therefore, theΔ-Y configuration should be
a topology of choice for the step-up application.
Fig. 2 shows key waveforms of the proposed converter for
illustration of operating principle. Upper and lower switches of
each leg are operated with asymmetrical complementary switching
to regulate the output voltage. Three legs at the low-voltage
side are interleaved with 120◦ phase shift, which results in increased
effective switching frequency.
The converter has seven operating states within each operating
cycle per phase, and equivalent circuits of each state are
shown in Fig. 3. It is assumed that the output filter inductance
is large enough so that it can be treated as a constant current
source during a switching period. It is also assumed that the
clamp capacitance is large enough so that there is no ripple on
the clamp voltage during a switching period.
State 1 [t1 − t2 ]: S1, S2 , and S6 are conducting, and lower
switches S2 and S6 are carrying half of upper switch S1
current since two transformer primary currents become equal
due to the current flow at the secondary. Since voltage across
transformer leakage inductor Vlk1 is a small negative value,
which is a difference between the input voltage and half of the
clamp capacitor voltage referred to the primary, transformer
primary current Ip1 is slowly decreasing. The transformer
secondary winding current is also decreasing but larger than
load current Io during this mode. Therefore, clamp capacitor
Cc is being charged through the body diode of Sc by the
decreasing current.
State 2 [t2 − t3 ]: When clamp current ISc decreases to zero,
the clamp branch is completely disconnected from the circuit.
The input power is still being delivered to the output.
Diodes D1 and D2 carry load current Io through the transformer
secondary windings. The voltages across the leakage
inductors Vlk1 is zero.
State 3 [t3 − t4 ]: S1 is turned off at t3 . External capacitor
Cext across S1 is charged and parasitic capacitor Coss of
S4 is discharged by reflected load current to the primary
2nIo . Switch voltage VS1 increases linearly with a slope of
2nIo/(Cext + 2Coss). The upper switch is almost turned off
with ZVS if external capacitor Cext is chosen large enough to
hold the switch voltage at near zero at the switching instant.
At the end of this mode, the body diode of S4 is turned on.
State 4 [t4 − t5 ]: Lower switch S4 is turned on with ZVS since
VS4 became already zero at State 3. Turning on of the clamp
switch at t4 causes the rectifier voltage referred to the primary
to be applied to the leakage inductor resulting in rapid
decrease of the transformer primary current to zero, and this
causes the clamp capacitor to discharge to supply the load.
This reset operation eliminates the circulating current through
Authorized licensed use limited to: Sri Manakula Vinayagar Engineering College. Downloaded on March 23,2010 at 07:14:21 EDT from IEEE Xplore. Restrictions apply.
KIM et al.: THREE-PHASE ZERO-VOLTAGE AND ZERO-CURRENT SWITCHING DC–DC CONVERTER FOR FUEL CELL APPLICATIONS 393
TABLE I
COMPONENT RATING ACCORDING TO VARIOUS TRANSFORMER CONNECTIONS
the transformer and switches, resulting in significantly reduced
conduction losses. Note that the clamp is turned on
with ZVS.
State 5 [t5 − t6 ]: At t5 , the main switch current, transformer
winding current, and diode current become zero, and the
clamp capacitor fully supplies the load.
State 6 [t6 − t7 ]: Clamp switch Sc is turned off at t6 , and the
load current freewheels through all the diodes. The clamp
switch can also be turned off with ZVS if capacitance across
the clamp switch is properly chosen. The gate signal for lower
switch S6 is removed during this mode, and S6 is turned off
with ZCS.
State 7 [t7 − t8 ]: Upper switch S3 is turned on at t7 and S2, S3 ,
and S4 start conducting. Note that S3 is turned on with ZCS
since S3 current linearly increases with a slope of Vin/Lk1.
This causes commutation of diode currents, that is, increase
of diode currents ID3 and ID4 and decrease of other diode
currents. At the end of the commutation, the rectifier voltage
is clamped by Vc through the body diode of Sc . This is the
end of one-third of the cycle. The second part of the cycle is
repeated in the same fashion.
III. FEATURES OF THE PROPOSED CONVERTER
In a low-voltage, high-current application such as fuel cells,
conduction loss at high-current side of the converter is a dominant
loss factor. Generally, in the phase-shifted full-bridge converter
conduction losses associated with the circulating current
generated during the nonpowering mode are of great concern.
In the proposed converter, the conduction loss is significantly
reduced due to the reset operation mentioned in State 4. That
is, the energy stored in the leakage inductance located at highcurrent
primary side, which is the circulating current through
the transformer winding and lower switch, is transferred to the
clamp capacitor located at the low-current secondary side resulting
in significantly reduced total conduction losses and kVA
rating of the transformer (see the shaded area in Fig. 2).
In the phase-shifted, full-bridge converter ZVS can inherently
be achieved using transformer leakage inductance. However,
in order to achieve ZVS over wide load range, the leakage
inductance should be increased. In the proposed three-phase
converter, the ZVZCS operation can be achieved over wide load
range for all switches (ZVS turn on and ZCS turn off for lower
switches and ZCS turn on and ZVS turn off for upper switches)
without increasing leakage inductance.
In order to achieve the ZVZCS operation, appropriate dead
times are required for both upper and lower switches. In the
ZVZCS full-bridge converter [7], dead times required for both
leading and lagging leg switches actually limit effective duty
cycle, which may cause increased conduction losses. The effect
of the duty limit on efficiency is considerable especially in the
low-voltage high-current application such as fuel cells. However,
in the proposed three-phase converter required dead times
do not affect the effective duty cycle of upper switch by which
energy is delivered to the load. That is, required dead times do
not impose duty cycle limit on operating range of the duty cycle.
In order to achieve ZVS turn-ON of a lower switch, the lower
switch should completely be discharged before the lower switch
turns on. Therefore, the required dead time for lower switches
is determined by
tdead,L ≥ (2Coss + C1 ) Vin
2nIo,ZVS
(1)
where Io,ZVS is a minimum load current to which ZVS turn-OFF
can be achieved from the full load. Since there is no duty limit,
the proposed converter can achieve ZVS turn-OFF over wider
load range compared to the ZVZCS full-bridge converter [7].