Current-fed double inductor push-pull DC/DC converter with closed ...

10 th International Symposium
„Topical Problems in the Field of Electrical and Power Engineering“
Pärnu, Estonia, January 10-15, 2011
Current-fed double inductor push-pull DC/DC converter with closed
loop control system
Aleksandrs Andreičiks, Ingars Steiks, Kristaps Vitols, Leonids Ribickis
Riga Technical University
aleksandrs.andreiciks@inbox.lv, ingars.steiks@rtu.lv, ktx@inbox.lv, leonids.ribickis@rtu.lv
Abstract
In order to use hydrogen fuel cells in domestic
applications either as main power supply or backup
source, their low DC output voltage has to be
matched to the level and frequency of the utility
grid AC voltage. Such power converter systems
usually consist of a DC-DC converter and a DC-
AC inverter. A double inductor step-up push-pull
converter with the output voltage and the input
current control is investigated in this paper,
presenting simulation and experimental results for
passive and active overvoltage clamping. The
prototype of the investigated converter is elaborated
for 1200 W power to match the rated power of the
proton exchange membrane (PEM) fuel cell
located in hydrogen fuel cell research laboratory.
Keywords
Fuel cell system, high frequency power converters,
ZVS converters
groups – voltage source converters and current fed
converters. In this paper a current fed topology is
preferred, since it is characterized by low input
current ripple, which is more appropriate for proton
exchange membrane fuel cells modules [3],[4].
.Considering the necessity of high voltage boosting
function with low input current ripple, the most
appropriate converters are current fed full-bridge
and push-pull configurations. Since the converter
efficiency can be considerably improved by
reducing the count of the primary switches and
implementing a transformer of a simple structure
(without split windings), a double inductor push-pull
converter (DIC) was selected and analyzed in this
paper Fig. 1. The converter is elaborated for 1200 W
power, since such is the rated power of the proton
exchange membrane (PEM) fuel cell in hydrogen
fuel cell research laboratory of Riga Technical
University.
Introduction
The research of the hydrogen energy has gained a
growing interest in the recent years. The hydrogen
fuel cells are fully ecological, taking into account
that heat and water are the only by-products, which
are excreted into the environment [1]. In order to
utilize the electrical energy, produced by fuel cells,
characterized by slow dynamic response, low output
voltage and large voltage variations, static power
converters are researched widely.
The fuel cells used as main power supply or backup
source in domestic application need to be connected
to the grid. Such power converter systems usually
consist of a DC-DC converter and a DC-AC
converter. Because of comparatively high input and
output voltage difference, most frequently as the
optimal solution converters with high frequency
transformer are acknowledged for the DC-DC stage
[1]-[4]. There are many known transformer isolated
dc-dc converter topologies, which could be suitable
to perform the necessary voltage boost from the fuel
cell voltage level to the inverter dc link voltage.
Such converters are the full-bridge, half-bridge, the
flyback, the forward and the push-pull basic
topologies, as well as a number of their derived
topologies [5], [6]. These can be divided into two
Fig. 1. Current fed double inductor push-pull
converter topology
The hard switching converters have a drawback of
voltage overshoots at turn-off due to the energy
stored in the parasitic inductances. These voltage
spikes are not only dangerous to the transistors but
they substantially increase the switching losses.
There are two basic ways to protect the transistor
switches from being damaged by the overvoltage.
The first is using transistors with blocking voltage
ratings that exceed these stresses. This, however,
results in poor utilization of the transistors, since on
state resistance of the MOSFET transistors increases
dramatically with increased blocking voltage.
1 Control of the converter
To control the designed converter it will be
compared two control types, the duty-cycle and the
current control. In all switching converters, the
output voltage is desired to be kept constant, in
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despite of the disturbances in the converter element
values, or inputs. For this, a control system is needed
which can automatically adjust the duty cycle as
necessary to keep constant the output voltage within
an acceptable range, regardless of the load current or
input changes. [5]
Two types of control for the DIC push-pull boost
converter, the direct duty-cycle control and the
current control. In the following will be briefly
described both of them, enumerating some
advantages and disadvantages for each. Basic
scheme is depicted by Fig. 2.
Fig. 2. Control of the converter
1.1 Duty Cycle control
In duty-cycle control model the output voltage is
measured and then compared to the reference
voltage. The error signal is used as input in the
compensator, which will compute from it the dutycycle
reference for the pulse-width modulator. Its
principle is illustrated Fig. 3.
Fig. 3. Direct duty-cycle control scheme
1.2. Current control
In current control model the converter output is
controlled by choice of the transistor peak current.
The control signal is a current and a simple control
network switches on and off the transistor such its
peak current follows the control input.
In the literature [5] and [6] is stated that the current
control, in the case of an isolated boost push-pull
converter has some advantages against the dutycycle
control. First, it has simpler dynamics
(removes one pole from the control-to output
transfer function). Second, it makes use of the
current sensor information in normal operation mode
– transistor failures due to excessive currents can be
prevented by limiting the reference switch current.
In the transformer can be induced a dc bias current
by small voltage imbalances due to the small
differences in boost inductors and/or switches.
The current control will alter the switch duty cycles
in a way that these imbalances tend to disappear and
the transformer volt-second balance to be
maintained.
The disadvantage is that the current control has a
susceptibility to noise in the reference and measured
switch current signals. In general, a small amount of
filtering is necessary for the measured current. The
current control becomes unstable whenever the dutycycle
becomes larger than 0.5. This drawback can be
overcome by adding an artificial ramp to the
reference current signal.
Considering the above arguments, the current
control seems to be more attractive for the present
application.
1.3 The current controlled converter model
Evaluating the model of the current controller
implies the calculation of the relation between the
inductor current and the control signal Fig. 4. In our
case there are two inductors and in the relation will
be used the sum of these two inductor currents.
Fig.4. Current control scheme
Is a known problem of the current control that it
becomes unstable when the duty cycle passes by 0.5.
The addition of an artificial ramp to the sensed
signal can improve the stability of the controller, and
for an adequate slope, it can be stable for all dutycycles.
However, in the present case the maximum
duty-cycle is 0.5, thereby only a small slope is
required, to improve the noise immunity of the
system. This small slope will not influence the
converter transfer functions and controller design.
The most important are that: it keeps the input
current under a desired value and it realizes the
current balance between positive and negative
alternance in the high frequency transformer
preventing the saturation of the transformer core.
The measured signals are the fuel cell voltage (VFC)
and current (IFC) and the output voltage (VDC)
Fig. 5. The fuel cell current (IFC) and output voltage
(VDC) are used as feedback signals in control.
The protection is realized by using all the measured
signals.
The voltages VDC and VFC are measured by means
of resistive voltage dividers while the fuel cell
current (input current) is measured with a current
transducer (LEM).
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FC
VFC
L1
L2
S1a
G1a
S2a
G2a
1:n
D1
D2
D3
D4
C
VDC
R
Load
i L1
L1
L2
Llk
i S
+
1:n D1 D3
i C i R
V P
V S
C R
V DC
G1
S1
Cclamp
S2
G2
Sa1
Sa2
D2
D4
VDCref
+
-
VDCm
iFC/2 = iin
Voltage iFC/2ref
controller
iFC/2m
Current
controller
OUT1
OUT2
CLK
EN
Gate signal
conditioning
G1
G2
G1a
G2a
i L2
i FC
Da1
Ca1
D1
Da2
Ca2
D2
C clamp
V FC C1
C2
S1 S2
-
VDCm
iFC/2m
PROTECTION
FAULT
VFCm
Fig. 5. DIC Converter schematics and control block
diagram
Experimental set-up of the control board was
fulfilled by the Field-Programmable-Gate-Array
(FPGA). Flowchart of the FPGA’s program is
illustrated in general by Fig. 6. It should be
mentioned that the voltage-to-frequency integrated
circuit was used at the FPGA’s input, as it generates
related square-wave signal with defined frequency
depending on the input voltage. The control system
was tuned so that 100 kHz square-wave signal is at
FPGA’s input related to the nominal output voltage
of the DIC converter.
Start
Fig. 7. Current-fed DIC Converter with Active
Clamping Circuit
commanded in anti-phase to the main switches. The
basis of the operation of the presented clamping
circuit is the resonant phenomena between the
leakage inductance of the transformer (L lk ) and the
clamping capacitor (C clamp ). The value of C clamp
should be set so that one half of the resonant period
formed by C clamp and L lk exceeds the maximum turnoff
time of the main switches. [4] The clamping
capacitor value is
2 2 2
clamp > s (1 − ) π ⋅ lk
C T D L
. (1)
Yes
Set default values
Do not change duty
cycle
Yes
To achieve ZVS for the main switch, it must be
turned on after turn-off the auxiliary switch. This
delay should be selected to be less than one quarter
of the resonant period formed by L lk and C 1
capacitor [4]
Is the duty cycle =
min?
No
Decrease the duty
cycle
Yes
Yes
Is Vdc_output >
Vdc_ref?
No
Is the duty cycle =
max?
No
Is the input current
= max?
No
T L C π
M 1− M 1a =
lk
⋅
1
⋅ 2
, (2)
where the value for C 1 has been taken from the
datasheet of the chosen switch. The maximum time
delay between turning off the main switch and
turning on the auxiliary, using the calculated
minimum value for the clamping capacitor can be
considered by
Set the new value
of the duty cycle
Increase the duty
cycle
T L C π
M 1− M 1a =
lk
⋅
clamp
⋅ 2
. (3)
Fig. 6. Flowchart of the program implemented in
FPGA.
2 Current-fed DIC converter with active
clamping circuit
A DIC converter with active clamping circuit is
shown on Fig. 7. In this case, the energy from the
leakage inductance is not dissipated on the clamping
resistance but transferred to the output. The active
clamping circuit can improve the efficiency of the
converter, as it does not dissipate the leakage
inductor energy on a resistance but feeds it to the
transformer primary. With active clamping circuit it
is able to operate at ZVS both at turn-on and turn-off
for all switches and simple to implement as it
contains only a capacitor and two transistors
3 Simulation of the DIC converter
The simulation of the DIC converter was done using
LTspice simulation tool. Since a steady operation
point was to be examined, the FC was modelled by a
constant voltage source. The core losses of the
inductances were neglected. The transformer was
modelled as an ideal transformer introducing the
leakage inductance in series. The power MOSFET
transistor switches (IXFN73N30) and active
clamping MOSFET transistors (IRF740) were
modelled using SPICE models provided by the
manufacturers.
The operation conditions of the simulation are
adjusted to the rated parameters of the converter.
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Active load with a small filter capacitor of 10 µF
was considered. The value of leakage inductance
(L lk = 4.8 µH) was obtained experimentally, by
measuring the transformers mutual and selfinductances.
It can also be noted, that the converter
operates as desired, providing 400 V output voltage
at rated input and load conditions and that the
transistor voltage does not exceed the maximum
allowed voltage of main power transistor 300 V and
maximum allowed current 73 A.
The schematic of DIC converter with active
clamping circuit in LTspice environment and
simulation results for the active clamping topology
are presented in Fig. 8. and Fig. 9. It can be noted
that the shape of the auxiliary switch current is
determined by the resonant circuit formed by the L lk
and C clamp .
4 Experimental results
Experimental testing of active clamping DIC
converter was carried out as well. The testing was
performed using Ballard Nexa PEM fuel cell module
with nominal power 1.2 kW and connecting the DIC
converter to a resistive load. Measurements of the
input current, input voltage and load voltage and
current were done. Then the efficiency of the
converter was calculated. The efficiency of the DIC
converter with the active clamping circuit is around
93 % (Table 1.)
Table 1. The Measured Quantities
Parameter (averal values) Measurements
Vin
26.7 V
Vout
336 V
Iin
28.73 A
Iout
2.12 A
Efficiency 93 %
Faster diodes (better turn-on and turn-off
characteristics than integrated diodes in transistors)
and 2.2 nF capacitors were added in parallel with
active clamping transistors. In parallel with the
power transistors, RC snubber circuits were
implemented composed of 4.7 nF polyester film
capacitors and 4.7 Ω 5 W resistors. Basing on
equation (3) the capacitance of the clamping
capacitor was 3.3 µF.
Experimental waveforms of the DIC converter with
the active clamping circuit are shown in Fig. 10. It
was possible to test converter at the full load as there
was no primary switch overvoltage problems.
Fig. 8. DIC Converter schematics with Active
Clamping Circuit in LTSpice environment
Fig. 10. Experimental waveforms of the DIC
converter with the active clamping – from the top:
voltage across one active clamping tranzistor,
voltage across one power transistor
Fig. 9. Simulated waveforms of the DIC converter
with active clamping circuit: from the top – current
through transistor M1, output voltage, voltage of the
clamp capacitor, transformer primary voltage
From the Fig. 11. it can be noticed that the load
variation does not change the output voltage of the
converter considerably. The following can be
concluded, that the closed loop control system works
as it changes the duty-cycle depending on the load
and the output voltage remains constant.
159

integrated circuit, for a current fed converter implies
new problems, as the signal conditioning in order to
realize the overlapping of the gate signals. However,
using dedicated PWM controllers have important
advantages as soft start-up and built-in over-current
protection.
References
Fig. 11. Experimental waveforms of the DIC
converter with the active clamping – from the top:
output voltage, output current.
Conclusions
The power converter needed in order to use the
hydrogen fuel cell as a main power supply or backup
source in domestic applications, usually consists of a
step-up DC/DC stage and a DC/AC inverter stage.
As an efficient solution for the DC/DC stage – a
double inductor push pull converter with active
voltage clamping circuit is analyzed in this paper
presenting simulation and experimental results.
It was acknowledged that the DIC converter with
active clamping circuit performs well and the
efficiency of the converter is 93 %. The efficiency
could be further increased basically in two ways: by
decreasing the resistance of the converter’s primary
circuit components or by reducing the primary
current, which can be achieved by connecting two or
more identical converters in parallel.
The ripple of the input current of the experimental
prototype is below ± 5 % of the mean value, which
is within acceptable limits for the fuel cell.
The elaborated prototype of double inductor pushpull
DC/DC converter with active clamping circuit
can be used as a background for further work on
clamp circuit optimization and the hardware
implementation of the control, using a dedicated
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