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Study and Implementation of Four Modules in Parallel 10kW
Boost DC-to-DC Converter for Fuel Cells
Shih-Jen Cheng, Shu-Wei Kuo, Yu-Kang Lo, and Huang-Jen Chiu
Department of Electronic Engineering, NTUST, Taiwan, ROC
Corresponding author: Yu-Kang Lo
Email: [email protected]
Add.: No.43, Sec.4, Keelung Road, Taipei, Taiwan, ROC
Keywords: Fuel cells, current-fed, interleaved, voltagedouble, constant current.
Abstract
In this thesis, this choice of architecture is the current-fed
full-bridge converter and use of voltage-double in the
secondary circuit, the turns ratio can be reduced by half,
thereby reducing losses, improving overall efficiency and
practicality. And use interleaved PWM control mode to
parallel with four modules, in order to reduce the current
ripple on primary side for fuel cells. To ensure that the battery
life and reliability in working condition, so design a feedback
of constant voltage (CV) / constant current (CC) feedback
circuit in terminal. Finally, implement a four modules in
parallel 10kW DC-DC boost converter, and has a soft start
time of 30 seconds to protect the fuel cells to a steady state. In
the high-voltage input full load output, the maximum
efficiency can reach 91%.
1 Introduction
Since the 18th century industrial revolution, men have
been using the fossils for fuels (petroleum, natural gas, coal)
to improve technologies and create a practical living
environment. Recently, technologies are still improving, thus
making living quality better and better, but this energy
consuming will eventually cause the natural energy to become
less and less, create pollutions, produce harms towards earth
and impact the whole ecosystems, such as global warming,
sour rain, damage of atmosphere, etc. Our earth has existed
for about 4.6 billion year, with the natural energy from the
fossils having been consumed by us merely in 200 years,
meaning that they can only enough for the upcoming tens to
hundreds of years. If human beings are still considering to
exist, we should make more progress on energy saving and
carbon reducing. Thus, the development of other energy
resources[1-3] technologies have become focus of human
beings. Some of these technologies include solar energy, wind
energy, hydraulic energy, burn energy, nuclear energy, and
fuel cells. Each of them focuses solely on clean and pollutionfree.
Till now, there are many ways to acquire energy
resources, and there exist various disadvantages from each
electrical generation way. Wind and hydraulic energy are
somewhat dependant on geometric shape, solar energy still
need much more improvement in energy transformation and
application efficiency. These energies are generating
electrical energy in collective sites, and will result in high
power loss during the transmission process. When they are
malfunctioning, the electrical energy consuming system will
be disturbed massively. These renewable energies naturally
have variations on voltage level, power density, and response
speed, thus making power electronics technologies become
significant to improve the transient and steady-state
characteristics, including also the fuel cells technology [4-5].
Consequently, the power converter should be designed with
so many considerations. Fuel cell is a device that converts
chemical fuels into electrical energy, with high power density,
clean power generation, high efficiency, and diverse energy
applications as main advantages [6]. But, the fuel cell is not
similar to conventional batteries, because it does not have
capability to store energy and only supply for low voltage
output with poor output voltage regulation range. In order to
stabilize this as voltage source, a DC-DC converter should be
inserted between fuel cells and loads.
In due to its broad regulation range low output voltage
and slow dynamic response [7], a high-voltage battery is
shunt at the fuel cells output, thus converting the voltage level
into 365V through the mentioned DC-DC stage and supplying
the loads and batteries. From fuel cells characteristics, the
hydrogen density inside the fuel cells is higher during very
light load and results in higher output voltage, and will
decrease to more stable lower output voltage during heavy
load. This is the reason a constant-voltage (CV) and constantcurrent (CC) feedback control and auxiliary supply are added
at the output points to improve the slow dynamic response
and output voltage stability. With simple topology and high
efficiency, the circuit application and implementation is
optimized. Finally, a 4-parallel-module of 10kW laboratory
prototype with current-fed full-bridge topology [8], including
the CV/CC control, auxiliary supply, is implemented.
2 Operating Principles
DTs
As shown in Fig.1 is the implementation of a 10kW
current-fed full-bridge [6-8] block diagram. First, a DC-DC
flyback converts the fuel cells voltage from 37~80V into
stable 5~15V as the auxiliary supply for digital signal
processor (TMS320F2808) and current-fed full-bridge. Then,
the 37~80V voltage is boosted to 365V and connected in
parallel to the high voltage batteries load. When rated output
power of 10kW is reached, the current is limited with a CC
circuit to avoid over-current through the high-voltage
batteries and functions deterioration, thus optimizing the life
and operational reliability of the high-voltage batteries.
DC
37V~80V
PEM
Fuel Cell
Current Fed
Full Bridge
Boost Converter
DC
365V
Parallel With
Battery&Load
PWM
Power Flow
Drive Signal
15V Vcc1
Auxiliary
Power 15V Vcc2
0
VgsB ,VgsC
t
DTs
0
VdsA ,VdsD
t
nVO/2
0
VdsB ,VdsC
t
nVO/2
0
IFB
VFB
Drive Circuit
VNp
nVO/2
0
t
-nVO/2
VO/2
VNs
0
t
-VO/2
Vin
0
Constant Current
Circuit
Control Circuit
(TMS320F2808)
t
TS
VLin
Feedback Signal
5V Vcc3
Overlap time (-0.5+D) Ts
VgsA ,VgsD
t
-Vin
ILin
Iin
0
t
t0
t1
t2
t3
t4
Fig.1 Overall converter block diagram
Fig.3 Waveforms of current-fed full-bridge converter
Conventional full-bridge converter should use 4 power
switches, which is double the number or switches in halfbridge and push-pull, thus making it more complicated. The
switches of half-bridge need more on the lower voltage stress
rating but higher current stress rating, meanwhile the ones of
push-pull with the lower current rating but higher voltage
rating. Full-bridge topology adopts the advantages of both
topologies and is very suitable for high-power applications.
As seen from Fig.2, there is an inductor at the front end
of the current-fed full-bridge converter, of which acting the
same as boost inductor, making the primary-side switches
should choose the twice input voltage rating. A voltagedoubler is added at the secondary side to minimize the
rectifier diode voltage rating as the same with output voltage
only, thus reducing transformer turns ratio and minimizing
the leakage inductance probably resulting from the winding
process.
At t = t0, shown in Fig.4, all the power switches are ON
and all the output diodes DO1, DO2 are OFF. This is the
overlap time and the input inductor Lin is storing energy.
Transformer T1 is shorted in due to conducting state of all
switches, resulting in zero voltage across both primary and
secondary windings. There is no energy transfer in the
transformer, and output capacitors CO1, CO2 are supply energy
to load.
At t = t1, shown in Fig.5, QA and QD are ON, QB and QC
are OFF. Current flows through input inductor, Q A, primary
winding, QD, and back to input port. This current is added
with the stored current in Lin during previous state, thus
making this almost similar to boost converter.
Simultaneously, the primary and secondary windings voltage
are built now with dot point as positive polarity, forcing the
output diode DO1 to conduct, and let the current from N S dot
polarity and CO2 flow to output load. At this time CO1 is
storing energy and CO2 is releasing energy. In this duration,
Vin and Lin are supplying energy to load and CO1.
At t = t2, shown in Fig.6, all the power switches are ON
and all the output diodes DO1, DO2 are OFF. This state is
exactly similar mode 1.
At t = t3, shown in Fig.7, QB and QC are ON, QA and QD
are OFF. Current flows through input inductor, QC, primary
winding, QB, and back to input port. This current is added
with the stored current in Lin during previous state, thus
making this almost similar to boost converter.
Simultaneously, the primary and secondary windings voltage
are built now with non-dot point as positive polarity, forcing
the output diode DO2 to conduct, and let the current from NS
non-dot polarity and CO1 flow to output load. At this time CO2
is storing energy and CO1 is releasing energy. In this duration,
Vin and Lin are supplying energy to load and CO2.
_
+ Lin
+
QA
DO1
QC
T1
Vin
+
N
_P
Cin
RO VO
1:n
QB
CO1
+
N
_S
QD
DO2
CO2
_
Fig.2 Current-fed full-bridge converter
As current-fed mode, the switches duty cycle should be
larger than 50%, with some important waveforms shown in
Fig.3. In the two-pair interleaving cycles, there exists overlap
time for the input inductor to store energy. During this
overlap time, transformer T 1 can be seen as shorted and
disabling the input inductor to supply energy to load. At this
moment, the load energy is supplied from output capacitor
CO1 and CO2. The operation is divided into 4 states, with each
operational principle and mode discussed below.
_
+ Lin
Do1
QC
T1
Vin
+
N
_p
Cin
Co2
3
C2=220n
A2
4
_
+ Lin
Vo
Do1
QC
T1
+
N
_p
Cin
C1=150n
QD
+
Do1
T1
+
N
_p
Cin
Co1
+
N
_s
Ro Vo
n:1
QB
Do2
QD
R1=1k
Rsense
Vsense
2.5V
Co2
_
+ Lin
QC
RA=Δk
TL431
Do2
Fig.5 Conducting current path during t1＜t＜t2
Vin
Vo
RB=2.4M
Vcc2
Ro Vo
_
QA
Io
5
Co1
+
N
_s
n:1
QB
A1
LM358
+
Vin
R2=47k
6
Fig.4 Conducting current path during t0＜t＜t1
QA
D
7
2
_
PC817
8
1
Ro Vo
Do2
QD
Vcc2
VFB
Co1
+
N
_s
n:1
QB
Vcc1
Vcc2
+
QA
Co2
_
Fig.6 Conducting current path during t2＜t＜t3
Fig.8 Constant current circuit block diagram
After the fuel cells have started up, the output voltage
will be raised to 365V by the current-fed full-bridge
converter, then with a controlled CV control from voltage
divider resistors, the over-voltage signal will be fed to PWM
controller IC and order the duty-cycle to be larger, regulating
the output voltage. At this regulated output voltage, the CC
circuit detects the current feedback signal from the current
sense resistor. The over-current signal will be fed to PWM
controller IC and order the duty-cycle to be smaller,
regulating the output current.
4 Digital control system analysis and design
_
+ Lin
+
QA
Vin
Do1
QC
_
Np
+
Cin
T1
Ro Vo
n:1
QB
QD
Co1
_
Ns
+
Do2
Co2
_
Fig.7 Conducting current path during t3＜t＜t4
As shown in Fig.9 is the circuit structure of the auxiliary
supply, and Fig.10 is the circuit structure of four-paralleled
module of current-fed full-bridge converter. The negative
feedback control is implemented from proportional ratio of
output voltage and a voltage reference, feeding a control
voltage to TMS320F2808. During output voltage variations,
TMS320F2808 will provide CPL3120 with switch driver
signal regulation to modulate the duty cycle, thus stabilizing
the output voltage.
3 Constant current circuit analysis and design
The output of this converter is shunt to high voltage
batteries, such that an over-voltage or over-current will
influence the batteries life and operational reliability, this is
why a constant current (CC) feedback circuit is designed.
As shown in Fig.8, LM358 OP will act as the feedback
part, a current sense resistor (Rsense) is connected in series in
the output current loop, transferring the output current signal
into Vsense and is being fed as non-inverting input at LM358.
Then, ongoing addition function with reference of 2.5V,
LM358 provides the sample points VFB to the primary side,
thus keeping the current controlled at a fixed value.
T1
Lm
NP
NS1
DO1 CO1
+
RO1 Vcc1
_
NS2
DO2 CO2
+
RO2 Vcc2
_
1:n1
Vin
Cin
QA
1:n2
Gate Driver
Photo-Couple CPL3120
-
PI
Digital Signal Process
TMS320F2808(PWM)
Sample
circuit
+
Fig.9 Auxiliary supply block structure
Vref
_
+ Lin
QA
DO1
QC
CO1
T1
Vin
+
N
_P
Cin
+
N
_S
n:1
QB
Gate Driver
Photo-Couple CPL3120
CO2
DO2
QD
TMS320F2808
PWM
-
PI
Sample
circuit
+
Vref
_
+ Lin
QA
DO1
QC
T1
+
N
_P
Cin
CO1
DO2
QD
CO2
_
+ Lin
QA
DO1
QC
T1
+
N
_P
Cin
CO1
+
N
_S
n:1
QB
DO2
QD
PHSDIR
Phase
ePWM2
1668
0
1
0°
ePWM3
1668
417
0
90°
ePWM4
1668
1668
0
180°
ePWM5
1668
417
1
270°
Table 1: Register value setting for each module.
DO1
QC
T1
+
N
_P
Cin
CO1
+
N
_S
n:1
QB
TBPHS
CO2
_
+ Lin
QA
TBPRD
+
N
_S
n:1
QB
Here, a DSP (TMS320F2808) is used to produce four
phase of synchronous driving signal for switches. Each
ePWM module has synchronous input (EPWMxSYNCI) and
output (EPWMxSYNCO), and by using the second module
ePWM2 during “0” value of counter to create synchronous
signal, producing EPWM2SYNCO output to other ePWM
modules. When ePWM2 counter is 0, EPWM2SYNCI creates
20-30ns pulse signal, other ePWM modules are according to
the setting of TBPHS and PHSDIR register to decide the
insertion of counter value and phase when receiving the
synchronous signal. For our example, when the switch
frequency is 30kHz, register of each module TBPRD is 1668,
as shown in Table 1.
QD
DO2
CO2
Fig.10 Block structure of current-fed full-bridge converter
The paralleled modules are adopting interleaved PWM
control method to minimize the current ripple on fuel cells
stage and optimize the 10kW rated output power, as
interpreted in Fig.11. In due to this four-paralleled module
system, each control signal is designed to be 90-degree phasedifferent, as mentioned in (1), with n the number of modules.
360

 90
n
(1
)
VgsA1
VgsA2
VgsA3
VgsA4
Fig.11 Interleaved control diagram
The adoption of digital control as microprocessor should,
beforehand, set the programming environment in order to
complete its functions. In Fig.12, it is shown that for DSP
initiation flow-chart, its setting parts include system control
register, input/output port, PIE vector diagram, and the most
importantly, the PWM modules and ADC channels. After
interfacing with environment, the procedure program is the
operational processes control required by hardware users. The
whole integration diagrams are shown in Fig.13.
Start
Start
System Initialization
Initialize System Control
Register
NO
Wait interruptions
while ePWM INT Flag
YES
Enable ePWM interruption
subroutine
Initialize GPIO
ADC interruption
Flag=1
Clear all Interruptions and
Initialize PIE Vector Table
Jump to ADC
subroutine
Sampling ADC1,
ADC2, ADC3
Initialize all Device
Peripherals
For (i=0, i<=20, i++)
{
Sampling Vin, Iin, Vout
}
Program User Specific Codes ,
Enable ePWM and ADC Module
NO
Sens Vin>30V
YES
Clear ePWM output
Wait Interruptions
Fig.12 Environmental initiation system flow-chart
Clear ePWM INT Flag
Return
ADC1
NO
soft start Time
Delay Flag=0
Return
YES
Delay 30s for Current Fed
soft start Setting flag=1
Output Voltage Feedback
control (Proportional control)
Setting D_max=40% & D_min=5%
Current_fed feedback control and
setting ePWM2~5 duty cycle
Clear ePWM INT Flag
Return
Return
Fig.13 Operational system flow-chart
6 Experimental Results
This paper outlines the implementation of a highefficiency converter for fuel cells application, which
improved the dynamic response speed and output voltage
stability. The output voltage will raise during very light load
and fall during rated load, thus resulting in a need to design
for broad voltage range. To verify this converter
effectiveness, a 10kW current-fed full-bridge prototype that
contains auxiliary supply and CC/CV circuit has been built.
The circuit operational specifications are as listed in Table 2.
Input voltage
37V~80V
Output voltage
365V
Maximum output power
10kW
Maximum output current
27.4A
Switching frequency
30kHz
Efficiency
90%
Table 2: Parameters of the current-fed full-bridge.
Fig.14 and 15 are the waveforms of Switch driver,
transformer primary voltage, and input inductor current at Vin
= 37V~57V、PO = 2.5kW for each module.
Fig.18 Output current waveforms for each module during Vin
= 47V、PO = 8kW
Fig.14 Each module of Vin = 37V、PO = 2.5kW
Fig.15 Each module of Vin = 57V、PO = 2.5kW
While operating in very light load, output voltage will
raise. To ensure normal operating system, the broad input
voltage range is used, as in Fig.16, where the waveforms of
output power at 1kW for 80V input voltag.
Fig.19 Output current waveforms for each module during Vin
= 57V、PO = 10kW
Because converter input is connected to fuel cells, the
low start-up condition of fuel-cells should be considered in
design, the output voltage needs about 30 seconds to reach its
stable point. Fig.20 shows the soft-start waveforms during
44V input voltage and 500W output power.
Fig.16 Each module of Vin = 80V、 PO = 1kW
Fig.20 Soft-start waveform of Vin = 44V and PO = 500W.
The four modules are paralleled, and the waveforms
during 37V to 57V input voltage, output power 6kW to 10kW
are shown in Fig.17、18 and 19.
Fig.17 Output current waveforms for each module during Vin
= 37V、PO = 6kW
Fig.21 shows the output power and efficiency graphs of
10kW four-paralleled module, from Vin = 37V to 57V and PO
= 1kW to 10kW.
Fig.21 Converter output power and efficiency graphs
7 Conclusion
This paper focuses on the design of a DC-DC converter
for fuel cells power generating system, with improvements on
fuel cells slow dynamic response and stability. The overall
topology uses current-fed full-bridge as main stage, voltagedoubling circuit on secondary stage to half-reduce the
transformer turns ratio, thus increasing overall efficiency. By
using interleaved PWM control method on the four paralleled
modules, the current ripple on fuel cells stage is minimized.
And with an auxiliary supply to implement soft-start, the slow
output voltage rising of fuel cells is solved. At the output of
the current-fed full-bridge converter, a CC/CV feedback
control is added to avoid over-voltage or over-current at the
back-stage high voltage batteries, thus optimizing the life and
operational reliability of the high-voltage batteries. Finally, a
prototype of four-paralleled module of 10kW output power
with current-fed full-bridge topology is built, having
measured several related waveforms and converter efficiency.
At higher input voltage of 57V and full-load output, the
measured efficiency reaches more than 91%.
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