High Efficiency DC-DC Converter using a Series-Parallel Compensation
Method for a Fuel cell
Koji Orikawa, Jun-ichi Itoh
NAGAOKA UNIVERSITY OF TECHNOLOGY
1603-1 Kamitomioka-cho
Nagaoka city Niigata, Japan
Tel./Fax: +81 / (258) .47.9533.
E-Mail: itoh@vos.nagaokaut.ac.jp
URL: http://itohserver01.nagaokaut.ac.jp/itohlab/index.html

Keywords«DC power supply», «Converter control», «Fuel cell system»

Abstract
This paper proposes a novel DC-DC converter for a hybrid power supply using both a fuel cell anda battery. In the proposed circuit, the power converter with the battery is connected in series to the fuelcell. The output voltage is controlled by a series converter that regulates only the differential voltage
between the fuel cell voltage and the output voltage. The power rating of conventional buck-boost DCDC converters are dominated by the input voltage or the output voltage. In contrast, since the voltage
rating of the proposed circuit requires the differential voltage between the fuel cell voltage and theoutput voltage, the power rating of the proposed circuit is smaller than that of the conventional
converters.
In addition, in order to suppress large current ripple in the fuel cell, a parallel converter is added tothe series converter. The experimental results confirmed that the proposed circuit could achieve 98.8%
at the maximum efficiency point in the small differential voltage region.

I. Introduction
Recently, most mobile electronic devices use batteries as a power source. The mobile electronic
devices are developed with increasingly high performance and functionality, accompanied byincreasing power consumption and the demand for longer operation times. The efficiency of DC-DC
converters become an important issue in terms of longer operation times for these devices. Resonant
type converter, which use zero voltage switching or zero current switching, is one of the most effective
circuit topologies to obtain high efficiency [1-2]. However, the conventional DC-DC converter needs
to convert all the power regardless of the output voltage because the conventional converter isconnected in parallel to a power supply and a load.
On the other hand, the fuel cell systems are being developed as a new power supply for mobile
devices. Many circuit topologies of DC-DC converters for fuel cell applications have been studied inorder to obtain high efficiency [3-5]. In Ref. [3], cascade converter topologies, which compensate
differential voltage between the fuel cell voltage and the output voltage, is a good solution to obtainhigh efficiency. However, one of the problems of these converters is that the fluctuation of the output
current is the same as the fluctuation of the fuel cell current due to the series connection. The output
power of a fuel cell should be constant, because quick power fluctuations of a fuel cell reduce its life
time. As a result, the series converter reduces the life time of the fuel cell although the output voltage
can be controlled.
A fuel cell requires a battery or an electric double layer capacitor (EDLC) to compensate thedynamic response, because it cannot respond to quick load fluctuations. A hybrid power supply using
both the fuel cell and battery also requires high efficiency, downsizing, and quick response to load
fluctuations.

Nowadays, hybrid power supplies using both the batteries and fuel cells are being actively studied[6-9]. One of the popular circuit configurations consists of two boost choppers for the fuel cell andbattery, and a buck chopper for output voltage regulation. When the load condition causes a change inthe terminal voltage of the fuel cell, the operation of the buck chopper regulates the output voltage
effectively. The buck mode operation is also used to save the power consumption of the load circuit.
The battery supports the control response of the fuel cell through the boost chopper. However, thissystem can not achieve high efficiency because the power is converted twice between the fuel cell andthe output, resulting in an increased of power loss.
In this paper a novel DC-DC converter is proposed for a hybrid system using both the fuel cell andbattery. The proposed system consists of a series converter and a parallel converter to control the input
current and the output voltage respectively. The same concept has been proposed for an
uninterruptible power system (UPS) [10] to control the input current and the output voltage. The input
current is controlled by a parallel converter and the output voltage is controlled by a series converter.
This paper applies the same concept to a DC-DC converter using both the fuel cell and battery.
In the proposed circuit, the series converter generates a positive and negative voltage to achieve
boost and buck operation. As a result, high efficiency is obtained, because the power rating of the DCDC converter can be drastically reduced [11]. On the other hand, the parallel converter with a battery
compensates the quick response according to the load fluctuations. A control strategy is also proposedto achieve both the quick response of the load and slow power fluctuation of the fuel cell. Finally, theexperimental results are presented to demonstrate the advantages of the proposed converter incomparison with a conventional circuit.

II. CONCEPT OF PROPOESD CIRCUIT
A. Series compensation
Figure 1(a) presents a block diagram of a conventional buck-boost chopper. These converters
convert all the input power, which is not dominated by the relations between the input and output
voltage. In particular, when the input voltage Vin is close to the output voltage Vout, the duty ratio D ofa single stage buck boost chopper is obtained by (1), using switch on time ton, and off time toff.
ton
Vout
D=
(1)
=
= 0 .5
ton + toff Vin + VoutIn this case, switching devices should be operated regardless of the relations between the input
voltage and the output voltage. Besides, the energy for the output should be charge by an intermediate
reactor or capacitor. As a result, the converter efficiency will decrease.
Figure 1(b) shows a block diagram of the series compensation converter, which is connected inseries to the power supply. The series converter outputs only the differential voltage between the fuelcell voltage and the output voltage. Therefore, when the fuel cell voltage is close to the output voltage,
the output power of the series converter becomes smaller. The small output power means a small loss
even if the efficiency is not so high. In particular, when the fuel cell voltage agrees with the output
voltage, the output power is directly provided by the fuel cell without switching operation; therefore,high efficiency is obtained by the series compensation circuit, although fluctuation of the output
current directly becomes a fluctuation of the fuel cell current due to the series connection, which
decreases the lifetime of the fuel cell. A voltage ripple of the fuel cell occurs due to the internal
impedance in the fuel cell.

B. Series-parallel compensation
Figure 1(c) shows a block diagram of the proposed series-parallel compensation converter. In the
proposed circuit, the parallel converter is connected in parallel to the fuel cell. When the output power
is constant, the parallel converter does not operate, and the series compensation method can obtainhigh efficiency. The output voltage Vout is obtained by (2), using series converter output voltage Vconv
and the fuel cell voltage Vfc.
Vout = V fc ± Vconv(2)
When the load condition is changed, the parallel converter will compensate the quick variation of
the fuel cell current. The fluctuation of the output power is compensated by the battery through theoperation of that parallel converter. In addition, when the battery voltage is decreased or overcharged,

either the battery will be charged from the fuel cell or the battery supplies the power to the load
through the operation of that parallel converter.

III. PROPOSED CURCUIT
A. Circuit configuration
Figure 2 shows the circuit configuration of the proposed circuit based on Fig. 1(c). The series
converter uses a boost converter and a step-down converter. The series converter generates a positive
and negative voltage according to the boost mode and buck mode, respectively. The output of theparallel converter is connected to the negative terminal of the fuel cell. The proposed circuit requires
only two reactors, but the conventional circuit uses three reactors. To suppress the ripple current of thefuel cell, an inductor is connected in series to the fuel cell. The reactor is used as a boost reactor too
when the boost mode is selected.
Figure 3(a) shows a block diagram of the proposed circuit for series compensation mode. In thiscase, the series converter works as a H-bridge converter. Figure 3(b), (c) present a block diagram ofthe proposed circuit for series-parallel compensation mode. When the load condition is changed, only
the parallel converter operates. The series converter compensates the differential voltage between thefuel cell voltage and the output voltage, and the parallel converter compensates the quick variation of
the fuel cell current. The battery voltage must be higher than the fuel cell voltage to prevent a rushcurrent from the fuel cell to the battery.

B. Design of the proposed circuit
In the proposed circuit, the ripple current of the inductors becomes maximum value when the
differential voltage between the fuel cell voltage and the output voltage is at maximum. The value ofthe ripple current of the inductor do influence on the lifetime of the fuel cell. The ripple current of theinductor ∆ifc is obtained by (3), using switch on time ton.
V fc
V fc
V fc Vout − V fc
t on =
Dboost =∆i fc =(3)
L fc
L fc f sw
L fc f sw
Vsb
where Lfc is an inductor of the fuel cell side, and Dboost is the duty ratio in boost mode.
Table I shows the specifications for the proposed circuit. In case of Table I, the ripple current of theinductor becomes the maximum value when the fuel cell voltage is 4 V in boost mode. The value ofthe ripple current of the inductor is limited to 30% of the output current, and then the ripple current is
0.5 A since the output current is 1.67 A at the output power 12 W. In this condition, from (3), theinductance of the fuel cell side is calculated by (4).
V fc Vout − V fc
4
7.2 − 4L fc === 23.3[ µH ](4)
3∆i fc f sw
Vsb
0.5 × 100 × 10
11(a) Conventional circuit.

(b) Series compensation.

(c) Proposed circuit.
Fig. 1: Block diagrams of proposed converter in comparison with a conventional circuit.

Fig. 2: Configuration of proposed circuit.
Vconv
Ifc
Vfc

H- bridge
converter
Vout

Vsb

(a) Series compensation mode.

(b) Boost mode with parallel compensation.

(c) Step down mode with parallel compensation.

Fig. 3: Block diagram of proposed circuit.

Table I: Specifications for proposed circuit.
Fuel cell voltage Vfc (V)Output power Pout (W)Output voltage Vout (V)Battery voltage Vsb (V)Switching frequency fsw (kHz)Inductor current ripple (A)4 to 10
12
7.2
11
100
30% of output current

IV. CONTROL STRATEGY
Figure 4 shows control diagrams of the proposed circuit. The proposed control method has twoloops; the inner loop is for controlling the fuel cell current and the parallel converter current. As forthe outer loop, it is for the output voltage control.
Operation between the series and parallel converter is constrained by the load condition in order toreduce the converter loss. In boost mode, the series converter works as a boost converter. That is, theswitch S2 maintains at on state. The difference voltage is then controlled by PWM modulation of S1. In
this case, the battery supplies the power to the load, because the differential voltage is positive. Inbuck mode, the series converter operates as a buck converter; switch S1 maintains at off state. The
differential voltage is then controlled by PWM of S2. In this case, the battery is charged from the fuelcell, because the differential voltage is negative. Boost and buck modes are selected by the control
signal of the series converter according to the relations between the fuel cell voltage and the output
voltage. The switch S3 and S4 are for the parallel converter. The parallel converter works only when
the load fluctuated. The parallel converter will be started when the load fluctuation is detected by thewindow comparator and is stopped after the time spends longer than the time constant of the LPF andHPF.

A. Output voltage control
The output voltage command of the series converter is calculated from the difference between the
output voltage command and the fuel cell voltage. Where the voltage drop of the inductance and theon-resistance of FET can be negligible, the modulation index α for the series converter is obtained by(5), using the fuel cell voltage Vfc and the battery voltage Vsb, and the output voltage command Vout*.
⎞⎛ V * −V
fc
⎟
⎜ out
− 1⎟
V
⎟
⎜
sb
⎠
⎝

α = 2⎜(5)

In order to improve the response and compensate the voltage error, the output voltage is regulatedby an automated voltage regulator (AVR) using a proportional-integral (PI) controller. Then the output
current command for the inner loop is output by the AVR.
Figure 5 shows control diagrams of the output voltage. When the response of the current control ismuch faster than the voltage control, the transfer function from the output voltage command to the
output voltage Vout is obtained by (6):
Vout=
Vout *

Kp(1 + sTi )CoutTiKKp
s2 + p s +Cout
CoutTi(6)where symbol “s” is Laplace operator, Kp is the proportional gain and Ti is the integral time constant of
the PI regulator. Then, the Kp and Ti are given by (7), (8)K p = 2ζω v _ nCout
(7)
Ti =

2ζ

ωv _ n(8)where ζ is a damping factor, ωv_n is a natural angular frequency for the output voltage control.

B. Current control
The current command is divided into the fuel cell current command and the compensation current
command. Low pass filters (LPF) and high pass filters (HPF) are used to divide the current command.
The LPF is used to separate the fundamental current fluctuation, for the slow response of the fuel cell.
The HPF is used to separate the transient current fluctuation for the fast response of the battery. The
HPF functions simultaneously with the LPF filter, as shown in Fig. 4. The time constants of the LPF
and HPF are set to the same value.
The frequency responses of the LPF and HPF do not influence the output current response, as
explained by the following. The relations between each current can be expressed by (9), using the fuelcell current Ifc and the parallel converter current Icomp.
I Lout = I fc + I comp
(9)
The fuel cell current command Ifc* and the parallel converter current command Icomp* are obtained by(10) and (11)1**
I fc =I Lout(10)
1 + sTsT**
I comp =I Lout(11)
1 + sTFig. 4: Control diagram for the proposed circuit.
⎛ 1 + sT i ⎞⎟⎟K p ⎜⎜⎝ sT i ⎠1
sC outFig. 5: Control diagrams of the output voltage.

⎛ 1 + sT i ⎞⎟⎟K p ⎜⎜⎝ sT i ⎠1
sL fc

Fig. 6: Control diagrams of the fuel cell current.

where T is the time constant of the filter and ILout* is the output current command. Therefore, the output
current is obtained by (12).
**I Lout = G1 (s )I fc + G2 (s )I comp(12)
where G1(s) and G2(s) are the transfer functions of the fuel cell current control and the parallel
converter current control, respectively.
In addition, the output current is expressed by (13) when both current regulator designs for the fuelcell and parallel converter are the same.
*
I Lout = G (s )I Lout
(13)
where G(s) is the transfer function of the output current control. That is, the time constant of the LPF
or HPF does not appear in the transfer function of the output current control. In addition, the operationof the parallel converter is started by variation of the parallel converter current command Icomp*. Then,
operation of the parallel converter is stopped by a timer which is used to prevent a decrease in theefficiency of the proposed circuit.
Figure 6 shows control diagrams of the fuel cell current. The transfer function from the reference ofthe fuel cell current to the fuel cell current is described by (14).
Kp(1 + sTi )I fc (s )L fcTi(14)=
Kp
Kp
I fc _ ref (s )
2
s +
s+
L fc
L fcTiThen, the proportional gain Kp and the integral time constant Ti are given by (15), (16)K p = 2ζωi _ n L fc
Ti =

2ζ

ωi _ n(15)(16)

where ωi_n is a natural angular frequency of the fuel cell current control. The natural angular frequencyof the fuel cell current control and the parallel converter current control should be set to the same

value. It is noted that the natural angular frequency of the current control is set to over than ten timesof the voltage control in order to avoid the influence of the current control for the output voltage
control.

V. EXPERIMENTAL RESULTS
The proposed circuit was tested under the experimental conditions shown in Table II. The responseof the current regulator was designed as much higher than that of the LPF. It should be noted that thetime constant was set to a shorter time in this experimental condition in order to confirm theeffectiveness of the parallel converter. In order to reduce negative influence on the lifetime of the fuelcell, the time constant should be set to a few seconds.
Figure 7 shows the efficiency of the proposed circuit to confirm the effectiveness of series
compensation. The experimental results confirmed that the proposed circuit could achieve 98.8% at
the maximum efficiency point in the small differential voltage region. The output voltage is kept
constant by the PI regulator. It should be noted that the output voltage waveform has no low frequency
component and the fluctuation is in a steady state load.
Figure 8 shows the result of the loss analysis. In order to evaluate the power loss of the proposed
circuit, the loss measurement and analysis for each part have examined. When the fuel cell voltage
agrees with the output voltage, the maximum efficiency of the proposed circuit is obtained. This

Table II: Experimental conditions.
Fuel cell voltage Vfc (V)Output voltage Vout (V)Battery voltage Vsb (V)Switching frequency fsw (kHz)Input reactor Lin (µH)
Output reactor Lout (µH)
Output capacitor Cout (µF)
ACR response (kHz)LPF time constant (ms)Fig. 9(a), (b)Load change (W)Fig. 10(a), (b)4 to 10
7.2
11
100
30
30
801
1
2.2
2 to 20
20 to 2100

Efficiency [%]

99
98
97
96
Switching frequency fsw :100[kHz]Battery voltage Vsb :11[V]Output voltage Vout : 7.2[V]Output power Pout: 12[W]

95
94

3

4

5

6

7

8

9

10

Fuel cell voltage [V]Fig. 7: Efficiency of the proposed circuit.

11

Power loss [W]reason is that the output power is directly provided by the fuel cell without any switching operation.
That is, the switching loss is zero. Besides, the proposed circuit provides higher efficiency than theconventional circuit even if each switch is controlled. This reason is explained as follows; in the small
differential voltage region, the ripple current of the inductors are reduced because the reactor voltage
is small. Therefore, the losses of the inductors are smaller than the conventional circuit. Especially, theiron loss in both the inductors becomes smaller.

Fig. 8: Loss analysis.
Figure 9 and Figure 10 show comparisons of the fuel cell current and the output voltage with andwithout parallel compensation when the load condition is changed. In Fig. 9(a), the voltage drop ofover 14% occurs at the point of the load change when the output power is increased by the load, where
only series compensation applied. In addition, the fuel cell current is changed at high speed. In
contrast, Fig. 9(b) shows that the variation of the fuel cell current is suppressed by parallel
compensation. The output voltage drop of within 7% is achieved.
On the other hand, Fig. 10(a) shows the waveform without parallel compensation when the output
power is decreased by the load. In this case, in Fig. 10(b), the variation of the output voltage is also
suppressed by operation of the parallel converter. It should be noted that the spike current occurs dueto the initial value of the integrator in the ACR of the parallel converter. This problem can be solved
by improving the initial operation of the parallel converter.
Overall, the experimental results confirm the validity of the series–parallel compensation method.

VI. Conclusion
A series-parallel compensation type DC-DC converter has proposed to achieve high efficiency,
downsizing and longer lifetime of a fuel cell-battery hybrid system. When the output power is almost
constant, the series converter provides only the differential voltage between the fuel cell voltage andthe output voltage, while the parallel converter suppresses the variation of the fuel cell current. In
addition, the control strategy combined a LPF and a HPF to suppress the quick load fluctuation has
proposed. The share rate between the fuel cell and battery is controlled by time constant of the LPF
and HPF. Besides, the design of control parameter for the ACR and AVR was discussed based on thecontrol model of the proposed circuit. The design of the proposed circuit was also clarified using theripple of the inductors.
The experimental results and the loss analysis confirmed that the proposed circuit could achieve amaximum efficiency of 98.8% in the small differential voltage region. In addition, the proposed
converter suppressed the variation of the fuel cell current and achieved an output voltage drop of
within 7% for the effect of load change.
Overall, the experimental results confirmed the validity of the series-parallel compensation method.

(a) With series compensation only.

(b) With series-parallel compensation.

Fig. 9: Voltage waveforms and current waveforms for increasing power condition.

(a) With series compensation only.

(b) With series-parallel compensation.

Fig. 10: Voltage waveforms and current waveforms for decreasing power condition.

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