Study on Isolated Boost Full Bridge Converter in FCEV

Xuesong Jiang, Xuhui Wen, Member, IEEE, and Haiping Xu

Abstract--Fuel Cell Engine cannot provide enough power at cold start. According to these, a 5KW Isolated Boost Full Bridge (IBFB) converter is developed as the auxiliary power supply. With the IBFB converter, the 24V DC of the auxiliary storage cell of the Fuel Cell Electric Vehicle (FCEV) is converted to a stabilized 300V high DC. The IBFB converter is controlled with a double voltage and current loop based on DSP- TMS320LF2407. Experimental results show that the system has excellent properties and can be used in areas like FCEV.

Index Terms--isolated boost, full converter, digital control, DC/DC converter

I.INTRODUCTION ORE and more people realized the importance of surrounding protection and the pressure of energy crisis. Although there are many difficult problems in the filed

of the EV research, governments in the most of the country are making plans to develop electric vehicle (EV), they invest Billions of dollars to exploit it. Without pollution, FCEV is believed to be the future of EV. But Fuel Cell Engine cannot provide enough power at cold start. According to these, an auxiliary storage cell used in FCEV is needed to supply its energy. Due to the voltage of storage cell is very low, the boost topology mentioned in paper [1] is not applied, which can only be used in small step up ratio. What’s more it has no isolation with main source.

An isolated boost full bridge converter is proposed in this paper. With the IBFB converter, the low voltage output of the storage cell is converted to a stabilized high DC voltage with the instantaneous voltage and current feedback double loop controlled based on DSP.

II.ANALYSIS OF THE MAIN CIRCUIT

Topology Selection The isolated boost full bridge converter will convert the

24V DC to 300V DC. The difference between input and the output is so great that the non-isolated DC/DC topology is not suited. While the isolated DC/DC topology such as forward This work was supported by Chinese Academy of Sciences (CAS). Xuesong Jiang is with the Institute of Electrical Engineering (IEE), Chinese Academy of Sciences, P.O.Box 2703, Beijing China. (e-mail: xsjiang@mail.iee.ac.cn). Xuhui Wen is with the Institute of Electrical Engineering (IEE), Chinese Academy of Sciences, P.O.Box 2703, Beijing China. (e-mail: wxh@mail.iee.ac.cn). Haiping Xu is with the Institute of Electrical Engineering (IEE), Chinese Academy of Sciences, P.O.Box 2703, Beijing China. (e-mail: Hpxu@mail.iee.ac.cn).

converter needs magnetism-reset circuit, the transformer is single direction magnetization, and the utilization efficiency is low. The magnetic core of the push-pull topology is easy saturation caused by DC magnetic biasing. And more, the transformer has leakage inductance, so the power switch will bear two times more source voltage. The voltage stress of the power switch is high. The fly-back topology is only suited for small power application, from several decades watts to several hundreds watts. The isolated boost full bridge converter is fit to great voltage difference between the input and the output, and its core is bi-directional magnetization, apt to be used in high power situation.

Principle of Operation Figure1 shows the main circuit of the isolated boost full

bridge. It consists of an input voltage sourceinV , an input filter

capacitorinC , and an input boost inductor

inL . Switches Q1~Q4 constitute the full bridge circuit. T is the isolated transformer whose ratio is N. Diodes D1~D4 constitute the rectifying circuit. C the output filter capacitor, and RL load resistor.

The timing diagram for the four-power switch is shown in Figure 2. Two independent PWM signals (Q1-Q4, Q2-Q3) produced by DSP are synchronized with °180 phase shift. The duty cycle D of the two PWM signals is always identical and can vary from 0 to 1. When 0<D<0.5, the circuit operates in buck mode. When 0.5<D<1, the circuit operates in boost mode.

inVinC

inL1Q 2Q

3Q 4QT

1D 2D

3D 4D

C LR

Fig.1 Isolated Boost Full Bridge converter

4ON 4ON2OFF 2OFF

sDT sTsT5.0

41 QQ

32 QQ

Fig.2 Timing diagram for power switch

M

As shown in Fig 2, when duty cycle exceeds 0.5, the PWM pulse of Q1and Q4 overlaps with Q2 and Q3. And it results the four switches are on simultaneously. Fig3 (a) shows the corresponding equivalent circuit. In this interval, the bridge is shorted, and the choke

inL is charged. The current of choke

inL increases and can be described as follows:

)1(],[, sn

ssin

inpin TDnTnTtt

LVIi +∈+=

Where ini is the current of the inL ,

inV is the voltage of the

inL , pI is the initial current, sT is the switch period,

and nD is the duty cycle of the number n switch period. When a pair of diagonal switches Q1-Q4 or Q2-Q3 remains

on, the current in choke inL attempts to go through the

primary winding of the transformer T, and transfers the energy to the output side. Fig3 (b) shows the corresponding equivalent circuit. The current of choke

inL decreases and can be described as follows:

,)/( tL

NVVIiin

oinTin

−+=

)2(])1(,[ ssn

s TnTDnTt ++∈ Where

TI is the initial current when the choke releases energy to the load.

In order to avoid high spikes of voltages and currents of the switches due to parasitic parameters, especially the leakage inductor of the transformer, a passive clamp circuit RCD is added on the primary side to clamp the primary switch voltages in the experiment circuits.

III.ANALYSIS OF THE CONTROL CIRCUIT Figure4 shows the control principle diagram based on DSP

of the isolated boost full bridge. A/D converter samples the feed back analog signals such as output voltage and current. The DSP control chip executes the PID regulation of the voltage and current loop, and produces PWM signals. Through logic processing, the CPLD chip produces two pairs of PWM signals with a duty ratio of more than 50 percent over a

inVinC

inL1D 2D

3D 4D

C LR

Fig.3 (a) All switches are on

inVinC

inL

T

1D 2D

3D 4D

C LR

Fig.3 (b) diagonal switches are on

inV

Fig.4 control diagram based on DSP

switching cycle. As a result, the duty cycle can be regulated and the output current and voltage can be controlled.

Control Principle of the System The isolated boost full bridge converter is controlled by a

double voltage and current loop. Fig.5 shows the control principle of the system.

Where uG is the voltage regulator, the transfer function is

skksG vi

vpu +=)( , iG is the current regulator, the transfer

function iss

kksG iiipi +=)( . When the converter switching

frequency, sf is much higher than the cut-off frequency of the open loop system, the PWM converter can be modeled as a proportional element with a gain of pwmk .

Where Nkk pwmA ×= , N is the transformer turns ratio, U∆

is the disturbance input which denotes the influences of some undesired factors, such as dc voltage variation. A LC filter is employed.

The transfer function of the inner current loop can be expressed as:

)3(2

)1)(()(

RkkkBsAs

RCskskks

iiiA

iiipAi ++

++=φ

where: RCkkkA ipAi=

RRCkkkkkkB iiAiipAi ++= Compared to the outer voltage loop, the cut-off frequency

and time constant of the inner current loop can be neglected. And the inner current loop can be modeled as a proportional element with a gain of ii ksG /1)( = . The simplified dynamic model of the system can be expressed as:

)4()(

)()( 2 RkksRkkkRCsk

kskRs

vivvpvii

vivpv +++

+=φ

Due to the controlled plant is a first-order inertia element, the close system can be modeled as a second-order, when

)(sIerU

fU

rI

fI

U∆1U

AK

ik

vkR

sCo1+

dI

Fig.5 control principle of the system.

vik ＝0, the phase angle stable margin of the voltage loop nears 90°, the response of the system is fast, and the current trace ability is strong.

Implementation of the Control Scheme Based on DSP The event manager of TMS320LF2407 provides excellent

full compare unit, which can produce PWM signals directly. The voltage regulator performs in the underflow interrupt of timer 2; the current regulator performs in the period interrupt of timer1. The interrupt priority of the current regulator is higher than the voltage regulator. Fig 6 shows the flowchart of the program.

IV.EXPERIMENTAL RESULTS A 5KW, 24V/300V isolated boost full bridge converter has

been developed. The load to the converter is 17 Ω resistors in

Fig.6 (a) flowchart of the T2INT ISR

Fig.6 (b) flowchart of the T1INT ISR

parallel with a 450 Fµ bus filter capacitor. Fig.7 is the Waveform of the output voltage when the

converter works in start up mode. Fig.8 is the waveform of the primary voltages of the transformer. Fig 9 is the waveform of the output voltage, which shows that the voltage fluctuation is less than 1%. Fig10 is he system efficiency curve.

)/

100

(di

vV

Volta

ge

)/500( divmsTime

Fig.7 Waveforms of the start up output voltage

)/

40(di

vV

Volta

ge

)/10( divsTime µ

Fig.8 Waveforms of the primary voltage of the transformer

)/

100

(di

vV

Volta

ge

)/10( divsTime µ

Fig.9. Waveforms of the steady output voltage

%/Ef

ficie

ncy

KWPower / Fig 10 System efficiency curve

V.CONCLUSIONS The isolated boost full bridge converter is designed for the

fuel cell electric vehicle, and from the experimental results, several conclusions can be drawn as follows:

1) The isolated boost full bridge topology can converter low input voltage to very high output voltage successfully.

2) The experiment results show that the converter has the good dynamic and static characteristic with the instantaneous voltage and current feedback control owing to the digital control.

3) The proposed isolated boost full bridge converter can be applied not only in FCEV, but also in UPS and in battery charger.

VI.REFERENCES Periodicals:

[1] Lizhi Zhu, Kunrong Wang, Fred C Lee, Jih-Sheng Lai. “New start-up schemes for isolated full-bridge boost converters,” Proc. IEEE Power Electron. Trans. 2003, pp 946-951.

[2] Ben Hongqi, “Study on Single PFC of full bridge structure”, Journal of Harbin Institute of Technology，2003 (Chinese)

[3] K. Wang, C. Y. Lin, L. Zhu, D. Qu, F. C. Lee, and J. S. Lai, “Bi-directional dc/dc converters for fuel cell systems,” in Proc. IEEE Power Electron. Transport, 1998, pp. 47–51.

Papers from Conference Proceedings (Published):

[4] Haiping Xu, Gang Ma, Changfu Sun, Xuhui Wen, Li Kong. “Implementation of a Bi-directional DC-DC Converter in FCEV,” ICEMS 2003, pp 375-378

[5] R. Watson, F. C. Lee, “A soft-switched full-bridge boost converter

employing an active-clamp circuit,” in Proc. IEEE PESC, 1996, pp.1948–1954

VII. BIOGRAPHIES Xuesong Jiang was born in JianHu County of JiangSu Province, China, in 1973. He received the B.S. degree in electrical engineering from Yancheng institute of technology in 1994 and the M.S. degree from Nanjing university of Aeronautics and Astronautics in 2002. Now he is a Ph.D. candidate of the Institute of Electrical Engineering of Chinese Academy of Science. Xuhui Wen (M' 04) was born in Chengdu, Sichuan Province, China, in 1963. She received the B.S., M.S. and Ph.D. degrees in electrical engineering in 1984, 1987 and 1993 respectively, from the Tsinghua University, Beijing, China. From 1993 till now, she has been working in Inst. of Electrical Engineering (IEE), Chinese Academy of Sciences (CAS), Beijing, China, as an associate professor in 1996 and a professor in 1999. During 2004--2005, she was a Visiting Researcher at the Ohio State

University. Her research fields include power electronics, motion control and electrical driver system.Dr. Wen is a member of the China Electrotechnical Society (CES) and the IEEE Power Electronics Society.

Haiping Xu received the B.S. and M.S. degrees in electrical engineering from Shenyang Polytechnic University, Shenyang, China, in 1992 and 1998, respectively, and is currently pursuing the Ph.D. degree in power electronics at the Institute of Electrical Engineering, Chinese Academy of Sciences (IEECAS), Beijing. In 1998, he joined the IEECAS as an Assistant Professor, and is currently an Associate Professor. His research interests include dc–dc converter, digital control systems, ac motor drivers, and DG. Mr. Xu is a member of the

CES of China.