2007 IEEE Canada Electrical Power Conference

A Simple Current Mode Controller for TwoSwitches Buck-Boost Converter for Fuel Cells

Ahmad Ale Ahmad, IRIEE and Adib Abrishamifar, IUST

A single switch buck_boost converter is the best approach toAbstract-Due to the internal losses of fuel cells, its load solve this problem. While VIN>VOUT+AV or VIN<VOUT-AV the

regulation is not better than 30% from the no load to full load converter does its duty perfectly in buck or boost mode,conditions. To increase the utilization of full cell, this paper respectively. In buck mode, the boost unit of converter isproposed a two switch buck-boost converter with a simple disable (the boost switch is turned off) and the buck unitcurrent mode controller. This structure performs conversion regulates the load voltage. In boost mode, the buck unit ofwith lower stresses than the single switch types. In this approach converter is disable (the buck switch is tured on) and theproblem of interference between buck and boost switches boost unit regulates the load voltage. AV depends on the I/Operformance are solved by determining four operation modes forthe~~~~~~~covrtr Th xeietl eut fa 11OV, 40KWxX voltage and also the delivered power. It is usually selected inthe converter. The experimental results of an 11UV, 4OKWVV

converter show the ability of the control method. The line and the range of 0.05VN<AV<0. 15 VIN If theload regulation in different situation is below than 0.9%. VOUT-AV<V1N<VOUT+AV, the buck or boost unit can't regulate

the load voltage alone. This problem arises from the inputripple and the different saturation voltage of semiconductor

Index Terms-Buck, Boost, Current Mode Control, fuel cell. switch in different load current. So, while VN remains as closeas to VOUT, it should be employed both unit of converter to

I. INTRODUCTION obtain a regulated output.r To overcome the air pollutions of conventional fossil fuel In this case the most important issue is the correct driving ofl power plant, renewable energy sources must replace fossil two unites, because the unsuitable switching causes the

energy sources. Fuel cell technology is primary options instability and oscillation of load voltage and increase thefor renewable energy sources [1]. Not only the zero pollution power loss of semiconductors. So a suitable method isof full cell have caused its popularity but also its efficiency required to control this converter.(32-46%), which can be higher than that of a conventional This paper proposes and implements a peak current modepower plant, increase, have increased its acceptance [1,2]. control for the two switch buck_boost converter of Fig. 1.Because of the internal losses of full cell, the load regulation isnot better than 30% [1], so to increase the utilities of fuel cell II. DESCRIPTION CIRCUIT OPERATIONa DC/DC converter is necessary to regulate the output voltage.In this paper, a nonisolated two switch buck-boost converter is A Operating Modesdeveloped for this target (Fig. 1 a). Although the Choosing peak current mode control as the best simple controlsemiconductors of this buck_boost converter are twice of method, there are four operation modes for buck_boostconventional single switch type, it performs conversion with converter.lower stresses than the conventional type. Moreover its 1. Boost mode: If VIN<VOUT-AV.inductor is more smaller than the single switch type [3] [4]. 2. Buck mode: If VIN>VOUT+AV.There are four basic topologies for single switch buck-boost 3. Buck_Boost mode: If the input voltage in buck modeconverters: conventional buck-boost, Cuk, Zeta and SEPIC. decreases slowly to reachThe advantages and disadvantages of these four topologies are VOUT+AV, the converter goes to the buck_boostsummarized in [3]. mode, and remains in this mode until VIN< VOUT-AV.

4. Boost_Buck mode: If the input voltage in boost modeincreases slowly to reach VOUT-AV, the convertergoes to the boost_buck mode and remains in thismode until VIN<VOUT+AV.

fD> VoutI?Lt B. Boost Mode

Fig. 1.Asigewthuk_oscnerImplementation of peak current mode control for boost- ~~~~~~~converter iS very simple. Fig. 2 shows this structure. The boost

Paralleling [4] and multilevel techniques [6][7] can be used to switch (S2) iS turned on in rising edge of the input clock.share current or voltage stresses, but neither of these methods When the peak of inductor current increases and reachs tocan not reduce the current and voltage stresses simultaneously. IREF, the switch is turns off. The average of signal which

obtains from the comparing the VREF and the VOUT sampleThis work was supported by the research center ofJDEVS . generates the IREF* The maximum of IREF is determined by the

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maximum output current or maximum switch current (S2). Step2. (Fig. 4b)When inductor current exceeded IREF (at t=Tl),the S2 is turned off. In this mode as the S2 turned off, the SI

Vill (Vo t also turned off. So, the diodes Dl and D2 become forwardbias and the inductor energy is transferred to the output.Therefore its current decreases linearly as (3).

dL (Vu DIODEon) (3)

Step3. (Fig. 4c) After (l-DBuckMax)*T the SI is turned on andthe diode DI becomes reverse bias, so theiL increases linearlyas follow:

+ VW-REF diL L -VINV -v -vOU4

dt IGBTon DIODEon OUTFig. 2. Peak current mode control for boost converter. The DBUck_Max is the maximum duty cycle ofthe buck switch in

the buck mode and I/T is switching frequency, (1-C. Buck Mode DBUCk_Max)*T=T2-T1. This step is the end of a period of

The peak current mode control for buck mode can be boost buck mode and the switches condition don't changeuntil next rising edge of the clock. Fig5 shows the iL in theimplemented as the boost mode. But it causes malfunctioning botbc oeboost-buck mode.

two switches in buck_boost or boost_buck modes. So, thedeep current is used instead of peak current in the buck mode.In this method, the buck switch (S1) is turned off in rising

D2

edge of the clock and turned on when the switch current

decrease to less than IREF (Fig. 3). The average of signal which (aobtains from the comparing the VREF and the VOUT sample,generates the IREF. The maximum of IREF is determined by the I 1maximum output current or maximum switch current (SI).

Vin VoVa T-< t' t hZ S .! ~~~~~~~~~~~~~~~~~~(b

Fig. 4. Boost_Buck mode operation at a)stepl b)step2 c)step3.

Fig. 3. DEEP CURRENT MODE CONTROL FOR BUCK CONVERTER. E. Buck Boost Mode

In the proposed control method for buck_boost mode, theD. Boost Buck Mode buck unit is master and boost unit is slave. In the steady state,

there are three operation steps as follow:In the proposed control method for boost_buck mode, the Stepl. (Fig. 6a) The SI is turned off in rising edge of clock.boost unit is master and buck unit is slave. In the steady state,there are three operation steps as follow:Stepi. (Fig. 4a)The S2 is turned on in rising edge of clock.The SI was turned on in the last step of previous period. So,the diodes DI and D2 are in reverse bias. As soon as the S2 isturned on the inductor current (iL) increases linearly as relation

diL L -V 2 V Tl T2 T

dt in XIGBTon ()FIG.5. THE INDUCTORCURRENT IN THE BOOST_BUCK.Meanwhile the COUT (Output Capacitor) supplies the load TeS stre f ntels tpo evospro.S h

curren andtsvolage i decresed a (2).diodes Di and D2 are in forward bias. As soon as the Si is

C 'VO UT (2) turned off the inductor energy is transferred to the output anddt its current decreases linearly as (4).

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di The current mode PWM unit includes an adder a leadL = (VOUT +2XVDIODEon) (4) compensator to generate IREF and two comparators to limit the

dt buck and boost switches current. The current mode approachStep2. (Fig. 6b) The SI is turned on when iL decreases to IREF- makes overload and short circuit protection easy. The P10041As the SI is turned on the S2 also turned on. Then the diodes series, the JDEVS IGBT Driver, is used for power switchDI and D2 become reverse bias. So the inductor current driver.increase linearly as (5).

LdiL VI4 Mode SelecoL -Vi - 2XVo (5)dt i IGTnIlStep3. (Fig. 6c) After DBoost_MM,)*T, the S2 is turned off and Dn r cBoos

the diode D2 becomes forward bias, so the iL increases linearly Cverteras (6), but the variation slope of iL in this step is less than ILstep2.

di 3v ILdL -V V -V -vdt iN IGBTon DIODEon OT(6

FIG. 8. THE PROPOSED DC TO DC CONVERTER BLOCK DIGRAM.The DBoost Mm is the minimum duty cycle of the boost switchin the boost mode and l/T is switching frequency, IV. EXPERIMENTAL RESULTDBoost_MM,*T=T2-T1. This step is the end of a period ofbuck_boost mode and the switches condition don't change In order to verify the validity of the control circuit, theuntil next rising edge of the clock. Fig7 shows the iL in the experimental circuits are established. The characteristics oftheboost_buck mode. system are:

Vin = 90 ... 14OVDCVout =I IOVDC

Vin __ VOU Pout =40KWInput capacitor 24000pF

v iS. * > a Output Capacitor 24000pFInductor 8pfH

__________________I vSwitching Frequency= 15KHz

Vin vbut ~~~~~~~~Buckswitch: 2* FZ600R12KE3Boost Switch: 1* FZ600R12KE3The Fig. 9 and 10 show the converter and control circuit,

1 D1 TSDIS2 respectively. Fig. I I shows the cuffrent of inductor in buck andboost-buck mode. Fig. 12 shows the transient waveform of the

_____________________________________ output voltage. The line and load regulation of converter are

Vin SIbelow 0.9°O in all of modes.

Induror ~ ~ st~o

FIG. 6. BUCK_BOOST MODE OPERATION AT A)STEPI B)STEP2 C)STEP3. oo

Ii~~~~~~~~~~~~~~~~~~~~IB

T M:I _S12rFIG. 7. THE INDUCTOR CURRENT IN THE BUCK_BOOST

FIG. 9. THE BUCK_BOOST CONVERTER.III. IMPLEMENTATION OF CONTROL CIRCUIT

The proposed control scheme is shown in fig8. The controlcircuit includes three units, mode selector, current mode PWMand power switch driver. According to section lIIthe operationmode is determined by input voltage. An AVR familymicrocontroller employs this duty. It also generates thenecessary clock pulses for the PWM unit.

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VI. REFERENCES[1] Ozpineci, B.; Tolbert, L.M.; Zhong Du; " Optimum fuel cell utilization

with multilevel inverters", IEEE 35th Annual Power ElectronicsSpecialists Conference, 20-25 June 2004. Volume 6, Page(s):4798--U ~~~~~~~~~~~~~~~4802.[2] Todorovic, M.H.; Palma, L.; Enjeti, P, "Design of a wide input rangeDC-DC converter with a robust power control scheme suitable for fuelcell power conversion ", Nineteenth Annual IEEE Applied PowerPElectronics Conference and Exposition, 2004. Volume 1, Page(s):374-379.

[3] Qiao, Haibo; Zhang, Yicheng; Yao, Yongtao; Wei, Li; "Analysis ofBuck-Boost Converters for Fuel Cell Electric Vehicles", ICVES 2006.IEEE International Conference on Vehicular Electronics and Safety, 13-15 Dec. 2006 Page(s):109 - 113.

[4] Jingquan Chen, Dragan MaksimoviC and Ro ert Erickson, "Buc -BoostFIG. 10. THE CIRCUIT OF PROPOSED CONTROLLER. PWM Converters Having Two Independently Controlled Switches",

Power Electronics Specialists Conference, 2001. PESC. 2001 IEEE 32ndAnnual.

[5] P. Lee, Y. Lee, D. Ciheng, and X. Liu, "Steady-State Analysis of anInterleaved Boost Converter with Coupled Inductors," IEEE Trans. OnIndustrial Electronics, Vol. 47,No. 4, August 2000, pp787-795.

[6] B. Lin and H. Lu, "A Novel PWM Scheme for Single-phase Three-Level Power-Factor-Correction Circuit," IEEE Trans. On IndustrialElectronics, Vol. 47, No. 2, April 2000.

[7] D. Maksimovic and R. Erickson, "Universal-Input, High-Power-Factor,Boost Doubler Rectifiers," Proc. IEEE APEC, 1995 Record, pp. 459-465.

Time/Dfive -204SecAAnperw simp10 Ahmad Ale Ahmad was born in Babol at the north of

Iran, on Aug 6, 1980. He received the BS and MSdegree in electrical engineering from the Iran University

FIG.°1. THE CURRENT OF INDUCTOR IN BUCK AND BOOST-BUCK MODE. of Science and Technology in 2002 and 2006respectively.

From 2002 to 2006, he was with power electronicresearch group in JDEVS, designing medium and highpower converter. He joined to Iranian Research Institute

of Electrical Engineering since 2006. His activities are currently focused onhigh voltage VSC.

Adib Abrishamifar received the B.Sc., MSc. AndPhD degrees in Electronics Engineering from IranUniversity of Science and Technology (IUST) in 1989,1992 and 2001, respectively. He has been with theDepartment of Electrical Engineering, IUST, since

Time ~~~~~~~e1 ~~~1993. His current research activities include analog0 ~~~~~~~~~~~~~~~~~~~~~~~circuitdesign and power electronic.

FIG. 12. THE TRANSIENT WAVEFORM OF THE OUTPUT VOLTAGE IN 600 oSTEPLOAD.

V. CONCLUSION

A new simple approach for controlling of a single buck-boostconverter to regulate the fuel cell output voltage, is developedin this paper. The implementation of this new method controlis very easy. It regulates the output voltage perfectly and there