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A Fhase-Shift Controlled Bi-directional DC-DC ConverterH. L. Chan,K. W. E. Chengand D. SutantoThe Hong KongPolytechnic University,Hung Horn, Hong Kong,

Tel: (852)2766-6162

Abstract-The newly developed phase-sh ift controlled bi-directional DC-DC converter has the conventional features ofphase-shifted converters, including zero-voltage switching (ZVS)and constant switching frequency. However, it ha s been improvedby developing a bi-directional power flow capability and applyingsynchronous rectification, hence the on-state voltmge drop of theactive devices are small.1. INTRODUCTION

The conventional bi-directional DC-DC converters [1-41 aremost suitable for high voltage and high power applications.However the proposed converter [5] has been improved withbi-directional power flow, it is especially useful forregenerative braking in electric vehicle (EV), where themechanical energy is converted into electrical energy bytraction inotor and then fed back into the EV batteries. Theconverter employs the phase shift technique for voltagecontrol; utilizes parasitic capacitance of active switches andleakage inductance of transformer for resonant switching; doesnot require a large filter inductor and applies the synchronousrectification [6-71 ot bi-directional power flow.

The bi-directional phase-shifted DC-DC converter iscomprised of an inverter brid ge (QI-Q4) nd a converter bridge(MI-M4) ,which are connected with a high frequency powertransformer. The 50% duty-ratio gate signals applied to theconverter bridge are synchronized with those applied to theinverter bridge. Fig. 1 shows the topology of the proposedconverter. A11 the active switches in both the inverter andconverter bridges are identical, each of them consists of aMOSFET with a body diode and a capacitor. The capacitors,C,-C4, epresent the slim of stray capacitance of the MOSFETand an add itiona? resonan t capacitor. L lk is the total leakageinductance of the transfonner's prima ry and secondarywindings. C,,, and C,,, re the input and output filtercapacitors. V,,, and V,,, are the input and ou tput voltage. i(t) sthe primary current of transformer.

Fig. 1. Bi-directional Phase-Shifted DC-DC Converter

0-7803-5491-5/99/$10.000 999 IEEE

Fig. 2. Idealised Voltage and Curren t WaveformsThe complete cycle of operation for the bi-directional phase-shifted DC-DC converter is shown in Fig. 2. V,, to V aregate signals applied to Ql to Q4. n addition, V,, is also usedto drive MI nd M4,Vs2 to drive M2 and M3, uch that th econverter bridge can operate in the manner of synchronousrectification. Th e synch ronou s rectification allows theconverter to have low on-state loss and ease of coritrol at thetransistors MIo M4.

11. OPERATIONS OFTHECIRCUITThe circuit operation in positive half cycle is the sam e as thenegative half cycle excep t the signs of voltage and current arereversed, so only the positive half cycle is explained here. Theoperation principle can be divided into five modes.1 ) Mode 1 : Q i nd Q 4 are on. Vi, is applied across theprimary winding of transformer, and the secon dary winding oftransformer is clamped to Yo, as MI and Ad4are on. As energyhas been stored in L f k during the previous mode of operation,then the primary current i ( t )flows n reverse direction.2) Mode 2: When energy in Llk is empty, the source issupplying energy to the load, and i(t) flows in forwarddirection.3) Mode 3: Ql remains on, but IQ4 is being turned off.Energy in L,k is charging up C4 and discharging Csimultaneously. D3 will conduct once the voltage across C , is

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zero. Subsequently, Q3 is turned on under zero voltageswitching (ZVS). It is clear that a dead time between turn-offof Q4 nd turn-on of Q3 is needed for Q3o achieve the ZVS.diV,,,- Vaul=Lk-r (4)

2 ) Off Stage (f4 < t 2 5) or t5 - 4 = ( 1 - D - &T/2: Thisstage is exactly the mode 4 of operation, energy in Lik isfreewheeling to drive the load, and ( 5 ) is the circuit equation.

di- nYnu,= L:k-fwhere C,lgl,, is sum of C,, C4 and stray capacitance oftransformer winding .4) Mode 4 : Ql and Q3 are on. Energy in Llk continues todeliver to the load, and the Iarge output filter capacitormaintains the secondary of transformer at Val,,.

3 ) Tran$itionStage (t5 < t 5 t6)or t6 - s = 4TD: hisstage is the changeover of the Off stage to the On stag e, itis actually !he mode 5 of operation. Circuit equation of thisstage is given in (6 ) .5 ) Mode 5: Q; remains on, but Q, is being turned off. C l isbeing charged up and Czis being discharged simultaneously bythe energy in LIR. Once voltage across C reaches zero, Dzconducts and Q2 witches on under ZV S condition. Also adead time between the turn-off of Q1 nd turn-on of Q2 srequired for ZV S of QZ.

didt- vu,- Vmll= L, -

The inpui: energy W ,, o the converter during a half cycle is:

(7 )

Then, solving (4), 3, 6 )and (7>,we gotwhere CIcjj s sun1 of CI, Z and stray capacitance oftransformer winding.Furthermore, there is a minimum load requirement for theconverter to achieve ZVS, it is given in (3). And the outp ut energy WO,,,or a half cycle is given by:

(3) (9)Assumed that WO,,= Wi,,,we obtained

111. MATHEMATICALALCULATIONSThe voltage conversion ratio and control region werecalculated with the p arameters: D = phase shift, n = turn ratio,T = switching period. In addition, C,,, is assumed to besufficiently large to hold V,,,,, at nearly constant. Three sat esare defined as follows: where k:=-badT. This is the DC characteristic of the4Llkconverter.A. Voltage ConversionRatio

E. Maximum an d Minimum Phase Sh@1) On Stage ( t l < I 4 )or (f4- l =DTl2): Since modes 1and 2 are identical when they are considered under circuittheory, and the primary current changes in mode 3 is small, sothis stage includes modes 1, 2 and 3. In addition, we alsoneglect the dead time &TI2 as it is insignificant whencompared with D. The circuit equation describing the Onstage is,

1)Maximum Phase Shift (Dmm):The maximum outputvoltage, when phase shift = DRlaX,an be obtained bydifferentiating ( 1 0) with respect to D, and Dm, s given in (1 1).

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2)Minimum Phase Shijt (Dm& The minimum load currenti(t5) in terms of phase shift can be derived from (4), ( 5 ) and (6),and it is substituted in (3) to yield Dmh.

3 90% -.-0s 85% -C. ControlRegion?-+ - A *v -

Plotting ( I 0), (1 1)and (12) on the sam e graph, we get Fig. 3,which shows the optimal control region of the converter. Thisgraph can then be u sed or design o f the proposed converter.IV. EXPERIMENTS

There were tw o experiments carried out to test theperforniance of the proposed co nverter's efficiency and reversemode of operation.A . Experiment One - Conversion EflcienciesA prototype of the proposed converter was built up w ith thespecifications: switching frequency of 100kHz, nput voltage o f70 V and output voltage of 30V. The circuit componentsincluded: all of MOSFETs (Q,-Q4 an d M1-M4)are LRF530N,Llk is 21.8pH, C,-C4 are 3.2nF, C,, and CO,,, re 5.4pF and

i 4.8pF respectively. Moreov er, the power transformer wasconstructed using a bobbin o fETD49 and Ferrite core matsrialof 3C85, its turn number of primary winding (NI) andsecondary winding (NJ were 70 and 40 respectively.

Under the load range from 60 W to 120W, he conversionefficiencies were measured and p lotted in Fig. 4. It is obviousthat the overall efficiency of the proposed converter is morethan 91% over the specific load range.~~ DC Characterisrrand Coiitrol RegionI in this control region, In Ihis control region.1 thovilhconverterw d witciiins.pcralcs D D,, thenverselyonverterroportionalutput isI

,,- .. .. __ _..

IO hc phase shift.

0.0 0.1 0.2 0.3 0.4 OS 0.6 0.7 0.8 0.9

Fig. 3. Converter Efficiencies

Fig. 4. Converter Efficiencie sInvestigating th e switching of MOSFET as shown in Fig. 5can support the high conversion efficiency.

Zero Voltage Switchingof MOSFET80 460 3

U 40 2 2o, a 2 0 $ 2.s g 20 -1.5?+:2 0 0 8-2 ?Q3 4 0-60 -3

-80 -4Time (us), I

Fig. 5 . Zero-Voltage Switching of MOSFET Q4It was clearly shown that the drain-source voltage of Q4 hasbeen resonated to zero before the gate-source signal is applied.With the aim of this switching method, the switching losses ofMOSFETs are minimized, and the overall efficiency of theconverter is maximized. The other MOSFET switchingwaveforms are similar.

B. Experiment Tw o -Reverse OperationAnother experimental setup shown in Fig. 6was used to testthe performance of bi-directional power flow of the converter,

where the converter input was connected to 7OV batteries andthe converter output was connected to the load and a switch i~series with anexternal power supply of45V.When the switch remains opened, the converter operates in

forward node, and it will change in reverse mode once th eswitch is closed.

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Fig. 6. Experiment for Reverse OperationInitially, the co nverter ran with Vi,,= 70V, Zin = 1.08A, Vu,,,=

36V and Iu,,, = 1.95A. When t = 0.5s, the external voltagesource of 45V was applied momentarily to the output of theconverter. The output current and input current flowed inreverse direction simultaneously with amplitude of 1.9A arid1 . I A respectively, however, the input voltage almost keptconstant at 70V either in forward and reverse operations. Fig.7shows the changeover of operations.

0T k second)

Fig. 7. Cliangeoverfrom forward to reverse power flow operationsV. CONCLUSION

inductance of the transformer and parasitic capacitance ofactive switches to perform resonant switching. All the activeswitching devices are switched at zero voltage switching. Theproposed converter, if employed in EV, is capable of not onlytransferring energy fiom EV batteries to powertrain formotoring, but also returning energy back into batteries in caseof regenerative braking.

Moreover, by making use of a simple drive circuit for thsconverter bridge, it can gain the benefit of synchronousrectification.ACKNOWLEDGEMENTS

This project is supported and funded by The Hong KongPolytechnic University, and the project account code is G -V30.5.REFERENCES

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[7] Yamashita N., Murakami N. and Yachi T., ConductionPower Loss in MOSFET Synchronous Rectifier withParallel-Connected Schottky Barrier Diode, IEEE Tram.On Power Electronics, Vol. 13, N O . 4, July, 1998, pp .

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A novel bi-directional phase-shift controlled DC-DCconverter was proposed. Th e converter uses leakage

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