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High Performance Boost PFP (Power Factor Pre-regulator)with an improved ZVT (Zero Voltage Transition) converter

Jin-Hyoe Kim, D.Y. Lee, H.S. Choi an d B.H. Ch oPower Electronic System Lab, School of Electrical Engineering, Seou l National UniversityTel: +8 2-2-880-1785, Fax: +82-2-878-1452E-mail :inhvoe@,chollian.net

Abstract - In this paper, the conventional ZVT PWM Boostconverter is improved to minimize the switching loss of theauxiliary switch using the minimum number of the components. Toreduce the switching loss and EM1 noise, the saturable core andthe parallel capa citor across the clamp diode (D2) re used in theauxiliary switching network. llsing this technique, soft-switchingfor the auxiliary switch is guaranteed at wide line and load ranges.Since the active switches are turned on an d off softly, the switchinglosses are reduced significantly and the higher efficiency of thesystem is achieved. This paper proposes the improved ZVT PWMBoost PFP. The prototype of 200kHz 1kW system wasimplemented to show the improved performance.

1. INTRODUCTIONRecently international regulations governing the amount ofharmonic currents (e.g. IEC 1000-3-2)became mandatory andactive power factor correction pre-regulator (PFP) circuitbecame inevitable for the ACDC converters. In order tominimize the penalty of using additional PFC circuit such asincrease of overall system size and deterioration of efficiency,various kinds of soft switching techniques have been proposedfor the past several years [l-121. These techniques reduce theswitching losses and EM1 noise enabling high frequencyoperation and consequen tly reduce the overall system size.In general, the soft switching approaches can be classifiedinto two groups; zero voltage switching (ZVS) approaches [1,2]and zero current switching (Z C S ) approaches [3,4]. The ZVSapproaches are desirable for the majority carrier semiconductordevices such as MOSFETs, since the turn-on loss caused bythe output capacitance is more dominant. Unfortunately,switching losses in the ZVS approaches can be reduced only atthe expense of much increased voltage/current stresses of theswitches, which make these approaches not suitable for the PFCpre-regulator. In order to achieve ZVS for both the active andpassive switches without increasing their voltage and currentstress, various kinds of ZV T approaches have been proposedThe Boost converter employing the ZVT technique was firstintroduced in [6] hown in Fig. 1. This converter provides ZVScondition for the main switch without increasing voltage stressof the active switches. However it has a disadvantage such asthe hard-switching o f the auxiliary switch which deteriorates theoverall efficiency and increases the EM1 noise.

[6-121.

0-7803-6618-2/01/$10.002001 IEEE 33 1

There have been many attempts to solve these problems [7-lo]. To improv e the conventional ZV T circuit, the circuit in [7]uses a regenerative snubber employing a magnetically coupledcell to reduce the tum-off switching loss of the auxiliary switch.However, too many components are added to improve theauxiliary network. T he circuit in [SI uses an active snubber andthe current stress of the main switch and the voltage stress ofthe auxiliary diode are increased. The circuit in [ IO ] uses twoauxiliary switches and snubber capacitors. This increases theoverall cost and system complexity.In this paper, the conventional ZV T PW M Boost converter isimproved to reduce the switching loss of the auxiliary switchusing the minimum number o f the components. To minimize theswitching loss and EM1 noise, the saturable core and theparallel capacitor across the clamp diode (D2) are used in theauxiliary switch. The saturable core prevents the oscillation atthe anode of D2 and the reverse recovery current of D2. Theparallel capacitor controls the dv/dt of the auxiliary switch S2.The ZVT is guaranteed at wide line and load ranges. A 1kWZVT PWM Boost PFP prototype has been implemented toverify the improved performance of the proposed method and adetailed design procedure of the proposed technique is alsopresented in this paper.

L

Fig. I . The conventional ZVT Boost PFP circuit diagram

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Fig. 2 . Th e proposed ZVT Boost PFP circuit diagram

v5-2

S's, .. ... .. .. .... ..

0 : , , . . ;, . ., . . ., . . . .. . . . .. , . . .. . . .. , .. . . .. . , . . .. . . . . .

l y ~ n. .. . . . . .. . , . . ., , . . . .. .. . . .. . . . . ,. . . . . .. . . . . .. . . . . .. . . . .. . . . .s2 v,,*... ... . . .. . . . .VSI. ...... . . . . . . .. .... .. .... .... . . . . .. . . .. . , . , ., . . . , .. . . . . .. . . . . . .. . . , .. .. .. .. ... ... ..., .. ., ., ., .. ...

4 r2ID 2 f. .. ... . . .. , , . . .. . . . . . . . Fir. i,a a T, T, T,,

g. 3 . The theoreticalkey waveformsof the proposed circuit.11. PRINCIPLES OF OPERATION

The circuit diagram and theoretical key waveforms of theimproved ZVT Boost PFP are shown in Fig. 2 and Fig. 3,respectively. One small saturable core and one parallelcapacitor across the diode D2 are added to the conventionalZVT circuit.In the steady state, eight operating modes exist within oneswitching cycle. In the analysis, all devices used are assum ed tobe ideal and the Boost inductor L and output capacitor CO areassumed to be large enough to treat as a current source and avoltage source, respectively. The eight equivalent circuits areshown in Fig. 4.(a) Mode 1 (To-Tl) Before To , the main sw itch is off and the

diode D1 is conducting the full load current and the voltageof Cr2 equals zero. At T o, S2 is tumed on. At this moment,the voltage Cr2 is charged to Vo. The peak value of thecurrent which charges Cr2 i s as ( 2 ) . With S2 on, the currentin Lr ramps up linearly to the input current Ii and the currentin D1 ramps down. When the current of the diode D1reaches zero, the diode D1 tums off softly due to theinductor Lr. The time when this mode lasts and the peakcurrent, i , which charges the capacitor, Cr2, are asfollows:

Iit --

where f.sz is the rising time of S2.(b) Mode 2 (TI-T,) : At TI , he Lr current reaches Ii and Lr and

C rl begin to resonate. During this cycle, the voltage of Crlis discharged until the voltage of Crl reaches zero. Crl iscomposed of the external capacitance and the drain-sourcecapacitance (Coss) of S1. C rl controls the dv/dt of the drainvoltage of SI . The time required for the drain voltagereaches zero i s 114 of the resonant period of Crl and Lr. Atthe end of this period, the body diode of SI beginsconducting. The time intervalof this mode is as follows.

Mode 3 (T2-T 3) At T,, the body diode begins conductingand the drain voltage of S1 reaches zero. The voltage acrossLr is zero so the inductor current freewheels through thebody diode of S I. During this interval, S1 must be turned onto achieve the zero-voltage turn-on of S1 , that is, (6 ) mustbe satisfied. The time for ZVT, tZVT, is less than a tenth ofthe switching period, Ts, so that the auxiliary network m aybe active only during a short switching-transition time tocreate the ZV T condidiont for the main switch. Lt an d Crldetermine the peak current of S 2 , is2,pk s (4).

where(4)

(d) Mode 4 (T3-T4) Before T3, the voltage of Cr2 equalsVo. At T3, SI tums on and S2 tums off an d Lr

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mode 1 (to - t l )(a)

mode 4 (t3 - t4)( 4

mode 2 (tl - t2)(b)

mode 3 (t2 -U)(C)

mode5 (t4 - t5)( e )

mode6 (t5 - t6 )(0

mode 7 (t6 - t7) mode8 (t7 - to)(g) (h )Fig. 4. Equivalent c ircuits during one switching cycle. (a) mode I . (b) mode 2 . (c) mode 3. (d) mode 4. (e)mode 5. ( f ) mode 6. (9 )mode 7. (h) mode 8.

and Cr2dischargingCr2. This interval lasts until C r2 is fully discharged, thatis, the voltage of Cr2 equ als zero.

resonate and the energy stored by Lr begins turn-off losses are significantly reduced.(h) Mode 8 (T7-To) : This stage is also the same with the

conventional Boost converter. The input current f lowst,, = -arcsin(-* 6 , )w z,.L p k

1@ =-

through the main dio de supplying the load.(7 ) 111. DESIGN ROCEDURE AND EXAMPLEI . Determine the value of the auxiliary inductor, Lr.

Since Lr controls the di/dt of the current of D1, it reduces thereverse recovery current of D1. Thus the time interval of th emode 1 given by (1) must be at least three times longer than thereverse recovery time of D 1.2. Select the auxiliary capacitor Cr lFirst, S1 must be turned on w hile D1 is conducting for ZV T asin (6). In order to minimize the circulating energy of theauxiliary network, the on-time of S2 must be shorter than onetenth of the sw itching period. Eq . (6) is modified as (1 0).

(e) Mode 5 (T,-T,) : The current that remains after dischargingCr2 flows through the diode D 2. Later a detailed mode willbe explained.t,, =___ cos(arcsin- v, ) (9)r rS2 pk

v, ' , , I S 2 pk The following two conditions must be satisfied.(f)Mode 6 (Ts-Td : At T5, the current of S1 equals 11. Duringthis period, the converter operation i s the Same with theconvention al Boost conve rter.

(10)110zvr = t s < o , + 4 28) Mode 7 (T6-T7) At T6, the main switch turns off. At thistime, the S1 drain-to-source capacitance charges to Vo.Since th e Cr l init ial ly holds the drain voltage to zero, the Second, Crl should be chosen to m inimize this peak value.

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Cr l together with Lr determine the peak value of the current ofS2 as can be seen in (4 ) an d (5).3. Design the auxiliary capacitor, Cr2.

Cr2 plays a key role to the soft-switching of S2. Th e peak ofthe current, icr2.pk,hat charges the Cr2 voltage to Vo at To andthe time that the Cr2 voltage discharges to zero during T34 arein proportion to the value of Cr2. T o minimize the current peak,the value of Cr2 must be small, but to guarantee the ZCS tum-off of S2, the value of Cr2 m ust be large. The objective functionis as follows:

T34 s longer than tf ,sz (1 1)minimization of ic,.2.,,k

where t,. s2 is the falling time of S2.Fig. 5 (a) shows the upper limit of the value of Cr2 accordingto ( 1 1) . The value of T34 depends on the characteristics of theMOSFET. An d Fig. 5 (b) show s the lower limit of the value ofCr2 according to (12). The standard value of this peak value

depends on the current rating of the MOSFET for S2. Theselection of the v alue of Cr2 is arbitrary in the gray zone of Fig.5(a) and (b). This is the guideline of selecting the value of Cr2and through the hardware test we can find the optimal value ofCr2 in this range.

50

%4 0 ( /5 I3025 lower limitower limit20

2 3 4 5 6 . 7 8 9 100 2 nF)

1 2 4 5 6 7 8 9 10CrZ (nF)

TABLE 1. Utilized components and parametersComponents I Parameters

~~ ~

LLrLs

Crl, Cr2C O

. SIs2D1

D2, D3Bridge diode

~~ ~ ~~~Dongbu M157-168A, 240uHMagnetics55586,8pH

Toshiba MS 15*10*4.5W2nF / 3nF, polypropylenecapacitor

3*220pF/450V, electrolytic capacitor2SK2837 (SOOV, 20A)

IRF840 (SOOV, 8A)FML36S (600V, 20A)MUR860 (600V, 8A)

RBV 1506IOther components and parameters are shown in Tab le I.IV . EXPERIMENTALERIFICATION

For comparison, three experimental prototype circuits ofsingle-phase CCM (continuous conduction mode) Boost PFP ;the hard-switching Boost PFP, the conventional ZVT BoostPFP, and the improved ZVT Boost PFP are built. The switchingfrequency of the hard-switching Boost PFP is 9OkHz nd that ofZVT Boost PFP is 200kHz. Th e output voltage of all circuits is400V and the output power is 1kW. In the breadboardedconverter, the main power switch is implemented by a high-speed and highcurrent switching MOSFET, 2SK2837. Sincethe auxiliary switch handles only a small resonant transitionenergy, a small MOS FET, IRF 840, is employed.I . Ef$ciency Measurement

Fig. 8 shows the efficiency measurements of the improvedZVT, the conventional ZVT and hard-switching Boost PFPcircuits. The efficiency was measured using PM3 300 (Voltech).It can be seen that the improved ZVT technique significantlyimproves the efficiency. The maximum efficiency at fill1 loadwa s 97.6%. Due to the heat caused by this switching loss of theauxiliary switch S2, it is practically impossible to operate theconventional ZVT circuit in the power level above 800W.

Vin vs. Efficiency10099 599

95 595

170 180 190 200 210 220 230 240 250 260Vin NAC)Fig.10. Com parisonof the Boost PFP, the conventionalM oost PFP andthe improvedZVT Boost PFP.b)

Fig.5 (a) Cr2 vs. T34 (b ) Cr2 vs. Ipk

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Voltage of SIInput Voltage (200V/div)(250V/div)

Input Current(SNdiv) (SA/div)Curmet of S1

Voltage of S2Voltage of S2 (400V/div)(4OOV/div)

Curmet of S2(5 A/d iv) (SA/div)Curmet of S2

(c) (d)Fig 9 Experimental result (a) the input current and voltage waveforms (b) the current and voltage waveforms of SI(c) the current and voltage waveforms of S2 in the improved ZVT PFP (d) the current and voltage waveformsof S2 in the conventional ZVT PFPThe im proved circuit can be used in above IkW system sincethe switching loss and the heat of the auxiliary switch issignificantly reduced.

2. Experimental WaveformsFig. 9 (a) shows the input voltage and the input currentwaveforms of the improved ZVT Boost PFP operating at 220Vinput and 1kW output. The PF (Power Factor) of the improvedcircuit is almost unity (0.993) and THD (Total HarmonicDistortion) is 4.7%. As can be seen in Fig. 9 (a), the inputcurrent is in phase with the line voltage.Fig 9.(b) shows the zero vo ltage switching waveforms of themain switch S1. As can be seen, before S1 turns on the currentflows through the body diod e of SI an d SI turns on under zerovoltage condition.Fig. 9c) shows the current and the voltage waveforms of theauxiliary switch S2 of the proposed circuit. Compared withwaveforms of the conventional ZVT circuit in Fig. 9(d)operating under the same conditions, it can be seen that the

switching loss and the switching noise of S2 is severe. On th eother hand, the improved circuit has good switchingperformance, that is, the switching noise and sw itching loss ar esignificantly reduced.

3. Input Harm onic Current MeasurementFig. 10 shows the line harmonic currents of the improvedZVT circuit. The output power is 1kW and the input AC

voltage is 220Vrms. It shows that this circuit m eets the EN60555-2 requirement as well a s IEC 1000-3-2. The regulation ofEN60 555-2 is applied to the display appliances, such as TV .2.5

2

9 1.5ci5 10.5

01

_ _ _ _ _____ Fig. IO Input Harmonic Currents at P e lkW

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V. CONCLUSIONIn this paper, the high performance PFP with an improvedZVT converter was proposed. The switching loss of theauxiliary switch is minimized using the minim um number of th ecomponents. To reduce the switching loss an d EM1 noise, onesmall saturable core and one p arallel capacitor across the clampdiode (D2) are used in the auxiliary switch. Using thistechnique, soft-switching for the auxiliary switch is guaranteedat wide line and load ranges. Since the active switches aretumed on and tumed off softly, the switching losses are reducedsignificantly and the higher efficiency of the system is achieved.The prototype of 200kHz 1kW system was implemented toexperimentally show the improved performance improvement.

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Power Conversion Conf. Rec. pp. 244-251, 1991.[3 ] G . Hua, F.C. Lee, Wove1 full-bridge zero-current-switched PWMconverter, 4Ih European Conference in Power Electronics and Applications,[4] Rodrigo Cardozo Fuentes, Helio Leaes Hey, An Improved ZCS-PWMCommutation Cell for IGBTs Application, IEEE Transaction on PowerElectronics, Vol. 14, pp. 939-948, September 1999.[SI G. Hua, F.C. Lee, Novel zero-current-transition PWM converter,Invention Disclosure, 1993 .[6] G. Hua, C.S. Leu, Y. Jiang, F.C. Lee, Novel zero-voltage-transition PWMconverters, IEEE tra nsactions on Power E lectronics, Vo1.9, pp.2 13-2 19,Mar. 1994.[7] Paulo J.M. enegaz, Marcio A. Co,Domingos S.L. Simonettietti, Jose L.F.Vieira. Imurovine the ODeration of ZVT DC-DC converters. Power

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Electronics Specklists Conference, PESC 99. 30th Annual IEEEVol. 1, pp. 293 -297, 1999.181 ChineJune Tsene. Chem-Lin Chen. Novel ZVT-PWM Converters with. A Active SnuTbbers,IEEE transaction on power electronics, Vol. 13, pp. 861-869, September 1998.[9] R.L. Lin, Y. Zhao, F.C. Lee, Improved soft-switching ZVT converterswith active snubber Applied Power Electronics Conference andExposition, Thirteenth Annu al, Vol. 2, pp. 1063 -1069, 1998.[l o] Gurunathan. R. Bhat, A.K. S ,A soft-switched Boost converter for high-frequency operation, Power Electronics Specialists Conference, PESC 99.30th Annual IEEE , Vol. I , pp. 463 -468, 1999.[1 ] Guichao Hua, F.C. Lee, Soft-Switching Techniques in PWM Conveters, [12] Ching-Jung Tseng, Chem-Lin Chen, A novel ZVT PWM Cuk power-factor corrector, Industrial E lectronics, IEEE Transactions on ,Vol. 46, pp.[I31 James P. Noon, UC3855- HIGH PERFORMA NCE POWERFACTOR PREREGU LATOR, Application note U-1 53, UnitrodeCorporation.[I41 Philip C. Todd, UC3854 Controlled Power Factor Correction CircuitDesign, Application note U-134, Unitrode Corporation.[151 Lloyd Dixon, Average Current Control of Switching Power Supplies,Application note U-140, U nitrode Corporation.

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