Fabrication, Simulation and Testing of a Zero Voltage ...

2012 IEEE International Conference on Power Electronics, Drives and Energy Systems December16-19, 2012, Bengaluru, India

Fabrication, Simulation and Testing of a Zero Voltage Switching strategy based high frequency Buck Converter Pinaki Mukherjee

Mainak Sengupta

Dept. Electrical Engineering, Bengal Engineering and Science University, Shibpur, Howrah - 711103, W.B., India. email: [email protected]

Department Electrical Engineering, Bengal Engineering and Science University, Shibpur, Howrah - 711103, W.B., India. email: [email protected]

Abstract—High switching frequency in power electronic converters reduces the size and weight of passive elements used in the converter but increases the switching losses of the power electronic devices. Remedy for high switching losses is the use of soft switching techniques which reduce switching losses and inturn increases efficiency. Zero voltage switching (ZVS) is one type of soft switching technique. In this paper a ZVS strategy based buck converter is presented operated at 50kHz. Performance analysis and simulation of the converter has been done by using SequelGUI2 software. Finally, a small laboratory phototype of rating 50W, 50kHz has been designed, fabricated and tested. The experimental results and simulated performance are found to be in good agreement. Index Terms—ZVS, Buck converter, MOSFET, high frequency, design, SEQUEL Simulation, fabrication and testing.

diode in case of a buck converter. Fig.1 shows the circuit configuration of ZVS- buck chopper[1].

I. I NTRODUCTION High frequency switching in buck chopper circuit gives various advantages like reduction of size and weight of passive elements used in the converter. Inspite of the advantages it suffers from high switching losses [4] which can be near about 50 % of total losses at high frequency. The reason behind high switching loss is the non-zero loss during turn-on and off transition of switching devices i.e. presence of considerable overlap in voltage (across switch) and current (through switch) slew rates during switching transition. One way to avoid this switching loss is to make sure that voltage is held close to zero during turn-on transition of switching devices. The current increases while the voltage is low, so the loss during the turn -on transition is low . The action is called zero-voltage switching (ZVS)[5]. MOSFET is a natural choice as a switching device at high frequency because of its high switching speed (upto MHz level). Parasitic elements of switch like (Coss ) of MOSFET has significant use in this type of converters. II. BASIC PRINCIPLES Zero voltage switching may be done using resonance principle by introducing a capacitor in parallel with the MOSFET power terminals and a series inductor with the freewheeling

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Fig. 1.

Fig. 2.

Circuit configuration of ZVS-buck chopper.

Experimental circuit diagram of ZVS-buck chopper

The principle of operation of the ZVS buck chopper can be classified into four modes as described later. Here sw#1 represents the MOSFET and sw#2 represents the freewheeling diode (FD). The buck chopper load is assumed to be a constant current load (I0 ) which is practically realised by using an appropriate inductor in series with a R load. The time constant of the load is much higher than switching

time-period of the converter (Tsw ) so that load current is always continuous. The operating modes of the converter may be explained as below following[1].

the system equation are following.

MODE 1 : sw#1 on, sw#2 off: The circuit equations in this mode are following. Corresponding circuit state is shown in Fig. 3.

Fig. 5.

Fig. 3.

Converter circuit state in mode 1 (state 1,0).

vd = Vdc

(1)

vt = 0, it = Io , id =0. Where, dc-link voltage after MOSFET is vd . Voltage across MOSFET is vt . it is source current & id is current through Freewheeling Diode. MODE 2: sw#1 turns off, sw#2 off: As soon as MOSFET (Sw#1) turns off, the circuit enters in mode 2. Here, capacitor(Ct ) is charging constantly by means of constant load current such that vd drops constantly till the freewheeling diode (sw#2) turns on. Corresponding circuit state is shown in Fig. 4 & the circuit equations in this mode are following.

Fig. 4.

Converter circuit state in mode 3 (state 0,1).

id it vd

= Io (1 − cos ωr t) = Io cos ωr t = Io Zc sin ωr t

(3)

vt

= Vdc + Io Zc sin ωr t

(4)

√ d √ 1 . From (4), if where, Zc = L Ct & ωr = 2πfr = Ld Ct Io Zc > Vdc then vt < 0, This is the condition for zero voltage switching(ZVS). Practically, vt can be just negative as MOSFET antiparallel body diode starts conduction (vt = vdiodeon = vγ ). MODE 4: MOSFET body diode on, sw#2 on: When MOSFET antiparallel body diode starts conducting the circuit enters into mode 4. In this mode voltage across MOSFET (sw#1) is ideally zero i.e voltage across capacitor (Ct ) clamped at zero voltage.Corresponding circuit state is shown in Fig.6. MOSFET should be gated on during this interval. The equation are relevant.

Converter circuit state in mode 2 (state 0,0).

Vdc −

Io t − vd Ct it

=

0

(2)

Fig. 6.

Converter circuit state in mode 4 (state 1,1).

= Io

MODE3: sw#1 off, sw#2 on: When sw#2 (FD) comes into the circuit, the circuit enters into resonating mode. Corresponding circuit state is shown in Fig. 5. In this mode

vd Io

did = Vdc dt = it + id = Ld

(5)

Mode 4 persists till id equal to zero. Then mode 1 repeats. III. D ESIGN AND S ET- UP FABRICATION From basic principles of operation of ZVS- Buck chopper, it is clear that selection of capacitor across MOSFET (Ct ) & inductor (Ld ) in series with freewheeling diode (FD)are very important. A. Selection of Ld and Ct : The operating frequency(fsw ) is chosen as 50kHz. According to mode-3 the resonant frequency fr must be greater than switching frequency fsw . Let us take fr ≥ 3 fsw = 180kHz, Io =2.5A & Vdc =20V. We know, for ZVS operation, Io Zc > Vdc ; ⇒ Zc > VIdc =8. Taking Zc =40 and o fr =200kHz, the Ld and Ct values turn out 30µH and 20nF accordingly. B. Calculation of mode-time, voltage overshoot and expected output voltage: Time duration for each modes mode-1: (Tsw -t2 -t3 -t4 ) = 6.89 µs. ∗20V mode-2: t2 = CdIVo dc = 22nF = 0.18µs. 2.5A mode-3: using(4) vt =0 at ωr t3 = 3.35 rad and t3 = 2.49 µs. At t3 , id =4.834A, This is the initial inductor current of mode4. mode-4: MOSFET Body diode initial current(it ) (using(6)) = (4.83-2.5) A = 2.33A. Time duration for MOSFET body diode conduction t4bd = 40µH∗2.33A = 4.66 µs. 20V During this interval MOSFET should be gated on. Using (5) duration of mode4 t4 = 40µH∗4.83A = 9.66µs . 20V

Fig. 7.

Gate Driver Circuit

make it suitable for 50 kHz. Output of TLP 250 drives bases of fast switching NPN(2N2222) and PNP(2N2905) transistor connected in common emitter mode. Gate current is supplied through the BJTs. Gate resistanceRg used here is 22Ω. Output of gate-drive is shown in Fig.11 Heat sink design:[3] Heat sink is designed for a buck chopper without ZVS of 50kHz, 50W rating whose input dc link voltage is 20 volt. Heat transfer from sink to ambient in both radiative and convective way.

The value of voltage overshoot: Vdc + Zc Io = 126V. Average output voltage(Vd ): Volt-second area for ∫ 0.18µs i) mode-2(using(2)): 0 (V − Io t )dt = 1.8µV s. ∫ 2.48µs dc Ct ii) mode 3(using(3)): 0 (−Io Zc sin ωr t)dt = −130µV s. iii)both mode 1 & 4(using(1) & (5)): Vdc (Tsw − t2 − t3 ) = 331µV s. as fsw =50kHz i.e. Tsw =20µs So, Average output voltage(Vo )=10V

Simulated switching trajectory of the MOSFET of buck chopper without ZVS (Y axis: VDS and X axis: ID )

Fig. 8.

C. Experimental setup Fig.2 represents the circuit diagram of fabricated setup. Fabricated converter rating is 50 kHz, 50W with 20V input dc. Output inductance(Lo ) and capacitance(Co ) values are accordingly 20mH and 47µF. Load resistance(Ro ) is 3-5 ohms. Input dc link capacitance(Cin )=1000µF. The value of(Ld ) and (Ct ) are 28µH and 22nF accordingly.

Fig. 9. Simulated switching trajectory of the MOSFET of ZVS-buck

chopper (Y axis: VDS and X axis: ID )

Gate Driver:[1] Fig.7 shows the circuit diagram of gate driver to drive the MOSFET at 50kHz. Opto-coupler based isolation is provided in the gate drive. TLP250 (opto-coupler base MOSFET gate drive IC) is used here whose output is ±15V pulse. But TLP250 is rated for 25 kHz with 1A peak current. So, output current of TLP 250 is reduced to low value( .001 A) to

Fig.8 shows simulated switching trajectory of MOSFET of buck chopper without ZVS of 50 kHz, 50W rating. Area under the switching trajectory represents switching losses. On other hand Fig.9 shows simulated switching trajectory of MOSFET of ZVS buck chopper of same rating. Comparing two switching trajectory it can be said that switching loss is very nominal for ZVS-buck chopper. The relevant formulae to

design the heat sink are given below. Rja Rja Ploss

Tj − Ta Ploss = Rjc + Rcs + Rsa Vdd Id (ton + tof f )fs = + Vdssat Id δ 2 =

Where symbols have their usual significance. Heat transfer from sink to ambient in both radiative and convective way. So, Rsa is a parallel combination of Rθ.rad & Rθ.conv . Solving above equations relevant to heat sink design, a heat sink is used of following dimensions. Heat sink length= 4 cm, Heat sink breath= 4.5 cm, Heat sink width= 1 cm, No of fines=10, fin hight=1 cm. IV. S IMULATION & E XPERIMENTAL RESULTS : Simulated and Experimental results are given in this section along with relevant waveforms. Simulation results: Average input dc-link voltage(Vdc )=20 V. Operating Frequency(fsw )=50kHz. = 16µsec/20µsec = 0.8 Duty ratio(d)= Tton sw Output dc voltage(Vo )=10V Calculation behind average output voltage is explained in section III-B. Experimental results: Average input dc-link voltage(Vdc )=20 V. Operating Frequency(fsw )=50kHz. Duty ratio(d)= Tton = 16µsec/20µsec = 0.8 sw Output dc voltage(Vo )=9V Temperature rise of heat sink is nearly 7o C in this case but in case of non-ZVS buck chopper of same rating temperature rise was nearly 200 C which is much larger.

Fig. 11. Experimental Waveforms of MOSFET VGS (top)(± 15 V ) and MOSFET VDS (bottom)(120/0 V )at 50 kHz. (ch1: probe ratio 10:1, Y-scale: 5V/ div, ch2: probe ratio 10:1, Y-scale: 5V/ div and t scale: 5µs/ div)

voltage across diode along with resonant inductor. Fig.13 shows corresponding experimental waveforms.

Fig. 12. Simulated waveforms of MOSFET VGS (blue) and and Dc link voltage after MOSFET Vd (green)(+20/-110 V) at 50 kHz.

Fig.10 shows the simulated waveforms of MOSFET gate pulse (VGS ) and voltage across MOSFET (VDS ). It is found from the simulated waveforms that MOSFET is gated on when voltage across it goes to zero. Fig.11 shows corresponding experimental waveforms.

Fig. 13. Experimental Waveforms of MOSFET VGS (top)(± 15 V ) and Dc link voltage after MOSFET Vd (bottom)( +20/-100 V) at 50 kHz. (ch1: probe ratio 10:1, Y-scale: 5V/ div, ch2: probe ratio 10:1, Y-scale: 5V/ div and t scale: 5µs/ div)

Simulated waveforms of MOSFET VGS (green) and MOSFET VDS (blue )(130/0 V) at 50 kHz.

Fig. 10.

Fig.12 shows the simulated waveforms of MOSFET gate pulse (VGS ) and Dc link voltage after MOSFET (Vd ) i.e.

Fig.14 shows the simulated waveforms of dc link voltage after MOSFET (Vd )i.e. voltage across diode along with resonant inductor and diode current (IF ). Fig.15 shows corresponding experimental waveforms. Fig.16 shows the simulated waveforms of voltage across MOSFET (VDS ) and current through MOSFET (ID ). Fig.17 shows corresponding experimental waveforms.

Simulated waveforms of Dc link voltage after MOSFET Vd (blue)(+20/-110 V) and Diode IF (green)(7/0 A) at 50 kHz. Fig. 14.

Fig. 17. Experimental Waveforms of MOSFET VDS (top)( 0/120 V)

and MOSFET ID (bottom)(5 /-2 A ) at 50 kHz (ch1: probe ratio 10:1, Y-scale: 5V/div, ch2: probe ratio 1:1, sensor gain 100 mV/A, Y-scale: 500mV/div and t scale: 5µs/ div)

company, Kolkata. Very special mention must be madeof Prof. V. Ramanarayanan for his technical discussions and motivation. Grateful thanks are also due to the NaMPET initiative of the Govt. of India, DIT, MCIT. The authors also acknowledge the support received from the Dept. of EE, BESU, Shibpur, Howrah towards this work. Fig. 15. Experimental Waveforms of Dc link voltage after MOSFET

Vd (top)( +20/-100 V) and Diode IF (bottom)( 7 / 0 A) at 50 kHz (ch1: probe ratio 10:1, Y-scale: 5V/div, ch2: probe ratio 1:1, sensor gain 100 mV/A, Y-scale: 500mV/div and t scale: 5µs/ div)

V. C ONCLUSIONS In this paper a ZVS-buck down converter has been simulated using SEQUEL and its performance analysed. Thereafter, the converter has been designed, fabricated and tested with load. The experimental results are in excellent agrement with the simulated ones. The future objective of developing the ZVSbuck chopper of 50 kHz switching frequency is test MOSFET at such frequency. The MOSFET is to be used in 50 kHz phase modulated resonant transition converter[2]. VI. ACKNOWLEDGMENTS The authors would like to express their gratitude for the moral, technical support received from Power-Con welding

Simulated waveforms of MOSFET VDS (blue)(130/0 V)(blue)and MOSFET ID (green)(5 /-2 A) at 50 kHz.

Fig. 16.

R EFERENCES [1] Krein. P.T, ‘Elements of Power Electronics’, New York, Oxford,1998. [2] Ramanarayanan.R, ‘Course Material on Switch Mode Power Conversion’,IISc Bangalore,2006. [3] Mohan N., Undeland,Robbins, ‘Power Electronics converters: applications and design’, Wiley, India Edition, 2003. [4] Per Karlsson, Martin Bojrup, Mats Alakla and Lars Gertmar ‘Zero Voltage Switching Converters. ’ [5] Guichao Hua and Fred C. Lee, Fellow, IEEE ‘Soft-Switching Techniques in PWM Converters’, IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 42, NO. 6, DECEMBER 1995. [6] Mukherjee P., ‘Power Electronic Converters For Welding Application’, M.E. Thesis, BESU, Shibpur, 2012.