Soft switched DC-DC PWM Converters

Soft switched DC-DC PWM Converters Mr.M. Prthp Rju (), Dr. A. Jy Lkshmi () Abstrct This pper presents n upgrded soft switching technique- zero current trnsition (ZCT), which gives better turn off chrcteristics for the switching devices thn conventionl ZVS. An uxiliry circuit is ugmented for the converter, which implements Zero Current Trnsition by incorporting the concept of resonnce for ll the switching elements. The proposed zero current trnsition is designed for DC-DC buck, fly bck, Boost converters nd the sme is simulted for frequency of 0kHz by using MATLAB- Simulink. The min emphsize is on the nlysis of switching stress (/) nd switching losses of the vrious converters simulted. Anlyticl comprison presented with hrd nd soft switching proves the superiority of the proposed soft switched converters over conventionl. Index Terms PWM, DC to DC, buck, boost, fly bck, Zero Current Trnsition (ZCT). I. INTRODUCTION PULSE WIDTH Modulted (PWM) dc/dc converters re vstly used in industry becuse of their high-power cpbility nd fst trnsient response. To reduce the volume nd the weight of these converters, higher switching frequency opertion is preferred. In high power frequency requirements, power semiconductor switches re subjected to high switching stresses nd switching losses, which limits the operting switching frequency. Generlly, snubber circuit reduces the switching losses nd stresses, but increses the totl power loss in the converter. In resonnt nd qusi-resonnt converters, the switching losses re reduced; however, the converter control system is usully frequency control insted of PWM control. Furthermore, in these converters, high voltge or current stresses re pplied to semiconductor devices due to the nture of the resonnce. Zero Current Trnsition (ZCT) nd Zero-Voltge Trnsition (ZVT) techniques incorporte soft switching function into stndrd PWM converters [] [9]; thus, the switching losses cn be reduced. In these converters, n uxiliry circuit is dded to the min PWM converter, which functions only t switching instnces nd recovers the switching losses. For high-power pplictions, n Insulted Gte Bipolr Trnsistor (IGBT) is the preferred device. However, the IGBT exhibits tiling current t turnoff, which increses turnoff switching losses. Hence, ZCT techniques provide better results thn ZVT techniques. Severl ZCT converters hve been previously proposed in [] [9], but they suffer from one or more of the following drwbcks. ) In some topologies, the min switch turn-on is not soft nd, thus, limits the gin in efficiency []. ) The min switch pek current is incresed considerbly [] [4]. 3) The switches turn off re not soft [5]. 4) There re ionl semiconductor devices in the min power pth tht increse conduction losses [6], [7]. 5) The proposed technique cnnot be pplied to fly bck converters [], [], [5]. A new fmily of ZCT converters lcking the forementioned sdvntges ws introduced [9]. In these converters, the min switch nd the uxiliry switch re turned on nd off under Zero-Current (ZC) contion, so tht the switching losses nd stresses re significntly reduced. The energy of the proposed uxiliry circuit is bsorbed from the input voltge source nd is trnsferred to the output, which boosts the effective output duty cycle. However, in other converters, usully, the uxiliry circuit energy is just circulting energy [],[],[5] [9]. One of the dvntges of the proposed uxiliry circuit is tht it cn be pplied to fly bck converters. A ZCT fly-bck converter ws introduced in [4], which hs sdvntges in comprison to the proposed ZCT fly bck s scussed in Section V. The ide of the proposed uxiliry circuit is scussed in section II. The ZCT buck converter ws nlyzed in []; however, since it is the bse of this converter fmily, its opertion is briefly scussed in Section III. In Section IV, fly bck ZCT converter is explined. As the requirement of boost converter is high in industries, detiled performnce chrcteristics re detiled in section IV. The performnce prmeters derived for fferent ZCT PWM converters re presented in Section V. Simultion results nd comprtive nlysis for the sme is presented in the proceeng sections. II. AUXILIARY CIRCUIT DERIVATION Generlly, in non isoted fundmentl converters, one wy to crete ZC contion for switch turn-on is to hve snubber inductor in series with the switch or ode. However, t turnoff, this inductor will cuse voltge spike on the switch. Therefore, pulse current source is required to provide the output current, nd, thus, the switch cn be turned off under ZC contion while preventing the voltge spikes. To reduce the number of circuit elements, the pulse current source pth nd the required snubber inductor to decrese turn-on losses cn be combined s shown in Fig. for buck converter. A pulse voltge source cn be pplied to the snubber inductor nd crete the required pulse current source t switch turnoff. Design of resonnt inductnce nd cpcitnce should be done such tht the underdmped nture of the LC circuit t certin resonnt frequency is ensured. Switching frequency of min

nd uxiliry switches is to be considered with the reference of the resonnt frequency for which the LC components re designed. Fig. Schemtic ZCT buck converter ()Snubber Inductor in series with ode nd (b)snubber inductor in series with switch. III. ZCT BUCK CONVERTER DESCRIPTION The proposed ZCT buck converter is shown in Fig.. The circuit is composed of the min switch S, the min ode D, the uxiliry switch S, the uxiliry ode D, the uxiliry inductors L nd L, nd the uxiliry cpcitor C. L is the snubber inductor, nd the pulse voltge source is bsiclly represented by C nd S. L nd D re used to rechrge the cpcitor t every cycle. The converter hs seven stinct operting intervls during one switching cycle. Before the first intervl, it is ssumed tht the uxiliry cpcitor is chrged to Vin, nd the min switch is conducting. Key theoreticl wveforms of the converter re shown in Fig. 3. Thus, the min switch cn be turned off under the ZC contion before the L current is reduced bck to the output current I0. Intervl 3 [t t3]: In this intervl, the L current is equl to I0, nd the uxiliry cpcitor linerly schrges until its voltge becomes zero t the end of this intervl. Intervl 4 [t3 t4]: This intervl strts when the min ode begins to conduct under the zero-voltge contion, nd the uxiliry switch cn be turned off under the ZC contion. In this intervl, the min ode current nd the L current re equl to Io. This intervl ends when the min switch is turned on. Intervl 5 [t4 t5]: When the min switch is turned on, the L current begins to linerly decrese, nd the L current strts to increse in resonnce fshion with C. Therefore, the min switch turns on under the ZC contion. Intervl 6 [t5 t6]: This intervl strts when the current of L, which flows through the min ode, reches zero, nd the min ode turns off under the ZC contion. At the end of this intervl, the resonnce between L nd C ends when C is chrged to Vin, nd D prevents the current from going negtive. Intervl 7 [t6 t0 + T]: The output current runs through the min switch, nd the circuit behves like regulr PWM buck converter. Fig. Proposed ZCT buck converter. Fig.3 Key theoreticl wveforms of the ZCT buck converter. Intervl [t0 t]: This intervl begins with turning the uxiliry switch on. This strts resonnce between the uxiliry cpcitor C nd the uxiliry inductor L. During this resonnce, the L current increses until it reches the output current I0 nd reduces the switch current to zero. Intervl [t t]: In this intervl, the min switch body ode strts to conduct, nd the resonnce between the uxiliry inductor L nd the uxiliry cpcitor C will continue. () (b)

(c) (e) (g) Fig.4 Equivlent circuit for ech operting intervl of the proposed ZCT buck converter. () (t0 t). (b) (t t). (c) (t t3). (d) (t3 t4). (e) (t4 t5). (f) (t5 t6). (g) (t6 t0+t) IV ZCT FLYBACK CONVERTER DESCRIPTION An equivlent circuit for ech operting intervl of the improved ZCT Fly-bck converter (Fig. 5) is shown in Fig. 7, nd the min theoreticl wveforms re illustrted in Fig. 6, Ip is the trnsformer mgnetizing current in the primry side, nd LL is the trnsformer lekge inductnce. The trnsformer mgnetizing inductnce is ssumed to be lrge enough, so tht the trnsformer totl mpere-turns re considered constnt in switching cycle. Before the first intervl, it is ssumed tht the C voltge is V, nd the min switch is conducting. The converter operting modes re s follows. (d) (f) reches Ip, The min switch is turned off under the ZC contion. L.C Intervl [t t]: The resonnce between L nd C will continue, nd the resonnce current increses beyond Ip cusing the body ode of the min switch to conduct. When the L current returns bck to Ip, the body ode of the min switch turns off. Intervl 3 [t t3]: The L current is constnt nd equl to Ip. Therefore, C is linerly schrged until its voltge reches nv0, nd the rectifying ode D is forwrd bised. During this intervl, the energy stored in C is trnsferred to the output, which is equivlent to boosting the effective duty cycle. Intervl 4 [t3 t4]: Since D is conducting nd the voltge cross the primry side of the trnsformer is constnt, resonnce strts between L, LL, nd C. During this resonnce,l nd LL currents decrese, nd the D current increses to Ip. (L L l ).C Intervl 5 [t4 t5]: During resonnce between L, LL nd C, C is chrged. Where 3 (L L l ).C Intervl 6 [t5 t6]: Diode D is conducting, nd the converter behves like regulr flybck converter Intervl 7 [t6 t7]: The min switch is turned on under the ZC contion due to the trnsformer lekge inductnce. At the sme time, slow resonnce strts between L nd C, in which its effect cn be ignored until the next intervl. At the end of this intervl, the min switch current is Ip, nd ode D turns off. Intervl 8 [t7 t8]: C is chrged in resonnce with L. Where 4 L.C Intervl 9 [t8 t0 + T]: The min switch is conducting, nd the converter behves like regulr flybck converter. Fig.5 proposed Improved ZCT flybck converter Intervl [to t]: The uxiliry switch is turned on, nd resonnce between C nd L begins. When the L current Fig.6 Min theoreticl wveforms of the ZCT flybck converter 3

FOR ZCT FLYBACKCONVERTER DESIGN PARAMETERS: Assumed tht C F Resonnt Frequency Fr = 0khz Lekge inductnce L L H L.C L 5.33 mh 4 L.C PERFOMANCE PARAMETERS Fig.7 Equivlent circuit for ech operting intervl of the improved ZCT flybck converter. () (t0 t). (b) (t t). (c) (t t3). (d) (t3 t4) (e) (t4 t5). (f) (t5 t6). (g) (t6 t7). (h) (t7 t8). (i) (t8 t + T). V. DESIGN AND PERFORMANCE PARAMETERS: FOR ZCT BUCK CONVERTER: DESIGN PARAMETERS: Assumed tht C F Resonnt Frequency fr = 0khz ; L.C L. L 5.33 mh w L mh C 5.33 Input voltge Vi = 50 v Duty cycle K=0.5, 0.5, 0.75,. Trnsformer turns Rtio: N/N = Output voltge = Vi K Vo 50 0.5 83.33 50 0.5 50 50 0.75 750 V 0 FOR ZCT BOOST CONVERTER: The performnce of the ZCT boost converter is lso similr to the remining converters. Design nd performnce permeters re detiled here below. N N.V K S.K PERFOMANCE PARAMETERS Input voltge Vi = 50 v Duty cycle K= 0.5,0.5,0.75, Output voltge Vi K Vo 50 0.5 6.5 50 0.5 5 50 0.75 87.5 50 50 Fig.8 ZCT Boost converter DESIGN PARAMETERS: Assume tht Cx F Frequency fr = 0khz V T T on 0.V i Lx.533 mh 4

PERFOMANCE PARAMERTERS Input voltge Vi = 50 v Duty cycle K=0.5, 0.5, 0.75, Vi K Vo 50 0.5 333.33 V 0.V i K 50 0.5 500 b. SIMULATION MODEL FOR PROPOSED ZERO CURRENT TRANSITION BUCK CONVERTER AT Fs = 0 KHz 50 0.75 000 VI. SIMULATION RESULTS With reference to the design nd performnce prmeters, the proposed ZCT converters nd conventionl converters for frequency re simulted. Simulted models long with results re detiled here below.. SIMULINK MODEL FOR HARD SWITCHED BUCK CONVERTER AT Fs=0 khz Fig. Simulink model for proposed ZCT Buck converter Fig.9 Simulink model for hrd switched buck converter Fig. Min switch gte pulse nd current wveforms (soft switched buck converter) Fig.0 wveforms for hrd switched buck converter Fig.3 Auxiliry switch gte pulse nd current wveforms 5

d. SIMULINK MODEL FOR ZCT FLYBACK CONVERTER Fig.4 Wveforms of ZCT buck converter c. SIMULINK MODEL FOR HARD SWITCHED FLYBACK CONVERTER AT Fs=0 khz Fig.7 Simulink model for ZCT flybck converter Fig.5 simulink model for hrd switched flybck converter Fig.8 Wveforms of ZCT buck converter e. SIMULINK MODEL FOR HARD SWITHCED BOOST CONVERTER Fig.6 wveforms of hrd switched flybck converter Fig.9 Simulink model for hrd switched boost converter 6

VII. ANALYSIS OF SWITCHING LOSS AND STRESS The proposed topologies with Zero current Trnsition nd conventionl hrd switched re nlyzed bsed on the simulted results obtined. Clcultion of the very importnt fctors switching (power) loss nd stress re done nd re detiled here below. Switching loss during the switching trnsition: P loss V i.i m f s t f Switching stress: Fig.0 Wveforms of hrd switched boost converter f. SIMULINK MODEL FOR ZCT BOOST CONVERTER / rting cross the switch when the switch is turned off: Trnsition in current during fll time Fll time As per the performnce prmeters figured in the erlier section. Detiled nlysis is given below. FOR HARD SWITCHED BUCK CONVERTER Vi = 50 v, Im =. A (s per the figure 0) Fs = 0 khz nd fll time of switching trnsition(ssumed) Tf = 3s Switching powerloss : P loss 7.87 wtts Switching stress: 0.7A/ sec FOR SOFT SWITCHED BUCK CONVERTER Fig. Simulink model for ZCT boost converter The current through the switch is modulted such tht Im = 0.9A (s shown in figure ), other prmeters remin sme. Switching Power loss: Switching stress: P loss 3.37 wtts 0.3A / sec FOR HARD SWITCHED FLYBACK CONVERTER Im = 4.5A (s shown in figure 6 ), other prmeters remin sme. Switching Power loss: P loss 6.87 wtts Switching stress: 0.3A / sec FOR SOFT SWITCHED FLYBACK CONVERTER Fig. Wveforms of ZCT boost converter Im = A (s shown in figure 8), other prmeters remin sme. Switching power loss: Switching stress: P loss 3.75 wtts 0.3A / sec 7

FOR HARD SWITCHED BOOST CONVERTER Im = 7A (s shown in figure 0), other prmeters remin sme. Switching power loss: Switching stress: P loss 6.5wtts.33 A / sec FOR SOFT SWITCHED BOOST CONVERTER Im = A (s shown in figure ), other prmeters remin sme. Switching power loss: 7.50wtts P loss 0.66 A / sec Switching loss nlysis: VIII CONCLUSION This pper emphsizes fferent PWM DC/DC buck, boost, Flybck converters with Zero Current Trnsition. The concept of resonnce is implemented by using cpcitnce nd inductnce (Auxiliry circuit) in ll the converters. An suitble converter PWM strtegy is designed for the entire control rnge of frequencies, such tht the power switching device IGBT current nd voltge wveforms do not overlp, which result in reduced switching losses results in improved converter efficiency. Design of the sme presented for certin switching frequency nd simulted by using MATLAB- Simulink. Switching stress nd power loss nlysis is lso crried out for the proposed topologies. Comprison between hrd switched nd soft switched converters proves tht dc-dc converter switched with ZCT hs reduced switching stress nd less switching losses which dversely improve the efficiency nd relibility entire converter system. IX REFERENCES [] G. Hu, E. X. Yng, Y. Jing, nd F. C. Lee, Novel zerocurrent-trnsition PWM converters, IEEE Trns. Power Electron., vol. 9, no. 6, pp. 60 606, Nov. 994. [] H. Mo, F. C. Lee, X. Zhou, H. Di, M. Cosn, nd D. Boroyevich, Improved zero-current trnsition converters for high power pplictions, IEEE Trns. Ind. Appl., vol. 33, no. 5, pp. 0 3,Sep./Oct. 997. Stress nlysis: Fig.3 grphicl view of power loss nlysis [3] H. S. H. Chung, S. Y. R. Hui, nd W. H. Wng, A zerocurrent-switching PWM flybck converter with simple uxiliry switch, IEEE Trns. Power Electron., vol. 4, no., pp. 39 34, Mr. 999. [4] M. Ilic nd D. Mksimovic, Interleved zero current trnsition buck converter, in Proc. IEEE Power Electron. Conf., 005, vol., pp. 65 7. [5] D. Y. Lee, M. K. Lee, D. S. Hyun, nd I. Choy, New zerocurrent trnsition PWM DC/DC converters without current stress, IEEE Trns. Power Electron., vol. 8, no., pp. 95 04, Jn. 003. [6] C. M. Wng, C. H. Su, nd C. W. To, Zero-currenttrnsition PWM DC-DC converters using new zero current switching PWM switch cell, Proc. Inst. Electr. Eng. Electr. Power Appl., vol. 53, no. 4, pp. 503 5, Jul. 006. [7] P. Ds nd G. Moschopoulos, A zero-current-trnsition converter with reduced uxiliry circuit losses, IEEE Trns. Power Electron., vol., no. 4, pp. 464 47, Jul. 007. Fig.4 Grphicl view of stress nlysis 8

[8] E. Ab nd H. Frznehfrd, New zero current trnsition PWM converters, in Proc. IEEE ICIT Conf., 006, pp. 3 36. [9] P. Ds nd G. Moschopoulos, A comprtive study of zero-current trnsition PWM converters, IEEE Trns. Ind. Electron., vol. 54, no. 3, pp. 39 38, Jun. 007. M.Prthp Rju, received his B.Tech degree, 003 from G. Pull Reddy Engineering College nd M.Tech(Power electronics) from JNTUH, Hyderbd, AP, In. He is currently pursuing Ph. D from JNTUH nd the pper is crried out towrds the reserch objective. He is currently working s sr. Assistnt professor, Dept of EEI, College of engineering Stues, University of Petroleum nd Energy Stues, Dehrdun, In. In. Dr.A.Jylksmi, received her B.Tech degree from NIT Wrngl nd M. Tech from Osmni University. Awrded Ph.D from JNTUH, Hyderbd, AP, In, in the yer 007. She hs quite good experience in the industry nd reserch. She is currently working s n Associte Professor in the Dept of EEE, JNTU College of Engineering, JNTUH, Hyderbd, 9