JPE Soon-Kurl Kwon, Bishwajit Saha *, Sang-Pil Mun *, Kazunori Nishimura ** *, *** and Mutsuo Nakaoka. 1. Introduction

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1 18 Journal of Power Elecronics, Vol. 9, No. 1, January 2009 JPE Series Resonan ZCS- PFM DC-DC Converer using High Frequency Transformer Parasiic Inducive Componens and Lossless Inducive Snubber for High Power Microwave Generaor Soon-Kurl Kwon, Bishwaji Saha *, Sang-Pil Mun *, Kazunori Nishimura ** *, *** and Musuo Nakaoka * Kyungnam Universiy, Masan, Korea ** Hiroshima Insiue of Technology, Hiroshima, Japan *** Indusrial College of Technology Universiy, Japan ABSTRACT Convenional series-resonan pulse frequency modulaion conrolled DC-DC high power converers wih a high-frequency ransformer link which is designed for driving he high power microwave generaor has he problem of hard swiching commuaion a urn-on and urn-off of acive power swiching devices. This problem is due o he influence of he magneizing curren of he high-frequency ransformer. This paper presens a novel prooype for a high-frequency ransformer using parasiic parameers wih a lossless inducive snubber and a series resonan capacior assised series-resonan zero curren swiching pulse frequency modulaed DC-DC power converer, which is designed using a high power magneron for microwave ovens. In order o implemen a complee and efficien sof swiching commuaion, he performance of he new converer opology is pracically confirmed and evaluaed in he prooype of a power microwave generaor. Keywords: Series resonan inverer, High frequency ransformer link, Volage doubler recifier, Magneron, ZCS-PFM 1. Inroducion Wih grea advances of power semiconducor swiching devices such as MOSFETs, IGBTs, and ESBTs as well as Manuscrip received April 3, 2008; revised Ocober 14, 2008 Corresponding Auhor: soonkurl@kyungnam.ac.kr Tel: , Fax: , Kyungnam Univ. * Dep. of Elecrical Engineering of Kyungnam Univ. ** Hiroshima Insiue of Technology, Japan *** Indusrial College of Technology Univ., Japan high-frequency passive circui componens, he leading developmen of he high-frequency resonan pulse inverer ype swiching mode DC-DC power conversion circuis and sysems have araced special ineres for high volage DC power applicaions [1]-[5]. High-frequency resonan pulse sof-swiching DC-DC converers using MOS gae power semiconducor devices are acively inroduced in his new paricular field of small power elecronic applicaions. On he oher hand, IGBTs or MOS gae bipolar ransisors are more suiable for zero curren swiching (ZCS)

2 Series Resonan ZCS- PFM DC-DC Converer using 19 commuaion raher han zero volage swiching (ZVS) due o heir associaed inheren ail currens a urn off swiching commuaions [4]. Thus, he series resonan inverers and converers operaing under he principle of ZCS commuaion are pracically recommended from he swiching losses poin of view. Mos convenional series-resonan circui opologies of high frequency inverers can operae under a commuaion principle of ZCS based on a disconinuous curren mode (DCM) conrol. However, in high frequency ransformer link opologies i is difficul o implemen he DCM in he convenional ype circui due o he influence of he magneizing curren ha flows hrough he high-frequency ransformer s primary side. Especially, he influence of he magneizing curren is remarkable for he DC-DC converers wih a capacior ype oupu-smoohing filer and a consan high-volage load as in he power magneron of microwave appliances. This paper presens a high-frequency ransformer wih parasiic parameers and a lossless inducive snubber assised series-resonan DC-DC power converer wih a volage doubler recifier circui, which can operae under ZCS commuaion based on a pulse frequency modulaion (PFM) in order o effecively improve he problem of he ransformer magneizing-inducive componens-based hard swiching commuaion. This new circui opology can acively uilize he ransformer parasiic circui componens as leakage and magneizing inducance o achieve ZCS operaion wih he aid of a lossless inducive snubber and series resonance capacior in he high volage ransformer s primary side. The performance evaluaions as he swiching volage and curren waveforms of he power semiconducor swiches, DC power regulaion and power conversion efficiency characerisics are pracically confirmed and evaluaed as applied o he power magneron of he microwave generaor for plasma applicaion. 2. Equivalen Circui Model of Magneron he RF (microwaves) energy ha cooks he food. Microwaves are very shor waves of elecromagneic energy ha ravel a he speed of ligh. Microwaves used in microwave ovens are in he same family of frequencies as he signals used in radio and elevision broadcasing. Elecromagneic forms of energy, such as microwaves, radar waves, radio and TV waves, ravel millions of miles hrough he empiness of space wihou he need of any maerial medium hrough which o ravel. Fig. 1 indicaes he inpu volage vs. inpu curren characerisics of he power magneron. As shown in his figure, he magneron has non-linear inpu characerisics, wha can be represened wih piecewise linear approximaion which includes a high resisance area in a non-oscillaing region and a low resisance area in he oscillaing region for he magneron. When he volage beween he anode and cahode exceeds abou 7.4 kv (cu-off volage), he magneron anode curren begins o flow from he anode o cahode. On he oher hand, when he volage beween he anode and cahode is lower han he cu-off volage he anode curren flows slowly. The cu-off volage of magneron flucuaes a lile wih he operaing emperaure which depends on he characerisics of he ferrie magne in he magneron, which are no considered here for he approximae v-i characerisics. The elecrical equivalen circui model of he magneron can be simply represened by using pure resisances R 0 and R 1, an ideal diode D, an ideal baery V Z (cu-off volage) and an ideal swich as shown in Fig. 2. As illusraed in his figure, he posiion of he ideal swich in he equivalen circui of he magneron is seleced wheher he volage beween he anode and cahode is higher han he cu off volage or no. In his paper, sable microwave power is required for semiconducor manufacuring producion in indusrial applicaions and he magneron is used under he condiion of coninuous oscillaion. Therefore, only a sable oscillaion sae is considered for he converer operaion. The hear of any microwave generaor is he high volage sysem. Is purpose is o generae microwave energy. The high-volage componens accomplish his by sepping up AC line volage o high volage, which is hen changed o an even higher DC volage. This DC power is hen convered o 3. Series-Resonan DC-DC Converer wih High-Frequency Transformer Link 3.1 Operaion in Disconinuous Curren Mode Fig. 3 shows he schemaic circui configuraion of a

3 20 Journal of Power Elecronics, Vol. 9, No. 1, January 2009 convenional ype DC-DC power converer wih a high-frequency ransformer(hft) link and volage doubler circui, which is designed for a high power magneron drive in indusrial power applicaions (his circui is called circui 1). This DC-DC converer circui is composed of a high frequency high volage ransformer, resonan high-frequency full bridge inverer wih a series resonan capacior C r in he primary side of he high frequency ransformer, full-wave volage doubler ype recifier circui, curren smoohing inducor L o, and a magneron o generae microwave power in he high frequency ransformer s secondary side. The magneron of Fig. 3 is represened by is elecrical equivalen circui shown in Fig. 2 under he condiion of coninuous oscillaions. The circui parameers and he design specificaions of he circui (see Fig. 3) are indicaed in Table 1. V A [kv] Oscillaed area (Cu off volage) V = 7.41 [kv] Z Non-oscillaed area R o =V AK /I A ) I [A A Fig. 1 Experimenal characerisics of magneron Anode D R 0 R 1 V Z Non-oscillaion O scillaion Cahode Fig. 2 Elecrical equivalen circui model of magneron Fig. 3 High frequency link series resonan DC-DC converer(circui 1) Table 1 Design specificaions and circui parameers of circui 1 Iems Symbol Value Equivalen DC volage of inpu source E 283 V Leakage inducance of HFT L l 2.3 mh Primary self-inducance of HFT L mh Turn raio of HFT N 2 / N 1 20 Capaciance of series resonan capacior Cr nf Capaciance of oupu filer capacior C 1, C 2 11 nf Inducance of oupu filer inducor Lo 0.3 H Cu-off volage of magneron V z 7.41 kv Equivalen resisance of magneron R Ω In he oscillaing sae of he magneron, is inpu side DC volage can be considered nearly consan since i is represened by a linear v-i characerisic wih a small slope as indicaed in Fig. 1. Thus, he microwave power from he magneron is proporional o is anode curren. Therefore, he oupu power of he magneron can be regulaed by conrolling he anode curren wih a closed feedback loop. Consequenly, runaway of he magneron is prevened and sable high microwave power can be effecively generaed and conrolled. The seady sae volage and curren waveforms in DCM operaion in he previously-developed series resonan DC-DC converer are illusraed in Fig. 4. In his case, he curren hrough he ransformer primary winding is disconinuous. As shown in Fig. 4, he gae pulse signal sequences of he IGBTs are designed o regulae he pulse frequency under consan on-ime condiion. The IGBTs are urned on wih ZCS and urned off wih hybrid ZVS & ZCS (IGBTs are urned off wih ZVS and ani-parallel diodes are urned on wih ZCS) in all power regulaion ranges when he swiching frequency of his DC-DC converer is less han half of he resonan frequency. This is decided by he

4 Series Resonan ZCS- PFM DC-DC Converer using 21 resonan capacior and he leakage inducance of he high-frequency ransformer. If he swiching frequency is increased more han half of he resonan frequency, he bridge curren of his converer becomes a coninuous waveform and his converer operaes in coninuous curren mode (CCM). Consequenly, he operaion becomes hard swiching. However, in he case of using a high-frequency ransformer wih leakage and magneizing parasiic circui parameers (L l, L m ), i is acually difficul o realize he DCM operaion o achieve a zero curren sof commuaion. In order o implemen his operaing mode, i is necessary ha he magneizing curren hrough he high-frequency ransformer primary winding be nearly zero. Neverheless, he magneizing curren remarkably flows in he DC-DC converer wih a capacior inpu ype oupu-smoohing circui and a consan high volage load as he magneron is equal o zero. Because he oupu recifier circui is cu off when he secondary side volage of he high-frequency ransformer is less han he oupu-smoohing capacior volage, and he series resonan inverer is isolaed separaely from he volage doubler recifier wih he capacior inpu filer. In his case, only he magneizing curren of he ransformer circulaes hrough he circui of he ransformer s primary side during he cu off mode of he volage doubler ype recifier. Consequenly, he disconinuous curren operaing mode (see Fig. 5) of his converer can no appear and be realized pracically. 3.2 Magneizing Inducor Hard Swiching Operaing Mode Fig. 5 shows he measured operaing waveforms of he curren flowing hrough each IGBT swich and he volage across i of circui 1 (see Fig. 3). In comparison wih he simulaion volage and curren waveforms in Fig. 4, he measured swich curren waveforms observed in Fig. 5 are disored. I is easily proven ha his power converer operaes under a hard swiching mode due o he remarkably confirmed high volage surges in he operaing curren and volage waveforms. A urn-on swiching ransiion, he high-frequency ransformer s primary side curren has an iniial value due o he magneizing curren hrough he high-frequency high-volage ransformer. V ge1, 4 V ge2, 3 i Q1, i Q4 i Q2, i Q3 i r Mode T on T off Fig. 4 Operaing waveforms in disconinuous curren mode Microwave Power 1.6 kw Microwave Power 5.2 kw Microwave Power 2.6 kw Upper races Curren waveforms 100 A/div, 4us/div Lower races: Volage waveforms 100 V/div, 4us/div Fig. 5 Experimenal volage and curren waveforms of Q1 (Circui 1) Therefore, he curren flowing hrough he swiches jumps o his iniial value, and he IGBTs in he bridge arms of he series ransformer resonan inverer has hard swiching commuaion a urn-on in all power regulaion seing ranges. Exremely high volage surges acually occur wih his high di/d sress. A urn-off swiching ransiion, he case which occurs while he curren flowing hrough he

5 22 Journal of Power Elecronics, Vol. 9, No. 1, January 2009 IGBT swiches is forcibly cu off o zero before he zero curren crossing poin. Consequenly, he IGBT swiches are urned-off a hard swiching ransiions. 4. Improved Series Resonan DC-DC Power Converer wih Lossless Inducive Snubber In order o solve he significan problem menioned above, a single inducive snubber assised series-resonan ZCS-PFM DC-DC power converer wih a high-frequency ransformer(hft) link is proposed in Fig. 6 (his circui is called circui 2). This DC-DC power converer has a single inducive lossless snubber L s in he inpu DC busline of he full bridge series resonan inverer. The curren flowing hrough each IGBT swich rises gradually a urn-on wih he aid of his inducive snubber, and ZCS urn-on commuaion can be achieved compleely. Fig. 6 Proposed sof swiching DC-DC power converer (Circui 2) The gae pulse iming sequences of each IGBT of his DC-DC converer are illusraed in Fig. 7. The duy cycle of gae pulse signal of he IGBT is designed for a consan duy cycle of 50%. The pulse widh of he gae signal varies wih swiching frequency. As shown in Fig. 8, he IGBT swiches can be always urned off while a curren coninues o flow hrough he ani-parallel diodes and he primary winding of he high frequency ransformer. Thereby, he IGBT swiches can be urned off wih ZVS & ZCS hybrid commuaion. Furhermore, even hough he dead ime beween wo gae pulse signals is se o zero, he inpu DC busline of his newly developed DC-DC converer can no be shored due o he effec of he single lossless inducive snubber conneced in he inpu DC side of he full bridge inverer. Table 2 Design specificaions and circui parameers of circui 2 Iems Symbol Value [Uni] Equivalen DC volage of inpu source E 283 V Lossless inducive snubber L s 1 mh Leakage inducance of HFT L l 2.3 mh Primary self-inducance of HFT L mh Turn raio of HFT N 2 / N 1 20 Capaciance of series resonan Cr nf capacior Capaciance of oupu filer capacior C 1, C 2 11 nf Inducance of oupu filer inducor Lo 0.3 H Cu-off volage of magneron Vz 7.41 kv Equivalen resisance of magneron R Ω Vge1 Vge2 iq1, iq4 iq2, iq3 Convenional ype On ime consan conrol Proposal ype 50% duy cycle consan conrol Microwave Power 1.6 kw Microwave Power 5.2 kw Microwave Power 2.6 kw Upper races : Curren waveform ( 100 A/div, 4µs/div ) Lower races : Volage waveform ( 100 V/div, 4µs/div ) Fig. 7 50% duy cycle consan PFM conrol scheme Fig. 8 Experimenal volage and curren waveforms of Q1 (Circui 2)

6 Series Resonan ZCS- PFM DC-DC Converer using 23 Microwave power [kw] 6 Measured efficiency [%] : Circui 1 : Circui Oupu power of DC-DC converer [kw] Fig. 9 Oupu power of DC-DC converer vs. microwave power Therefore, i is possible for his DC-DC converer circui o perform zero curren sof swiching commuaion over a wide power regulaion range in he PFM sraegy. The design specificaions of he DC-DC converer in his experimen are indicaed in Table 2. The observed volage and curren operaing waveforms of he IGBT swiches in he proposed converer are shown in Fig. 8. Observing Fig. 8, i is easy o prove ha his proposed DC-DC converer can operae under a principle of ZCS operaion a boh urn-on and urn-off ransiions. The microwave power regulaion characerisics are shown in Fig. 9, while he measured efficiency characerisics for he oupu power seings of he circui 1 and 2 are comparaively shown in Fig. 10. The power conversion efficiency of he proposed sof swiching DC-DC converer is improved in comparison wih ha of convenional DC-DC converers in he wide range of high power seings. 5. Reducing Peak Curren Sresses When he swiching frequency is more han half he resonan frequency, he convenional DC-DC converer operaes a hard-swiching ransiion mode in good condiion. However, he proposed DC-DC converer operaes a sof-swiching ransiion even if he swiching frequency is more han half he resonan frequency, because is sof-swiching operaion is based on coninuous curren mode Oupu power of DC-DC converer [kw] Fig. 10 Comparison of power conversion efficiency vs. oupu power characerisics for wo converer opologies The measured peak curren values for a wide oupu power conrol range of he convenional and proposed DC-DC converer are indicaed in Fig. 12. I is proven from his figure ha he peak curren value of IGBT on circui 3 is lower han half of ha of previous ypes over all power regulaions. The new design specificaion of his circui is indicaed in Table 3. These circui parameers are designed o acively uilize he coninuous curren mode, in oher words, he maximum swiching frequency on circui 3 is se near he resonan frequency of he resonan circui o increase he uilizaion facor of elecric power. Observing Fig. 11, i is confirmed ha he peak values of he curren flowing hrough he IGBT swiches of circui 3 are much lower han hose of previous converer circuis (circui 1 and circui 2). Table 3 New Design specificaions and circui parameers of circui 2 Iems Symbol Value Equivalen DC volage of inpu source E 283 V Lossless inducive snubber Ls 3.5 mh Leakage inducance of HFT Ll 11.3 mh Primary self-inducance of HFT L mh Turn raio of HFT N 2 / N 1 20 Capaciance of series resonan Cr nf capacior Capaciance of oupu filer capacior C 1, C 2 11 nf Inducance of oupu filer inducor Lo 0.3 H Cu-off volage of magneron Vz 7.41 kv Equivalen resisance of magneron R Ω

7 24 Journal of Power Elecronics, Vol. 9, No. 1, January 2009 Microwave Power 1.6 kw Microwave Power 2.6 kw Upper races : Curren waveform ( 50 A/div, 4µs/div ) Lower races : Volage waveform ( 100 V/div, 4µs/div ) Microwave Power 5.2 kw Fig. 11 Experimenal volage and curren waveforms of Q1 (Circui 3) Peak curren of IGBT [A] : Circui 1 : Circui 3 (7.42kW, 179.2A) (7.45kW, 80.4A) Oupu power of DC-DC converer [kw] Fig. 12 Comparison of peak values of curren flowing hrough IGBT Power [kw] Efficiency [%] : Measured efficiency : Oupu power of DC-DC converer Swiching frequency [khz] Fig. 13 Oupu power and measured efficiency vs. swiching frequency characerisics wih newly design specificaion of circui The measured peak curren values for wide oupu power conrol range of convenional and he proposed DC-DC converer are indicaed in Fig. 12. I is proven from his figure ha he peak curren value of he IGBT on circui 3 is lower han half of previous ypes over all power regulaion range. Efficiency improvemen can be expeced by improving he characerisics of IGBT swiches wih reduced peak curren values and raed values. Fig. 13 shows he power conversion efficiency and he oupu power characerisics of he experimenal prooype of he proposed DC-DC converer designed under he specificaions of Table 3. The maximum value of he efficiency of circui 3 shown in Fig. 13 is abou 94.3[%], and his value is higher han ha of circui Conclusions In his paper, a ransformer parasiic parameer and a lossless inducive snubber assised series-resonan ZCS-PFM DC-DC converer was proposed in order o improve he significan problems of hard swiching commuaion a urn-on and urn-off of he acive power swiching devices in a series-resonan PFM conrolled DC-DC power converer wih a high-frequency high-volage ransformer link. Based on he experimenal resuls of he proposed pulse frequency modulaed DC-DC power converer wih a high-frequency high-volage ransformer, i was acually confirmed ha all he acive power swiches could achieve ZCS commuaion operaion. ZCS commuaion could be implemened wihin he DC power regulaion range and high power conversion efficiency could be performed in a wide power regulaion range. The ransformer parasiic circui componens as leakage and magneizing inducive componens were effecive o achieve sof swiching (ZCS) operaion wih he aid of a single lossless inducive snubber. The peak values of he curren flowing hrough he IGBT swiches could be reduced by more han half of convenional DC-DC converers over all power regulaion ranges by acively uilizing he coninuous curren mode operaion wih proper design selecion of circui parameers. This paper was sponsored by Kyungnam Universiy Research Fund 2007

8 Series Resonan ZCS- PFM DC-DC Converer using 25 References [1] A. Harada, How o Use C.W. Magnerons, Proceedings of Microwave Effec and Applicaion Symposium, Japan, Augus [2] T. Masushige, E. Miyaa, M. Ishiobi, and M. Nakaoka, Volage-Clamped Sof-Swiching Inverer-Fed DC-DC Converer for Microwave Oven and Uiliy AC Side Harmonic Curren Evaluaions, Proceedings of IEEE-IAS Inernaional Appliance Technical Conference, IATC, pp , Kenucky, USA, May [3] T. Myoi, M. Ishiobi, L. Gamage, and M. Nakaoka, Lossless Inducive Snubber-Assised Series Resonan DC-DC Converer wih ZCS-PFM Conrol Scheme, Proceedings of Inernaional Power Elecronics and Moion Conrol Conference (EPE-PEMC), Dubrovnik, Croaia, Sepember [4] E. Hiraki, M. Nakaoka, Pracical Power Loss Analysys Simulaor Developmen of Swiching Mode Power Converer Using Measured Characerisic Values of Power Semiconducor Devices, The Trans.of Insiue of Elecrical Engineering of Japan, IEEJ, Vol. 122-D, No. 12, December [5] Yuzurihara, A. Takayanagi, and H. Fujikawa, Microwave Generaor Sysem for Plasma Applicaion (Vol. 2), Kyosan Technical Circular, Vol. 53, No. 3, [6] Shibaa, Indusrial Microwave Power Engineering, Denkishoin Publishers Co., Ld., December [7] K. Takahashi, Base and Applicaion of High Frequency, Tokyo Denki Universiy Book Press, Ocober, Soon-Kurl Kwon Received Ph. D (Dr-Eng) degree in Elecrical Engineering from Young-Nam Universiy, Daegu, Republic of Korea. He joined he Elecrical Engineering Deparmen of Kyungnam Universiy, Masan, Republic of Korea, in 1983 as a professor. He was a visiing professor of Virginia Polyechnic Insiue and Sae Universiy, USA in His research ineress include applicaion developmens of power elecronics circuis and sysem. He is a member of he KIEE and KIPE. Bishwaji Saha received his B.Sc. degree in Elecrical and Elecronic Engineering in 2004 from Dhaka Universiy of Engineering & Technology, Bangladesh. He received his M. Sc. degree in he Deparmen of Elecrical Engineering, Kyungnam Universiy, Masan, Republic of Korea. His research ineress and research developmen includes DC-DC power converer opologies, high frequency inverers applicaions and renewal energy relaed power condiioner. He is a suden member of IEEE and member of IEB-Bangladesh. Sang-Pil Mun received his B.S. degree in Elecrical Engineering from Pukyong Universiy, Pusan, Korea in 1997 and M.S. and Ph.D. degrees in Elecrical Engineering from Kyungnam Universiy, Masan, Korea in 1999 and 2003 respecively. He joined he Elecrical Energy Saving Research Cener (EESRC), Kyungnam Universiy from 2003~2005 as a researcher. Since 2006 he is an inerim full-ime insrucor in he Deparmen of Elecrical Engineering, Kyungnam Universiy. His research ineress are in he areas of phoovolaic power generaion sysems, power elecronics, and sof-swiching echnology. He is a member of he KIEE, KIPE, KIIEE.. Kazunori Nishimura was born in Ehime, Japan, He received his B.S. from he Deparmen of Elecronics Engineering, Yamaguchi Universiy. He received his M.E. and Docor of Informaion Engineering degree from he Deparmen of Informaion Sysems, Hiroshima Ciy Universiy, Japan. Now he is an associae professor in he Hiroshima Insiue of Technology, Japan. His curren research ineress are in Modern Power Elecronics; Sof-swiching Technique for High frequency Power Conversion sysems. Musuo Nakaoka received his Ph.D. degree in Elecrical Engineering from Osaka Universiy, Osaka, Japan in He joined he Elecrical and Elecronics Engineering Deparmen of Kobe Universiy, Kobe, Japan in 1981 and served as a professor in he Deparmen of Elecrical and Elecronics Engineering, The Graduae School of Engineering, Kobe Universiy, Kobe, Japan. Since 1995, he has been a professor of he Elecrical and Elecronics Engineering Deparmen, he Graduae School of Science and Engineering, Yamaguchi Universiy, Yamaguchi, Japan and Hyogo College of Indusrial Technology, Hyogo, Japan. His research ineress include applicaion developmens of power elecronics circuis and sysems. Prof. Dr.-Eng. Nakaoka received he 2001 Premium Prize Paper Award from IEE-UK, 2001/2003 IEEE-IECON Bes Paper Award, he hird paper award in 2000 IEEE-PEDS, 2003 IEEE-IAS James Melcher Prize Paper Award, Bes Paper Award of Inernaional Appliance Technical Conference. He is now a chairman of he IEEE Indusrial Elecronics Sociey Japan Chaper. He is an acive member of he, IEE-J, IEICE-J, Solar Energy Insiue of Engineers, European Power Elecronics Associaion, IEIE-J, KIPE, JIPE and IEEE.