Reliability Improvement of FB inverter in HID Lamp Ballast using UniFET II MOSFET family

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1 Reliabiliy Improvemen of FB inverer in HID Lamp Ballas using UniFET II MOSFET family Won-Seok Kang Sysem & Applicaion Group Fairchild Semiconducor Bucheon, Korea Jae-Eul Yeon Visual Sysem Team, PIA Fairchild Semiconducor Bucheon, Korea fairchildsemi.com Hee-Jun Kim Elecrical Engineering Hanyang, Universiy Ansan, Korea Tae-Young Ahn Elecrical Engineering hongju Universiy hongju, Korea Absrac This paper inroduces a newly developed UniFET TM II MOSFET family of which he body diode characerisic has been highly improved and presens is effeciveness for he mixed frequency swiching inverer leg in he HID ballas. Is reverse recovery ime, T rr of he body diode is 35 nsec and he peak value of reverse curren, i rr is also much smaller han ha of he ypical MOSFET and even smaller han FRD s. Therefore, i can ensure he beer reliabiliy in he swiching inverer applicaions in which he performance of he MOSFET body diode is significan. In order o verify he validiy, an experimen wih an 150W HID ballas was implemened and is resuls and effeciveness are presened in his paper. I limi Lamp curren [A] Peak curren conrol P max Transien I limi Raed power onsan power conrol Seady sae I. INTRODUTION High-inensiy discharge (HID) lighing can offer beer efficacy and longer lifespan wih beer color qualiy approaching ha of incandescen lighing. Alhough HID was originally inended o use in oudoor or indusrial applicaions, i has spread o indoor applicaions such as office and reail lighings because heir color-rendering characerisics have improved and smaller sizes have become available [1]. HID however, requires a ballas o iniiae discharge and limi he curren afer he sars. Moreover, acousic resonance phenomenon occurs in he seady sae operaion a frequencies higher han some kiloherz [1-2]. Acousic resonance causes serious problems such as visible arc disorions and unsable arc ha are resuling in decreased lifespan. Even in he wors case, unsable arc may crack he discharging arc ube. The operaion of he in a low frequency square waveform is he mos common mehod o avoid he acousic resonance phenomenon in HID ballas. The ampliude of volage wih he square waveform also has o be conrolled in accordance wih he impedance o preven he curren runaway [1-3]. The mixed frequency full bridge inverer is one of he popular opology in he HID ballas because i can generae he alernaive low frequency volage wih square waveform and conrol he ampliude of he oupu volage a he same wih only single inverer sage i doesn need an addiional D/D converer o conrol he ampliude of he volage. This inverer is however, concerned abou MOSFET failure when i operaes in coninuous curren mode (M). Therefore, he inverer should operae in criical curren mode (RM) o avoid MOSFET failure. Afer igniion of HID, he impedance becomes exremely low. In ha case, he inverer ineviably operaes in M and he shoo-hrough curren occurs beween boh high and low side MOSFETs. Because he inverer operaing frequency is limied o he minimum while he curren is exremely high. For his reason, he addiional FRDs have o be required o preven MOSFET failure. Recenly, a new planer power MOSFET, UniFET II MOSFET family was developed. The new power MOSFET has robus body diode characerisics. Especially is peak reverse-recovery curren is enough o preven device failure. In his paper, UniFET II MOSFET family wih robus body diode characerisics and is effeciveness for he mixed frequency full bridge inverer in HID ballas is inroduced. In order o verify he validiy, an experimen wih an 150W indoor HID ballas was carried ou and is resuls are presened. II. Figure 1. Lamp volage [V] Power conrol paern of HID Raed volage MIXED FREQUENY INVERTER FOR HID BALLAST Figure 1 shows he power conrol paern of HID. The operaion mode of HID ballas is simply divided ino 2 saes, ransien and seady sae. During he ransien sae, he impedance becomes exremely low hus he volage becomes low o limi he curren o I limi in figure 1. As he impedance seadily rises, he volage is also geing higher and higher and he operaion mode is shifed from ransien o seady sae. Afer he peak curren becomes lower han I limi, he will be driven hrough he consan power conrol mode. The operaion of he ballas mus be complying wih he above sequence. Figure 2 shows he mixed frequency full bridge inverer in

2 HID ballas. In Figure 2, Q a is he igniion swich ha R Limi Q a High frequency leg D 1 D 3 D 2 generaes a pulsed high volage o ignie he HID. The mixed frequency inverer consiss of he high frequency leg, composed of MOSFETs ( and Q 2 ) and FRDs D 1 ~ D 4 ), and he low frequency leg composed IGBTs (Q 3 and Q 4 ). Since he won discharge unless i is ignied, an igniion circui is required. Q a and he ransformer, T, compose he igniion circui. Afer he power is urned on, Q a begins swiching and a very high volage (above 3kV) is applied o he hrough ransformer, T. Once he is ignied by he high volage, he swiching of Q a is sopped and he produces ligh by T Q 2 Q 4 D 4 Figure 2. Mixed frequency inverer for HID ballas V L - L eq + V - i Q 2 Q 4 Low frequency leg Q 3 L he square A volage waveform. The leakage inducance of T, inducor L and capacior are consis of low pass filer. The operaion modes and key waveforms of he mixed frequency full bridge inverer are shown in Figure 3. The operaion of he inverer is simply divided ino 4 modes bu 2 modes are repeaed alernaely. Therefore we only need o consider 2 modes in half cycle. For half cycle, one IGBT in he low frequency leg mainains on sae and he oher IGBT mainains off sae while one MOSFET in he high frequency leg is swiching wih high frequency. Mode I (DTs) : Powering mode Under he condiion ha one IGBT(Q 4 ) is keeping on sae and MOSFET(Q 2 ) is keeping off sae, MOSFET ( ) is urned on and he inpu volage, supplies energy and he curren, i flows hrough MOSFET( ), he equivalen inducance (L eq ), load (Lamp//) and IGBT(Q 4 ). During Mode I, he volage of he equivalen inducance is V V V (1) L i Mode II {(1-D)Ts}: Freewheeling mode Under he condiion ha one IGBT(Q 4 ) is keeping on sae and MOSFET(Q 2 ) is keeping off sae, MOSFET( ) is urned off and he he curren, i freewheels hrough he body diode of MOSFET(Q 2 ), he equivalen inducance (L eq ), load (Lamp//) and IGBT(Q 4 ). During Mode II, he volage of he equivalen inducance is V V (2) L From he vol-sec balance low, he volage is derived Q (a) Mode I + V L - + V - i L eq Q 2 Q 4 (b) Mode II as below; 1 V V DTs V D Ts (3) i V DV (4) The volage ripple in he volage is small enough o ignore. Thus, he average volage is obained as below; 1 T /2 V V d V T /2 0 i (5) onsequenly, he average volage can be conrolled by means of conrolling he duy raio of MOSFETs in he high frequency legs. Q 3 Q 4 V gs1 i Q1 V DTs (1-D)Ts Ts I limi Several ens khz Dead ime V gs2 i Q2 V <v > T/2 DTs (1-D)Ts ΔV T (Several hundreds Hz) (c) Waveforms Figure 3. Operaion modes and waveforms Ts I limi III. MOSFET FAILURE MEHANISM If he high frequency leg of he inverer shown in Figure 2 works in M, he shoo hrough curren ineviably occurs due o he poor reverse recovery characerisics of he body diode of MOSFET. Figure 4 shows he equivalen circui of MOSFET. Basically, base and emier of parasiic BJT (bipolar juncion ransisor) are common ogeher o he source meal. Thus, he parasiic BJT should no be acivaed. In pracice, however, he small resisance, R b exiss beween gae and source as shown in Figure 4. If large curren flows hrough R b, he volage across R b becomes large enough o rigger he parasiic BJT. Once he parasiic BJT urns on, a ho spo is formed and more curren concenraes ino i due o he negaive emperaure coefficien, which finally resuls in he device failure. Anoher failure mode is called as breakdown dv/d. I is a

3 Drain Gae dg gs Body Diode R b db Parasic BJT dv/d R gs combinaion of breakdown and saic dv/d: The device undergoes avalanche curren and displacemen curren a same ime. If MOSFET ransiion during he body diode reverserecovery process is exremely fas, drain-source volage may exceed maximum raing of a device. A his ime, MOSFET may ener he breakdown mode due o high-volage spikes. I is exacly he same mechanism as avalanche breakdown. In addiion o his process, high dv/d affecs he failure poin of he device. More displacemen curren is buil up wih greaer dv/d. The displacemen curren is added o avalanche curren and he device becomes more vulnerable o failure. Because of he poor reverse recovery characerisics of he body diode of MOSFET, he addiional FRDs should be added in he mixed frequency inverer in HID ballas as shown in Figure 2. If he mixed frequency inverer works in M, he shoo-hough curren occurs and MOSFET will evenually be desroyed. However, he inverer in Figure 2 ineviably works in M for several minues afer he igniion of he. Therefore, 4 addiional FRD mus have been required o preven MOSFET failure. IV. Body diode Irrm Source Displacemen curren by db*dv/d Figure 4. Equivalen circui of planer MOSFET UNIFET II MOSFET FAMILY TEHNOLOGY The reverse recovery characerisics of MOSFET are very inferior o FRDs The body diode of power MOSFETs has a very long reverse-recovery ime and large reverse-recovery charge. In spie of is poor reverse recovery characerisics, he body diode is widely used as a freewheeling diode in many swiching applicaions. As more and more applicaions use an inrinsic body diode as he criical componen in he sysem, he body diode characerisics have been improved. Fas recovery MOSFET (FRFET) ha has fas reverse recovery characerisics was already inroduced in 2008[4]. Bu i has some drawbacks such as higher on resisance and drain-source leakage curren. Besides, he manufacuring cos increases due o addiional process for life-ime conrol. For beer body diode performance, a highly opimized power MOSFET, called UniFET II MOSFET family, was developed recenly. I has improved body diode ruggedness and oupu sored energy in OSS, while minimizing negaive effecs like he increase of on-resisance. Especially, is peak reverse-recovery curren, I rr is grealy improved enough o he level ha does no cause device failure. And also, is dv/d srengh is more han wice han ha of normal MOSFETs hus, i can wihsand more han double he curren sress Figre 5. Reverse recovery characerisics during breakdown dv/d mode. Figure 8 shows he reverse recovery performance of UniFET II MOSFET family (FDPF8N50NZ), a convenional MOSFT (FQPF9N50) and FRD (BYV28X-500). Figure 5 clearly shows ha he reverse recovery characerisics of UniFET II MOSFET family are superior o ha of he convenional MOSFET and even FRF s Is T rr a 2A of I pk is 35.4 nsec and I rr is 2.2A respecively while, FRD has 51.8 nsec of T rr and 2.8A of I rr and he convenional MOSFET has nsec of T rr and 11.7A of I rr a he same condiion respecively. V. EXPERIMENTAL RESULTS To verify he validiy of UniFET TM II MOSFET family, an experimen wih 150W indoor HID ballas including he mixed frequency inverer was carried ou. The inverer circui of he ballas is same as in Figure 2. The low frequency leg consiss of 2 IGBTs and is operaing frequency range is 60Hz 120Hz. On he oher hand, he high frequency leg consiss of 2 MOSFETs and 4 FRDs ha are 2 blocking FRDs and 2 freewheeling FRDs, and is operaing frequency range is 30kHz 110kHz. In case of he convenional soluion, he forwarding curren flows hrough MOSFET and he blocking FRD and he reverse curren flows only hrough he freewheeling FRD. In case when only MOSFET is used, however, boh forwarding and freewheeling currens flow hrough MOSFET, channel or body diode. If he reverse recovery characerisic of he body diode is inferior and he inverer works under he M, MOSFET will be desroyed by excessively high reverse curren. The key parameers of devices in he convenional soluion and UniFET II MOSFET family are lised in Table 1. The on resisance of UniFET II MOSFET family TABLE I. Devices for high frequency leg di/d= 100A/us, I pk=2a onvenional MOSFET FRD UniFET TM II FQPF9N50 BYV29X-500 FDPF8N50NZU R dson(max) [Ω] V f [V] Q RR [n] T RR [ns] I RR [A] dv/d [V/ns]

Lamp volage Lamp curren (a) Transien sae (b) Seady sae Figure 6. Lamp volage and curren (a) Transien sae (b) Seady sae Figure 7.

High frequency leg (MOSFET) curren FQPF9N50 wih BYV29X-500 FDPF8N50NZU (a) onvenional wih FRDs V ds _Q1 I d _Q1 V ds _Q1 I d _Q1 (b) UniFET2 TM II MOSFET family Figure 9.

However, he T rr and I rr characerisics of UniFET II MOSFET family are much beer han ha of convenional MOSFET and even baer han FRD I has 38.2 nsec of T rr and 2.

Therefore, UniFET II MOSFET family can remove 4 addiional FRDs in he mixed frequency inverer even i operaes under he M.

Afer he igniion, he impedance is exremely low, hus he curren is limied o I max and he volage is he minimum value.

The swich curren flows wih M during he ransien sae, hough, he swich curren flows wih RM during he seady sae. The swiching curren a he high frequency leg is shown in Figure 8.

case of he seady sae. As we examined in Table 1, UniFET II MOSFET family has beer reverse recovery performance.

4 Lamp volage Lamp curren (a) Transien sae (b) Seady sae Figure 6. Lamp volage and curren (a) Transien sae (b) Seady sae Figure 7. Low frequency leg (IGBT) curren Shoo-hrough curren (a) Transien sae (b) Seady sae Figure 8. High frequency leg (MOSFET) curren FQPF9N50 wih BYV29X-500 FDPF8N50NZU (a) onvenional wih FRDs V ds _Q1 I d _Q1 V ds _Q1 I d _Q1 (b) UniFET2 TM II MOSFET family Figure 9. Shoo hrough curren during ransien period Lamp volage Lamp curren (FDPF8N50NZU) is 1.2 Ω while ha of he convenional MOSFET (FQPF9N50) is 0.8 Ω. However, he T rr and I rr characerisics of UniFET II MOSFET family are much beer han ha of convenional MOSFET and even baer han FRD I has 38.2 nsec of T rr and 2.2A of I rr a I pk =2A condiion whereas FRD has 52.1 nsec of T rr and 2.8A of I rr a he same condiion. Therefore, UniFET II MOSFET family can remove 4 addiional FRDs in he mixed frequency inverer even i operaes under he M. Figure 6 shows he volage and curren a ransien and seady sae which describes he HID has he negaive coefficien of resisance. Afer he igniion, he impedance is exremely low, hus he curren is limied o I max and he volage is he minimum value. As he ballas operaion goes o he seady sae, he curren decreases while he volage increases. Figure 7 shows he swich (IGBT) curren a he low frequency leg. The swich curren flows wih M during he ransien sae, hough, he swich curren flows wih RM during he seady sae. The swiching curren a he high frequency leg is shown in Figure 8. Since he swich curren flows disconinuously in case of high frequency leg, a shoo-hrough curren is observed a he urn-on insan during he ransien sae, while here is no shoo-hrough curren is observed in case of he seady sae. As we examined in Table 1, UniFET II MOSFET family has beer reverse recovery performance. The shoo-hrough curren comparison resuls beween he convenional soluion wih normal MOSFETs & FRDs and UniFET II MOSFET family case are shown in Figure 9. In case of he convenional soluion wih 2 normal MOSFETs and 4 FRDs, he peak value of he reverse recovery curren was measured as 11.44A. Meanwhile, he peak value of he reverse recovery curren in case of UniFET II MOSFET family was measured as 10.48A. VI. ONLUSION In his paper, he reliabiliy improvemen using UniFET TM II MOSFET family, a new planer MOSFET for he full bridge mixed frequency in HID ballas has been presened. UniFET II MOSFET family has reverse recovery beer characerisics such as fas recovery ime and small reverse recovery curren and small recovery charge. Therefore, i can eliminae 4 addiional FRDs ha are required o preven MOSFET failure in he mixed frequency inverer sysem. To verify he validiy of new MOSFET, an experimen wih 150W indoor HID ballas was implemened. Through he experimen, i was verified ha he UniFET II MOSFET family can improve he sysem reliabiliy in he mixed frequency inverer and he manufacuring coss can also be saved by he removal of he addiional diodes. REFERENES [1] hulz-harder, J., "DB subsraes: soluion for high emperaure applicaions", High Temperaure Elecronics, The Third European onference on Vol. 4-7 pp65 [2] Hu, J.; von Bloh, J.; De Doncker, R.W.; O, N., "ircui opology and conrol sraegies of D-D power module for he fuure 42 V auomoive PowerNe", Power Elecronics and Moion onrol onference 2004., Vol. 1, pp

5 [3] Kim, W.S.; ho, B.H.; Lee, K.. Design and analysis of high-volage ransformer for HID ignier, APE Twenieh Annual IEEE March 2005 pp Vol. 2 [4] Jae-Eul Yeon; ha-kwang Kim; Myung-Sub Jung; Woo-Taek Kim; Kyu-min ho; Hee-Jun Kim; Effeciveness of FRFET MOSFETs for he FL inverers in LD TVs, SPEEDAM 2008 conference, pp

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