Reverse Blocking IGCTs for Current Source Inverters

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1 A. Weber, T. Dalibor, P. Kern, B. Oedegard, J. Waldmeyer and E. Carroll ABB Semiconducors AG, 56 Lenzburg, Swizerland Tel: +4 (62) ; Fax: +4 (62) ; Absrac - Today IGCTs (Inegraed Gae Commuaed Thyrisors) are widely used for differen applicaions especially volage source inverers (VSIs) for which reverse conducing and asymmeric elemens wih discree freewheeling diodes have been developed. For curren source inverers (CSIs), reverse blocking elemens are required and o his end symmeric 6 kv IGCTs have been developed. Using reverse blocking IGCTs in a CSI offers significan benefis compared o presen GTO or hyrisor soluions and allows higher inverer raings and swiching frequencies. In addiion o he semiconducor swich, he performance of he inegraed gaedriver has been adaped o he demands of he CSI. Saus feedback signals for proecion purposes and LEDs for easier commissioning of he drive have been incorporaed and he gae-driver is opionally available wih an AC power supply inpu allowing furher sysem cos reducions. I. Inroducion In he pas five years, IGCTs wih discree or inegraed fas recovery diodes have been opimised for use in VSIs. Device improvemens have been achieved by advanced lifeime engineering echniques [] as well as wafer design and process conrol. In conras o he VSI, he CSI requires symmeric devices such as he hyrisors or GTOs in use oday. However, boh devices have heir limiaions: hyrisors, requiring large commuaion capaciors, have low swiching frequencies and GTOs, wih heir large snubber circuis, incur significan losses. There are only wo devices available oday for high swiching frequency and quasi snubberless operaion: he IGBT and he IGCT. II. Device Srucure Reverse blocking devices can be realised in wo ways. The firs is symmeric wafer processing - creaing wo blocking juncions on he same silicon wafer. For hyrisors and GTOs, symmeric processing is he sae-ofhe-ar. The second mehod is he series connecion of a diode wih an asymmeric urnoff device such as an IGBT [2] or an (asymmeric) IGCT. Since CSIs are used predominanly in Medium Volage Drives (MVDs), he seleced device echnology mus allow series connecion of several devices. Where redundancy is addiionally required, a sable shor-circui failure mode becomes necessary [3]. For hese reasons, press-pack devices are preferred and he IGCT wih is high blocking volage and low losses becomes he device of choice. Symmeric processing of he silicon does no allow he incorporaion of a buffer layer [4] necessiaing hick silicon slices for high volage devices which in urn leads o high dynamic losses. While his may be accepable a he low frequencies used in he pas, i is a major drawback for fuure high volage and high frequency applicaions. Aemping o reduce dynamic losses by lifeime reducion leads o an increase in conducion losses. These drawbacks can be overcome by he series connecion of discree GCT and diode wafers. Since boh wafers can be opimised for minimal hickness, heir on-sae volage drops versus urn-off losses are very favourable. A furher problem, he lower urn-off capabiliy of symmeric devices [5], is also avoided. For applicaions wih low currens, boh GCT and diode wafer can be encapsulaed in one press-pack device whils, for high power applicaions, i may be advanageous o use he same wafers discreely encapsulaed, hus allowing 5% more cooling. PCIM of 6 Nürenberg, 6, 2

2 III. Tes circui A curren source inverer is schemaically shown in Fig.. Each of he posiions can be a single swich or a series connecion of several swiches wih small RC snubbers for volage sharing. delay, DUT2 is urned off forcing he curren o commuae o DUT wih a di/d given by he snubber of DUT2 (R S, C S ) and he commuaion inducances (2 x L C ). When DUT2 is gaed on again, DUT undergoes reverse recovery wih high di/d given by he commuaion inducance L C ( diode operaion ) and susains full reverse volage. Thus his circui ess he devices under condiions close o CSI operaion and, in paricular, issues of iming beween DUT urnoff and DUT2 urn-on can be invesigaed. LDC Fig. Basic circui of curren source inverer. Tesing of devices in an inducively clamped circui based on he design of a volage source inverer is no saisfacory as he waveforms are dissimilar and inerpreaion of resuls wih respec o losses, SOA ec. is problemaic. For his reason he special es circui of Fig. 2 was buil o evaluae devices under condiions close o hose of he CSI applicaion. The circui of Fig. 2 consiss of DC capacior C DC, DC link inducance L DC, wo commuaion inducances L C and wo DUTs, each wih RC snubbers. For he device 5SHZ 8F6 raed for a maximum urn-off curren I TGQM = 8 A, he condiions used were: L C = 3µH, R s = Ω and C s =. µf. The es circui is placed in a climaic chamber so ha he DUTs can be esed from 25 C o 25 C. All measuremens shown in his paper were performed a 25 C. The circui allows he invesigaion of urn-on and urn-off under forward bias in posiion DUT2. This mode is called GCT operaion because i requires gae-conrolled urn-off. In he posiion of DUT however, urn-on and urn-off occur under negaive anode-cahode bias. This mode is referred o as diode operaion. These are he wo commuaion modes of a CSI. A ypical measuremen cycle is described below. Iniially DUT and DUT2 are in he off-sae. The DC capaciance C DC is charged, DUT2 is urned on and DUT blocks reverse volage. The curren ramps up in he load inducance L DC wih low di/d. Afer reaching he es curren level, DUT is gaed on and afer some C DC LC DUT DUT 2 Fig. 2 Circui o es IGCTs in condiions close o hose of a CSI. LS RS C S IV. Characerisics and Raings IV.. Thermal Impedance Deailed invesigaion of he hermal resisance and impedance of a wo-wafer design shows ha when he oal power dissipaion in boh wafers is considered, he hermal resisance is only slighly greaer han ha of a single wafer press-pack, as long as double-side cooling is used (see Fig. 3). I is advanageous, however, o uilise wo wafers raher han jus he one: he division of diode and GCT funcions in separae wafers allows an overall reducion of losses. Furhermore, he diode s saic and dynamic losses are abou 3 % higher han hose of he GCT and his is allowed for in he wo-wafer approach by designing such ha he diode wafer has a lower hermal resisance o anode-side han ha of he GCT o cahodeside. The hermal model for a wo-wafer device is a lile more difficul o accuraely describe han wih a single hermal resisance or impedance funcion. I can however be formulaed exacly by a marix of four hermal resisance or impedance erms, reflecing self and crossheaing of he wo juncions. The wo selfheaing erms are only marginally larger han PCIM 2 of 6 Nürenberg, 6, 2

3 for a single-wafer device and he cross-heaing erms do no conribue o T j rise for shor imes, i.e. for surge curren cases. ANODE CATHODE.5 x P TOTAL.5 x P TOTAL Fig. 3a Single wafer press-pack Legend: -silicon wafer; -molybdenum -copper pole-piece PDIODE + PGCT (Fig. 3b) = PTOTAL (Fig. 3a) P DIODE ANODE Fig. 3b Two-wafer press-pack CATHODE P GCT For an accurae calculaion of GCT juncion emperaure i is necessary o know he losses in he GCT and he diode. The exac juncion emperaure of boh wafers can hen be derived. For he GCT we find: T jgct () = T jgctsar + P GCT hgg ( τ)dτ + P Diode hdg ( τ) dτ where Z hgg describes he self heaing of he GCT wafer and Z hdg describes he muual heaing of he GCT wafer by dissipaion in he diode. A similar formula can be derived for he juncion emperaure of he diode wafer: T jdiode () = T Thermal Resisance [K/W] jdiodesar.e-.e-2.e-3.e-4 + P Diode hdd ( τ)dτ + P GCT hgd... ime [s] Fig. 4 Thermal resisance of reverse blocking IGCT ype 5SHZ 8F6 ( τ) dτ Evaluaion of he complee se of equaions requires deailed informaion abou he disribuion of losses in he GCT and diode wafer under operaion. This is soluble and allows opimal design (minimal margins) bu is complex. A simpler bu conservaive approach is o assume ha all he losses are generaed in he wafer wih he higher hermal impedance which resuls in he curve of Fig. 4. IV.2. Dynamic Properies IV.2.. Turn-off ino Forward Volage During urn-off under forward bias, he GCT wafer susains volage as is curren falls (boh posiive). The series diode (being forward biased) generaes no losses. Le us consider DUT2 conducing and DUT blocking: Fig. 5 shows anode volage of DUT2 rising as anode curren commuaes o he snubber. Once he anode volage of DUT2 exceeds he volage of capacior C DC, curren commuaes o DUT, (unil now reverse-biased), provided i was previously gaed on. IA [ka] ime [µs] Fig. 5 Turn-off of DUT2: 8 A agains 37 V IV.2.2. Turn-off ino Reverse Volage We now consider he curren esablished in DUT wih he GCT receiving a small backporch gae curren (which keeps he cahode-gae volage posiive). Turning on DUT2 commuaes he curren from DUT o DUT2 wih a high di/d, forcing reverse recovery of DUT. The reverse curren hrough he GCT will a firs flow ino he gae-driver and reduce he gae-cahode volage unil he gae cahode juncion avalanche volage is reached and he curren flows hrough he device from anode o cahode. The energy loss in he GCT wafer is, however, negligible since he GCT gae-cahode region susains only VD [kv] PCIM 3 of 6 Nürenberg, 6, 2

4 of he 5 V peak appearing in reverse (Fig. 6) - he remaining 498 V are susained by he diode wafer which hus generaes virually all he dynamic losses of his phase. Depending on he applicaion, a urn-off command may be sen o DUT during or shorly afer his reverse recovery phase. I should be noed ha no addiional diode is required ani-parallel o he asymmeric GCT o by-pass reverse curren as has been suggesed [5]. IF [ka] ime [µs] Fig. 6 Reverse recovery waveforms of DUT for 8 A urn-off ino 37 V. The commuaion di/d is A/µs. IV.2.3. Turn-on The reverse blocking IGCT can eiher be urned on from: () he forward-biased sae by a posiive gae pulse (following negaive gae-bias), or from: (2) he reverse-biased sae by curren commuaion from he complimenary device (following posiive gae-bias). In he es circui, boh urn-on modes are invesigaed. IA [ka] ime [µs] Fig. 7 Turn-on of 8 A from 37 V VD [kv] VD [kv] The firs mode is invesigaed on DUT2 in Fig. 7 where urn-on from 3.7 kv is shown. The curren peaks o.8 ka due o reverse recovery of DUT and discharge of he local snubber resuling in jus under J of urn-on energy. The second mode is similar o diode forward recovery since he device is already gaed on. Fig. 8 shows he reverse volage of DUT slewing from 37 V o V, a which poin curren can rise a a rae given by he inducances L C and he fall ime of DUT2 anode curren. The urn-on losses are small for his mode (.5 Ws for he case of Fig. 8 I A [ka] ime [ µ s] Fig. 8 Turn-on of DUT: 8 A agains 37 V However, since DUT has o be urned on o allow for curren commuaion, his mode is imporan wih respec o iming delays beween urn-off of DUT2 and urn-on of DUT. Turning off DUT2 while DUT is no gaed will lead o over-volage and device failure and i is herefore a par of he conrol sequence of a CSI ha a device be gae-biased on in advance of a complimenary device being urned off (in his case: DUT on before DUT2 off) V D [kv] V. Cosmic Ray Induced Failures In he design of VSIs, cosmic ray wihsand capabiliy is an imporan design crierion because of he high coninuous DC volage o which he semiconducors are exposed. In he CSI, here is no consan volage applied o he devices which allows a degree of loss (hickness) opimisaion for a given blocking volage (V DRM, V RRM ). The semiconducors here are subjeced o an alernaing volage and since cosmic ray failures have an exponenial dependence on applied volage, a slighly more complex calculaion mus be performed (and wo separae cases considered) if he sysem has boh a line-side as well as a load-side inverer. PCIM 4 of 6 Nürenberg, 6, 2

5 Le us consider an acive fron-end (line-side) conneced o a 23 V RMS line. Wih ± % line olerance, he peak volage is 36 V. A uniy power facor, we have pure recifier operaion wih he device blocking in reverse direcion an approximaely sinusoidal volage of 36 V peak during 66 % of he ime (Fig. 9) and sees no forward blocking volage. On he load side, he siuaion is reversed and he device sees no reverse blocking volage a uniy power facor. Applying Fig. 9 o he now well-esablished cosmic ray induced Failures-In-Time (FIT) models under DC volage [6] for he wo wafers used, he FIT raes lised in Table are calculaed for he 5SHZ 8F6 symmeric IGCT. Blocking volage [V] Percenage of ime Fig. 9 Blocking volage for he calculaion of he FIT rae due o cosmic radiaion. Condiion: uniy PF VDM=36 V Line-side Load-side FIT rae 5 Table Fi rae due o cosmic ray failures. VI. New Gae-driver The IGCT is a swich which makes no preence a modulaing rise and fall imes. As such, i has only wo saes and is gae-driver is device and no circui specific. This allows sandard gae unis o be supplied wih he GCT (hence IGCT) in keeping wih he marke demand for higher inegraion. However, he IGCTs inroduced 3 years ago [7] were designed for VSIs and, as described above, logic changes have now been implemened o allow for hese new CSI applicaions. Thus a new generaion of IGCT gae-drivers has been developed, suiable for boh CSIs and VSIs. The new drivers addiionally provide LED gae-saus for simpler commissioning and diagnosics as well as an opical feedback for supervision of: Gae-drive supply volage Gae-o-cahode saus Opical link saus Opionally, facory uning of IGCT urn-on and urn-off ime delays can be provided o minimise he spread of hese imes for quasi snubberless series connecion. An AC powersupply inpu is planned o be opionally available in order o furher reduce sysem coss. Wih a swiching frequency of 75 Hz and an average urn off curren of 4 A, he power consumpion of he gae drive will be abou 4 W, mos of which needed for urn-off. VII. Raings of he 5SHZ 8F6 The characerisics and raings of he new 5SHZ 8F6 derived from he measuremens made in he circui of Fig. 2, are shown in Table 2. Parameer Symbol Raing Uni Condiion Repeiive peak off-sae volage VDRM 6 V Neg. gae supply volage conneced Repeiive peak reverse volage VRRM 65 V Neg. gae supply volage conneced Maximum conrollable urn-off curren ITGQM 8 A VD = 3 V, LS = 2 nh, CS =. µf, RS = Ω, TJ = 25 C, LC = 3 µh On-sae volage VT 6.3 V IT = 8 A, TJ = 25 C Turn-off swiching energy EOFF 5 J IT = 8 A, VD = 3 V, di/d = A/µs, Tj = 25 C Reverse recovery energy ERR 6.5 J IT = 8 A, VD = 3 V, di/d = A/µs, Tj = 25 C Sandard gae supply volage VIN 9..2 V DC Planned opional VIN V AC volage, square wave, -o-peak Typ. gae uni power consumpion PIN_MAX 4 W IT AVG = 4 A, fpwm = 75 Hz Table 2 Raings of 5SHZ 8F6 PCIM 5 of 6 Nürenberg, 6, 2

6 VIII. Conclusion Afer 5 years of service and wihin only 3 years of marke inroducion, he IGCT has esablished iself as a polyvalen power swich for Energy Managemen (Power Qualiy), Tracion and Indusrial Drives in boh VSI and CSI opologies. The hrus for boh componen and plaform sandardisaion has been aken a sep furher by realising he wo-wafer press-pack. This allows for opimisaion of boh GCT and diode wih high producion yields, sandardisaion of wafer producion processes and sandardisaion of gae-drivers and housings. Thus he same wafers, gae-unis and housings are used for series (including redundance) and non-series connecion in boh VSI and CSI opologies. An emerging applicaion for symmeric IGCTs lies in he field of Power Qualiy AC breakers. So far, hese have been realised wih aniseries conneced reverse-conducing devices [8]. However, where he raio of faul curren o load curren mus be increased, an aniparallel connecion of reverse blocking devices will be preferred. [7] E.Carroll, S.Klaka. S.Linder, Inegraed Gae-Commuaed Thyrisors: A New Approach o High Power Elecronics, IEMDC, Milwaukee, 997 [8] W.Raihmayr, P.Daehler, M.Eichler, G.Lochner,, E.John, K.Chan, Cusomer Reliabiliy Improvemen wih a DVR or a DUPS, Power World 98, Sana Clara, 998 References [] N.Galser, M.Frecker, E.Carroll, J.Vobecky, P.Hazdra, Applicaion-Specific Fas-Recovery Diodes: Design and Performance, PCIM, Tokyo, 998 [2] P.Puonen, M.Salo, J.Mokka, H.Tuusa, A Curren-source PWM-converer for Variable Speed Wind Energy Drive, PCIM Europe 999 [3] H.R.Zeller, High Power Componens: From he Sae of he Ar o Fuure Trends, PCIM Europe, 998 [4] A.Weber, N.Galser, E.Tsyplakov, A New Generaion of Asymmeric and Reverse Conducing GTOs and heir Snubber Diodes, PCIM Europe, 997 [5] K.Saoh, M.Yamamoo, K.Morishia, Y.Yamaguchi, H.Iwamoo, High Power Symmerical GCT for Curren Source Inverer, ISPSD, Torono, 999 [6] H.R.Zeller, Solid-Sae Elecronics, 38, , 995 PCIM 6 of 6 Nürenberg, 6, 2

Explanation of Maximum Ratings and Characteristics for Thyristors

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