MOSFET = Metal Oxide Semiconductor Field Effect Transistor 2) IGBT = Insulated Gate Bipolar Transistor

Transcription

1 Fas Recovery Epiaxial Diodes (FRED) Characerisics - Applicaions - Examples Rüdiger Bürkel, Thomas Schneider During he las 10 years, power supply opology has undergone a basic change. Power supplies of all kinds are now consruced so ha heavy and bulky 50/60 Hz mains ransformers are no longer necessary. These ransformers represened he major par of volume and weigh of a radiional power supply. Today hey have been replaced wih smaller and ligher ransfomers, whose core maerials now consis of sinered ferries insead of iron laminaions and which can operae up o 250 khz. For he same power raing, high frequency operaion significanly reduces he weigh and volume of he ransformer. This developmen has been significanly influenced by new, fas swiching power ransisors, such as MOSFETS or IGBTs, working a high blocking volages (V CES > 600 V). However, nearly all opologies equipped wih hese ransisors also need ulrafas diodes o conduc he reacive load curren and o recify he AC oupu when DC volage is required. The swiching behavior of hese diodes mus be ailored o mach he swiching charcerisics of he ransisors. This is no only rue for swich mode power supplies bu also for inverer circuis. For hese inverers, manufacurers have chosen PWM frequencies of abou 8 khz o creae a smooh sinoidial waveform of he oupu curren or have used a PWM frequency above 20 khz in order o operae above he audible level. 1) MOSFET = Meal Oxide Semiconducor Field Effec Transisor 2) IGBT = Insulaed Gae Bipolar Transisor Apar from he characerisics of he ransior swiches, he on-sae and dynamic characerisics of he free wheeling diodes have a significan impac on he power loss, he efficiency and he degree of safey in operaion of he whole equipmen. They also play a decisive role when i comes o increasing he efficiency of a SMPS and o reduce he losses of an inverer, which clearly mandaes ha ulrafas diodes be used. The ulrafas diodes described here embrace all characerisics of modern epiaxial diodes, such as sof recovery, low reverse recovery curren wih shor reverse recovery imes. Technologies The abbreviaion FRED (Fas Recovery Epiaxial Diodes) sands for a series of ulrafas diodes, which have gained wide accepance during he las few years. There exis several mehods o conrol he swiching characerisics of diodes and each leads o a differen inerdependency of forward volage drop V F, blocking volage V RRM and rr values. I is hese inerdependencies (or compromises) ha differeniae he ulrafas diodes available on he marke oday. Fig.1 shows a qualiaive relaionship of forward volage V F and reverse recovery ime rr. The mos imporan parameers for he urn-on and urn-off behavior of a diode (Fig. 2) V FR, V F, fr and, rr will be influenced by differen manufacuring processes. Bu as shown in Fig. 1, each echnology mus come up wih a is own compromise beween forward volage and recovery ime o obain a device ha will operae saisaisfac-orily. Figure 2 shows a ypical swiching cycle for he diode. During forward conducion, he resisiviy of he n- epiaxial layer (see Fig. 4) is decreased by excess minoriy charges (in his case holes) being sored here. When his forward curren is commuaed o anoher swich, he diode canno regain is reverse blocking capabiliy unil his excess sored charge is removed, which can only be done by recombinaion of he sored holes wih he background elecrons or by reverse curren flow hrough he diode. Since he ideal diode has zero reverse recovery V F V F rr ns Fig. 1 Qualiaive V F - rr correlaion o show his compromise for various FRED echnologies Fig. 2 Typical swiching I/V waveforms for a FRED diode 1999 IXYS All righs reserved P - 12

2 Technologies Characerisics p Anode n n n Kahode Cahode Fig. 3 Cross-secional view of a Schoy diode curren, he recombinaion of sored charge mus be acceleraed, which is done by he inroducion of recombinaion ceners ino he n- epiaxial layer. Of course, he end resul will be ha he sored charge boh recombines and is swep ou by reverse curren, resuling in a shor negaive curren pulse, called reverse recovery curren. As he reverse recovery curren reaches is maximun ( ), areas free of carriers develop a he pn-juncion, which can hen sar o block volage. The ensuing decrease of he recovery curren is essenially deermined by he acual disribuion of he remaining carriers in he n-region. The decrease of he recovery curren (di R /d) versus ime is of special imporance, because i deermines he peak volage and dv/d ransiens ha will occur. This will be discussed in more deail below. As already menioned, recombinaion ceners are creaed wihin he diode o decrease reverse recovery curren. The Schoky diode, whose cross-secion is shown in Fig. 3, is a majoriy carrier diode, which can be swiched very quickly, similar o a MOSFET. The observed reverse curren is due o he charging of he meal barrier-silicon capaciance, which is independen of emperaure. Up o now, Schoky diodes have only been used for applicaions wih low reverse volages (ypically less han 60 V). However, newer ypes wih higher blocking volages up o 300 V and more are already available. For applicaions requiring ulrafas diodes wih blocking volages in excess of 300 V, bipolar pn-juncion FREDs are he only answer. p Characerisics The mos imporan processes o speed up he urn-off behavior of a bipolar diode are gold or plainum doping and elecron irradiaion. In he case of ulrafas diodes, he n-layer ha suppors he reverse volage, should be made as hin as possible o minimize he forward volage drop as well as he sored charge in he pn-juncion. To obain a wafer hickness ha allows a good mechanical handling of he wafers, he epiaxial echnology is he mos favorable choice. This echnology makes use of a relaively hick n doped wafer subsrae for mechanical srengh, on which a hin, monocrysal n-layer (he so-called epiaxial layer), is grown. The epi-layer hickness and resisiviy are adjused according o he desired blocking volage capabiliy. The passivaion of he pn-juncion uses planar echnology, which is similar o he manufacuring process of ransisors. Guard rings reduce he elecric field srengh o preven volage break-down and he surface is glass-coaed o ensure blocking volage sabiliy. To improve he urn-off behavior, eiher gold or plainum aoms can be diffused inersiially ino he epilayer and hese aoms ac as rapping sies, in which he excess holes can recombine wih elecrons. Recombinaion ceners can also be creaed by elecron irradiaion, which displaces silicon aoms from heir normal cyrsalline laice sies. Very high emperaures will allow he displaced silicon aoms o vibrae back ino he laice srucure. Therefore, he irradiaion is ofen followed by an annealing process o anneal ou he low Epiaxy Epiaxie Schich layer n- n- Subsrae n n emperaure sensiive porion of he crysal disurbances. If he process parameers, irradiaion energy and annealing emperaure are properly chosen, he swiching characerisic will remain sable. Table 1 shows all he essenial characerisics of ulrafas diodes by process echnologies. Of course, any manufacuring process for ulrafas diodes has advanages as well as disadvanages. FRED diodes, using gold doping o conrol minoriy carrier lifeime, represen an excellen compromise beween forward volage, low peak reverse recovery currens wih sof recovery. These diodes are characerized by a sof recovery behavior from -40 C o 150 C, showing even a very high -di F /d (> 800 A/µs) no endency o snap-off. The higher leakage curren of he gold doped diode is, in compari-son o he plain doped or irradiaed diode, he only disadvanage. However in mos applicaions, he power loss caused by he leakage curren is small in comparison o forward curren and reverse recovery losses (Fig. 5) [1]. Table 1 Comparaive advanages of ulrafas diodes by process echnologies Gold Doped Plainum Doped Elecron Irradiaion Schoky Diodes Sof reverse Tendency o Tendency o Much beer recovery snapp-off snapp-off urn-off han gold Good V F - rr Good V F - rr V F - rr rade-off Very small, rade-off, very rade-off a no as good as low V F a shor rr wih nearly he same V F as P or Au doping; low V R much gold; higher for clearly longer rr higher V F same rr han gold High I R a high Low I R Low I R Very high I R, even emperaures a room (100 C) emperaure, relaively low V R Planar epiaxial diode, DWEP... Glasspassivaion Guard ring Anode Anode Kahode Cahode Mealizaion Fig. 4 Cross-secional view of a FRED juncion diode wih planar passivaion 1999 IXYS All righs reserved P - 13

3 Free Wheeling Diodes Free Wheeling Diodes These diodes are mainly used as free wheeling diodes, conneced in parallel o fas swiching ransisors working wih an inducive load such as e.g. inducors in a boos or buck converer, ransformers and moors. The majoriy of hese circuis is conrolled by pulse widh modulaion working a a fixed frequency. Forced by he inducive load, he curren mus coninue o flow via a free wheeling circui. In case he ransisor is urned-on, he free wheeling pah mus be blocked o preven a shor circui. The ypical ineracion beween he power ransisor and he free wheeling diode is described in he following example. T1 V in inpu Sollwere value Fig. 6 Buck-converer circui diagram D1 GND I L L PWM PWM conroller - Regler I R I VU C ou V ou load Las Fig. 6 shows he simplified circui of a buck converer. This circui provides an oupu volage V ou which is lower han he supply volage V in. Fig. 7 shows he conrol signals of T1 and he volage and curren waveforms of T1 and D1. The conducing and blocking phases of he wo acive elemens T1 and D1 can be devided as follows: 20 The swiching acion of Diode D1 is characerized by four imporan phases: A. he diode blocks while T1 is in he onsae, B. he ransiion from blocking o conducing mode: urn-on, on he blocking volage cababiliy, on he emperaure of he diode chip and, above all, on he echnology of he diode. Togeher wih he applied reverse volage he reverse power loss is: P SP = V in I R B. Turn-On i A di /d = 250A/us F -5 DSEI 12-10A -10 AEG SPEED BYT BYT 12PI ns500 4 Fig. 5 Comparison of reverse recovery currens for several differen FREDs [1] A a cerain ime 0 he conroller swiches on T1. The series circui of L and C ou is conneced o he supply volage V in and makes he curren I L increase linearly. This curren is deer-mined by he inducor L and he oupu volage V ou. Afer a cerain ime, fixed by he conroller, T1 is swiched off again. In he disconinuous mode of operaion, he energy sored in L (W = 0.5 L I L2 ) is ransferred via he free wheeling pah ino he capacior C ou. A a cerain ime 2, T1 is swiched on again and he whole precedure is repeaed. C. he diode conducs forward curren while T1 is blocked, D. he ransiion from conducing o blocking mode: urn-off. A. Blocking Mode While he MOSFET T1 is conducing, he supply volage V in appears as reverse volage a diode D1. As wih all semiconducors, a low curren in he diode flows from he cahode o he anode (leakage curren I R ). The leak-age curren depends Wih he ransisor T1 swiched off, he curren I L in he inducor mus keep on flowing. The volage across he diode drops down and he diode akes over he curren of he inducor. The curren rise ime in D1 equals he curren fall ime in T1. The volume charge formed in he pnjuncion of he diode during he blocking phase is flooded by carriers causing a change of resisance of he pn-juncion during curren rise ime. This urn-on of he diode is accompanied by a shor overvolage in forward direcion which depends on he chip emperaure, on he -di F /d and again on he chip echnology. Compared o he blocking volage, he overvolage is very low (< 50 V) and for mos applicaions, i is no imporan o he operaion of he diode (waveform D1 in Fig. 7). However, his dynamic urn-on volage of he diode does add o he peak volage ha appears across he ransisor T1 and adds o is urn-off losses. The overvolage V FR deermines he urnon losses of he diode. These urn-on losses increase linearly wih he swiching frequency IXYS All righs reserved P - 14

4 C. On-Sae Once he urn-on phase is over, he diode conducs he forward curren. There is a forward volage drop V F due o he hreshold volage of he pn-juncion and he resisance of he semi-conducor. This volage drop depends, as already menioned, on he chip emperaure, he forward curren and he process echnology. To indicae he forward volage drop a various currens and, consequenly, o calculae he on-sae losses, he parameers V T0 and r T ofen appear in daashees. A simplified model for he forward volage drop shown in Fig. 8 is: D. Turn-Off Apar from he on-sae characerisic, he urn-off behavior is considered o be he mos imporan parameer in deermining he suiabiliy of he diode for a high frequency applicaion. If he curren is commuaed o he ransisor, i decreases linearly wih he di/d a which he ransisor urns on he curren. In he case of r T Wha resuls is he reverse recovery curren I R, whose waveform depends on he chip emperaure, he forward curren, he -di F /d and he echnology. Fig. 9 shows how he reverse recovery curren depends on he chip emperaure of a gold doped (9a) and a plainum doped (9b) epiaxial diode of he same forward characerisic. a) V,I 25 C V F = r T V T0 The on-sae power dissipaion can hen be calculaed accordingly: V T0 V F 125 C P D = V T0 r T 2 Fig. 8 Typical forward volage drop V F versus curren is model, V T0 r T b) V,I Gae T1 T1 I D V DS 25 C 125 C D1 I R V R Fig. 7 Transisor gae signals, curren and volage waveforms along wih he diode curren and volage waveforms for one swiching cycle of he buck-converer shown in Fig. 6 Fig. 9 Reverse recovery curren and volage for wo FRED diodes a T VJ = 25 C und 125 C a) gold-doped diode b) plainum-doped diode The calculaed losses, however, are only approximae values, as V T0 and r T depend a grea deal on he emperaure and are only given for a cerain emperaure (T VJM ). Since his emperaure can differ from he acual operaing emperaure, he calculaed losses are only valid for he given emperaure. power MOSFETs and IGBTs, -di F /d values of more han 1000 A/µs can easily be reached. As menioned before, he carriers which have flooded he pn-juncion during he on-sae phase mus be removed before he diode can sar o block reverse volage. The difference beween he wo echnologies is really sriking, if one compares he recovery behavior a various - di F /d and a he same emperaure IXYS All righs reserved P - 15

5 Example Whereas, in he case of plainum, he decrease of he recovery curren speeds up (Fig. 10b), he gold diodes wih conrolled minoriy carrier reducion keep heir sof recovery behavior, even a high -di F /d values (Fig. 10a). The faser he decrease of he recovery curren (he diode ges "snappy"), he higher he overvolage caused by he sray inducances of he circui lay-ou. If he maximum volage reaches he maximum blocking volage of he ransisor, snubbers mus be used o guaranee he safey in operaion of he equipmen. Furhermore, much oo high dv/d values cause EMI/RFI problems, which complicae he shielding, if cerain RFI limis have o be kep. The reverse recovery curren flowing in he diode does no only deermine he urn-off losses of he diode bu also adds o he urn-on losses of he ransisor, which now carries he diode curren. a) I The reverse recovery curren mus be added o he curren in he inducor and he urn-on ime exended by some porion of rr (Fig. 11a and b). Figures 11a and 11b emphasize he significance of a low peak recovery curren accompanied by sof recovery behavior. In he firs place, he sof recovery behavior of he gold doped diodes enails a small overvolage and a low reverse recovery curren. Therefore he diode is marked by low urn-off losses. Secondly, he low reverse recovery curren leads o essenially reduced urn-on losses in he ransisor. Thus, he choice of he diodes decisively influences he power losses in boh devices. V F rr a V R b Operaing condiions of he buckconverer: DC-link volage V in 600 V Curren fall ime of MOSFET f 60 ns Oupu volage V ou 300 V Swiching frequency f 50 khz Inducor curren I L 15 A Duy cycle of MOSFET d 0.5 Max. juncion emperaure T VJ 125 C A. Maximum Blocking Losses From daashee page 11: I Rmax = 7 ma a T VJ = 125 C, V R = 800 V P SP = V in I Rmax d = 600 V 7 ma 0.5 P SP = 2.1 W B. Turn-on Losses The calculaion of he acual urn-on losses is much more difficul han he calculaion of he blocking losses or onsae losses. There is no saic operaion, he curren and he volage of he diode are funcions of ime and can only be calculaed approximaely by using exponenial and hyberbolic equaions. V,I V DS I Las V,I V FR Eon b) I Fig. 11a Reverse recovery waveforms for a free-wheeling diode Fig. 11b Transisor curren and volage waveforms showing he impac of reverse recovery curren as i commu-aes off he free-wheeling diode 10%V FR fr V FR VF Example Fig. 10 Reverse recovery currens for differen -di F /d a T J = 125 C a) gold-doped diode b) plainum-doped diode The following example illusraes how o calculae he power losses of he free wheeling diode D1 in a buck-converer (Fig. 5). The epiaxial diode, ype DSEI A, is described in he slighly shorened daashee (page D1-16), where one can also find he respecive daa o calculae he power losses. Fig. 12 Acual diode urn-on V/I waveforms and heir linearized approximaions o simplify urn-on power loss calculaions 1999 IXYS All righs reserved P - 16 fr VF

6 Furher Applicaions To esimae he diode urn-on losses, he urn-on waveform is given in a simplified form in Fig. 12. This simplificaion is conservaive, i.e. he acual urn-on losses are smaller han he calculaed ones, and makes i possible o do he calculaion using he daashee values. The curren rise ime is deermined by he urn-off ime of he MOSFET and he load curren: di F /d = / f = 15 A/60 ns = 250 A/µs The diagram in Fig. 6 of he daashee shows he urn-on recovery ime fr and he urn-on overvolage V FR for a -di F /d = 250 A/µs : V FR = 31.5 V fr = 360 ns To calculae he urn-on energy, he curren, which is assumed o be consan, is muliplied by he riangle waveform of he overvolage V FR and by he ime fr : E on = V FR fr 0.5 = E on = 15 A 31.5 V 360 ns 0.5 = E on = 85 µj The urn-on power losses can be calculaed by muliplying he pulse energy E on by he swiching frequency: P on = E on f = 85 µj 50 khz = P on = 4.3 W C. On-Sae Losses The on-sae forward volage a = 15 A and T VJ = 125 C is shown in he diagram in Fig. 1, page 12. The on-sae forward volage a 15 A is given by he V F curve for T VJ = 100 C: V F = 1.77 V Thus he following on-sae losses can be calculaed: P D = V F d = 1.77 V 15 A 0.5 = P D = 13.3 W If one calculaes he on-sae losses using he formula P D (V T0 r T 2 ) (1-d), one ges a smaller value: V T0 = 1.5 V and r T = 12.5 mω P D 1.5 V 15 A mω 15 A P D» 12 W D. Turn-off Losses Similar o he urn-on losses, he urn-off losses can only be calculaed approximaely using he daashee values. Once again, he waveforms of he volage and of he curren are simplified (Fig.13). If one assumes he same di F /d as given during swich-on, he diagrams of Fig. 3 and Fig. 5 of he daashee show: = 15 A, rr = 100 ns. These daa are valid for T VJ = 100 C and mus be muliplied by a facor adjusing hese daa for T VJ = 125 C. This facor K f is given in he diagram of Fig. 4 of he daashee. For T VJ = 125 C he daa for are o be muliplied by 1.1. This simplified calculaion of he urn-off behavior resuls in a urn-off energy of: E off = K f V R rr /2 0.5 E off = 15 A V 50 ns 0.5 = E off = 248 µj The muliplicaion by he swiching frequency resuls in a urn-off power loss: P off = E off f = 248 µj 50 khz P off = 12.5 W V F rr rr E off Fig. 13 Acual diode reverse recovery I/V waveforms and heir linearized approximaions V R V R The oal losses of he diode in he buckconverer are: P o = P SP P on P D P off P o = 2.1 W 4.3 W 13.3 W 12.4 W P o = 32.1 W Using he hermal resisance R hjc = 0.9 K/W, which is shown in he daa-shee, he chip emperaure is 29 C above he case emperaure. Assuming a hermal resisance of R hck = 0.25 K/W and a chip emperaure of less han 125 C, he maximum heasink emperaure may no exceed 88 C: T Kmax = T VJM - (R hjc R hck ) P o T Kmax = 125 C - ( ) K/W 32.1 W = T Kmax = 88 C While his applicaion used a buck-converer circui as an example, he same approximaions and calculaions can be used for he boos-converer. Furher Applicaions Recifier Diodes Ulrafas epiaxial diodes are used as recifier diodes bu only if he swiching frequency is higher han 1 khz and he blocking volage exceeds 200 V. These condiions are very common in swichmode power supplies delivering oupu volages of more han 200 V, because here are no Schoky barrier diodes wih he required blocking volage capabiliy (Fig. 14). Swich-mode power supplies generally operae wih a PWM conroller. Therefore he mode of operaion of he epi-diode used as recifier is very similar o ha of he free wheeling diode. As he curren and volage waveforms are recangular, he calculaion of he power losses can be carried ou as described in he case of he free wheeling diode. ~ Fig. 14 Cener-apped DC oupu circui wih common-cahode diode connecion IXYS All righs reserved P - 17

7 Snubber Diodes Summary FRED Modules Snubber Diodes "Snubber" circuis are used o proec power semiconducors from being desroyed by shor overvolage spikes. The di/d values of more han 1000 A/µs, which can be reached wih ransisors (MOSFETs or IGBTs), cause overvolages due o he parasiic sray inducances of he circui wiring. The equaion V = L di/d underlines how high hese overvolages can be, even a very low sray inducances. For example, for he he case of a -di/d of 1000 A/µs during swich-off wih a sray inducance of 100 nh, he compued volage spike is: V = 100 nh 1000 A/µs = 100 V This 100 V spike, which will be added o he DC bus volage, will require he use of a higher volage MOSFET or IGBT. These devices no only cos more bu he efficiency of he circui will decrease due o heir higher swiching losses. Snubber circuis can limi he generaed overvolage ransferring he energy sored in he sray inducances o a capacior. Apar from is capaci-ance, he urn-on behavior of he diode deermines he remaining overvolage. The diagram in Fig. 6 of he daashee illusraes he forward recovery volage V FR, which can be expeced, and he forward recovery ime fr for various di/d values. P SP (6.5%) P on (13.5%) P off (38.6%) P D (41.4%) Fig. 15 Comparaive losses for he freewheeling diode in he buck-converer example Summary As depiced in deail, ulrafas diodes can have differen characerisics depending on he manufacuring process. These characerisics should be considered o make bes use of hem in he various applicaions. If sandard FREDs in an exising circui are replaced by DSEI diodes, he power losses of boh he diode and he ransisor can hus be reduced. The sof recovery behavior of all DSEI diodes also prevens he ransisor from being overloaded by oo high a dv/d or overvolage spike and also reduces EMI/RFI. The marke for ulrafas diodes is consanly expanding. Apar from heir sandard applicaion as free wheeling diode in inverers, hese diodes are also more and more used in snubber circuis and in recifier circuis in swich-mode power supplies. All FRED diodes delivered by IXYS are characerized by very low reverse recovery currens, even a high -di F /d. Simulaneaously, hey show a sof decrease of he reverse recovery curren, hus avoiding inducive overvolages wih very high dv/d. These overvolages could cause a malfuncion or even he desrucion of he acive swiching device, eg. a MOSFET, an IGBT or bipolar ransisor. The given example of he buck-converer shows, ha when choosing an ulrafas diode, all operaing modes aken ogeher mus be considered and no only he individual parameers. The losses of he diode in he buckconverer can be divided as illusraed in Fig. 15. The on-sae and urn-off losses make up 80 % of he oal losses of he diode. The deermining facors of hese losses are: P D ~ V F P off ~ rr, IXYS offers ulrafas diodes (FRED = Fas Recovery Epiaxial Diodes) in he TO- 220, TO-247 and SOT-227B packa-ges. These diodes are available wih blocking volages from 400 o 1200 V. Furhermore, IXYS provides FRED modules in various opologies up o 300 A and 1200 V (able 2, page 10). FRED Modules The FRED modules ype MEO, MEE, MEA and MEK (Fig. 16) are an exension of he discree DSEI o higher curren while sharing he same diode characerisics menioned before. They can be applied in all circuis wih MOSFETs, IGBTs or bipolar Darlingons, working a swiching frequencies of more han abou 1 khz. If he paralleling of several discree diodes or miniblocs is necessary, hese modules represen a possible alernaive o minimize he assembly ime and he size of he final equipmen. Generally he FRED modules can be used as free wheeling diodes for high curren IGBTs or for bipolar Darlingons and as fas recifiers in power supplies and welding equipmens. Wha follows is a series of applicaions showing he use of FRED modules. MEO MEK MEE MEA Fig. 16 Circui diagrams for he available FRED modules 1999 IXYS All righs reserved P - 18

8 Applicaions Parallel and Series Connecion of he Modules C. Applicaion as recifier in power supplies and welding equipmens Fig. 19a Half-wave recifier (Fig. 19a). Depending on he load curren, several MEO modules can be paralleled. a Fig. 17 MEE ype or MEO ype A. Applicaion as free wheeling diode in one or hree phase inverers for drive and UPS sysems, working wih PWM conrolling and swiching frequen-cies in he khz range (Fig. 17) Common cahode opology (Fig. 19b). Depending on he load curren, several MEK modules can be paralleled. Common anode opology (Fig. 19c). Depending on he load curren, several MEA modules can be paralleled. Full bridge recifier (Fig.19 d). Depending on he load curren, eiher MEE or MEO modules can be paralleled. Fig. 19b a Fig. 19c a Full bridge recifer for higher oupu volages (Fig. 19e). Here boh diodes in he MEE module are conneced in series o obain a higher/blocking volage. Fig. 19d Fig. 18a MEE ype or MEO ype For all FRED modules he coninuous DC curren AVM is given a a heasink emperaure of T S = 65 C and a juncion emperaure T VJM = 125 C (difference of emperaure = 60 C). When comparing hese modules wih ypes of he compeiion, one has o make sure ha he wo modules o be compared share he same difference of emperaure beween heasink (case) and juncion, because his is wha deermines he maximum allowable forward curren. Fig. 19e Furhermore, he curren raings for he FRED modules include he blocking losses of he diode a T VJ = 125 C and a a duy cycle of d = 50 %. Parallel and Series Connecion of he Modules Fig. 19a-e Examples of differen FRED modules as oupu recfiiers in power supplies Fig. 18b MEO ype or MEE ype B. Applicaion as free wheeling diode in swich-mode power supplies and servo drives a. symmerical full bridge wih MOSFETs and Schoky blocking diodes (Fig. 18a) b. assymmerical full bridge wih MOSFETs for forward converers and DC moor drives (Fig. 18b) All FRED modules consis of several individual diode chips, which are conneced in parallel inernally o obain he desired curren raing of he module. In order o ge a good curren sharing beween he diode chips, hey are seleced in such a way ha he forward volage drop V F is nearly he same for all chips in parallel. This V F selecion faciliaes he parallel connecion as well as he series connecion of several modules IXYS All righs reserved P - 19

9 Parallel and Series Connecion of he Modules Parallel Connecion To simplify he parallel connecion of several modules, hey are esed for V F caegories which are indicaed on he ype label. The par number is followed by a digi or leer, which sands for he V F caegory of he device. Example: MEO260-12DA3 or MEK160-06DAD. If several modules are conneced in parallel o ge a higher oupu curren, only modules of he same V F caegory should be used. There are, of course, excepions o he rule: I is also possible o use wo adjoining V F caegories; however, due o he safey in operaion, his should only be he excepion. The curren handling capabiliy of he modules conneced in parallel is calculaed as follows: I P n I n 0.8 n = number of modules I n = curren of one module (see able 3) I P = oal curren of he modules in parallel Example: 3 modules MEO DA1 are conneced in parallel: The allowable coninuous forward curren AVM a T VJ = 65 C equals: I P = A 0.8 = 629 A Fig. 20 Volage sharing neworks for FRED modules when is diodes are o be used in series Series Connecion The series connecion of FRED modules requires a saic resisor circui o equalize he differen blocking currens of he diodes and a dynamic RC snubber circui o equalize he differen reverse recovery charges (Q r ) of he diodes. The calculaion of hese snubber circuis has o ake ino accoun a grea number of condiions which can differ from applicaion o applicaion. The snubber circuis can only be opimized, if one is really familiar wih hese condiions. The rough seup of hese snubber circuis is illusraed in Fig. 20. Wha is of special ineres is ha, for he individual diode chips wihin a module, he selecion in V F caegories also means ha here exiss a selecion in and hus in Q r. A deailed daabook (publicaion no. D94013DE) for he whole range of ulrafas diodes exiss o help he design engineer o choose he righ diode for his applicaion. Lieraure [1] Heumann, K: Impac of urn-off Semiconducor devices on power elecronics, EPE Journal, Vol. 1 (1991), No. 3, page IXYS All righs reserved P - 20