A Parametric evice Stuy for SiC Power Electronics Burak Ozpineci urak@ieee.org epartment of Electrical an Computer Engineering The University of Tennessee Knoxville TN 7996- Leon M. Tolert tolert@utk.eu Sye K. slam sislam@utk.eu Oak ige National Laoratory P.O. Box 9 Oak ige TN 78-647 M. Hasanuzzaman mhasanuz@utk.eu Oak ige nstitute for Science an Eucation Oak ige TN 78-7 Astract: Materials an evice researchers uil switching evices for the circuits researchers to use in their circuits ut they rarely know how an the evices are going to e use. The circuits people incluing power electronics researchers take the evices as lack oxes an use them in their circuits not knowing much aout the insie of the evices. The est way to esign optimum evices is an interactive esign people esigning an uiling the evices have a close interaction with the people who use them. This stuy covers the circuit aspects of the SiC power evice evelopment. As a contriution to the aove-mentione interactive esign in this paper the evice parameters which nee to e improve in orer to esign etter evices will e iscusse.. NTOUCTON Typically power electronics researchers have to choose off-the-shelf power evices with the specifications est fit for their applications. They usually o not have a say aout how they woul like the evice parameters e change. Materials an evice researchers uil switching evices for the power electronics researchers to use in their circuits ut they rarely know how an the evices are going to e use. As represente in Fig. a arrier exists etween the people who esign an uil power evices an the people who use them in their circuits an systems. Close interaction etween the oth sies of the arrier is neee to otain the most performance for evices an systems. With this interaction the esign loop will e close an the possiility for uiling application specific optimum power evices will arise. ecently a significant increase in the interest of silicon carie (SiC) power evices has occurre ecause of their system level enefits. n the literature SiC research is mainly concentrate on the materials an evices aspects [ ]. ecently more circuit applications [ 4] are eing pulishe. Prepare y the Oak ige National Laoratory Oak ige Tennessee 78 manage y UT-Battelle for the U.S. epartment of Energy uner contract E-AC5-O75. The sumitte manuscript has een authore y a contractor of the U.S. Government uner Contract No. E-AC5-O75. Accoringly the U.S. Government retains a non-exclusive royalty-free license to pulish from the contriution or allow others to o so for U.S. Government purposes. Circuit esign farication an testing evice esign farication an testing Moreover the system level enefits of SiC are also eing evaluate in some recent papers [5-7]. However SiC power evices are still in their evelopment stage; therefore this is a goo opportunity at this time to close the loop. At Oak ige National Laoratory (ONL) a team of materials evice an power electronics researchers are working together with the University of Tennessee Auurn University an Vanerilt University to uil application specific optimum SiC power MOSFETs. This paper will summarize some of this work.. APPLCATONS Systems applications SiC processing Fig. Closing the evice esign loop. This paper is a part of a stuy system impact of SiC power electronics on hyri electric vehicle (HEV) applications was investigate [5-8]. n the mentione stuy two HEV power converters were ientifie moele an simulate to show the system level enefits of SiC power electronics quantitatively. The two selecte applications were a c-c power supply an a traction rive. The c-c power supply shown in Fig. is an isolate fullrige c-c converter which is selecte mostly ecause of its high frequency transformer which provies isolation an aitional taps in the seconary to fee more than one converter. The main traction rive shown in Fig. uses most of the power in an HEV when the vehicle is in motion. A traction rive consists of a attery feeing a three-phase inuction machine through a three-phase inverter. Because of the cooling requirements of the power evices in the inverter usually a large heatsink is require. n an HEV any reuction in volume an weight of any component will enefit the efficiency of the vehicle. Because SiC evices can operate at higher temperatures an they have -78-74-7//$7. (C) EEE
V c / V c / lower losses the heatsink volume an weight can e reuce if all SiC evices are use in all HEV power converters. The simulation results of these converters have shown on average % ecrease in weight an volume of the heatsink an a 5-% increase in the efficiency. mproving the relate evice parameters can increase these further. n the next two sections these parameters will e ientifie for SiC Schottky ioes an MOSFETs an then necessary suggestions for improvement will e state. Note that all these moification suggestions also apply to Si evices ut the main focus of this stuy is given to SiC power evices.. OES Some important ioe parameters for power electronics systems are the reakown voltage on resistance uilt-in voltage peak reverse recovery current an reverse recovery time. A. Conuction Loss Parameters ) Traction rive A ioe conuction loss expression for a traction rive inverter shown in Fig. has een erive in [5] an it is repeate elow for convenience. P = V con 4 8 π π 8 is the current through the ioe M is the moulation inex for sinusoial PWM φ is the power factor angle is the ioe series resistance an V is the ioe uilt-in voltage. This equation consists of two parts loss associate with V c / V c / o Q Q 4 a Q Q v N N - Fig. solate full-rige step-own c-c converter. Q Q 4 a 4 Q i a Q 6 6 N Q 5 Q - v o v L - o 5 i c L C AC MOTO Fig.. Three-phase inverter riving an inuction machine loa. i c v o - () the on resistance an loss associate with the uilt-in voltage rop V. ioes with lower an V woul e preferale ut these parameters epen on similar evice parameters e.g. oth of these parameters epen on the oping ensities. Higher oping ensity means lower ut higher V an lower reakown voltage BV; therefore oth an V cannot e lowere at the same time i.e. a trae-off is require. Consier a 4H-SiC Schottky ioe with a BV of more than 5V for a traction rive. E r c.5 BV ε = > 5V an qn N 8 N <.7 () BV is the reakown voltage ε r is the permittivity E c is the critical electric reakown fiel q is the electron charge N is the oping ensity The maximum oping ensity value to sustain the chosen BV is calculate aove. The resistance value corresponing to this N is the minimum. t cannot e ecrease with oping any further; however the oping ensity can still e selecte lower than this value which woul increase BV an an ecrease V. Then the question is: Can moifying V an ecrease the conuction losses? To answer this question it is require to fin how much changes in an/or V will affect the conuction losses. >? < V 8 π π 8 earranging terms an assuming V 8 π >? < π 8 V f ( M cosφ) >? < 8 π f ( M cosφ) = π 8 M is the moulation inex which varies etween an 4/π (square wave operation) an cosφ is the power factor which varies etween an. The power factor of an inuction machine is always lagging; for this example calculation it is assume to e.9 at rate loa. 4 M an cosφ <. 9 π Then () (4) -78-74-7//$7. (C) EEE
.6 M cosφ < (5) π an f ( M cosφ) varies etween.787 (no-loa) an.5 (rate loa) as shown in Fig. 4. At first glance it might seem that ecause the V multiplier is larger than the multiplier at all Mcosφ values in Fig. 4 the V losses shoul always e higher. This oservation woul have een true if an only if V an the prouct were equal. This however is not the case an that is why all three of these variales are inclue in (4) to fin uner what conitions what part of the conuction losses is higher. The following example illustrates how to make use of (4). For a particular hyri electric vehicle traction rive the rate peak machine current is 6.8A which makes f ( M cos φ ) = 6.8.5 = 9. A. gnoring the off conition the minimum evice current is the magnetizing current which is 7A. uring the magnetizing current operation the phase angle is almost π/ raians an the power factor is almost zero then f ( M cos φ ) = 7.787 = 55. 9A Consiering (4) the following are some recommenations to maximize the efficiency of a SiC ioe in a traction rive application: V ) f 9.A > then the losses are higher at all times keep the oping ensity an constant ecause ecreasing means ecreasing BV which woul limit the evice s application. V ) f 55.9A < then the V losses are higher at all times ecrease the oping ensity so that V will e smaller. V V.5.5 V V > 55.9A < 9.A V losses are higher losses are higher V ) f 9.A < < 55. 9A then the average current of operation will etermine the recommene oping ensity as follows: a) A rive working close to its rate current value uses the conition V 9.A < V losses are higher ecrease the oping ensity so that V will e smaller. ) A rive working at light current loas uses the conition V < 55. 9A losses are higher keep the oping as it is ecause ecreasing means ecreasing BV which woul ecrease the voltage locking capaility of the evice. Fig. 5 isplays the aove statements on an - V plane. A commercial SiC Schottky ioe -V characteristics are otaine at ifferent temperatures. From these characteristics V an values of the ioe are calculate. These values are taulate in Tale an shown as a small rectangular area in f(mcosφ).9.8.7.6.5.4....6.4...8.6.4. multiplier V multiplier..4.6.8 Mcosφ (a).6/π.5 Tale..4.6.8 Mcosφ ().6/π..4.6.8...4.6.8. Ω Fig. 5. The variation of f(mcosφ) with Mcosφ (a) The enominator an the numerator of f(mcosφ) vs. Mcosφ () f(mcosφ) vs. Mcosφ. Fig 4. The V plane for the traction rive. -78-74-7//$7. (C) EEE
Fig. 5. Also shown in Tale is the corresponing V / ratios at ifferent operating temperatures. At temperatures up to an incluing 9 C the V / ratio is greater than 55.9A therefore V losses are higher. At the other temperatures the ratio is etween 9.A an 55.9A. The traction rive will operate close to the rate operation of the inuction machine; therefore consier the comparison with 9.A. For all the other temperatures the ratio is greater than 9.A; thus the V losses are higher again. As a conclusion for this case if the oping concentration N for the SiC ioes in this stuy is ecrease then V an the conuction losses ecrease. The limit of this ecrease is etermine y the V / ratio. Equation (4) can e use for any sinusoial PWM application as long as the operation current power factor an moulation inex information is availale. ). c power supply The conuction loss expression for the isolate full-rige c-c converter shown in Fig. is as follows: P con = TABLE SiC OE PWL MOEL PAAMETES AN V / ATO ( V ) (6) is the uty ratio of the ioe. Using the same approach as in the previous susection the ominant losses can e foun as follows: >? < V T oven C m Ω V V V / A 7 4..7 54 6 9.4.6 67 8..56 55 6 8.9.68 76 9..59 59 5.5.55 48 74.7.55 48.8.5 4 5..48 4 V >? <. (7) The significance of (7) can e summarize as follows: V ) f > then the resistive losses are higher keep the oping an constant ecause ecreasing means ecreasing BV which woul ecrease the voltage locking capaility of the evice. V ) f < then the V losses are higher ecrease the oping so that V will e smaller. For ifferent operation conition the amount of current passing through each evice an the voltage across them are calculate an the results are liste in Tale. Accoring to Tale varies etween 47A an 9A for a 5 kw c-c converter in the HEV simulation then applying the aove criteria V f 47 A > then the first criterion applies. V f 9 A < then the secon criterion applies. V f 47 A < < 9A then it epens on how close the magnitue of the current is to the minimum or maximum values for the majority of the time. For example if the average loa is varying or constant an is in a range etween.5 an 5 kw then the current is closer to the upper limit an the secon criterion applies. f on the other han the average loa is in a range etween an.5 kw then the current is closer to the lower limit an the first criterion applies. This criteria presente here can e applie to almost any c-c converter using SiC evices. B. Switching Loss Parameters The ioe switching losses occur ue to the reverse recovery of the ioe which is cause y the store charge in the epletion region. Schottky ioes are majority carrier evices so they o not have store charge. However they isplay a characteristic similar to reverse recovery ue to the ringing of the parasitics an the internal pn junction cause y the p-rings. The p-rings are use to reuce the large reverse leakage currents. For Schottky ioes the switching losses can e reuce either y reucing the parasitic elements or improving the reverse recovery characteristics of the pn junction forme y the p-rings. A ioe switching loss expression has een erive in [5] using Fig. 6: V F Strr Prr = f c (8) S t S f c is the switching frequency V is the reverse locking voltage F is the forwar ioe current S is the snappiness factor an t rr is the reverse recovery time. TABLE MAXMUM EVCE VOLTAGE AN CUENTS FO FFEENT LOA POWE AN NPUT VOLTAGE CONTONS P out (kw) V c (V) V MOSFET (V) MOSFET (A) V OE (V) OE (A) 6.67 84 47 45 45 4.44 84 47 5 6.67 84 9 5 45 45. 84 9-78-74-7//$7. (C) EEE
Turn-on loss Turn-off loss F everse recovery loss c-a region a- region Anoe p p p n - - F /t c t a t rr a t n() t n* Carrier ensity istriution t t n* n* x Fig. 6. Typical ioe switching waveform. V M -V (a) t o t t n this expression all the parameters except S an t rr are circuit epenent. These two parameters can e expresse [9] in other evice parameters for a pn ioe as follows W S = (9) W t = () rr n kt n is the electron iffusion constant ( = µ ) W n n q is the with of the rift region an is a istance in the rift region measure from the p n junction qa [ n() n* ] n = as shown in Fig. 7 n() is the carrier F ensity at the p n junction when the ioe is on an n* is the average carrier concentration in the n region. Gathering the S an t rr relate terms in (8) an inserting (9) an () the following is otaine: W 4 W St rr St rr n = = S S ( S ) 4W () 4 W 4 ( W ) = = n n Therefore ecreasing an/or W can ecrease the switching losses an can e ecrease y ecreasing the area an/or [ n( ) n* ]. Note that the conclusions here also apply to the c-c converter ecause (8) represents a switching cycle inepenent of the application. V. MOSFETS The following stuy will focus on the traction rive ut the conclusions erive can also e applie to the c-c converter. A. Conuction Loss Parameters i F The conuction loss expression of a MOSFET in a traction rive has een erive in [5] an it is repeate elow for convenience. P = M cosφ () con Q S on 8 π The only evice relate parameter in this expression is Son which can e represente y other evice parameters as follows 4BV = () S on on sp ε sµ Ec for a evice with cm area onsp is the specific on resistance of the MOSFET rift region an ε s E c an µ are material relate constants. Equation () is a rough estimate of a MOSFET resistance which also contains other resistive components like the channel resistance an the contact resistance. The rift resistance cannot e change much; however the channel an contact resistances can e lowere with more research. () Fig. 7. Carrier istriution in a ioe uring turn-off (a) Linearize carrier ensity istriution of a ioe at ifferent time instants () Linearize turn-off current waveform of the ioe. t -78-74-7//$7. (C) EEE
B. Switching Loss Parameters The energy loss equation of a MOSFET has een shown in [5] as follows V E = E E = ε E V (4) tot on off s c BV K K E ( V V ) ( V V ) g g m GH th K = an K = g m is the transconuctance is the current ensity V GH is highest gate voltage applie V GL is lowest gate voltage applie an V th is the threshol voltage f (4) is rearrange (5) is otaine. tot = ε E V s c V BV g m m th GL ( V V ) g ( V V ) GH (5) The most important parameter contriuting to the MOSFET switching energy loss is the transconuctance g m. This parameter can e represente as follows [] w w A ox g = µ C V = µ V m ox ox l l ε t (6) ox µ is the moility w is the channel with l is the channel length C ox is the oxie capacitance V is the rain voltage ε ox is the oxie ielectric constant t ox is the oxie thickness an A ox is the oxie area. n (6) µ an ε ox are material epenent; therefore for a specific application four evice parameters affect the transconuctance w A ox l an t ox. The first two of these parameters are irectly proportional to g m an the others are inirectly proportional to it. From (6) the following statements can e erive: ecreasing t ox increases g m ut t ox has to e of a minimum thickness to e ale to support the rate gate voltage; therefore it cannot e change much. ecreasing l increases g m ut the value of l is limite y the evice processing technology. ncreasing A ox increases g m ut A ox epens on the evice area; it cannot e aritrarily increases without some ifficulty. th m th GL ncreasing w increases g m. To increase w the evice area has to e increase proportionally. As a summary to ecrease the MOSFET switching losses g m nees to e increase. ncreasing the evice s area an consequently increasing A ox an w seem to e the est metho to o this. V. CONCLUSONS n this paper losses of the evices in a traction rive are investigate as functions of evice parameters. Some moifications to evice parameters are suggeste to improve the losses in this rive. The next step is for evice researchers to consier these suggestions an evaluate the viaility of these moifications. The interaction of evice an power electronics researchers will e extremely useful in proucing application specific power evices esigne for optimum performance. This stuy is the first step to achieving this goal. EFEENCES [] M. Bhatnagar an B.. Baliga Comparison of 6H-SiC C-SiC an Si for power evices EEE Trans. on Electron evices vol. 4 no. March 99 pp. 645 655. [] K. Shenai. S. Scott an B.. Baliga Optimum semiconuctors for high power electronics EEE Transactions on Electron evices vol. 4 no. 9 Sept. 989 pp. 8 8. [] A. Elasser M. Kheraluwala M. Ghezzo. Steigerwal N. Krishnamurthy. Kretchmer an T. P. Chow A comparative evaluation of new silicon carie ioes an state-of-the-art silicon ioes for power electronic applications EEE AS Annual Meeting Conference Proceeings 999 pp. 4 45. [4] A.. Hefner. Berning. S. Lai C. Liu an. Singh Silicon Carie merge PiN Schottky ioe switching characteristics an evaluation for power supply applications Proceeings of the Annual Meeting of the EEE nustry Applications Society pp. 948-954. [5] B. Ozpineci L. M. Tolert S. K. slam an M. Hasanuzzaman Effects of silicon carie (SiC) power evices on PWM inverter losses The Annual Conference of the EEE nustrial Electronics Society (ECON') pp. 87 9. [6] B. Ozpineci L. M. Tolert S. K. slam an F. Z. Peng "Testing characterization an moeling of SiC ioes for transportation applications " EEE Power Electronics Specialists Conference (PESC') une -7. [7] B. Ozpineci L. M. Tolert S. K. slam an M. Hasanuzzaman "System impact of silicon carie (SiC) power evices" nternational ournal of High Spee Electronics an Systems in press [8] B. Ozpineci System impact of silicon carie power electronics on hyri electric vehicle applications August [9] B.. Baliga Power Semiconuctor evices PWS Pulishing Company Boston 996. []. A. Grant an. Gowar Power MOSFETS-Theory an Applications ohn Wiley & Sons New York 989. -78-74-7//$7. (C) EEE