SiC and GaN Power Transistors Switching Energy Evaluation in Hard and Soft Switching Conditions

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1 SiC and GaN Power Transisors Swiching Energy Evaluaion in Hard and Sof Swiching Condiions Ke Li, Paul Evans, Mark Johnson Power Elecronics, Machine and Conrol group Universiy of Noingham, UK Absrac SiC and GaN power ransisors swiching energy are compared in his paper. In order o compare swiching energy E sw of he same power raing device, a heoreical analysis is given o compare SiC device conducion loss and swiching losses change when device maximal blocking volage reduces by half. Afer ha, E sw of a 65V is measured in hard swiching condiion and is compared wih ha of a 12V SiC- MOSFET and a 65V SiC-MOSFET wih he same curren raing, in which i is shown ha E sw of a is smaller han a 12V SiC-MOSFET, which is smaller han 65V SiC- MOSFET. Following by ha, in order o reduce device urn- ON swiching energy, a zero volage swiching circui is used o evaluae all he devices. Device oupu capaciance sored energy E oss are measured and urn-off swiching losses are obained by subracing E oss, which shows ha is sill beer han SiC device in erms of swiching losses and 12V SiC- MOSFET has smaller swiching losses han 65V SiC-MOSFET. Keywords Wide bandgap power semiconducor device; GaN- HEMT; SiC-MOSFET; Swiching energy; hard swiching; sof swiching I. INTRODUCTION Wide bandgap power semiconducor devices like silicon carbide (SiC) and gallium niride (GaN) are able o operae in high emperaure, high frequency and realize high energy conversion efficiency, so hey are gradually applied in power elecronics sysems for elecrical power conversion. The abiliy of blocking volage of he commercial SiC ransisors such as JFET and MOSFET is bigger han 1.2kV, while ha of commercial GaN ransisors such as HEMT and GIT is smaller han 65V. Curren raings of boh SiC and GaN devices can be found in a wide range from a few amperes o a few ens of amperes. Depending on elecrical power requiremens, SiC and GaN power devices can be applied in differen power elecronics sysems such as baery charger, swiching mode power supply, elecrical moor drive. I is illusraed in Fig. 1 he repored applicaion of SiC and GaN devices in power elecronics sysems in lieraure [1] [6], where all devices are in hard swiching mode when convering elecrical power. SiC and GaN devices are compared in erms of power raing, efficiency and swiching frequency. I is shown ha boh SiC and GaN devices can realize high efficiency power conversion. SiC devices are applied for above 1kW, below 5kHz power conversion while GaN devices are applied for below 1kW, above 5kHz conversion. KTH 212 SiC JFET (12V) 1kHz,2kW,.99 APEI 214 SiC MOSFET (12V) 2kHz,6.1kW,.96 CPES 215 GaN HEMT (65V) 5kHz,6W,.98 KTH 214 SiC BJT (12V) 25kHz,6kW,.98 LAPLACE 213 GaN HEMT (1V) 5kHz,35W,.98 CPES 213 GaN HEMT (4V) 5MHz,12W,.88 Fig. 1: Comparison of SiC and GaN devices in erms of swiching frequency, power raing and efficiency Due o he dismach of device power raing, here are few publicaions abou experimenal comparison beween SiC and GaN power semiconducor devices on swiching energy. The objecive of his paper is a firs o heoreically analyze how losses of SiC power devices change when blocking volage reduces from 12V o 6V and hen experimenally compare swiching losses of a commercial 12V SiC- MOSFET (C2M812D, 12V/36A), 65V SiC-MOSFET (SC12AF, 65V/29A) and 65V (GS6658P, 65V/3A) wih similar curren raing in differen hard swiching condiions, which would be helpful for engineers o choose wide bandgap power devices when designing power elecronics sysems. As shown in [6], [7], zero volage swiching (ZVS) is an efficien way o furher reduce device swiching losses, hus all he above devices are all evaluaed in sof swiching condiion as well. The paper is srucured wih following secions: a firs, heoreical comparison of conducion loss and swiching losses of SiC-MOSFET when reducing blocking volage is analyzed. Aferwards, based on an opimized swiching circui of each device, swiching energy of all he aforemenioned devices is measured and compared in boh hard and sof swiching condiions. Conclusions are given a las. II. THEORETICAL ANALYSIS A. Conducion loss comparison The srucure of a MOSFET is shown in Fig. 2, where i is shown ha device ON-sae resisance R ON is mainly consiued by device channel resisance R ch and drif region resisance R drif.

2 P Source W D N- N+ Gae Oxide N+ N+ C gd L ch W cell Drain R ch Drif region R drif C gs WS Source C ds Fig. 2: Srucure of a MOSFET Muliplying by device acive area A, he relaion of each specific resisance (mω mm 2 ) is expressed by he following equaion: R ON,sp = R ch,sp + R drif,sp (1) where device specific drif region resisance R drif,sp can be expressed by a funcion of device maximal blocking volage V DSS, maerial permiiviy ɛ, carrier mobiliy in he drif region µ and criical elecrical field E c as given in [8]: R drif,sp = 4V DSS 2 ɛµe c 2 (2) Shown in [8], he minimal R drif,sp is a funcion of V DSS 2.5. Following relaion is obained if one compares R drif,sp value of a 6V device and a 12V device. R drif,sp,6v R drif,sp,12v = W D,6V W D,12V Device R drif,sp is also proporional o device drif region hickness W D. Thus, device W D raio of a 6V device and a 12V device follow he same relaion as eq.(3). Device specific channel resisance R ch,sp can be approximaely expressed by a funcion of device channel lengh L ch, uni cell widh W cell, channel mobiliy µ ch and accumulaed charge in he channel Q ch as shown in [9]: P (3) R ch,sp = L ch W cell µ ch Q ch (4) According o he resuls presened by auhors in [1], R ch,sp varies lile on V DSS volage. By applying he parameers given by auhors in [11] for a 12V SiC-MOSFET, R ch,sp is found o be abou 4% of he oal R ON,sp of a 12V device. Thus, by combining he above equaions, following equaion can be obained, which shows ha R ON,sp of a 6V device is abou a half of he value of a 12V device. R ON,sp,6V R ON,sp,12V 1 2 Aferwards, device swiching losses are compared in he nex subsecion. (5) V DS I D V GS V pl I S V com V S P V S I S Fig. 3: Ideal swiching waveforms and swiching losses calculaion B. Swiching loss comparison I is illusraed in Fig. 2 he SiC-MOSFET iner-elecrode capaciances C gd, C ds and C gs beween each erminal. Differen like C gs, C gd and C ds are V DS volage dependen capaciances and heir values can be approximaely calculaed by he following equaion: C x = ɛ A x W S (6) where C x refers o eiher C gd or C ds, A x refers o he acive area of each capaciance and W S is depleion region hickness, which is dependen on device swiching volage V S. Acive area of C gd and C ds is obained by muliplying a coefficien b o he device area A. The relaion beween W D and W S is obained by he following equaion given in [8]: W S = W D VS V DSS (7) By combining eq.(6) and eq.(7), sored charge Q x of each capacior during swiching can be obained by: Qx d qx = VS C x d vs Q x = 2b ɛ A W D V DSS V S Thus, he comparison of specific charge (Q x,sp ) beween 6V and 12V device can be obained by following equaion: (8) Q x,sp,6v Q x,sp,12v =.7 WD,12V W D,6V (9) By combining eq.(3) and eq.(9) ogeher, i is shown in he following equaion ha in conrary o device R ON,sp, 6V device Q x,sp is four imes bigger han 12V device. Q x,sp,6v Q x,sp,12v 4 (1) Power ransisor ideal swiching waveforms (device swiching a volage V S and curren I S ) is shown in Fig. 3, where ransisor gae-drain charge Q gd plays an imporan role in device swiching, because is discharge and charge ime by

3 gae curren I g influence on device swiching losses. Thus, device swiching losses E sw of one period can be approximaely calculaed by he following equaion: Q gd E sw = V S I S = V S I S Qgd = V S I S R g (11) I g V com V pl where R g is gae resisor, V com is conrolled gae volage and V pl is Miller-plae volage. I is o be noed ha device oupu capaciance C oss sored energy E oss would be dissipaed during urn-on swiching and E oss would be recovered during urn-off swiching. By adding E oss in urn-on swiching and subracing i from urn-off swiching, eq.(11) can be sill used o esimae device oal swiching loss. I is shown in his equaion ha E sw is proporional o Q gd. By dividing device surface, 6V and 12V device specific swiching loss E sw,sp can be compared by he following equaion: E sw,sp,6v E sw,sp,12v 4 (12) Subsequenly, swiching losses of a 6V and a 12V device wih he same curren raing are compared in he nex subsecion. C. 6V/12V device comparison Device maximal conducion curren I D is limied by is hea dissipaion, which is calculaed by he following equaion: I D2 R ON R h = T j(max) (13) A firs condiion, only device hermal resisance R h of die is considered (wihou influence of packaging), which is deermined by device hickness (which is supposed o be device drif region hickness W D for a ransisor), acive area A and maerial hermal conduciviy k. Thus, R h = W D (14) k A By combining eq.(13) and eq.(14), following equaion can be obained: A I D = T j(max) k RON,sp W (15) D If 6V and 12V device has he same curren conducion capabiliy, he comparison of device area A can be expressed by he following equaion by combining eq.(3) and eq.(5) ogeher: A 6V RON,sp,6V W D,6V = A 12V RON,sp,12V W.3 (16) D,12V By combing eq.(12) wih eq.(16), following equaion can be obained if swiching losses of a 12V device and a 6V device are compared. E sw,12v E sw,6v = 2 W D,6V W D,12V.83 (17) A second condiion, if he influence of he packaging is considered, R h is mainly dependen on device packaging ype, where device area and hickness has lile influence on R h value [12]. Thus, R ON of 6V and 12V device should be he same if boh devices have same curren raing. I is hen found ha A 12V should be wice as big as A 6V, so he following swiching loss relaion can be obained. E sw,12v E sw,6v.5 (18) I is shown in eq.(17) and eq.(18) ha swiching loss of a 6V device can be superior o a 12V device wih he same curren raing in boh condiions. SiC and GaN power devices will be experimenally compared in he nex secion in order o evaluae heir performance. A. Swiching circui III. EXPERIMENTAL VALIDATIONS Swiching energy of a 12V/36A SiC-MOSFET (C2M812D), a 65/29A SiC-MOSFET (SC12AF) and a 65V/3A (GS6658P) is evaluaed and compared in his secion. The swiching circui o es all he devices is shown in Fig. 4a. Gae loop and swiching loop of each device is minimized in order o reduce gae loop inducance and swiching loop inducance L para,sw. The realizaion prooype o es 12V SiC-MOSFET is shown in Fig. 4b, where he package of he device is TO-247. Device is driven by a gae volage from -5V o 2V. is obained by measuring gae volage resonance frequency, which is supposed o be resonaed beween and device inpu capaciance C iss. in he prooype is abou 2nH. The realizaion prooype o es 65V SiC-MOSFET is similar as ha shown in Fig. 4b, excep he package of he device is TO-22. Device is driven by he same gae volage as 12V SiC-MOSFET. The measured in he prooype is also abou 2nH. The realizaion prooype o es 65V is shown in Fig. 4c, where he package of he device is GaNPX. Device is driven by a gae volage from V o 7V while he measured in he prooype is abou 3nH. The used gae driver in all he above design is IXDN69SI and lumped gae resisance in all he measuremens are abou 4Ω including device inernal gae resisance, exernal gae loop resisance and gae driver oupu resisance. Device swiching curren I D is measured by a curren shun (SSDN , 1.2GHz) while he swiching volage V DS is measured by a differenial volage probe (TA43, 7V/1MHz). The bandwidh of he used oscilloscope is 1.5GHz. The device is a firs esed in hard swiching condiions by double pulse. B. Comparison of device hard swiching The measured swiching waveforms when all he device swiching a V DS = 3V, I D = 1A are shown in Fig. 5. As shown in he resuls, 12V SiC-MOSFET swiches faser han 65V SiC-MOSFET in urn-off swiching, which is supposed o be ha ransfer capaciance C rss of 12V device

4 Swiching mesh L para,sw R g V C R bus g L para,sw Load V DS I D (a) Elecrical circui of he swiching circui (Driver circui a boom side) Load C bus (b) Swiching circui of SiC- MOSFET V DS (V) Capaciance C(pF) I D (A) V SiC-MOSFET 65V SiC-MOSFET (a) Turn-on Load C bus Swiching mesh (c) Swiching circui of Fig. 4: Elecrical circui of he swiching circui and heir realizaion is bigger han 65V device in high volage. This can be also confirmed when comparing device iner-elecrode capaciance daashee values as illusraed in Fig. 6, where i is shown ha C rss of 12V SiC-MOSFET is smaller han 6V SiC- MOSFET when blocking volage superior o 2V. In erms of urn-on swiching, curren ransiion ime for boh 12V and 65V SiC-MOSFET is quie similar while measured di/d of 12V device is higher han 65V device, which suggess a similar inpu capaciance C iss of boh device. Again, i is found in he daashee value ha C iss of boh device is around 2nF and ha of 12V device is slighly smaller han 65V device. When comparing wih 65V, i is shown ha swiches faser han 12V SiC-MOSFET in urn-on swiching, which confirms he daashee value ha C iss of is only abou one sixh of 12V SiC- MOSFET. In erms of urn-off swiching, no obvious difference observed beween 12V SiC-MOSFET and GaN- HEMT. This is supposed o be following wo facs: one is ha is urned OFF by V while SiC-MOSFET is urned OFF by -5V. If were urned OFF by negaive volage, he swiching ransiion migh be even shorer. Anoher fac is he fases fall ime of he gae driver is abou 7ns shown in is daashee. If a faser gae driver were used o drive, urn-off ransiion can be also acceleraed because device has much smaller C rss and C iss values han 12V SiC-MOSFET. The swiching energy comparison resuls of differen swiching condiions are shown in Fig. 7 for all he aforemenioned devices. As i is in hard swiching condiion, measured I D urn-on curren excludes C oss discharge curren V DS (V) I D (A) V SiC-MOSFET 65V SiC-MOSFET (b) Turn-off Fig. 5: Swiching waveforms comparison among 12V SiC- JFET, 65V SiC-MOSFET and 65V in hard swiching SiC-MOSFET (12V) SiC-MOSFET (65V) C iss Coss Crss Volage Vds(V) Fig. 6: Device iner-elecrode capaciance comparison and I D urn-off curren includes C oss charge curren. Thus, dissipaed C oss energy E oss is no included in device urn- ON energy and E oss is included in device urn-off energy. As shown in he resuls, 65V SiC-MOSFET has bigger swiching energy han 12V SiC-MOSFET, which confirms he analyical analysis. Thus, swiching losses of a 12V SiC- MOSFET is smaller han 65V device wih he same curren raing. Swiching energy of is smaller han ha of 12V SiC-MOSFET. However, a some swiching condiions,

5 Swiching Energy (µj) Swiching Energy (µj) Swiching Volage V DS =2V ON OFF 12V SiC-MOSFET ON 12V SiC-MOSFET OFF 65V SiC-MOSFET ON 65V SiC-MOSFET OFF Swiching Curren (A) Swiching Volage V DS =3V Swiching Curren (A) V in C bus V com1 V com2 Lpara,sw V com2 V com1 L para,sw R g R g I D, I D, (a) Elecrical circui I L L C ou V ou Fig. 7: Swiching energy including C oss sored energy comparison among 12V SiC-JFET, 65V SiC-MOSFET and 65V in hard swiching measured swiching energy for he wo devices are quie close. This is because of he volage drop across L para,sw during he rise of I D, hus i helps o decrease I D and V DS overlapping ime. Obained apparen swiching energy of SiC-MOSFET is decreased because of he snubber effec of L para,sw. L para,sw can be esimaed based on he observed LC resonance frequency of he swiching waveform a he end of he swiching, which is supposed o be he resonance beween device C oss and L para,sw. I is hus found L para,sw of swiching circui is abou 36nH while ha of SiC-MOSFET is abou 8nH, so V DS volage drop of SiC- MOSFET is wice han ha of for he same di/d urn-on swiching rae. According o he swiching energy comparison in his subsecion, i is concluded ha is more suiable han SiC-MOSFET for below 3V elecrical energy conversion because of less swiching losses. However, for boh GaN- HEMT and 12V SiC-MOSFET, i is found in Fig. 7 ha device urn-on loss is bigger han urn-off loss, hus i is helpful o apply hese devices in sof swiching o reduce urn- ON loss. C. Comparison of device sof swiching The circui using o analyse device sof swiching is illusraed in Fig. 9a, where volage of he oupu capacior C ou is mainained consan by using an exernal power supply. A insan 1, device is swiched ON and load inducor sars o be charged by V in V ou. A insan 2, is swiched OFF in hard swiching condiion. Simulaneously, device sored energy in is C oss is ransfered by load curren I L o C oss of device. Afer deadime d, device is swiched ON a zero volage swiching (ZVS). Aferwards, Load inducor is reversely charged by V ou and i changes he direcion. A insan 3, is swiched OFF in hard swiching condiion and simulaneously, device sored energy in is C oss is ransfered by load curren I L o C oss of. Afer deadime, is swiched ON a ZVS and finally i is swiched OFF a hard swiching condiion a insan 4. I L d 3 3+d 4 (b) Conrol signals wih load curren waveform Fig. 8: Elecrical circui using o es device in sof swiching and load curren waveform Thus, by changing I L direcion, device and urn-on losses can be avoided and boh device only have urn-off losses which is due o he overlapping of V DS and I D. Sored energy in is ransfered o before swiching ON and vice versa. Device oal swiching losses can be grealy reduced in his curren mode. When V in = 3V and V ou = 5V, he measured 12V SiC-MOSFET swiching waveforms are shown in Fig. 9a. A insan, device is swiched OFF in hard swiching and sored energy in C oss of device is ransfered o. By muliplying measured curren and volage of each device, swiching power waveforms are obained and shown in Fig. 9b. Thus, urn-off energy including E oss and E oss can be obained separaely. Device urn-off swiching loss due o he overlapping of curren and volage waveforms can be obained. In he waveform shown, i is found ha E oss of 12V device is abou 4.4µJ when device is wihsanding 3V V DS. Oupu capaciance sored energy E oss of he aforemenioned devices is hen measured by his mehod and hey are compared in Fig. 1. As shown in he resuls, he measured E oss of all he devices is close o each oher, which confirms ha all devices have close C oss values as shown in Fig. 6. According o hese resuls, even hough E oss of 65V GaN- HEMT is only measured a 1V, is value would be close o ha of 65V SiC-MOSFET when i swiches a 2V, 25V and 3V. By subracing E oss from device urn-off energy as shown in Fig. 7, 65V SiC-MOSFET sill has bigger swiching losses han 12V SiC-MOSFET and 65V is

6 Eoss (µj) Power (W) Volage (V) Curren (A) I D, I D, I L V DS, V DS, ime (µs) ime (µs) (a) Device swiching waveforms Eoss urn-off energy ime (ns) (b) Device urn-off energy and sored energy when i swiches a 3V/15A in sof swiching Fig. 9: Measured 12V SiC-MOSFET swiching waveforms and sored energy V SiC-MOSFET 12V SiC-MOSFET Swiching Volage (V) Fig. 1: Device sored energy E oss comparison among 12V SiC-JFET, 65V SiC-MOSFET and 65V sill more efficien han 12V SiC-MOSFET in his swiching mode. IV. CONCLUSION SiC and GaN power semiconducor devices swiching energy are compared in he paper. In order o compare swiching energy of devices wih he same power raing, a heoreical analysis is given o show ha specific ON-sae resisance of SiC power ransisors will reduce half if device maximal blocking volage decreases half. In conrary o ha, device specific capaciance value would increase. Thus, when comparing device wih same curren raing, swiching losses would increase in a 6V SiC device han 12V device. In order o validae he heoreical analysis, swiching energy of a 65V/29A SiC-MOSFET is compared wih a 12V/3A SiC-MOSFET in hard swiching mode. Measuremen resuls show ha swiching energy, especially urn-off energy of 65V SiC-MOSFET is bigger han 12V device, which confirms he heoreical analysis. By comparing wih a 65V/3A, i is found ha swiching energy of is smaller han 12V SiC-MOSFET. I is also shown in he resuls ha device urn-on energy are bigger han urn-off energy for boh 12V SiC-MOSFET and 65V. In order o reduce device urn-on energy, a zero volage swiching circui is used o evaluae device in sof swiching mode. Device oupu capaciance sored energy E oss can hus be measured, so device urn-off losses due o overlapping of swiching curren and volage can be obained. By subracing E oss, i is shown ha is sill beer han 12V SiC-MOSFET, which is beer han 65V SiC-MOSFET in erms of he swiching losses in his swiching mode. Based on all he resuls, i can be concluded ha 12V SiC-MOSFET has smaller swiching losses han 6V device and is more suiable han SiC device o be applied in below 3V energy conversion. REFERENCES [1] J. Rabkowski, D. Pefisis, and H.-P. Nee, Parallel-Operaion of Discree SiC BJTs in a 6-kW/25-kHz DC/DC Boos Converer, Power Elecronics, IEEE Transacions on, vol. 29, no. 5, pp , 214. [2] J. Rabkowski, D. Pefisis, and H.-P. Nee, Design seps owards a 4- kva SiC inverer wih an efficiency exceeding 99.5%, in Applied Power Elecronics Conference and Exposiion (APEC), 212 Tweny-Sevenh Annual IEEE, pp , Feb 212. [3] J. Brandelero, B. Cougo, T. Meynard, and N. Videau, A non-inrusive mehod for measuring swiching losses of GaN power ransisors, in Indusrial Elecronics Sociey, IECON h Annual Conference of he IEEE, pp , Nov 213. [4] B. Whiaker, A. Barkley, Z. Cole, and B. Passmore e al., A High- Densiy, High-Efficiency, Isolaed On-Board Vehicle Baery Charger Uilizing Silicon Carbide Power Devices, Power Elecronics, IEEE Transacions on, vol. 29, no. 5, pp , 214. [5] S. Ji, D. Reusch, and F. Lee, High-Frequency High Power Densiy 3-D Inegraed Gallium-Niride-Based Poin of Load Module Design, Power Elecronics, IEEE Transacions on, vol. 28, no. 9, pp , 213. [6] X. Huang, Z. Liu, F. C. Lee, and Q. Li, Characerizaion and Enhancemen of High-Volage Cascode GaN Devices, IEEE Transacions on Elecron Devices, vol. 62, pp , Feb 215. [7] M. Kasper, R. M. Burkar, G. Deboy, and J. W. Kolar, Zvs of power mosfes revisied, IEEE Transacions on Power Elecronics, vol. 31, pp , Dec 216. [8] J. Luz, H. Schlangenoo, U. Scheuermann, and R. De Doncker, Semiconducor Power Devices. Springer, 211. [9] B. Baliga, Advanced High Volage Power Device Conceps. Springer, 211. [1] D. Ueda, H. Takagi, and G. Kano, A new verical power MOSFET srucure wih exremely reduced on-resisance, IEEE Transacions on Elecron Devices, vol. 32, pp. 2 6, Jan [11] M. Ruff, H. Milehner, and R. Helbig, SiC devices: physics and numerical simulaion, IEEE Transacions on Elecron Devices, vol. 41, pp , Jun [12] H. Wu, M. Chen, L. Gao, and M. Li, Thermal resisance analysis by numerical mehod for power device packaging, in Elecronic Packaging Technology and High Densiy Packaging (ICEPT-HDP), h Inernaional Conference on, pp , Aug 212.