High-Speed Gate Driver Using GaN HEMTs for 20-MHz Hard Switching of SiC MOSFETs

High-Speed Gate Dr iver Using GaN HEMT s for 20 -MHz Hard Switching of SiC MOSFET s T akaf umi Okuda and T akashi Hikihara Department of Electrical E ngineering, Kyoto U niversity , Ka tsura, Nishikyo, Kyoto 615 -8510 Email address: t -okuda@dove.kuee.k yoto-u.ac.jp In this paper, we investigated a gate driver using a GaN HEMT push-pull con figuration for the high-frequency hard switching of a SiC po wer MOSFET . Lo w on-resistance and low i nput cap acitance of GaN HEMT s ar e suitable for a high-frequency gate driver from the logic level , a nd ro bustness o f SiC MOSFET with high avalanche capability i s suitable for a valve transistor in power converters . Our proposed gate driver consists o f digital isola tors, co mplementar y Si MOSFET s, and GaN HEMT s. T he GaN HEMT push-pull stage has a hig h driving cap ability owing to its s uperior switching characteri stics, and complementary Si MO SFET s can en hance the control signa l fro m the digital isolator . W e investigated limiting factors of the switching frequency of the p roposed gate driver by focusing on each circuit component a nd prop osed an improved driving configuratio n for the gate driver . As a r esult, 20 -MHz hard switching of a SiC MOSFET was achieved using the improved gate driver with GaN HEMT s. I. INTRODUCT ION W idegap se miconductor materials such a s silicon carbide (SiC) and galliu m nitride (Ga N) have high critical electri c field stre ngth, which is attractive for hi gh-voltage p ower devices [1-3]. In SiC, hi gh-quality bulk cr ystals with a large diameter (~6 inch) can be obtai ned with subli mation method , and SiC homoepitaxial lay ers can be gro wn o n S iC bul k crystals with chemical vapor dep osition (CVD) [4 ,5]. Thus, a vertical structure is typically utilized for SiC po wer devices. SiC metal-oxide-se miconductor field-effect transistors ( MOSFET s) with a high bloc king voltage (> 1 kV) a nd low on -resistance (< 100 mΩ cm 2 ) have been rep orted [6 -9]. SiC MOSFET s also exhibit a su ff icient avalanche capabilit y [10,1 1] , which is an advantage as a valve transistor for hi gh-voltage a nd high-current po wer converters. In GaN , even though no mass pro duction o f vertical power d evices was achieved due to a lack o f high-quality and large GaN bulk cr ystals, the hetero epitaxial growth o f the mixed allo y in the III -nitrides is available on a Si substrate [ 12 , 13 ] . T wo-dimensional ele ctron gas (2DEG) is generated at a heteroj unction o f AlGaN /GaN, and GaN -based high electron mobility tr ansistors ( HEMT s ) have bee n develop ed [ 14 - 18 ]. Although GaN HE MT s still have poor avalanche capability , t heir on -resistanc e and inp ut capacitance are lower than t hose of M OS-based devices, which enabl e high-frequency oper ation with low power consumption. Since SiC MOS FET s are suitab le for valve transistors i n p ower converters a nd GaN HEMT s are applicab le to gate drivers with high dr iving capab ility , we pr eviously propo sed a high -speed gate driver (P rototype-A) based on a GaN -HEMT push-pull con figuration for driving a Si C MOSFET [ 19 ]. By usi ng the pro posed gate driver, 10-MHz hard switching of SiC MOSFETs was obtained . Although there ar e other reports on gate driver s using GaN HEMT s [ 20 - 23 ] , the driving target is GaN -HEMT valve s witches or soft- switching circ uits such as resonant con verters. In this study , on the other hand, we foc us on the high-frequency hard s witching o f SiC MO SFET s because power con verters with har d switching are suitable for handling various kinds of loads . W e enhance the switch ing fre quency up to ISM radio bands (T ype-B w orld wide: 13.56 or 27.12 MHz) that are reserved internationally for radio frequenc y energy . A high-speed gat e driver with a high drivi ng capability is necessar y to drive SiC MOSFET s at high switching frequenc y . In this stud y , we i nvestigated the limitin g factors of the switchi ng freque ncy in the proposed gate driver a nd ac hieve 20 -MHz hard s witching of SiC MOSFETs with an i mproved gate d river . The 20 -MHz frequency (2 0.34 MHz) corresponds to the third harmonic of 6 .87 MHz that is al so reserved for T ype- A I SM radio band s. II. CONFIGURA TION OF GA TE DRIVER A sc hematic of our fabricated gate driver is s hown in Fig. 1. T he GaN -HEMT push-pull configuration is e mployed for the driving stage. T he dual control si gnals are generated with a function generator (T ektronix, AFG310 2C) and isolated by d igital i solators (Silico n Labs, Si8610 ). T he digital is olator, which consists of an RF trans mitter and receiver fo r isolation, is commonly used for digital signal transmission to isolate the transmission lin e. T he output signal s fro m t he digital isolators are enhanced through complementar y Si MOSFET s (ROHM, US6M1) and finally co nnected to the GaN -HEMT push-p ull stage. T he GaN HEMT s are d istributed from ROHM for Protot ype-A and EP C (EP C2014C) fo r Proto type-B, both of which are fabricated as normally -off (enhancement-mode) transistors. Four isolated f loating voltage sources (Matsusad a, P 4K36 -1) we re used for the p roposed gate driver . Inp ut po wer supply V dsig for the digital isolators was set at 5 V , and an output power suppl y for the digital isolato rs at the high and low sides ( V dSiH and V dSiL ) was e mployed. V dSiH and V dSiL also s upply po wer to the co mplementar y Si MOSFET s at the high and low sides . Here V dSiH and V dSiL we re varied f ro m 3 to 5 V to inv estigate t he sw itching characteristics of the complementary Si MOS FET s. V dGan , which supplies a driving voltage at t he GaN -HEMT push-pull config uration , is fixed at 18 V to drive the SiC MOSFET s. Note that a bo otstrap capacito r is not employed for V dSiH in this study . According to our previous study [ 19 ] , a high-side voltage source V dSiH w as generated by a b ootstrap capacitor from t he low-si de voltage source V dSiL , which simplifies the voltage supply at the hi gh side. However, the forward voltage drop of the bootstrap dio de is 0.8-1.0 V , and the high -side suppl y voltage is lower t han the low-side. As d escribed below , the suppl y voltage for the compleme ntary Si MOSFET s strongly affects t heir switching characteristics. I n ord er to overco me the upper li mit of the driving frequency in our propo sed gate driver , we removed bootstrap capac itor for the high -side voltage suppl y and used another isolated floating voltage suppl y V dSiL to balance the suppl y voltage of the high -side and lo w-side Si MOSFET s. The fabricated gate d rivers are sho wn in Fig. 2. P rototype -A, which was fabricated in our p revious study [ 19 ] , has test terminals on the b oard, but the parasitic inductance increases with extra circuit patter ns. In order to reduce the parasitic inductance, in t his stud y we fabricated a small-sized gate driver, Prototype-B. In Section III, P rototype-A is investigated to clarify the limi ting factors o f the hig h-frequency operatio n using their test ter minals. In S ection IV , we use Prototype - B Si 8610 Is o l ation V sig L V g SiL V g Ga n L Si 8610 Is o l ati on V sig H V g SiH V g Ga n H V gs GaN H EMT Si MOSFET s D igit al I s olat or V d SiH V d SiL V d Ga n V d sig V d sig Fig. 1 Schematic o f fabricated gate driver us ing GaN HEMT push -pull con figuration. to achieve 20-MHz hard switchi ng of SiC MOSFET s and measure the switching wave forms by an oscilloscope (T ektro nix, MDO4104) with a passive voltage prob e (T ektron ix, TPP1000 ) and a current probe (T ektronix, TCP0 030). III. INVE STIGA TION OF LIMIT ING F ACTORS OF HIG H-FREQUENCY OPERA TION In this section, we investigate the limiting factors of high-freque ncy operation i n our propo sed gate driver at frequencies exceedi ng 10 MHz by focusin g on each circuit component, t he digital isol ators, the Si MOSFET s, and the GaN HEMT s. A. Propagation Characteristic s of Digital Isolators W e investigated the p ro p agation characteristic s of digital is olators Si8610 and measured the switchin g behaviors at the low-side di gital isolator o f t he gate driver . The outp ut characteristics o f the digital i solator ar e shown in Fig. 3. The switching frequenc y was se t at 1 or 20 MHz, and supply voltage V dSiL for the digital isolat or was set at 3 .5 V . The digital isolator w orked at 1 MHz, and a clear outpu t sign al V gSiL w as obtained, w hereas output s ignal V gSiL w as strongly degraded at 20 MHz owing to the p oor propagation capability o f the digital isolator at high frequency . The output voltage of the on-state mode decreased to 2.5 V , and the output voltage of the off-state mode increased to 1.2 V . A lthough the dat a rate of the digital isolator is 150 Mbps according to the d atasheet, its d riving capability of the digital isolator is insuffient to drive the co mplementary Si MOSFET s at 20 MHz. The pr opagation dela y at the digital isolator is appro ximately 8 ns according to the datasheet . This value is consistent with the d elay obtained by the experi mental result s hown in Fig. 3. [ P r otot ype- A] [ Prot otype- B] Fig.2 Photograph of fabricate d gate drivers : Proto type-A a nd Prototype- B. Since the output si gnal from the digital isolator was degraded at high s witching frequenc y , the outp ut signal fro m the digital isolator must be enhanced . This is why compleme ntary Si MOSFET s are employed in the next drivi ng stage . T he output characteristics of t he Si MOSFET s at 1 MHz are shown in Fig. 4. Suppl y voltage V dSiL for the Si MOSFET s varied from 3 to 5 V . T he ir output voltage e xhibited fast er switc hing behaviors than the digital isolator, suggestin g that t he driving capabilit y is enhanced through complementar y Si MOSFET s. B. Limitation of Source V oltage for Si MOS FET s Si MOSFET s ca n en hance th e co ntrol signal fro m the digital isolator, but supp ly volta ge V dSiL for Si MOSFET s is complicated, as described belo w . A higher suppl y volta ge would enhance the dr iving capability , while the penetration current through the complementar y Si MOSFET s increase s with add itional suppl y volta ge. Hence, an ap propriate supply condition is required to avoid the thermal runaway induced by the penetration current at h igh switching freque ncy . 0 2 4 6 Sign al (V) V sigL 0 500 1000 0 2 4 6 Voltag e (V) Ti me (n s) V gS iL 0 2 4 6 Sign al (V) V sigL 0 20 40 60 80 0 2 4 6 Voltag e (V) Ti me (n s) V gS iL Prototype -A: 1 MHz 20 MH z (a) (b) 0 2 4 6 Sig nal (V) V sig L 0 200 400 600 800 10 00 0 2 4 6 Vol tage (V) Ti me (n s) V gS iL V gG anL V ( # 1 ) = 5 .0 V V ( # 1 ) = 4 .0 V V ( # 1 ) = 3 .0 V Pro to ty pe- A : 1 MHz Fig.3 Output character istics of digital isolator (Si86 10) : (a) 1 MHz and (b) 20 MHz, measured in prop osed gate driver Prototype - A. Fig. 4 O utput characteri stics of d igital isolato r ( V gSiL ) a nd Si MOSFET s ( V gGanL ) at frequenc y of 1 MH z measured in proposed gate driver P rototype- A. Supply voltage ( V dSiL ) varied from 3.0 to 5 .0 V . W e measured the avera ge sup ply currents from the voltage source to the Si MOSFET s with various suppl y volta ges V dSiL to i nvestigate t he effect of the p enetration c urrent at the switchi ng. T he supply curr ents to t he Si MOSFET s plo tted against the switching freq uency ar e shown in Fig. 4. Supply voltage V dSiL was varied from 3.0 to 4 .5 V . T he supply current increases with increa sing the switching freq uency as well as increa sing the supply volta ge V dSiL . The package temperature of the S i MOS FET s is monitored by a n infrared camera (optris P I - 160) , which allo ws for exact measurements fro m an obj ect size of 1.5 mm with a measurement speed of 1 20 Hz. It was fo und t hat the package temperature of the Si MOS FET s rose to 100 o C at suppl y curr ents higher than appro ximately 2 00 mA, resulting in a thermal runaway of the Si MO SFET s. In t he case of a suppl y voltage V dSiL of 4.5 V , the supply curre nt reached 2 00 mA at 10 MHz. In or der to operate the Si MOSFET at 20 MHz, we e mployed a suppl y voltage V dSiL of 3.8 V . T he penetration current in t he Si MOSFET s wa s found to be one of the limiting factors to enhance the switching frequency o f the ga te driver . For 20 -MHz hard switching in the p roposed gate driver , a bootstrap diode wa s re moved to balance the supp ly voltage of the high-side and low-side Si MOS FET s. It is important to choose co mplementary Si MO SFET s with lo w power consumption (lo w penetration curr ent) for high -frequency switchi ng. C. Limitation of Dri ving Capabilit y of Si MOSFET s Complementar y Si M OSFET s can enha nce the output signal from the digital isolator, as described above. T he output voltage V gSiL fro m the d igital isolato r and V gGanL fro m the Si MOSFET s are s hown in Fig . 6. The s witching frequenc y varied from 1 to 30 MHz a nd the supply voltage V dSiL to th e Si MOSFET s wa s set a t 3.5 V . The o utput signal from the digital i solator w as degraded by increasing the switchi ng frequency , but the Si MOS FET s enhanced the output signal . Since threshold voltages of t he Si MO SFET s exist b etween the output signals at the on- state (2.5 V at 20 MHz) and t he off-state ( 1.2 V at 20 MH z) from the digital isolators , th e Si MOSFET s narro wly worked at high switching freque ncy . 0 10 20 30 0 100 200 Cu rren t (mA) Freq u ency (MH z) Supply V oltage 4.5 V 4.0 V 3.8 V 3.5 V 3.0 V The rmal lim it (>100 o C) Fig.5 Current co nsumption of Si MOSFET s p lotted against switching freque ncy . Supply voltage ( V dSiL ) was varied from 3.0 to 4.5 V . Thermal limit is defined at a device temperature o ver 100 o C. Ho wever , the maximu m value o f output voltage V gGanL fro m the Si MOSFET s decreased to 2 .5 V at 3 0 MHz. The d riving capability of the Si MOSFET s is inadeq uate at 30 MHz. The maximum val ues o f the output voltage V gGanL are plotted against the switchin g frequenc y in Fig. 7. The supply voltage V dSiL to the Si MOSF ET s was set at 3.5 V . Note that we ob served the o vershoot of the output volta ge, a nd the maximum value o f the o utput voltage increased to 3.9 V compared to a suppl y voltage of 3.5 V . The output volta ge decreased with increasing the switching frequenc y over 20 MHz, resulting in 2.5 V at 30 MHz. In order to further enhance the switching frequency , we must emplo y Si MO SFET s with a higher d riving ca pability or GaN HEMT s with a smaller input capacitance in the gate dr iver . 0 2 4 6 Sign al (V) V sigL 0 20 40 60 0 2 4 6 Voltag e (V) Ti me (n s) V gSi L V gG an L 0 2 4 6 Sign al (V) V sigL 0 20 40 60 80 0 2 4 6 Voltag e (V) Ti me (n s) V gSi L V gG an L 0 2 4 6 Sign al (V) V sigL 0 50 100 150 0 2 4 6 Voltag e (V) Ti me (ns) V gSi L V gG an L 0 2 4 6 Sign al (V) V sigL 0 100 200 0 2 4 6 Voltag e (V) Ti me (n s) V gSi L V gG an L (a) (b) (c) (d) 5 MH z 10 MHz 20 MHz 30 MHz 0 10 20 30 0 1 2 3 4 5 Out put Voltage of Si MO SFE T ( V) Frequ en cy (MH z) S up p l y v ol tage : 3.5 V Fig.6 Ou tput c haracteristics of a d igital isolato r ( Si8610) : (a) 1 MHz, (b) 10 MHz, (c) 20 MH z, and ( d) 30 MHz, measured in prop osed gate driver Proto type- A. Fig.7 Output voltage ( V gGanL ) of Si MOSFET s plo tted against switching frequenc y . Su ppl y voltage ( V dSiL ) wa s set at 3.5 V . D. Output Characteristics o f GaN -HEMT Push-Pull Driver The above investigations wer e conducted in a gate driver Proto type-A that has testin g t erminals. Here, we co mpare th e output characteristics o f Proto types- B and Prototype- A. The output characteristics o f t he ga te drivers o f Prototype -A and P rototype-B are sho wn i n Fi g. 8 . T he measurements with the oscillosco pe were triggered b y input signals from the function ge nerator . The s witching frequency was set at 14 MH z, and suppl y voltages V dSiL and V dSiH were set at 3. 8 V and the supply voltage V dGan was set at 1 8 V . T he output of the gate driver was o pen (no dr iving target). T he voltage surge was sig nificantly more reduced in the outp ut characteristics of Prototype- B t han P rototype-A , probab ly caused b y the red uction of the parasitic inductan ces i n t he gate driver o wing t o its downsizing. T he gate driver Prototype-B exhibit ed higher drivi ng capability with a lower su rge volta ge than Prototype- A. IV . HIGH-FREQUEN CY HARD SWI TCHING OF SIC M OSFETS Next t he hard switching of SiC MO SFET s wa s investigate d using Protot ype-B. The schematic of the testing circui t for hard s w itchi ng is s hown i n Fig. 9. T he link voltage was set at 50 V , and the current limiter wa s a 100 - Ω r esister . W e used a 120 0-V 10 -A Si C MOSFET (ROHM, SCT245 0KE). No gate resistance wa s emplo yed here, and gate d river Prototype- B was directly co nnected to the SiC MOSFET to investi gate the maximum driving cap ability o f the gate dr iver . Supply volta ge V ds ig wa s set at 5 V , supply voltages V dSiL and V dSiH wer e set at 3.8 V , and supply voltage V dGan wa s set at 18 V . 0 20 40 60 80 100 0 10 20 30 Vol tage (V) Ti me (ns) Protot ype-A Protot ype-B 0 20 40 60 80 100 0 2 4 6 Sig nal (V) Fig. 8 Output characteristics of GaN HEMT s in the gate drivers of P rototype-A and Pro totype-B. Duty ratios of high-side and lo w-side input signals were set to 0 .6. The s witching characteristics of the SiC MOSFET s driven b y gate driver Pro toty pe -B are shown in Fig. 10. T he switching frequenc y was set at 2 0 MHz and the duty ra tio of the co ntrol signal was 55 % for both the high and lo w side s to suppress the penetration curr ent throu gh the Ga N -HEMT push-pull configuratio n. We found t hat t he gate voltage o f the SiC MOSFET reached 18 V . Since on - a nd off-states of the Si C MOSFET were obser ved at 20 MHz, 20-MHz hard switching of SiC MO SFET was obtained using pr oposed gate driver Protot ype-B. In our p revious study [ 19 ], a ga te driver based on Si su ff ered fro m thermal runaway at freq uencies higher than 3 MHz . The gate driver with a Ga N-HEM T push-pull co nfiguration ha s hi gher driving capabilit y a nd more robustness o wing to the superior device performance of the GaN HEMT s. The switching wavefor ms o f t he SiC MOSFET exhibited slower turn -off characteristic s than those of the t urn-on. Those characteristics are mostly determined by the capacita nce-voltage (C- V) characteristics of the SiC MOSFET . T he limiting factors of the s witching charac teristics of t he Si C MOSFET must b e clar ified to i mprove the device structure of the Si C MOSFET for enhancement o f the switching freq uency . 50 V 112 μF 0 Ω Gate Dri ve r 10 0 Ω SiC MOSF ET 0 10 20 V gs (V) V gs 0 20 40 60 80 10 0 0 20 40 60 0 0.2 0.4 0.6 V ds (V) T ime (n s) V ds I ds I ds (A) Fig.9 Schematic of a switchi ng test circuit for SiC MOSFET s using our proposed gate dr iver . Fig. 10 Ou tput voltage ( V gGanL ) of Si MOSFET s plotted against switch in g frequency . D uty r atio of input sig nal varied from 0.5 to 0 .7. Supply voltage ( V dSiL ) was 3.5 V . V . CONCLUSION W e investigated the limiting factors of switching frequency in our prop osed gate d river based o n a GaN -HEMT push-pull configuration. T he digital i solator did not work at high frequency , b ut co mplementar y Si MOSFET s enha nced the drivin g capabilit y , res ulting i n the 20 -MHz hard s witching o f the Si C MOS FET . W e successfull y achieved the hard switching at 13 .56 MHz (ISM band) using the prop osed gate d river . In orde r to further improve the s witching frequency , it is necessary to accelerate t he digital isolator and r educe the power consumptio n of Si M OSFET s. 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