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DOI: 10.1109/APEC.2017.7930963

Citation: S. Park and J. Rivas-Davila, "Power loss of GaN

transistor reverse diodes in a high frequency high voltage

resonant rectifier," 2017 IEEE Applied Power Electronics

Conference and Exposition (APEC), Tampa, FL, USA,

2017, pp. 1942-1945.

IEEE Xplore URL:

http://ieeexplore.ieee.org/document/7930963/

Power Loss of Gate-Source-shorted GaN Transistorsin Tens of Megahertz and Hundreds of Volts

Class-DE Resonant RectifiersSanghyeon Park and Juan Rivas-Davila

Electrical EngineeringStanford UniversityStanford, CA 94305

Email: spark15@stanford.edu

Abstract—This paper presents power loss measurements ofgate-source shorted GaN transistors in place of diodes for highfrequency high voltage rectification. To evaluate the performance,we use gate-source-shorted GaN transistors as passive rectifyingdevices in a class-DE resonant rectifier and operate the circuit at10s of megahertz switching frequencies and 100s of volts outputvoltages. Thermometric calibration method identifies power lossin all GaN transistors that increases with switching frequency andthe rectifier output voltage. Furthermore, comparisons betweenexperiments and simulations suggest that the diode power loss isneither unintended hard switching loss nor conduction loss fromthe forward voltage drop and on-resistance of the device. Thecorrelation between the amount of power dissipation and GaNtransistor output capacitance suggests that the observed powerloss is related to output capacitance.

I. INTRODUCTION

High-frequency high-voltage dc-dc conversion makes pos-sible small and lightweight power supplies and thus manyexciting new applications [1]–[4]. Efficient rectifying devicesare critical in building high efficiency dc-dc conversion sys-tems. This paper evaluates the performance of gate-source-shorted Gallium Nitride (GaN) transistors as passive rectifyingdevices.

Silicon (Si) and Silicon Carbide (SiC) Schottky diodes, bothwidely used in power systems, have their shortcomings whenit comes to high-frequency high-voltage converter design. SiSchottky diodes have limited reverse voltage capability. SiCdiodes, despite their high blocking voltage and low reverseleakage current, does not prove ideal due to a power loss thatrapidly increases with the voltage and switching frequency [5].

Commercially available GaN transistors behaves like adiode when its gate node is shorted to the source node. Sourceand drain nodes correspond to diodes’ anode and cathode,respectively, in terms of their functionality. Manufacturersclaim that this diode-like behavior comes with zero reverserecovery loss [6], [7]. Even though those gate-source-shortedGaN transistors’ forward voltage drop is several times largerthan that of Si or SiC diodes, the corresponding power losswould be insignificant for applications that demand outputvoltage levels of hundreds of volts or higher [1]–[4]. The GaNtransistor with such configuration would ideally operate as an

efficient passive rectifying device if we minimize switchinglosses by adopting a resonant rectifier topology. .

This paper investigates GaN transistors’ performance inhigh frequency high voltage rectifiers, and compare the re-sults with SiC diode counterparts. We use gate-source-shortedGaN transistors as a passive rectifying switch in class-DEresonant rectifiers [8] (the referenced paper calls them ”class-D” resonant rectifiers). We measure diode power losses bythermometric calibration and map device temperatures andpower losses in a one-to-one fashion. The experiment identifiesa significant loss from GaN transistors under tens of megahertzand hundreds of volts operation. We observe power lossesincreasing with the voltage and switching frequency, which weclaim is attributable to neither conduction loss nor switchingloss, calling for further investigation.

The rest of the paper is organized as follows. In Section IIwe explain how the class-DE resonant rectifier operates anddescribe the voltage and current waveforms that are applied torectifying devices. In Section III we discuss the thermometriccalibration method that we use to estimate the power lossesin a device. In Section IV we show the measured GaNtransistor losses under various voltage, frequency, temperatureand current conditions. We also compare GaN transistors withSiC diodes in regard to their loss levels under the sameconditions. In Section V we discuss possible mechanisms forthe observed GaN losses.

II. CLASS-DE RESONANT RECTIFIER

We use class-DE half bridge resonant rectifiers [8] illus-trated in Fig. 1a to evaluate power losses in GaN transistors.The circuit includes an LC parallel tank in which the resonantcapacitance is the sum of Cres and diode junction capacitancesCj,Dgnd and Cj,Dout. The ac sinusoidal current input is(t)resonates the LC parallel tank to generate a large voltageswing across diode Dgnd. This voltage swing is clipped eitherto ground by diode Dgnd or to the output voltage Vout bydiode Dout. The subsequent current pulse flows from groundthrough Dgnd and through Dout to reach the output node,resulting in the dc output current Iout.

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Fig. 1: Class-DE resonant rectifier we use to evaluate power loss in GaNtransistors. (a) Schematic of the rectifier circuit. The ac current input resonatesLC parallel tank until the amplified voltage is clipped either to ground bydiode Dgnd or to Vout by diode Dout. (b) The voltage across and currentthrough diodes in the rectifier. We use SiC diodes or gate-source-shorted GaNtransistors as a passive rectifying device. Provided that there is no reverserecovery transient, the resonant operation is ideally free of switching loss. (c)One of our rectifier implementations, a 500 V 27.12 MHz rectifier with a pairof gate-source-shorted GS66502B GaN transistors used as passive rectifyingdiodes.

Fig. 1b depicts the voltage across and current through diodeswhen the rectifier is operating at switching frequency fs. Weuse gate-source-shorted GaN transistors as rectifying devicesand model them as a diode. vD,pp, the peak-to-peak voltageswing across the diode, is the same as the rectifier outputvoltage Vout. The average diode current iD,avg is equal to therectifier output current Iout. As illustrated in the diode voltagevD and current iD waveforms, the circuit achieves zero voltageturn-on and zero current turn-off. Provided that we use diodesthat do not exhibit any reverse recovery, the class-DE resonantrectifier should be free of switching loss.

Fig. 1c shows one of our rectifier implementations usingGS66502B GaN transistors. The circuit has output voltage500 V and switching frequency 27.12 MHz with a pair ofgate-source-shorted GS66502B GaN transistors being used aspassive rectifying diodes. The impedance matching inductor(not shown in Fig. 1a) matches the input impedance of therectifier with the output impedance of the RF power amplifierthat provides the ac input signal.

We evaluate power losses of the following three GaNtransistors: GaN Systems GS66502B eGaN transistor; NavitasNV 6110 eGaN transistor; and Transphorm TPH3002LScascoded dGaN transistor. We also evaluate the followingSiC Schottky diodes in the identical setup for compari-son: STMicroelectronics STPSC406B; Cree C3D04060E,CSD04060E, and C3D1P7060Q. For the fairest possiblecomparison we selected parts of which voltage and currentratings are as similar to each other as possible. Appendix A

summarizes voltage and current ratings of the selected parts.

III. THERMOMETRIC CALIBRATION

(a)

(b)

Fig. 2: Diode power loss estimation by thermometric calibration (a) A constantcurrent is injected to the diode while the device voltage and temperature aremeasured. Repeating this measurement multiple times yields the temperaturevs. power loss calibration curve. (b) Once the diode temperature is measuredduring the rectifier operation, the diode power loss is read from the calibrationplot.

We measure the power loss of rectifying devices in class-DE resonant rectifiers by thermometric calibration illustratedin Fig. 2. This method estimates the power dissipation of thedevice by the temperature it reaches in thermal equilibrium.Calibration curves correlate the temperature and power dissi-pation to enable loss estimation.

We obtain the temperature versus power calibration curveas follows. First, we attach a populated rectifier circuit boardonto a large metal block that maintains a constant temperaturethroughout the experiment. Second, we use a constant currentsource to inject a dc current into the rectifying device andrecord the temperature of the hottest spot on the device surface.We repeat this step for multiple power levels until the numberof data points become large enough to draw a linear calibrationcurve. Finally, we operate the rectifier and measure the devicetemperature when the circuit reaches thermal equilibrium. Weestimate the device power loss by using the calibration curveobtained above.

The direct use of voltage and current probes, the moststraightforward measurement method, is inappropriate in thisstudy. Since this study focuses on rectifiers with tens of MHzswitching frequency, capacitance and inductance in the circuitare only several tens of pF and few hundreds of nH. Voltageand current probes would add capacitance and inductance largeenough to disrupt the circuit behavior significantly.

IV. POWER LOSS MEASUREMENTS

In this section, we show the power loss observed in GaNtransistors and identify factors affecting loss levels. We also

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Fig. 3: Power losses in rectifying devices under various voltage, frequency and temperature conditions. Blue and red markers indicate GaN transistor losses andSiC diode losses, respectively. (a) Power losses at 170 V, 350 V and 500 V of peak-to-peak voltage across the device. The average diode current (equivalent tothe rectifier output current) is 50 mA. (b) Power losses at 13.56 MHz, 27.12 MHz and 40.68 MHz of switching frequency. The peak-to-peak diode voltage is500 V. (c) Power losses at case temperatures from 30 to 60 C. We change the device case temperature by manipulating the burst duty cycle. The peak-to-peakdiode voltage is 500 V and average diode current is 50 mA. Note the broken vertical axis.

compare the losses of GaN with those of SiC Schottky diodesunder identical conditions for reference purposes. Except forthe power loss vs. temperature experiment, we maintain thetemperature of the hottest spot of the device surface to bewithin 27-30 C by adjusting the rectifier burst duty cycle.

A. Power Loss vs. Voltage, Frequency and Temperature

Fig. 3 shows power losses in rectifying devices at vari-ous voltages, frequencies and temperatures. Plots show GaNtransistor losses as blue markers and SiC diode losses asred markers. Fig. 3a illustrates power losses measured at170 V, 350 V and 500 V of peak-to-peak voltage swingacross GaN transistors and SiC diodes. Here all the data areat 27.12 MHz switching frequency. Fig. 3b displays powerlosses at 13.56 MHz, 27.12 MHz and 40.68 MHz of switchingfrequency, when the peak-to-peak voltage swing is 500 V.Fig. 3c shows the power losses in all three GaN transistorsincrease with the case temperature. We change the device casetemperature from 30 to 60 C by manipulating the burst dutycycle of the rectifier. The peak-to-peak diode voltage is 500 Vand average diode current is 50 mA. Note the broken verticalaxis between 4 W and 10 W.

Gate-source-shorted GaN transistors turn out to be inap-propriate as passive rectifying devices in 100s of volts and10s of megahertz operations. GaN power losses are at bestcomparable to SiC diode counterparts, and increase rapidlywith voltage and frequency. Power losses per GaN deviceare significant in all cases in Fig. 3, ranging from 5.6 %(in NV 6110 at 170 V and 27.12 MHz) to 77.6 % (inTPH3002LS at 500 V and 40.68 MHz) of the rectifier outputpower.

B. Power Loss vs. DC Current

Fig. 4 shows power losses measured when the dc currentthrough GS66502B and NV 6110 GaN transistors varies from

50 mA to 1000 mA. Plots from Fig. 4a to Fig. 4d representthe power losses against the dc current through devices. Inall voltage and frequency conditions, the power loss increaseswith the dc current through the device, although not as rapidlyas with voltage and frequency.

Plots from Fig. 4e to Fig. 4h rearrange the data to comparethem with simulated GaN losses. The thick gray dashed lineacross each plot indicates the place where measured andsimulated losses are equal. We obtain simulated losses by mea-suring the IV curve of gate-source-shorted GaN transistorsand modelling them as ideal diodes in series with a constantvoltage source and a linear resistor. In order to prevent thedevice temperature from affecting the IV curve measurement,we adjust the duty cycle of the dc current injection so thatthe temperature falls into the same range as in the device lossmeasurement experiment.

In all four plots, the array of data points from the bottomleft to the top right correspond to the increasing current levelsfrom 50 mA to 1000 mA. Plots show that the additional powerloss on top of the simulated loss (the gap between blue markersand the thick gray line) is a very weak function of the devicecurrent and constitutes a significant portion of total loss onlyat low current levels. At high dc current levels, most of thepower loss is explained as a conduction loss and well-predictedby the aforementioned simple diode model.

V. DISCUSSION ON LOSS MECHANISM

We cannot explain the GaN power loss as the conductionloss predictable from the device IV curve. Looking at plotsfrom Fig. 4e to Fig. 4h, following characteristics are apparentin all frequency and voltage conditions: (a) the differencebetween simulated and experimental data sets remains nearlyconstant throughout the whole current range; (b) the differencebetween those two increases with switching frequency and the

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Fig. 4: We vary the dc current through GS66502B and NV 6110 GaN transistors from 50 mA to 1000 mA and measure the power loss at various voltagesand frequencies. The switching frequency is 27.12 MHz for (a), (c), (e) and (g). The peak-to-peak device voltage is 500 V for (b), (d), (f) and (h). (a) Powerlosses of GS66502B at 27.12 MHz frequency and various peak-to-peak device voltages. (b) Power losses of GS66502B at 500 V peak-to-peak devicevoltage and various switching frequencies. (c-d) Power losses of NV 6110 under the same condition as in (a) and (b). (e-f) Power losses of GS66502Bcompared with the losses in simulation. (g-h) Power losses of NV 6110 compared with the losses in simulation.

peak-to-peak voltage across the device. These characteristicsindicate the existence of a power loss mechanism that isnot a conduction loss. We cannot model the loss mechanismas a constant voltage drop and a linear resistance, or morecomplex models that reflect IV characteristics of GaN at lowfrequencies.

Hard switching loss is also unlikely to be the mechanismof the observed GaN transistor power loss. Fig. 5 shows verygood match between simulated (dashed lines) and experimen-tal (colored solid lines) waveforms of the voltage across thedevice during rectifier operations at 27.12 MHz and threedifferent output voltages. The good match between two voltagewaveforms confirms that the rectifier circuit indeed operatesas intended; the resonant operation recycles energy stored injunction capacitors, and zero-voltage switching occurs. There-fore, it is doubtful that the reason for the discrepancy betweenexperimental and simulated data lies in hard switching.

We suspect the power loss in GaN transistors is causedby the lossy output capacitance of the device. Fig. 6a showsthat the output capacitance (Coss) profile of GS66502B [9]is almost 2.3 times that of NV 6110. Fig. 6b and Fig. 6ccompares the power loss of GS66502B at various voltages andfrequencies which is also around 2.3 times that of NV 6110 inevery case. This comparison suggests that the observed lossesin GaN transistors might be proportionally related to Coss, or

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more generally, the semiconductor die area.

VI. CONCLUSION

This paper presented power losses of gate-source-shortedGaN transistors in place of rectifying diodes in a high-frequency high-voltage class-DE resonant rectifier. All three

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Fig. 6: Comparison of output capacitance Coss and power losses inGS66502B and NV 6110 GaN transistors. (a) Coss vs. source-to-drainvoltage. The output capacitance profile of NV 6110, when scaled up by afactor of 2.3, almost coincides with that of GS66502B. (b-c) Power lossesin different voltages and frequencies. In all cases, the loss in GS66502B isalmost 2.3 times larger than that in NV 6110.

GaN transistors tested exhibit power losses that increase withvoltage, switching frequency, temperature and dc current.Whereas the case of dc current is explained by conductionlosses, the exceedingly high loss levels that increase withvoltage and frequency calls for further investigation. GaNtransistor losses are comparable or larger than their SiC diodecounterparts and are large enough to significantly degradethe rectifier efficiency at 100s of volts and 10s of megahertzoperations.

ACKNOWLEDGMENT

We would like to thank Texas Instruments for fundingthis work through the Energy/Power Management Systemsfocus area of the Stanford SystemX Alliance. We would alsolike to thank Navitas Semiconductor for providing parts forevaluation.

REFERENCES

[1] L. Raymond, W. Liang, L. Gu, and J. Rivas, “13.56 mhz high voltagemulti-level resonant dc-dc converter,” in 2015 IEEE 16th Workshop onControl and Modeling for Power Electronics (COMPEL), July 2015, pp.1–8.

[2] S. H. Jayaram, “Sterilization of liquid foods by pulsed electric fields,”IEEE Electrical Insulation Magazine, vol. 16, no. 6, pp. 17–25, Nov2000.

[3] F. Guo, X.-H. Ji, K. Liu, R.-X. He, L.-B. Zhao, Z.-X. Guo, W. Liu, S.-S.Guo, and X.-Z. Zhao, “Droplet electric separator microfluidic device forcell sorting,” Applied Physics Letters, vol. 96, no. 19, 2010. [Online].Available: http://scitation.aip.org/content/aip/journal/apl/96/19/10.1063/1.3360812

[4] C.-H. Lin, J.-H. Wang, and L.-M. Fu, “Improving the separationefficiency of dna biosamples in capillary electrophoresis microchipsusing high-voltage pulsed dc electric fields,” Microfluidics andNanofluidics, vol. 5, no. 3, pp. 403–410, 2008. [Online]. Available:http://dx.doi.org/10.1007/s10404-008-0259-7

[5] L. C. Raymond, W. Liang, and J. M. Rivas, “Performance evaluation ofdiodes in 27.12 mhz class-d resonant rectifiers under high voltage andhigh slew rate conditions,” in 2014 IEEE 15th Workshop on Control andModeling for Power Electronics (COMPEL), June 2014, pp. 1–9.

[6] Bottom-side cooled 650 V E-mode GaN transistor, GaN Systems, 2016,rev. 160302.

[7] GaN Power Low-loss Switch, Transphorm, Dec. 2014.[8] L. Raymond, W. Liang, J. Choi, and J. Rivas, “27.12 mhz large voltage

gain resonant converter with low voltage stress,” in 2013 IEEE EnergyConversion Congress and Exposition. IEEE, 2013, pp. 1814–1821.

[9] G. Systems, “650v enhancement mode gan transistor,” GS66502Bdatasheet, September 2015.

APPENDIX AWe provide the voltage and current ratings of GaN transis-

tors and SiC diodes evaluated in this paper. Shown in Fig. 7 aremaximum allowable blocking voltages and continuous currentsat case temperature of 100-135 C according to the datasheets.We made our best effort to select parts with similar ratings forthe fairest possible comparison.

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Fig. 7: Comparison of maximum blocking voltages and continuous draincurrents of GaN transistors and SiC diodes evaluated in this paper. Bluemarkers and red markers correspond to GaN transistors and SiC diodes,respectively.

APPENDIX BWe estimate the impact of inductor losses on GaN transistor

loss measurements. Thermal imaging throughout the experi-ment reveals that inductor losses are dominant source of powerdissipation among passive components in the rectifier circuit.Therefore, we focus on measuring the thermal resistance be-tween inductor soldering pads and GaN transistors. As shownin Fig. 8a, we attach a 10 Ω resistor to the soldering pad of theresonant inductor Lres and the impedance matching inductorL. We deliver constant power to one of the resistors for aroundtwo minutes until the system reaches thermal equilibrium. Wethen record the average case temperature of the GaN transistoras in Fig. 8b.

The thermal resistance between the inductor and GaNtransistors is 0.38 C/W for both Lres and L. This thermalresistance translates to +5 % error or less in the GaN transistorloss measurement. This estimate is based on several conserva-tive assumptions: first, the thermal conductivity between the

inductor and the circuit board is as high as that in the caseof surface-mount resistors in Fig. 8a; second, the rectifieroperation time is several minutes for thermal equilibrium,which has never been the case during the transistor lossmeasurement; lastly, the inductor loss and the burst dutycycle are simultaneously at their maximum (6 W and 0.33,respectively), which is the opposite of the usual conditionwhere those two variables are negatively correlated.

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Fig. 8: Experimental setup to measure the thermal resistance between inductorsoldering pads and GaN transistors. (a) We attach a 10 Ω resistor to thesoldering pad of the resonant inductor Lres and the impedance matchinginductor. (b) We deliver constant power to the resistors and measure theaverage temperature rise in the GaN transistor surface by thermal imaging.