EET ASIA 2022

Wise-Integration’s WiseGan ZVS evaluation board allows to evaluate its WI62100, half-bridge 100mΩ (or WI62175 half-bridge 175mΩ), enhancement mode-high electron mobility transistors (EHEMTs) with the Si8274GB4D-IS1 gate driver in a half-bridge configuration.

ZVS corresponds to soft switching (switching of the component at zero voltage); it reduces switching losses while enabling high-frequency switching.

The WiseGan ZVS evaluation board is assembled with one Si8274GB4D-IS1 driver and one half-bridge power GaN WI62100 (or WI62175). The LTC6907 oscillator is included to generate the control signal. Headers are included for power connections, while probe points are included for waveform measurements.

Figure 1: WiseGan ZVS WI62XXX Evaluation Board Assembly

GaN Evaluation Board

The block diagram and schematic of the evaluation board are provided in Figures 2 and 3.

The half-bridge circuit WI62100 integrates two GaN power transistors 100mΩ in a 6x8mm PQFN package. The capacitors C7 and C8 form a low-pass filter with L1 and L2, rolling off the frequency response at 3dB/Octave above approximately 20kHz. The shunt resistors R9, R10, R13, and R14 are introduced, allowing measurement of the inductance current with oscilloscope voltage probe. The resistors R11 and R12 form a voltage divider bridge in order to bias the start voltage on each capacitor C7 and C8 to VPOS/2. The inductor current is triangular, the peak value is 1A at VPOS=240V.

Figure 2: WiseGan ZVS WI62100 Block Diagram

The LTC6907 oscillator and Si8274GB4D-IS1 GaN driver are supplied by the micro-USB (+5VDC) via J4. The oscillator is configured to generate a 1MHz PWM signal (RSET1=200kΩ). RDT adjust the dead time for driver U2, which allow to minimize the dead-time between one transistor turning off and the other turning on, thus eliminating any inefficient and to ensure the transistors always switch under ZVS.

Si8274GB4D-IS1 drives the respective high and low side GaN transistors via low value resistors (R7 and R8) which tame the parasitic inductances on the transistor gate loops, damping any resonances. Capacitors C5 (in conjunction with diode D2) provide the bootstrapping action for the high side of Si8274GB4D-IS1 gate drive. All other capacitors are for local decoupling of the various stages of this high-frequency circuit.

Diode D3 protects against accidental polarity reversal at the input, but only up to 1A, so at first, power up here using a suitably safe low current limit setting (for example, 100mA).

Test Points COM, V_shunt and PGND allow for convenient oscilloscope monitoring the VCOM and the inductor current waveforms.

Figure 3: WiseGan ZVS WI62100 Schematic

The evaluation results are shown in Figure 4.

Figure 4: V COM waveform at 1MHz, with VPOS=350V

Thermal Considerations

The evaluation board includes WI62100 device. Although the electrical performance surpasses that for traditional silicon devices, their relatively smaller size does magnify the thermal management requirements. The evaluation board is intended for bench evaluation with low ambient temperature and convection cooling. The addition of heat sinking and forced air cooling can significantly increase the current rating of these devices, but care must be taken to not exceed the absolute maximum die temperature of +125°C. The thermal performance of the WiseGan ZVS evaluation board is shown in Figure 5.

Figure 5: WiseGan ZVS Thermal Plot Showing of 70°C on WI62100 package with PWM at 1MHz, with VPOS=350V, ILpeak=1.5A

The hottest components are the shunt resistors and the inductors. WiseGan component remains cool due to the ZVS operation.