In many applications such as military, weather and marine radar, RF power amplifiers are used in pulsed mode. Typically, the RF signal is pulsed while the DC bias used to power the transistor to a set point is left constant. However, in other applications pulsing the DC bias allows for reduced interference to the receiving path, noise and power consumption. For Wolfspeed GaN on SiC HEMT devices careful consideration needs to be taken when pulsing either the gate or drain of the device. Wolfspeed GaN on SiC HEMT are depletion mode devices, which means a negative voltage (such as -5 V) needs to be applied on the gate to turn off the transistor. Applying drain voltage with 0 V on the gate will result in catastrophic damage to the device. The main challenge of drain pulsing is the high current on the drain side and switching time that can be achieved. This application note goes over a circuit that was used to achieve 1.9 uS turn on time for a high-power (20 A peak) device using drain pulsing.
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