A 10-MHz eGaN Isolated Class-Φ2 DCX

Derived from the class-E resonant converters, the isolated class-Φ
₂
resonant converters have much reduced voltage stress of the control FET owing to a shunt branch paralleled with the switch to provide low pass path for the second harmonic voltage. With the increase of the input voltage, the reverse conduction time of the power FETs varies seriously and the gate drive voltage can hardly match efficiently with the drain voltage of the synchronous rectifier (SR) FET. This causes the earlier turn-on of the SR FET before zero voltage switching (ZVS) is achieved, which results in high turn-on loss at multi-MHz. Moreover, with the enhancement mode gallium nitride (eGaN) HEMTs, the mismatch between the drive voltage and drain voltage results in high reverse conduction mechanism loss after the SR FET turns off in the switching period of hundreds of nanoseconds. The reverse conduction mechanism of the control eGaN HEMT can be triggered before ZVS turn-on, which causes high reverse conduction loss. It is interesting to find that when the output voltage is controlled to follow the input voltage proportionally, the drive voltage can match well with the drain voltage of the power FETs over a wide input voltage range, which is proved mathematically by the state-space analysis. Then, a voltage following control is proposed to control the isolated resonant converters as dc transformers (DCXs). The turn-on loss of the SR FET and reverse conduction loss of the SR and control FETs can be minimized. An 18-24 V input, 18 W/2 A output, 10-MHz eGaN DCX was implemented to verify the advantages. With the proposed control, the efficiency is improved from 79.9% to 85.5% at 18 W output and 24 V input.