Investigation of soft-switching behavior of 600 V cascode GaN HEMT

Gallium Nitride High Electron Mobility Transistor (GaN HEMT) is an emerging wide band gap power device in recent years. Using a cascoded structure, the GaN HEMT can be combined with a low voltage MOSFET to make the combination behave as a normally-off device. This paper investigates the soft-switching behavior of cascode GaN HEMT in the phase-leg structure. The analysis reveals some internal device behaviors during the soft-switching transition, which are not found in the non-cascode device. Due to the internal feedback of the cascode structure, the channel current of the internal GaN HEMT drops to zero quickly, leading to extremely low turn-off loss. However, it has been found that there are switching energy loss dissipated in the internal GaN HEMT during the turn-on transient, although the external waveforms of the cascode GaN HEMT exhibit zero voltage switching. The fundamental reason is that ratio of the sum of MOSFET output capacitance and internal GaN HEMT input capacitance to the internal GaN HEMT output capacitance is quite low. Based on the simulation, by adding additional capacitance on the gate source terminals of internal GaN HEMT, these losses can be mitigated. Experimental tests using a commercially available GaN device are presented which show nearly 400 mW of loss at 1 MHz switching frequency in four different load current conditions.