Thermal analysis and improvement of cascode GaN HEMT in stack-die structure

The high-voltage cascode gallium nitride (GaN) high-electron-mobility transistor (HEMT) enables high-frequency and high-efficiency power conversion. The parasitic inductances induced by traditional packages of the cascode GaN HEMT device significantly deteriorate the device switching performance. A new stack-die packaging structure has been introduced and proved to be efficient in reducing package related turn-on loss and turn-off parasitic ringing. However, the thermal dissipation of the device packaged in such structure becomes the limitation for further pushing the operating frequency and the output current level for high-efficiency power conversion. This paper focuses on the analysis of thermal performance of the cascode GaN HEMT device in different packages used in a high-frequency power converter. There exist several possible approaches for improving thermal performance of the device. Nevertheless, it is not convenient and cost-effective to put every method into practice. A thermal model has been built based on the actual structures and materials of the packaged device and the assembled converter in order to evaluate the effectiveness of different thermal performance improving methods. The simulation results of the improved design demonstrate the possibility of further increasing the switching frequency for the converter while maintaining the temperature of the device under 125°C. Finally, the corresponding experiments have been conducted to validate the simulation results using the fabricated stack-die devices. The presented simulation method and result successfully provide a guidance of thermal design improvement for the high-voltage cascode GaN HEMT device in the stack-die structure.