Self-Sustained Turn-OFF Oscillation of Cascode GaN HEMTs: Occurrence Mechanism, Instability Analysis, and Oscillation Suppression

Abstract

This paper presents a comprehensive study on the occurrence mechanism, instability analysis and suppression methods of self-sustained turn-off oscillation which occurs on cascode gallium nitride high electron mobility transistors (cascode GaN HEMTs). In the beginning, the oscillation waveforms are analyzed, which indicate that the occurrence of the oscillation is determined by test circuit instability. Based on the double pulse test, the impact of the load current IL, DC-bus voltage VDC and gate resistance RG on the self-sustained oscillation is identified. To investigate the instability of the resonant circuit, a smallsignal ac model of the resonant circuit is derived. Based on the model, the influences of various parameters on the self-sustained oscillation are analyzed. The analyses reveal the possible methods which can suppress the oscillation. The effectiveness of the proposed methods is validated by the experimental data and simulation results in the end.

Ferrite Beads Design to Improve Turn-Off Characteristics of Cascode GaN HEMTs: An Optimum Design Method

“…Self-sustained oscillation is observed during the turn-off transient. According to [8], the self-sustained oscillation occurs due to the instability of the test circuit. Due to the cascode configuration, a positive feedback mechanism can be triggered.…”

“…Due to the cascode configuration, a positive feedback mechanism can be triggered. If the test circuit is unstable, the positive feedback action can draw additional energy from the power supply, which compensates for the power losses dissipated by the resistive components in the test circuit [8]. The oscillation is thereby able to maintain self-sustaining.…”

“…To investigate the instability of the test circuit using the power loop beads, a small-signal model is derived following the approach presented in [8], [19], [21]- [24]. Fig.…”

“…In the beginning, the initial ferrite beads design which achieves I R > I L should be selected. initial parameters of the circuit and power loop beads are thereby obtained to calculate the transfer function H(s) based on the system of linear equations ( 1)- (8). It should be noticed that the dominant pole pair P D of H(s) depend on the circuit and power loop beads parameters and may not be the pole pair P6.…”

“…This makes the cascode GaN HEMTs based half-bridge circuit very unstable and prone to generate underdamped turn-off oscillation. In the worst-case scenario, the underdamped turnoff oscillation can become self-sustained [4], [8]. The selfsustained oscillation can induce very severe electromagnetic Interference (EMI) problems, which need to be avoided in power conversion applications.…”