The recent development of fast switching converters require the power switches to operate at a high switching frequency. With high breakdown voltages and fast switching speeds, the silicon carbide (SiC) MOSFETs seem to be a perfect choice for high frequency converter applications [
1,
2]. However, the utilization of SiC MOSFETs can also introduce some unwanted side-effects to the converter. Under certain test conditions, the self-sustained oscillation can occur in the turn-off transient of SiC MOSFETs [
3,
4]. As shown in
Figure 1, during the oscillatory transient, the gate voltage rings back above the threshold voltage
, which gives rise to the the unexpected turn-on of the MOSFET. The unexpected turn-on draws additional energy from the power supply, which compensates the energy dissipated by parasitic resistance in the test circuit. As a results, the oscillation can maintain self-sustenance during the oscillatory transient. Since the self-sustained oscillation causes severe electromagnetic interference (EMI) problems and can completely disrupt the converter operation in the worst case, great care needs to be taken to suppress this kind of oscillation. To achieve this, it is necessary to investigate the underling mechanism and oscillatory criteria of the self-sustained oscillation for SiC MOSFET.
Unfortunately, only few works have been presented to study the self-sustained oscillation phenomenon [
3,
4,
5,
6,
7]. In [
3,
4]; with the common source inductance
being neglected, the papers studied the sensitivity of various circuit and device’s parameters on the self-sustained oscillation of SiC MOSFETs by casting the switching circuit as the negative conductance oscillator. However, the studies have a few drawbacks. Firstly, since the voltage drop
on the inductance
can directly affect the gate voltage, the
has a very significant impact on the turn-off oscillation. The research proposed in Reference [
8] demonstrated that the
can disrupt the positive feedback process of the oscillatory system and damp the turn-off oscillation. Under certain test conditions, the
can also induce the unexpected turn-on of the low-side MOSFET [
7,
9] and can excite the self-sustained oscillation during the turn-off transient [
7]. Therefore, the inductance
should not be neglected in the analysis. Secondly, the potential mechanisms which can lead to the occurrence of the self-sustained oscillation were not revealed in the papers. Last but not least, some circuit parameters, like the stray inductances of gate and power loop, have a very strong impact on the turn-off oscillation [
8]. Unfortunately, the parametric sensitivity of these parameters on the self-sustained oscillation were not investigated in the paper. In Reference [
5], the self-sustained oscillation phenomenon of a GaN transistor in a half-bridge circuit was studied. The self-sustained oscillation was induced by the unique reverse conduction characteristics of the GaN devices. However, since the SiC MOSFETs do not have such kind of reverse conduction characteristics, the self-sustained oscillation presented in this research does not occur on the SiC MOSFETs. In Reference [
6], with the gate-drain capacitance neglected, a simplified SiC MOSFET equivalent model was utilized to derive the transfer function of the oscillatory system. Based on the transfer function, an analytical oscillatory criteria of the self-sustained oscillation was obtained. Since the gate-drain capacitance was neglected, the oscillatory criteria was not very accurate and could only provide an approximate estimation on the occurrence condition of self-sustained oscillation for SiC MOSFETs. In Reference [
7], the self-sustained oscillation induced by the the common source inductance
was reported for CoolMOS. The research revealed the critical impact of the common source inductance
on the self-sustained oscillation. However, the mechanism of the self-sustained oscillation was still unknown. The criteria of the oscillation was also not studied in the paper.
This paper presents a comprehensive study on the potential mechanisms and oscillatory criteria of the self-sustained oscillation for SiC MOSFETs. In
Section 2, two distinct test conditions which can trigger the self-sustained oscillation are identified by the double-pulse test. The related positive feedback mechanisms which originate the oscillation are clarified based on the test results. With the common source inductance
included, a small-signal ac model is proposed in
Section 3 to obtain the transfer function of the oscillatory system. By studying the two pole pairs of the criteria transfer function, the impact of various circuit and device’s parameters on the self-sustained oscillation is analyzed in
Section 4. The theoretical analyses reveal the oscillatory criteria of the self-sustained oscillation. The methods to suppress the self-sustained oscillation are thereby obtained in
Section 5 according to the oscillatory criteria. In
Section 6, the experiment is performed to validate the proposed oscillation suppression methods.