Analyse impact of RC snubber on switching oscillation of the SiC MOSFET.

What is it about?

As the most popular wide bandgap (WBG) power device, the silicon carbide (SiC) MOSFET has been widely adopted in the power electronics applications, and brings in the benefits including reduced switching losses, enhanced switching frequency, and improved power density. However, the switching oscillation and the electromagnetic interference (EMI) become more serious due to the rapid switching speed of SiC MOSFET. Thus, adding RC snubber branch is considered as an effective method to suppress such unwanted oscillation in the early works. In this paper, the switching transient of SiC MOSFET with RC snubber is investigated with an analytical model based on finite state machine (FSM). The accuracy of proposed analytical model can be verified by comparisons between calculated and measured waveforms during the switching transition. In addition, the impacts of the RC snubber on switching oscillation, switching loss and high-frequency (HF) EMI noise have been comprehensively investigated based on the model, which shows that the added RC snubber can effectively avoid the switching oscillation, and reduce the level of HF EMI without increasing switching loss.

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Why is it important?

Compared with the existing works on RC snubber, this paper aims at comprehensive investigation of the impacts of RC snubber on switching oscillation, switching loss, and HF EMI noise based on an analytical model designed with finite state machine (FSM), which is capable of depicting the whole switching actions of the SiC MOSFET in high accuracy. The main contributions are summarized in the following aspects: 1) To accurately reflect the switching behaviors of the SiC MOSFET with the influence of the RC snubber an analytical modeling has been elaborated. The critical factors on switching performance of the SiC MOSFET, such as the C-V and I-V characteristics, stray inductance in the circuit and device, skin effect (ac resistor) during oscillation stages, have been fully considered in this model. Meanwhile, the model is designed by FSM that can intuitively illustrate the whole switching transient. The effectiveness of the model is verified by comparisons between calculated and measured results based on DPT. Compared with other existing models, this model has been improved, and can accurately illustrate the switching performance in terms of transition response, switching oscillation, switching loss, as well as the impacts of RC snubber. Therefore, the model is expected to provide much helpful assistance for RC snubber design in practical applications. 2) To comprehensively investigate the damping effect provided by RC snubber, a 4th-order equivalent circuit is extracted based on the proposed analytical model. The damping effect of the RC snubber has been analytically discussed by analyzing the distributions of the poles in the 4th-order system, and the relationship between low-frequency (LF) and HF oscillations with the variation of the RC snubber can be well demonstrated. Based on the analyzed and simulated results, the design guideline for oscillation suppression and HF EMI alleviation has been summarized. In addition, the switching energies on SiC MOSFET and RC snubber are also predicted and measured, which can provide helpful evaluation of the switching loss. With the quantitative analysis of the calculated and measured results, it can be found that the added RC snubber will not bring additional switching loss in applications.

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