Characterization and Modelling of Switching Dynamics of SiC MOSFETs

Abstract

Silicon Carbide MOSFETs (SiC MOSFETs) fall into the class of wide band gap (WBG) power devices. These devices are commercially available in the voltage range of 600-3300V and superior over the state of the art Si insulated gate bipolar junction transistors (IGBTs) due to their better electrical and thermal performances. In power electronic converters, semiconductor devices operate as switches. They can be turned on or off using a control signal. Unlike ideal switches, practical devices require a finite amount of time to transit between on and off states. This is termed as switching transient. Non-zero finite product of voltage and current during switching transient results in switching loss. Characterization and modelling of switching dynamics help gain insight into the switching process and estimate switching loss. It is useful over experimental measurement techniques like double pulse test (DPT) or calorimetric measurements in the early stages of power converter design. Estimated loss through the switching transient model can be used to determine switching frequency and selection of power devices. Also, switching dynamics is strongly impacted by the device and circuit parasitics. Insight into the switching process helps in the proper design of gate driver and power circuit layout.

Superior material properties of SiC MOSFET leads to smaller die size compared to the state of the art Si-based power devices. It results in faster switching transients and lower switching loss. However, it excites device and circuit parasitics that may lead to prolonged oscillation, high device stress, spurious turn on and EMI-related issues Etc. So, the benefit of using SiC MOSFET as power devices come with numerous design challenges resulting in slow commercial adaptation. It is predicted that the overall market share of WBG devices (SiC and GaN together) will be roughly 10% of the total market for power semiconductors by 2025.

To overcome the design challenges and fully utilize the benefits of fast switching SiC MOSFETs, a better understanding of dynamics is essential. However, the switching dynamics of SiC MOSFET is different compared to its Si counterpart. This is due to the highly non-linear device characteristics. Also, the fast switching transient of SiC MOSFET excites the circuit parasitics and makes the switching dynamics highly involved. This work focuses on characterization and modelling of switching transient of SiC MOSFET.

Simulation and analytical modelling approaches are used to model the switching dynamics and estimate switching loss. The behavioural modelling approach is a widely used simulation based approach (i.e., Spice simulation) and it can capture the switching transient with sufficient accuracy. This approach uses lumped parameter model (circuit model) of the device and external circuit and can be simulated in circuit simulator like MATLAB/Simulinkr. This implies numerical solution of a set of coupled non-linear differential equations. On the other hand, analytical modelling approach is based on the simplified approximate solution of a set of coupled non-linear differential equations obtained from the behavioural model. In order to obtain the approximate solution, the entire switching process is divided into different modes with clearly defined transition conditions. Different approximations are used in each mode to arrive at analytical closed-form solutions or reduced order coupled non-linear differential equations. This model is computationally efficient and can be implemented easily in freely available programming platforms such as C or Python. Also, the parameters required for analytical models can be obtained from the device datasheet. This modelling approach is beneficial for the converter design when switching loss and junction temperature need to be evaluated over several operating points for many available devices from different manufacturers.