SiC Power Device｜Application

SiC MOSFETs： Snubber Circuit Designs　ーSummaryー

2024.11.13

SiC MOSFETs： Snubber Circuit Designs – Summary –

Due to their fast switching speeds compared with conventional power semiconductors, there are sharp changes in voltages and currents of SiC MOSFETs. As a result, the effects of the package inductance of the device itself as well as the wiring inductance of the peripheral circuit cannot be ignored, and large surges occur. As methods of suppressing such surges, this series has explained methods involving addition of snubber circuits and design methods.

Below are summarized the key points explained in the articles of this series, along with links to the articles.

▶SiC MOSFETs： Snubber Circuit Designs　– Introduction –

Key Points of This Article

・In recent years, there have been steadily expanding applications of SiC MOSFETs in various power conversion applications, due to their fast switching operation.

・However, their fast operation results in the occurrence of large surges between drain and source. Such surges must be suppressed.

・Snubber circuits are one way to suppress such surges.

▶Surges Occurring between Drain and Source

Key Points of This Article

・Surges that occur between drain and source are caused by resonances due to inductive components and the MOSFET parasitic capacitance.

・In many cases it is not realistic to use a layout that minimizes wiring inductance; hence it is important to locate a snubber circuit as close as possible to switching devices to reduce wiring inductance.

▶Types and Selection of Snubber Circuits

Key Points of This Article

・In order for a snubber circuit to be maximally effective, it must be mounted as close as possible to the switching device.

・Snubber circuits include circuits that are combinations of passive components such as resistors, inductors, and capacitors (R, L, C), as well as active circuits using semiconductor devices.

・In this article, C snubber circuits, RC snubber circuits, discharge RCD snubber circuits, and non-discharge RCD snubber circuits are introduced as circuit designs that do not require control and are attractive with respect to cost.

▶C Snubber Circuit Design

Key Points of This Article

・The larger the value of CSNB in a C snubber circuit, the greater is the surge suppression effect.

・However, the inductor LSNB in the snubber circuit must be smaller than LMAIN, and the capacitor ESL must be added to LSNB, so that care is required.

▶RC Snubber Circuit Design

Key Points of This Article

・When designing an RC snubber circuit, CSNB and RSNB are determined taking the power dissipation PSNB into consideration, and the resonance angular frequency ωSNB must be set well above the surge resonance angular frequency ωSURGE.

▶Discharge RCD Snubber Circuit Design

Key Points of This Article

・The design of a discharge RCD snubber circuit is basically the same as that of an RC snubber circuit.

・However, due to surge absorption by diodes, there is no need to check the resonance angular frequency using equation (5) as in the case of an RC snubber circuit.

・A diode with a small recovery current must be selected for use in this circuit.

▶Non-Discharge RCD Snubber Circuit Design

Key Points of This Article

・In a non-discharge RCD snubber circuit, the power dissipated by RSNB is only a fraction of the surge power, so that the allowable power dissipation for RSNB can be comparatively small.

・As a result, the capacitance value of CSNB can be made large, so that the clamping effect can be enhanced, and the switching frequency fSW can be raised.

・In general, a circuit with non-discharge RCD snubber circuits added has lower efficiency under light loading but higher efficiency under heavier loads. This is because of the surge suppression effect of the snubber circuits under heavy loading, as a result of which switching losses are reduced.

▶Differences in Surge Occurrence Depending on Package

Key Points of This Article

・Surges that occur between the drain and source of a SiC MOSFET are different depending on the package type.

・Compared with the TO-247N package, the TO-247-4L enables faster switching due to modification of the driving circuit path, and consequently surges tend to be larger.

【Download Documents】 Basics of SiC Power Devices

This handbook explains the physical properties and advantages of SiC, the differences in characteristics and usage of SiC Schottky barrier diodes and SiC MOSFETs with a comparison to Si devices, and includes a description of full SiC modules with various advantages.

SiC MOSFETs： Snubber Circuit Designs　ーSummaryー
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