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2020.06.24 SiC Power Device

Introduction

SiC MOSFETs: Behavior of Gate-Source Voltage in a Bridge Configuration

As the first article in this series on basic knowledge and applications of SiC power devices, we begin with "SiC MOSFETs: Behavior of Gate-Source Voltage in a Bridge Configuration".

Introduction

Power switching devices such as MOSFETs and IGBTs are used as switching elements in various power supply applications and power lines. The circuit types in which they are used are also many and varied, and in addition to use as independent single components, they are also frequently used in series and parallel connections and the like.

Among these are bridge configurations, in which switching elements are series-connected in high and low sides, the devices are generally turned on and off in alternation. Below is shown a synchronous boost circuit with a basic bridge configuration; the waveform diagram is an example of the drain-source voltages (Vds) and the drain currents (Id) of the low-side (LS) MOSFET and the high-side (HS) MOSFET, which are turned on and off in alternation by gate signals.

The currents flowing in the elements and the changing voltages affect each other in a complex manner depending on the switching operation. In circuits that handle high voltages and large currents in particular, voltage and current behavior occurs that is affected by parasitic components arising from the mounting board and connections, and this behavior may result in unstable operation or drops in efficiency, in some cases possibly causing such problems as increased losses and abnormal heat generation.

In recent years, it has become possible to convert large amounts of power through fast switching using high-performance power devices such as SiC MOSFETs. However, effective use of these devices requires in-depth understanding of the switching operation. Here, we will focus on the behavior of the gate-source voltages of each of the MOSFETs in a MOSFET bridge configuration, and will examine the following topics, using simple synchronous boost circuits as examples.

・MOSFET bridge configurations and synchronous boost circuits

・Gate driving circuits and turn-on/turn-off operation

・Currents and voltages arising due to dVds/dt and dId/dt

・Behavior of gate signals at turn-on

・Behavior of gate signals at turn-off

* Changes in the topics planned for discussion may occur.

Key Points:

・Power switching devices are used as switching elements in various power supply applications and power lines.

・Circuit types used vary greatly, and there are many methods of use as well.

・In a bridge configuration in which switching elements are series-connected on high and low sides, the elements affect each other through their alternating on-off operation.

・Detailed understanding of switching operation is necessary for successful fast switching conversion of large amounts of power.

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