Understanding Bandwidth Requirements When Measuring Switching Characteristics in Power Electronic Applications

Understanding Bandwidth Requirements When Measuring Switching Characteristics in Power Electronic Applications

By Bernhard Holzinger, Michael Zimmermann, Ryo Takeda, and Mike Hawes, Keysight Technologies

When using an oscilloscope for measuring signal switching characteristics in a power electronic application, one important aspect to consider is that the measurement system has sufficient bandwidth to be able to accurately measure the dynamic characteristics of the device under test. With the advent of wide bandgap semiconductors, we are facing a transition to higher operating frequencies and more importantly shorter switching times.

This article was originally published in Bodo’s Power Systems Magazine, in May 2021.  It is reprinted with permission.

In a previous article, “Overcoming Challenges Characterizing High Speed Power Semiconductors” [1], Keysight had a general look into the bandwidth required for Double Pulse Test Systems. In that article, we discussed bandwidth, but with a focus on the requirement for the Double Plus Test (DPT) circuit design. We highlighted how there are different contributors to the overall system performance. In this article, we are focusing on the aspect of measuring and bandwidths that are required for specific signals.

This article proposes a method for estimating the required measurement bandwidth. Two aspects must be considered: 1) the properties of the signals to be measured, and 2) the properties of the measurement setup (i.e. oscilloscope) itself.  Both aspects will be discussed to estimate the bandwidth requirements needed for measuring switching characteristics in power electronic applications accurately.

Signal Properties

A signal has many different properties which determine the measurement requirements, for instance the ‘signal levels’ to be measured. The discussion in this article focuses on the properties dictating the bandwidth requirements.

The bandwidth requirements are determined by two properties:

• Transition times:  The times it takes for a signal (e.g. VDS) to switch from high to low or vice versa, when a transistor is turned on or off. It is not the switching frequency of the transistors that determine the bandwidth requirements, but the transition times.

• Ringing: There are always parasitic capacitance and inductance present in each circuit, forming a LC-circuits. When a transistor is switching, those LC-circuits become excited and start to oscillate. The oscillations are damped by resistive elements in the circuit and therefore fade away after some time