Maximize The Accuracy Of Switching Loss Measurements In Power Supplies
by Michael Schnecker, Rohde & Schwarz, Columbia, Md., How2Power Today, Feb 29 2012
Focus:
Measuring the loss of switching transistors in switched-mode power supplies require significant dynamic range due to the low loss of these switching devices in the on state and the large voltage across them in the off state. While averaging is an effective way to improve the dynamic range of such measurements, the dynamic variation of the switching waveform results in distortions that limit the range of the measurement. This article explains the benefits of using an oscilloscope with a high effective number of bits (ENOB) and a high-resolution mode to measure switching loss. The combination of high ENOB and high-resolution mode can provide nearly the same performance as large averaging while preserving the waveform fidelity. The article reviews the mechanisms for switching transistor loss in dc-dc converters and explains how loss levels dictate bandwidth and dynamic range requirements for oscilloscopes used to measure switching losses. It then explains how waveform averaging and high-resolution mode reduce noise and improve the dynamic range of measurements; discusses the limitation of waveform averaging, and presents example measurements with waveform averaging and high-resolution (or enhanced-resolution) mode. The article comments on instrument selection for measuring transistor losses at kilohertz switching frequencies and how requirements are impacted by megahertz switching frequencies.
What you’ll learn:
- How to understand bandwidth and dynamic range requirements for oscilloscopes used to measure switching transistor losses in switched-mode power supplies
- How to understand the benefits of high- or enhanced-resolution mode when measuring switching transistor losses
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