Five Things Every Engineer Should Know About Bode Plots

Focus:

In this article, the author presents an example where a Bode plot predicts that the control loop of a dc-dc converter is stable, yet a Nyquist plot reveals a poor stability margin, essentially undermining the Bode plot results. The author explains why, and then how non- invasive output impedance measurements obtained with a VNA (and running Picotest software) can provide a more thorough assessment of control loop stability. Steve has covered these points in other articles, but this one gives a very convenient summary of the five pitfalls of using Bode plots to assess loop stability. Plus it begins with a brief history of control loop analysis techniques from Bode plots to Nyquist charts to Middlebrook’s minor loop gain stability assessment to Venable’s frequency response analyzer.

What you’ll learn:

• How to understand the limitations of Bode plots as a method of assessing control loop stability
• How to understand the value of output impedance measurement in assessing control loop stability of power converters
• How to understand the history of loop stability assessment methods in power supplies

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Author & Publication:

Steve Sandler, Picotest, Power Electronics Technology, Jan 21 2016

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