Five Things Every Engineer Should Know About Bode Plots
by Steve Sandler, Picotest, Power Electronics Technology, Jan 21 2016
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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