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When Bode Plots Fail Us

by Steve Sandler, Paul Ho, and Charles Hymowitz, AEi Systems, Vendor website, Jan 20 2012

Focus:
When testing power supply designs, bode plot measurements of gain and phase margin are commonly used to predict control loop stability. But as this article demonstrates, there are instances where these invasive measurements (i.e. ones that break the loop) do not predict instabilities, which that are clearly visible in load-step measurements. The authors discuss their solution--a non-invasive approach in which they measure the output impedance of the power converter at the feedback points. This article describes two case studies that demonstrate the problem—-a five-output winding flyback converter and an op amp buffer. In each case, Bode plot measurements (or simulation) show adequate phase margin, but ringing appears in the load step response. Meanwhile, the output impedance measurements and Nyquist plots confirm inadequate values of phase margin. In the first case study, a current injector and network analyzer measure small-signal output impedance and group delay, and the analyzer uses these measurements to calculate Q and phase margin. Among the issues discussed here are the reasons for the discrepancies among the different measurements, other techniques used in assessing stability, and implications of phase margin for closed-loop performance.

What you’ll learn:

Notes:
This application note was also published as an article in the May 2012 issue of Power Electronics Technology, www. powerelectronics.com. To better understand the technique used to measure output impedance in the first case study, see Sandler and Hymowitz's earlier article, "New Technique for Non-Invasive Testing of Regulator Stability" in Sept 2011 PET magazine.

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