Extracting Bode Plots from Output Impedance

by Steven Sandler and Charles Hymowitz, Picotest, Power Electronics Technology, Feb 27 2012

Focus:
The authors have written previously about the measurement of output impedance as a means of non-invasively measuring the phase margin of a power converter’s control loop. Although this technique was not intended to generate Bode plots for a converter, in the case of a linear regulator, the same measurement setup used to non-invasively measure phase margin can be used to “extract” a set of Bode plots from impedance measurements. The user measures the open-loop and closed-loop output impedances of a regulator, and then calculates the Bode gain and phase plot values using a simple equation. Here, the authors measure the open- and closed loop impedances of a voltage regulator, use Agilent’s ADS tool to generate Bode plots from the impedance measurements, and then use ADS to generate Bode plots for the same regulator using simulation. The extracted and simulated Bode plots agree closely up to a point past the gain crossing. The authors also see close agreement when they compare extracted Bode plots for an LM317 with measured results.

What you’ll learn:

• How to generate Bode plots for linear voltage regulators when the device does not allow access to the feedback loop

Notes:
1. This article appeared in print in the March 2012 issue of PET magazine. 2. This Bode plot extraction technique is typically not useful for obtaining Bode plots of switching regulators.

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