How to Measure the Loop Transfer Function of Power Supplies

Focus:

If you’ve never measured the gain and phase of a power converter’s control loop and generated Bode plots, and you don’t have access to a network analyzer or even a purpose-built injection transformer, this 8-page application note will tell you how to accomplish these tasks using more rudimentary test equipment. In other words, the note explains how to take Bode plot measurements using a signal generator, standard transformer, and oscilloscope. The methodology does not promise convenience or high accuracy, but gives new engineers an introduction to Bode plot measurements and enables them to gather some key information. The app note spells out the electrical and mechanical requirements for creating the test set up, taking the measurements and (to a lesser extent) analyzing the Bode plot. App note also explains how to measure the plant transfer function with an eye toward selecting compensation components. Finally, there is a description of how to modify the test circuit to create an artificial injection node for cases where you don’t have a low impedance node and high impedance node between which you can insert the injection resistor.

What you’ll learn:

• How to take power supply Bode plot measurements “on the cheap” using readily available test equipment

Notes:

Originally published October 2008 as National Semiconductor application note 1889 written by Frederik Postal, then subsequently republished by Texas Instruments after TI acquired National Semiconductor. No author is listed on the revised April 2013 version.

View the Source

Author & Publication:

no author specified, Texas Instruments, Vendor website, Apr 01 2013

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