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May 22, 2001

Feedback Loop Stabilization

Question: How do you stabilize the feedback loop on a forward converter?

Original Question: How do you determine the type of compensation network [for a forward converter] and calculate it's values? T. J. 05/22/01

Answer: This is too complex to cover in an email. Being a forward (buck-derived) converter you will not have to worry about right-half zeroes and components varying with duty cycle or load -- so that makes it easier.

The general approach is to measure or calculate the bode plot of your power stage and then give the control circuitry the gain and phase shaping you need to go through unity gain at your selected cross-over frequency with the gain and phase margin you want. Starting points are often a cross-over at 1/10 the switching frequency with a phase margin between 45 and 90 degrees and a gain margin between 6 to 12 dB under worst case conditions of line, load, temperature, and component variation. (Added later same day: Ray Ridley in his On Semiconductor seminar recommends at least 50 degrees phase margin for power supplies and at least 10 dB gain margin.) Normally when you select one component in the compensation, say a resistor, the rest drop out from the calculation you do to get the gain and shape the frequency.

You should be able to find more help in vendor application notes or one of the many texts on switching power supply design.

Posted by Jerrold Foutz at May 22, 2001 04:48 PM