Power conversion design help

Tips and tricks: Setting up a power conversion circuit requires forethought to avoid headaches. Advice follows to ensure control loop stability.

John Bottrill

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Setting up a power conversion circuit requires forethought to avoid headaches. John Bottrill, senior applications engineer at Texas Instruments, offers advice to ensure control-loop stability.

- The optocoupler loop gain requires that the designer know the optocoupler’s current transfer ratio (CTR).

- Plot the power converter’s open-loop gain as a function of frequency without the effect of the feedback circuit by multiplying all the converter’s various gain elements together.

- The converter operates at a specific-switching frequency. The designer knows that the total open-loop gain must cross zero dB at a point below one-sixth of that frequency. Most designers leave a margin for component tolerances. This usually can be accomplished by designing the loop to cross over at approximately one-tenth of that value.

- Ideal conditions in textbook examples can deliver the desired phase margin, but real-world conditions can differ. It may be necessary to filter the voltage source that provides the current to the optocoupler. This helps to remove a “sneak path” and to control the gain of the feedback loop with the components around the three-terminal adjustable shunt regulator.

See detailed online examples, four equations, and 18 figures to illustrate these tips in a Control Engineering tutorial: Gain consideration using a shunt regulator and optocoupler feedback, by John Bottrill, TI; edited by Mark T. Hoske, content manager, CFE Media, Control Engineering.

www.power.ti.com