Loop Control: Hand Calculations Or Automation?

by Christophe Basso, ON Semiconductor, Toulouse, France, How2Power Today, Dec 15 2009

Focus:
When designing switched-mode power supplies, it is possible to get the impression that a prototype, which delivers acceptable transient response on the oscilloscope, is ready for production. But such a conclusion is unwise and potentially costly. Stray and parasitic elements often remain hidden during prototype tests. In the factory, the dispersions of these parameters, combined with silicon lot-to-lot variations, can make converters fail in quantity at final test. To avoid such situations, designers should spend time analyzing the control loop in detail. Using a current-mode flyback converter design as an example, this article describes four methods for deriving the control-to-output transfer function: 1) analytically deriving the small-signal model of the converter 2) using a SPICE large- or small-signal averaged model 3) using automated design software provided by the controller IC supplier and 4) building a prototype and extracting the ac response of the power stage using a network analyzer. This article focuses on option 3 demonstrating a spreadsheet-based design tool from ON Semiconductor.

What you’ll learn:

• How to stabilize the control loop in a power supply design
• How to design a TL431-based compensation circuit for a power supply design

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