The Small-Signal Model of An Active-Clamp Forward Converter (Part 1): Basics of The Forward Converter And Its Transfer Function

Focus:

Introduced more than 20 years ago, the active-clamp architecture elegantly overcomes multiple limitations of the standard forward converter topology. Among other benefits, it increases the allowable range of duty ratio, provides self-driven synchronous rectification, and makes possible zero-voltage switching (ZVS), which in turn enables an increase in switching frequency and the associated shrinking of magnetic components. But as with any dc-dc converter, you need to obtain the power stage’s small-signal response before attempting to stabilize the loop. The purpose of this 3-part article series is to show how to build a small-signal model of the active-clamp forward converter operating in voltage mode and derive its ac transfer function. This series begins with a detailed study of the classical single-switch forward converter with a demagnetization winding, explaining its operation and how to obtain its transfer function under voltage-mode operation. Along the way, the pros and cons of different methods of obtaining small-signal response of a power converter are discussed with the author making the case for the two approaches--the PWM switch model and SPICE simulation--that he uses in this article series.

What you’ll learn:

• How to derive the ac transfer function of a single-switch forward converter
• How to understand the operation of the single-switch forward converter
• How to understand the options for obtaining small-signal response of a power converter

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Author & Publication:

Christophe Basso, ON Semiconductor, Toulouse, France, How2Power Today, Feb 14 2014

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