The Small-Signal Model Of An Active-Clamp Forward Converter (Part 3): Active-Clamp Forward Converter Transfer Function

Focus:

In the third and final part of this article series, the author completes the task of building a small-signal model of the active-clamp forward converter operating in voltage mode and derives its ac transfer function. The converter is modeled as two separate converter stages: 1) a buck-boost converter operated with the transformer magnetizing current and controlled by the duty ratio and 2) a classical buck converter. The final transfer function will be the product of the transfer functions for these two stages. To obtain those transfer functions, the author substitutes Vorpérian’s small-signal PWM switch model into the large-signal models derived in part 2. Just prior to deriving the transfer function analytically using the small-signal PWM switch model, the validity of this approach is confirmed by running a SPICE simulation on the small-signal models of the converter stages and comparing the results with those obtained for the Spice simulation of the large-signal models in part 2. Then, after the transfer functions are derived for for each converter stage, they are plugged into Mathcad and results are compared with the Spice simulation results. Finally, Bode-plot measurements are taken on a prototype and those results are compared with Spice simulation results using the small-signal models derived in this part 3 article.

What you’ll learn:

• How to derive a small-signal model and ac transfer function of the active-clamp forward converter

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Author & Publication:

Christophe Basso, ON Semiconductor, Toulouse, France, How2Power Today, Apr 15 2014

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