Closing The Loop Of An Active-Clamp Forward Converter
by Christophe Basso, ON Semiconductor, Toulouse, France, How2Power Today, Jun 17 2019
Focus:
This article describes in detail how to compensate an ACF. After reviewing the
operation of an ACF power stage, it introduces the ACF transfer function, shows Bode
plots for a design example based on the NCP1566 active-clamp PWM controller and
discusses selection of the crossover frequency. The impact of the pulse-width
modulator (PWM) on the transfer function is also explained, along with a technique for
accounting for the PWM. The design of a type-3 compensator, as well as a strategy for
applying this compensator to the ACF, is described and demonstrated using a Simplis
simulation. All of this leads to the presentation of a complete circuit for the design
example and its key features, circuit elements and additional design considerations
are discussed. Finally, a common technique for making the Bode plot measurements of
the loop’s gain and phase margins is reviewed and the benefits of using a low-cost
oscilloscope and frequency response analyzer (Cleverscope) to make these measurements
are highlighted.
What you’ll learn:
- How to compensate an active-clamp forward converter operating in voltage-mode control
- How to define the transfer function of an active-clamp forward converter
- How to understand the benefits of making Bode plot measurements using the Cleverscope frequency response analyzer
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