Modeling The Effects of Leakage Inductance On Flyback Converters (Part 2): The Average Model

Focus:

The first part of this article described the switching effects produced by leakage inductance: a reduced effective duty ratio, which extends the secondary-diode conduction time and delays the appearance of the secondary-side current after the main switch has turned off. As a result, the output voltage is lower than what the original formula predicted and power dissipation increases in the RCD clamping network. Given the impact of the leakage term on operating waveforms, it would be interesting to investigate its influence on the small-signal response of the flyback converter. However, in order to perform small-signal analysis, a good average (or large-signal) model is needed. Here in part 2, the author develops this average model by applying Vorpérian’s PWM switch model to the CCM flyback converter with adjustments that account for the effects of leakage inductance. But first, he uses the cycle-by-cycle model to demonstrate the effects of leakage inductance on the load-step response of the converter, showing how it impacts ringing, steady-state output voltage and output impedance.

What you’ll learn:

• How to develop a large signal of a CCM flyback converter under voltage-mode control that accounts for leakage inductance
• How to understand the impact of leakage inductance on the stepload response of a CCM flyback converter under voltage-mode control

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Author & Publication:

Christophe Basso, ON Semiconductor, Toulouse, France, How2Power Today, Dec 17 2015

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