Modeling The Effects of Leakage Inductance On Flyback Converters (Part 1): Converter Switching

by Christophe Basso, ON Semiconductor, Toulouse, France, How2Power Today, Nov 16 2015

Focus:
The frequency response of a flyback converter operating under voltage mode (VM) control and driven in continuous conduction mode (CCM) is that of a second-order system. If the vast majority of analyses predict a transfer function whose quality factor is solely affected by the various losses (ohmic paths, magnetic losses, recovery time-related losses and so on), very few tackle the damping effect brought by the leakage inductance. However, transient simulations predict the damping of output oscillations as the leakage inductance increases. Because formulas available in the literature do not reflect this effect, a new flyback converter model is necessary and will be described in this article. Part 1 of this series begins by explaining the leakage inductance-induced damping effect on flyback waveforms. These effects include 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; lower than expected output voltage; and increased power dissipation in the RCD clamping network. The article then derives a new dc transfer function that accounts for the impact of leakage inductance on output voltage. Finally in this part, the leakage inductance effects are observed in measurements taken on a prototype circuit.

What you’ll learn:

• How to understand the impact of leakage inductance on the operation of a CCM flyback converter under voltage-mode control
• How to model the effect of leakage inductance in a CCM flyback converter under voltage-mode control

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