The Over-Power Phenomenon in DCM/CCM-Operated Flyback Converters (Part 4): The Leakage Inductor Contribution

Focus:

Under certain operating conditions where the feedback loop is lost, the output power delivered by a flyback converter may double at high-line input versus low-line input. This four-part article series explains the origins of this excess power and how it can be controlled so that output power remains within a reasonable range. In the previous parts of this article series, during the derivation of the power transfer equations, estimated values for the total converter efficiency were used. In this fourth and final part, the author calculates the theoretical maximum transmitted power by explaining the role of the transformer leakage inductance at the switch opening. This leakage inductance delays the rise of the secondary-side current and reduces the available output current. By diverting a portion of the primary-side stored energy, it affects the total efficiency of the converter. The example of a 65-W converter based on NCP1250 is presented, its maximum output power (under low and high line) is calculated and a test circuit is built to verify the output power calculations. Discrepancies between calculated and measured values at low line are explained.

What you’ll learn:

• How to understand the influence of the leakage inductance in the power transfer of a flyback converter

Notes:

The other parts of this article series explains what happens when a flyback converter operates in discontinuous conduction mode (DCM), continuous conduction mode (CCM), and in boundary conduction mode (BCM). See the October 2010, November 2010, and December 2011 issues of How2Power Today.

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Author & Publication:

Christophe Basso, ON Semiconductor, Toulouse, France, How2Power Today, Jan 31 2011

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