The Over-Power Phenomenon in DCM/CCM-Operated Flyback Converters (Part 2): Fixed-Frequency CCM Operation

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. Part 2 focuses on flyback converters operating in continuous conduction mode (CCM), describing how two factors--the propagation delay during the turn-off event and the mode transition from continuous to discontinuous operation--causes a flyback to deliver more power at high line than at low line. The equations for power transfer in CCM are derived. The design example of a 65-W adapter is presented. The implementation of over-power protection in the NCP1250, which relies on an auxiliary winding to sense the input voltage, is discussed in further detail.

What you’ll learn:

• How to understand why the output power capability of a flyback converter operating in continuous conduction mode (CCM) may increase under high-line conditions

Notes:

The other parts of this article series explain what happens when a flyback converter operates in discontinuous conduction mode (DCM), the case of quasi-resonant operation, and the influence of leakage inductance on power transfer. See the October 2010, December 2010, and January 2011 issues of How2Power Today.

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Author & Publication:

Christophe Basso, ON Semiconductor, Toulouse, France, How2Power Today, Nov 23 2010

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