The Over-Power Phenomenon in DCM/CCM-Operated Flyback Converters (Part 3): Quasi Square-Wave Resonant Mode

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 3 explores operation under quasi square-wave resonant operation. QR converters operate at a frequency where the primary inductor current always remains at the border between continuous and discontinuous modes. This is known as boundary or borderline conduction mode (BCM), or critical-conduction mode (CrM). Again, the article explains why the power delivered at high line under a fault condition, is larger than the output power delivered at low line in this mode of operation. BCM mode operation is discussed. The impact of the varying switching frequency on maximum output power is explained. Using a 65-W quasi-resonant converter design as an example, the switching frequency at low line and high line are computed and these values are used in computing the maximum output power under these input conditions. The application of over-power protection in the 65-W design example is described.

What you’ll learn:

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

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 the influence of leakage inductance on power transfer. See the October 2010, November 2010, and January 2011 issues of How2Power Today.

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Author & Publication:

Christophe Basso, ON Semiconductor, Toulouse, France, How2Power Today, Dec 16 2010

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