The Over-Power Phenomenon In DCM/CCM-Operated Flyback Converters (Part 1)

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. This allows the power converter to meet IEC950's Limited Power Sources (LPS) specification and eliminates the need to oversize the output diode. Part 1 focuses on flyback converters operating in discontinuous conduction mode (DCM), describing how the propagation delay during the turn-off event causes a flyback to deliver more power at high line than at low line. The equations for power transfer in DCM are derived. The impact of propagation delay on max output power when the feedback loop is lost is explained, and this explanation is illustrated with the design example of a 30-W adapter. Two techniques for limiting output power--adding an offset to the current-sense voltage and reducing the internal clamping value are described. Implementation of the latter method in the NCP1250 controller is discussed and test results are shown.

What you’ll learn:

• How to understand why the output power capability of a flyback converter operating in discontinuous conduction mode may increase under high-line conditions and why it matters
• How to limit the maximum output power delivered by a flyback converter across a wide input voltage range

Notes:

The other parts of this article series explain what happens when a flyback converter transitions from continuous conduction mode (CCM) to DCM, the case of quasi-resonant operation, and the influence of leakage inductance on power transfer. See the November 2010 through January 2011 issues of How2Power Today.

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Author & Publication:

Christophe Basso, ON Semiconductor, Toulouse, France, How2Power Today, Oct 22 2010

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