Boosting Efficiency in Off-Line Power Converters

Focus:

Power factor correction (PFC) is needed to meet the EN61000-3-2 standard for input current harmonic content for power supplies, yet the inclusion of a PFC pre-regulator hurts power supply efficiency, making it harder to meet efficiency standards like 80 PLUS. This article discusses control techniques that improve the efficiency of the PFC pre-regulator. It begins by discussing the two stage approach to PFC, how the efficiencies of each stage multiply to determine overall efficiency, and it identifies the reverse recovery current in the boost diode as a major contributor to PFC losses. One control technique that eliminates these losses is the transition-mode PFC, and its advantages and disadvantages are discussed; FET switching losses are also discussed; which leads to a discussion of the technique known as the PFC boost follower which reduces these switching losses but requires more hold up capacitance. The efficiency improving benefits of the semi-bridgeless PFC pre-regulator are also discussed. Finally, the ability of interleaved PFC boost stages to reduce passive component size while improving full load efficiency is discussed including the use of phase management (aka phase shedding) to improve efficiency at lighter loads.

What you’ll learn:

• How to implement power factor correction in power supplies while still achieving high efficiency
• How to understand advantages and disadvantages of various PFC control techniques such as transition-mode, boost follower, semi-bridgeless, and interleaved operation

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Author & Publication:

Michael O'Loughlin, Applications Engineer, Texas Instruments, Dallas , Power Electronics Technology, May 01 2008

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