Adjustable Clamp Circuit Enables Use of Lower-Voltage MOSFET In High-Voltage Flybacks

Focus:

Flyback converters often require an expensive high-voltage MOSFET to enable operation from a high input voltage such as 240 Vac. The voltage seen at the MOSFET drain is equal to the dc voltage from the rectified ac line plus the flyback voltage, the clamped leakage inductance voltage, and some design margin. An adjustable clamp circuit presented in this article can dramatically reduce the required MOSFET drain-to-source voltage (VDS) rating. By lowering the necessary VDS rating, this circuit permits use of a discrete MOSFET with better switching performance, lower RDS(ON) and lower cost. Alternatively, the circuit can enable the use of a controller IC containing an integrated MOSFET. Included here is a comparison of the cost of a 1200-V MOSFET required in a conventional flyback design with the components that would replace it in the propsed solution. Four versions of the proposed clamp circuit are discussed.

What you’ll learn:

• How to reduce the required MOSFET voltage rating in flyback converters by adding an adjustable clamp circuit

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Author & Publication:

Ed Wenzel, STMicroelectronics, Schaumburg, Ill., How2Power Today, Mar 30 2011

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