Selecting and Using Ferrite Beads for Ringing Control in Switching Converters

This whitepaper shows how small 0603 or 0805-sized ferrite beads can be used to reduce the slew rate of the high side MOSFET´s switching transitions in a synchronous buck converter, which in turn reduces the amplitude and the duration of the ringing. In particular, slowing the rising slew rate gives excellent results while increasing the switching loss only slightly. This task is normally done by carefully selecting and tuning a resistance in series with the gate of the MOSFET, or series with the positive supply voltage of the gate driving circuit. However, a ferrite bead of the same size as the resistor performs as well or better and can be selected from product datasheets with far less time spent on the lab bench.

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