Quantification of Self-Damping of Power MOSFET in a Synchronous Buck Converter

Ringing in the switching waveform of the switching power supply (e.g., synchronous buck converter) is known to cause broadband electromagnetic interference problems in the 30-300 MHz range. The measured switching waveform shows overshoot and, then, exponential decay of the ringing. It has been observed that this exponential decay rate varies significantly between using different low-side FETs; thus, the low-side FET dominates the attenuation of the ringing. This paper provides a novel method for quantifying the losses by using the measureable quantity “ R_oss,” which is equivalent to the loss of the output capacitance C_oss of the FET. This paper describes the method of measuring R_oss and explains its relevance to the self-damping of the switch ringing. The method permits selection of a low-side FET to optimize the electromagnetic compatibility performances of the supply.