A buck converter with a given output filter is operated with pulsewidth modulated (PWM) and quasi-resonant switching schemes at the same nominal load and switching frequency. Electromagnetic interference (EMI) generated by the natural switching action of the converter is examined by spectral analysis. Interference caused by excitation of parasitic elements is examined experimentally. Quasi-resonant converters are found to have a lower switching frequency harmonic bandwidth than the equivalent PWM converter. The most significant parasitic responses are the turn-on current and turn-off voltage of the catch diode, and the gate current of the MOSFET. A significant decrease in radiated and conducted noise is obtained when the gate drive voltage rise and fall times are increased, which is possible without loss of efficiency using quasi-resonant switching.
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