Reduce EMI in Switching PS

Posted on Jun 17, 2012

Power supplies that operate at a fixed frequency have their largest EMI emission at this fundamental, fixed frequency. Emissions also occur at multiples of the switching frequency but at diminished amplitudes. The simple circuit in Figure 1 makes the switching converter operate over multiple frequencies rather than one, thereby reducing the time average at any one frequency. This scheme effectively lowers the peak emissions.

The circuit in Figure 1 is a self-starting oscillator with an oscillation frequency of approximately 500 Hz. When you apply power, C3 begins to charge up from 0V, and the output of the TL331 comparator is in a high-impedance state because its noninverting

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