Spread Spectrum is used to reduce the effect of EMI by converting a narrowband signal into a wideband signal, which will spread the energy across multiple frequencies. In the context of switching regulators, the switching frequency set by the oscillator is manipulated to reduce the peak energy and is distributed to other frequencies and their harmonics. Figure 3-1 illustrates how manipulating the clock frequency over time has the effect spreading energy generated by a switching power supply.

Figure 3-1 EMI Reduction by Spread Spectrum, Frequency Modulation

There are many ways to implement spread spectrum; different techniques, such as triangular analog dither, Pseudo-random, adaptive random spread spectrum (ARSS), dual random spread spectrum (DRSS) and many other techniques. Each of these techniques will typically perform better at either low frequencies or high frequencies. Note that the total EMI energy is unchanged, since there is no attenuation. Instead, the spread spectrum technique alters the shape of the conducted and radiated interfering power spectrum, reducing the level of peak emissions.