Another feature added to the UCC28070 is frequency dithering by presenting both fixed frequency and frequency dithering. Frequency dithering is a technique that makes a power supply more easily pass EMI testing. It basically dithers the operating frequency of the PFC controller to frequencies above and below the set point. This spreads the EMI energy over a wider frequency spectrum, reducing the peak amplitudes of the EMI footprint (blue line). If the frequency were fixed with no dithering, the energy would cause high EMI spikes at fixed frequencies like the waveform above (red line). This feature simply helps the system pass EMI testing.