The UCG28826 includes frequency dithering to spread the energy in the spectrum around the switching frequency to reduce electromagnetic emissions making it easier to qualify various emissions standards. Since this device uses peak current mode control to turn-off the primary GAN HEMT, the dithering in frequency is achieved with small change to the value of peak current threshold in every switching cycle. This changes the peak current, and thus the on-time, off-time and switching frequency in every cycle. The per cent change to peak current threshold varies based on a fixed 390Hz triangular carrier signal with 32 steps alternating in sign at every transition from minimum (0%) to maximum value (called dithering depth, 6.25% or 12.5%) in every carrier cycle. The alternating sign of dither perturbation at each step and the low frequency output pole of flyback converter topology averages the cycle-to-cycle power delivery, causing small impact in output ripple due to dithering. The dither perturbation to peak current is asynchronous with the switching frequency and the instantaneous change in peak current is calculated based on the dithering signal during the primary GAN HEMT turn-on time.

The dithering depth can be chosen with a resistor from IPK pin to GND as given in Table 6-2. Figure 6-10 shows the dithering carrier and peak current perturbation waveforms to dither the switching frequency.

Figure 6-10 Frequency dithering to reduce emissions