7.3.5 Valley-Switching and Valley-Skipping

The UCC28742 utilizes valley switching to reduce switching losses in the MOSFET, reduce induced-EMI, and minimize the turn-on current spike at the sense resistor. The controller operates in valley-switching in all load conditions unless the V_DS ringing diminished.

Referring to Figure 16 below, the UCC28742 operates in a valley-skipping mode in most load conditions to maintain an accurate voltage or current regulation point and still switch on the lowest available V_DS voltage.

Figure 16. Valley-Skipping Mode

The UCC28742 forces a controlled minimum switching period corresponding to the power supply operating frequency. In each switching cycle, after the minimum period is expired, the UCC28742 looks for the next resonant valley on the auxiliary winding. The controller initiates a new power cycle at this valley point which corresponds to a reduced voltage level on the power MOSFET. If at the point in time when the minimum period expires ringing on the transformer winding has decayed such that no further resonant valleys can be detected a new power cycle is initiated following a fixed time, t_ZTO. This also applies when primary inductance L_P is designed with a high value that causes de-mag end ringing cycle longer than t_ZTO. When either happens, the valley switching is lost but the converter output voltage will still be in regulation.