7.3.2 Valley-Switching and Valley-Skipping

The UCC28740 uses valley-switching to reduce switching losses in the MOSFET, to reduce induced-EMI, and to minimize the turnon current spike at the current-sense resistor. The controller operates in valley-switching in all load conditions unless the V_DS ringing diminishes to the point where valleys are no longer detectable.

As shown in Figure 11, the UCC28740 operates in a valley-skipping mode (also known as valley-hopping) in most load conditions to maintain an accurate voltage or current regulation point and still switch on the lowest available V_DS voltage.

Figure 11. Valley-Skipping Mode

Valley-skipping modulates each switching cycle into discrete period durations. During FM operation, the switching cycles are periods when energy is delivered to the output in fixed packets, where the power-per-cycle varies discretely with the switching period. During operating conditions when the switching period is relatively short, such as at high-load and low-line, the average power delivered per cycle varies significantly based on the number of valleys skipped between cycles. As a consequence, valley-skipping adds additional ripple voltage to the output with a frequency and amplitude dependent upon the loop-response of the shunt-regulator. For a load with an average power level between that of cycles with fewer valleys skipped and cycles with more valleys skipped, the voltage-control loop modulates the FB current according to the loop-bandwidth and toggles between longer and shorter switching periods to match the required average output power.