The UCG28826 is designed to operate with soft switching and primary FET turn-on at a valley to reduce switching losses. The converter operates in valley switching except during peak load transients during which control can transition to CCM mode (if enabled using CDX pin). During valley switching mode, the target valley and peak current threshold are governed by the control law of Figure 6-4 and Equation 1:

Equation 1.

During valley switching, with increasing output power, the peak current threshold continues to increase linearly as per above equation. The switching frequency also varies based on I_PK and valley targets corresponding to the instantaneous FB pin voltage. When output power is increasing from light loads to rated power, the control transitions from 6th valley till 1st valley with corresponding linear increase in I_PK threshold. As output power continues to increase further to take FB voltage to the edge of 1st valley operation, the converter transitions into CCM mode operation with I_PK clamped to its max. value I_PK,MAX and increase in switching frequency F_SW with further increase in output load. This clamp on I_PK,MAX limits the transformer size in a high density power supply design. See Section 6.4.2.4 for details of CCM mode of operation. If output power reduces while operating in 6th valley, the control transitions to frequency foldback mode to operate at higher valleys and lower frequency to reduce switching losses further.

The FB pin voltage thresholds for valley transitions include a hysteresis and vary depending on increasing or decreasing P_OUT to enable valley locking and prevent any audible noise due to hopping between valleys. Refer to the Electrical Characteristics table for FB pin voltage thresholds which determine the mode of operation for UCG28826. For zero optocoupler collector current with large P_OUT, the FB pin is pulled up to V_FBOPEN through a 60kΩ resistor.

Figure 6-5 Valley counter with continuous valley occurrence

Typically, the control counts the valleys and turns on the primary GaN HEMT once the target valley is reached, shown in Figure 6-5. For the case when SW node waveform is damped so the valleys disappear before reaching the target valley, a DCM ring fixed timer of 3.75μs starts to continue counting valleys and turn on the primary GaN HEMT once the valley target is reached, shown in Figure 6-6. During startup (soft start) when V_OUT is small, if the valleys don't appear, the controller turns on the primary GaN HEMT after 100μs from last turn on, to switch at 10kHz (min. frequency clamp during soft start) for initial few cycles to avoid latch up condition.

Figure 6-6 Valley counter with missing valleys

The device operates in CCM mode for maximum 4ms to support any transient output load conditions, as seen in notebook chargers and other applications. The converter returns to 1st valley QR operation after expiry of this 4ms CCM timer. At all times during operation of UCG28826, the maximum switching frequency can be limited with a frequency clamp setting programmable with a resistor from FCL pin to GND, as detailed in Section 6.3.8.