The UCG2882x includes frequency clamp to limit the maximum switching frequency. Frequency clamps are useful during design optimizations to pass emissions standards and reduce switching losses by limiting the switching frequency to a certain value. The device offers four maximum frequency clamp settings at 100kHz, 140kHz, 250kHz, and 500kHz which are selected with a resistor from FCL pin to ground as per values in Table 7-4. There are no condition where the switching frequency exceeds the chosen value of clamp frequency, except in burst mode when the clamp frequency is set to 250kHz. The minimum switching frequency is also clamped to a fixed 25kHz to prevent switching in the audible frequency range and noise from the flyback converter. Low switching frequency can occur during operation in higher valleys or frequency foldback mode.

Figure 7-9 illustrates example waveforms that demonstrate the operation of the frequency clamp. In this case, the valley target based on the FB pin voltage at the 2nd valley. After the secondary winding current reduces to zero, the VDS voltage on SW pin begins to resonate and valleys are counted by the valley detection block. The primary-side GaN HEMT does not turn-on at the 2nd valley because the frequency clamp timer is still counting. Only after the expiration of the frequency clamp timer that the primary GaN HEMT turns-on at the next valley, which is the 3rd valley in this case. Based on this logic, the turn-on of the primary GaN HEMT happens only at the next valley post expiration of the frequency clamp timer.

Figure 7-9 Primary GaN HEMT Turn-On Delay Due to Frequency Clamp