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UCC28780: Questions about simplis simulation of the active clamp flyback controlled by UCC28780

Part Number: UCC28780

Hi, esteemed experts. I use the official UCC28780 simulation circuit given inside simplis to simulate. when I use DC source, the circuit can work properly,  Vcs can reach about 0.28V during the first 2μs PWML waveform.

The DC source simulation diagram and waveform diagram are as follows.

But when I just change the DC source to AC source, i.e. 50Hz AC, without changing any other circuits, the emulated circuit does not work properly. During the first 2μs PWML waveform comes out, Vcs cannot reach 0.28V and the circuit does not work properly anymore, then I observe the voltage of Cbulk capacitor, during the first pulse of 2μs PWML, the Cbulk voltage value is very small, so I guess that the UCC28780 can not start normally due to the undervoltage, how should this situation be handled, and is it possible to extend the start-up time of the UCC28780? Thanks.