UCC28780: UCC28780: Unable to start，RUN sets high in a wrong point of time, no PWML pulse

Hi XiaoFeng,

Could you please show me a waveform that can present the RUN sets high before VDD reaches Vdd(on). 

I stronger suggest you capture the low side FET Vds , transformer primary winding current  during your debug , these two signals can tell you much information that others .  here attached another application note of UCC28782 which also help for you to debug UCC28780. 

https://www.ti.com/lit/an/sluaaf9/sluaaf9.pdf?ts=1639722845518&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FUCC28782%253FkeyMatch%253DUCC28782%2526tisearch%253Dsearch-everything%2526usecase%253DGPN

BTW: May i ask what the purpose of R15 and R16 which connected to the RUN pin , looks like they are dummy load for RUN , so the RUN pin voltage rise to high with a very slow slew rate. I think the two resistors can be removed.

Thanks.

thanks for your reply,

Actually, I removed the R15 and R16 to do the test. 

Here is the waveform which can present the RUN sets high before VDD reaches Vdd(on).

ch1: RUN

ch2: HVG

ch3: VDD

Due to the small decline rate of VDD (ater VDD reaches the Vdd(on)), I overlapped the VDD and HVG signal in the waveform, try to present that RUN sets high before VDD reaches Vdd(on), and sets low when VDD reaches Vdd(on).

thanks again for your reply.

thanks for your help,

I will follow the debug guide steps to try again. thank you.