UCD3138HSFBEVM-029: There is a problem with the secondary synchronous rectifier drive waveform during startup

I think there could be lots of potential causes. The first thing we need to see is the waveform on the input to the gate driver - the signal from the UCD to the gate driver. Could you send us that signal as well? Also, it would be very helpful to understand where in the startup this occurs, and what the state of the power supply is - is it in continuous mode or discontinuous mode?

The waveform on the input to the gate driver  is as follows:

Zoom in on the image as shown below:

The startup time is about 10ms. This pulse occurs in the first few hundred microseconds. Because the secondary uses synchronous rectification technology, the circuit should be in continuous mode.

It's not explicitly clear, but it looks to me like the circled section on the gate driver:

matches the circled area on the UCD DPWM pin:

If that is the case, I think it's a system noise issue, rather than a firmware or hardware logic issue.  We do have layout guidelines for the UCD itself that you need to follow carefully:  

I'm not an expert on system noise issues.  If you can confirm that the two waveforms match, I'll refer this question to a colleague who is an expert, and see if they have any additional advice for you.