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UCD3138HSFBEVM-029: There is a problem with the primary drive waveform during startup

Part Number: UCD3138HSFBEVM-029
Other Parts Discussed in Thread: UCD3138,

We designed a full-bridge converter using the UCD3138, similar to UCD3138HSFBEVM-029.but there is a problem with the primary drive waveform during startup. During the soft start, one of the primary drive waveforms is as follows. The UCD3138 sends drive waveform for a white, then it stops sending drive waveform .Waits for about 600 microseconds, it starts sending again, then stops again,as shown in the figure below. 

Zoom in on the image as shown below.Zoom in on the image as shown below. It can be seen from the figure that drive  wave works normally at the beginning, then something wrong happens,there is one more pulse between the two drive waves. Can you  tell me what can cause this to happen?