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Original question:
Hello -
We are experimenting with the UCC28712 in a 24V power supply design. We are using a SiC MOSFET as the primary switch in the flyback circuit. During startup, we observed the following waveform where the blue trace is the gate signal, and the yellow trace is the voltage across the primary coil (drain to +DC bus). This shot was taken when input voltage was ~ 100Vdc. As we go up in input voltage, the gate signals eventually all look like the middle one in this screen shot. Any thoughts on what would be causing the gate output to shutoff like this?
Thanks!
Brett
Hi, Brett:
Thanks for reaching us.
Would you please share input/output spec and test condition to me to be a reference?
Gate signal would be turned off by CS signal, and turned on by Vs. Thus, please check VCS waveform and to see if it touches CS limit. Sometimes it happens because of transformer operates in CCM shortly during start-up. Please measure VDS and VCS( or IDS) and it could provide the information to see if transformer operates in CCM shortly or the gate off just because switching noise on CS pin.
And you also mentioned that gate pulse is very short after start-up. May I know what output condition is? Is it no load or light load? If yes, please increases the output loading and to see if gate pulse follows the symptom as below. It would be okay when gate pulse operates as the curve below.
( from no load to full load, FM mode -> frequency increased; AM mode-> pulse width increased; FM mode-> frequency increased )
If the gate pulse does not operate like this, it could be impact by switching noise or improper transformer design. SLUAAC5 can be a refence and help you clarify this phenomenon caused by noise.
Regards,
Wesley
