UCC2897A: VOUT drop slope change when power off

Hello Johnny,
Sorry about the delay in answering your question.
Your system is shutting down without a load on the output and this will make it very difficult to detect a duty cycle change.
Can you check the soft start pin ?
SS should slowly decrease when LINEUV reaches its threshold and switching should terminate when SS is discharged..
If you look at the shutdown waveforms with a load on the output then you should see detect a duty cycle change in the gate drive.
Hope this helps
Regards
John

John,

The channel 3 waveform in previous 1st waveform (in purple) is SS pin voltage. You can check.

Thanks.

Johnny

John,

Below is the waveform of QF and QR PWM. Is it possible that the low driving voltage causes the VOUT drop slope change?

Thanks.

Johnny

Hi Johnny,
The UCC2897A allows the PWM to decrease at LINEUV as the soft start capacitor voltage decreases from 4.5V to 2.5V.
At 2.5V the PWM is disabled and the soft start capacitor continues to discharge to 0.5V
Soft start is only resumed if LINEUV is no longer enabled.
The data sheet does not show why the slope of the ramp should change from 2.5V to 0.5V and the soft start time is defined as the time that CSS changes from 2.5 to 4.5V. This is equation 13 and 14 in the data sheet.
Do you have an issue that this increased slope causes in your system?
Regards
John