UCC28740: Switching stop at during startup

Hello,

Thank you for reply. Please see the attached file. Those are waveform at startup for our board.
When Vout reached to near 5V, controller is stop switching.
Is the switching stop trigger Vfb?
Is this caused by a low crossover frequency?

Regards,
ACGUY

UCC28740 Startup.pdf

Hi,

Thanks for the waveforms. It looks like the device is exiting the soft start sequence (which it should) and then operating in CC mode at maximum Vcs as the output voltage rises. It does appear there is a little bit of overshoot in the output voltage when switching stops. At the moment when switching stops, do you see the VDD voltage get discharged to the UVLO threshold?

Best Regards,
Ben Lough

Hi Ben,

I took waveforms. Please see file. Vdd was not drop to UVLO when switch stop.

My opinions are follow;
 - Vfb rose to over 880mV.
 - Increased Ifb to more than 20uA.
 - fsw decreased to about 170Hz.

I think the excessive rise in Vfb is due to the OV of Vout. So I thought that this switch stop was the OVP of Vout.

Countermeasure;
 - Suppress collector current for photocuppler.
 Is it correct?

Please let me know your opinion.

Regards,
ACGUY

UCC28740 waveform.pdf

Hi,

If you do not see a fault response on VDD then yes, I would suspect excess opto-coupler current is causing the device to stop switching and suppressing the collector current would help. Are you able to share a schematic for this design? Adjustment to the compensation parameters of the feedback loop may also help.

Best regards,
Ben Lough

Hi,

Thank you for your cooperation.
I do like to send you a schematic in private messarge. I will send you a friend application so please approve.

Best regards,
ACGUY