HVLED burst mode too early

[Franciss]

Hi,

I am developing AC-DC flyback with HVLED815PF controller.
Using application note AN4350 I managed to have a CV output for 6W max.

At high load, everythings good and functioned as expected.
I got 25% iTHD and 0.96 PF.

However at 2W and below its suddently getting worse.
After checking it out, it turns out the controller operating in burst mode and normal mode alternately.

I have checked all pins and did not found anything out of the ordinary, except that COMP pin is below 1V that triggers this burst mode operation which is my question
What could be the reason for this? and how to mitigate this?

Its too early to enter burst mode at just 30% load.

[Peter BENSCH · Accepted solution]

Welcome @Franciss, to the community!

The voltage at pin COMP depends, among other things, on the level at pin DMG (see e.g. the data sheet, fig. 5 or AN4350, section 2.3). If in your application, whose component parameters and schematics you have unfortunately not inserted, the Burst Mode Threshold is reached at 2W, the burst mode is inevitably activated.

Have you taken a look at the EVLHVLED815W8CV, which may help you with component selection?

Regards
/Peter

[Franciss]

Hi Peter,

Thank you for the answer.
I managed to found out that I miss dimensioned few components, and as soon as I change it, it fixes the problem.
AN4350 do have error for the given equation (Eq.3).
It would be so cool if ST can fix that in the future, in case other people find the same problem.

Thanks before.

Best regards,
Francis