Uc3854 problem

[erdemtr55]

Hi Guys,
I tried to simulate a PFC circuit based on the schematic in the attached file, but for some reason, the gate signal is at a very low level (in the mV range). What could the problem be?

Note: 100 kHz, 250 W, Vin 220 VAC, Vout 400 VDC.

[moffy]

If you post the LTSpice *.asc file and the UC3854 model we could try them and possibly find the problem.

[erdemtr55]

I sent you my file , maybe you can solve my problem
thanks

[moffy]

I sent you my file , maybe you can solve my problem
thanks

I can't find what you sent, but you are better off just posting the files on the forum. There are more knowledgeable people than I am.

[erdemtr55]

I am  sharing it at now,
Sorry previously

[moffy]

From the testing of your simulation file it looks like there might be a problem with the UC3854 model, especially since it has been developed by an individual rather than the manufacturer.
The following article: https://dspace.upt.ro/jspui/bitstream/123456789/1802/3/BUPT_ART_Șchiop_f.pdf
goes into detail of how the authors developed a pspice model for the UC3854, and seems to have enough detail to reconstruct it but it would involve a bit of work, unfortunately there doesn't appear to be a link for their model.

[moffy]

I have to revise my last statement, I modified the LTSpice file to be like the example given in the datasheet and it output full level gate pulses.

[erdemtr55]

I tested mod2 and firstly voltage boost 410 v but then voltage buck 320-310v
I think that it is not normal.What if  how ı solve it?

[moffy]

I tested mod2 and firstly voltage boost 410 v but then voltage buck 320-310v
I think that it is not normal.What if  how ı solve it?

The model for the UC3854 is unverified, it cannot really be trusted until proven, the question is does the model behave as the real device?
Here is an application note: https://www.ti.com/lit/an/slua144/slua144.pdf?ts=1724033606595

[MariuszD]

I tested mod2 and firstly voltage boost 410 v but then voltage buck 320-310v
I think that it is not normal.What if  how ı solve it?

The 410V does not result from the controller's operation; even if you remove the switching transistor, the phenomenon will remain. You need to limit the charging current of the capacitor through the NTC.
320V is simply the rectified mains voltage, the boost converter is not working.