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Boost PFC converter with variable Vout

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We will design a 3.8kw Boost PFC with UCC28070 chip. We need Vout to be variable, since the following stage is an LLC converter with variable load voltage.
Do you think the attached opamp method, to change vout, will be OK when used with UCC28070? We can’t see any possible interaction with the UCC28070’s internals as long as we are careful about the OVP.

UCC28070 datasheet


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I am using lt1248 in ltspice to do an equivalent one, but i cannot simulate ucc28070.

Incidentally, the UCC28070 is unuseable in our application because one simply cannot vary the output voltage of the ucc28070 based PFC...this is because if we do it like in post#1, then we will violate the requirement that the VINAC divider ratio be the same as the VSENSE divider our design has just been blown out of the window.
This is as per page 19 of the UCC28070 datasheet.

Serves us right for trying to use a multi-functional controller and "hack" it to have extra functionality....stupid of us, we should have just picked a dead simple 8 pin PFC controller in the first place.

UCC28070 datasheet

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I reckon we might be able to use the FAN9672, which has a separate pin to vary vout. I believe this can be used with current sense transformers, which we would prefer as its less noisy especially when two interleaved stages are used.

FAN9672 PFC controller ic

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I wonder why the UCC28070 PFC controller needs to do inductor downslope synthesis , but the FAN9672 does the same job as UCC28070, and in the same way, but doesn't need to do inductor downslope synthesis?

Why are you using OPAMP ?

Why do't you use variable resistor in place of R9.

I have been using a FAN6982 to great effect. You can just fiddle the output voltage feedback to suit. You can use a pot and a cap to remove wiper noise surely?


Having done a 20kW PFC with variable Vout (350-600V) using the UCC28070, I can say that yes it can be done. We did it using a potentiometer to change the feedback gain, simplest possible method. Never had any issues, though it was for a group project, not a product. Note the datasheet never explains why having unequal Vinac and Vsense ratios would be problematic. It was a few years back so I can't recall exactly what the issue was, but we did investigate it and found no problem for our application.

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