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A few question regarding UC2854 PFC

Hong Yu67765
Prodigy 20 points

We are designing a 220VAC input 10KW power supply. Front end is a 2854 based PFC stage.

I put a 75 ohm to it and raises input from 0VAC, the PFC starts to work from 20VAC, boosts to 200VDC or so, at 30VAC it regulates at 400VDC, PF 0.982. However when rise to 35-40VAC it buzzes, PF drops to 0.7 and regulation drops to 80%, further rising the input causes more severe buzz and regulation gradually increases.

A few questions.

1. Although rated only for offline application, does the 2854 IC often work below 90VAC at way below rated load? I don't want it to see 220VAC for now.

2. Instead of sensing AC voltage and current from rectified line, can I sense at the AC input with differential opamp? (as control is on another board, don't want to bring in too many high voltage).

   We use SCR as bridge, thus when the PFC IC always sense differential AC input V, I but actually there could be none V or I presents to the inductor, (as there is a bit phase cut due to SCR), I wonder what might happen from there?

  • 0
    Intellectual 970 points

    I think you need to give it the full 220VAC to know if its ok.

    You are giving results from very low input voltages.......at such low input voltages the current demand will be ridiculously high.....and indeed buzzing may well be expected there...............

     

    have you scoped the RECTIFIED DC HIGH VOLTAGE bus?

     

    It would be interesting to see what you have there.

     

    i take it that this is a H-Bridge based PFC circuit(?)

  • 0 in reply to treez
    Prodigy 20 points

      It's a single phase PFC with two paralleled FETs.

      Further troubleshooting shows sth like this:

      I gave it a 30W load only, for 1KW load,  there is too much noise.  

     The PF is high at 0.985 at 20-25VAC, then as AC goes up, PF gradually goes down, at 110VAC, PF about 0.7. At 220VAC, PF is only 0.4.

      There is sth wrong at 30-40VAC, within a few seconds the PFC tries to regulate beyond 330V, same time it gradually try to pull more than load need ( close to 100W or so, not sure where this power goes), then at certain point the PFC would collapse for a second (PF goes to 0.2) then returns to pull the correct amount of power, but now it only regulates at 300V thereafter until 220VAC. It should boost to 400V.

     

       During this collapse the Vsense, and Vac-sense pins look normal, but Iac-sense goes low for 1 sec then comes back. Iac is also very noisy (peak goes to  a few  hundred mV) as the real value should be in few mV( may due to my old scope). Maybe the current sense resistor  is not that good, or transferring mV through the cable is susceptible to too much noise .

     

       Ipk-limit, starts very clean, then becomes noisier. During the same collapse, I pk-limit becomes clean again (due to Iac disappearance), then goes noisy as before. It’s hard to tell if the over-current threshold has been reached as it’s noisy before the collapse.

     

       Gate drive, as expected, starts at large duty cycle, disappears with the Iac, then gradually goes to very little cycle.

     

      The voltage feedback and voltage feedforward signals both look clean and linearly varying through the whole process.