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UCC28950: Can the UCC28950 be operated directly from the AC line?

Catalin Muntean
Intellectual 280 points
Part Number: UCC28950
Other Parts Discussed in Thread: TINA-TI

Hello,

Usually in high power applications there is a PFC stage in front of the DC/DC converter. However, I am planing to use the UCC28950 with no PFC stage in front, directly from the AC line. Is this possible if proper inrush current limiting is implemented ?

Thank you

  • 0
    TI__Mastermind 25570 points
    Hello Catalin

    Interesting - normally the answer is no because as you point out a PFC stage is needed to give a reasonable power factor to the load presented to the line.

    I'd suggest that you take some time to model this idea - there is a transient TINA-TI model for this part on it's web page. The PSFB won't match the power factor of a dedicated PFC stage so that is the first thing you should look at.
    The load transient response will be poor because you will have to close the feedback loop at a low frequency - 10Hz or so and there will be significant voltage ripple on it (maybe 10% to 20% pp) so you will probably need a second DC/DC stage to remove this ripple - especially if you are designing a battery charger which probably removes any benefit of not having the PFC stage.

    You will also have to consider that things like lightening surge protection will be more difficult because you don't have the boost PFC output capacitance to absorb some of the surge energy.

    Let me know what you think - and if you wish, send me some more application details.
    Regards
    Colin
  • 0 in reply to Colin Gillmor
    Intellectual 280 points
    Hi Colin,
    The application is a 1 kW PSU with 62 output voltage.
    I did some numbers and the poor power factor gives an RMS current through the rectifing capacitor of about 21 A RMS ! That would require to big of a cap and too expensive. A capacitor able to handle 27 A RMS (put in some margin) costs around 90 €.. almost the same as the PFC, so seems that I need to go with a PFC stage.

    Increasing the ripple voltage of the rectifier (decreasing the cap value) a lot, might ( might...) be possible to reduce that RMS to some acceptable values, but then, I am concerned about the input voltage ripple influence in the output voltage and hence if the controller would be able to handle it. What do you think about the input voltage ripple influence on the output ?

    Thank you for your time.
  • 0 in reply to Catalin Muntean
    TI__Mastermind 25570 points

    Hi Catalin

    I'll assume that the PFC stage has some ripple on it at twice line frequency. The amplitude of this ripple is a function of the amount of capacitance at the output of the PFC stage and of the DC load current taken out of the PFC stage by the PSFB - ±20Vpp isn't unusual and if you wanted to use less capacitance you could let it go to ±30Vpp. Anyhow - whatever it is the bandwidth of the PSFB control loop will be high enough to attenuate this ripple to tolerable levels. there is a worked example in the UCC28950 DS and the Bode Plot shows about 45dB of gain (attenuation) at 120Hz for a loop cross over frequency of about 3.5kHz. If you have  30Vpp ripple on the input to the PSFB at 120Hz then the output of the PSFB will be attenuated by 45dB to approx 170mVpp.

    Regards
    Colin