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UCC28810: Low PF operation at light load

Adesh Jagtap98
Prodigy 210 points
Part Number: UCC28810

We are using this part for providing 24V DC supply converted from 230 V, 50Hz mains supply. The converter topology used is Flyback. The application aims to have high power factor. The issue seen with it that at lower load like .1A, power factor is reduced to very low value (around 0.35), whereas at high load that i.e. 1A the PF seen is around 0.9. The power factor is seen decreasing with increase in mains voltage as well. We observed the current waveform in both cases which are attached. When operating at voltage around 220V for heavy load or at around 100V for light load, the average sensed current doesn't remain sinusoidal which is caused by Turn off of the MOSFET. I have attached some Graphs with the operating condition. If having any solution regarding this, please help.

Image 1- At switch node when operating at light load

Image 2- distorted  current waveform caused due to irregular switching(in image 1), causing low PF(green color graph)

image 3- Waveforms of current(Green) and secondary voltage(yellow) when operating at high load and high PF

  • 0
    TI__Mastermind 31580 points
    Hi Adesh

    I have asked one of our applications engineers to respond to your post. You should see a response soon.

    Regards

    Peter
  • 0 in reply to Peter Meaney
    TI__Mastermind 34465 points
    Hello Adesh,
    Sorry I was unavailable to answer your question until now.
    Unfortunately power factor will decrease at light load and this effect is quite normal.
    You will need to design for high power factor at max load. Best power factor is achieved at low line and high load.
    As the load is decreased and/or the line is increased power factor is degraded.
    With a flyback type of pfc controller this is mostly due to the capacitance across the high voltage input. This capacitance will tend to draw current around the peaks of the AC line. At light loads you do not need as much capacitance as at high loads but unfortunately if you have a wide load range the cap value is set by the high load.
    Most applications require high pf at full load and light load is not an issue

    Regards
    John
  • 0 in reply to John Griffin1
    Prodigy 210 points

    Hello,

    Thanks John.

    I understood that the power factor will decrease as load is light. but can you please explain the reason behind the discontinuity in current i.e. switch is not at all ON when AC input voltage is around Zero crossing. You can see this in graph I shared. Is there device specific issue which we can debug and improve the PF?