• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Subscribers 64 subscribers
  • Views 3418 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UCC28019: Boost PFC vs Buck PFC

Oliver Kim7
Expert 3455 points
Part Number: UCC28019

Hi, my name is Oliver Kim, TI Field application Engineer.

When I studied the PFC Controller, most of the applications selected the boost PFC controller such as UCC28019 typical application diagram.

Could you let me know why we can usually use boost PFC, not the BUCK?

Of course, I know in case of step down voltage application, buck PFC controller is also needed.

But it seems that most of the documents and articles say about boost PFC controller basically.

For example, On-board charger always uses boost PFC. But I think Buck PFC also can be used because there is a Full bridge DC-DC converter so we can control again the voltage level by the ratio of the coils.

Someone tells that it's a kind of energy storage efficiency reason: high voltage with low current is better than high current with low voltage, so we use boost PFC!

..Well, why high voltage is better??

This doesn't clearly solve my curiosity. Is there any other reason?

I really need your help.

Thank you.

  • 0
    TI__Mastermind 34465 points
    Hello Oliver,
    There are two big reasons why the boost pfc is preferable over any other kind of pfc.
    (1) The pfc power supply is normally connected to an unregulated line that can have high voltage surges and lightning strikes.
    The bulk cap on the boost pfc acts to absorb this energy and prevent an over voltage from destroying the power supply.
    A buck pfc will most likely be destroyed by a line surge because the semiconductor switch, MOSFET or diode, will be directly across the high voltage.
    You can add various forms of lightning protection to the buck pfc to mitigate this situation but no amount of over voltage protection can be as effective as a bulk capacitor used in the boost.
    Field reliability is a very big reason for the popularity of the boost pfc
    (2) Line harmonics and power factor are better with a boost pfc.
    This is because the line current is flowing when the MOSFET is on and off.
    Line current is only flowing when the MOSFET is on for a buck or flyback pfc.
    The end result is that the buck current is more distorted.

    Hope this answers your question
    Regards
    John
  • 0 in reply to John Griffin1
    TI__Expert 3455 points
    Thanks for your fast reply.

    But I still have something to ask.

    (1) Why we cannot add bulk cap on the buck PFC? I think we can also block voltage surges by adding big input capacitor on the buck.

    (2) Could you specify the term, line current? I mean current for which node?

    Regards
    Oliver
  • 0 in reply to Oliver Kim7
    TI__Mastermind 34465 points

    Oliver,

    You cannot add a bulk cap on the input to a buck pfc because it will no longer correct the power factor.

    Line current is another name for pfc  input current.

    Regards

    John

  • 0 in reply to John Griffin1
    TI__Expert 3455 points
    Thank you! It perfectly cleared my curiosity.