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UCC3817: UCC3817

douglas Sudjian
Prodigy 190 points
Part Number: UCC3817
Other Parts Discussed in Thread: UC2854, , UCC28180

My question or observation concerns the PFC part differences  between the  ucc3817 vs. UC2854 units

The internal multiplier output (Imo)  is  hard-connected either to  the inverting or non-inverting input on the Current amplifier (CA).

In other words, the CA Input (CAI)  used for current sensing is either connected to the non-inverting(UCC3817) or inverting (UC2854) pin.

Is connecting the current sense input to the inverting pin on the CA amplifier done for lowering the amplified noise prior to PWM?

amplifier gain is (- Z1/Z2) versus (1+Z1/Z2).

It seems like a TI design improvement.

I plan on using multiple current transformers for current sensing in a boost or buck-boost configuration.

  • 0
    TI__Genius 9790 points
    Hi,

    UCC3817 and UC2854 are released for many years. TI recommend you to use UCC28180 which is the latest CCM PFC controller.

    BR,
    Yunsheng
  • 0 in reply to Yunsheng Qu
    TI__Mastermind 34465 points

    5556.Comparison of UCC3817 and UCC3817A family slua294 (1).pdf

    Hello Douglas,

    The difference in current amplifier inputs between these two devices is due to the difference in PWM comparator inputs.

    The UC2854 has the CA output going into the negative of the PWM comparator and the UCC3817 has the inputs reversed.

    The attached document (page 5) shows this and all the other differences.

    Regards

    John Griffin

  • 0 in reply to John Griffin1
    Prodigy 190 points
    Hi John,
    I think I remember reading in a newer PFC part manual what I am asking about, but I can't find the article.
    I think one has to reverse the comparator inputs if they swap the current sense polarity at the current
    amp inputs. If I find the TI article I will let you know
    Thanks
    Doug (thanks for response over holiday weekend)
    BTW, using dual current transformer sensing (Reference SLUA308) (with very low primary resistance) in a Boost converter generates
    huge inductor currents. How do people manage that? Just with the L-boost inductor series resistance?
  • 0 in reply to douglas Sudjian
    TI__Mastermind 34465 points

    Hi Doug,

    I am not familiar with SLUA308 but I see that it says :

    "to theoretically show how to set up a pfc boost regulator using current sense transformers"

    So I would proceed with caution.

    It looks like the mosfet current is sensed and the diode current is sensed. These two currents are summed but I cannot see exactly how.

    I think a simpler method of current sensing is attached.

    Current Sense.pdf

    I would use a single current transformer with dual primary windings. The first primary is set in the MOSFET drain and the second primary is set in the diode.

    The primaries are out of phase so the volt-seconds balance and the transformer is not saturated. The secondary is full wave rectified and applied to the ISENSE pin of the pfc controller.

    I'm not sure what you mean about huge inductor currents. The inductor current is set by the input voltage and the power output. All the current transformer does is measure this current and step it down by the turns ratio.

    Regards

    John