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PMP3162: Purpose/sizing of R3

Daniel Hintz
Prodigy 120 points
Part Number: PMP3162
Other Parts Discussed in Thread: UCC2897A

In another thread, R3 is mentioned as a sense resistor, but I'm unclear as to how it's used here (also new to sense transformers in general).  Shouldn't all of the power transformer's current flow through the current sense transformer's (CST) primary?  R3 sits across the primary of the CST, which can only serve to provide an alternate path for current.  R6 provides for the transformer reset, and R5 provides for the sense voltage on the UCC2897A.

Without understanding how it's being used here, I'm unsure how to calculate the proper value.

  • 0
    TI__Intellectual 2875 points

    Hi Daniel,

    We found that by adding R3 we can reduce the high frequency spike that occurs across the CST during turn off of the primary FETs.  R6 could probably be removed, because R3 will also act to control the reset of the CST. 

    thanks and regards,

    Brian 

  • 0 in reply to Brian King
    Prodigy 120 points

    Thanks for the response, Brian.  I'm assuming a specific value wasn't critical, then, just adding a bit of dampening.

    Since the CTS primary is ~2 orders of magnitude lower in resistance than R3, I'm assuming the difference in measured current is minimal enough to ignore?

  • 0 in reply to Daniel Hintz
    TI__Intellectual 2875 points

    Hi Daniel,

    The value is not that critical.  Lower is better for minimizing the voltage spike, but you have to keep the impedance significantly larger than the CST impedance to ensure the primary current flows through the CST (as you pointed out.)

    thanks,

    Brian