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UCC24610 / SYNC Control

Other Parts Discussed in Thread: UCC24610

Hello,

I'm assisting customer working on 5 volt 2 or 3A flyback, off 90-264VAC.  Would like to use synchronous rectification to lower the temperature of electrolytic caps.

 UCC24610 might work, however, it is turning off the gate of the FET before the current has gone to zero. Varying the programming resistors does not seem to have enough range to get it to stay on longer.

We need applications help preferably from someone who knows this part and the operation of all the pins well.

Can we please take this off-line with the customer?

  • The SR will be shut off when the current through the Rdson crosses the -5mV threshold.  Using an SR FET that has ahigher Rdson actually will improve efficiency because with a higher Rdson the current is lower and the body diode conduction time will be shorter.  We recommend using a FET with an Rdson of 8mOhms to 12mOhms, minimum.  Any lower than this and the Gate will turn off and force the body diode to conduct for a longer time.

    Also, if the gate is being shut off very early in the switching cycle, it may be due to excessive ringing that is tripping the thresholds.  If that is the case then the TON resistor needs to be adjusted.

  • Lisa,

    Thank you for the help. I see the 5mV spec on the datasheet and had missed it - thank you.  However, something is wrong with the explanation of how Rdson affects efficiency as the current through the FET is independent (to a first order) of the Rdson.

    Raising Rdson will increase the losses in the FET during the high portion of the current pulse which occurs at the beginning. Our Rdson is nominally 11.5milliohms. The peak current is 6A and the base of the triangle current wave is 4.7uS long. If the threshold is 5mV then it will turn off at .435A. The losses will be 65mW during the first portion of the triangle current waveform and 5.2mW during the last .34uS when the body diode is conducting for a total of 70mW.

    If our Rdson was half or .00565ohms, then the losses would be 32mW (half of the previous example) during the first part and 21mW (4 times the previous example) during the second part for a total of 53mW which is LESS not more which is the opposite of what Lisa's explanation implies.

    Customer will look for ground loops in their layout.