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UCC28064A: UCC28064A OCP set point margin

Dexter Choi
Expert 2795 points
Part Number: UCC28064A

Dear Sir,

If OCP is set 380W and output load is increase over 380W. OCP is good working.

When output load is decrease during OCP. release OCP for output load between 340W~320W.The gap of released OCP set point is 40W ~ 60W.

1.Could you let me know that why are different gap of OCP and released OCP?

2.how to minimize the gap of released OCP set point is 40W ~ 60W?

Thanks.

  • 0
    TI__Mastermind 40580 points

    Hello Dexter,

     

    Thank you for your interest in the UCC28064A PFC controller.

     

    This controller uses a quantized peak detection method at VINAC to estimate the RMS value of the input line. This method allows an internal adjustment of the control loop gain to keep the gain approximately constant over the full 85-264Vac input voltage range.

     

    But since the quantization is in 8 levels, the gain does vary slightly within each level, so the COMP voltage will vary within each rms level for a given output power. At maximum output power, COMP level is usually ~5V at the lower boundary of each rms level and is < 5V at the upper boundary of each rms level.

     

    When your load is increased to the OCP level, Vout will fall and COMP will rise to the maximum 5V.
    This voltage will allow the maximum on-time for the given rms level of the quantized gain. As long as COMP is clamped to the upper limit (5V) the cycle-by-cycle peak current can continue to trigger the OCP limit even if the load is gradually reduced.

    Only when the load is reduced enough to allow Vout to rise, then COMP can fall and reduce the on-time and reduce the current peaks below the limit threshold.

     

    I believe that it is the variable gain within each rms level that results in this OCP behavior.
    I believe that the gap between OCP and released-OCP will vary depending on where the input rms voltage is within each quantized rms level. In other words, a smaller power gap when Vac is near the lower boundary and a larger gap when Vac is near the upper boundary of a level.

     

    I don’t think this effect can be minimized with external circuit changes.
    I think the best solution is to set the OCP limit higher by the maximum gap from the required release-OCP level. For example, set OCP trigger at 440W to release OCP at 380W.
    Or if allowable, compromise somewhere in the middle, like OCP at 410W and release at 350W.

     

    This may be a consequence of using a constant power load on a supply with a peak current limit.
    The behavior may be different with a constant current load and/or a constant resistance load.

     

    Regards,
    Ulrich