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UCC2806 Duty Cycle

Nicholas Smith57318
Prodigy 195 points
Other Parts Discussed in Thread: UCC2806

I'm using the UCC2806 in current-mode control for an H-bridge high power 300V isolated DC-DC converter.  The voltage and current loops seem to be working fairly well.  The current loop is limiting the current cycle-by-cycle as expected, and the voltage loop is accurately regulating the output voltage to 300V.  The issue is if I increase the input voltage such that the duty cycle goes below ~26% (for a half-cycle), then the controller goes unstable.  I'm sure this is control problem with my compensation loop and/or how much slope compensation I am applying, but is there any reason the UCC2806 would not be able to handle a duty cycle below a certain number, such as 26%?  Any tips would be appreciated.

Thanks,

Nick

  • 0
    TI__Genius 15910 points

    There isn't any reason why the UCC2806 can't operate at 26% duty  cycle or less for each output (spec actually says it can be varied from 0% to 50% although some small amount of dead time is required to prevent cross conduction).

    Is your Rt greater than 12.5k to meet the current limitation of the Ct timing capacitor?  What frequency are you operating at?

    Unfortunately this is a relatively old controller so documentation on design tips takes some digging.  I will let you know if I find anything but I would suggest looking at the loop and slope comp; the part is not likely to be the limiting factor.

  • 0 in reply to Lisa Dinwoodie
    TI__Genius 15910 points

    Too much slope compensation may lower your crossover frequency...so more isn't always better.  Check to see that you didn't add too much slope comp...