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UCC289x overshoot

Kevin Benstead
Prodigy 20 points
Other Parts Discussed in Thread: UCC2891

Hi

I've inherited a design for a 24V to 3.3V power supply using the UCC2891 device.

As far as I can tell the circuit is the same as the circuit in the application note sluu407a.pdf for a 48V with a few component changes to get it to start at about 12V rather than 36V.

At startup my system has a low load (less than 100mA).  But once operating 17A or more is required.

My problem is that during startup the 3.3V supply seems to do some strang things.

This is the waveform I get from the output (5mS/div horizontal, 2V/div vertical)

The supply appears to ramp upto about 3V over about 7mS,  stay at 3V for 6mS, surge upto 5.5V over 4mS and finally drop to 3.3V over a further 4mS.

The over-voltage on the output is potentially damaging the components in my system that use 3.3V.

Any ideas why this happens or any suggestions for 'tweaks' to the circuit to stop this from happening would be appreciated.

I've thought of fitting a zener diode across the output but it would need to be a large (and therfore expensive) device to cope with the energy in the over-voltage surge.

Thanks

Kevin

  • 0
    TI__Expert 5900 points

    Hi Kevin,

    Can you talk more details about the issue?

    Does the issue occure at 12V input or 24V input? what is the load when the issue happend?

    Can you also attach the waveform of SS/ FB?

    It seems like there is something wrong when ss cooperated with FB.

    Regards

    Kening

  • 0 in reply to Kening Gao
    Prodigy 20 points

    Hi Kening

    I have done some more tests with resistive load instead of the system load I tested it with initially.  The initial plateua has disappeared (I think this is because it normally drives a couple of boost converters) but I still get the overshoot.

    Heres waveforms for 100mA load

    and with 3A load

    The startup seems VERY slow and as you can see the overvoltage surge is still there.

    I can't get SS/FB waveforms at the moment the board is conformally coated and I can't get the scope probe through it.  However I suspect that you are correct there is something wrong with the SS/FB circuit.

    I'll investigate further.