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UCC21520: High side output glitch

Joe Porter
Prodigy 20 points
Part Number: UCC21520

I am using two of these driver to drive one side of a dual active bridge converter. I am experiencing a glitch in the high side output when first turning on. I've checked the VDDA and it is steady at 15V with small 1/4V to 1/2V dips. There is a good amount of noise during startup but I would like to know the nature of this behavior to try to find a patch. All rails have been up for several seconds prior to enabling the driver.

INA/INB RC = 51 Ohms + 220pf

VDD1 = 3.3V with 0.47uf and 0.01uf

ENABLE pin RC = 51 Ohms + 220pf

CH1(blue): High side output

CH2(light blue): Inductor current

CH4(green): Low side output

  • 0
    TI__Mastermind 33620 points

    Hello Joe,

    When the transformer current starts at 0A, the timing required to achieve -20A peak in the transformer is less than when the transformer current starts at +20A. Because your peak-to-peak current starts smaller than at steady state operation, but your pulse width does not change, your peak current measurement is extraordinarily large during the noise event just before the trigger, putting the transformer in hard saturation. The much higher peak current at this point will contribute to enormous dv/dt on the switch node, likely >200V/ns. The ±20V of dv/dt related ringing on the output also greatly exceeds the absolute maximum ratings of the IC. I suspect the ringing on the output is causing a circuit in the output control to latch up, resulting in simultaneous conduction of the VDD-OUT transistor and the OUT-VSS transistor.

    To prevent this behavior, consider amending the startup sequence for the converter with a soft start sequence to limit the peak current in the transformer to an acceptable value. There are several techniques to do this using duty cycle control or frequency control, which vary depending on the topology of converter selected. The simplest solution may be to just reduce the initial high side pulse duration by about one half-cycle.

    Regards,

  • 0 in reply to Derek Payne
    TI__Mastermind 33620 points

    Hello Joe,

    Do you still require assistance with this issue? Please let me know what other support I can provide.

    Regards,

  • 0 in reply to Derek Payne
    TI__Guru**** 189401 points
    Hi Joe,

    It's been a while since we've heard from you, so we must assume you've resolved your issue.

    If possible, please post how it was resolved for others to learn from.

    If your issue isn't resolved, let us know, and we will work with you to resolve it.