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UCC28180: Gate is ringing

Jeffrey Cho
Prodigy 150 points
Part Number: UCC28180

Schematic is shown below.  Input voltage comes from rectified 12VAC, Vout = 28VDC, Pout = 70Wmax.

Everything works but I just noticed that Gate pin is ringing badly.  What could cause this?

Thank,

Q6 GATE

U7 - PIN8

Q6 DRAIN

  • 0
    TI__Mastermind 25570 points

    Hi

    All three oscillations are running at the same frequency - about 75MHz

    The highest pk/pk amplitude at the IC output is about 15V, that at the gate is about 7V and that at the drain about 5V. Also the ringing is least damped at the IC output.

    So - I assume that you have taken the measurement with a very short ground lead or a 'tip and barrel' approach ( see http://www.eetimes.com/document.asp?doc_id=1273282 for an example )

    The first place I would look is the layout, especially at the grounding arrangements. Is there a long trace in the source of the MOSFET and the input capacitor C25. Are the IC decoupling capacitors connected directly between the VCC and GND pins and is the path from the GATE output through the MOSFET and back to the GND pin as short as possible.

    Let me know how you get on and if you wish you can send me a copy of the layout (gerber files are best) at colingillmor@ti.com

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    Prodigy 150 points

    Hi Colin,

    Yes, I do use a short GND spring.

    I sent you Gerbers on 9/14.  Please let me know what you spot.

    Thanks.

  • 0 in reply to Jeffrey Cho
    TI__Mastermind 34465 points
    Your gate drive circuit has 3.3 Ohms for turn on and a shorted diode for turn off. This is way too fast and is most likely causing gate oscillations.
    For the power levels you need, use 100 Ohms for turn on and place 33 Ohms in series with the diode for turn off. This will attenuate any likey hood of gate ringing.
    Regards
    John
  • 0 in reply to John Griffin1
    Prodigy 150 points

    Thank you John for the Gate resistor recommendations.  Here are turn-0n and turn-off waveforms with the values you recommended.

    Two questions:

    1. Turn-on waveform still shows some oscillation in the plateau region.  Should I be concerned?

    2.  The original 3.2 Ohm value came from UCC28180 Evaluation schematic.  Why is it ok there? 

  • 0 in reply to Jeffrey Cho
    TI__Mastermind 34465 points
    (1) The turn on waveform looks a little slow so you can decrease the turn on resistor to speed it up.
    Turn off looks ok so maybe you can use the same resistor values
    If you want to attenuate the ringing around the threshold voltage you can add a small ferrite bead here.
    Why do you think this is a problem ? Your original waveforms were almost destructive in nature.
    (2) The UCC28180EVM is a high voltage pfc with input from 85Vac to 264Vac and a power rating of 300W. The pfc was optimized for the design requirements.
    Your design is a 12Vac input with a 70W output.

    Regards
    John
  • 0 in reply to John Griffin1
    Prodigy 150 points
    HI John,

    I am still trying to understand why 3.2 Ohm Gate resistor is ok in the high power / high voltage EVM but not in a low power / low voltage circuit. Regardless of other parameters, the Gate pin is just driving the Gate of a MOSFET. So why is 3.2 Ohm ok with with a MOSFET with much bigger Gate charge but rings terribly in a lower voltage circuit?

    Thank you.