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UCC27710: buck converter damaging the IC during transient response

Part Number: UCC27710

We are using UCC27710 to drive a synchronous buck converter with a bus voltage of 65V and Max load output current 5A at the lowest output voltage of 12V. We have failures on the IC alone when the buck converter transitions from no load to higher load (during transient response from open circuit to load). We believe that the high di/dt and dv/dt is causing the failure but we are properly driving the IC using the right gate resistors and an RC snubber to eliminate this effect. Any suggestions on how to solve this issue, I am thinking of a soft start PWM to gradually increase the load current. in steady state the operation is not affected.

upon looking on the scope on the HO on a damaged IC the HO is locked in HI. A PIN to PIN impedance measurement comparison between a damaged and good IC does not show any damaged pin

  • Hello Soufiane,

    Thank your for the interest in the UCC27710. I will look to help resolve the issue you are seeing in your application. During a full load transient there is the possibility of large duty cycle variation and increasing current from cycle to cycle. With synchronous buck converters, there is always the issue of the low side switch body diode conduction during the dead time, and the high side turn on into this body diode conduction reverse recovery time. This will result in high dI/dt and high dV/dt on the switch node rising. Very high dV/dt on the high side switch can result in the miller charge forcing the gate driver output low with a large negative spike.

    Can you comment on the Mosfet part numbers you are using, and the gate resistance from the driver to the MOSFET? Without seeing scope plots it is not possible to say this is exactly your issue, but is a common concern with sync buck topology.

    You can try a couple of things. Increase the turn on resistance on the high side driver, which will reduce the switch node turn on spike. The turn off resistance with a diode in series can be kept at the existing value.

    Please confirm if this helps, Or for more advice can you provide scope plots of the HO, HS and LO outputs during load transient conditions (that do not cause the issue).

    Regards,

    Richard Herring

  • Thanks Richard for your quick response, the VCC used is 15V and the gate on and off resistance are calculated based on the app note to achieve 1.0 A Source and 0.5 A sink currents therefore Ron = 16OHM and Roff = 7.5OHM for the high side FET I am using TK32E12N1 and for the low side mosfet I am using IRFB5615PBF-ND.

    I want to also note that during start up the IC survives this transient when VCC is still charging, this failures only happens when we transition from load to open circuit and back to load
  • Hello Soufiane,

    Thank you for the FET part number information, and the information that while VCC is still ramping during start up you do not see the failures.

    In looking at the MOSFET for the high side I see the Qg is 34nC with Vgs 10V which is 3.4nF equivalent capacitance. With 16 Ohm turn on the time constant will be ~54ns to reach ~10V. I see the low side MOSFET body diode trr is 80ns.

    Without seeing the scope waveforms, the one suggestion I have is to see if increasing the turn on resistance helps resolve the issue. The gate drive source current available may be lower in the power up case depending on the VDD on the driver when the switching starts.

    As an experiment can you try higher turn on resistance, at least for the high side MOSFET? Try 22 Ohms, 33 Ohms etc to see if there is any improvement.

    I may still need to see scope plots requested, if the suggested changes do not resolve the issue.

    Please confirm if this helps resolve the issue, or you can always ask more questions on the thread.

    Regards,

    Richard Herring