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Original question:

UCC27282-Q1: Having trouble with half-bridge gate driver circuit

UCC27282-Q1: Bootstrap Diode Failure

Ben G
Prodigy 80 points
Part Number: UCC27282-Q1

I'm encountering a problem where the switch node in my gate driver fails and the reverse bias measurement of the bootstrap diode is not open when the bus voltage is increased to around 45V. I suspect this issue might be due to the internal bootstrap diode not handling the surge in current well when it charges the external bootstrap capacitor. Given the high gate capacitance of my MOSFETs, ~ 600nF, I'm using a 1uF bootstrap capacitor. This leads me to the following questions:

1. What is the maximum current rating for the bootstrap diode?
2. The datasheet mentions a dynamic resistance of 1.5 Ohms for the diode. Does this imply the presence of an internal series resistor within the diode?
3. I'm looking for advice on how to address this issue. Although the datasheet recommends adding a Schottky diode externally for high switching frequencies, my gate driver operates at a relatively low frequency of 2kHz. What would be the best approach to resolve this?

  • 0
    TI__Prodigy 430 points

    Hi Ben,

    Please see the answers to your questions below. Let me know if you have any further questions. 

    Ben G said:
    1. What is the maximum current rating for the bootstrap diode?

    We like to suggest the diode average current be limited to under 80mA. The average value can be found by using the switching frequency (Fsw) and the total charge(Qg) of the high side FET that's connected to the HO pin (I = Qg * Fsw).

    Ben G said:
    2. The datasheet mentions a dynamic resistance of 1.5 Ohms for the diode. Does this imply the presence of an internal series resistor within the diode?

    There is not a resistor in series with the internal bootstrap diode. The dynamic resistance of the diode limits the peak forward current.

    Ben G said:
    3. I'm looking for advice on how to address this issue. Although the datasheet recommends adding a Schottky diode externally for high switching frequencies, my gate driver operates at a relatively low frequency of 2kHz. What would be the best approach to resolve this?

    Will you be willing to share a schematic? I am interested in seeing the gate resistance and VDD capacitor size. A Schottky diode would likely solve this issue since the currents could be too high as you mentioned. Another option could also be to have a series bootstrap resistor to limit the current as well. This application note covers some of these ideas which could be of interest to you. 

    Regards,

    Zihao