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UCC21521: UCC21521 drives SiC MOSFET highly ineffective

Darek Cygan
Prodigy 30 points
Part Number: UCC21521

Tool/software:

Dear TI,

I have built a prototype PCB where I wanted to have SiC MOSFET in half-bridge architecture. I used TI dual gate driver with SiC but I got into an issue with high ineffective switching. I would say it is down to the driver but I am struggling to find a way to correct the design. Here, in attachment, are the details and screenshots.

HBridge_dual_gate_driver_UCC21521.pdf

  • 0
    TI__Expert 8686 points

    Hi Darek,

    Thank you for providing all of these details.

    My first thought is if there is sufficient current to properly bias the diode to the desired voltage. This would explain the driver output voltage not sufficiently providing the negative bias but still tries to dip below 0.

    • Based on the zener diode IV characteristics found in the datasheet, you can calculate the resistor value to properly bias the diode (R = (Vsource - Vz) / Iz)

    Please let me know if this helps.

    Regards,

    Hiroki

  • 0
    Prodigy 30 points

    Hi Hiroki,

    At VDDA,VDDB - 22V, Vz=4.3 I put about 21mA through the Zener, it could be more as its VZ1 is 45mA.

    When I measure stable-state I really see the volt drop on Zener nearly 4.3V, so I just provide minimum current which gives Zener voltage.

    When I have 3W Zener I could max put even 500mA. I think I could run into problem with not enough current through Zener when MOSFET Vgs high->low, as at that case the current is stolen from Zener and driver sinks that current along with gate current to switch off the MOSFET. What is the right balance of that current, how much Zener current I could account for sinking? Any idea how the currents split?

    Regards,

    -Darek

  • 0 in reply to Darek Cygan
    TI__Expert 8686 points

    Hi Darek,

    Interesting, with 4.3V across the Zener diode there should be a visible difference in the waveforms since the VDD voltage seen by the gate driver should be the voltage across R38 plus Vz. 

    As long as there is enough current to sufficiently bias the diode, there should not be any issues.

    Based on the waveforms, my concern is that there is no steady supply in the ON state as well as an unstable OFF state. To better isolate the source of the issue, can you test without the Zener negative voltage connection?

    Regards,

    Hiroki

  • 0 in reply to Hiroki Honda
    Prodigy 30 points

    Dear Hiroki,

    I got into the problem, I used the external power supply with 3 channels and powered VCCA - ch1 and VCCB - ch2. It seems the channels have a capacitance coupling between GND1 and GND2, though they dont have DC-connection between.

    When I used 3 separate power supplies then my VgsB and VCCA looks very clean. See the screenshot at 2A output - VgsB is super clean comparing what had been before.

    Many thanks