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UCC21732QDWEVM-025: UCC21732 output always pulled to VEE

Yl Zh
Prodigy 10 points
Part Number: UCC21732QDWEVM-025
Other Parts Discussed in Thread: UCC21732

Tool/software:

My UCC21732 always output low. The figure shows the booting waveforms, where C1 is FLT，C2 is 24V voltage source of the PCB(which is used to generate isolated VDD and VEE) and C3 is the gate voltage.

The power sources, including a 5V VCC from LDO, isolated 18V VDD and -4V VEE from UCC14240 are fine. RDY is high. IN+ is connected to 5V, and IN- is connected to DGND. Output voltage is pulled down to -4V after booting.

The SiCFET(IMW65R039M1HXKSA1) was never swiched on, so it can't be the overcurrent protection. CLMPE-VEE voltage is 0V, so I think external Miller clampe circuit is not the cause either.

  • 0
    TI__Mastermind 22666 points

    Hi Yl Zh,

    I am having a hard time reading this oscilliscope image. Can you provide a version that is more clear? Possible a saved image file?

    It seems like you nFLT pin is being pulled low? In that case, can you try to short OC and COM? Maybe there is something the matter with the current OC circuit.

    Can you share your schematic?

    Best regards,

    Sean

  • 0 in reply to Sean Cashin
    Prodigy 10 points

    Hi Sean,

    Thanks for the reply, and you are right. There exists a spike at OC. After connecting OC to COM, the circuit works. In addition, the spikes only exists in OCs from the upper legs. If the FET is in the lower legs, COM=VGND, the interferences are smaller.

    Sorry for the picture quality yesterday! This time should be better.

    C1 is nFLT, C2 is 24V source, and C3 is OC.

    What can I do to cope with this interference?

  • +1 in reply to Yl Zh
    TI__Mastermind 22666 points

    Hi Yl Zh,

    It looks like your OC is accurately measuring the output current of the gate driver itself as it charges the V_GS. After it measures the gate driver output current, OC is interrupting the output because the Cgs charging current creates a voltage spike higher than the 0.7V threshold.

    What is the value of the shunt resistor? If it was 1 ohm, 3A of Iout would create a 3V spike, and I estimate your effective overcurrent threshold is now about 3A. What is the overcurrent threshold you would like to set? 

    If you want to filter this transient Vgs gate charge current, you can increase the R20, C25 filter values from 10 ohms and 240pF (66MHz) up to 100 ohms and 1nF (1.6MHz). That should still be fast enough to interrupt overcurrent, but too slow to detect the gate driver's transient output current.

    Thank you for the clear images.

    Best regards,

    Sean