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UCC21750: Missing pulses at the driver output

Gianni Facchini
Prodigy 10 points
Part Number: UCC21750
Other Parts Discussed in Thread: SN74AHCT541

Tool/software:

Hello,

I’m currently using a couple of UCC21750 to drive a 10kW half bridge IGBT converter.

Some time happen that some pulse was missing at the output of one of the two drivers. The first think that I’ve though was that pulse is missing on the input of the driver and with my surprise I’ve noted that the pulse at the input of the driver was truncated in amplitude at about 2.5V.

For each input of the driver there is a resistor of 5.6k and a capacitor of 100pF between driver input and GND.

The power supplies are good and stable in all conditions and no fault was come out from the driver.

As you can see in the image (the vertical scale is slightly different between them), the blue trace is the pulse before the resistor of 5.6K and the green trace is the pulse after that resistor (input of the driver). Looks like that sometime the impedance of the driver input goes low than 55k pulldown.

Of course, I had tried to change some IC driver but the issue persists.

Do you have any idea what can cause this behaviour?

  • 0
    TI__Expert 6730 points

    Hello Gianni, 

    Are you interlocking two gate drivers together? If so, and if the inputs are floating, it's possible that the internal pull-up resistor of IN- and the internal pull-down resistor of IN+ are forming a voltage division network and pulling the input pins to 2.5V. 

    I usually see customer using ~100ohm of input resistor to the gate drivers IN+/IN- instead of resistors in the kOhm range. You can decrease the size and see if the issue improves. Also, what are you using to drive the inputs? If the drive signal comes from another IC, maybe that IC's pull-up/pull-down resistance plays a role as well. 

    Thanks, 

    Vivian

  • 0 in reply to Vivian Qi
    Prodigy 10 points

    Thank you Vivian for the reply,

    Yes, the two gate driver are interlocked together. I had considered that the internal pull-up and pull-down forming a voltage divider but  the voltage measured at the input of the gate driver swing between 4.5 and 0.5V so I think it is in the range of ON-OFF of the gate driver. To drive the input, I’m using a SN74AHCT541.

    What I don’t understand is why only some pulses are clipped to about 2.5V although the others are at the correct levels (see image above). This strange behaviour occurs only when the power is applied to IGBTs bridge.

    Anyway I’ve tried to reduce the resistors value to 100ohm, as your suggestion, and the problem doesn’t occurred any more.

    The doubt still, could be an immunity issue of the gate driver itself?

    Thanks.

  • 0 in reply to Gianni Facchini
    TI__Expert 6730 points

    Hi Gianni, 

    I don't believe it's a noise immunity issue, since the phenomenon is seen on the primary side of the gate driver where there should be minimum noise involved.

    After changing the input resistors, you should verify the input voltages (IN+/IN-) on UCC21750 are still between VCC and GND. I see that the voltage before the input resistors goes down to ~-1.5V or so, and I want to make sure the negative voltage does not appear on IN+/IN- to avoid damaging the device. 

    Thanks, 

    Vivian