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UCC20520: Channel B has short off pulse after switching on if there is load on Channel A

Stefan Oltmanns
Prodigy 20 points
Part Number: UCC20520

Hi,

i've got a problem with the UCC20520: Depending on the load of channel A, channel B has a short off pulse after switching on. Position and length of the pulse does not depend on the duty cycle, the pwm frequency or the deadtime (it's possible to increase the deadtime so the off pulse is completely gone, as it's during the deadtime). The length of the off pulse however does depend on the load of channel A.

The problem is shown in picture above: On the bottom channel B is displayed (unloaded), the top shows channel A (loaded with two FETs, each with 7R5 gate resistor), duty cycle is just a few percent, deadtime is off. You can see when channel A switches on B switches off, but when A switches off again, B first switches on, but for a short pulse off again. The pulse becomes shorter or appears only sometimes by decreasing the load of channel A. I tried two different UCC20520, no difference. Could someone explain whats going here?

Best regards

Stefan

  • 0
    TI__Mastermind 29190 points
    Hi Stefan,

    I'm an applications engineer supporting isolated gate drivers. Sorry to hear you're having trouble with UCC20520. Let's try to figure out what's going on.

    Common-mode transients can temporarily interrupt the signal from the control side to the output side when they exceed datasheet limits. This will cause a short off pulse such as the one seen in the scope image. Is there a common-mode transient that exceeds the 100V/ns datasheet limit? Can you measure this transient slew rate if present, or try increasing the gate resistor size if that isn't possible?

    Another possibility is that noise is coupling to the DIS circuit. How is DIS connected? If DIS pin is unused, it should be tied directly to GND close to the IC. If DIS is used or left floating, try connecting a 1nF capacitor in parallel with the DIS pin to provide a low impedance path for high frequency noise.

    Let me know if neither of these are responsible for the observed behavior.

    Regards,
  • 0 in reply to Derek Payne
    Prodigy 20 points
    Hi Derek,
    thank you very much for this fast answer. I investigated the DIS pin, and even though it is connected to an optocoppler output, there is a small peak for the same length a few nanoseconds before the off pulse. The PCB traces of the optocoppler input overlaped the ones of the gate resistors on the other side of the PCB, there was a coupling. I added a few capacitors to the optocoppler circuit, problem is gone.
    Best regards,
    Stefan