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UCC21732-Q1: UCC21732 and LM2941 power output issue

Part Number: UCC21732-Q1
Other Parts Discussed in Thread: LM2941, UCC21732, UCC21710-Q1, UCC21710, UCC21750

Hello, I have a strange problem with the driver, I used UCC21732 with desat protection and secondary side supply provided by Murata DC-DC converter with +20/-5V and the +20V is adjusted to +18V using LM2941. The circuit is as follows when I power up the driver it consumes a lot of currents and the output of LM2941 shows 4.7V, and the primary side of the UCC21732 driver supply starts rising to 7V.

DO you have any idea why I am seeing this problem with the UCC21732 driver? 

  • Hello,

    Looking at the shared portion of the schematic I don't see any clear shorts that could be causing this behavior of the power supplies.

    • Would you be able to share more details on the isolated bias supply portion of the design?
    • Have you seen this behavior on multiple UCC21732 IC's or only on one?

    Another interesting design choice I saw was the separation of GND_HSU into GND_HSU1 and GND_HSU2. I don't believe that is the source for this issue, but I would not recommend to separate the grounds as this could potentially have ground bouncing between each net due to the HV switching transients. This could specially be a problem for the DESAT function which in your design is referenced to GND_HSU1 while the COM-pin which the DESAT internal circuitry is referenced is connected to the GND_HSU2 net.

    Let me know if there's any questions.

    Best regards,

    Andy Robles

  • HI Andy,

    Thanks for the reply, I have the same problem with all driver boards (with UCC21732Q). When I Change the driver to UCC21710-Q1 the secondary output is working but the primary is increasing to 6.5V.

    I separated the grounds to bypass the high-frequency currents due to switching.  IF I don't use the OC pin can I keep it open or connect it to the ground? 

    That is the complete circuit. The primary circuit is a simple logic circuit.  

  • Hi,

    If the OC-pin is un-used short to COM (ground) directly.

    It's un-usual that the UCC21732 was not working in this application since the UCC21710 and UCC21732 are from the same family of devices.

    • When using the UCC21732 what did you do with the CLMPE pin?
      • The UCC21732 has an external miller functionality and outputs a 5V signal to drive an external NMOS to act as the miller clamp
      • If un-used this signal should be left floating in the UCC21732

    In order to see where the short could be coming from we could start by disabling some features of the design to narrow down our search of the root cause. We can try the below steps one by one to see where the issue is coming from:

    1. Disable the OC-pin feature by connecting directly to COM
      • The shared schematic shows the UCC21750 which has the DESAT-pin while UCC21710 and UCC21732 has the OC-pin
    2. Disconnect the load from the gate driver output
    3. Short AIN directly to COM
    4. Float APWM
    5. Short FLT and RDY directly to GND

    These debugging step should help us isolate the problem if it is coming from the gate driver itself.

    Let me know if there's any questions.

    Best regards,

    Andy Robles

  • Hi Andy,

    Thanks will check as per your suggestions.

    I connected the clamp pin directly to Gate. We initially made the board with UCC1750 but the ICs are not available so we used UCC21732 IC .

  • Hi,

    In UCC21732 ensure the CLMPE (external clamp feature) is left floating and not connected directly to the gate.

    Let me know of any additional questions and updates on the test results.

    Best regards,

    Andy Robles

  • HI Andy,

    Thanks the problem is solved after I disconnected the Clamp pin now the circuit works fine...