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UCC21750: The value of DESAT series resister

Nam, Dino
Expert 6445 points
Part Number: UCC21750

Dear team, 

My customer would like to delete Rlim on DESAT port to increase the effective DESAT threshold. Currently they are using 1K series resister and 1ea HV blocking diode. 

After deleting the series resister,  there is thermal issue on HV diode during normal operation.

I know that the internal current source flows to the collector through the series resistor and HV blocking diodes during normal operation. So I think the charge current(Ichg) is only 500uA which don't cause thermal issue when there is no the series resister. 

Please review below question and let me know your opinion. 

1. Is the series resistor(Rlim) mandatory option? If we delete the series resistor, is there any side effect like thermal issue? 

  - Will Ichg(500uA) cause thermal issue on HV diode? or Is there another reason?

  - If deleting of the series resister cause thermal issue, please let me know your opinion regarding the reason. 

2. The series resister(Rlim) is mandatory option, Is there any guide for Minimum value of the series resister(Rlim)?

Thank you.

  • 0
    TI__Expert 6730 points

    Hi Dino, 

    Please see the answers to your questions below - 

    Nam, Dino said:
    1. Is the series resistor(Rlim) mandatory option? If we delete the series resistor, is there any side effect like thermal issue? 

    Theoretically, Rlim can be deleted without observing any thermal issue. Since the charging current (Ichg) is kept constant at 500uA, the power dissipated in the diode should stay constant. However, deleting Rlim would not make too big of a difference on the desat threshold voltage (500uA * 1kOhm = 500mV increase in desat voltage). 

    Nam, Dino said:
      - Will Ichg(500uA) cause thermal issue on HV diode? or Is there another reason?

    As mentioned above, there should not be any observed issue. What HV diode is being used by the customer? Also, if you can share the schematic to us, we can help with further debugging. 

    Nam, Dino said:
    2. The series resister(Rlim) is mandatory option, Is there any guide for Minimum value of the series resister(Rlim)?

    Rlim is not mandatory, and we do not have a recommended minimum value for Rlim. 

    Thanks, 

    Vivian

  • 0 in reply to Vivian Qi
    TI__Expert 6445 points

    Hi Vivian, 

    Thank you for your response. 

    Is there any other role of Rlim? Is there any other reason to cause thermal issue? The customer said that they only deleted Rlim. 

    Currently, The customer is using USIM-E3, VISHAY diode as HV diode. And please refer to the below schematic and let me know your opinion.  

    Thank you. 

  • +1 in reply to Nam, Dino
    TI__Expert 6730 points

    Hi Dino, 

    Nam, Dino said:
    Is there any other role of Rlim?

    Besides adjusting the DESAT threshold, Rlim also has some current limiting abilities. It can limit the current spike caused by capacitive coupling from the diode when the transistor is switching, and it can limit the current flowing through the diode if the reverse recovery time of the diode is too long. However, in this case, either situation is unlikely to happen - the capacitance of the diode is small (15pF), and the reverse recovery time of diode is short (75ns). 

    Another role of Rlim might be related to the internal current supply (Ichg). It is possible that removing Rlim results in short in the circuitry, causing high charging current, thus high power dissipation in the diode. We will test the effect of Rlim on Ichg and get back to you. 

    Nam, Dino said:
    Is there any other reason to cause thermal issue?

    Other reasons that might cause thermal issue are: 

    1. The DESAT pin is more susceptible to noise after Rlim is removed, causing excessive power dissipation. 

    2. Noise on the collector/drain of the IGBT/MOSFET causing excessive power dissipation. 

    To test for these reasons, it can help if the customer can probe voltage on the DESAT pin before and after Rlim is removed. 

    Also, how is "thermal issue" defined? What were they using to measure temperature of the diode, and what temperature did it reach? And what is the switching frequency that they're using? 

    Thanks, 

    Vivian

  • 0 in reply to Vivian Qi
    TI__Expert 6445 points

    Hello Vivian, 

    Thank you for your kind explanation. 

    When can I get test result regarding the effect of Rlim on Ichg? 

    Vivian Qi said:
    what is the switching frequency that they're using? 

    => 25KHz

    About your another questions and waveform of voltage on the DESAT pin, I will discuss it with my customer and get back to you by next Monday.

    Thank you. 

  • 0 in reply to Nam, Dino
    TI__Expert 6730 points

    Hello Dino, 

    I'll get back to you regarding the effect of Rlim by end of day next Monday.

    Thanks, 

    Vivian

  • 0 in reply to Vivian Qi
    TI__Expert 6445 points

    Hello Vivian, 

    Do you test the effect of Rlim on Ichg? If done, please let me know your test result. 

    And please review below and let me know your opinion. 

    Vivian Qi said:
    it can help if the customer can probe voltage on the DESAT pin before and after Rlim is removed. 

    Please refer to the attached test result regarding thermal result and current waveform of Diode. 

    Bat Gate Desat Diode thermal Issue_E2E_220315.pptx

    Vivian Qi said:
    how is "thermal issue" defined? What were they using to measure temperature of the diode, and what temperature did it reach?

    After removing Rlim, temp of the diode increased from 43.2C to 87.6C. They used thermal camera.

    Vivian Qi said:
    what is the switching frequency that they're using? 

    As mentioned before, their switching frequency is 25KHz. 

    Please review above and let me know your opinion regarding the cause of thermal issue.

    Thank you. 

  • 0 in reply to Nam, Dino
    TI__Expert 4070 points

    Hi Dino,

    Thanks for reaching out!  Vivian is out at the moment, but will back to assist you later this week.

    -------

    Aaron Grgurich

  • +1 in reply to Nam, Dino
    TI__Expert 6730 points

    Hello Dino, 

    Apologize for the delayed reply. In my lab setup, I tested if the internal circuitry generating Ichg will be affected by the removal of Rlim, and the result is that removing Rlim does not significantly influence Ichg. 

    From the waveform you provided, I can see that the peak current when the Rlim resistor is removed is high (342mA) due to reverse recovery. This can explain the heating up of the diode. 

    We recommend putting in a Rlim resistor of 200 Ohms - 1 kOhm. Hopefully this will solve the thermal issue. 

    Thanks, 

    Vivian

  • 0 in reply to Vivian Qi
    TI__Expert 6445 points

    Hello Vivian, 

    Thank you for your strong support. one more question.. 

    So, Is there any possibility that the reverse recovery current(342mA) of Diode when the Rlim resistor is removed can damage on out UCC21750?

    Thank you. 

  • +1 in reply to Nam, Dino
    TI__Expert 6730 points

    Hi Dino, 

    The high reverse recovery current in this case will likely be damped by the charging capacitor, Cblk, since it is a pulse; however, if high current level persists, it is possible that the chip will be damaged. 

    Hope that helps! 

    Vivian