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ISO5852S: Whats the Zener on the Desat pin for?

Tony Philip45
Intellectual 755 points
Part Number: ISO5852S

Hi,

I found this schematic in one of your EVM designs foe the ISO5852s. Is this 3V3 Zener (MMSZ4683T1G) used to increase the threshold voltage of Desat triggering?

Is there any math by which I can estimate the amount of current that the IGBT will have to pass through before turnning-off (max current)?

Regards,

Tony

  • 0
    TI__Expert 3355 points
    Hi Tony,

    Thank you for your interest in ISO5852S. I work on the applications team in the high power drivers group and can help you with your question.

    This Zener diode is actually used to lower the threshold voltage of the DESAT threshold. As seen in figure 9.2, the DESAT pin has a 500 uA current source which charges the blanking capacitor and applies forward bias the diodes in the DESAT circuit. The DESAT circuit triggers when the pin reaches 9V. Adding the zener diode changes the equation to:
    Vce-fault(th) = 9V - 0.5mA*R - Vf - Vz
    Where:
    Vf is the blocking diode forward voltage.
    R is the series resistor
    Vz is the knee voltage of the zener diode

    In this circuit, I estimate that the Vce-fault(th) would be: 9V - 0.5mA*1kOhm - 0.7V - 3V = 4.8V.

    You can review the Ic vs Vce vs Vge chart in your selected IGBT datasheet, and see what collector current will trip the DESAT protection.

    If this helped answer your question, could you please press the green button?

    Thanks and best regards,
    John
  • 0 in reply to John Geiger
    Intellectual 755 points

    Hi John,

    I was trying to figure this out, but can't quite get the idea correct.

    Heres a screenshot of the graph. My VGE=15V , VCE=+325V and I want the current limit to be around 20A. How would I achieve this on the ISO5852S:

    https://www.st.com/resource/en/datasheet/stgw30m65df2.pdf

    Thanks and Regards,

    Tony

  • 0 in reply to Tony Philip45
    TI__Expert 4875 points

    Hello Tony,

    The DESAT threshold voltage is based on the desired current trip point and the corresponding on-state voltage, VCE, as I’ve shown from the I-V curves you provided below. If you set the DESAT current to be 20A as you mentioned, you would set V_DESAT = ~1.5V. Triggering DESAT at 20A sets off the short circuit protection (i.e. soft turn off) of the IGBT. However, as you can see, this region is not actually near the short-circuit limit of the IGBT when VGE is 15V. The IGBT will not go into desaturation until a much higher current, which is why ISO5852S part has a default V_DESAT of 9V. The equation which John provided above allows you to tune V_DESAT to your desired level, which I described above.

    What is your typical operating current? The DESAT threshold is very low when set at ~20A, and you will probably experience false-triggering of DESAT during typical operation, as the threshold is very close to the rating of your device.

    Regards,

    Audrey

     

  • 0 in reply to Audrey Dearien7
    Intellectual 755 points
    Hi Aurdey,
    Thanks for your reply. My operating current will be as low as 10Amps continuous. I had attached the datasheet earlier. Do you think its advisable to use this option with our application? It would be a great advantage in our prototyping runs to have this option on our boards and even at final production.
  • 0 in reply to Tony Philip45
    TI__Expert 4875 points
    Hi Tony,

    Desaturation should be used to protect the device from a short circuit condition, during which the IGBT desaturates. Based on the characteristics of your device, I would not advise using DESAT protection for such a low threshold voltage, which is not near the IGBT's actual desaturation region. As mentioned previously, you will likely experience false trips because there is little margin between the VCE threshold you want to monitor versus normal operation. In this case, to monitor an over current condition when current rises above 20A, then I would recommend a separate circuit, such as shunt resistor monitoring.

    Regards,
    Audrey
  • 0 in reply to Audrey Dearien7
    Intellectual 755 points
    Hi Audrey,
    Thanks for your reply. Can you please share with me your mail id, I wanted your opinion on a design I have completed.
  • 0 in reply to Tony Philip45
    TI__Expert 4875 points
    Hello Tony,

    I will request you as a connection through e2e and afterwards you can share your design via private messenger.

    Regards,
    Audrey