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ISO5452-Q1: Adjusting DESAT Threshold with Zener Diode

Grant Cosens
Prodigy 40 points
Part Number: ISO5452-Q1
Other Parts Discussed in Thread: ISO5452,

I'm working on an application which requires a 2.5V desat threshold with 2us blanking time. I'd ideally like to use a Zener diode to set the threshold as I would otherwise need a very large string of rectifier diodes in series.

However, I've done an exhaustive search of various Zener diodes and cannot find any which reach their knee point at 500uA (desat charge current of the ISO5452).

I've seen some replies on here which suggest using Zener diodes to adjust desat voltage - can you recommend a particular series which is suitable for the very low current in this application?

FYI - I've also considered using a ~10k resistor to adjust the desat threshold. The disadvantage with this approach is that it significantly extends the desat blanking time due to the RC time constant.

I end up needing a 33pF blanking capacitor using this solution and this makes me very concerned about noise immunity - my switching frequency is up to 100kHz and bus voltage is 1200V.

  • 0
    TI__Expert 3355 points

    Hi Grant,

    Thanks for your interest in ISO5452-Q1. I'm an applications engineer in the high power drivers group and can help answer your question.

    I also can't find any zeners which have their listed knee at the 0.5-mA desat charge current. Looking at a few zener diode datasheets,however, it seems like they still hit most of their Vz at 500-uA. If this variation is tolerable, the resistor can then be used to dial in the trigger voltage. There is some more information, detailed in a previous E2E thread.

    Besides this, it may be possible to use some kind of BJT Baker clamp, though I haven't personally seen it used in this application. I'll poll the team at the beginning of next week to see if they have any more inputs on this question.

    Thanks and best regards,

    John

  • 0
    TI__Prodigy 150 points
    Hi Grant:

    The zener diode with 0.5mA knee current is hard to find. A 5mA Iz zener diode still reverse conducts at 0.5mA but the voltage will be a little lower than Vz. A resistor needs to be connected in series with the zener diode to trigger the 9V desat threshold voltage. Before the zener diode reaches Vz, the Idesat 0.5mA is charging the blanking capacitor, after the capacitor reaches Vz, the zener diode reverse conducts and the current is shared with the blanking capacitor until 9V threshold voltage is reached.

    Thanks and Regards,
    Susan
  • 0 in reply to Susan Guo
    Prodigy 40 points

    Thanks Susan, John,

    Susan - your description has confirmed my understanding of the Desat circuit. I was hoping to avoid simply increasing the series resistor value as this causes a very large variation in detection time depending on Vds in my simulations. I can achieve my threshold requirement with a 9k6 resistor but I then need a 33pF capacitor to achieve my 2us timing requirement. I'm worried that this solution will be highly susceptible to switching transients as the 33pF  desat capacitor will not be very large compared to the blocking diode capacitance.

    I managed to find some zeners with very low knee current - the Vishay MMBZ series:

    I was also wondering whether I could use a precision shunt voltage reference in place of the Zener in order to guarantee stability across the temperature range? My simulations indicate that this approach would work very well but I'm not sure how accurate the shunt reference models are.

    Are you aware of any customers who've used a shunt reference in this sort of application?

  • 0 in reply to Grant Cosens
    TI__Prodigy 150 points
    Hi Grant:

    The blanking time of the 33pF capacitor and 9.6kohm resistor + zener diode should give you <2us blanking time. The 0.5mA current charges the blanking capacitor before Vz. Say you are using a 5.1V zener diode and 33pF blanking capacitor, it takes 330ns for the cap to charge to 5.1V. Then the zener diode reverse conducts and the time constant becomes RC. If using 9.6kohm resistor, time constant is 317ns. The blanking time to reach to 9V threshold should be ~1us. A higher Vz zener diode can be used to reduce the blanking time.

    I think the shunt voltage reference can also work. I did not see that in the application to my knowledge, but I did not find problem preventing this solution.

    Thanks and Regards,
    Susan