I have the below circuit:

enter image description here

Forward diode D0002 - Datasheet SIJ series for reverse protection and C0001-C0004 capacitors for ESD protection.

Zener diode - Datasheet - 36V reverse breakdown.

Question :

BATT1 is the Vcc supply for certain ICs in my design whose absolute maximum ratings is 40V.

I have provided the Zener diode circuitry to clamp if the battery voltage goes above 36V (during transient ISO7637 pulses namely pulses 2a,3b and 5a.)

I have performed simulations in Simetrix with the proper models for each part, but the Zener diode circuitry is not clamping the voltage to 36V during these ISO7637 positive pulses. Why is it so? Is the Zener diode slow to respond or react to the input transient pulses 2a, 3b and 5a?

Please let me know like if there is something like Zener response time which makes it unable to do this clamping. Or what might be the reason?

  • 3
    \$\begingroup\$ It won't clamp at all well with 100 kohm in series with the collector. Clamping will be negligible in fact. \$\endgroup\$
    – Andy aka
    Aug 12, 2020 at 14:07

2 Answers 2


Consider the two components in red below: -

enter image description here

The resistors in red are the only paths for current to flow when the transistor is attempting to clamp. Ask your self how much current they are going to shunt away and whether they are actually the right values.

A voltage clamp works by taking a large shunt current. Having a 100 kohm resistor in series with the transistor is not going to pass more than a few tens of micro-amps when the 36 volt is exceeded. This does not constitute a clamp in my book.

  • \$\begingroup\$ Thank you for the answer. Then, could you please provide a solution to clamp the mentioned transient pulses? \$\endgroup\$
    – user220456
    Aug 12, 2020 at 14:23
  • \$\begingroup\$ I'm providing an answer to your question. A solution would probably take about 3 hours of my time and yours without guessing. Try picking a TVS diode or producing a crowbar clamp. \$\endgroup\$
    – Andy aka
    Aug 12, 2020 at 14:36

Your intention ought to be how to protect a device from the conducted back EMF surges to a battery defined by ISO7367 and not clamp the battery. You want to block passing this voltage to your target device. In Theory this could be done by opening a series switch or closing a shunt switch that dissipates the voltage difference between your 40V limit and the applied pulse. However the risetime of opening a switch may not be practical with a surge dump.

It seems your solution has no specifications and your setup does nothing useful.

please review the standard test methods https://training.ti.com/iso7637-test-pulses?context=1127713-1106817.

Then define your device specifications for pass voltage range and current and block voltage range and resulting current or pulse energy to be absorbed.

Then the design choices for clamping overvoltage protection will become standardized by suppliers of TVS.

There are also some false assumptions in your question.

  1. The diode does not prevent reverse voltage on the input caps and is not necessary to protect caps of this value from reverse voltage as they are non-polar.

  2. If the PNP acts as a comparator switch, it is essentially a non linear resistance divider. So adding R below the Zener defeats that purpose of reducing the incremental R when the zener conducts with excessive current limiting for bias.

  3. If the PNP conducts as a switch , what is it’s Rce? And why have it do nothing useful except pull up 100k?


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