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BATT= 16V

27V Rated TVS Diode spec - TVS Diode Datasheet

The circuit :

enter image description here

My questions:

  1. I want to understand what happens when the BATT voltage goes above the TVS diode clamping voltage. For both positive profile voltage and negative profile voltage.

My understanding, suppose, if the BATT = 50V. The D101 will be forward biased. The D102 TVS diode will clamp at 27V (its nominal rated TVS voltage). But won't this create a short circuit? There is nothing to limit the current through the D102 diode. Won't this circuit fail?

For negative voltage, the Diode D101 itself will block the voltage. So, the D102 will not face any voltage.

How to understand on the TVS diode behaviour during positive and negative transients in this circuit when there is no series element to limit its current?

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  • \$\begingroup\$ TVS diodes without current limiters will be used for Transients/surges not for static voltages. If BATT is at 50V continously TVS will get burnt based on BATT's current capability. \$\endgroup\$
    – user19579
    Commented May 8, 2020 at 10:35

1 Answer 1

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Won't this circuit fail?

Yes, the circuit will fail without something that can limit the current.

The whole idea about surge (indirect lightning) and ESD protection is that there is something that limits the current. In the typical case of ESD it's the 1500 ohm resistor in the human body model: -

enter image description here

In the case of indirect lightning (as in ISO/EN 6100-4-5) it's a resistor of value 2 ohms to several tens of ohms. This value is chosen by the test house to be appropriate for the ports and wiring of the equipment being tested. A shielded/screened cable would be subject to a 2 ohm source and, if the intended rating of the equipment as 2 kV Level 3 below), this means that a peak current surge of 1000 amps is possible applied directly to the shield/screen: -

enter image description here

There is also the energy storage in the capacitor - this limits the duration and you can find that for very short bursts (say 1 ms) a powerful TVS can handle 5 kW or more. For even shorter bursts, it's tens of kW.

But won't this create a short circuit?

Well, not exactly a short circuit because a short circuit won't dissipate energy or power but, it'll be the clamping voltage of the TVS.

How to understand on the TVS diode behaviour during positive and negative transients in this circuit when there is no series element to limit its current?

You MUST have a series current limiting resistor and this is usually in the test equipment but, there are many cases where the designer will add extra resistor just to alleviate TVS stress.

I want to understand what happens when the BATT voltage goes above the TVS diode clamping voltage. For both positive profile voltage and negative profile voltage.

For the positive profile that has been explained above. For the negative profile I believe that D101 (rated at 400 volts) may fail due to reverse over-stress voltage.

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  • \$\begingroup\$ Thank you. I have 3 questions from your answer. 1. In the Block diagram you show, on the right side, I don't have a Resistor in series with my DUT (My DUT is just the circuit which I have shown). So, without the series resistor installed before my DUT, My DUT will fail when I apply an ESD pulse? 2. In your second section of the answer, you say its not a short circuit exactly. But, without any current limiter for the TVS, the TVS will actually short circuit right? And while short circuit, does it clamp the voltage? And lastly, for negative profile voltages, won't the input 2 capacitors help? \$\endgroup\$
    – user220456
    Commented May 8, 2020 at 9:53
  • \$\begingroup\$ 1) If you are performing a standard test (HBM/ESD or lightning surge), the current limiter is inside the equipment that does the testing. 2) The TVS, if it fails may indeed go short-circuit and will of course clamp to close to 0 volts. 3) The capacitors - we've discussed this before and yes, they can help greatly. \$\endgroup\$
    – Andy aka
    Commented May 8, 2020 at 10:00
  • \$\begingroup\$ Thank you. Got it . But only for the second question, when you say the TVS fails, at what condition will it fail? For example, if my TVS nominal clamp voltage is 27V. And from the datasheet, the Constant power dissipation max is 5W. So, the Maximum current at 27V, is 5.4A. So, will the TVS fail, if the series resistor outside the DUT does not limit the current to 5.4A (in the event of an +8kV ESD pulse) \$\endgroup\$
    – user220456
    Commented May 8, 2020 at 10:07
  • \$\begingroup\$ If constant dissipation is 5 watts then the current will be 5/27 = 185 mA. Please link me the data sheet. \$\endgroup\$
    – Andy aka
    Commented May 8, 2020 at 10:45
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    \$\begingroup\$ Yes, Sorry. I have finished this question and got the required clarification. Accepted the answer. Thank you \$\endgroup\$
    – user220456
    Commented May 21, 2020 at 9:25

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