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I am wondering what is the criteria for selecting diodes TVS, for example my circuit runs at 36V and I want protect my circuit from spikes

But I am little confused about the Breakdown Voltage (VBR) if it select a value just a little above 36V and Reverse Stand-off Voltage (VRWM) at 36V.

My confusion in regarding to the Clamping voltage (Vc) because I see some graphs and the clamping is actually at Clamping voltage (sounds logic), but why is used the VBR widely instead of Vc?

for example: :http://www.protekdevices.com/xyz/documents/kb/tech/ta1004.pdf VBR = 6V, but it is clamping at 10V+

the actual max voltage input possible for my circuit is 38V, but the parts for 36V actually says clamping voltage 56V+ :/

for example this :https://www.digikey.com/product-detail/en/littelfuse-inc/SMAJ36A/SMAJ36ALFTR-ND/762298

Voltage - Reverse Standoff (Typ) =36V
Voltage - Breakdown (Min) =40V
Voltage - Clamping (Max) @ Ipp =58.1V

my power requeriments are in wide range from 12V to 36V max:

36V/900mA Max or 12V/2.6A

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    \$\begingroup\$ One of the important metrics you don't mention is the power rating. \$\endgroup\$ – Joren Vaes Sep 18 '17 at 19:54
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Reverse Standoff Voltage is the voltage at which no noticeable current flows, uA or nA scale usually, for the diode you have listed it is 1uA. This means that below the standoff voltage the diode will not conduct more than 1uA.

Breakdown Voltage is the voltage at which the diode begins to conduct, at the breakdown voltage for your diode the current is 1mA. When 1mA is flowing through the diode, the voltage will be 40V-44.2V.

Clamping Voltage is the voltage across the diode where the power limit is reached, which for your diode is 58.V @ 6.9A. The pulse rating for the diode is 400W and 58.1V * 6.9A = 400.89W

The current flowing through the diode is more of a curve(not discrete steps) and these voltages/currents just help give a better understanding of the current that flows through the diode as the voltage increases. If your circuit continually operates at 38V you will have between 1uA-1mA of leakage through the diode. your actual power dissipation at that level will be no greater than 38mW, so this diode should work for your design, depending on your power budget and the spike levels you want to protect against.

You may want to use a circuit like the following:

schematic

simulate this circuit – Schematic created using CircuitLab

The resistor and capacitor filter the repetitive spikes and the diode is for infrequent large or sharp spikes. The actual values should be determined based on the amount of power going through the filter and acceptable noise.

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  • \$\begingroup\$ for example for that component if the input is 41V the voltage will be clamped a 40V? or it have to reach to 58V+ to be clampled? \$\endgroup\$ – Ale Sep 18 '17 at 20:24
  • \$\begingroup\$ as the voltage increases the current increases, so at 41V you will have roughly 1mA leaking through the diode \$\endgroup\$ – Redja Sep 18 '17 at 20:25
  • \$\begingroup\$ I see! actually my circuit will powered for a truck battery (24v), but the alternator gives values from 32-36V. I want protect my circuit when it goes above 36V+ or Spikes 36V+ \$\endgroup\$ – Ale Sep 18 '17 at 20:28
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    \$\begingroup\$ Then you must not use a TVS in my opinion... These will be repetitive spikes, a TVS diode is meant for rare events... \$\endgroup\$ – next-hack Sep 18 '17 at 20:29
  • \$\begingroup\$ The voltage is never truly "clamped". If you exceed the clamping voltage you are above the power rating for the diode and it will most likely fail. \$\endgroup\$ – Redja Sep 18 '17 at 20:29

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