# How to choose the correct TVS diode and how to size it?

So I was thinking about transient suppression and I found TVS diodes. I was wondering how to choose the correct TVS diode since they have 3 voltage parameters:

• Vc --> clamping voltage, voltage I think you want to cut the transient spike. the voltage at which the device will conduct its fully rated current.
• Maximum reverse standoff voltage --> the voltage below which no significant conduction occurs.
• Breakdown voltage --> the voltage at which some specified and significant conduction occurs.

I would like to know if I'm trying to use TVS, how to choose the proper diode values if I want to work with Vin = 12V and clamp the spike at 13V?

• Is it possible?
• What 3 voltages must I select for clamping at 13V when Vdiode grows and keep 12V at load when Vtransient voltage is less or equal than 12V?
• Some diodes I have seen with Vc = 13V has Vreverse and breakdown voltages less than 12V. I don't know if this could make it work wrongly. Or whether Vreverse doesn't matter.
• I have here a TVS with Vreverse being 12V but clamping voltage 19V. I guess it only works when spike transients exceed 19V, is that correct?
• You’ve adequately described the three points in the voltage curve of a TVS and then gone on to ask questions that appear to prove you don’t know what they mean. No mention of acceptable current draw at 12 volts and no mention of the surge current you are trying to protect against is what I refer to. – Andy aka Oct 17 '19 at 18:16
• Maybe you' re right. I think I know the theoric explanation but I don't know how to apply it in practice to get the propper diode. So that's the reason why I'm asking. According to my understanding of definitions I think I will never found a diode able to be transparent at 12V (Vreverse) and clamping 12V + %(12V) when Vreverse grows over 12V. I was lokoking for one diode like that, but I doesn't found any one with these charecteristics. So I have asked for clarifications. – Eugenia Suarez Oct 21 '19 at 8:02
• > I have here a TVS with Vreverse being 12V but clamping voltage 19V. I guess it only works when spike transients exceed 19V, is that correct? Yes, usually that's how they work. They have a nominal working voltage (12 V) that they don't conduct at, then a minimum voltage at which they break down and begin to conduct (Vreverse) and then a clamping voltage at which they significantly conduct (Vc). If you can tolerate some voltage above 13 V, then the 19 Vc parts seem like a good fit for you. – ricehornet Oct 23 '19 at 6:19

A TVS has six key parameters

Ppp Peak Pulse Power. This is the power rating of the devices and should not be exceeded. Take care of temperature derating and design margin de-rating

Ipp Peak pulse current.

Vwm Working voltage. This is the voltage at which a TVS is guaranteed to to conduct more than the leakage

Vbr(min) This is the minimum voltage the TVS will start to avalaunch the test current

Vbr(max) This is the maximum voltage the TVS will start to avalaunch the test current

Vclamp(max) This is the maximum clamping voltage at the rated current.

Working voltage

Knowing the maximum voltage the LRU could be subjected to influences the Vwm of the device (or devices in series) needed.

Source impedance

Knowing the voltage and the current of a waveform is required to understand the source impedance of the thread. What's the peak threat voltage, whats the peak threat current -> source impedance of the thread.

Knowing the threat voltage, the minimum breakdown voltage and the source impedance, the peak threat current can be calculated

$$\Ipk = \frac{ V_{oc} - \Sigma V_{br(min)}}{R_{source} + R_{circuit}}\$$

This is a pessimistic maximum

Clamping voltage

With a pessimistic maximum current known, a representative clamping voltage can be calculate via linear interpolation between $$\V_{br(min)}\$$ at test current and $$\V_{clamp}\$$ at max current. Likewise you have a need to clamp at 13V.

TVS Power

With the conduction current and the blocking voltage known, the Peak pulse power can be calculated. This needs to be compared against the energy and the peak pulse power of the waveform.

How to determine the energy and the peak power of a double-exponential, or multi-stroke. There are a number of ways. One proven way is via the Wunsch-Bell relation where a "K-factor" which is dependent on the type of waveform, is used to approximate teh energy