# How to select a TVS diode?

#### ke5nnt

Joined Mar 1, 2009
384
I've read a lot about these devices. I don't fully understand the difference between clamping voltage and breakdown voltage. It seems that breakdown voltage is when avalanche breakdown occurs, which I would want to select to be below whatever voltage damage to my circuit would occur at. However, clamping voltage seems to be described as the same thing, just in a slightly different way.

Is anyone able to clarify the differences between those two parameters? To help understand my application, I am designing a device which is to be powered from a vehicle's DC system and is generally not used while the vehicle is off. However, in playing it safe for a solid design, operating voltage of the circuit under "normal" conditions can be described as anywhere between 9V and 16V DC. Damage would occur to the circuit at or above 20V. Total current draw should be no greater than 500mA, but probably around 300mA.

Thanks as always.

#### #12

Joined Nov 30, 2010
18,224
The breakdown voltage is where it starts to conduct. The clamping voltage is what develops with the maximum current running through it. You will need an impedance of some sort in front of the device to stop the maximum current from hitting a high clamping voltage, and a TVS might not be the right thing for you.

and don't ever use the word, "vehicle" on this forum!

Joined Dec 26, 2010
2,148
The specification of these things is pretty opaque. One could almost think that the manufacturer was trying to avoid being tied down on the device behaviour, but that is probably not fair.

Here's how I see it, with reference to the Microsemi definitions:

1. There is a maximum working voltage VWM, at which the leakage is small and has a specified maximum value. This is a safe range of voltage in normal operation.
2. There is a somewhat higher breakdown voltage, VBR, defined as being in some range, at which the leakage current reaches some higher value. Normal operation should not reach these values, for fear that the leakage may be excessive and the device may overheat.
3. There is a clamping voltage VC, which specifies the maximum voltage that may be reached under some specified surge current.
http://www2.microsemi.com/datasheets/sd21a.pdf

Your application would therefore ideally require a clamping voltage of 20V or less, but a perusal of a data-sheet for some devices suggests that this would bring the breakdown within the normal range of a lead-acid voltage on charge. Perhaps you can find some more suitable types?

#### Wendy

Joined Mar 24, 2008
23,477
Think of two zener diodes back to back, but quicker and probably with higher current ratings.

#### debe

Joined Sep 21, 2010
1,390
Chec the specs on a S20K 14. this is what VDO use in an instrument panell ive just been working on which is 12V powered. Its ratings are Vrms 14V, VDC 16V, imax 8/20us 2000A. Vjump start(5min) 25V.

#### ke5nnt

Joined Mar 1, 2009
384
The specification of these things is pretty opaque. One could almost think that the manufacturer was trying to avoid being tied down on the device behaviour, but that is probably not fair.

Here's how I see it, with reference to the Microsemi definitions:

1. There is a maximum working voltage VWM, at which the leakage is small and has a specified maximum value. This is a safe range of voltage in normal operation.
2. There is a somewhat higher breakdown voltage, VBR, defined as being in some range, at which the leakage current reaches some higher value. Normal operation should not reach these values, for fear that the leakage may be excessive and the device may overheat.
3. There is a clamping voltage VC, which specifies the maximum voltage that may be reached under some specified surge current.
http://www2.microsemi.com/datasheets/sd21a.pdf

Your application would therefore ideally require a clamping voltage of 20V or less, but a perusal of a data-sheet for some devices suggests that this would bring the breakdown within the normal range of a lead-acid voltage on charge. Perhaps you can find some more suitable types?
I can increase the voltage at which damage occurs to the device under normal operation to +/-30V by changing one component it turns out. Perhaps that would open the gap between clamping and breakdown voltage to a more workable level?

From what I'm gathering from your information is:

Working voltage is normal operation where the TVS does not conduct (spare for a specified leakage current). I should select a TVS diode that has a working voltage about equal (or slightly higher) to what I expect max normal voltage will be (about 16V in my case).

Breakdown voltage is the point where avalanche breakdown begins to occur. The device shouldn't reach this point EXCEPT in some type of load spike from a transient which will be very brief and is handled by the clamping voltage. Normal operation above the breakdown voltage can cause overheating of the TVS and TVS failure.

Clamping voltage is set to be just below max circuit damage voltage (below 30V for my case) and stops a transient from damaging the circuit (in theory).

Does it sound like I'm on the right track? Thanks for the feedback.

#12 said:
and don't ever use the word, "vehicle" on this forum!
First Amendment to the United States Constitution said:
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
This guarantees me the right to say "vehicle" where and when I want.

#### #12

Joined Nov 30, 2010
18,224
Tell that to the moderators. They will quote the "Terms of Service" for this website and end the conversation.

Is it possible that you have done over 200 posts here and never heard of this rule?

#### ke5nnt

Joined Mar 1, 2009
384
My topic does not fall under that rule as the intention of my circuit/thread is not to modify an existing system. It is no more harmful than a cell phone charger. The rule is not in place to completely shut out automotive circuits, only the intent to not contribute to modifications or additions which might be construed as a danger to passengers or defeat safety features.

#### #12

Joined Nov 30, 2010
18,224
It's safer not to say, "car" or "vehicle". Some days the moderators seem to be in a don't feel like reading, kill 'em all and let God sort 'em out, grumpy kind of mood.

#### Wendy

Joined Mar 24, 2008
23,477
Please don't make this personal. We have to follow the TOS, which are clear. My interpretation is cars and how they work is fair game, but modifications are not, in big letters. I have sent you a PM #12.

The thread has drifted off topic big time. I assume there is more to be said on topic?

I missed the automotive reference in the first post. IMO it is borderline but legal, as a cigarette lighter accessory is generally not considered a modification. I'll point it out to the other mods for discussion, but I believe it is OK.

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#### ke5nnt

Joined Mar 1, 2009
384
I had felt the same way, and I feel my question has been lost in the dramatic discussion over the TOS, as I still have a question that has gone unanswered in this thread. As I did say above, I have asked no question regarding how to modify any existing circuit (such as headlight/taillight circuits, etc.)

My last question on the matter, which is back to TVS diodes in general, was the following. If we could just get that much answered, this thread will pretty much be dead anyways. As always, I appreciate the feedback of everyone here at AAC. Your knowledge and insight is endlessly useful.

ke5nnt said:
I can increase the voltage at which damage occurs to the device under normal operation to +/-30V by changing one component it turns out. Perhaps that would open the gap between clamping and breakdown voltage to a more workable level?

From what I'm gathering from your information is:

Working voltage is normal operation where the TVS does not conduct (spare for a specified leakage current). I should select a TVS diode that has a working voltage about equal (or slightly higher) to what I expect max normal voltage will be (about 16V in my case).

Breakdown voltage is the point where avalanche breakdown begins to occur. The device shouldn't reach this point EXCEPT in some type of load spike from a transient which will be very brief and is handled by the clamping voltage. Normal operation above the breakdown voltage can cause overheating of the TVS and TVS failure.

Clamping voltage is set to be just below max circuit damage voltage (below 30V for my case) and stops a transient from damaging the circuit (in theory).

Does it sound like I'm on the right track?

#### Wendy

Joined Mar 24, 2008
23,477
I would put a pi filter in front of the clamp, to try to eliminate spikes and whatnot.