Choosing a TVS - Page 1
Forum section: electronics / projects

author: angust_uk
author_rank: Regular Contributor
timestamp: December 03, 2020, 09:29:17 am
content: I'm looking to add transient protection on the input supply of a circuit which may be used in environments where this supply could well spike above the maximum rating of the switching regulator used.I'm using a TI LM5005MHX which has an absolute maximum input voltage of 76V, so I'd like to make sure the voltage is clamped to below this point. I'm planning on adding a TVS, but am not entirely clear of the best way to select the clamping voltage without limiting the range of the input supply.Should I choose a device whose maximum clamping voltage is below 76V? This is probably a silly question, but I'm trying not to reduce the maximum input supply too much, so I don't' want to select a device which might start to conduct too soon, as this would reduce the maximum supply that can be used. For example the SMBJ45CD has a clamping voltage of 72V but a breakdown voltage of just over 50V, which would mean that it would start to conduct when the supply is still quite a way from the maximum allowable voltage.

author: tszaboo
author_rank: Super Contributor
timestamp: December 03, 2020, 09:48:07 am
content: TVS is not overvoltage protection. TVS is for what the name suggest it is for. So your transient is not 72V or not 76V, it is typically thousands of volts. So you should chose a TVS that has the breakdown voltage above 76V.When the ESD even occurs, the voltage quickly rises to above the breakdown, and it clamps to a voltage that your device can handle.If you want to protect the device from supply voltage rising, that is typically asurge, and not a transient voltage. Surges are different, their source is different, like a lightning or a motor dumping it's energy. The power/energy in it is much higher, and protecting against it is more difficult. Typically with MOVs or crowbar clamping circuits.If you want to protect the device for overvoltages, like wrong supply plugged in, that's again, different. That is overvoltage protection. That operates with even higher power, and the usual way to protect against it is to just disconnect your input.Dave really should create a video explaining this.

author: angust_uk
author_rank: Regular Contributor
timestamp: December 03, 2020, 09:58:14 am
content: Thanks - that makes a lot of sense. I think I was conflating over-voltage and transient voltages. I'll approach them as 2 different problems, which makes things slightly easier!

author: trobbins
author_rank: Frequent Contributor
timestamp: December 03, 2020, 10:31:30 am
content: So much depends on the possible power source environment, which you need to describe in more detail than 'could spike above .... 76V'. And it also depends on the circuity you present to the power source, as even the typical buck converter circuit requires a "Cin" which could well suppress many 'spikes', but again you need to provide details. 76V is also the absolute max rating, so you need to apply a margin for starters that you design not to go over for all known tolerances and defined source environments.You also need to align the source environment with any over-voltage protection part you may implement, especially if you want the equipment to survive events that cause the protection part to be active (unless you purposefully design in a part that is meant to fail as part of providing protection, or is meant to latch (which may mean some level of human or remote intervention to reset).

author: Siwastaja
author_rank: Super Contributor
timestamp: December 03, 2020, 01:28:39 pm
content: Obviously, when the LM5005MHX has maximum input voltage of 76V, the TVS must clamp to a lower voltage than this. "Maximum clamping voltage" is the right parameter, it has to be <76V.Note that maximum clamping voltage is specified at some test current. If your transients/overvoltages are from a low-impedance source that can supply more current than this, the clamping voltage will be higher than rated, you need to either choose even lower clamping voltage, or a part rated at higher test current. Vice versa, if you know the overvoltage transients are high-impedance low-current transients, you can succeed even if the maximum clamping voltage is specified slightly more, say 78V, because at lower current, it will clamp at lower voltage.Finally you will find that TVS diodes are not precision devices. If you choose a part with maximum clamping voltage of 76V, the "working voltage", which is, the TVS is guaranteed to be non-conducting (or actually conduct a specified very small current), is much lower than 76V, generating a dead-zone you can't utilize, i.e., you have to overrate the switching converter's max Vin. For example, if you have a maximum input of 50V, the TVS will be rated to clamp at 75V, and you'll then need a 75V switcher IC.TVS diodes are fine with continuous overvoltages as well but the continuous power dissipation capability is many orders of magnitude worse than the headline surge handling. As a result, like NANDblog suggests, it's a good idea to catch only short and large voltage transients with TVS, and if you have a condition of having overvoltage last longer than a few milliseconds, don't use a shunt style dissipative protection (like your TVS is), instead use series regulator to drop the voltage (to keep the device operational) or disconnect it. The simplest way to implement this idea is to just overspecify the voltage rating of your switcher!For example, if you expect 50V max nominal, 75V occasional hundreds of milliseconds or more, 100V peak (milliseconds), choose TVS working voltage 75V, therefore clamping voltage will be at about 100V, and the switcher max Vin needs to be 100V.For example, an automotive load dump is kind of edge between choosing shunt (TVS across the line) and series (pass regulator with enough voltage headroom) protection, both solutions are used. If you have a much shorter pulse than load dump, the shunt style is obvious, and much longer, pass style is obvious.

author: angust_uk
author_rank: Regular Contributor
timestamp: December 03, 2020, 03:17:14 pm
content: Many thanks for the detailed responses!