Electronics > Power/Renewable Energy/EV's

How to combine OVP with Surge Protection without burning out TVS?

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OleMi:
Looks like a trivial task from a text-book, but not really.

Provided:

A device which is rated for 12V (DC) application with desirable limit up to 16.7V.
Current 4.5A.
The first IC on the device withstands only 21V according to AMRs.
Looking for OVP 18V.
Looking for a Surge Protection as well.
Desirable reasonable AMRs for the device, say, 28V (for the case if integrator will connect 24V truck battery).

So, according to text-books:

TVS + fuse (5A) + "electronic relay" for OVP (protections with MOSFET that will be off for OV conditions). Well, let's take a look.

The core problem is: TVS for what voltage?

Say, OVP's components (MOSFET) withstand DS Breakdown 50V. We take TVS for 36V. Surges above 36V will not damage OVP, but surges above 36V will reach the IC and damage it, because it is rated for 21V and is not protected from surges. Why OVP will not help? Because it is a surge (an impulse of short duration) and OVP is not that fast. There is no fast OVP exist, because TVSs are designed for protection from fast short-duration impulses and OVP must be protected from them as well. The OVP itself can't provide Surge Protection. So, doesn't work.

OK, obviously we want to move TVS Breakdown/Clamping close to OVP's value 18V then. The maximum limit we allow for the user is 16.7V. Let's take 15's series of TVSs which start opening at about 16.4 and clamped at about 18-19v. Wonderful, protection from surges of 18V and more is guaranteed. The IC is protected from Surges. Now let's look what happens in over voltage conditions of close to 18V, say 17.8V. Clamped TVS will burn out, before the fuse will burn! Why? Because TVS is for short-duration impulses and not for continuous over voltage conditions. Why fuse will not burn out? It's slower than TVS and the fuse is installed not for that case, but for reverse polarity, when the TVS will short pretty any voltage and the fuse will blow earlier than TVS.

So, how not to burn TVS?

richard.cs:
The simple answer is that your 21V rated IC is marginal for a device where the user is expected to apply up to 16.7V.

The complex answer is of course you can but the design is more complex, for example a precision OVP circuit that cuts in around 17V before the TVS current gets too high, placed before the TVS, and then a higher voltage TVS before that to protect the OVP circuit.

You then have a TVS at 36V that protects the device from surge if the OVP switch is off and protects the OVP switch, another that protects the device to 19V if the OVP switch is on, and the OVP switch which protects both the device and the lower voltage TVS from overvoltage.

capt bullshot:
https://www.analog.com/en/products/ltc4367.html
These are claimed fast enough for your kind of protection.

Marco:
Use a smarter OVP ... when voltage is higher than 16.7 for x amount of time, crowbar. It's slower than the TVS, but it's fast enough to limit the energy the TVS can see.

OleMi:
@richard.cs
The reason of OVP because the user is expected to connect up to 28V. I'm not talking about voltage regulation here, but about protection from users' mistakes.
Second TVS doesn't solve the problem at all. Just draw it and simulate say 30V impulse, 40V impulse, 24V over-voltage and you will see all the same problems.

@capt bullshot
I don't consider to use specific ICs, especially those with two external MOSFETs, especially overprices Linear's stuff... I also don't believe that such solutions are really surge protected. They are OVP, not Surge Protection.

@Marco
I don't understand how OVP, which is slower than TVS can help to protect TVS in over-voltage conditions. Do you say that I should use crowbar OVP as a surge protection in the same time and remove TVS from the design? One thing that I don't want neither is to have fuse blown in OV or surge conditions, only in the reverse polarity case. Crowbar in my knowledge will blow the fuse - the same like OVP based on zener clamp with transistor buffer. The last, btw, is not surge protected, because transistor will not survive the same surges as TVS. Crowbar also looks like a bit unreasonable for DC application.

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