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BMS short circuit interruption capability
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Biduleohm:
Hi guys :)

Long story short I work on a BMS (battery management system) and I need some help on the disconnect part of it which is basically a 300 A 64 V SSR. If you want the whole story you can see it here.

The problem I have is that I want to be able too interrupt a 10 kA short-circuit current with the mosfets surviving. The interrupt part is ok but the mosfets surviving isn't because of the inductive spike.

I took the (defavorable) case of the battery connected with 2 m of 10 mm diameter wire, which is about 2.4 µH, from my calculations.

The battery can be considered as a 64 V voltage source with a short circuit current of 10 kA.

I have 2x 10 mosfets connected source to source to form a bidirectional switch on the negative side of the battery. Each mosfet can handle 1 kA for 10 µs and 1.1 kA for 5 µs, I calculated the total time from over current detection to mosfet turned off to be under 4.46 µs wort case. I use these mosfets.

I used the formula from this answer to calculate the power dissipated by a 64 V TVS and I get about 100 kW (120 J in 1.2 ms). That's a lot but do-able with multiple TVS diodes in parallel; the real problem is that 64 V TVS diodes have a clamping voltage of 103 V at best which is obviously a lot higher than the 80 V the mosfets can handle. 100 V mosfets start to be in the too high Rdson domain, but more importantly they are a lot more expensive than the 80 V ones and the project is highly cost sensitive.

But then is occured to me that the inductance would need about the same time to store the energy than it needs to unstore it (about 1.2 ms) and I'm capable of switching off the current 250 times faster than that (5 µs) so it shouldn't have the time to store a lot of energy, and so it should be a lot easier to dissipate it and not kill the mosfets.

I attached the schematic as a PDF but I can provide the KiCad files if needed. The section of concern is in the middle, the mosfets are Q15x and Q16x. I only put 4 of the 20 mosfets for now because if I modify something I don't want to have to redo all of it. B- is connected to the battery negative and P- is connected to the loads negative. I can also provide the schematics of the over current detection section if needed.

If you wonder about the 100 nF caps and the schottky diodes on the mosfets it's to prevent them to turn on in case of a high dV/dt on the drains (like when someone is connecting the battery to something for example). The two gates drivers in parallel are for redundancy and a lower turn-off time.

Is the disispated power formula and my numbers correct?

Am I correct about the fact that the energy stored will be a lot lower than 120 J?

If so, how much energy should I expect to have to dissipate in how much time?
strawberry:
transistor amplified zener
Biduleohm:
Yes, I tought about an active circuit but I need to know what kind of energy I'll have to dissipate to design it accordingly.
jbb:
How about some alternatives:
- Reduce MOSFET off speed to reduce dV/dt. Will increase heat dump into MOSFETs, so check Safe Operating Area (SOA)
- High rupture current DC fuse to break the dead short (delay MOSFET turn off a little so the fuse goes)
- Limit MOSFET drive voltage a little (depends on devices) to use inherent MOSFET current limit (ie FETs move from saturation to linear region) and don’t let it get to 10kA. You can also add desaturation protection for fast overcurrent detection
- maybe you could use a crowbar clamp, i.e. when the voltage starts to surge up (as the MOSFETs start to reduce the current), use an SCR to short the output rail to 0V. It’s vicious, but it’ll keep the voltage spikes out.
Biduleohm:

--- Quote from: jbb on March 24, 2020, 11:49:44 pm ---- Reduce MOSFET off speed to reduce dV/dt. Will increase heat dump into MOSFETs, so check Safe Operating Area (SOA)

--- End quote ---

I can't reduce the dV/dt or the mosfets will fry.


--- Quote from: jbb on March 24, 2020, 11:49:44 pm ---- High rupture current DC fuse to break the dead short (delay MOSFET turn off a little so the fuse goes)

--- End quote ---

Same problem, the mosfets will protect the fuse but not the other way around.


--- Quote from: jbb on March 24, 2020, 11:49:44 pm ---- Limit MOSFET drive voltage a little (depends on devices) to use inherent MOSFET current limit (ie FETs move from saturation to linear region) and don’t let it get to 10kA. You can also add desaturation protection for fast overcurrent detection

--- End quote ---

The mosfets are fully saturated in normal use to have the lowest Rdson possible so I guess that will not be possible.


--- Quote from: jbb on March 24, 2020, 11:49:44 pm ---- maybe you could use a crowbar clamp, i.e. when the voltage starts to surge up (as the MOSFETs start to reduce the current), use an SCR to short the output rail to 0V. It’s vicious, but it’ll keep the voltage spikes out.

--- End quote ---

That's likely what I'll be doing (but not to 0 V as it would maintain the short circuit, it'll be somewhere between 64 and 80 V) but where I'm stuck is calculating what power/energy I will need to dissipate with this circuit.
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