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BMS short circuit interruption capability

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Biduleohm:

--- Quote from: mzzj on March 26, 2020, 09:44:45 am ---dI=dt*V/L
dI = 5us*64v/2.4uH = 133A

--- End quote ---

I didn't thought about using this formula this way, thanks ;)




--- Quote from: Siwastaja on March 26, 2020, 11:19:08 am ---2.4uH of inductance stores way too much energy at 10000A, but the idea is that you won't need to ever put 10000A through that 2.4uH! Note the current has a second exponent in the energy stored, so you need to really prevent the current from reaching the resistance-limited short circuit current; act well before that happens.

--- End quote ---

Yep, I had the feeling it would work that way but I needed to be sure.


--- Quote from: Siwastaja on March 26, 2020, 11:19:08 am ---Electrolytic caps are cheap and small, it's trivial to store the energy stored in the inductance in the capacitors, unless someone adds a MASSIVE ferrite core in the system, but this wouldn't be expected.

E = 0.5 * L * I^2 = 0.5 * 2.4uH * (550A)^2 = 0.36J
E = 0.5 * C * U^2, solve for C:
C = E/(0.5*U^2). Allow for 20V voltage rise on the capacitor voltage,
C = 0.36J/(0.5*(20V)^2) = 1800uF

You can clamp the voltage to a capacitor bank with a diode even if the switch is bidirectional. The advantage is you may be able to find fast diodes better than extremely fast TVS diodes (I don't know if I'm correct here; you may able to find a directly suitable TVS, as well, in which case the diode-capacitor solution does not have such advantage), and you can discharge the capacitors as slowly as you want.

--- End quote ---

I don't want to use electrolytics for longevity purposes. But now I know the energy level is much more reasonable I have a lot of other possible solutions so that shouldn't be a problem.


--- Quote from: Siwastaja on March 26, 2020, 11:19:08 am ---Note, 2.4uH, whatever this is coming from, is already quite a lot. Using https://www.eeweb.com/tools/loop-inductance , this would be a circular loop 75cm in diameter. In a properly designed battery pack and wiring, the current and the return current run closer to each other.

--- End quote ---

Yes, it's a defavorable case but I prefer to design for the worst case (and add some margin on top...) because I'm tired of the reverse in today's products (designed to just barely work, and so breaks at the first occasion), I want something that work, no matter what you do (as long as you stay in the limits of course), and which do it for years.


--- Quote from: Siwastaja on March 26, 2020, 11:19:08 am ---The idea is, do not design your active efuse circuit around the short circuit current of the battery, because it sees this current only if you have failed the design somehow, i.e., it won't be able to detect the overcurrent in time, and switch off the transistors in time - in which case it doesn't matter, it will likely blow up anyway. Instead, design your efuse circuit to prevent the current from ever rising much over the expected maximum operating current. Then, protect (the wire insulation and the battery pack, and other things thermal) against a design failure using a traditional passive fuse, and for that, make sure it handles the 10kA current.

--- End quote ---

Yep, obviously there will be a classic fuse too, but it would not have saved the mosfets (and at more than 3 € each that would get expensive quickly...) as it's way too slow.



@all Thanks everyone for the help ;)

Siwastaja:
Yes, a classical fuse cannot protect the MOSFETs, hence your MOSFETs need to protect themselves, in other words, you need good control for them. Good that you have a fuse; even the most perfect designer isn't perfect, and this problem is not trivial. The fuse cannot save the device, but it saves from a larger catastrophe. It must be non-user replaceable, so that once it's blown, the user must replace the device, which has failed short.

Nothing wrong in electrolytics if you don't require the stablest ESR in the world, and design for the ripple current spec (and derate it a bit). Actually, limiting yourself outside of electrolytics may lead to much more serious omissions on your part; large-value ceramics especially came with mechanical reliability problems and voltage spiking due to ESR too low, and actually combining smaller values of ceramics with electrolytics tends to solve these problems. Whatever you are doing likely requires some DC link capacitance, and with pure ceramic, you are likely seeing spikes 2x or even exceeding 2x the supply voltage. TVS damping is one option to limit the overshoot to somewhere about 50%, but electrolytics easily bring it down to near zero. After all, if you require low-loss low-ESR capacitance on your link, then you need preferably at least 2-3x the C with higher ESR. Doing that with another set of ceramics, with explicit series resistors, increase the BOM cost and area quite a bit more.

The fact you see electrolytics die early in SMPS power supplies only mean they are pushed over the limits there.

Biduleohm:
To detail a bit more: initially it's a personal project but I share it because I think others might be interested. It's not a project for beginners so anyone making/using it should be competent enough to do that. The fuse will not be integrated to this project, it's not its responsability nor mine, it's a "if you don't want to fuse your battery it's your problem" even if, of course, I'll put a clear warning and recommendation about fuse specs, etc...

I don't think I'll use electrolytic caps as my design must work reliably from -40 °C to +60 °C continuous for 15-20 years minimum, they are not space efficient, they are costly, they would add problems to solve (i.e. big current spike when the mosfets closes while the caps are charged). Also I'm curious about what electrolytic cap would handle a peak of hundred of amps?

Most likely I'll use some TVS or some active circuit based around zeners and transitors (didn't look about MOVs either, but I will as I'm curious if they can be helpful here). I may add a RC snubber network to help spread the energy over a longer duration and to help with EMI too, but I need to learn more about RC snubbers before.

mzzj:

--- Quote from: Biduleohm on March 26, 2020, 07:55:20 pm ---
I don't think I'll use electrolytic caps as my design must work reliably from -40 °C to +60 °C continuous for 15-20 years minimum, they are not space efficient, they are costly, they would add problems to solve (i.e. big current spike when the mosfets closes while the caps are charged). Also I'm curious about what electrolytic cap would handle a peak of hundred of amps?

Most likely I'll use some TVS or some active circuit based around zeners and transitors (didn't look about MOVs either, but I will as I'm curious if they can be helpful here). I may add a RC snubber network to help spread the energy over a longer duration and to help with EMI too, but I need to learn more about RC snubbers before.

--- End quote ---
You have already written it in stone that you don't want flywheel diodes and electrolytic caps but yet have no idea of even basics like RC snubbers and current rise over inductance?  ???

Cap bank to handle 550A current and some odd half joule energy is not much. Deciding factor is most likely ESR.  80v mosfets for 64v system don't leave too much margin but lets allow 3 volts for ESR @550A, that would require <5.5mOhm ESR.  10x 1000uF 100v electrolytics should cover that.

Or just keep your fingers crossed and let the mosfets avalanche.  :-/O :scared:
One is rated for 300A 10us avalance, 10x in parallel can probably hand 5x that ie 1500A for 10us.

jbb:
On electrolytics: it tends to be the heat which ages them.  But heat will wreck your battery pack much faster, so you should be able to get durable capacitors.

If there’s a chance of a vibrating environment (especially a vehicle!) you’ll need to pay careful attention to capacitor mounting. Otherwise they’ll vibrate, fatigue and fall off. When you provide this support, make sure you don’t block the emergency relief vents (typically a scribed pattern on the end of the capacitor tin) so you don’t make a little bomb.

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