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| BMS short circuit interruption capability |
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| Biduleohm:
--- Quote from: mzzj on March 26, 2020, 08:55:45 pm ---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. --- End quote --- It's not that I don't want flywheel diodes, I just don't see how you can make that work with a bidirectional switch; if you have some schematic of how you make it work I'm all for it (it would be an almost perfect solution actually...). For the electrolytics caps I gave my reasons, if you think there's some that can fit the constraints, again, I'm open for suggestions. I don't like maths too much and it's been a long time I used the inductance formulas so I forgot a good part of what I knew. For snubbers I know how they work, etc. I just need to refresh about how to calculate the values, that's all. You know, we can't be good in every part of every domain. I really try to not bother others with questions if I can learn by myself (on the whole project, which is 5 different boards BTW, I only asked for help 3 times and it was only after hours of research) but here it was a bit too much and it was on something I can't screw-up so I wanted to be sure. The only 1000 µF 100 V cap I can find whith a low enough ESR (< 55 mOhms) is this one: https://www.mouser.fr/ProductDetail/Nichicon/UHW2A102MHD?qs=KdTYp8tcPgnQ6r7UMjz7nA%3D%3D for ten of them that would be 32 € (without tax which are 20 % here) which is more than 1/3 of the budget of the whole board, that's too expensive. Also they are 18*40 mm so 10 of them would need a lot of PCB real estate. Yeah I'm not that savage ;D even if they should handle it actually (Eas = 0.85 J and there's 10 of them, so well over 0.36 J), I prefer to keep them out of avalanche mode. It's more like a last resort if the external protection fail to protect them. --- Quote from: jbb on March 26, 2020, 09:31:26 pm ---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. --- End quote --- I didn't think about vibrations, but yeah, that's another constraint. So now the worst case isn't when the inductance is maximum, but minimum. With 10 cm of wire that's 73 nH, so the current would be almost 4.7 kA for an energy of 0.81 J. Ok, that's still reasonable :phew: |
| Siwastaja:
The idea with the electrolytic caps is that they do have ESR. Use them for damping the ceramic pack you use for low ESR. If you use 1*C of ceramic plus 2*C of ceramic with series R, the cost is likely 10x more than 1*C of ceramic and 5*C of electrolytic. If the low-ESR ceramic take most of the ripple current in the system, the electrolytics last forever (unless the environment is exceptionally hot, like over 100 deg C). In any case, what is the device doing? Or is it just a stand-alone efuse module? Turns out in most complete applications (thinking about motor controller, most likely) the DC link capacitance is there already, and it's often a combination of low-ESR ceramic and high-ESR electrolytic, for damping. But a beefy TVS (or a few in parallel) will likely do just fine catching the voltage spike, we got sidetracked into the capacitors. Yes, precharging causes a current spike, but you can't just ignore the problem; if this is a module, it will likely supply a capacitor bank, and it's a PITA if the user needs to create another precharge circuit. Precharging should definitely be a feature in your thing, IMO. Otherwise it's just nuisance tripping on capacitive load, preventing operation. You may indeed need to add a certain amount of inductance so you have a known minimum to work with so you can guarantee a current limit value (depending on your reaction speed). Note, from the MOSFET point of view only, the faster you switch, the higher the voltage spike would be (less snubbing action in MOSFET itself) - but OTOH, the slower you switch, the higher the current has time to rise to, hence you store more energy, and create a higher spike after all. So switch quickly. Unless you have some strange requirement such as very low Iq comparator, you can easily detect the overcurrent in, say, 500ns, then switch the MOSFETs in 1us. I actually designed such efuse circuit two years ago, and the extra challange there was the requirement for extremely low quiescent current - it doubled as a power switch - in order not to overdischarge the battery pack, even in the most demanding case - pack is ran flat, then the product is put in storage without recharging. It was a nightmarish or extremely fun design challenge (whichever way you look at it) involving precision resistor networks and extremely low Iq comparators - which are not that fast - but in the end it worked out. I did some heavy Excel magic with combining all the component tolerances. |
| Biduleohm:
Ok, I'll see for the ceramic/electrolytic mix, it makes sense. No, there's no external cap bank, it's for residential and mobile solar installations. There's actually a precharge feature (look just below the main disconnect on the schematic) but that's to precharge external capacitive loads like the large caps in the inverters for example. Here we're talking about caps placed across the mosfets which would be naturally charged if there's any load connected while the mosfets are open, and when the mosfets close they would short-circuit the caps. Edit: wait a second... now I see, the precharge would also discharge the caps between P- and B-, that's nice, one less problem :) Yep, but instead of adding inductance (not practical) I can specify a minimum wire length under which the mosfets aren't guaranted to be protected. 10 cm is a very defavorable case and not accounting for internal battery inductance (inductance of the cells and of the busbars between the cells), I just wanted to see what to expect in the very worst case possible and I now know that the energy stay pretty constant and under a Joule, but the current rises quickly when the inductance is reduced. Yes, it would not make sense here to switch slower as the gain in dI/dt will be far outweighed by the energy I need to dissipate. So yep, I'll keep the switch quickly design. The mosfets should switch under 300 ns, the mosfet driver add 160 ns of propagation delay, the comparator (TSX3704IDT) to detect the over-current condition adds 2 µs and the amplifier (OPA4990IDR configured as a diff amp with a gain of 41) for the current shunt adds 2 µs too. That's worst-case for all the values, typically it should be quicker than that. And yes, Iq is a concern as the BMS is connected to the battery 24/7, I tried my best to keep everything as low power consumption as possible. Which, as you experienced too, is a bit of a PITA when selecting components (you know the "I found a very nice component for this" followed by "ah, it would need 10 mA, hmm yeah, I'll find something else..." ::)), add to this a lot of BoM re-use to keep the costs as low as possible and you have even more compromises to make... |
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