On 18-Dec-11 03:53, Forrest Christian wrote: > These are typically very large, very deep battery banks. 50AH is > wimpy. In my other life, I have a location which has an array running > at 24VDC, 480AH. There is a *lot* of current there, admittedly. > > Fortunately there are also a lot of connectors, small-guage wires, > fuses, and the like typically between my gear and the array, all which > serve to limit the instantaneous current. > > In this application FQB22P10's are pretty much jellybean parts. -88A > pulsed drain current. Eas of 710Mj, Ear of 12.5Mj. I would add an inductor and a flyback diode, a few uH of inductance=20 should get you enough time to turn the MOSFET off. Your previous idea of a Schottky parallel to the sense resistor should=20 also work, at <100mV the Shottky will pass little current compared to=20 the shunt, even at high temperatures. However, the next weakest=20 component may fail - it's better to prevent huge short-circuit currents=20 from happening. Djula > > Admittedly it's all a race of 'what will melt first', or maybe better > 'will we get it all shut down before something exceeds a rating'. > Fortunately, this is also a very rare occurance, and so statistcally > we're just trying to make the product more bulletproof in those cases - > I.E. adding a lot of cost isn't in the cards in this case. > > I'm actually eyeing the 22P10 I'm using on the front end of this for > reverse polarity protection and thinking that with a few truly jellybean > parts it could become not only reverse polarity protection, but also a > rough current limiter to a few amps. If I can get the impulse energy > available to the downstream shunts and FETS down a ways, then things > become much less critical, and the 22P10 only needs to dissipate at > worse case that initial (admittedly huge) pulse while it adjusts, and > then a bit of additional power at the lower amperage for however long it > takes the circuitry to decide there's a true overcurrent/short going on > here and shut things down. But as is mentioned below the challenge > might be really in the realm of not worth the effort. > > -forrest > > On 12/17/2011 10:05 AM, Sean Breheny wrote: >> How accurately do you need to sense current? >> >> I am concerned about the ability of your MOSFET switch to stop the >> short-circuit current from a large battery bank. Remember, during the >> time that the FET is switching off, there is a huge power dissipation >> because the voltage is rising at the same time that the current is >> falling. I don't know what kind of battery bank you are talking about, >> but let's say it's a 24V 50 Amp-Hour lead-acid battery. The short >> circuit current could be 5000 Amps. At the worst point, there will be >> about half voltage (12V) across the FET and about half current (2500 >> Amps) through it. That's 30kW. Let's also say that the FET will switch >> off in 500 nanoseconds and that the overall average power dissipation >> is half of the worst case point. So, 15kW for 500 nanoseconds - that's >> 7.5 milliJoules. Might not seem like much but given that the actual >> die is quite tiny, and the bond wires are even smaller, it might well >> be enough to destroy them. >> >> This is ignoring the additional heat which will come from just I^2 R >> heating during the time when the current is ramping up before your >> system trips and decides to turn the FET off. It is also ignoring the >> inductive energy in the leads which will show up as a huge voltage >> pulse across the FET, increasing the power dissipation and possibly >> exceeding Vds max. Finally, it is also ignoring the difficulty in >> keeping a FET turned on fully during the very high current conduction >> - the voltage drop across the FET (drain-source) will likely exceed >> Vgs max which will prevent you from applying enough gate drive to keep >> the FET in the ohmic region. >> >> What I am saying is that a FET swtich that is designed to catch and >> stop the short circuit current from a large battery is NOT a small or >> inexpensive thing :) >> >> Sean >> >> >> On Sat, Dec 17, 2011 at 6:40 AM, Forrest Christian = wrote: >>> On 12/17/2011 3:09 AM, Mike Harrison wrote: >>>> Schottkys get very leaky at higher temperatures, so probably not a >>>> good choice. Is the current really so high that you can't use a more >>>> chunky sensing resistor? If you really want to bypass the shunt, a >>>> MOSFET is probably a better bet - arranged such that it turns on >>>> quickly when the measured current exceeds a threshold, witha >>>> monostable to hold it on for a while, limiting the current duty cycle >>>> through the resistor to the monostable time / the turn-on time. >>> Just to be clear what we're typically talking about here... >>> >>> The normal load is around 250mA. Inrush currents can be extremely high= , >>> in the range of amps, but not close to high enough to exceed the >>> resistor rating. >>> >>> The issue comes where our customer is using a large battery bank with >>> virtually unlimited current available, and decides to dead short the >>> output. I think we've actually got the firmware in the PIC doing the >>> overcurrent protection tweaked so failures just aren't likely - It's a >>> fine line between being slow enough that the inrush doesn't trip the >>> protection, and a short or long-term overcurrent will. With some >>> creative integration (in the mathematical sense), things seem to be >>> working well - but I still hate not protecting something I know is clos= e >>> to the edge. >>> >>> At this point, I think I might look at using a mosfet on the input of >>> the whole device to limit current to something reasonable - dozens of >>> amps for instance - probably a more reasonable solution. >>> >>> And yes, we're already using about as chunky of a sense resistor as is >>> practicable. >>> >>> -forrest >>> -- >>> http://www.piclist.com PIC/SX FAQ& list archive >>> View/change your membership options at >>> http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .