Has anyone ever seen a design for an “outboard” Lipo protection circuit? There are many shaped to be shrinkwrapped in with a bunch of 18650 cells, but I want something with a Deans/XT60/PowerPole connector I can plug a 2s or 3s RC car battery into on one end, and get the pack’s native voltage out the other but with over-current and under-voltage protection.
I want something that allows a very high current (some of these batteries can push well over 100A) and I’m not so worried about over-voltage protection when charging, the dedicated charging hardware handles that.
I know there are many chips which do this, but I have been unsuccessful finding a breakout board design for them on Sparkfun or OshPark or similar. Sounds like something that should exist, yes?
(Dumb question just in case it doesn’t: if you are using a MOSFET as a switch, does it get hot when the switch is on, or when the switch is off? Bonus points for a practical MOSFET explainer which doesn’t go down to the semiconductor physics.)
I’m gonna jump on the not-such-a-dumb question.
If you think of a MOSFET as a switch, when it’s closed (turned on), it conducts and carries current. But there is a little bit of resistance in the drain-source path (the switch) and the higher the current, the more power is lost as heat across this resistance. It’s referred to in spec sheets as “Rdson” – resistance, drain-source on. The power loss can be approximated as P=I^2Rdson. When the switch is open (off) then no current flows and no power is lost. The physics are more complicated but that’s it in a nutshell.
I don’t know enough about battery circuitry to take a stab at the real question so I’ll just sit back and wait for the smart people to chime in.
MOSFETs also have a considerable resistance for the brief moment they’re turning on or off, depending on how hard you’re driving the gate (if it has a resistor attached, etc) so keep that in mind.
Thanks @Jeff_Mizener , that’s what I needed to know, brilliant. @1icri_Old_Account , I think that won’t be a consideration for my application as I don’t want to switch it on and off at any rate—the aim is to turn off a strip completely when it’s not showing a pattern so as to not drain the battery from the “dark draw”.
So poking around here’s a part with a 4.2 mΩ Rds(on) @10V which can pass 120A and will take a logic-level gate voltage: http://www.onsemi.com/pub_link/Collateral/NVMFS5832NL-D.PDF . Maxing it out at 120A means it’ll be throwing off 120^2 * 0.0042 = ~60W. That’s… a lot, but it’s feasible for what I want to do, and it costs less than a buck. Maybe there’s a more appropriate part out there but it passes the sniff test.
@Robert_Atkins Ah, in that case yes the dissipated power during switching time is negligible.
@Robert_Atkins If you are planing to pass 120A through that little chip, you had better heat sink the hell out of it or it’s gonna go up in a noisy puff of smoke. I have no experience with SMD power FETs, only with power devices in the older leaded packages (such as TO-220) that I can screw securely to a metal heat sink.
Yeah, I’d want something with a similar spec in much beefier packaging obv.