I think when this circuit in current limiting mode, the power dissipation of the mosfet is, P = Vds * Ids (that's the main part you will need to do the calculation, bases on your load current and supply voltage, and the voltage drop of Rsense, if you are not switching)Many thanks, seems almost too simple. As I understand it, the MOSFET dissipates heat through Rds(on) and switching losses. With this circuit configuration, will any MOSFET be able to dissipate any amount of power (assuming it is voltage tolerant and doesn't melt)? Or is there a ceiling (set perhaps by the maximum switching frequency/loss)?
I'd like to add to this if and only if they are in freely convecting air of 25 degrees (C). If you put them in a sealed housing, like a small plastic box, they will heat up the air in there and you risk thermal runaway, especially if Rds is only a small portion of the total resistance of the circuit.To make this simple, buy a TO-220 or TO-247 package mosfet. They are about the size of a penny, only fatter and they will survive a watt, all day, without any help from a heat sink.
I think you understand thermal runaway. It means that, as a component dissipates power and gets hot, the temperature coefficient of it or of some other component in the circuit causes it to dissipate even more power and get even hotter. This is a vicious circle, and can result in the destruction of the component.Thanks all.
So does the circuit reach equilibrium by having the NPN transistor partially on, pulling down Vgs (to just above Vgs(th)) to increase Rds(on) until its resistance is just right? So the MOSFET basically acts like series resistance?
In terms of thermal runaway in a small case, as I understand it, Rds(on) increases with temperature, so the circuit would increase Vgs to compensate (keeps the resistance the same). So does thermal runaway simply mean overheat and melt/fail (as oppose to LED thermal runaway where the current rises with temperature)? Will there be any current change as temperature increases?
by Jake Hertz
by Jake Hertz
by Jake Hertz
by Aaron Carman