bjt
I'm using a BJT as a switch, so it will either be off or saturated.
I'd like to know how the collector-emitter voltage (when saturated) varies with temperature in general, (but perhaps take the BC179 for example – the datasheet only shows how it varies with collector current.)
Thanks
![Repicating your circuit in LT spice]
1In all the cases you cannot say that the base voltage is irrelevant for the transistor's operation or as you said transistor;s saturation operation.
If I consider your case the the transistor is mainly dependent on the base voltage and also if you have not taken care of the transistor for its ability to sustain the huge base current it might even blow off.
Now consider when the switch is suddenly opened the voltage induced across the inductor is huge and it is seen by the 10k resistor. This gives rise to a huge base current which might even blow the transitor. So you need to check the maximum Vbe which can be given to the base of the transistor before switching on the circuit.
Solution: If you can place a zener of some voltage lesser than the supply voltage then I think your circuit will work.
One input: Placing inductor at the base of the transistor is always not a good thing to do. Better try to place it in the collector with a free wheeling diode across it.
If you think your transistor can sustain a huge base voltage of 100s of volts and base current of 100s of mA then you can go with the circuit you have else No.
I suspect that power dissipation would be the limiting factor.
Normally you'd only be concerned with (Vce x Ic) since (Vbe x Ib) would be insignificant in comparison.
But if you're driving the hell out of Ib it'll become far more of a significant term.
Best Answer
In general, the saturation voltage of a BJT goes down a little with increasing temperature.
This is the main reason that you can't parallel C-E of multiple BJTs and expect them to share current well. There are always some variations from device to device. One of the parts will take a little more current than the others, which makes it hotter, which decreases it's saturation voltage, which makes it take a larger fraction of the current, which makes it hotter with respect to the other devices, etc.
MOSFETs work the other way around in that the channel resistance at full-on goes up with increasing temperature. You therefore can parallel MOSFETs and have them share current reasonably.
To use multiple BJTs in parallel to get higher current capability, you give each one its own emitter resistor and tie the bases together. Any device with a higher C-E current will have a higher drop accross its emitter resistor, which decreases its B-E voltage, thereby throttling it back.