BJTs with similar current carrying capacity will typically have a much larger voltage drop and hence power dissipation then an appropriately sized MOSFET when used is switching applications.
When use in the linear region, the two will have the same power dissipation, and here BJTs are actually preferred.
Bob
I was mostly noticing that, provided that enough power is provided to fully switch them, there didn't seem to be huge difference in how hot they get between MOSFETs and BJTs (for similar amounts of amperage).
however, if not enough base current is supplied to the BJT, it gets very hot fairly quickly, in addition to not supplying full power to the load. this was a bit of a problem with my older circuits, until I started realizing this and taking it more into account.
likewise, if too much base current is supplied, it also gets hot, and seems prone to get stuck in the "on" state (often temporarily, sometimes permanent), so there is a need to make sure it doesn't get fed excessive amounts of current (but still has plenty of current for switching).
provided this, and avoiding overly large voltage spikes, they seem to do ok.
the MOSFETs seem to be more easily damaged by smaller voltage spikes and by misbehaved loads, and are basically dead if the gate-to-source voltage goes out of range (say, +/- 20v), which can become a problem for some driver designs if the load operates at a higher voltage (say, 24 or 36 volts), or if the control rail is subjected to an elevated voltage (say, if the 12v control rail is for whatever reason briefly exposed to 24 or 36 volts).
apparently, the type of MOSFETs used in the commercially made motor-driver board had also included built-in gate-to-source TVS diodes, possibly for these reasons.
in either case though, if a big heat sink and a cooling fan is needed to deal with a load, it doesn't seem like a big difference.
when using BJTs for analog current limiting though (for a driver controlled via a potentiometer): yeah... built a driver which uses point-to-point wiring and the wiring is covered in silicone caulk and the thing runs submerged in water (in a PET plastic bottle), and when running a load, gets hot enough that the water starts boiling off the transistors...
the pot has a high enough resistance though that it doesn't even reach full power (couldn't think up a good solution, ideally would need a pot with a larger value range, say, 1k to 100k or something). had partly considered adding an "overdrive" switch (connects up a resistor, or another pot driving another transistor, vs just shorting the pot for "full power").
but, if water starts boiling at 15 amps (w/o shorting the pot), what does it look like at 30?... maybe more epic boiling?... (guess it is however much boiling you can expect from around 700W or similar...).
(FIX: probably wont be seeing 700W under normal conditions, and my thinking is that I would probably see a fair bit more vigorous boiling, vs a relatively small stream of rising bubbles...).
this was a limiter I had built mostly for higher-power testing, namely when running things off lead-acid batteries, where I still wanted some level of control, vs just hooking whatever directly up to the batteries, where I potentially have several hundred amps going through whatever, where if something goes wrong in the electronics stuff basically just straight-up explodes... (well, vs just more casually spewing smoke and fire... grr...).
I divide some of my testing into several stages:
low power: variable power supply, often at lowered voltage and maybe limiting amperage (below the 5A imposed by the variable supply);
medium/high power: lead-acid batteries, current limiting via the driver above (done only occasionally, and this is where stuff often blows up);
full power: lead-acid batteries used with no regulation, rarely done.
have considered possibly making a higher-power power supply using a big transformer, but would need some parts I don't currently have to make it worthwhile (vs the current setup with using lead-acid batteries).
