BJT vs FET in dummy load

[jay]

All the dummy loads discussed on the forum seem to use a power FET (Or maybe I missed something while searching?). What are the reasons to not use power BJT? Base current? Min voltage?

[Simon]

Base current could become a problem for high power loads as I believe larger BJT's have lower HFE although darlingtons could be used.

Other than that no idea

[Seekonk]

There is an abundance of FETs out there.  I scavenge FETs from old UPS inverters.  Can't remember when I have found  something as once common in the 2N3055 range.  They are easy to drive without having to go the extra steps as multiple darlingtons.   You are still limited to the same power per package size. So I would say it is availability.

[c4757p]

V_CE(sat) means the load won't work at low voltages with a BJT. The required base current will cause an error term in the current sensing if sourced from the load's internal power supply, and if sourced from the DUT, this will increase V_CE(sat) to that of a Darlington or worse, making the load even less useful. MOSFETs don't require gate current at DC, their only equivalent to V_CE(sat) is (I_D)(R_DS(on)), and R_DS(on) can be very low with a modern device.

[David Hess]

For a given current and die size at higher voltages (although not that high), the voltage drop from MOSFET Rds(on) will be greater than Vce(sat) of a transistor which is why IGBTs or composite transistors are used instead of MOSFETs in some applications.

As far as dummy loads, the gate current of the MOSFET contributes insignificant error so current sensing can be single ended which is simpler.