I am having some difficulties getting my output stage to work correctly. Here is some info:
I have a +48VDC source with a series pass transistor to drive a load between +45V and 0V. The control system uses op amps and can have an output voltage of anything I wish really. (+5/-5, +5/GND, +12/-12, +12/GND). Its going to depend on my output stage.
Devices I tried:
D44VH10 (Vce=80V, Vbe=7V, Ic=15A)
TIP142 (Vce=100V, Vbe=5V, Ic=10A)
2N6039 (Vce=80V, Vbe=5V, Ic=4A)
What I am having problems with is actually getting my output stage to go from +45V to 0V.
I have tried a few combinations of NPN pass transistors controlled by a PNP in a darlington type setup and this meets all my requirements. However it requires my op amps to run at +48V which they will happily do while on fire. If I lower my op amp's output swing to +12/GND then the load only gets around 26V maximum.
I then tried a PNP pass transistor with an NPN control in a similar darlington setup all controlled by an NPN to ground. This worked and allowed my to use +5/GND as my output swing but even if I applied 48V and 1A to my control NPN my load never received more than 22V.
I then tried the same as above but with a MOSFET to ground and I got a 27V at my load. I modified again an used the MOSFET to drive a photo transistor which in turn drove an NPN series pass transistor from the 48V source. I managed to get my 45V out to the load this way but the temperature variations and unreliability of the photo transistor package does not meet my requirements and worst of all OFF was a state passing around 7mV and 2mA.
So, I am missing something here. What is the correct way to drive an output stage from a lower voltage control system?
The whole reason I am doing this is because the only op amp that can 40V+ available to me cost $13/piece. If I can get my control circuity to a lesser value I can get them for $1.