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Transistors not delivering full power issue

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mitrynicolae:
Hello everyone,

I have an issue regarding the output stage of a power supply. Regardless the output transistor used or the configuration of the regulator the transistors refuse to let the full power through them. I have attached a schematic which include both of the connections variants(using BTJ or N channel mosfet). Please note that the transistors used are MJ11028G or IXTK90N25L2 (which are more than adequate for this job) but the design application does not have them in the library. I have abstracted the regulator since the issue is not in that part (or at least that's what I suppose).
Few things regarding the setup:
- the regulator has a loop option in which it tries match the input voltage with the set one (yes using op amps). No mater how high I set the input the output won't pass certain limits described in the schematic.
- the regulator has also a buffer option (open loop) in which the input is ignored and a proportional voltage to the set one is delivered to the output transistors. Again setting the full throttle won't do the job.
- you will now say "yeah, but its normal, your external power supply cannot deliver the required amps and the voltage drops". This is not the case. The external power supply is a Korad 3005p which can deliver 30V at 5A which is more than enough for the job. Using the BTJ (when the measured voltage is 12.1V) the external PSU report a 1.21 amps used. Also changing the 10 ohms resistor with a car headlight bulb rises the current drained to over 4.5amps but the voltage is still low which again indicates that the output transistors are not the bottleneck.
- there is no measurement issue. I have measured the voltage with both an external multimeter and also using the internal circuitry of the regulator. Both reported the same values (+ - some milivolts which can be ignored)

1. At 20V I am expecting 2 amps consumption but still the voltage should not drop. Why it drops?
2. Why changing output transistors should have any effect on the output voltage?
3. Is there any issue in how I am measuring the voltage?

P.S. Again I believe there is no issue with the regulator since it can deliver on the output @15V at 100mA which in my view should be more than enough to drive the transistors (at least the mosfet).

Zero999:
It appears the regulator is going open loop. What's the maximum voltage it can output?

You have an emitter (BJT) source (MOSFET) follower. The voltage on R1/R2 will always be lower, than the base/gate voltage. The transistor starts to turn on, when a certain threshold between the base and emitter/gate and source is exceeded. The voltage on R1/R2 rises, thus reducing the potential difference between the base-emitter/gate-source and turning the transistor off. The circuit settles with the voltage on the emitter/source a bit lower than the voltage on the base/gate.

BJTs have a turn-on threshold of about 0.7V (double that to 1.4V for a Darlington pair such as the MJ11028G) and MOSFETs a couple of volts, hence why the voltage across R2 is lower, than that across R1.

mitrynicolae:
Nice catch @Zerro999. It can be. I will test it in open loop configuration with common emitter (load connected to collector) and I will let you the result.

Zero999:
Have you measured the gate/base voltages?

I estimate they're about 13.5V.

mitrynicolae:
I have good news and bad news.
The good news is that you spotted the bug correctly. Moving the load to the collector fixed the bug and now I the amps are at the correct value. The bad news is that now I cannot measure the input voltage because the circuit will enter in short circuit. Please check the attached schematic. Maybe you have a proposal. Differential amplifier is not an option since the circuit is already on PCB.

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