Hi!
I'm a master degree student and we are trying to make a voltage regulator in this one course with specific components. We have designed the circuit for it and simulated it. It works fine in the simulation but when we made the PCB and tried to measure it, the current limitation doesn't work.. The current limiter works from approx 400mA to 1A and it should be 0A to 1A. Voltage and current are controlled by two potentiometers and the voltage information is sent to operational amplifier and op amp then controls transistors, which set the output voltage.
We found out that the current limiter's Q3 (you can see it in the uploaded file of our simulation picture) isn't working in the linear area. Q3 works as a current sink that limits the operation of voltage regulator, which is basically op amp 1, Q1 and Q2. So we need to bias Q3 again. The problem is the biasing though. It is kind of hard because the Vcc isn't constant in this application. Also we can't quite simulate this because simulations look OK as it is. If I make modifications then they look OK as well. So the testing is by error and it is really slow.. Plus we don't really know what we are even doing '
Does anyone have any good hints how to approach this problem? Here are some measurement results from simulation and from measurement:
From Q3, simulation
Vb = 0,736mV, Ib = 148uA, Vc = 4,7V, Ic = 40mA.
measurement
Vb = 850mV (so not in linear section), Ic = 60mV. We didn't/couldn't measure current because it's a PCB..
I can give more information if needed. Thanks in advance
BR,
Manu
I'm a master degree student and we are trying to make a voltage regulator in this one course with specific components. We have designed the circuit for it and simulated it. It works fine in the simulation but when we made the PCB and tried to measure it, the current limitation doesn't work.. The current limiter works from approx 400mA to 1A and it should be 0A to 1A. Voltage and current are controlled by two potentiometers and the voltage information is sent to operational amplifier and op amp then controls transistors, which set the output voltage.
We found out that the current limiter's Q3 (you can see it in the uploaded file of our simulation picture) isn't working in the linear area. Q3 works as a current sink that limits the operation of voltage regulator, which is basically op amp 1, Q1 and Q2. So we need to bias Q3 again. The problem is the biasing though. It is kind of hard because the Vcc isn't constant in this application. Also we can't quite simulate this because simulations look OK as it is. If I make modifications then they look OK as well. So the testing is by error and it is really slow.. Plus we don't really know what we are even doing '
Does anyone have any good hints how to approach this problem? Here are some measurement results from simulation and from measurement:
From Q3, simulation
Vb = 0,736mV, Ib = 148uA, Vc = 4,7V, Ic = 40mA.
measurement
Vb = 850mV (so not in linear section), Ic = 60mV. We didn't/couldn't measure current because it's a PCB..
I can give more information if needed. Thanks in advance
BR,
Manu
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