Electronics > Projects, Designs, and Technical Stuff
Oscillations! Linear variable PSU
xavier60:
--- Quote from: Datguy123 on May 17, 2020, 11:34:06 am ---
--- Quote from: xavier60 on May 17, 2020, 07:01:30 am ---The Darlingtons should be driven in the same way that works well in audio amplifiers. A Voltage Amplifier stage consisting of a capacitively loaded trans-conductance amplifier. The opamp will need to have some proportional gain for loop stability.
https://electronics.stackexchange.com/questions/254375/purpose-of-voltage-amplifier-vas-in-three-stage-amplifier-design
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Hi, so are you saying the Darlington should be configured to have both voltage and current gain? I have no formal education in EEE so I am not sure how much gain the op-amp should have. My plan was to use the diff pair as the VAS and use the temperature stability of the op-amp to correct the errors in gain using NFB. The darlington was used to provide large current gain at high output voltages when the current in the left side (Q9 and Q17) is low.
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Just like with an audio amplifier, the Darlington's current gain buffers the VAS.
Having a single inverting VAS transistor can be corrected by swapping the opamp's inputs so long as its local feedback still goes to its inverting input. A resistor will be needed in series with the feedback capacitor.
If you drive the VAS transistor's Base directly with the opamp, Vas compensation will need to be done with a capacitor from Collector to ground.
Look at some typical AB audio amplifier circuits for ideas.
Datguy123:
--- Quote from: ogden on May 17, 2020, 09:10:05 am ---
--- Quote from: Datguy123 on May 17, 2020, 05:41:36 am ---do you think there's anything apparent in my circuit that could attribute to the oscillation?
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Sorry, but your circuit is rats-nest as well :) Main beauty of best hobby engineering projects is simplicity. Why two stages in series? My advice - divide your project in logical steps, work on single suply until it is running, only then add/connect 2nd.
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Hi, I thought of using two stages because of the SOA and power derating of the output transistors. If I series them, I would get more power from the output transistors at lower output voltages. I would try separating the two series parts, but the circuit works at with lighter loads so I'm not sure if I would need to take such drastic measure.
gbaddeley:
1 stage is enough. Parallel up as many power transistors as you need (run them at <50% Pmax) use a big enough heatsink (Tcase <80deg.C).
xavier60:
For the TIP35, max dissipation drops from about 120W at 30V to about 40W at 60V. That's a drop from 4A to 0.6A.
ogden:
--- Quote from: Datguy123 on May 17, 2020, 12:10:40 pm ---Hi, I thought of using two stages because of the SOA and power derating of the output transistors.
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Don't. Why don't you look how it is done in other "DIY supplies"? This forum alone is full of powerful enough linear supply circuits that uses parallel output transistors instead of weird two supplies in series arrangement.
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