Electronics > Projects, Designs, and Technical Stuff
Oscillations! Linear variable PSU
dietert1:
Let's try to write a comment concerning oscillations:
You need to understand what is the meaning of unity gain stability. If you buy an OpAmp that is called unity gain stable (the OP747 is of that type), that means it has internal bandwidth limiting and enough speed that allows you to feed back the full output signal to the input - for maximum accuracy. You added additional gain stages with gain of 20 (680 Ohm/33 Ohm). That means you are feeding back 20x the OpAmp output signal back to its input. You cannot do that, otherwise you get oscillations.
I can see another error in your design. You added two capacitors to reduce gain at high frequencies: C10 and C21. You cannot do this. Already at moderate frequencies those 2 caps will cause a phase shift of near 180°, which means negative feedback becomes positive feedback => oscillations.
Unless you have a lot of practical experience with circuits of this type, you need to simulate your design. Open the control loop and try to understand the Bode diagram to decide whether the closed loop will be stable or not. Just soldering circuits you can spend years trying to find a good solution.
Regards, Dieter
Datguy123:
--- Quote from: xavier60 on August 01, 2020, 09:31:05 am ---The separate 8V isnt needed for the ORing node pullup. Just use the + opamp supply. If there is too much max drive voltage for the level shifter transistors, resistors could be put from the ORing nodes to ground.
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Thanks, I will take note when I build the prototype. But any advantages to the current limiting part of the circuit with a current sinking design of the VAS transistor(quote below)?
--- Quote from: Datguy123 on July 29, 2020, 01:12:56 am ---I saw another circuit online: http://www.kerrywong.com/blog/wp-content/uploads/2013/12/ps_pos.png which also deploys a sziklai pair output.
I would like to ask if the main advantage of incorporating the additional 8V supply and having op amps to sink current is to aid in the current limiting of the circuit. From my understanding, the CV op amp will try to pull the base voltage up when current limit is on and the constant current op amp doing the opposite. In the case of Kerry Wong's design, is it true to say that the base drive would be weak as both CC and CV op amps are working in different directions and would end up in an equilibrium (CV op amp at +max O/P v and CC op amp at -max O/P v) and the base does not get driven well? However in the circuit you proposed, the base drive would be stronger as the diode D1 prevents CV op amp from sourcing current to the base when CC op amp is sinking current.
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--- Quote from: xavier60 on August 01, 2020, 09:31:05 am ---For R1,2,4,5, start with 1K.
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I did some calculations. The max current the power supply is capable of is 6A, and having a conservative hfe of the sziklai pair (TIP42C: 30, TIP35C: 20), Q6 would need to draw 5mA of current from R6. The voltage drop across that resistor would be 5V if it was 1k ohm, which is too much. (50V input, 45V output max). Would you then advice a lower value for R1,2,4,5?
Datguy123:
--- Quote from: xavier60 on August 01, 2020, 08:34:54 pm --- The high side current sensing needs to always have some voltage present on the output to work properly.
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Why is that so?
xavier60:
There needs to be some positive voltage at the top of R13 for Q7 to source current to ground.
Datguy123:
--- Quote from: David Hess on August 02, 2020, 02:33:15 am ---
Power supply circuits of that complexity usually include a negative supply for the control circuits so the high side error amplifier can operate down to 0 volts, or actually a little lower. For bipolar dual output supplies, this is free.
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Hi, I'm actually building a dual rail power supply. So by having a negative rail for U3 (In fact, I'm powering all op amps with +15V and -12V rails), the current limiting circuit can work properly at 0V output?
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