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Oscillations! Linear variable PSU

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Datguy123:

--- Quote from: xavier60 on July 29, 2020, 01:50:19 am ---
--- Quote from: Datguy123 on July 29, 2020, 01:12:56 am ---
--- Quote from: xavier60 on July 27, 2020, 03:22:57 am ---Use this version of the design that I have been using as a starting point for the drive and compensation.
https://www.eevblog.com/forum/beginners/lm324-power-supply-with-variable-voltage-and-current/msg3089254/#msg3089254
The BD139 can be used for Q1 in case more drive current is needed.

I have settled on the D45H  for the driver because of its consistently higher hFE.
If you already have the TIP42's, measure the hFE to be sure it's not too low.
The actual output voltage needs to be used to derive  the reference for the 1st stage so that both stages will share properly while current regulating.
I think it can be done with just 3 resistors. 2 of the same value to sum the unreg input and the output of the 2nd stage. The 3rd goes from the summing node to ground.

The current regulation can be done the same as mine applied just to the 2nd stage or maybe to both in a way only fast limiting is applied to the 1st stage.

--- End quote ---

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.

Also, I need to use high side current sensing as this would be part of a dual rail PSU, so would using a differential amplifier to sense the voltage from a shunt resistor on the + rail and comparing it to a voltage signal from an DAC be effective?

--- End quote ---
Ill have to get back to some of that later.
For the best chance of success with high side sensing if you want it to be reasonably accurate and settable, do it like this,
 https://www.eevblog.com/forum/projects/improving-adjustable-dual-voltage-bench-power-supply/msg3020386/#msg3020386
I'm not sure yet what that sensing circuit would drive into, likely an opamp/comparitor circuit.

 EDIT; Actually the LT6105 works the same way.

--- End quote ---

I have incorporated the high side level shifting current limiting circuit, as well as the resistive summer and divider in the attached circuit. Please advice me if the general design has any flaws. I guess I'll use the op amp circuit instead of the LT6105, unless I want the extra level of precision in another revision of my power supply. Right now I want to use what I have currently. I will be using a dac (most likely MCP4725) for I_set, with lowest step interval of 50mA.

Also, the op amps I will be using will be Lm358 instead of op747. I hope to hear from you soon on the use of the extra 8V supply opposed to not having it at all for the VAS transistor.

xavier60:
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.
If the CC loop is needed to be stable, U4 might need a compensating capacitor from output to inverting input. But this will slow the response to sudden oveloads.
Fast limiting could be added. An NPN transistor could sense the voltage across R12. It would then drive a PNP transistor that would bypass the B-E drive for Q6.

For R1,2,4,5, start with 1K.

David Hess:
I like your level shifters.  Keep in mind that the differential pairs could also be made with NPN/PNP pairs.  What is really going on is an emitter follower to increase the current driving a common-base stage to increase the voltage.

Track down the high current wiring and the return paths for any high currents.  These are the connections which must be wide and short to minimize the effects of inductance.  Of course this includes the collector and emitter wiring of the output transistors but it also includes any capacitors connected to that wiring.

xavier60:
     The high side current sensing needs to always have some voltage present on the output to work properly.
Put it before the 2nd stage.
Fast limiting will be easier to implement with a PNP monitoring the voltage across the CS resistor and its Collector going to the Base of  Q6.

David Hess:

--- 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.
--- End quote ---

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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