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Oscillations! Linear variable PSU

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xavier60:
I think that the voltage set point for the 1st stage can be derived by halving the sum of a proportion of the input voltage and of the output voltage. This can be done closely enough with a resistor adder.
For example if the input is 60V and the output of the 2nd stage is set to 10V, the sum is 70V. The output of the 1st stage gets set to 35V.
There will be 25V drop across each stage.
Fast current limit can be applied to both stages, slow CC can also be applied to the 2nd stage.

Datguy123:

--- Quote from: xavier60 on May 26, 2020, 01:14:54 am ---I think that the voltage set point for the 1st stage can be derived by halving the sum of a proportion of the input voltage and of the output voltage. This can be done closely enough with a resistor adder.
For example if the input is 60V and the output of the 2nd stage is set to 10V, the sum is 70V. The output of the 1st stage gets set to 35V.
There will be 25V drop across each stage.
Fast current limit can be applied to both stages, slow CC can also be applied to the 2nd stage.


--- End quote ---

Hi, sorry for the period of inactivity.

I have drafted out a circuit over the free time I have, and I hope i have computed the values for the resistors correctly. However, I am unsure where to place capacitors/resistors to ensure the circuit has phase/gain margin. Please advice me on that :) And also, should I factor in current limiting into the design before doing any loop stabilizing?

Thanks.

xavier60:
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.

Datguy123:

--- 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?

xavier60:

--- 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.

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