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TL431 linear power supply
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not1xor1:

--- Quote from: Kleinstein on December 22, 2018, 11:01:01 am ---The initial circuit right from the start is not that bad - it is mainly missing the frequency compensation. So with the first response one could call it problem solved. There are still some points to optimize and the selection of parts could be better, but nothing serious.

--- End quote ---

The initial circuit is not that bad ... if one just looks for trouble.  >:D
So far have I been unable to make it work under all the possible conditions of load and feedback resistor.
TL431 is handy for a fixed voltage regulator, but unless you severely compensate it (achieving awful transient recovery times) it is unsuitable for a bench psu.

A PSU with fixed feedback opamp and variable voltage reference is not more difficult to build, but is much more reliable and easier to set up.
And with a range of 2.5-25V, as required by the original poster, there is no need for a further voltage boost stage.
I already showed two simple circuits which are simple yet offer much better performances than the other proposal.
Of course one is free to do what he/she likes... :horse:

 ;D

not1xor1:

--- Quote from: imo on December 22, 2018, 11:39:20 pm ---Bode

--- End quote ---

that's fine for a power oscillator  ;D
iMo:
Stability test simulation made according to the AD app note

Vout 15V/1.5A with:

1. C4=100pF
2. C4=1nF
3. C4=1nF ser 56ohm
4. none comp C4 - here at 300kHz the phase margin is 0deg - it oscillates
not1xor1:

--- Quote from: imo on December 23, 2018, 10:33:55 am ---Stability test simulation made according to the AD app note

Vout 15V/1.5A with:

1. C4=100pF
2. C4=1nF
3. C4=1nF ser 56ohm
4. none comp C4 - here at 300kHz the phase margin is 0deg - it oscillates

--- End quote ---

you must not forget that TL431 is specified for a minimum current of 1mA (although in some cases might be fine with as little as 500µA).
R4 might work for the whole voltage range just for 1A of output current or so...
once you replace it with a constant current source the open loop gain soars and the nightmare begins  ;D
BTW it would be better to start from 1Hz or even below...
T3sl4co1l:
FWIW, TL431 works just fine at <1mA, it's just not going to be spot on 2.500V.  In particular, it drops to about 2.2V at light currents, around which it's pretty much off (~uA).

Knowing this behavior is helpful when working with a clamping or startup threshold sort of circuit: any time the threshold needs to be sharper than a comparable zener (a 2.5V zener is a damn good resistor in comparison), but also accurate at higher currents.  (For example, the fact that current rises from ~uA at ~2.2V, to 1mA+ at 2.50V, only means a clamping threshold is rounded off by a comparable amount.)

Tim
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