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TL431 linear power supply

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spec:

--- Quote from: Kleinstein on December 18, 2018, 07:56:14 pm ---With the extra charge pump stage as shown the voltage might get too high for an LM317.
So if at all one should use the charge pump to only give something like 2 times the normal voltage (diode to ground and not the normal positive supply).

I don't see a real need for the charge pump. Already just extra diodes and Filter capacitor would give some extra headroom for the current source, as with an extra capacitor one can have less ripple than the main supply. For the low current needed by the OP, the zener might be sufficient - the LM317 would give some extra loss in voltage, but could still work.

--- End quote ---
:-//

not1xor1:

--- Quote from: mike_mike on December 17, 2018, 04:23:37 pm ---There should be a NPN transistor where it writes "vdd" (with the base on vdd) ?

--- End quote ---

the current regulated by the zener is just few mAmperes so there is no need to transform it in a series regulator (if you meant that)

not1xor1:

--- Quote from: spec on December 18, 2018, 02:19:56 am --- [...]

--- Quote from: not1xor1 on December 15, 2018, 09:28:39 am ---I'll see if I can simulate it.
--- End quote ---
Be great if you did do a bode plot of the circuit. :)

--- End quote ---

I simulated just AC. The bodeplot doesn't look so good. It needs some more work on compensation.

not1xor1:

--- Quote from: Kleinstein on December 18, 2018, 07:56:14 pm ---With the extra charge pump stage as shown the voltage might get too high for an LM317.
So if at all one should use the charge pump to only give something like 2 times the normal voltage (diode to ground and not the normal positive supply).

I don't see a real need for the charge pump. Already just extra diodes and Filter capacitor would give some extra headroom for the current source, as with an extra capacitor one can have less ripple than the main supply. For the low current needed by the OP, the zener might be sufficient - the LM317 would give some extra loss in voltage, but could still work.

--- End quote ---

The LM317 request concerned spec's schematic, not mine.
Since the purpose of that circuit of mine was to get an output of 25V from a LM358 without any voltage boost stage, the charge pump ensures a proper and stable supply voltage for the LM358 even when the transformer voltage gets too low.
Of course the circuit may work even without charge pump as showed in the other schematic I posted

spec:
UPDATE #2 of 2018_12_21 (R4 changed from 1k to 56R)
UPDATE #1 of 2018_12_19 (add resistor to pot wiper)

mike_mike

Schematic for 25V 5A PSU version 06, using an LM317 regulator, attached below. There are a few other minor changes here and there, including replacing all BC547/BC557 transistors with BC337/BC227 types. This will give more power dissipation margin for Q3 and Q8 and will make the constant current handover a touch sharper. But you can still use the original BC547/BC557 types if you like. The 1N400x diode in Q3 collector has been removed as it was unnecessary.

I have done a thermal budget for BC547/BC557 in Q3 and Q8 positions and the maximum junction temperature, with 300mW dissipation and an assumed ambient temperature of 70 degC, is 125 degC, which gives a 25 deg C margin which is safe, especially as it is unlikely that the ambient temperature will be as high as 70 degC. All the same, with either transistor family, it would be wise to attach small heatsinks to the cases of Q3 and Q8.

All resistors have been normalized and the need for an AOT resistor has been eliminated.

A 1N400x diode has also been added at the output.

All solid capacitors are X7R ceramic, through hole, and all polarized capacitors are aluminum electrolytic.

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