It seems that Darlington transistors have fallen out of favor for power supplies, since I have not found any high current (8 A output) ones in stock at Mouser. What have they been replaced with (Sziklai pair, power MOSFETs, or something else?) and how do you adapt a circuit with a Darlington to use it?

Specifically, I have a calibrated AD584 precision voltage supply, but its maximum output is 10 mA. I want to use it as the basis of a high-power precision voltage power supply. That is, I want to keep the voltage precise via the AD584, but be able to supply several amps at that voltage.

The AD584 manufacturer includes this schematic for using it as the basis of a high-current (4 A) precision voltage power supply in the datasheet:

High-current precision voltage power supply schematic

I would use this circuit, except, like I said, Mouser says the specified Darlington is obsolete and they don't have anything like it to replace it. Which makes me think there is a better circuit design with a different kind of amplifier stage.

My problem is that I don't fully understand how this circuit works, and given the very limited current capacity of the voltage source, I worry that I could easily make a mistake that ruins the whole thing. How does the AD584 output voltage feed back into the Darlington to regulate the current?

Your challenge, should you choose to accept it, is to take this circuit as inspiration and design an alternative precision 10 V, 4 A (or greater) power supply using only the AD584, a single packaged device that is not a Darlington, and possibly a single resistor. You can assume the load has minimal capacitance or inductance. Extra credit for explaining why this other type of device has replaced Darlingtons in this type of application, and/or how in general you replace Darlingtons with this other kind of device. Since someone asked, this only needs to work at "normal" temperatures (say 15ºC to 35ºC).

In case you are wondering, one reason I want to do this is to combine the power supply with a precision resistor to create a cheap reference for calibrating ammeters. Another reason is that this is (hopefully) a cheap way to build a very low noise 40 W DC power supply, because all the switching power supplies I have tried introduce too much noise into audio circuits, despite my best attempts at filtering.

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    \$\begingroup\$ It's typically under \$1\:\text{mA}\$ supply, but the output can be counted on to source up to \$5\:\text{mA}\$ over its temp range. All of that would get pulled out of the V+ pin. So, a maximum of \$6\:\text{mA}\$. Multiplied by the \$470\:\Omega\$ resistor would get near to \$3\:\text{V}\$ drop, which is the worst case that would be provided to the Darlington and is more than enough. So you don't need a Sziklai here. You can stay with a discrete Darlington version. What kind of assurances are you looking to get? \$\endgroup\$ Commented Dec 30, 2023 at 10:12
  • \$\begingroup\$ I should have checked this datasheet. It only guarantees \$\beta=300\$ at \$I_{_\text{C}}=3\:\text{A}\$ (while the typical chart shows \$\beta\approx 1800\$.) That could mean \$15\:\text{mA}\$ or more, worst case, into the V+ pin. And the AD584 doesn't guarantee things there, as I read it using worst case. Of course, it's likely to be fine. But you may need to evaluate any circuit you create over whatever temp range you want to support and over part variation parameters. Worth some time in both design and testing, I think. \$\endgroup\$ Commented Dec 30, 2023 at 10:23
  • \$\begingroup\$ You can get some on ebay. \$\endgroup\$
    – Dave Tweed
    Commented Dec 30, 2023 at 15:20
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    \$\begingroup\$ For an input of 15V and an output of 10V @ 4A, the transistor will dissipate 20W. I use a very rough estimate of 1 square inch per Watt. So, you want a heatsink about 4 x 5 inches. A little less if it has a lot of fins. \$\endgroup\$
    – Mattman944
    Commented Dec 30, 2023 at 15:30
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    \$\begingroup\$ Forget a website's opinion of what is or is not an exact replacement, and concentrate on your requirements. You need a PNP power Darlington rated for at least 30 V and at least 6 A (8 preferred). DigiKey has 13 separate part numbers in stock in a TO-220 package; more in other packages. \$\endgroup\$
    – AnalogKid
    Commented Dec 31, 2023 at 0:54


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