Hi,
I'm making a triple output dc-dc power supply using the schematics from here. I'm aiming for the following specifications:
5V 12a
3.3v 12a
12v 2a
I have pots to adjust the voltages += 10% because the hardware it's for is supposedly very finnicky with voltages.
First off, I'd just like to ask if the circuit makes sense? is there anything obvious I'm missing here?
I've never created a circuit with such high current before, I've done my research on track width, expansion, heatsinks etc. But I thought I'd just try and simulate the circuit, however the 6A4 diode in the spice symbol library seems to be broken (this one). Could someone possibly upload a diode that works?
Also, is there a simple way to simulate load as part of the simulation?
"I have pots to adjust the voltages += 10% because the hardware it's for is supposedly very finnicky with voltages."
If your load is that fussy about the exact output voltages that you need to have adjustment pots then the circuit you are intending to use as a power supply is going to be pretty rubbish.
"First off, I'd just like to ask if the circuit makes sense? is there anything obvious I'm missing here?"
Well, you asked.
The reason your circuit is pretty rubbish is because although the LM317 output voltages are closely regulated, the output voltages from the emitters of the 2N3055 transistors are not.
The output voltages from the emitters of the 2N3055 transistors are not regulated because there is no negative feedback loop controlling the output voltage. Although the output voltages of the LM317s are regulated, they are passed through the series diodes and the base emitter drops of the 2N3055 transistors and these voltage drops will increase with load current and will changes by about 2mV/degC for each device.
Everything about this "PSU" circuit is just badly designed.
The series diode allows the transisotr base current to increase rapidly if required but may stop it reduing quickly when the load current drops suddenly.
There are no decoupling capacitors anywhere in the circuit.
There is no short circuit protection for the outputs.
The 3.3V output is passing 12A. So your 3.3V load is dissipating 39.6W.
Neglecting the power dissipation in the LM317 which is probably supplying about 1A base current to the transistor, the transistor is dissipating (12V-3.3V)*12A = 104.4W.
The LM317 will have to be set to output about 1.4V higher than 3.3V to allow for the diode and transistor Vbe drops which will be around 0.7V each so it will be dissipating (12V-3.3V-1.4V)*1A = 7.3W.
The diode and the base emitter junction of the transistor will dissipate another 1.4V*1A = 1.4W
So for a roughly 40W load you are burning over 110W as waste heat!
You can do the same sums for the 5V output which is not quite so bad but for a 60W load it is still wasting about 90W.
That is hopelessly inefficient and will need massive heatsinking, probably with forced air cooling.
You would be well advised to read the LM317 datasheet and study the example circuits in it, which include some designs that could be adapted to and would far better suit your requirements:
http://www.ti.com/lit/ds/slvs044y/slvs044y.pdf?ts=1588798780682
Having said that, at the powers you are dealing with, unless you have a very good reason for needing the low noise you can get easily from an analogue regulated power supply, you should be looking at a switch mode PSU design.
In fact you could probably buy an off the shelf PC supply that will already give you these outputs well enough regulated for your needs. And it will come with a universal mains input.
For example:
https://www.ebuyer.com/951101-corsair-cv-series-cv450-450-watt-power-supply-cp-9020209-uk
About the only thing you need to watch is that you always draw the minimum load from each rail to stop it drifting up too far. If necessary that can be done just by adding extra load resistors on each output. If you are prepared to consider wasting 200W with your current circuit then a few tens of watts for the minimum load resistors can be considered as hand warmers.
You should also note that the 6A4 diode is not recommended for new designs:
https://www.diodes.com/assets/Datasheets/products_inactive_data/ds28009.pdf