I apologize if I will write a lot, I invite you to read only if you have a lot of free time...
For some time now I have needed a versatile bench power supply, which would be useful for several general tests. I understand that the word "general" in electronics does not mean anything, because an instrument suitable for any situation is only a utopia, but from my experience I can say that it has hardly happened to me that I need voltages higher than 24V and currents higher than 3A, for small applications.
I would therefore like to build a power supply with two separate channels (possibly usable in series or in parallel), for example with maximum 2x15V.
I would like to have a very stable power supply with very low noise, so I would like it to be linear, not switching.
I have available (in addition to a nice metal case with "generous" dimensions) a 2x12V 6.25A (150VA) toroidal transformer. These two outputs, after being rectified, should give me 12 x 1.41 = 16.92V. After subtracting a dropout voltage, typically 1.5V, I should get my 15V per channel.
I also have two LT1083 linear regulators, which hold up to 7.5A continuous. They are very beautiful and have a TO-3P package easily mountable on a heatsink. But unfortunately it is an obsolete model, not replaced by any equivalent, so I would prefer not to use them, to make a future replacement easy, should one break.
I have found many solutions with linear regulators and a current of many amps, both with many regulators (like LM317 and equivalent) in parallel, and with bridge-connected mosfets.
Then I would have to make other choices:
1) Start from 1.2V of minimum voltage or adopt a more complex circuit to start from 0V
2) Adjust only the voltage or also the current (not of primary importance to me)
3) Use a simple analogue control with multiturn precision potentiometers and display to measure voltages and currents, or insert a microcontroller to control the settings with PWM, with a digital interface (in the future possibly also controlled by software)
But the crucial point is another: the maximum current and its dissipation.
If I want each channel to go from 0 to 15V, for low voltages I would have a maximum dropout of about 15V, which at 6A would require a dissipation of 90W !!!
Normally I would not reach this condition, because when could I ever need a 1V to 6A output? For example, it would be more likely to use 12V at 6A ((17-12) * 6 = 30W of dissipation). But I should still be in the worst conditions and I think that dissipating 90W of heat is really too much for a bench power supply, even with forced ventilation.
What do you think? What would you do for me? Settle for a much lower maximum current, not taking full advantage of my toroid? Make sure to limit the current as a function of the voltage, with a digital control, so as to keep the total dissipation below a set threshold?
After all, a good compromise could be having a precise linear variable power supply, with a low current, and a second switching power supply in case I need a higher current.
But even if I limit myself to 3A per channel, dissipating 45W of heat may not be that simple.
Any advice and opinions will be welcome...thank you in advice!