1
\$\begingroup\$

Im looking to construct a dual power supply capable of supply about +/-25V and hopefully up to ~5A. I already have a SMPS capable of 30V but it has only one output terminal.

I have opted for a LM317/LM337 regulator design to keep the adjustability. Unfortunately this limits me to 1A (1.5A MAX) due to the limitations of the LM chips. After searching previous threads I think a buck converter is the way to go here but have never used one before so I have some questions about their use:

  • Will i run into any issues connecting the output of the regulator circuit directly to the buck? The only one I can think of is that being a step down, the LM regulator circuit will just need to be set to a higher voltage than the desired final output.
  • Ive found only one other reference to this question and without a real clear answer; to boost the current for the negative half of the regulator output, can this be connected to a seperate buck by just reversing the polarities? i.e would the top level circuit look something like this:enter image description here On a less important note, im looking at grabbing these from ebay as its likely only a one time build and i prefer the finished look of PCB. I have found these two seperate listings for the regulators with the only difference I can note being the OP AMP and a few extra resistors in the second link. any ideas as to what this is for? is this likely to act as some sort of feedback? a buffer? Regulator 1, Regulator 2

Also if there is a less complicated way of achieving this for around the $50 range Im open to suggestions.

Thank you for any and all help

\$\endgroup\$
  • 1
    \$\begingroup\$ Your buck converter 2 needs to be a negative one, uncommon but fully possible. Another option would be to build an inverting converter. Oh, and the elephant in the room is having a switch mode converter(s) after your linear regulators. Possible, but you are wasting a lot of heat for nothing. \$\endgroup\$ – winny Sep 4 '16 at 7:11
  • \$\begingroup\$ exactly, why would you put a switching south of a linear? I can't think of a valid reason. \$\endgroup\$ – Vladimir Cravero Sep 4 '16 at 8:40
  • 1
    \$\begingroup\$ What the two other comments said. You'd usually go the other way around: SMPS to break down the "larger voltage difference", linear to do the low-noise, exact regulation. If you add the complexity of two SMPS after your linear regulator, the choice of LM317 was pretty bad. There's other linear regulator ICs than this stinky grandma of all adjustable regulators. \$\endgroup\$ – Marcus Müller Sep 4 '16 at 8:43
  • \$\begingroup\$ Also, nice, you have an adjustable intermediate voltage, but as far as I understand, this will be compensated by the fixed-voltage Bucks - so this design doesn't seem that well-thought-through this far \$\endgroup\$ – Marcus Müller Sep 4 '16 at 8:44
  • \$\begingroup\$ Also, I think this has been said a million times: don't grab voltage regulators off Ebay. It doesn't pay. Voltage regulator ICs can be had from any electronics distributor, and the 5€-10€ shipping cost usually aren't that bad if you include a couple of caps and resistors that you end up needing, anyway. \$\endgroup\$ – Marcus Müller Sep 4 '16 at 8:46
1
\$\begingroup\$

Considering +-25V and 5A imply 250W, the option to design your own inverting supply are relatively slim – inverters put an especially high demand on both the switch and the inductivity, so this might be hard to achieve without killing your budget.

So, the next approach would probably be generating a ground +- ~30V with a transformer with a middle tap on the secondary side. Then, for the +25V, normal buck controller, and for the -25V, negative output buck:

I'd have to admit that I've never done a negative output buck with significant currents – however, here's an application node from Vishay explaining the principle behind that idea. Note that this might or might not work with every Buck controller out there – you should ask in the manufacturer forums if you want to build this with a different controller IC.

\$\endgroup\$
  • \$\begingroup\$ I think i like the idea of a centre-tapped transformer and i am confident enough in building and rectifying this to DC. Ive been doing a lot of looking around as to how to then regulate this DC and have it adjustable (while trying to avoid the easy option of twin linear regulators!) Using positive and negative output bucks is the best i can find so far... My searches keep returning never-ending results using linear regulators, why are these so popular if they are so inefficient? You have been very helpful so i will be accepting your answer tomorrow after i can hopefully solve this last issue \$\endgroup\$ – Jordan Cartwright Sep 4 '16 at 11:47

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.