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I'm making an adjustable power supply, constant voltage/current and all that jazz, and I'm trying to make everything as small and portable as possible, avoiding bulky heatsinks, and using a switching tracking pre-regulator to reduce the overall volume. I read a lot about noise suppression, and I know there'll still be quite a lot of it, but I concluded that SMPS with short/wide AC tracks and other recommended layout considerations followed by a linear regulator would still be more stable than just a SMPS with additional filter (please correct me if i'm wrong).

Anyway, I have a pretty, center-tapped transformer extracted from some old analogue receiver rated at decent power, and I was wondering whether something like this is possible using two switching regulators. The "top" and "bottom" regulator should be able to have different offsets.

The similar questions I stumbled upon online were mostly about connecting ATX PSUs in series, which are fixed voltage supplies using flyback topology, so yeah, no luck there.

If possible, the topology for the positive output regulator would obviously be the buck converter, but what about the bottom one? Can a buck converter sink the current? If not, is there a right combination of topologies to do what I'd like to without meddling with the transformer?

I mean, I guess I could connect the two identical windings in parallel for max power and use buck + inverting for +/-, right? I would like to avoid that because the transformer would get a lot uglier, and I'm afraid I'd mess it up.

PS: Please do not ask me what do I need the stabile voltage for, it is just for fun, and learning about PSU design along the way :P

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closed as unclear what you're asking by Voltage Spike, Daniel Grillo, dim, jonk, The Photon Nov 30 '16 at 5:42

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You can easily get hold of linear regulators that truly regulate a negative supply voltage and hence can easily be wired together with a positive regulator just like in your picture (which I attach here for ease of explanation): -

enter image description here

I was wondering whether something like this is possible using two switching regulators

Here's where the problem begins - almost all (if not all (contradiction later)) buck switching regulators are based around regulating a positive voltage with respect to 0 volts. The upshot of this is that if you had a transformer output with two seperate secondary windings it makes things doable. In short, (unless someone else knows of one) there is not buck switching regulator equivalent of an LM337 (or a 79xx type) regulator.

It's probably better to use the full output from the secondary (ignoring the centre tap) and have both a positive regulator and an inverting regulator; the inverting regulator is still fed a positive voltage and 0 volts but, in this special case, it produces a negative voltage. Here's an example: -

enter image description here

Having said that I'd never found a truly negative switching regualtor, in searching for a fairly powerful inverting regulator, I found this and it's certainly worth considering because it's from a reputable source: -

enter image description here

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  • \$\begingroup\$ Wow, thanks for a quick and extensive answer! I have a quite limited choice of those SMPS regulators, so I mostly get to choose from the quite common ones. I'll probably go for LM2576/LM2596/TL494/SG3524/MC34063 or similar. \$\endgroup\$ – Njubster Nov 4 '16 at 16:34
  • \$\begingroup\$ I'm noob to this site, so my comment is missing a part. Anyway: I've read somewhere today that under 50kHz, EMI is not much of an issue. Is that correct? I'm asking that because I'm not sure which switching frequency to use. LM2575 for example, it has a fixed frequency of 52kHz, LM2596 runs at 150kHz, while the other ones can be adjusted manually. I do have enough space for a bit larger inductors/caps, but I'm not sure which frequency will make the least noise. Also, are linear regulators better at rejecting HF or LF noise? Thanks again :) \$\endgroup\$ – Njubster Nov 4 '16 at 16:44
  • \$\begingroup\$ EMI is an issue at any frequency where another piece of equipment (or a local circuit) might be susceptible to that frequency. Lower frequencies are more easily "restrained" with conventional circuitry and, lower frequencies are naturally less able to produce genuine EM waves due to the size contraints. But, at the end of the day, all switching regulators will interfere with something if close enough up to that "something". Adding linears to the output is probably a good idea. Trying to make the switcher track a couple of volts above the linear regulator can be a headache but it's worth doing. \$\endgroup\$ – Andy aka Nov 4 '16 at 16:55
  • \$\begingroup\$ Thanks again :) I guess I'll go with LM2575 then, with additional filters, perpendicular inductors and stuff. I'll try to bother you a bit more: LM317 for example has around ~60dB CMRR. Would the linear regulator significantly benefit if, for example, I was to use a darlington pair regulated with TL074 op amp, which has the CMRR of ~100dB, instead of IC regulator? \$\endgroup\$ – Njubster Nov 4 '16 at 17:07
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    \$\begingroup\$ I'd stick with a pre-made regulator like the LM317 but make sure the adjust pin also has a decoupling capacitor (reduces output noise). The TL074 may appear to have a CMRR of 80 dB but I can guarantee it'll be several tens of dB worse at 52 kHz. \$\endgroup\$ – Andy aka Nov 4 '16 at 17:09

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