I've got a component within a larger device that requires both +15 and -15V, drawing about 100mA per rail.

In addition, the main power supply of the larger device is providing +/- 28V, with a common ground.

My initial thought was to use good old linear regulators - 7815 for +15 and 7915 for -15. However I want something a little more efficient...

...and it just so happens I have a bunch of these fancy R-7815-0.5 switching regulators (PDF) by Recom. They accept 18-35V on the input, and produce +15V on the output (500mA). In addition, they can be used to convert positive to negative, albeit at a lower output current (200mA).

I want the +28V from my supply to step down to +15... no problem there, one of the Recom devices will work perfectly. In addition, it makes sense to me that the -28V be stepped down to -15V.

However, I can't determine whether a switching regulator like this can be wired up to accept a negative voltage input, and thus produce my desired negative output voltage. To be clear, I don't want to do any voltage conversion - I want to my input +28V to produce my +15V, and my -28V to produce my -15V, to ensure that approximately the same load is being placed on both my + and - supply evenly.

In the world of compact, all-in-one switching regulators, there don't seem to be "79xx" equivalents which would make this easy!

  • Is it possible to do this with the devices I have - i.e. wire the R-7815-0.5 to accept a negative input?
  • If not, what would be the best way to efficiently produce +/-15V from my supply, placing an even burden on each +/-28V rail?

I've been using Recom converters before, really good devices. I don't think you can wire this one to draw power from negative rail, though.

So, if you want to use this particular device you'd have to draw from positive line only.

However if you open to try other Recom products, you can get any isolated dual 15V output part that accepts 56V, like RS3-4815DZ-H3, and connect input between the rails. So, you'd only need one converter and no other parts.

  • \$\begingroup\$ Aha, cunning! Yeah, totally happy to pick up a different component. Noob question: Given that I’d then be feeding this 56V-capable converter with my +/-28V rails instead of just a single positive supply and GND, would this cause any grounding/reference issues with other components on the board that need a common reference to GND? For instance an audio input that has a positive signal and a ground reference. Wouldn’t your solution end up with me having “two grounds” - one being referenced by every other component, and one being “created” by the Recom regulator? \$\endgroup\$ – abza Jun 19 '18 at 6:03
  • \$\begingroup\$ But you only will be powering it from two rails. The output ground will be connected to the circuit ground, that is why I specifically mentioned isolated device. \$\endgroup\$ – Maple Jun 19 '18 at 9:15
  • \$\begingroup\$ I see you've accepted the answer. Did it work for you? I hope it did. \$\endgroup\$ – Maple Jul 3 '18 at 1:03

A negative converter like this is rather uncommon... if you want cheap off the shelf parts, you could try a canned isolated converter which doesn't care how you reference the output voltage...

Here is an example digikey search.

  • \$\begingroup\$ Thanks for the answer. Any examples of off-the-shelf components like you mention here? I can’t seem to find any converters that are +/- agnostic. \$\endgroup\$ – abza Jun 19 '18 at 5:57
  • \$\begingroup\$ I've added a link to a digikey search, hope it helps. I selected isolated types, and of course sort by price. The first ones look nice. \$\endgroup\$ – peufeu Jun 19 '18 at 19:46

You can use a boost supply in reverse with an opamp to measure the difference.enter image description here

In this configuration the “input” is really the output.


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