I am building an appliance where we have a device where we need to have a +48 V/300 W DC power rail and a +24 V/300 W DC power rail. The AC (mains) should be world-compatible (both low- and high- lines) and the DC part should be isolated from the mains (double-isolation)

I am struggling to come up with the best power distribution strategy to generate those two power rails:

  1. Two single-output AC/DC power supplies in series (mains-to-24 VDC) with a +24 VDC mid-point and common ground.
  2. One dual-output AC/DC power supply (Output 1: 48 VDC, Output 2: 24 VDC) with common ground.
  3. One single-output AC/DC power supply (48 VDC) with a step-down DC/DC converter (48 VDC to 24 VDC), with common ground.
  4. Other?

Both DC power rails should not be isolated and share a common ground to avoid ground loops.

EMC, floor area, efficiency and cost are concerns (by order of priority).

What would be the best option?

  • 1
    \$\begingroup\$ Welcome to EE.SE. What options did you have in mind? Unless items 1,2,3 and 4 are your options. My opinion is that the fewer supplies you have the simpler it will be. I would avoid option 1. Floating power/ground is tricky. \$\endgroup\$
    – user105652
    Sep 12, 2018 at 4:02
  • \$\begingroup\$ At the moment I'm considering Option 4) One single-output AC/DC power supply (mains-to-48VDC) and One single-output AC/DC power supply (mains-to-24VDC) with connected grounds (secondary). \$\endgroup\$
    – HW2015
    Sep 12, 2018 at 8:23
  • \$\begingroup\$ Welcome to EE.SE! Your question is what I would say a good engineering question, the ones you don’t learn about in school. However for SE, it’s unanswerable and a bit open ended. All your options are viable and you need to look deeper into price, space, cooling, startup behavior, fault cases and so on to find your answer. \$\endgroup\$
    – winny
    Sep 12, 2018 at 9:28

1 Answer 1


Cheapest: Possibly Option 1-- 24V/300W power supplies are widely available from many suppliers. Actually, you'll need 450W out of the lower power supply, so that may put you above the cost of Option 4: a 48V/300W power supply and a 24V/300W power supply. Again, both are common voltage and power ratings, and you can probably even find them in the same form factor.

Most Compact/simplest to wire: Option 2, if you can find a suitable supply. 24V+48V/600W is not as common of a configuration, but there are manufacturers that will do semi-custom configurations using a modular design if you're willing to order in sufficient quantity.

100-250VAC input supplies are extremely common, I would venture to say that most of the enclosed or open-frame power supplies on the market will meet your needs there.

Regarding EMI, this is not likely to be a significant defining factor between your options. It comes down mostly to the quality of the supplies you select, the nature of your load, overall wiring, etc. You pretty much have the same options for adding any additional filtering you find to be necessary in any of your proposed configurations.

As far as efficiency, this largely depends on the nature of your load and where it fits in the efficiency graph of your PSU. Going for a single larger supply may give you more PFC-equipped power supplies to choose from, if that matters to you.

So on the whole, your best bet is to start looking at what power supplies are available and seeing what best fits your cost target and your mechanical envelope.

  • \$\begingroup\$ Option 2. is hard to find as this is a medical appliance. How about option 4) One single-output AC/DC power supply (mains-to-48VDC) and One single-output AC/DC power supply (mains-to-24VDC) with connected grounds (secondary) ? Thanks \$\endgroup\$
    – HW2015
    Sep 12, 2018 at 8:23

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