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If I have a product that requires several voltage rails internally, why does it make sense for my external power supply to only source a single rail.

For instance, if I have a product requiring the following DC supply rails internally

  • 5V @ 2A, 10W
  • 3V3 @ 4A, 13W
  • 1V8 @ 4A, 7W

and having an external AC/DC adapter, what are the reasons for generating a single higher voltage (e.g. 24VDC @ 1.25A, 30W) within the adapter when I would still need to step that voltage down using 3 DC/DC converters within the product?

The benefits I see for two stage regulation are, - Better line regulation due to two filter stages - Lower cost for DC power entry plug/socket and cable due to fewer conductors - Lower cost for DC power entry plug/socket and cable due to lower current rating - Better line/load regulation due to colocation of supply and load.
- Reduced noise cross-coupling due to single voltage in cable

The benefits I see for single stage external regulation are, - Lower BoM cost due to removal of one regulator stage - Increased power efficiency from removal of one regulator stage - Increased thermal performance due to removal of one regulator stage - All regulator losses occur outside of the product - Reduced product size due to removal of regulators (within the product)

Is there anything else I've missed?

If a products primary design constraints are size and heat dissipation, why wouldn't this be the logical choice?

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    \$\begingroup\$ One of the biggest issues is that multiple external supply voltages requires a multi-pole connector. These are nearly always nonstandard, as opposed to the ubiquitous "barrel" connector used for single-voltage supplies. \$\endgroup\$ – Dave Tweed Sep 11 '13 at 14:09
  • \$\begingroup\$ My experience is that there are still PLENTY of power supplies that will do stuff like +/-12V and +5V. They're just not the ubiquitous wall warts. \$\endgroup\$ – Scott Seidman Sep 11 '13 at 14:44
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    \$\begingroup\$ HP Printers have multi-rail external PSU's. However, I'd wager that sourcing a standard wall wart and then generating nicely regulated supplies on-board is preferable because it gives you more control, close to the target, and you're already manufacturing that board anyway so there's no need to make a 2nd as a PSU. \$\endgroup\$ – John U Sep 11 '13 at 18:45
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    \$\begingroup\$ Another issue you've missed is that a device which expects to receive an unregulated voltage can be made robust against undesirable things that may happen to that voltage much more easily than one which expects a perfectly regulated voltage. \$\endgroup\$ – supercat Sep 16 '13 at 23:49
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There are many reasons for this, and it isn't always obvious.

Years ago it was common for power supplies to output several rails. Usually +12, +5, and -12v, but other variations were common. Typically, most of the power was available on the +5v rail. +12v had the second largest amount of power. And -12v usually had the least.

But as digital logic started to run from lower voltages, an several interesting things happened.

The biggest thing is that the current went up. No great surprise, really. 12 watts at 12v is just 1 amp. But 12 watts at 1v requires 12 amps! Modern Intel CPU's might require 50+ amps at somewhere near 1 volt. But as current goes up, so does the voltage drop in the wires, and thus power is wasted. If the power supply is located at the end of a 1-2 foot cable then your power losses become large compared to if the power supply is located right next to the load. Also, having tight voltage regulation becomes more problematic due to the inductive effects of the cable. So the appropriate thing to do would be to have a higher voltage come out of the AC/DC power supply and then regulate it down to a lower voltage at the load. The industry seems to be using +12v as that higher power distribution voltage, although other voltages are not unheard of.

The other thing is that the number of power rails required on a PCB has become large. A recent system that I designed has the following rails: +48v, +15, +12, +6, +3.3, +2.5, +1.8, +1.5, +1.2, +1.0, and -15v. That's eleven power rails! Many of those were for analog circuits, but six of them were for digital logic alone. And as new chips are developed, the number of power rails is increasing and the voltages are decreasing.

What this has done to the AC/DC power supply industry is that they are standardizing on supplies with a single output rail, and that rail is usually +12v, +24v, or +48v-- with +12v being the most common by far. Since everyone started doing local DC/DC converters on their PCB, and most taking +12v in, this makes the most sense. Also, due to the volumes of supplies being made, a single +12v out supply is much easier to get and cheaper than just about any other supply.

There are, of course, other factors that should not be ignored. However, it is difficult to agree on much less explain their impact. I'll just briefly touch on them below...

When a PS company has to decide on what rails to manufacture they would end up with so many variations that they might as well build custom supplies. Unless they standardize on just a couple of common voltages with a single output.

When a PS does have multiple outputs, the current supplied on each output is usually wrong. Even just the +5, +12, and -12 supplies it used to be that most of the current was on the +5v rail. But today it would be on the +12v rail because of all of the downstream point of load supplies. Add the variations on how the power is distributed to the different rails to the already huge voltage options and for a simple 3 output supply you could easily end up with hundreds or thousands of variations on how to configure the supply.

When building supplies, volume matters. The more you make, the cheaper they can be. If you have a hundred variations of a supply then you have divided your volume for any one variation by 100. That means that your cost has gone up significantly. But if you build 4 variations then the volume can remain high and cost low.

If you have a specific need for what will be a high volume product then it is common to have a completely custom supply. In this case, a multiple-output supply might make sense.

Multiple output supplies tend to only regulate one rail, and allow the other rails to track that one and have looser regulation specs. This might not matter for some, but for the low-voltage rails used by modern digital logic this can be a killer.

So there you go: single-rail supplies are becoming more and more popular because of technology advances, ohms-law, and economics.

Update: I was talking about power supplies in general. The same basic concepts applies to both internal or external supplies.

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    \$\begingroup\$ Laptop supplies seem to have standardized on 19~20V. Another advantage of internal regulators -- the exact voltage doesn't matter. \$\endgroup\$ – markrages Sep 11 '13 at 14:45
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    \$\begingroup\$ Great answer! Just to add to the economics point, in addition to the manufacturing costs, the distribution costs rise with SKU count as well. Catalogs and websites need to be bigger, distributors need to stock more units, you need more inventory in the pipeline, warrantee service becomes more complex, etc. etc. :) \$\endgroup\$ – scanny Feb 8 '16 at 6:31
  • \$\begingroup\$ Afaict laptops use ~20V input because it's conviniant for charging multi-cell lithium ion batteries. \$\endgroup\$ – Peter Green Jul 24 '18 at 17:01
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First off, regulating 24 V down to 5 V pretty much requires switching regulation, else you'd be burning P=19·I watts. Sometimes you need linear regulation, which would demand a much smaller voltage drop.

As for why you don't often see power supplies with 5, 3.3, and 1.8 V outputs, to pick your example, there are lots of reasons:

  • Your values are common, but not exactly a standard. What happens when someone else wants to add a 1.2 V rail, or 1.5 V, or...?

    If you were to design a power supply line that covers the 10 most common rail voltages and offer all possible combinations, it would be:

    • single voltage rail: 10 choices
    • any two voltage rails: 45 choices
    • any three: 120 choices
    • any four: 210 choices
    • any five: 252 choices
    • any six: 210 choices
    • any seven: 120 choices
    • any eight: 45 choices
    • nine of 10 rail options: 10 choices
    • all 10 together: 1 choice

    That's 1,023 choices! (2N-1, where N=10 here.)

    Put yourself in the manufacturer's shoes.

    Your challenge: make over a thousand different bulky products which differ in ways that aren't easily automated. You could design software that would take the input parameters and spit out a board layout and BOM, but it's probably cheaper to pay some poor engineer to grind through the options.

    Those thousand-plus power supplies then have to be shipped, stocked, and re-shipped by the supply chain.

    Some will be more popular than others, so many will be out of stock at times, and when in stock, will be taking up lots of shelf space, so they'll be doubly expensive, which further drives down demand, which drives up cost, which...

    Some combinations of rail voltages will be so unpopular that none of the distributors will stock them, so you have to offer them on demand only. That's effectively custom manufacturing, which means you're going to have to charge the customer more than it would cost for them to build it themselves.

    In the end, you go out of business.

    You can reduce the number of items to stock greatly by reducing N above. With N=5 rail choices, you only have to design, build, distribute, and re-ship 31 different products. But now you miss out on many desired choices, so you're scarcely better off than the bulk of your competitors, which only ship 1-3 rail combinations, but your costs are higher so you go out of business again.

  • You talk about saving money if the off-board power supply had the necessary rails, but you don't actually save money. To provide the desired rails, you still have to have the regulators. They just live inside the power supply now.

    If you think this doesn't matter, compare the prices for regulated vs unregulated power supplies. A common unregulated wall wart costs about $6, whereas a regulated version might be double that, or more.

  • If you put the regulators inside the power supply, they're far from the point of load, so you have IR drops to contend with. This can be a big deal when currents get high, as often happens when voltages get low. Far better to regulate near the point of load.

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If I were designing something that needed several different power rails my intuition would drive me to have all power circuits fed from one external source.

The main reason is that it saves me the trouble, during design of having three or four power supplies feeding the prototype. There are other reasons too: -

  1. There may be power timing issues that need to be controlled within the target product
  2. EMC - probably a lot easier to design to meet regulations with one incoming supply
  3. Cables and connectors are a source of unreliability and having a single supply improves reliability taken as a whole system.

It's probably easier to find an off-the-shelf wall-wart too.

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