Why don't UPS's have a DC output, and PSU's a correspondng DC input? (or why aren't there any combined UPS/PSUs?)

[Dan Forever]

As far as I understand it, batteries do not generate AC, and computers do not run on AC, but AC is what is supplied to the home because it's more efficient.

 

That means, assuming you use a UPS, you convert your AC input, in my case that's 220-240V into DC (I'm assuming 12V, but I'm not sure) for the battery.

I'm also guessing that UPS's will pass through mains directly to a PC (when the mains is supplying power), otherwise why would their specifications sheets have a "response time"?

 

If you're running off the UPS's battery it's gotta spin the DC voltage back up to 240V AC, so that it can pretend to be a mains supply for the PSU, which then switches it back down to 12V DC (and 5V an 3.3V etc).

 

So why aren't their any solutions that skip the need to output AC from the UPS? Wouldn't that mean you can last longer on the battery? or is the saving so insignificant as to not be worth it?

[flibberdipper]

I feel the issue is less about the change in efficiency/run time from not having to convert it several times, but more the R&D costs compared to the very small number of people that would actually take advantage of such a feature. The only potential group where such a feature might make sense is in the enterprise/datacenter setting.

[tkitch]

how many people would  buy a special new PSU just to use a DC input from a specialty made PSU to have a DC Output?

 

There's no way people would pay 300+ Bucks for the PSU, and then another 200-300 bucks for the UPS  just to do that.

 

as for combining?  unless you make the PSU way bigger, there's not enough physical space for batteries in the PSU

[AbydosOne]

12 minutes ago, tkitch said:

how many people would  buy a special new PSU just to use a DC input from a specialty made PSU to have a DC Output?

To be a little reductive, you've just described a laptop

 

35 minutes ago, Dan Forever said:

So why aren't their any solutions that skip the need to output AC from the UPS? Wouldn't that mean you can last longer on the battery? or is the saving so insignificant as to not be worth it?

Power transmission is part of the issue. 1000W at 120VAC is 8-9A; 1000W at 12VDC is 80-90A which would require cables the size of broom handles to minimize resistive losses over any appreciable distance. In a home scenario it's not so impractical, but even over the height of a server rack, we're talking some serious power distribution.

 

I don't know this for certain, but I suspect the resistive losses of low voltage DC are probably less efficient than the conversion losses of rectifying and inverting, not to mention the increased material costs for DC distribution.

[porina]

7 minutes ago, AbydosOne said:

Power transmission is part of the issue. 1000W at 120VAC is 8-9A; 1000W at 12VDC is 80-90A which would require cables the size of broom handles to minimize resistive losses over any appreciable distance. In a home scenario it's not so impractical, but even over the height of a server rack, we're talking some serious power distribution.

This. A more well known example might be GPUs. Just how hard is it to get enough power into one of the higher end ones at 12V?

 

IMO 12V is only acceptable as long as we remain at lower powers. If we start going to higher power devices, then a change to 24V or 48V DC might be useful. Given high power isn't needed by the vast majority of consumers it seems unlikely to gain traction there.

[Agall]

1 hour ago, Dan Forever said:

As far as I understand it, batteries do not generate AC, and computers do not run on AC, but AC is what is supplied to the home because it's more efficient.

 

That means, assuming you use a UPS, you convert your AC input, in my case that's 220-240V into DC (I'm assuming 12V, but I'm not sure) for the battery.

I'm also guessing that UPS's will pass through mains directly to a PC (when the mains is supplying power), otherwise why would their specifications sheets have a "response time"?

 

If you're running off the UPS's battery it's gotta spin the DC voltage back up to 240V AC, so that it can pretend to be a mains supply for the PSU, which then switches it back down to 12V DC (and 5V an 3.3V etc).

 

So why aren't their any solutions that skip the need to output AC from the UPS? Wouldn't that mean you can last longer on the battery? or is the saving so insignificant as to not be worth it?

A UPS contains a transformer, rectifier, filter, DC bus that includes the battery, then the inverter (also including some switching to allow for line-interactive type UPS switching to occur).

 

The DC bus on some UPS does sometime Vdc outputs, but for the most part, your standard computer UPS is operating off the wall AC 99.99% of the time. My APC 1500VA UPS as an example has USB ports on it for powering Vdc devices, but in reality, a device that's powered off of type-C has its own internal 'UPS', being a battery, so they're relatively low wattage USB ports. If there was a 100W type-C port, then you could power your laptop off it and skip a type-c power supply entirely for that device.

 

So for almost all scenarios, the devices that a UPS requires to power are Vac input, like a PC power supply, where its operating off of the wall Vac 99.99% of the time, where the UPS is designed to switch within single digit milliseconds to its battery+inverter whenever it senses low/high voltage/frequency.

 

The important part is understanding that most computer UPS's are line-interactive type UPS, where they're operating off the wall 99.999% of the time. Double conversion type UPS's that are always supplying Vac power from the device's inverter are very expensive, including maintenance, and are more limited.

 

99.99% of the time, the load's Vac source is from the Input AC through the AVR+filter, then to the Load.

[Dan Forever]

These are some truly interesting responses, thanks everyone.

 

I didn't even realise (but it makes sense now that you've spelled it out to me) that the low voltage DC would mean we need impractical conductors. Though maybe having a higher DC voltage (like someone pointed out), that you step down, might be better? Though again, I don't know what that step down would cost efficiency wise, maybe it makes the idea pointless.

 

I was thinking it could well make sense in an enterprise/datacentre context, if there was enough efficiency to be gained that the savings make sense long term.

[inane.ink]

On 10/3/2023 at 4:31 PM, Dan Forever said:

So why aren't their any solutions that skip the need to output AC from the UPS? Wouldn't that mean you can last longer on the battery? or is the saving so insignificant as to not be worth it?

There is a solution / way of achieving this, but for an average user it's a bit expensive.

 

I just finished a SFX build using the Fractal Terra Jade, with the key factor being: DC input.

 

Power Supply: instead of a traditional ATX or SFX power supply, you can also opt for a DC-ATX power supply:
https://hdplex.com/hdplex-800w-dc-atx-with-12v-63vdc-input.html

They have cheaper / weaker models too, but I want to be able to run a i7 + 4060ti easily and be on the safe side regarding future upgrades.

 

You'll notice that >300$ for a 800W PSU is a bit higher than some standard PSU

However, it will take any input voltage from 12V to 63V, which is amazing for industrial settings or running off a car/truck battery.

 

And regarding efficiency: that thing is crazy, at >90% efficiency under full load it can be run without a fan, continuously!

 

Next, you'll need to modify your case to support DC input rather than AC input. This meant that I had to make a custom DC input panel for my PC case, and solder appropriate connectors to the wire. As my target was 20V, and the GPU+CPU can easily draw 300W combined, the wire and connectors have to be rated to at least 15A. If you want to max out the 800W, at 20V that would equate to 40A... which is not insignificant. Higher voltages would reduce the current requirements, which is why the electrical grid uses 110V in the US or 230V here in Switzerland.

 

At this point, you've invested approx. 300$ and a few hours of work, but still can't run the PC, because now it does not accept a standard AC cord.

 

My goal was to make my build battery-powered, and as I have a few Bosch Professional (blue) ProCore batteries lying around I will be using those.

One 12Ah 18V battery has a voltage range of roughly 17V...21V, and can easily output 2kW of peak power. At 200Wh capacity the run-time highly depends on the load though.

To keep the battery topped-up when a power source is available, I'm using a 230W GAN laptop PSU coupled with a DC/DC battery charger. The charger can deliver 200W, any higher power draw is supplied by the battery. If my total average power is below 200W, this solution works great. Running Cyberpunk will exceed 200W though, so I also need to hot-swap between batteries occasionally.

 

In summary:

Running your build off of DC is not that hard anymore, but it does require investments into hardware, compromises, and creative problem solving - plus it's not without it's risks.

 

If you already have the batteries and charger for the batteries, you'd only need the DC-ATX PSU. Oh, and you need to make sure that the PSU does not drain your batteries below what they're rated for. In my case I do not have any undervoltage protection, so I need to be quite careful : D

 

---

You may be asking yourself.. why?
- As I wanted a very quiet SFF build that does not compromise on CPU and GPU power, I had to maximise the space available for water cooling. The DC-ATX PSU is slimmer than a normal SFX PSU, and fit into my case better.
- My use case is a "mobile" gaming rig, as I stay at a different location 3d/week. There is power, the main reason for using the battery is to compensate power draw peaks and to act as a UPS.

- Instead of paying 2000$ for a workstation laptop for use in a CAD/Engineering BsC, I wanted to be able to upgrade components down the line. My build is quite heavy compared to a beefy laptop, but it's a lot quieter and has even more power. With two 12Ah battery packs and some power efficient settings the runtime is quite a bit better than my dying Lenovo P1 Gen 3...

- I like bodging stuff together and take pride in unconventional solutions hehe

 

I've attached some photos for the curious, it's still a work-in-progress though

Spoiler

Using an iPad with a HDMI capture card as screen:

 

Here's my hardware config.:

 

 

The loudest component... 4060ti:
(And yes, that's a 240mm radiator on the bottom. Not ideal placement but it's pretty unobtrusive with the sidepanels attached.)

 

 

I'll be improving cable managment and tubes, I wanted to see if things even work before putting in way too much time.

 

 

Even has a flip-out antenna if the WiFi signal is weak! xD

 

 

Somehow everything did fit together in the end:

 

 

Please excuse the flipped image, new to the formatting on the forum. Here you can see the DC input:

 

 

And my DC/DC battery charger (although this is a weaker one, not the 200W one mentioned in the text. This one can charge 2 batteries in series though.:

 

 

 

 

[inane.ink]

On 10/3/2023 at 4:53 PM, tkitch said:

how many people would  buy a special new PSU just to use a DC input from a specialty made PSU to have a DC Output?

 

There's no way people would pay 300+ Bucks for the PSU, and then another 200-300 bucks for the UPS  just to do that.

 

as for combining?  unless you make the PSU way bigger, there's not enough physical space for batteries in the PSU

oops lol

 

 

To be fair, the 400W DC-ATX is approx. 100$, the 200W is even lower. But yes, it does get expensive, especially compared to a standard PSU.
There is quite a market though, especially in industrial settings (control cabinet PC) and portable builds.

If you have a good UPS already, you can hook up the 200W HDPlex DC-ATX PSU to the battery pack (12V...63V input range handles a lot of battery configurations) and run a server off of it, the only problem being draining the UPS batteries too low as there is no low-voltage cut-off.

[tkitch]

2 minutes ago, inane.ink said:

oops lol

 

 

To be fair, the 400W DC-ATX is approx. 100$, the 200W is even lower. But yes, it does get expensive, especially compared to a standard PSU.
There is quite a market though, especially in industrial settings (control cabinet PC) and portable builds.
If you have a good UPS already, you can hook up the 200W HDPlex DC-ATX PSU and run a server off of it, the only problem being draining the UPS batteries.

and most industrial solutions don't use ATX standards for anything, so that's not gonna apply there.  Same for servers.  Also?  200W?  What're you running, a single mid-range consumer CPU with no GPU?

 

If the equipment is important enough to need a battery backup, it's either included in the unit, or the purchase of a UPS is part of the spec for the equipment.  

[inane.ink]

25 minutes ago, tkitch said:

and most industrial solutions don't use ATX standards for anything, so that's not gonna apply there.  Same for servers.  Also?  200W?  What're you running, a single mid-range consumer CPU with no GPU?

 

If the equipment is important enough to need a battery backup, it's either included in the unit, or the purchase of a UPS is part of the spec for the equipment.  

 

Fun fact: a 40k LeCroy oscilloscope I took apart once used a bog-standard ATX power supply. They get used for more things than one may assume at first. (Yeah, I was pretty shocked, especially as that was the broken part and using a normal replacement PSU fixed the darn thing.)
 

I won't disagree with you on whether "most" industrial solutions don't use ATX, as I honestly do not know how "most" solutions are done. I'd imagine that it really depends on the industry.

However, I've worked on at least two instances where ATX power supplies were used:
- A big international producer of factory equipment was using ATX power supplies for their machine vision platform. If they'd want to change to DC main rails, or if the machine vision platform (modular approach) would be incorporated in a mobile apparatus, a DC-ATX would make a lot of sense.

- Research lab: One of the lab measurement PCs was running off a DC power supply, which was custom-built quite a few years ago. Why? The entire setup was in a galvanically isolated cage (50kV potential / floating) and had weight restrictions. A DC-UPS that does not require any transformers saves a lot of weight, and not having 50Hz noise is also very advantageous in some settings. Now that DC-ATX are commercially available, 300$ is nothing compared to a custom PCB etc.

 

Generally speaking, if the focus is on maintainability and repairability (Example: Research / Education facilities), then using established standards does make a lot of sense. But yeah, most corporations will prefer proprietary solutions, which is a shame when it comes to right-to-repair

 

For big server racks, a normal UPS is preferrable. Fully agree with you. But sometimes there are other factors to take in account, like the weight restrictions mentioned above. Or if you have a battery bank / power wall, an old car battery left over, those kind of situations. A 12V car battery charger is not very expensive, and coupled with a DC-ATX PSU you've got yourself a pretty high-capacity UPS.

 

200W server?
Well, for example a i7-12700K overclocked is still below 200W. As a web and data server, even with multiple M.2 and 10Gbit NIC, that is still overkill. For my private use I don't need some fancy data-crunching server, I've got my new mean green mini machine for that
My example of plugging it into an existing UPS is maybe not optimal, but the idea of using any already available DC power rail is my main point.

 

 

[Glenwing]

On 10/5/2023 at 2:31 PM, Dan Forever said:

These are some truly interesting responses, thanks everyone.

 

I didn't even realise (but it makes sense now that you've spelled it out to me) that the low voltage DC would mean we need impractical conductors. Though maybe having a higher DC voltage (like someone pointed out), that you step down, might be better? Though again, I don't know what that step down would cost efficiency wise, maybe it makes the idea pointless.

 

I was thinking it could well make sense in an enterprise/datacentre context, if there was enough efficiency to be gained that the savings make sense long term.

It does make sense in an enterprise/datacenter context, which is why large datacenters already use this kind of architecture. It's just in the home that the gains are small and the inconvenience is large, so it's not really worth it to most consumers.

[wseaton]

OP has a really good question. Along with that, I live in a northern state, and it\s patently hysterical to walk into a data center that's running  massive A/C units.....but....it's 20F outside.  Also, for a bonus, the offices surrounding the data center are using forced heat from natural gas. You would think all that heat from those servers could be used for something else other than to keep the AC units cool  

 

The OPs question regarding DC power is the exact same one every smart residential Solar owner has to deal with. 

 

The problem is basically computer power supplies. A PSU for a PC, even a high current one would actually be simplier and cost less if it\s input was 24 or even 48volts. Buck regulators are pretty simple, and very efficient. However, they have to be primarily powered by AC mains, so such a PSU would be expensive given economy of scale. It's not hard to build though. I recall several times powering GPUs off external Mean Well supplies to trouble shoot a PSU issue. Computer components don't care where they get their power from, and with GPUs being the biggest power hogs they can likely be powered more efficiently by a dedicated supply - preferably an external one. 

 

Been in data centers that use an intermediate 48volt step for bridging hybrid power backup topologies, but I didnt have time to see how it all worked. I just know that 120/240 isnt the rule of law.