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While building an online UPS system, I'm basing the choice to eliminate as many ac inverters from the system as possible because of the assumption that dc to dc conversion is more efficient than ac to dc conversion. I'm not totally sure that this is the case though. I've read some articles stating that buck conversion might only be 85% efficient yet many product offering tout 97% efficiency. Also, one of the big selling points on AC is the efficiency of conversion. Hoping to get some solid info on this specific efficiency comparison.

No general statements can be given.

Generally, the larger the input/output voltage ratio is, the smaller the efficiency for basically all converter topologies.

However, whether you end up with 70%, 85% or 99% efficiency depends on the actual converter and the load.

The reason installation wiring uses higher voltages like 120V or 230V is that the energy losses during transportation will be lower if you need less current to deliver the same power – so, the very standard way of having 120V to racks of equipment and then individual converters where you need your power makes a lot of sense.

AC/DC converters are technically so close to DC/DC converters that the difference between those is shadowed by other technical considerations, such as how much you need to "overdimension" your power supply, how much money you want to invest in copper, and whether you're going for constrained size etc.

There will be almost no difference in the conversion efficiency of an AC-DC versus a DC-DC convertor.

For example if you used a PC power supply to convert 120/240 to 12VDC (I'll simply ignore the other voltages produced), most modern SMPS supplies are at least 85% efficient:

Here you can clearly see that the efficiency at some reasonable load range is much better than 85%, and up to 90% for some portion of the load managed.

A DC-DC convertor specifically designed for 24-12VDC conversion could be expected to achieve similar efficiencies at reasonable loads.

However, there are other differences that may impact your decision.

1. A task designed 24-12V convertor may run at a much higher frequency than a PC power supply and be more space efficient for the same power output.


2. For any given 12VDC load current the 24-12V convertor will have more input current flowing than the PC power supply. This may significantly impact the I^2R losses on the primary side.


3. The device cost (FETs voltage rating, isolation components, inductors) will be significantly higher in the PC supply.

You need to decide where your cost vs performance decisions are made.

I would suggest, based on my own experience, the DC to DC route is better for low power situations like just a few watts. If you need kilowatts, you'd better go AC to DC. In between the efficiency is unlikely to be regulated by this particular choice as pointed out by Marcus. I have used buck convertors with 90% or greater efficiency, they are pretty well developed now days.