Eliminating the UPS Efficiency Penalty with -48Vdc: Part II

In Eliminating the UPS Efficiency Penalty with -48Vdc, there is a discussion of how a non-redundant AC and DC configuration can have nearly equivalent efficiency in facilities without a UPS. However, when redundancy is figured in, the advantages of DC power become more pronounced.

Let's start by looking at the power supply unit (PSU) component by itself. Based on the information in the quantitative analysis by The Green Grid, high-efficiency AC and DC power supplies look like this when compared to each other:

The graph shifts to the right when redundant power supplies are considered. Since there are numerous different voltage converters in a server (modern servers often have in excess of 25 voltage rails used internally), it's really impractical to try to duplicate every voltage converter in a server--at least if you want it for a reasonable price. However, servers with redundant power supplies provide three principal benefits:

1. Connectivity to separate primary power sources (i.e., different utility feeds)
2. Protection against failure in upstream power equipment (i.e., failure in a PDU)
3. Cabling problem or service failure (i.e., accidentally unplugging the wrong server)

In an AC system, separate power supplies are required to have redundant feeds, since each power feed might be slightly out of phase with the other feed by the time the power signal gets to the server (relative phasing can shift in different parts of the data center based on relative cable lengths). If a server has two power supplies equally sharing the load as is commonly done, then each power supply <50%>

In contrast, a DC system has no phasing issues to deal with. Therefore, DC-based equipment has two main options: full duplicate power supplies (like AC) or using a technique called diode OR'ing (or FET OR'ing) to safely combine power from two separate DC sources as inputs to a single power supply. [Since there are numerous downstream power converters that are not redundant, there's no need for the power supply itself to be redundant--it just needs to be fed from multiple inputs.] Many DC power supplies do this today, as this approach is commonly used in the highly-reliable telecommunications system with -48Vdc systems. The result is a wider gap between the net AC power supply efficiency and the DC power supply efficiency:

Taking this a step further, look at the typical operating point for servers vs. their power supply ratings. For example, look at the various published reports for SPECpower_ssj2008: you'll notice there are numerous cases where the power supply shipped with the system is 2-4 times the maximum power draw in the sytem. If the power supply in a system is 2x the necessary power, then the system would normally operate in the left half of the graph immediately above. If the average power is considerably less than the maximum power draw, then the system could spend the bulk of its time operating at the 25% load level or less in the graph above.

At these lower loads, the efficiency benefits of -48Vdc systems become more apparent, even when there's no UPS in the picture. If an installation uses UPSes, the efficiency gap widens further in favor of -48Vdc.

Eliminating the UPS Efficiency Penalty with -48Vdc

The Green Grid recently released Quantitative Efficiency Analysis Of Power Distribution Configurations For Data Centers, which shows how different power chains from 480Vac down to 12Vdc stack up in terms of efficiency. This showed -48Vdc to have the highest efficiency for systems at 60% of capacity and below--in an idealized world.

This is true when a UPS is required--but what happens if a UPS isn't needed?

Say what? Who would ever want to deploy servers without UPS
backup?

There are certain circumstances where a UPS is not needed:

• Services with sufficient geo-redundancy that a power failure at any one site doesn't have appreciable impact on the overall service availability


• Lower-priority services for which an infrequent service outage would be acceptable

In situations like this, how does a -48Vdc system stack up? Let's look at the data in the report from The Green Grid mentioned above:

• The best AC power supplies to go from 240Vac down to 12Vdc peak out at around 93% efficiency [Figure 31].


• The best DC rectifiers (with batteries) to go from 240Vdc down to -48Vdc peak out around 96.5% efficiency [Figure 29].


• The best DC power supplies to go from -48Vdc down to 12Vdc peak out at almost 95% efficiency [Figure 31].

Taken together, the 96.5% rectifier efficiency x the 95% power supply efficiency equate to ~91.7% efficiency, slightly less than the 93% efficiency of a pure AC to 12Vdc power supply solution.

However, this is using rectifiers with tightly regulated -48Vdc outputs designed to work with batteries along with wide-ranging inputs. This is a mis-match! It's understandable why this has traditionally been done (for applications needing battery backup), but it's overkill for applications not needing battery backup.

Since most -48Vdc power supplies can handle input voltages from -42Vdc to -56Vdc (or a wider range), think what could happen with a DC rectifier with a loosely regulated output well within this range. If a DC rectifier was allowed to vary its output voltage between -44Vdc and -54Vdc, the net efficiency of the -48Vdc system could meet or beat the approach with a straight AC power supply.

Without battery backup, a -48Vdc system could match an AC system; even with full-time battery backup, the -48Vdc system is within ~1.5% of the AC system without battery backup.

Next: the story gets even better when redundancy is considered...