It's easy to over-specify your AC/DC supply—but you shouldn't

Don Knowles -November 13, 2012

You can buy more supply than you need, yet less is actually often better, says Don Knowles, VP Engineering at N2Power.

Just as it is important to properly size and specify your AC/DC supply, it's also important for designers not to over-specify this vital component. It may seem counter-intuitive, but "too much" of a good thing can have negative consequences in efficiency, cooling, overall product size, and even available vendors, besides the obvious downside of higher cost.

The first and largest factor to consider is matching the supply output capability to the load it must support. For example, if the maximum load (DC voltage × current) is 500W, then a 1000W supply provides much more design-margin insurance than you actually need.

What are the consequences of a supply that has so much headroom? The good news is that, obviously, you'll have plenty of amps at the nominal voltage-rail values you need. Simple enough, end of story—but not quite. There are significant drawbacks to having all this extra, unused power available.

The biggest one has to do with inefficiency and its many consequences. Every supply has an efficiency vs. load graph, such as the one in Figure 1. For a well-designed switching supply, this efficiency is usually at its highest in the range of 80-95% of maximum rated load. [This general guideline does not apply to linear regulators and supplies, but those are unusual above fairly low power levels of a few watts.]

Figure 1: The efficiency of a supply varies with load, and most peak in the zone of 80-95% of their maximum rated capacity; this chart shows the N2Power XL280-48 curve.

When operating at low loads, which may be most of the time in an application like a data center, the power supply can generate a lot of extra heat, and this is where the engineer's nightmare of both obvious and unintended consequences starts. The obvious effect is that you are wasting more AC-mains power, so your system costs more to operate, and that cost is straightforward to quantify. The larger supply is also more expensive to buy, and it’s easy to put a cost number on that, as well.

But beyond those easily assessable factors are ones that are much harder to grasp. As a consequence of the additional heat, which you must get rid of, you are now dealing with more complex design and budget issues related to convection cooling (which may no longer be possible), fans, airflow layout, and heat sinks. These alternatives add direct cost, materials, unreliability, and constraints on packaging and layout to the design, and even limit your degrees of freedom as you need to squeeze more into the product box, or make the box bigger. In addition, the larger-capacity supply has a larger footprint, with clear negative consequences.

Further, as you select larger supply sizes, you'll likely find fewer vendors to choose among, and fewer direct alternatives or second sources to your primary or preferred source. This may not bother you, but your purchasing department or contract assembly source may be uncomfortable and even push back.

For these reasons, most AC/DC supply vendors offer a broad family with many similar units, except for capacity, so you can match the supply size to the load with little excess capacity. For example, members of the XL series of AC/DC supplies from N2Power are available with closely spaced 125, 160, 275, and 375W ratings.

Note that adjacent-rated units from some vendors often differ only in their power rating, but have the same physical size and connector, so you can "interchange" painlessly if it turns out your actual power needs are different than you anticipated, as the N2Power XL125 and XL160 photos show (Figures 2a and 2b); both have the identical 3" × 5" inch (7.5cm × 12.5cm) footprint.

Figure 2: a) The XL125 125W AC/DC supply and b) the XL160 160W supply from N2Power differ primarily in their power rating; their footprint, physical size, connector, and many other specifications are otherwise identical. 

Of course, it's easy to say "just design to use less overall power, and then size the supply to the maximum load." The problem is that for many designs, the ratio between the maximum (peak) load and the typical load is large; 2:1 or even 3:1 ratios are common. So you must size your supply for the peak load, but most of the time it is running at far less, and is in the inefficient zone.

There are ways to circumvent this problem, such as by using an auxiliary booster for peak loads, a super-capacitor, or other techniques. However, each of these brings new design problems of switching them to the load, and the overall response to load transients. Therefore, to avoid over-specifying, try to get the maximum load of the system down to as close as possible to the typical load value.

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