Choosing the “right” AC/DC supply: the priorities may not be obvious

Don Knowles -November 13, 2012

An interesting set of tradeoffs complicates the selection puzzle, explains Don Knowles, VP of Engineering at N2Power.

When you are specifying an AC/DC supply for your project, you have both an easy task and complex one. Obviously, you need to select one with the right output voltage and current rating, as well as input voltage range. That’s actually the easier part.

But there are two other issues you need to address: what is the power-distribution topology you’ll be using? What are your thermal and supply placement/mounting options?

We won’t talk about the internal topologies of the AC/DC converters. There are some commonly used design approaches, but to the user, the supply can be considered as a “black box” with input, output, and performance attributes.

Distribution topology

If all you need to do is supply a single DC rail to a single PC board from your AC supply, you have a relatively easy situation. For example, you may have to deliver 12V DC at 20 A (nominal 240 W) to a mid-size board, and all further power needs are handled locally on that board. Consider yourself lucky, and proceed to “thermal issues.”

Many applications, though, need that single voltage across two or more boards, or need different voltages (and currents, of course). Right there, you have the choice of one larger supply, or multiple smaller ones, Figure 1.



Figure 1: The AC line can feed a single larger AC/DC converter (top), or several smaller ones (bottom)—even with the same output voltage.

Many applications use multistage-power-distribution approach, where the AC/DC supply feeds one or more intermediate bus converters (IBCs), which in turn transform the DC voltage from the AC/DC supply into lower-voltage DC rails. For example, individual IBCs may take 48 VDC and provide 12 V DC, 5V DC, 3V DC, and other voltages needed by the circuitry, Figure 2.

Figure 2: Many installations use a single AC/DC converter to supply a one DC voltage to an array of intermediate bus converters, which in turn provide the final DC rail voltages.

If the system uses IBCs, the decision as to which kind of the AC/DC converter is needed is, again, simplified. In most designs, there is only a single intermediate voltage, such as 48 V DC or 12 V DC (some more complex systems use a variety of IBCs supply rails. Therefore, you can focus on finding an AC/DC supply (or supplies) which provides the desired intermediate rail voltage(s), at sufficient current.

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