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Selecting the right AC/DC supply

22 Nov 2012  | Don Knowles

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The bigger question is the line tolerance you need. The wider the tolerance around the nominal value—5%, 10%, or more—the more difficult the supply challenges, especially when it is called to supply maximum output at minimum line; note that a full-range supply could be specified for 90 to 264 VAC. Also, on-going line voltage fluctuations make it more difficult for the supply to maintain its regulated output, even if the load is not changing.

As with most engineering designs, its these "corner cases" of operation that require you to carefully study the vendor's data sheets, and see if max/min specs are maintained under worst-case conditions, or only at nominal points.

Moving on to the DC side, look at required voltage(s) and associated current rating(s). There's not much to argue about; either the supply is rated to deliver what you need, or it isn't. Do check if the supply's output regulation meets your requirements—that's the ability to maintain the nominal output within tolerance, despite shifts in the current being drawn from the supply.

Most vendors offer screw terminals for the DC output, but connectors are also available as standard options. Screw terminals reduce costs of interconnection cables, but require more labour in final assembly of your product and are prone to wiring errors.

Other features you'll need to check on the regulatory compliance. The major ones cover efficiency; power factor correction (PFC)(how much the supply shifts the line current with respect to voltage); safety issues such as isolation and fault modes; RoHS (reduction of hazardous substances); and EMI (electromagnetic interference). Check that the vendor meets the applicable standards and certifications; as nearly all do, (figure 4 for a table which lists compliances for a representative supply, the XL375 Series from N2Power).

Figure 4: There is a large set of regulatory compliance standards that supplies must meet; this list for the N2Power XL375 is representative. [From section 1.2, of Product Specification XL375 Series 375-Watt AC to DC Power Supplies]

If not, just go to another vendor because there's little you can or should do if the supply falls short in any of these areas. Even if you could do something, you don't have a way to qualify the fix and get certification, so your engineering effort would be wasted.

Nearly all supplies, such as the XL375 series from N2Power, incorporate standard protection features such as over-voltage protection (OVP), output current limiting and shutdown (crowbar), and thermal shutdown. This supply offers shut down on command, loss of input power, or whenever excessive loads or temperatures are sensed, and provides the host system with warning of an impending shutdown, to enable it to perform housekeeping before power is lost.

Finally, check the ability of the supply to handle line transients. All supplies can do this to some extent—there is an IEC transient-test waveform, of course. But your application may see transients that are greater than that, due to nasty nearby loads (relays, motors), lightning, and other factors. You may want to add additional transient protection via a low-cost passive component on your AC mains, ahead of the supply's input.

Conclusion

Today's AC/DC supplies are remarkably efficient, compact, versatile, and rugged, while incorporating complex control strategies, algorithms and protection. At the same time, they meet a wide variety of performance and safety standards. First decide what supplies you need and where—both functionally and physically—you'll be ready to take the next step of finding a vendor and specific models that meet your needs and your cost targets, as well.

About the author
Don Knowles joined N2Power as VP Engineering 12 years ago after more than two decades' experience in power electronics design and manufacturing spanning industrial, ICT and medical electronics sectors. Prior to joining N2Power, he ran his own power electronics business for 20 years, designing power supplies and high-power AC and DC loads, and working with contract manufacturers. Don holds a degree in Electronics from American River College, Sacramento, California, USA.

To download the PDF version of this article, click here.


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