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Resolving trade offs

Developments in switch mode technology are resolving the long standing trade off between efficiency and emc. By Andy Skinner.

Whatever the application, power supply users want products that are small and efficient, yet have excellent emc performance. Until recently, it was difficult for designers to reconcile these requirements, but new developments in switch mode technology now are providing solutions.
In recent years, switch mode power supplies have almost completely displaced traditional linear supplies in the majority of applications. The reasons are not hard to find: for a given rating, switch mode supplies are much smaller and lighter than their linear counterparts; and they are much less costly to manufacture.
Switch mode supplies clearly have a lot to offer, but they do have one potential shortcoming. Because of the way they work, they are capable of generating radio frequency interference (rfi) or, to put this in a more commonly used term – they have poor emc performance.
To understand why this is and what can be done about it, it’s necessary to take a brief look at how switch mode supplies work. In essence, they’re not very complicated; they take power from the mains then rectify and smooth it to produce a high voltage dc supply of, typically, around 340V.
This dc supply is applied to an inverter stage which converts it back to ac, but at a much higher frequency than the mains supply. The exact frequency used depends on the design of the power supply, but it is almost always more than 20kHz. The high frequency ac from the inverter is then applied to a transformer which, because of the high frequency, can be very small, light and inexpensive. Finally, the output from the transformer is rectified to produce the output from the power supply itself.
There are, of course, other facets of the power supply, including some form of feedback arrangement from the output to the inverter stage to ensure that the output voltage remains constant even if operating conditions vary. The major sections described, however, are enough to explain the potential trade off between efficiency and emc performance.

Andy Skinner

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7812\Resolving Trade Offs.pdf

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