DESIGN VIEW is the summary of the complete DESIGN SOLUTION contributed article, which begins on Page 2.
Wall adapters get the nod when production volumes are low or getting the product out quickly is key. But they're not a wise choice for high-volume products. DSL modems are a perfect example to illustrate this disparity. As a consumer product, cost is a sensitive issue in modem design, and that ripples down into the power-supply-architecture selection.
Designers face two popular choices. The first is a 50/60-Hz transformer, rectifier, and filter that generates a low dc voltage, which is converted to well-regulated outputs. In the second, ac input power is rectified and filtered, and a high-frequency switcher converts the resulting high-voltage dc to regulated voltages for the DSL electronics. The second approach is generally cheaper in high-volume applications, but it significantly complicates the modem design.
Modems are generally required to run from an ac wall power supply that has a wide voltage and frequency range. A table provided with the online article presents typical VoIP DSL modem power-supply requirements. As with many modern electronic systems, a number of low voltages power various analog and digital functions, while two higher negative voltages will power a telephony interface.
For instance, a −24-V output provides power for the loop current while the phone is in use. A −72-V output powers the phone's ringing circuitry.
The article presents the cases for the wall-adapter and offline approaches and tackles the pros and cons of each. Adapters are simpler, smaller, and virtually free from safety concerns. Offline power supplies are lighter in weight and less expensive. However, going offline will also increase schedule risk, due to added debugging time resulting from potential noise issues.
Full article begins on Page 2