[Design View / Design Solution]
When Should Your Design Use A Wall Adapter For AC Input Power?
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.
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.
HIGHLIGHTS:
The AC-DC Wall Adapter
A wall adapter's function is to step down the raw 115/230-V ac line voltage into a safer, lower dc voltage that either the end-use equipment or another power-supply input can readily accept.
The Wall-Adapter Approach
A wall adapter converts wall power to an unregulated 9 V dc. Because the adapter is outside the modem and isolated, the 9-V input to the product doesn't represent a safety issue. It can be simply routed within the modem. The 9-V input then drives multiple power stages to supply the user voltages.
The Offline Approach
In an offline switcher, the 115 V ac is rectified and filtered to provide an unregulated dc voltage of 240 to nearly 400 V. When using this approach, power-transformer design is crucial. Proper spacing must be maintained between windings to prevent arcing.
Considering The Tradeoffs
Many differences become apparent when comparing the wall adapter versus offline in approximately the same scale. Size, weight, cost, time-to-market, and risk are some of the parameters discussed as well as illustrated in Table 2 in the online article, "Offline Approach Is Cheaper, But Higher Risk."
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