[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.
The Wall Adapter Approach Figure 4 shows a block diagram of a wall-adapter-powered DSL modem power-supply design. A wall adapter converts wall power to an unregulated 9 V dc. With load ranges from 0% to 100% and input voltage tolerances of greater than ±10%, the 9-V output can have a variation of more than 6 to 15 V. Because the wall adapter is outside the modem and isolated, the 9-V input to the product doesn’t represent a safety issue and can be simply routed within the modem. The 9-V input then drives multiple power stages to supply the user voltages. Buck converters and linear regulators generate the lower voltages for the digital and analog circuits, while a flyback power supply feeds the telephony interface circuits.
The Offline Approach Figure 5 represents the block diagram of an offline switcher for powering the DSL modem. The 115 V ac is rectified and filtered to provide an unregulated dc voltage of 240 to nearly 400 V. The flyback converter primary FET switches this high voltage, which gets rectified into dc on the secondary side. The circuit senses the main regulated output voltage and uses feedback to the primary side to maintain regulation over input-line and output-load variations. The telephony output voltages are unregulated and will vary some with line/load, while the lower voltage secondaries use linear regulators. The power transformer and the feedback optocoupler provide the required isolation between the primary input and secondary outputs.
Care must be taken when designing the power transformer. Proper spacing must be maintained between primary and secondary windings to prevent arcing. Interwinding capacitances, improper grounding, and poor layouts will allow differential and common-mode currents to flow in the primary and/or secondaries and create noise voltages on the outputs—as well as put EMI back into the source voltage. The input filter must be designed to suppress these currents to meet agency approvals. The designer must also be careful to use the proper voltage clearances between the optocoupler leads and between the transformer primary and secondary leads on the printed-wiring board (PWB) itself, as well as between adjacent layers. The high voltage and isolation requirements present on the offline converter make the design more complicated than the wall-adapter power supply.
Consider The Tradeoffs Figure 6 shows the two approaches in approximately the same scale, making many differences quickly apparent. The wall adapter is large as its transformer must be designed for 50/60-Hz operation. But it’s generally located outside the product and will not affect the product size. The adapter is very aesthetically unpleasing because it can occupy more than one slot on a power strip, or will be hanging from a wall plug. As you look further downstream, though, it becomes clear why so many products use the adapter. The wiring from it to the product is simpler due to the little, or no, concern for safety. Also, the power supply in the product is simpler because it doesn’t need to supply safety isolation or significant EMI filtering.
As shown in the photo of the offline power supply (Fig. 6b), EMI filter components and clearances can represent nearly a third of the offline switcher board area. In addition, the offline power supply is another 20% to 30% larger thanks to an onboard transformer that must be designed to handle the full output power.
Table 2 compares the two approaches. The first comparison is physical size. Figure 6 demonstrates how the wall-adapter approach will result in the smallest impact to the modem size, with an advantage of at least 4 in.2. Substituting linear regulators for the buck power supplies could further reduce the design’s size. Component height also favors the wall-adapter approach, because the input EMI filter components and power transformer of the offline approach drive its height 0.2-in. taller. Overall power-supply weight favors the offline approach with its high-frequency transformer, versus the very-heavy line-frequency transformer of the wall adapter approach.
In addition, Table 2 illustrates the relative costs of the two approaches, including product cost and engineering development time. The offline approach holds a slight cost advantage in very high-volume applications, because development cost isn’t a significant portion of the total cost. Moreover, the offline inventory costs will drop because you’ll have to inventory a $0.25 line cord rather than a $2.00 wall adapter. But in lower-volume applications, the wall adapter has an advantage because it represents a simpler design with much less qualification costs. Amortizing agency approvals over small production runs increases the costs of the offline approach.
UL will take a much closer look at products with internalized high voltage versus those that isolate high voltage within an approved wall adapter. The additional safety concerns will lengthen the time-to-market, as the designer will have to ensure that the design is correct before qualifying it.
DSL modems are also sensitive to power-supply noise, and the offline approach will switch 400 V on the primary, increasing the likelihood of noise problems. These factors all raise the schedule risk of the offline approach, due to the fact that the PWB layout with the offline is more critical. Consequently, the offline approach will take a little more time to debug.
So when is a wall adapter an appropriate choice? It’s when production volumes are low, or when getting the product out quickly is key. Typically, it’s not an appropriate choice when production volumes are going to be high.
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