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.
|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."|
Full article begins on Page 2
A DSL modem is a prime example of the need for a tradeoff study between using a wall adapter versus an offline power supply. DSL modem usage in North America sees an annual growth rate of nearly 50%. Its worldwide growth rate at the end of 2002 was nearly 100%. Presently, there are 36 million worldwide subscribers, with the vast majority being residential. As a consumer product, cost is a very sensitive issue in the design of these modems, which ripples down into the power-supply-architecture selection. This issue also applies to many other power-supply applications.
Basically, designers are faced with two popular choices. In the first, a 50/60-Hz transformer, rectifier, and filter generate a low dc voltage that’s 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. Although the second approach is generally cheaper in high-volume applications, it significantly complicates the modem design. The power supply is typically implemented on the same circuit card as the remainder of the electronics, and the high dc voltage brings issues of agency approvals, noise, and size.
Table 1 presents typical VoIP DSL modem power-supply requirements. Modems are generally required to run from an ac wall power source that has a wide voltage and frequency range. As with many modern electronic systems, a number of low voltages power various analog and digital functions. In addition, two higher negative voltages power a telephony interface. The —24 V output provides power for the loop current while the telephone is in use. A —72 V output powers the phone ringing circuitry.
Compared to the lower voltages, these outputs have widely varying load ranges from essentially no load, when the phone system is not in use, to full load on either output, depending on whether the line is in use or simply ringing. Efficiency is generally not a critical issue as long as the heat can be removed; consequently, low-cost linear regulators are widely used.
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 output-voltage tolerance that the equipment can operate over will determine if additional voltage regulation is required. Some circuits, such as battery chargers, may not require a tightly regulated input voltage, and an unregulated dc input voltage may work just fine. In this case, Figure 1 illustrates the simplest way to generate that voltage.
This circuit generates one output voltage, but often multiple well-regulated outputs are needed. The most common ways to generate these voltages are with switching regulators, linear regulators, or a combination of both. If the unregulated input voltage is higher than the output voltages, multiple buck converters and/or linear regulators often provide the best solution. Linear regulators would be used if the output current weren’t large, so that excessive power isn’t dissipated in the device.
In the situation where only a single regulated output is needed, one option is to place the switching converter inside the ac/dc wall adapter, making this the entire power supply. The other option is to add the switcher as part of the end-use circuit. Depending on the goals of the overall product, either choice may be valid. For example, if a smaller or lighter product is desired, a regulated wall adapter would be used. If aesthetics, integration, or heavy loading is an important goal, then placing the switcher with the end-use circuit would be the best solution.
Figure 2 shows the output voltage variation with an unregulated wall adapter. When loaded lightly, the output voltage is at its maximum because the output capacitor peak detects the transformer secondary. The capacitor stays fully charged during the entire line period due to low current draw. As the load increases, the dc output voltage starts to droop.
A large amount of primary winding resistance and leakage inductance is designed into the transformer to limit energy in a fault condition. A large portion of the leakage inductance is due to the separation between the primary and secondary windings required for the approval of Underwriters Labs (UL). This can be in the form of either a split bobbin with the primary and secondary windings on opposite halves of the core, or a large amount of insulating tape between the layer stacks. With increasing load current, a larger share of the transformer’s voltage drops across the winding resistance and leakage inductance, reducing the output voltage.
Because the output diodes only conduct when the secondary voltage on the transformer exceeds the voltage on the output capacitor, the output capacitor supplies the load current during a large portion of the line period. The larger the load becomes, the more voltage droop and output ripple there will be across the output capacitor because it must support the load entirely.
Eventually, as the load extends beyond its design limits, either the output diodes or the transformer windings will overheat and fail as open circuits, reducing the output to zero volts. This failure, when overloaded, doesn’t usually happen instantly. As Figure 3 shows, peak output powers of approximately 150% of maximum power can be obtained for a duration of several seconds. However, this peak power occurs at an output voltage that’s significantly lower than its specified nominal voltage rating.
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.
GCI Technologies, 1301 Precision Drive, Plano, TX 75074-8636; Tel: (972) 423-8411.
Kollman, Robert, and Betten, John, Power Supplies for Residential Telephony Systems, Texas Instruments Inc., 12500 TI Boulevard, Dallas, TX 75143
Global DSL Statistics Briefing - Year end 2002, DSL Forum and Point Topic