[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.
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.
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