Universal high-power AC inputs: Bane or boon?

Article By : Robert Taylor

Designing a power supply that works over the "universal input range" can be useful for certain applications that are taken to different regions, such as cell phone charger or a laptop supply. But there is also a catch.

Alternating current (AC) power is present throughout almost the entire world and provides a convenient power source for many electronics. Unfortunately, the power around the world is not the same in each location. There are different voltage levels and frequencies, depending on where in the world you may be, but is there a better way to design this power supply?
Most of the reliable power sources (including tolerance) can be broken up into two voltage categories: low line or high line. Modern day electronics usually operate on direct current (DC) versus AC. This difference between currents requires a power supply to make the energy usable; the input range that the power supply needs to support will directly affect the cost and performance.
Designing a power supply that works over the "universal input range" (85-265 VAC) can be useful for certain applications that are taken to different regions, such as cell phone charger or a laptop supply. Many manufacturers would like the convenience of having a universal input supply so they can reduce the number of product skews and components for procurement purposes; however, a price savings can be demonstrated for having separate supplies.
Another argument for having separate products involves regulations in each region. Much like differing voltage levels, each region has its own set of regulatory specifications that have to be tested and certified—to have a product that will only be used in the United States certified for use in Europe does not make much sense. Add to this that some regions such as Europe require power factor correction (PFC), which is not required in other regions. Additionally, the cost savings can be increased for separate supplies.
As an example, consider a 250 Watt power supply that does not require PFC. Each of these power supplies has a unique configuration that allows it to operate over its specified range.

*Figure 1: Universal input power supply.*

*Figure 2: Low-line only input power supply.*

*Figure 3: High-line only input power supply.*

Power supplies that operate at low line need to carry more current than those that are high line only. The supplies designed for high-line only need to have higher voltage components.

*Figure 4: Power supply with a voltage range switch.*

The universal input design has to accommodate the worst of both separate supplies; it needs to be able to handle the higher current of low-line operation and support the high-voltage requirements of high-line operation. The configuration that uses the voltage selection switch has some benefits (lower voltage caps, lower current requirements), but comes with the risk that the user will not put the switch in the right state. It also requires twice the capacitance of the low-line only configuration.
Input capacitors and main power switches change in the design, based on the input range and drive the cost difference. Other components can benefit from the narrowed range as well, such as the transformer, diode bridge or electromagnetic interference (EMI) filter, but these will mainly affect the performance. The design's power level also has a major effect on the cost difference. For a 5-10 W adapter that charges a cell phone, there is not going to be a noticeable difference. However, for a 250 W audio amplifier, the cost difference can be significant.

Trade-offs between the four supplies.

If the power level is increased to 700 W for a motor driver, the difference is even more significant. Also, if we consider a PFC circuit for use in Europe, then making one universal supply will be a failure. The cost for PFC increases if it also needs to operate over the full line range.

Results for a 700 W full-bridge design. Note, if PFC is required for high-line use, the switch configuration is not possible.

The evidence is clear, if a product is not destined to be used in multiple regions throughout its life, it makes sense to use separate designs to cover each region. There is still a need for universal input power supplies, but the designer needs to make a decision based on the power level and portability to determine if it makes sense. The requirement for PFC and the certifications required in different regions can also make the decision very simple.

About the author
Robert Taylor is an applications manager in the Power Design Services group and a technical staff member at Texas Instruments. Power Design Services is responsible for providing high-volume customers with power reference designs. Taylor received his bachelor's and master's degrees in electrical engineering from the University of Florida.