Power Electronics

New Standards Shine Spotlight on External Power Supplies

External power supplies are an indispensable part of the consumer electronics industry. They power our cell phones, MP3 players, modems, cordless phones and many other products. However, all but the savviest consumers take them for granted, oblivious to the differences between power-supply technologies. How many consumers have ever refused to buy a product because they didn't like the power supply?

This indifference largely reflects the mindset of OEMs, who often treat power supplies as an afterthought. Even OEMs that do use compact, energy-efficient power supplies rarely market this feature to consumers. It's a chicken-and-egg problem: Consumers don't demand better power supplies because they are unaware of their benefits, and OEMs don't stress the benefits because there is no demand. As a result, a large percentage of external power supplies, particularly in low-power applications, still use century-old technology for power conversion.

This status quo was shaken up in December 2004 when the California Energy Commission (CEC) issued regulations governing the energy consumption of external power supplies. The standards are similar to the new Energy Star guidelines and the recently tightened specifications in the EU Code of Conduct. However, there is one important difference: the California standards are mandatory. Beginning in July 2006, all external power supplies manufactured for sale in California are required to meet stringent thresholds for operating efficiency and no-load consumption.

Due to their inherent inefficiency, linear transformers (“energy vampires”) generally fail to comply with the CEC's rules. Given California's size, as well as the fact that other U.S. states and other countries are moving to adopt similar standards, it seems inevitable that linear transformers will give way to energy-efficient switched-mode power supplies.

This trend will have consequences throughout the supply chain. Consumers themselves will see a measurable benefit in the form of lower energy bills, and perhaps even begin to appreciate the convenience of smaller, lighter power supplies. For suppliers of power supplies and components, the regulations represent an opportunity to gain market share at the expense of linear transformers and noncompliant discrete designs.

For OEMs, the impact is less clear. To be sure, the transition to energy-efficient power supplies need not be costly. Some switchers now can deliver a bill of materials at parity with or lower than that of linear transformers, even at low power levels.

However, when it comes to safety and reliability, not all switchers are created equal, and OEMs will want to choose carefully. For all their faults, linear transformers have good safety characteristics, especially in the area of thermal shutdown protection. Thermal shutdown is essential to guard against malfunction in overload situations and in abnormally high temperatures, such as when a power supply is exposed to direct sunlight or operated in an enclosed space.

Fortunately, thermal shutdown is easily implemented in switchers with the use of integrated circuits. In fact, ICs can improve upon the mechanical thermal fuses used in linear transformers. Once a mechanical fuse has blown, the entire adapter must be replaced. Advanced switchers feature hysteretic thermal shutdown, which automatically restarts the power supply when the temperature returns to a safe level. (Hysteretic shutdown should not be confused with the less-effective latching shutdown features commonly offered in switchers lacking control circuitry.)

OEMs and consumers will appreciate the capability of switchers to operate across the entire range of input voltages (85 Vac to 265 Vac), a feature not offered by linear transformers. While this capability is inherent to switched-mode technology, switchers incorporating the most advanced ICs offer the additional benefit of reliable operation in abnormally high and low input voltages. These over/undervoltage protection features, combined with hysteretic thermal shutdown and a low bill of materials, ensure that manufacturers can meet the new regulations cost-effectively without compromising safety and reliability.

Balu Balakrishnan joined Power Integrations in 1989, serving in a variety of engineering, marketing and business management roles before being named president and COO in April 2001. He was named CEO and appointed to the company's board of directors in January 2002. Balakrishnan is the chief inventor of the company's innovative TOPSwitch and TinySwitch technologies and holds more than 75 patents. He holds an MSEE degree from the University of California, Los Angeles, and a BSEE degree from Bangalore University, India.

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