July 1, 1996

Engineers' Invention Reduces Power Consumed by Computers

Out of context, a 90 percent efficiency rating might sound pretty good to most people. To a couple of North Carolina State University researchers, however, that figure meant 10 percent of the energy used by personal computers is being wasted--quite a lot of energy considering the number of computers in homes and offices the world over.

Several years ago, Dr. B. Jayant Baliga, professor of electrical engineering and director of the Power Semiconductor Research Center at NC State, and his student Manoj Mehrotra, who recently received his doctoral degree, set out to solve the problem. In late 1994, the two received a patent on their Trench MOS Barrier Schottky Rectifier (TMBS), a semiconductor component that can significantly increase the efficiency of power supplies.

Computers and other electronic appliances use integrated circuits such as microprocessors and memory chips to process information. The efficiency of the power supply that converts power from a 120V AC socket to the lower DC voltage required by the circuits determines the efficiency of the appliance.

While converting the 120V line to the power supply, a loss of efficiency occurs in a device called a Schottky rectifier, which drops voltage as it carries current. According to Baliga, this voltage drop yields an efficiency of 90 percent for a 5V supply, 85 percent for a 3.3V supply like that of the new Pentium chip, and a predicted efficiency of only 67 percent for the 1.5V supplies currently being designed and engineered.

In the case of the 1.5V chips, about one-third of the power going into the computer will be thrown away as heat, Baliga said. Consumers, then, pay more for electricity than is necessary, and homes and offices are affected by the extra heat generated.

The TMBS rectifier is more efficient because less voltage is dropped as it carries a current. Approximately 60 percent less heat--and therefore wasted energy--will be generated with this device.

"Let's say we have 100 watts of power. If we had an efficiency of 90 percent, then we are wasting 10 watts of power. With the new devices we can get it up to 96 percent and only lose 4 watts of power," Baliga said. "For the chips that require 3.3V supplies, you'd be wasting 15 watts. With our device you'd get 94 percent and only 6 watts of wasted power. Moving on to the future 1.5V chips--we would be wasting 33 watts. Our device would give an efficiency of 87 percent with only 13 watts wasted."

The reduction in electricity requirements will result in lower bills for consumers and a lower energy demand on utility companies, which could mean fewer generating stations and less pollution.

The device will also yield a smaller power supply in the computer itself. Because the power supply fills a large space in the unit, the computer will be smaller and lighter, thus shrinking the amount of desk space required for the computer.

Baliga said that computers are just one example of the invention's application. The device could extend the life of batteries in portable or cellular phones, and it has potential use in automotive electronics.

Baliga and Mehrotra achieved their results by performing a simple modification to the Schottky rectifiers currently used in power supplies. The cost of the two devices is very similar, making the TMBS rectifier commercially viable.

"The TMBS invention is significant owing to the fact that Schottky diodes are the most basic electronic devices, and for Dr. Baliga and his student to create a substantially new device concept from a basic device is indeed dramatic," said Ali Salih, design manager for Schottky and Gas Devices with the Motorola Power Products Division in Phoenix, Ariz. "They have creatively combined the Schottky and MOS technologies to provide devices with low forward and reverse conduction losses for efficient power conversion, which is critically needed in modern electronic equipment."

The invention is scientifically important as well, Baliga said, because it demonstrates a breakthrough in fundamental physics by supporting voltages about three times larger than predicted on the basis of accepted theoretical analyses. These standards indicate that a semiconductor requires a certain doping concentration (adding impurities to achieve a desired quality) to support voltage.

"When we invented this device we found a way to support the same voltage at a doping concentration tenfold higher," Baliga said. "With a higher doping, the conductivity of the semiconductor materials goes up proportionately and the resistance goes down proportionately, and that allows us to attain the very low voltage drop."

The patent for the TMBS device is one of 32 for NC State in Baliga's name.

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