Circular Ceramic Devices Make More Efficient Transformers

"Conventional electromechanical transformers are big, very heavy and produce a great deal of magnetic noise that, without shielding, can wipe floppy disks, tapes and hard drives," says Dr. Kenji Uchino, professor of electrical engineering and member of Penn State's Materials Research Laboratory. "Rectangular piezoelectric ceramic transformers have been commercially in use since 1994 especially for computer notebook applications. Our circular configuration, made with improved materials, advances this field and opens the way for more powerful, light-weight transformers."

Electromagnetic transformers consist of two coils of wire that convert high voltages to lower ones or low voltages to higher ones. The size of the step-up or step-down depends on the difference between the number of turns of wire in each coil. Large electromagnetic transformers are very efficient, but miniaturization decreases the efficiency.

Uchino, Burhanettin Koc, postdoctoral associate in electrical engineering, and Yongkang Gao, graduate student in materials science and engineering, worked on disk-shaped transformers made of an enhanced piezoelectric material consisting of a lead, zirconium, titanium ceramic doped with manganese and cerium. The researchers reported their research today (Dec. 1) at the annual meeting of the Materials Research Society in Boston.

"The screen uses most of the energy consumed by a notebook computer," says Uchino. "The liquid crystal display uses fluorescent lights to backlight the screen and these lights take high voltage to turn on."

Most portable computers run on 12-volt batteries, but it takes 500 volts to turn on the screen backlight, while it takes only 250 to 300 volts to continue their operation. Uchino collaborated with NEC Corporation in Japan in developing the rectangular piezoelectric transformers currently in use in their notebooks, but the circular wafers promise increased efficiency and higher conversion ratios.

In the circular wafer, about one-third of the wafer is used for a crescent shaped input electrode with the remaining two-thirds as output. The disk transformer provides a voltage step up ratio of about 60 rather than the rectangular rate of just over 40 because it uses more than one vibration mode. The circular wafers take up the same volume and weigh the same as rectangular transformers.

Piezoelectric material moves when under an electric current, and, when displaced by outside pressure, these materials produce an electric current.

Transformers are made from piezoelectric materials by applying a chopped electric charge to one side of a piezoelectric wafer. This on and off charge creates a vibration in the material, which is converted to an ac current on the other side of the wafer. Piezoelectric ceramics are manufactured using conventional sintering methods, and electrodes can be screened onto the wafers.

"Piezoelectric transformers are not only more efficient, smaller and lighter, but they are also much less expensive to manufacture than conventional coil wound transformers," says Uchino.

He warns, however, that piezoelectric transformers are not necessarily the answer to all power problems. For one thing, in any transformer, as the voltage is increased, the current decreases, so transformers are only applicable where current is unimportant. Also, piezoelectric transformers have only been tried in a few applications, and there may be unforeseen problems with their use. The researchers are, however, looking into developing piezoelectric transformers for use on the battlefield to replace the enormous transformers needed to operate x-ray equipment. These machines can require as much as 10 kilovolts, but not much current. In these applications, 20 or more disk transformers might be stacked and still take up only a fraction of the weight and volume of a conventional transformer. In the other direction, the researchers are looking at step-down applications for audio amplification.