Piezo transformers could shrink power supplies

The company's research based on the fact that piezo crystals can work in two ways: Stimulated by an electric signal, they change their shape in that they bend or vary their thickness. In the reverse direction, the effect works as well; if a piezo crystal is bent or deformed it generates an electric voltage at the electrodes.

Epcos researchers used both effects in a single piezo crystal. On one end, an AC voltage is fed into the crystal, exciting the crystal at its resonance frequency which can be several hundred kilohertz. At the other end of the crystal, the mechanical oscillations are transformed into electrical voltages which can be used to energize a circuit. Like in a normal transformer, the output voltage can be higher or lower than the input voltage, depending on the specific implementation.

The main advantage of a piezo transformer over a conventional one is its size, which is significantly smaller than the size of a coil-and-iron-based transformer with a comparable output power, explained Johannes Lehrhofer, Product Marketing Manager for Epcos' Piezo Commodities business. In addition, the piezo transformer can be mass-produced more easily, albeit in a relatively complex process. And the simple, ultra flat shape of a piezo transformer allows it to be integrated into miniaturized electronic devices more easily.

The list of the drawbacks is rather short: Since the effect is based on the device's resonance frequency, the piezo transformer cannot be used in the signal path of a circuit, as long as the signal has a certain bandwidth. Thus, it cannot be used to match impedance in audio amplifiers, Lehrhofer said. "We go clearly after power applications", he explained.

At resonance frequency, the power efficiency of the novel part is even better than a standard transformer's efficiency, ranging at 95 percent or better, claims Epcos. Power density varies between 10 and 50 Watts per cubic centimeter depending on design and power. For a comparison: in standard electromagnetic transformers, power density can be achieved in the range between 10 and 30 Watts per cubic centimeter. Presently, piezo transformers could be implemented with an output power of up to approximately 50 watts.

The challenge for piezo transformer designis the packaging: While it must provide good thermal transfer, it also must not damp vibrations. "This is quite tricky", Lehrhofer said. "One needs to bring the mounting to the nodal points of mechanical displacement."

For the applications to be implemented with piezo transformers, Epcos puts the emphasis on low power applications. For instance, the component could be used in DC-DC converters for mobile phone displays with output voltages ranging from 15 to 300 V at .1 to 1 watt output power. Other applications imaginable are LED power supplies for lighting purposes with a power range of up to 5 watts, backlight inverters for LC displays with output voltages of 2,5 kilovolts and about 10 watts or ignition of small combustion engines (for instance, lawn mowers) with voltages of up to 16 kilovolts with 10 watts.

Product availability, however is a delicate topic for Epcos. Since the development under way apparently was done by order of customers, Lehrhofer declined to provide any statement as to when piezo transformers will be available at the market. "I only can say that we are beyond the stage of research", he said.