Piezoelectric transformer

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A piezoelectric transformer (PT) is a design of a resonance transformer which is based on piezoelectricity and, in contrast to conventional magnetic transformers, represents an electromechanical system. It is used to convert a supplied electrical alternating voltage of a certain frequency , which is determined by the mechanical dimensions of the transformer, into a higher or lower alternating voltage. Areas of application are resonance converters for generating the high voltage for supplying the fluorescent tubes (CCFL), which were previously often used for background lighting in TFT monitors .

Piezoelectric transformers (PT) generate high electric fields via the piezoelectric effect. These fields are able to ionize gases and liquids through electrical excitation. On the secondary side of the PT, the alternating electric field generates strong polarization, excitation and ionization of atoms and molecules. This process creates a piezoelectrically ignited microplasma, PDD (Piezoelectric Direct Discharge Plasma). PDDs have properties that correspond to typical dielectric barrier discharges (DBD). PDDs can be ignited in a wide pressure range of 0.01 mbar and 2000 mbar.

General

Structure of a piezoelectric transformer with piezo crystal, equivalent circuit diagram below.

The first work on electromechanical energy conversion by means of piezoelectricity goes back to Charles A. Rosen in 1958, from which the name Rosen transformer for a design is derived and which is shown schematically in the adjacent figure. At that time, however, unsuitable materials, so-called ferroelectrics such as lead zirconate titanate (PZT), were not available for practical use. Piezoelectric transformers only achieved a certain economic importance in the 1990s in various niche applications such as backlighting of TFT displays in the field of resonance converters .

In the case of piezoelectric transformers, the alternating voltage supplied on the primary side is first converted into a mechanical vibration via the electrodes vapor-deposited on the crystal . The frequency is essentially dependent on the geometry and the mechanical structure. As a result, a mechanical wave forms within the ferroelectric, which generates an output voltage through the piezoelectric effect on the secondary-side electrode . Depending on the geometry of the crystal plate and the position of the electrodes on the crystal, this output voltage is higher or lower than the input voltage. A galvanic isolation is not possible with the Rose transformer.

Piezoelectric transformers are only suitable for transmitting small amounts of power. Typical power ranges are in the range of a few watts up to a few 10 W at resonance frequencies around a few 10 kHz to a few 100 kHz. It is comparatively easy to generate high, sinusoidal alternating voltages, such as those required for supplying fluorescent tubes . In conventional magnetic transformers, especially at high output voltages, the insulation of the windings represents a technological difficulty or a high cost factor, since electrical flashovers between the windings can easily occur in the event of excessive voltages . In the case of piezoelectric transformers, this insulation problem does not exist in principle, which means that cost-effective resonance converters for CCFL lighting can be manufactured with a long service life.

Piezoelectric transformer as a cold plasma source

Parasitic discharge phenomena on the PT are undesirable, but this effect can also be used in a targeted manner. With the concept of Piezoelectric Direct Discharge (PDD, Piezoelectric Direct Discharge), a plasma can be ignited directly. Similar to a silent electrical discharge (DBD: dielectric barrier discharge), if the oscillating field strengths are sufficiently high, a cold discharge occurs. Due to the high field inhomogeneity and the frequency influence, the surrounding gas can be ionized even under atmospheric conditions without the absolute ignition voltage having to be below the Paschen curve .

PTs of the Rosen type (Rosen type PT) are particularly suitable for generating PDD plasma (Piezoelectric Direct Discharge Plasma), since this type provides high power densities and very high transmission ratios. Transformation ratios of more than 1000 can be achieved in practice. Resonance frequencies between 10 kHz to 500 kHz are optimal for igniting PDD plasma. If the power driver is optimally adapted to the resonance and the impedance of the PT, the conversion into the discharge process takes place with a high degree of efficiency in the overall system. The operating behavior of the system under PDD conditions differs greatly from the electrical small-signal behavior of the system. At the threshold of ignition of the discharge, the damping of the PT increases, the coupled power increases and the resonance frequency shifts. To stabilize the PDD z. B. the frequency can be readjusted (frequency tracking).

Ozone generators based on PDD and operated with air deliver an average ozone concentration with the highest efficiency of the systems known to date. When operating with an He / Xe mixture, hard UV radiation VUV (peak at 172 nm, Xe * excimer ) is emitted. With PDD, the gas temperature in the plasma volume is typically at an ambient temperature of 300 + 20 K. Electron densities of approx. 10 14 and 10 16 m −3 are achieved. PDD thus delivers a typical “cold” non-equilibrium plasma. These properties of PDD open up a wide range of possible applications. PDD devices are used in medical research, for germ reduction, odor reduction and in microbiology. Typical industrial applications include surface activation to optimize wetting and adhesive properties in plastics, e.g. B. in printing, painting and gluing processes.

Individual evidence

  1. ↑ a b C. Kauczor, T. Schulte, H. Grotstollen: Piezoelectric transformers - circuits and applications . 47th International Scientific Colloquium, Technische Universität Ilmenau, September 23, 2002 ( online (PDF; 1.4 MB) [accessed October 4, 2010]).
  2. ↑ Comparing magnetic and piezoelectric transformer approaches in CCFL applications . Application Note Texas Instrument, 2005.
  3. ^ Charles A. Rosen: Electromechanical Transducer , U.S. Patent No. 2,830,274, 1958
  4. ↑ Piezo transformer element . Fuji & Co, queried on October 3, 2010.
  5. ^ S. Ben-Yaakov, M. Shvartsas, G. Ivensky: A Piezoelectric Cold Cathode Fluorescent Lamp Driver Operating from a 5 Volt Bus. In: Proceesings. of PCIM 2000. Nürnberg 2000, pp. 379-383.
  6. ↑ M. Teschke and J. Engemann, WO 2007 / 006298A3, PCT publication
  7. ↑ M. Teschke and J. Engemann, Contrib. Plasma Phys. 49, 614 (2009)
  8. ↑ H. Itoh, K. Teranishi and S. Suzuki, Plasma Sources, Sci. and Tech. 15 (2006) S51.