Piezoelectric Ceramic Transformer With Specific Width To Length Ratios

Abstract

A piezoelectric ceramic transformer of the plate type having a total length 2L and width W is driven into fundamental mode, longitudinal vibration by an A.C. input, the wave length of which is .lambda., that is, 2L = .lambda./2. When the ratio W/L falls within the range of 0.2 to 1.2 a high voltage output and a high input-to-output power efficiency are attained. Alternatively, the piezoelectric ceramic transformer referred to above is driven into second, higher harmonic mode longitudinal vibrations where 2L = .lambda.. When the ratio W/L falls within the range of 0.05 to 0.85 the high performance referred to above is also attained.

Description

BACKGROUND OF THE INVENTION

This invention relates to an improved piezoelectric ceramic transformer for high voltage generation and, more particularly, to a structure for the above type transformer which gives a large voltage step-up ratio, hitherto unattained in such transformers.

It is an object of the present invention to provide a piezoelectric ceramic transformer having a width-to-length ratio such that, when driven into fundamental mode longitudinal vibration, the transformer gives a high voltage output and power efficiency.

It is another object of the present invention to provide a piezoelectric ceramic transformer having a width-to-length ratio such that, when driven into second harmonic mode longitudinal vibration, the transformer gives a high voltage output and power efficiency.

It is an additional object of the present invention to provide a novel connection for a piezoelectric ceramic transformer, which relates to the direction of polarization and the ground side of an output circuit of the transformer and gives a higher voltage output than the other connections give.

SUMMARY OF THE INVENTION

In accordance with the present invention, a piezoelectric ceramic transformer of the plate type having a width W and a total length 2L is driven into a fundamental mode, longitudinal vibration under the condition that the width-to-length ratio W/L falls within the range of 0.2 to 1.2.

In accordance with a second feature of the present invention, the above transformer is driven into a second, higher harmonic mode of longitudinal vibration, under the condition that the width-to-length ratio W/L falls within the range of 0.05 to 0.85.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a piezoelectric, ceramic transformer with electrical connections thereto;

FIG. 2 is a view showing schematically two vibration modes of the ceramic transformer wherein a solid curve a depicts the magnitude of displacement of the lengthwise vibration in the fundamental mode (.lambda./2 mode) and a dotted curve b depicts the magnitude of displacement in a second, higher harmonic mode (.lambda.mode);

FIG. 3 is a graph showing a voltage step-up ratio versus the ratio between the width and the length of the ceramic transformer, in fundamental mode vibration;

FIG. 4 is a graph showing the voltage step-up ratio versus the ratio between the width and the length of the ceramic transformer in a second, higher harmonic mode vibration;

FIGS. 5A and 5B are diagrammatic side views diagrammatic of the ceramic transformer showing the relationship between the polarization direction and the electrode connection where FIG. 5A relates to a conventional connection system and FIG. 5B to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, showing the general structure of a piezoelectric ceramic transformer PCT, it is well known in the art that a transformer function can be produced by plating electrodes E.sub.IN, E.sub.IN ' and E.sub.OUT on a plate of ferro-dielectric material and by effecting a polarization process in two directions, perpendicular to each other, as shown by the arrows P. Such ceramic transformers, manufactured as above described, have a high degree of frequency selectivity, and may seem to be a constant voltage source for a high resistance load and a constant current source for a low resistance load.

When operated as a constant voltage source, the voltage step-up ratio G.infin. of the ceramic transformer with no load across output terminals may be given by:

where,

Q.sub.m is indicative of the degrees of mechanical vibration, K.sub.31, K.sub.33 are electro-mechanical coupling factors in the direction perpendicular to the polarization direction and in the same direction, respectively,

L is the length of the sample,

and T is the thickness of the sample.

It is known in the art that the dimension of a ceramic plate transformer affects its step-up ratio and that the step-up ratio is proportional to the length, and inversely proportional to the thickness, of the plate, as seen from the above equation (1), but heretofore qualitative information relative to the width of the plate was not known.

In driving a ceramic transformer, there are principally two modes of lengthwise vibration, as shown schematically in FIG. 2, which provide useful voltage step-up ratios. In this drawing the curve a illustrates the magnitude of displacement in fundamental mode vibration, in which the length 2L of the transformer is equal to .lambda./2, ".lambda." being the wave length of the A.C. input voltage, and the curve b illustrates the magnitude of displacement in second, higher harmonic mode vibrations in which 2L = .lambda..

The data plotted in FIG. 3 was obtained by connecting, as shown in FIG. 1, an A.C. voltage source OSC across electrodes E.sub.IN, E.sub.IN ' and an output circuit, consisting of a voltage doubler rectifier, comprising diodes D.sub.1 and D.sub.2, and a load resistor R.sub.L, across the output electrode E.sub.OUT and the ground electrode E.sub.IN '.

The frequency of the input voltage was selected so as to drive the transformer into fundamental mode longitudinal vibrations.

In FIG. 3, the ordinate represents the voltage step-up ratio (V.sub.2 /V.sub.1) between the input voltage V.sub.1 (r.m.s. volts) and the output voltage V.sub.2 (r.m.s. volts) after rectification, and the abscissa represents the width-to-length ratio (W/L) between the width W and half of the total length 2L of the test piezoelectric ceramic transformer PCT.

As will be seen from the graph of FIG. 3, the step-up ratio, without load, is as high as 300, or more, for a W/L range of 0.2 to 1.2, but with a W/L ratio falling without the above range, no substantial amount of lengthwise vibration in the longitudinal direction is produced and the step-ratio is small, due to the influence of other mode vibrations.

Accordingly, when driven in fundamental mode with a W/L range of 0.2 to 1.2, the step-up ratio, with no load at the output, becomes 300 or more, and the step-up ratio with a load resistor of 100 M.OMEGA. for example, becomes 100, or more. It will be noted that with L = 5 - 80 mm and T/L = 0.02 - 0.4, the maximum output voltage was 30 KV.

Similarly, FIG. 4 illustrates the relationship between the voltage step-up ratio and the width-to-length ratio when the ceramic transformer was driven in a second higher harmonic mode. As will be seen from the graph of FIG. 4, the step-up ratio with no load is as high as 300 or more for a W/L range of from 0.05 to 0.85.

Accordingly, when driven in second, higher harmonic mode, with a W/L range of 0.05 to 0.85, the step-up ratio, with no load, is 300 or more, and the step-up ratio with a load resistor of 100 M.OMEGA., for example, is 100, or more. It will be noted that with L = 5 - 80 mm, and T/L = 0.02 - 0.4, the maximum output voltage is 30 KV.

It is also found that within the foregoing W/L ranges, in either vibration mode, the input-to-output power efficiency is also high.

Piezo-ceramic material develops an effective piezoelectric performance only when a high D.C. voltage (about 2 KV/mm) is applied thereto. The polarization appears in the directions shown by the arrows in FIGS. 5A and 5B where, in FIG. 5A which shows a conventional connection, the downward short arrow on the left represents the fact that a positive potential was applied to the upper electrode E.sub.IN when the polarization process took place. Thus, the lower electrode E.sub.IN ' which was supplied with a negative potential at the time of polarization is connected to the ground side of the output circuitry.

In accordance with the present invention, however, the lower input electrode E.sub.IN, in FIG. 5B, which shows the connection according to the present invention, and which was supplied with a positive potential when the polarization process took place, is connected to the ground side of the output circuitry, and the direction of polarization is upward.

Table 1 below shows the voltage step-up ratio (V.sub.2 /V.sub.1) obtained using the conventional connection shown in FIG. 5A the ratio (V.sub.2 '/V.sub.1 ') and obtained using the present connection, as shown in FIG. 5B.

TABLE 1

Dimension: L = 28 mm T = 3.5 mm W = 15 mm Load: 100 M.OMEGA.

step-up ratio Sample (b)/(a) No. (a) V.sub.2 /V.sub.1 (b) V.sub.2 '/V.sub.1 ' 1 270 283 1.05 2 283 300 1.06 3 277 290 1.05 4 297 305 1.03 5 270 290 1.07 6 277 290 1.05 7 303 330 1.09 8 273 295 1.08 9 300 320 1.07 10 297 307 1.03

as seen from Table 1, the step-up ratio of a transformer using the electrical connection according to the present invention is higher than that of a conventional connection by from 3 to 9 percent. However, no difference between the present connection and a conventional connection was noted with respect to other characteristics, such as resonance frequency, output voltage regulation, self heating, and the like.

A piezoelectric ceramic transformer, in accordance with the present invention, has a specific width-to-length ratio (W/L), which results in a high voltage output heretofore unattainable in the art. Further, the transformer is very reliable with respect to its insulation characteristics and is noncombustible, so that the present invention is very effective for practical use in high voltage and power generation.

Claims

What is claimed is:

1. A piezo-electric ceramic transformer of generally rectangular, parallelopiped configuration, said transformer having a longitudinal length of 2L, a thickness T and a width W, which comprises:

1. a first region electrically polarized in a thickness-wise direction;

2. a second region electrically polarized in the longitudinal direction;

3. a pair of input electrodes applied to opposite surfaces of said first region for receiving an energizing A.C. voltage from an external source;

4. an output electrode applied to an end face of said second region, one of the electrodes associated with said first region functioning as a common electrode for said output electrode, wherein

5. for applications where said transformer is driven into fundamental mode longitudinal vibrations by said source, the improvement comprises adjusting the dimensions of the transformer so that the width to length ratio W/L falls within the range of from 0.2 to 1.2.

2. The piezo-electric ceramic transformer according to claim 1, wherein the one of said pair of input electrodes connected to a source of positive potential, with respect to the other one of said pair of electrodes, is connected to the ground side of the output circuitry connected to said output electrode as the common electrode therefor.

3. A piezo-electric ceramic transformer of generally rectangular, parallelopiped configuration, said transformer having a longitudinal length of 2L, a thickness T and a width W, which comprises:

1. a first region electrically polarized in a thickness-wise direction;

2. a second region electrically polarized in the longitudinal direction;

3. a pair of input electrodes applied to opposite surfaces of said first region for receiving an energizing A.C. voltage from an external source;

4. an output electrode applied to an end face of said second region, one of the electrodes associated with said first region functioning as a common electrode for said output electrode, wherein

5. for applications where said transformer is driven into second harmonic longitudinal vibrations by said source, the improvement comprises adjusting the dimensions of the transformer such that the width to length ratio W/L falls within the range of from 0.05 to 0.85.

4. The piezoelectric ceramic transformer according to claim 3 wherein the one of said pair of input electrodes connected to a source of positive potential, with respect to the other one of said pair of electrodes, is connected to the ground side of the output circuitry connected to said output electrode as the common electrode therefor.