Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-12-15
2002-04-02
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S366000
Reexamination Certificate
active
06366006
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of Invention
The present invention relates generally to a voltage converter using multiple layers of piezoelectric ceramic. More specifically, the present invention relates to a multilayer piezoelectric transformer using resonant vibrations along the longitudinal axis for step-up voltage conversion applications. The input and output layers are bonded to each other in such a manner that the piezoelectric transformer operates at a lower frequency, with higher gain and power density than a piezoelectric transformer simply using thickness mode vibrations.
2. Description of the Prior Art
Wire wound-type electromagnetic transformers have been used for generating high voltage in internal power circuits of devices such as televisions or fluorescent lamp ballasts. Such electromagnetic transformers take the form of a conductor wound onto a core made of a magnetic substance. Because a large number of turns of the conductor are required to realize a high transformation ratio, electromagnetic transformers that are effective, yet at the same time compact in shape are extremely difficult to produce. Furthermore, in view of high frequency applications, the electromagnetic transformer has many disadvantages involving magnetic material of the electromagnetic transformer, such as sharp increase in hysteresis loss, eddy-current loss and conductor skin-effect loss. Those losses limit the practical frequency range of magnetic transformers to not above 500 kHz.
To remedy this and many other problems of the wire-wound transformer, piezoelectric ceramic transformers (or PTs) utilizing the piezoelectric effect have been provided in the prior art. In contrast to electromagnetic transformers, PTs have a sharp frequency characteristic of the output voltage to input voltage ratio, which has a peak at the resonant frequency. This resonant frequency depends on the material constants and dimensions of the materials of construction of the transformer including the piezoelectric ceramics and electrodes and the mode of operation of the transformer. Furthermore PTs have a number of advantages over general electromagnetic transformers. The size of PTs can be made much smaller than electromagnetic transformers of comparable transformation ratio, PTs can be made nonflammable, and produce no electromagnetically induced noise.
The ceramic body employed in PTs takes various forms and configurations, including rings, flat slabs and the like. Typical examples of a prior PTs are illustrated in
FIGS. 1 and 2
. This type of PT is commonly referred to as a “Rosen-type” piezoelectric transformer. The basic Rosen-type piezoelectric transformer was disclosed in U.S. Pat. No. 2,830,274 and numerous variations of this basic apparatus are well known in the prior art. The typical Rosen-type PT comprises a flat ceramic slab
20
appreciably longer than it is wide and substantially wider than it is thick. In the case of
FIG. 1
, the piezoelectric body
20
is in the form of a flat slab that is considerably wider than it is thick, and having greater length than width.
As shown in
FIG. 1
, a piezoelectric body
20
is employed having some portions polarized differently from others. A substantial portion of the slab
20
, the generator portion
22
to the right of the center of the slab is polarized longitudinally, and has a high impedance in the direction of polarization. The remainder of the slab, the vibrator portion
21
is polarized transversely to the plane of the face of the slab (in the thickness direction) and has a low impedance in the direction of polarization. In this case the vibrator portion
21
of the slab is actually divided into two portions. The first portion
24
of the vibrator portion
21
is polarized transversely in one direction, and the second portion
26
of the vibrator portion
21
is also polarized transversely but in the direction opposite to that of the polarization in the first portion
24
of the vibrator portion
21
.
In order that electrical voltages may be related to mechanical stress in the slab
20
, electrodes are provided. If desired, there may be a common electrode
28
, shown as grounded. For the primary connection and for relating voltages at opposite faces of the low impedance vibrator portion
21
of the slab
20
, there is an electrode
30
opposite the common electrode
28
. For relating voltages to stresses generated in the longitudinal direction in the high impedance generator portion
22
of the slab
20
, there is a secondary or high-voltage electrode
35
on the end of the slab for cooperating with the common electrode
28
. The electrode
35
is shown as connected to a terminal
34
of an output load
36
grounded at its opposite end.
In the arrangement illustrated in
FIG. 1
, a voltage applied between the electrodes
28
and
30
of the low impedance vibrator portion
21
is stepped up to a higher voltage between the electrodes
28
and
35
in the high impedance generator portion for supplying the load
36
at a much higher voltage than that applied between the electrodes
28
and
30
. The applied voltage causes a deformation of the slab through proportionate changes in the x-y and y-z surface areas. More specifically, the Rosen PT is operated by applying alternating voltage to the drive electrodes
28
and
30
, respectively. A longitudinal vibration is thereby excited in the low impedance vibrator portion
21
in the transverse effect mode (“d31 mode”). The transverse effect mode vibration in the low impedance vibrator portion
21
in turn excites a vibration in the high impedance generator portion
22
in a longitudinal effect longitudinal vibration mode (“g33 mode”). As the result, high voltage output is obtained between electrode
28
and
35
. On the other hand, for obtaining output of step-down voltage, as appreciated, the high impedance portion
22
undergoing longitudinal effect mode vibration may be used as the input and the low impedance portion
21
subjected to transverse effect mode vibration as the output.
The Rosen type PT has been found disadvantageous in that the only useable coupling coefficient is k31, which is associated with the very small transverse effect longitudinal vibration mode (“d31 mode”). This results in obtaining only a very small bandwidth. Conventional piezoelectric transformers like this operate only up to about 200 kHz.
Another inherent problem of such prior PTs is that they have relatively low power transmission capacity. This disadvantage of prior PTs relates to the fact that little or no mechanical advantage is realized between the vibrator portion
21
of the device and the driver portion
22
of the device, since each is intrinsically a portion of the same electroactive member. This inherently restricts the mechanical energy transmission capability of the device, which, in turn, inherently restricts the electrical power handling capacity of such devices.
Additionally, even under resonant conditions, because the piezoelectric voltage transmission function of Rosen-type PTs is accomplished by proportionate changes in the x-y and y-z surface areas (or, in certain embodiments, changes in the x-y and x′-y′ surface areas) of the piezoelectric member, which changes are of relatively low magnitude, the power handling capacity of prior circuits using such piezoelectric transformers is inherently low.
In addition, with the typical Rosen transformer, it is generally necessary to alternately apply positive and negative voltages across opposing faces of the vibrator portion
21
of the member in order to “push” and “pull”, respectively, the member into the desired shape.
Even under resonant conditions, prior electrical circuits that incorporate such prior PTs are relatively inefficient, because the energy required during the first half-cycle of operation to “push” the piezoelectric member into a first shape is largely lost (i.e. by generating heat) during the “pull” half-cycle of operation. This heat generation corresponds to a lowering of efficiency of the circuit, an increased fire hazard, and/or a re
Bolduc David J.
Clark Stephen E.
Dougherty Thomas M.
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