Abstract:
A piezoelectric power supply has a piezoelectric transformer circuit configuration which is alterable flexibly according to varying loading power requirements. It includes a control unit (control IC), a power switch (MOSFET) and an inductor to drive a piezoelectric transformer unit to actuate a high voltage and power load such as a cold cathode fluorescent lamp. The piezoelectric transformer unit consists of a plurality of piezoelectric transformers coupled in parallel. The specification of the piezoelectric transformers (single sheet piezoelectric transformers or low level laminated piezoelectric transformers) and parallel coupling number may be altered according to the required power of the load to change the voltage and current output by the piezoelectric transformers.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a piezoelectric power supply and particularly to a piezoelectric power supply that has a novel piezoelectric transformer circuit configuration which is alterable flexibly according to varying loading power. 
   BACKGROUND OF THE INVENTION 
   Ceramic piezoelectric transformer was first introduced by C. A. Rosen in 1956. Its operation principle is different from the magnetic transformer that transforms electromagnetic energy. The piezoelectric transformer first transforms electromotive force to mechanical energy (this process is called inverse piezoelectric effec), then transforms the mechanical energy to electric energy (this process is called positive piezoelectric effect). The ceramic transformer is a new type of electronic transformer made by sintering ferroelectric ceramic material and polarized under high pressure. Its basic operation frequency ranges from dozens KHz to hundreds KHz in the supersonic frequency zone. 
   The piezoelectric transformer makes fabricating high efficiency and small size converters possible. Compared with the piezoelectric transformer, the magnetic transformer of the same given power is thicker, heavier and has a lower efficiency. But its cost is lower, and can operate in a wide range of loading conditions. Hence for a long time the cold cathode fluorescent lamp (CCFL) that requires high actuating and ignition voltage is supported by the winding magnetic transformer with a high coil ratio. However, development of the piezoelectric transformer has great progresses in recent years. Now it can offer advantages such as higher efficiency, smaller size, lower electromagnetic noise, higher ignition voltage, non-flammable, and sinusoid operation, etc. For instance, R.O.C. patent publication No. 504101 discloses a “High luminosity fluorescent lamp driving apparatus” which employs the piezoelectric transformer. 
   The piezoelectric transformer includes a rectangular piezoelectric ceramic layer to perform energy transformation, a pair of primary electrodes (for input) and a pair of secondary electrodes (for output). Electric signals input to the primary electrodes are converted to mechanical vibrations through a piezoelectric fashion. The mechanical vibrations are transferred to the secondary ceramic layer to be converted to a high electric potential through the piezoelectric fashion to be output. The voltage gain of the piezoelectric transformer is the function of piezoelectric material coefficient g( ), primary layer number, material thickness and the entire length, as indicated in the equation below:
 
 V  (Voltage gain)=(Length×Layer No./Thickness) g ( )
 
   In terms of the hardware structure, the piezoelectric transformer has single sheet (or single layer) structure and laminated structure. The single sheet piezoelectric transformer may be fabricated easier at a lower cost, but has a lower voltage gain (typically 5–10), and might require a voltage boosting magnetic transformer to actuate a lamp set. On the other hand, the laminated piezoelectric transformer has a higher voltage gain (20–70) and can actuate a load of a greater power. For instance, R.O.C. patent publication No. 492204 entitled “High output laminated piezoelectric transformer” is such an example. But the design and fabrication of the laminated piezoelectric transformer is more difficult, and the production yield is lower. At present only a few well known manufacturers have the fabrication techniques for such products. Moreover, the laminated piezoelectric transformer to actuate CCFL requires different output powers depending on varying CCFL specifications of various vendors. The manufacturers have to customize the laminated piezoelectric transformer to meet customer&#39;s requirements. Hence fabrication cost is higher, and the specification is not very flexible. 
   SUMMARY OF THE INVENTION 
   The primary object of the present invention is to provide a piezoelectric power supply that offers a composite specification of the piezoelectric transformer that is alterable flexibly according to the loading power. 
   To achieve the foregoing object, the piezoelectric power supply according to the invention adopts a composite specification for the piezoelectric transformer. In one embodiment of the invention, a plurality of single sheet piezoelectric transformers or lower level laminated piezoelectric transformers with a few layers are coupled in parallel to provide a higher output power to meet the requirement of a higher power load. 
   Another object of the invention is to provide a piezoelectric power supply with a greater power output and flexible production and assembly to provide electric current required in selected loads such as CCFLs. 
   The piezoelectric power supply according to the invention employs the single sheet piezoelectric transformer as the basic construction element. By adopting parallel coupling, the secondary electrode of each parallel piezoelectric transformer is connected to the same load to provide a greater output power. Thus the multi-layer laminated piezoelectric transformer that is expensive and has a lower production yield is not required. 
   The foregoing, as well as additional objects, features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a basic circuit topologic diagram of a first embodiment of the invention. 
       FIG. 2  is a circuit topologic diagram of a first embodiment of the invention. 
       FIG. 3  is a circuit topologic diagram of a second embodiment of the invention for a push-pull circuit structure. 
       FIG. 4  is a circuit topologic diagram of a third embodiment of the invention showing a composite structure based on  FIGS. 2 and 3 . 
       FIG. 5  is a circuit topologic diagram of a fourth embodiment of the invention 
       FIG. 6  is a circuit topologic diagram of a fifth embodiment of the invention for a push-pull circuit structure. 
       FIG. 7  is a circuit topologic diagram of a sixth embodiment of the invention showing a composite structure based on  FIGS. 5 and 6 . 
       FIG. 8  is a basic circuit topologic diagram of a second embodiment of the invention. 
       FIG. 9  is a basic circuit topologic diagram of a second embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The embodiments of the invention illustrated below use CCFLs as the loads for detailed discussions. 
   Refer to  FIG. 1  for the basic circuit topologic diagram of the piezoelectric power supply of the invention to ignite a CCFL  10 . It is an AC power supply that is also a pulse-width modulation (PWM) power supply (but not as a limitation). The technique of the PWM power supply is known in the art, details will be omitted. It includes: 
   a piezoelectric transformer unit  20  which consists of single sheet piezoelectric transformers  201  that have respectively a primary electrode  21  on a first side and a secondary electrode  22  on a second side to transform a high frequency AC signal input from the primary electrode  21  to a high voltage to be output on the secondary electrode  22  to ignite the CCFL  10  connecting to the secondary electrode  22 ; 
   a power supply unit  30  to provide an electric power source required to actuate the CCFL  10  (depending on the specification of the CCFL  10 , generally is a DC power with the voltage from 0 to several volts); 
   a PWM control unit  40  to output a PWM signal through a pulse-width modulation technique; 
   a power switch  50  driven by the PWM signal and turned ON and OFF alternately in the positive half cycle and the negative half cycle through the PWM signal to output a modulated DC operation current. Namely, the PWM signal alters the void ratio of the DC current output from the power supply unit  30  at 0 volt and the full amplitude output (3.3 V); and 
   an inductor  60  which is an energy conversion element having one end connecting to an output end of the power switch  50  and another end connecting to the primary electrode  21  of the piezoelectric transformer unit  20  to transform the DC operation current from the power switch  50  to a high frequency AC signal, then input to the primary electrode  21  of the piezoelectric transformer unit  20 . 
   Refer to  FIG. 2  for a first embodiment of the circuit of the invention that aims to ignite a CCFL  10   a  of a greater power that cannot be actuated by one single sheet piezoelectric transformer. The piezoelectric transformer unit  20   a  consists of a plurality of single sheet piezoelectric transformers  201   a . The primary electrodes  21  of the single sheet piezoelectric transformers  201   a  are jointly connected to the one end of the inductor  60   a , and the secondary electrodes  22  of the single sheet piezoelectric transformers  201   a  are jointly connected to the input end (high voltage end) of the CCFL  10   a  to actuate and ignite the CCFL  10   a  in a cooperative manner. Of course, to drive the multiple single sheet piezoelectric transformers  201   a , the inductor  60   a  and the power switch  50   a  must have a matching power. 
   Likewise, transformer units  20   b ,  20   c  and  20   d  each consist of a plurality of single sheet piezoelectric transformers  201   b ,  201   c  and  201   d , respectively. The primary electrodes are connected to one end of corresponding inductors  60   b ,  60   c  and  60   d.    
   As the CCFL  10  have many different specifications, such as long tube or short tube; and the actuating circuit also has many types, such as the long tube is ignited by a push-pull circuit, and the number of loads driven by each element also is different, hence the circuit configuration could vary. But the basic structure shown in  FIG. 1  still is applied. The piezoelectric transformer of the invention is adaptable to these variations. More embodiments are discussed below. 
   Refer to  FIGS. 2 through 7  for the circuit topologic diagrams of various embodiments that employ a single power supply unit  30  and a single PWM control unit  40  to control a plurality of loads (namely CCFLs  10   a – 10   d  . . . ). The circuit of the power switch, inductor and piezoelectric transformer unit may have different designs as follow: 
     FIG. 2  shows CCFLs  10   a – 10   d  are driven respectively by a matching piezoelectric transformer unit  20   a – 20   d  and inductor  60   a – 60   d . One power switch  50   a  or  50   b  respectively drives two or more inductors  60   a  and  60   b , and  60   c  and  60   d .  FIG. 5  illustrates another circuit topologic structure which has one inductor  60   e  (or  60   f ) driving two parallel piezoelectric transformer units  20   a  and  20   b , and  20   c  and  20   d.    
   Refer to  FIG. 3  for another embodiment in which CCFLs  10   e – 10   f  are driven by a push-pull circuit that is usually employed to ignite the lamp of a long tube. As shown in  FIG. 3 , the CCFL  10   e  is driven by matching piezoelectric transformer units  20   e  and  20   f  that are actuated respectively by inductors  60   a  and  60   b . Similarly, another CCFL  10   f  is driven by matching piezoelectric transformer units  20   g  and  20   h  that are actuated respectively by inductors  60   c  and  60   d.    
   One power switch  50   a  or  50   b  drives two or more inductors  60   a  and  60   b , and  60   c  and  60   d .  FIG. 6  also is a push-pull circuit that has one inductor  60   e  (or  60   f ) driving two parallel piezoelectric transformer units  20   e  and  20   f  and  20   g  and  20   h . Each transformation unit  20   e ,  20   f ,  20   g  and  20   h  contains a plurality of single sheet piezoelectric transformers  201   e ,  201   f ,  201   g  and  201   h , respectively. 
     FIG. 4  is a composite circuit topologic structure based on  FIG. 2  and  FIG. 3 . Similarly,  FIG. 7  is a composite circuit topologic structure based on  FIG. 5  and  FIG. 6 . Details are omitted. 
   While the embodiments set forth above employ the single sheet piezoelectric transformer  201  as the basic structural element of the piezoelectric transformer unit  20 , in practice, a low level laminated piezoelectric transformer (such as four or two layers) may also be used.  FIG. 8  illustrates an embodiment which includes a double-layer laminated piezoelectric transformer  701  and a single sheet piezoelectric transformer  201  coupled in parallel to form a piezoelectric transformer unit  20 . While it provides a voltage same as the one consisting of three parallel single sheet piezoelectric transformers  201 , it consists of fewer elements and may be assembled and installed easier. Similarly, two double-layer laminated piezoelectric transformers  701   a  and  701   b  may be coupled in parallel to form a piezoelectric transformer unit  20  (referring to  FIG. 9 ). The lower level laminated piezoelectric transformer also has a primary electrode on a first side and a secondary electrode on a second side. The primary electrodes are coupled in parallel and connected to one end of an inductor, while the secondary electrodes also are coupled in parallel and connected to the input end of the load. 
   While the preferred embodiments of the invention have been set forth for the purpose of disclosure, modifications of the disclosed embodiments of the invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments which do not depart from the spirit and scope of the invention.