Polarity switching circuit

A polarity switching circuit includes: a first current-limiting resistor and a second current-limiting resistor connected to a DC high voltage; a first transistor switch, a second transistor switch, a fourth transistor switch, and a fifth transistor switch respectively controlled by a first PWM signal and a second PWM signal; a third transistor and a sixth transistor switch whose control terminals are respectively connected to the first transistor switch and the fourth transistor switch; a first filter connected to the second transistor switch and the third transistor switch and a contact of a piezoelectric actuator; and a second filter connected to the fifth transistor switch and the sixth transistor switch and another contact of the piezoelectric actuator. When the first and the second PWM signal are switching between a high level and a low level, output AC voltages with smoothed AC waveforms are supplied to the contacts of the piezoelectric actuator.

FIELD OF THE INVENTION

The invention is related to a polarity switching circuit, and more particularly to a polarity switching circuit for outputting an output AC voltage with a smooth waveform to drive a piezoelectric actuator.

BACKGROUND OF THE INVENTION

With the progress of technology, various electronic products have been developed for stimulating the growth of the information technology market. Undoubtedly, such trend will carry on. Also, with the advancement of the microelectronic technology, the electronic products will be more versatile and more miniaturized. Besides, the portability of the electronic products will be enhanced as well. Nowadays, the user can handle all kinds of business easily with various electronic products. In recent years, the so-called piezoelectric actuator have been developed and applied to electronic products. The piezoelectric actuator have the advantages of low voltage, high immunity to noise, small size, fast response, low heat radiation, high sophistication, high conversion efficiency, and high controllability.

The piezoelectric actuator generally requires an AC voltage that is applied thereto to drive the piezoelectric actuator to carry out high-speed periodically operations. Hence, the piezoelectric actuator needs a driving system to operate. The driving system is used to convert a DC voltage into an AC voltage for driving the piezoelectric actuator. Referring toFIG. 1, the conventional driving system1is used to convert a DC input voltage VDCinto output AC voltages Vo1and Vo2for driving a piezoelectric actuator9shown inFIG. 2A. The driving system1includes a boost circuit10, a voltage multiplier11, and a polarity switching circuit12. The boost circuit10is connected to the DC input voltage VDCto convert the DC input voltage VDCinto a transient voltage VTby the switching operations of the internal switch elements and the energy storage and filtering operations carried out by the internal inductors, capacitors, and diodes. The voltage multiplier11is connected to the transient voltage VTto multiply the transient voltage VTby 4 to generate a DC high voltage VB. The polarity switching circuit12is used to convert the DC high voltage VBinto output AC voltages Vo1and Vo2for driving the piezoelectric actuator9.

Referring toFIGS. 2A,2B, and3with reference toFIG. 1, in whichFIG. 2Ashows the internal circuitry of the polarity switching circuit ofFIG. 1andFIG. 2Billustrates the operation of the polarity switching circuit ofFIG. 1as the digital signal fswis low. Also,FIG. 3shows the timing of the voltage signals ofFIG. 2AandFIG. 2B. The polarity switching circuit12is connected to the DC high voltage VB, the input DC low voltage Vin, and the digital signal fswto convert the DC high voltage VBinto output AC voltages Vo1and Vo2driving the piezoelectric actuator9to operate repetitively. The polarity switching circuit12includes a first current-limiting resistor R21, a second current-limiting resistor R22, a third current-limiting resistor R23, a first transistor switch Q21, a second transistor switch Q22, a third transistor switch Q23, a fourth transistor switch Q24, a fifth transistor switch Q25, a sixth transistor switch Q26, and a seventh transistor switch Q27.

As the digital signal fswis high and is sent to the control terminal of the first transistor switch Q21and the control terminal of the sixth transistor switch Q26, the first transistor switch Q21and the sixth transistor switch Q26that are connected to the ground terminal G will turn on. As the first current-limiting resistor R21is connected to the first transistor switch Q21, the circuit branch consisted of the first current-limiting resistor R21will be connected to the ground terminal G. Meanwhile, the second transistor switch Q22and the fourth transistor switch Q24will turn off as the control terminal of the second transistor switch Q22and the control terminal of the fourth transistor switch Q24are connected to the circuit branch consisted of the first current-limiting resistor R21, thereby driving the voltage level of the circuit branch consisted of the second current-limiting resistor R22to a high level due to the DC high voltage VB. hence, the third transistor switch Q23will turn on as the control terminal of the third transistor switch Q23is connected to the circuit branch consisted of the second current-limiting resistor R22. Meanwhile, the control terminal of the seventh transistor switch Q27is connected to the digital signal fswwith a high level. Therefore, the seventh transistor switch Q27is also turned on. As the third current-limiting resistor R23is connected to the seventh transistor switch Q27, the circuit branch consisted of the third current-limiting resistor R23is connected to the ground terminal G. Also, the control terminal of the fifth transistor switch Q25is connected to the circuit branch consisted of the third current-limiting resistor R23, the fifth transistor switch Q25is turned off. Therefore, the current will flow in the direction as indicated by the arrows shown inFIG. 2A.

As the digital signal fswis low, as shown inFIG. 2B, the operations of all the transistor switches are reverse to the operations of all the transistor switches indicated inFIG. 2A. Under this condition, the current flow will be indicated by the arrows shown inFIG. 2B. In this manner, the output AC voltages Vo1and Vo2of the polarity switching circuit12will have a square waveform on the piezoelectric actuator9, as indicated by the waveform of the voltage signal of (Vo1-Vo2) shown inFIG. 3.

As the output AC voltages Vo1and Vo2of the polarity switching circuit12have square waveforms on the piezoelectric actuator9, the piezoelectric actuator9is rapidly charged as the voltage levels of the output AC voltages Vo1and Vo2are bobbing rapidly. Although the piezoelectric actuator9can reach the peak of its amplitude due to the rapid charging of the piezoelectric actuator9, the power loss is increased as well. More disadvantageously, as the polarity switching circuit12is configured to charge the piezoelectric actuator9rapidly with square AC waves, the piezoelectric actuator9will vibrate under a natural resonant frequency. Such vibration will cause tremendous noise.

Hence, it is needed to develop a polarity switching circuit to address the problems encountered by the prior art. The invention can meet this need.

THE SUMMARY OF THE INVENTION

An object of the invention is to provide a polarity switching circuit for addressing the problems of the huge power loss and the tremendous noise generated during the operation phase of the piezoelectric actuator.

To this end, the invention provides a polarity switching circuit for converting a DC high voltage into an output AC voltage for driving a piezoelectric actuator. The inventive polarity switching circuit includes a first current-limiting resistor connected to the DC high voltage; a second current-limiting resistor connected to the DC high voltage; a first transistor switch having a control terminal connected to a first pulse-width modulating (PWM) signal, a current input terminal connected to the first current-limiting resistor and the DC high voltage, and a current output terminal connected to a ground terminal; a second transistor switch having a control terminal connected to the first pulse-width modulating signal, a current input terminal, and a current output terminal connected to the ground terminal; a third transistor switch having a control terminal connected to the current input terminal of the first transistor switch and the first current-limiting resistor, a current input terminal connected to the DC high voltage, and a current output terminal connected to the current input terminal of the second transistor switch; a fourth transistor switch having a control terminal connected to a second pulse-width modulating signal, a current input terminal connected to the DC high voltage through the second current-limiting resistor, and a current output terminal connected to the ground terminal; a fifth transistor switch having a control terminal connected to the second pulse-width modulating signal, a current input terminal, and a current output terminal connected to the ground terminal; a sixth transistor switch having a control terminal connected to the current input terminal of the fourth transistor switch and the second current-limiting resistor, a current input terminal connected to the DC high voltage, and a current output terminal connected to the current input terminal of the fifth transistor switch; a first filter connected to the current input terminal of the second transistor switch, the current output terminal of the third transistor switch, a first contact of the piezoelectric actuator, and the ground terminal; and a second filter connected to the current input terminal of the fifth transistor switch, the current output terminal of the sixth transistor switch, a second contact of the piezoelectric actuator, and the ground terminal. When the first pulse-width modulating signal and the second pulse-width modulating signal are alternately and respectively switching between a high level and a low level, the first filter and the second filter are configured to filter the output AC voltage into a smoothed AC waveform, thereby providing an output AC voltage with a smoothed waveform for the piezoelectric actuator.

Another aspect of the invention is attained by the provision of a polarity switching circuit for converting a DC high voltage into an output AC voltage for driving a piezoelectric actuator. The inventive polarity switching circuit includes a first pulse-width modulating signal; a second pulse-width modulating signal; a first filter for receiving a pulse voltage generated by converting the DC high voltage and connected to a first contact of the piezoelectric actuator; and a second filter for receiving another pulse voltage generated by converting the DC high voltage and connected to a second contact of the piezoelectric actuator. When the first pulse-width modulating signal and the second pulse-width modulating signal are alternately and respectively switching between a high level and a low level, the first filter and the second filter are configured to filter the output AC voltage into a smoothed AC waveform, thereby providing an output AC voltage with a smoothed waveform for the piezoelectric actuator.

Now the foregoing and other features and advantages of the invention will be best understood through the following descriptions with reference to the accompanying drawings, in which:

DESCRIPTION OF THE PREFERRED EMBODIMENT

Several exemplary embodiments embodying the features and advantages of the invention will be expounded in following paragraphs of descriptions. It is to be realized that the present invention is allowed to have various modification in different respects, all of which are without departing from the scope of the present invention, and the description herein and the drawings are to be taken as illustrative in nature, but not to be taken as a confinement for the invention.

Referring toFIG. 4AandFIG. 4B, in whichFIG. 4Ashows the internal circuitry of the polarity switching circuit according to a preferred embodiment of the invention, andFIG. 4Billustrates the circuit operation of the polarity switching circuit ofFIG. 4Aas the first pulse-width modulating signal PWM1is low and the second pulse-width modulating signal PWM2is switching between a low level and a high level. As shown inFIG. 4AandFIG. 4B, the polarity switching circuit4is connected to a DC high voltage VBand is configured to convert the DC high voltage VBinto output AC voltages V1and V2according to a first pulse-width modulating signal PWM1and a second pulse-width modulating signal PWM2, thereby driving a piezoelectric actuator to operate repetitively. The DC high voltage VBis outputted from a voltage multiplier11shown inFIG. 1. The polarity switching circuit4includes a first transistor switch Q41, a second transistor switch Q42, a third transistor switch Q43, a fourth transistor switch Q44, a fifth transistor switch Q45, a sixth transistor switch Q46, a first filter40, a second filter41, a first current-limiting resistor R1, and a second current-limiting resistor R2.

The first current-limiting resistor R1is connected to the DC high voltage VB, and is connected to a current input terminal of the first transistor switch Q41and a control terminal of the third transistor switch Q43. The second current-limiting resistor R2is connected to the DC high voltage VB, and is connected to a current input terminal of the fourth transistor switch Q44and a control terminal of the sixth transistor switch Q46.

A control terminal of the first transistor switch Q41is connected to the first pulse-width modulating signal PWM1. A current input terminal of the first transistor switch Q41is connected to the DC high voltage VBthrough the first current-limiting resistor R1. A current output terminal of the first transistor switch Q41is connected to a ground terminal G. A control terminal of the second transistor switch Q42is connected to the first pulse-width modulating signal PWM1. A current input terminal of the second transistor switch Q42is connected to the first filter40. A current output terminal of the second transistor switch Q42is connected to the ground terminal G. A control terminal of the third transistor switch Q43is connected to the current input terminal of the first transistor switch Q41. A current input terminal of the third transistor switch Q43is connected to the DC high voltage VB. A current output terminal of the third transistor switch Q43is connected to the first filter40and the current input terminal of the second transistor switch Q42. The first filter40is connected to a contact of the piezoelectric actuator9and the ground terminal G.

A control terminal of the fourth transistor switch Q44is connected to a second pulse-width modulating signal PWM2. A current input terminal of the fourth transistor switch Q44is connected to the DC high voltage VBthrough the second current-limiting resistor R2. A current output terminal of the fourth transistor switch Q44is connected to the ground terminal G. A control terminal of the fifth transistor switch Q45is connected to the second pulse-width modulating signal PWM2. A current input terminal of the fifth transistor switch Q45is connected to the second filter41. A current output terminal of the of the fifth transistor switch Q45is connected to the ground terminal G. A control terminal of the sixth transistor switch Q46is connected to the current input terminal of the fourth transistor switch Q44. A current input terminal of the sixth transistor switch Q46is connected to the DC high voltage VB. A current output terminal of the sixth transistor switch Q46is connected to the second filter41and the current input terminal of the fifth transistor switch Q45. The second filter41is connected to another contact of the piezoelectric actuator9and the ground terminal G. Referring toFIG. 5A,FIG. 5B, andFIG. 5Cwith reference toFIG. 4AandFIG. 4B, in whichFIG. 5A,FIG. 5B, andFIG. 5Care the timing diagrams of the voltage signals ofFIG. 4AandFIG. 4B, respectively. As shown inFIG. 4A,FIG. 4B,FIG. 5A,FIG. 5B, andFIG. 5C, the first pulse-width modulating signal PWM1and the second pulse-width modulating signal PWM2are alternately switched between the high level and the low level. That is, when the first pulse-width modulating signal PWM1is switching between the high level and the low level, the second pulse-width modulating signal PWM2is low. On the contrary, when the second pulse-width modulating signal PWM2is switching between the high level and the low level, the first pulse-width modulating signal PWM1is low.

When the first pulse-width modulating signal PWM1is switching between the high level and the low level and the second pulse-width modulating signal PWM2is low, the low level of the second pulse-width modulating signal PWM2will force the fourth transistor switch Q44and the fifth transistor switch Q45to turn off. Also, the sixth transistor switch Q46will turn on as its control terminal is connected to the DC high voltage VB. Meanwhile, the high-frequency switching of the first pulse-width modulating signal PWM1between the high level and the low level will drive the first transistor switch Q41, the second transistor switch Q42, and the third transistor switch Q43to switch synchronously. That is, when the first transistor switch Q41and the second transistor switch Q42are turned on, the third transistor switch Q43is turned off. On the contrary, when the first transistor switch Q41and the second transistor switch Q42are turned off, the third transistor switch Q43is turned on. Therefore, when first transistor switch Q41and the second transistor switch Q42are turned on, the current will flow in the direction indicated by the arrows shown inFIG. 4A.

On the contrary, when the second pulse-width modulating signal PWM2is switching between the high level and the low level and the first pulse-width modulating signal PWM1is low, the operations of the transistor switched are reversed. That is, the low level of the first pulse-width modulating signal PWM1will force the first transistor switch Q41and the second transistor switch Q42to turn off, and the third transistor switch Q43will turn on as its control terminal is connected to the DC high voltage VB. Meanwhile, the high-frequency switching of the second pulse-width modulating signal PWM2between the high level and the low level will drive the fourth transistor switch Q44, the fifth transistor switch Q45, and the sixth transistor switch Q46to switch synchronously. That is, when the fourth transistor switch Q44and the fifth transistor switch Q45are turned on, the sixth transistor switch Q46is turned off. On the contrary, when the fourth transistor switch Q44and the fifth transistor switch Q45are turned off, the sixth transistor switch Q46is turned on. Therefore, when fourth transistor switch Q44and the fifth transistor switch Q45are turned on, the current will flow in the direction indicated by the arrows shown inFIG. 4B.

Hence, when the timing of the first pulse-width modulating signal PWM1and the timing of the second pulse-width modulating signal PWM2are set as indicated inFIG. 5A, that is, the frequency of the first pulse-width modulating signal PWM1and the frequency of the second pulse-width modulating signal PWM2are respectively drifting from a high value to a low value and then to a high value, the first pulse-width modulating signal PWM1and the second pulse-width modulating signal PWM2will enable the polarity switching circuit4to convert the DC high voltage VB. Under this condition, a second switching voltage Vs2is generated between the current input terminal of the second transistor switch Q42and the current output terminal of the third transistor switch Q43, and a first switching voltage Vs1is generated between the current input terminal of the fifth transistor switch Q45and current output terminal of the sixth transistor switch Q46. Also, as shown inFIG. 5B, the first switching voltage Vs1and the second switching voltage Vs2that are pulse voltages will drift in synchronization with the first pulse-width modulating signal PWM1and the second pulse-width modulating signal PWM2from a high-frequency band to a low-frequency band and then to a high-frequency band. The first switching voltage Vs1and the second switching voltage Vs2will be filtered by the second filter41and the first filter40, respectively, thereby generating output AC voltages V1and V2with smoothed AC waveforms, as shown inFIG. 5C.

Referring toFIG. 5C, the driving electric energy applying to the piezoelectric actuator9, that is, the remainder of the output AC voltage V1and the output AC voltage V2, will reach a first fractional value of the maximum voltage Vmaxlinearly within a first time period after the polarity switching circuit4starts operating, as indicated by the curve between the numerical marking1and the numerical marking2. Afterwards, the waveform of the driving electric energy applying to the piezoelectric actuator9will smoothly bob up and reach the maximum voltage Vmaxwithin a first predetermined time period, as indicated by the curve between the numerical marking2and the numerical marking3. Afterwards, the waveform of the driving electric energy applying to the piezoelectric actuator9will be remain flat within a second time period, as indicated by the curve between the numerical marking3and the numerical marking4. Afterwards, the waveform of the driving electric energy applying to the piezoelectric actuator9will smoothly decline and reach a second fractional value of the maximum voltage Vmaxlinearly within a second predetermined time period, as indicated by the curve between the numerical marking4and the numerical marking5. Finally, the waveform of the driving electric energy applying to the piezoelectric actuator9will drop to zero linearly, as indicated by the curve between the numerical marking5and the numerical marking6. As to the waveform of the driving electric energy applying to the piezoelectric actuator9indicated by the curve between the numerical marking6and the numerical marking11, it is not intended to elaborate as the characteristics of this segment of waveform are similar to those of the segment of waveform indicated by the curve between the numerical marking1and the numerical marking6. Also, the rising rate, the falling rate, the knee point radian, and the maintaining time of the maximum voltage Vmaxof the smooth AC waveform of the output AC voltages V1and V2of the polarity switching circuit4can be tuned by adjusting the pulse width of the first pulse-width modulating signal PWM1and the second pulse-width modulating signal PWM2.

The output AC voltages V1and V2of the polarity switching circuit4have smooth AC waveforms and are applied to the two contacts of the piezoelectric actuator9. According to the prior art as shown inFIG. 2A, the output AC voltages Vo1and Vo2of the conventional polarity switching circuit have square AC waveforms and are applied to the piezoelectric actuator9. Thus, the inventive polarity switching circuit can charge the piezoelectric actuator9moderately, which would reduce the power loss as a result of rapid charging. More advantageously, the vibrations of the piezoelectric actuator9under a natural resonant frequency can be suppressed, thereby avoiding the noise generated during the operation phase of the piezoelectric actuator9.

In alternative embodiments, the first filter40can include a first inductor L1and a first capacitor C1, as shown inFIG. 4A. InFIG. 4A, the first inductor L1is connected to the piezoelectric actuator9, the current input terminal of the second transistor switch Q42, and the current output terminal of the third transistor switch Q43. The first capacitor C1is connected to the piezoelectric actuator9, the first inductor L1, and the ground terminal G. The second filter41can include a second inductor L2and a second capacitor C2. The second inductor L2is connected to the piezoelectric actuator9, the current input terminal of the fifth transistor switch Q45, and the current output terminal of the sixth transistor switch Q46. The second capacitor C2is connected to the piezoelectric actuator9, the second inductor L2, and the ground terminal G.

In alternative embodiments, the first filter40can include a first capacitor C1only, as shown inFIG. 6. InFIG. 6, the first capacitor C1is connected to a contact of the piezoelectric actuator9, the current input terminal of the second transistor switch Q42, the current output terminal of the third transistor switch Q43, and the ground terminal G. The second filter41can include a second capacitor C2only, as shown inFIG. 6. InFIG. 6, the second capacitor C2is connected to another contact of the piezoelectric actuator9, the current input terminal of the fifth transistor switch Q45, the current output terminal of the sixth transistor switch Q46, and the ground terminal G.

In alternative embodiments, the transistor switches Q41-Q46can be implemented by NPN bipolar junction transistors (BJTs). Under this condition, the control terminal, the current input terminal, and the current output terminal of the transistor switches Q41-Q46are constituted by the base, the collector, and the emitter, respectively. Nonetheless, in alternative embodiments, the transistor switches Q41-Q46can be implemented by field-effect transistors (FETs), as shown inFIG. 7AandFIG. 7B. Under this condition, the control terminal, the current input terminal, and the current output terminal of the transistor switches Q41-Q46are constituted by the gate, the source, and the drain, respectively. Furthermore, provided that the transistor switches Q41-Q46are implemented by field-effect transistors (FETs), the polarity switching circuit4further includes a third current-limiting resistor R3and a fourth current-limiting resistor R4. The third current-limiting resistor R3is connected to the control terminal of the first transistor switch Q41and the control terminal of the second transistor switch Q42. The fourth current-limiting resistor R4is connected to the fourth transistor switch Q44and the fifth transistor switch Q45. As the circuit topology and operation principle of the polarity switching circuit12ofFIG. 7AandFIG. 7Bare similar to those of the polarity switching circuit12ofFIG. 4AandFIG. 4B, it is not intended to give details to the circuit topology and operation principle of the polarity switching circuit4ofFIG. 7AandFIG. 7Bherein.

Referring toFIG. 8, the structural view of a mechanical body incorporating the piezoelectric actuator ofFIG. 4Ais shown. InFIG. 8, the mechanical body may be a fluid transfer device8that is applicable to biomedical technology, computer technology, printing technology, or energy industry for transferring gas or liquid. The fluid transfer device8may be a pump in an inkjet printer for converting electric energy into mechanical energy. The piezoelectric actuator9includes an actuating piece90and a vibrating film91that are respectively connected to the output AC voltage V1and the output AC voltage V2. The output AC voltage V1and the output AC voltage V2are used to drive the actuating piece90and the vibrating film91to operate repetitively to allow the pressure chamber92to be compressed or expanded, thereby enabling the fluid transfer device8to transfer fluid.

In conclusion, the inventive polarity switching circuit employs sixth transistor switches and two filters to output AC voltages with smoothed AC waveforms. Thus, the power loss of the piezoelectric actuator is reduced and the noise of the piezoelectric actuator is suppressed.

While the invention has been described in terms of what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention need not be restricted to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures. Therefore, the above description and illustration should not be taken as limiting the scope of the invention which is defined by the appended claims.