WO2003019990A1

WO2003019990A1 - Electronic ballast with piezoelectric ceramics

Info

Publication number: WO2003019990A1
Application number: PCT/KR2002/001581
Authority: WO
Inventors: Dong-Soo Paik
Original assignee: Dong-Soo Paik
Priority date: 2001-08-27
Filing date: 2002-08-21
Publication date: 2003-03-06
Also published as: KR20030018220A; KR100458617B1

Abstract

High efficiency electronic ballast with piezoelectric ceramics is provided. A piezoelectric coupler is used in which ceramics in disc shape are polarized in vertical direction and housed in a plastic case. Matching impedance with an inductor in front of the piezoelectric ceramics is adjusted so that it could be drived in a frequency band in which the efficiency of a lamp can be maximized. The preferred driving frequency is off the resonant frequency of the piezoelectric ceramics by more than 20kHz. Damage of the piezoelectric ceramics may be suppressed by attaching a no-load and overload protection circuit.

Description

ELECTRONIC BALLAST WITH PIEZOELECTRIC CERAMICS

FIELD OF THE INVENTION

The present invention relates to high-efficiency electronic ballast with piezoelectric ceramics, in particular, to non-preheating ballast incorporating piezoelectric ceramics in ballasts for fluorescent lamps or high voltage discharge lamps, requiring no preheating or soft starting; and to cathode preheating ballast with additional functions of preheating and soft starting.

BACKGROUND OF THE INVENTION

Since a fluorescent lamp shows a higher light efficiency when it is ignited at a high voltage of 10 kHz or over than it is ignited at commercial power of 50 to 60 Hz, ballasts used recently are mainly electronic ballasts. Electronic ballasts operating at high frequency band are generally ballasts of cathode preheating type comprising various supplemental circuits for enhancing the performance and extending the life time of the lamp. Such ballasts allow a flow of current at the initial phase of a lighting to preheat the cathode filaments and further comprise a soft start function for life time extension of the lamp.

Various patents on electronic ballasts have been reported, and several patents on electronic ballasts using piezoelectric ceramics or ferroelectric ceramics are reported as well.

USP 4,858,066 and USP 4,447,549 have disclosed ignition technology of a discharge lamp using the non-linear responses of ceramic capacitors based on the nonlinear electric field to polarization effect of ferroelectric materials, but failed to provide a ballast incorporating such technology for practical use. These patents have concentrated in materials capable of utilizing the ferroelectric hysterisis, but commercialization of such material fails, because the practical drive frequency thereof is very high. In addition, since a reversal of the polarization is attempted continuously at high frequency, leading to fatigue behavior of the ceramics used, it is highly probable that a problem pertinent to the life time of the lamp arises here.

On the other hand, USP 6,034,484 relates, in contrast to the above mentioned patents, to a ballast capable of igniting a fluorescent lamp by a polarized ferroelectric material and connecting the same to an inductor so as to function as a piezoelectric resonator. The piezoelectric material used here can take various forms, and is connected in parallel to the fluorescent lamp while it is connected in series to the cathode filaments to perform preheat ignition. Further, the above patent also comprises ballast with a plurality of piezoelectric resonators connected in parallel, to allow use in an expanded driving frequency band. However, such ballasts with ceramic elements have the problems that the coercive electric field should be small enough to allow non-linear capacitance changes with commercial power, the driving frequency band shall be expanded by connecting a plurality of different piezoelectric resonators, and that each resonator operates only at its accurate resonating frequency. In addition, there exists no solution for the problem that a piezoelectric material to be driven under no load at a resonant frequency may be destroyed by overcurrent. From these reasons, an intact ballast of the above type has not been provided.

DETAILED DESCRIPTION OF THE INVENTION The present invention, conceived to solve the above problems, aims to provide a high efficiency electronic ballast comprising piezoelectric ceramics with high light efficiency and low power consumption in addition to the merits of a general electronic ballast. Another objective of the present invention is to provide a ballast, which can light a fluorescent lamp even when the filaments thereof are disconnected, by adopting a system requiring no filament of a fluorescent lamp in the manufacture of a high efficiency ballast.

Still another objective of the present invention is to provide an instantaneous preheating ballast designed to light a fluorescent lamp at an optimal status in stationary state, which operates for the first several seconds to several minutes in a cathode preheating manner similar to that of a conventional electronic ballast in order to prevent an early blackening of the lamp, which phenomenon is a shortcoming of the non-preheating instant starting type ballast capable of lighting a fluorescent lamp independently of the existence of filaments, and lights a fluorescent lamp using piezoelectric ceramics when the fluorescent lamp reaches the stationary state.

A further objective of the present invention is to provide a ballast capable of driving lamps with different diameters such as PL lamps in U-form, compact type lamps with integrated ballast, screw type fluorescent lamps, discharge lamps such as sodium vapor lamps, metal-halide lamps, in addition to the general linear tube form fluorescent lamps, in a stable manner. In particular, the present invention aims to provide a ballast of small size and of light weight capable of solving a problem with the conventional magnetic or electronic ballast for sodium vapor lamps that the ignition circuit thereof fails very frequently, by supplying a sufficient discharge voltage through the piezoelectric ceramics, making the ignition circuit for the initial lighting unnecessary. Another objective of the present invention is to provide a ballast operable in a very wide temperature range.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram showing connection of piezoelectric ceramics to a discharging lamp in an electronic ballast as per the present invention.

Fig. 2a is a graph showing behavior of resonant frequency vs. inductance of the inductor in the circuit of Fig. 1 for various capacitances of the piezoelectric materials used, while Fig. 2b is a graph showing an impedance change at the induced resonant frequency for the same materials presented in Fig 2a.

Figs. 3a and 3b illustrate the general circuits of ballasts with integrated piezoelectric coupler in accordance with the present invention for fluorescent lamps and for high voltage discharging lamps, respectively.

Fig. 4 illustrates the general circuit of a preheating type ballast with integrated piezoelectric ceramics controller in accordance with the present invention for fluorescent lamps.

Fig. 5 is a circuit showing the construction of the piezoelectric ceramics controller of Fig. 4.

Fig. 6 is a graph showing the initial discharge voltage and the ignition or non- ignition of the discharge lamp as per the frequency bands of the piezoelectric ballast included in the piezoelectric coupler (C=2.3nF) in accordance with the present invention.

Fig. 7 is a graph showing the initial discharge voltage and the ignition or non- ignition of the discharge lamp as per the frequency bands of the piezoelectric ballast included in the piezoelectric coupler (C=2.5nF) in accordance with the present invention. Fig. 8 is a graph showing the relationship between the input power as per the driving frequencies and the lighting efficiency in a non-preheating ballast and an instantaneous preheating ballast in accordance with the present invention as well as in a conventional electronic ballast. Fig. 9 is a graph showing the voltage and current changes as per ignition time of a screw type fluorescent lamp by a ballast in accordance with the present invention.

Fig. 10 is a graph showing the change in illumination of a fluorescent lamp as per the change of temperature of a ballast in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a non-preheating type ballast and a preheating type ballast to achieve the above objectives. The ballasts comprise circuits required for a general ballast, such as no-load and overload protection circuit, power factor compensation, noise filter, inverter, etc. in addition to piezoelectric ceramics. The ballasts also adopt an inductor connected in series to the piezoelectric element, which, in turn, is connected in parallel to the fluorescent lamp, in order to achieve LC resonance as well as impedance matching with the piezoelectric element. The non-preheating ballast, being of an instantly igniting type, operates at a power saving mode by removing preheating current from the filaments and reducing heating temperature generated by the filaments, allowing to save energy from the moment of ignition.

The instantaneous preheating ballast, which comprises an additional soft start function like a conventional electronic ballast, allowing preheating current to flow for a predetermined duration after ignition and a time delay module not included in a non- preheating ballast, to enable driving at a power saving mode by the piezoelectric element as in a non-preheating ballast after several seconds have elapsed from the ignition. Although the non-preheating type driving is highly effective in power saving, there arises a problem that it accelerates the blackening behavior. From these grounds, a time delay module has been adopted to enable a driving at the initial ignition as with a conventional electronic ballast, while the driving mode is converted after a predetermined duration from the ignition to a power saving mode to be driven by the piezoelectric ceramics.

Since the piezoelectric element comprised in a thus manufactured ballast enables in combination with the frequency inverter the ballast to function as a high efficiency ballast, a piezoelectric element used in the present invention is called a "piezoelectric coupler" in this specification. Such piezoelectric coupler functions as a high voltage generator which discharges a high voltage when lighting a discharging lamp, as a resonator resonating with the inductor, and as a non-linear capacitor whose capacitance changes rapidly in a non-linear form with the input voltage. The driving frequency bandwidth is determined by such characteristics of the piezoelectric coupler and the impedance matching with the inductor.

In the conventional arts, the piezoelectric ceramics are driven in the resonant frequencies. In contrast thereto, the piezoelectric ceramics are driven in the present invention in a non-resonant frequency band at least 20 kHz off the correlated resonant frequencies, in order to extend the life time of a lamp and to avoid destruction of the piezoelectric ceramics, and thus, the safety is greatly improved.

The piezoelectric coupler used in the present invention is manufactured by a piezoelectric material shaped in single layered disc form made of various compositions and polarized in thickness direction. Here, capacitance of the piezoelectric element is adjusted by the surface of and distance between the electrodes as in case of the general capacitors. For the piezoelectric ceramics of the present invention, materials, of which the variation of the polarization under the commercial power (100V to 240V) have been semipermanently stabilized, have been used. The resonant frequency of a piezoelectric coupler is highly dependant on the size of the element such that a reduction in diameter of a disc type piezoelectric coupler can increase the resonant frequency up to several hundreds of kHz. The present invention, however, has succeeded in driving the piezoelectric ceramics at a frequency band (20 kHz to 90 kHz) allowing to maximize the efficiency of a fluorescent lamp, by adjusting the inductance used at the front end of the piezoelectric coupler and the impedance matching, and by driving the ceramics at non-resonant frequency to allow it to maintain a stable status until the no load protection circuit operates.

A high efficiency ballast for discharging lamp can be manufactured using the above piezoelectric ceramics and driving circuits corresponding thereto. A ballast for fluorescent lamp in accordance with the present invention comprises AC power source inclusive of an over current protecting means, a surge prevention circuit, and a noise filter for reduction of electromagnetic inference (hereinafter, "EMI"); a power factor improving circuit connected thereto; a rectifier connected thereto; a pulse generator for generating signals at predetermined frequencies after having received signals from the input power; a switching frequency setting circuit; two field effect transistors for switching the AC voltage outputted from the rectifier; a piezoelectric coupler in parallel connection, which is connected in series to an inductor connected to the output of the field effect transistors; a discharging lamp; and a reset circuit, which can reset the pulse generator when an overload or a non-load occurs, while it is connected to the pulse generator.

The reset circuit includes a secondary winding formed to face the inductor in order to detect abnormal current in case of an overload or a non-load, a Zener diode for detecting an over voltage while it is connected to the secondary winding, and a silicone controlled rectifier operable by signals inputted from the Zener diode. Such a ballast is of non- preheating type and is applicable to fluorescent lamps as well as to high voltage discharging lamps, since it enables the initial ignition voltage generated by the piezoelectric coupler to be automatically adjusted to the types of discharging lamps.

The basic constituents of an instantaneous preheating ballast manufactured for application to fluorescent lamps alone are as described in the explanation of the above circuits, with the addition of a time delay module and a protection circuit for reverse electromotive force elimination placed between the output part and the lamp. Here, the piezoelectric coupler functions to enhance the efficiency of the lamp by matching the oscillation frequency to its own resonance frequency. The time delay module controls the appropriate time to enable safe operation of the piezoelectric coupler in a manner that an early destruction of the piezoelectric ceramics due to a reduction in the impedance during resonance is prevented and the life time of the lamp as same as the function of the protection circuit for reverse electromotive force elimination is extended. The protection circuit for reverse electromotive force elimination protects the piezoelectric ceramics and the circuits by reducing the reverse electromotive force generated at the choke coil when the power is switched on so that no such force is applied to the lamp, and by stopping oscillation in case of an overloading. Since the output power of a ballast for discharging lamps as per the present invention is controllable by the capacitance of the piezoelectric ceramics, this allows to minimize the size of the ballast. And also, since the used piezoelectric coupler has one polarization axis unlike usual piezoelectric transformers, the polarization fatigue can be minimized in the invented ballast operating at commercial electric source of less than the electric field for poling. In addition, since the above ballast allows driving thereof at a non- resonant point of piezoelectric ceramics, the range of usable frequency has greatly been expanded, and a high efficiency ballast can now be manufactured by effectively utilizing the impedance difference between the piezoelectric ceramic and discharging lamp. Below, the preferred embodiments of the present invention are described in detail making reference to the accompanying drawings.

An instant starting type is used in the present invention, wherein the cathode filaments 3 of a fluorescent lamp 2 are short-circuited and then connected in parallel to a piezoelectric coupler 5, while the front end thereof is connected in series to an inductor 1 as illustrated in Fig. 1, in order to limit the current flowing through the filaments.

The filaments are short-circuited in order to prevent reduction in light efficiency resulting from reduction in power efficiency due to the consumption of a part of the preheating current, which flows not only at the beginning of a lighting but at the middle of the lighting through the cathode filaments 3 of the fluorescent lamp. As the initial starting voltage of a discharging lamp in use herein can greatly vary dependent on the types of the lamp, application of an excessively high voltage to a discharging lamp can be prevented in a manner that the applied voltage is divided by connecting an appropriate capacitor 4 in series to the lamp 2.

Figs. 2a and 2b are graphs showing a correlation between the inductance of the inductor 1 used for effective operation of the resonant circuit in Fig. 1 as well as for impedance matching and the resonant frequency; and showing a correlation between the inductance and the impedance at the resonant frequency, respectively.

Generally, an LC resonator shows a correlation of /_r= l/2w LC Accordingly, the resonant frequency decreases as the inductance increases as shown in Fig. 2a, and the impedance at a resonant frequency generated through an inductor connection to a piezoelectric coupler tends to be saturated at an inductance of about 2mH or over.

Thus, an inductor matching the piezoelectric coupler used in the present invention should desirably have an inductance of 2mH or more, at which value the impedance is saturated and the driving frequency is stabilized. However, since the capacitance is adjustable in accordance with the compositions of the piezoelectric ceramics, an effective matching point could be established at an even lower inductance value.

The resonant frequency at OmH in Fig. 2a, being the resonant frequency of the piezoelectric coupler itself, varies in accordance with the diameter of the elements. However, the resonant frequency thereof can be reduced to lOOkHz or lower depending on the matching of the piezoelectric coupler with the inductor. Furthermore, since the present invention expands the operable frequency band utilizing non-resonant frequency in addition to the peak resonant frequency, it allows driving at a selected frequency band showing superior characteristics of the discharging lamp to be used, a more detailed explanation on which will be given later.

Figs. 3a and 3b illustrate general circuits of non-preheating instant starting type electronic ballasts with integrated piezoelectric coupler in accordance with the present invention. These ballasts, which are applicable to both fluorescent lamps and high voltage discharging lamps without regard to the volume of power consumption, can widely be employed by simply changing the size of piezoelectric coupler based on the power consumption or the tubular characteristics of the discharging lamps as well as the inductor capacity matching thereto. In particular, these ballasts allow manufacturing of ignitor-free ballasts for a sodium lamp in smaller size and lighter weight, since such ballasts can discharge sufficiently high voltage for initial ignition without an ignition circuit as it is required in a conventional magnetic ballast. Moreover, they can be used as low temperature ballasts that do not fail to ignite at a temperature on or below 0° C, since these ballasts allow the initial ignition voltage to be adjusted to the impedance of the discharging lamp (load) connected in parallel to the piezoelectric coupler. In Fig. 3a, the power source is commercially supplied AC, while a fuse FI is employed for over current protection. A varistor TTSTR connected in parallel to the AC line at the front end of a rectifier is employed as an element for over voltage protection. A noise filter LF and three capacitors CI, C2, C3 function for EMI prevention, while a coil TI, diodes D5, D6, D12, D13, and capacitors C12, C13 improve the power factor. The commercially supplied AC is bridge-rectified by diodes DI, D2, D3, D4; its power factor improved by coil TI, diodes D5, D6, D12, D13, and capacitors C12, C13; and then, smoothened by capacitors C5, C6. Further, surplus low voltage is obtained from the secondary winding of the inductor TI so that it can be used after rectification as 13V to 14V power source for the IC Ul. C7 is a smoothening capacitor for the IC power source. The IC Ul is a pulse generator, and the frequency of the generated pulse is determined by VR1, VR2, and CIO.

The oscillation frequency varies dependant on the types of a pulse generator used, while the oscillation frequency of IC used in this embodiment example is determined by the following formula:

^{/ _} 1.4 x ( W?l + 7i?2 + 75.2) x CIO

The oscillation frequency is preferably set in a manner that it does not fall within the resonance frequencies of the inductor T2 (EI2519) and the piezoelectric coupler PC.

The output pins 5 and 7 of IC output inversed or non-inversed pulse of rectangular forms, driving FET Q4 and Q3 via the resistors R4 and R5, respectively. If driving voltage of a predetermined frequency is applied to these FET gates, output AC is formed through alternate switching of FET Q4 and Q3. Here, as the inductor T2 and the piezoelectric coupler PC generate resonance, high voltage is discharged by initial input signal, whereupon the high voltage is evenly induced to the discharging lamp FL connected in parallel to the piezoelectric coupler PC, so that mercury in the discharging lamp FL is activated enabling an ignition, and then, the discharging lamp FL is maintained in stable state.

On the other hand, a second winding is installed to the inductor T2 to allow the voltage directly proportional to the current in the first winding to be distributed to resisters R9 and RIO. Here, if the voltage drop by resistor RIO is larger than Zener voltage (e.g. 10V) at Zener diode ZD2, i.e. in case of an overloading or a non-loading, the SCR is turned on and the diode D9 is connected effectively to the earth, so that IC having its third pin fixed to low voltage is stopped to drive, with which procedure the inverter ceases to be switched on and the ballast is protected. Now, a description of another embodiment of the present invention, the circuit illustrated in Fig. 3b, is given below.

In Fig. 3b, the power source is commercially supplied AC, while a fuse FI is employed for over current protection. A varistor connected in parallel to the AC line at the front end of a rectifier is employed as an element for over voltage protection. A noise filter and three capacitors CI, C2, C3 function for EMI prevention, while a coil Ell 916, diodes and capacitor C4 improve the power factor. The commercially supplied AC is bridge- rectified upon application; its power factor improved; its energy accumulated by C5; and then smoothened. Further, surplus low voltage is obtained from the secondary winding of the inductor so that it can be used after rectification as power source for the IC. A 12V Zener diode is employed to maintain the voltage of IC power source constantly, while C6 is a smoothening capacitor for the IC power source. The IC is a pulse generator, and the frequency of the generated pulse is determined by R4, R5, and C7. Here, it is preferable that R4 is designed as a variable resister to allow an accurate setting of the oscillation frequency.

,₌ 1

⁷ 1.4 x (/δ + + 75J2) x C7 The oscillation frequency is preferably set in a manner that it does not fall within the resonance frequencies of the inductor T2 (EI2519) and the piezoelectric coupler PC.

The capacitors are connected in normal state by the driver input terminals of the IC" to effect a driving, while the opposite poles of the capacitors are closed by FET (field effect transistor) to stop driving when abnormal current is present. The output pins of IC output inversed or non-inversed pulse of rectangular forms, driving FET Q3 and Q4 via the resisters R6 and R12, respectively. If driving voltage of a predetermined frequency is applied to these FET gates, output AC is formed through alternate switching of FET Q3 and Q4. Here, as the inductor EI2519 and the piezoelectric coupler PC generate resonance, high voltage is discharged by initial input signal, whereupon the high voltage is evenly induced to the discharging lamp FL connected in parallel to the piezoelectric coupler PC, so that mercury in the discharging lamp FL is activated enabling an ignition, and then, the discharging lamp FL is maintained in stable state.

Since ballasts using a piezoelectric coupler utilize the non-linear behavior of capacitance of a piezoelectric material depending on the input voltage, the voltage discharged initially in such ballasts vary automatically in accordance with the characteristics of the discharging lamps. On the other hand, a second winding is installed to the inductor EI2519 to allow current detected at overload or non-load and to distribute the voltage to resisters R9 and RIO. Here, if abnormal voltage is detected by 5. IV Zener diode so that voltage at the second pin of the IC reaches 0V, the IC with its oscillating terminal fixed to low voltage stops to oscillate, with which procedure the inverter ceases switching on and the ballast is protected.

Fig. 6 is a graph showing the correlation between discharge voltage and frequency at the time of igniting a discharge lamp using the ballast in Fig. 3b, as well as the frequency band in which a normal driving is possible.

The capacitance of piezoelectric coupler used herein was 2.3nF, while the planar resonance mode frequency of the element was 114kHz. An inductor of 2.15mH was used, and a resonance frequency of 70.8kHz and an impedance of 8.8Ω were indicated while coupled with the piezoelectric coupler.

Fig. 6 shows the results of ignitions of a 20W compact type fluorescent lamp CFL (line graphs) and of a straight tube type fluorescent lamp T8 with a diameter of 26mm (dotted graphs) under the above conditions. The initial ignition voltage required for lighting the lamp was 740V for the compact type fluorescent lamp, while that for T8 was 780V However, since the initial voltage required for igniting a discharging lamp of same type can even vary from manufacturer to manufacturer, the igniting voltages of specific fluorescent lamps used in the present invention shall not be taken as absolute figures generally applicable to all fluorescent lamps. There are, for instance, compact type fluorescent lamps with an ignition voltage of about 400V in trade. The frequency bands, in which lighting of the compact type fluorescent lamps used in the present invention is possible, lie from 18kHz to 22kHz and from 60kHz to 72kHz, with frequency bandwidths of 4kHz and 12kHz, respectively. Although the highest voltage has been reached at the resonance point, it can be seen that ignition is possible over a wide range near the resonance point and even in frequencies from 18kHz to 22kHz, considerably apart from the resonance point.

In case of T8 such frequency bands lie from 18kHz to 22kHz and from 58kHz to

78kHz, showing driving bands wider than those of the screw type fluorescent lamps near the resonance point. The fluorescent lamps, being non-linear impedance elements with RC characteristics, show different driving frequency ranges and generated voltages dependant on the types of discharging lamps.

Fig. 7 shows the results of ignitions of a 20W compact type fluorescent lamp CFL (line graphs) and of a straight tube type fluorescent lamp T8 with a diameter of 26mm (dotted graphs) under the above conditions using a piezoelectric coupler with different characteristics compared to that appeared in Fig 6, wherein capacitance of the piezoelectric element was 2.5nF, resonance frequency of the piezoelectric element itself was 78kHz, and the main resonance point while connected to the inductor was 58.5kHz.

While the compact type fluorescent lamps used in Fig. 6 show ignitable frequency bands of 18~27kHz, 32-38kHz, 52~63kHz, and 103-108kHz, T8 shows frequency characteristics similar to those of the compact type fluorescent lamps except for the lowest frequency band, where the frequency band has been narrowed to 18~22kHz. In piezoelectric ceramics, resonance frequency band of an element varies generally dependant on the resonance modes used, and the resonance frequency is largely dependent on the size of the resonant part in one resonance mode. The piezoelectric couplers employed in the present invention are basically of circular plate form in view of reliability (of function) and conveniences in mass production, using a resonance mode in planar direction, wherein the resonance frequency increases as the diameter of the piezoelectric coupler is reduced.

However, since a discharging lamp is effectively driven, in general, between 20~90kHz, requiring the resonance frequency of an element to match this band, a ballast capable of igniting a discharging lamp between 20~100kHz regardless of size of the element can be manufactured by effectively matching the element to an indictor to be used.

Now, a description of electronic ballast for instantaneous preheating type fluorescent lamp as per the present invention is given below making reference to Figs. 4 and 5.

Fig. 4 illustrates the general circuit of an electronic ballast for instantaneous preheating type fluorescent lamps. While the instant non-preheating ballasts illustrated in Figs. 3a and 3b are advantageous in that they can be applied to various discharging lamps, a drawback of such ballasts is that they cause dispersion of filaments in the fluorescent lamps when they are used for fluorescent lamps, leading to an early blackening of the fluorescent lamps. Thus, a piezoelectric ceramics controller has been employed in the embodiment example in Fig. 4, between the output part of the ballast and the discharging lamp in Fig. 3a, in order to improve this drawback.

The piezoelectric ceramics controller comprises, as illustrated in Fig. 5, a piezoelectric coupler, a time delay module, and a protection circuit for reverse electromotive force elimination. The function of piezoelectric coupler is as described above, while the functions of the time delay module as well as of the protection circuit for reverse electromotive force elimination are explained below:

The time delay module is a circuit for reducing occurrence of blackening of a fluorescent lamp by sufficiently preheating filaments of the lamp at the starting time, and thus, safely operating the piezoelectric coupler (PC). Upon supply of power, Zener voltage is formed at *ZD2 and charged in *C4, which charged voltage is charged further in *C3 via *R4. During *C3 is charged, gate voltage of *Q2 is formed across *R4, to operate *Q2. If *Q2 is operated, +B power source of terminal No. 11 flows through the relay via *R6. If *C3 is fully charged, no current flows through *R4 and the relay is switched off. In other words, the relay is driven only when the *C3 is being charged. While the relay is driven, a terminal and b terminal of the relay are connected to terminal No. 2 so that no connection is established between the ballast and the discharging lamp. That is, the discharging lamp remains unconnected to the piezoelectric coupler for a predetermined period of time after supply of the power, and, after the above time has elapsed, the discharging lamp is connected to the piezoelectric coupler to drive the latter safely. The relay driving time can be adjusted by the values of *C3 and *R4.

The protection circuit for reverse electromotive force elimination functions to protect the lamp from damages by the reverse electromotive force generated from the inductor T2 (EI2519) at the moment when the circuit is operated. Such operation is explained below referring to Figs. 4 and 5.

Upon application of power, strong reverse electromotive force is generated at both ends of the inductor T2, and the boosted voltage at the secondary winding is charged in *C6 via Dll. The boosted voltage is then divided by R9 and R10, and the voltage at both ends of R10 is raised higher than the voltage at normal driving, so that *ZD1 is turned on by the boosted voltage to drive *Q1. With driving of *Q1, the drain current flows through *R2 so that voltage drop at both ends of *R2 occurs. This voltage drop effects both the drain voltage of *Q1 and the oscillation voltage to drop, so that the lamp fails to operate due to the dropped voltage even when oscillation is possible. However, if preheating of the filaments is ended and the oscillation frequency returns to the operation frequency band, the lamp light normally.

As described above, the protection circuit for reverse electromotive force elimination functions to protect the lamp and to extend the life time of the lamp by prohibiting ignition of the lamp at the time when the reverse electromotive force is generated.

Fig. 8- is a graph showing the relationship between the input power and the lighting efficiency in a non-preheating ballast and an instantaneous preheating ballast in accordance with the present invention as well as in a commercially traded electronic ballast (by manufacturer A). In a wide range of frequency band from 42kHz to 51kHz, where a high level of efficiency of the ballasts is maintained, a stable driving is possible, this range being greatly wider than that of a product utilizing resonance frequency of piezoelectric ceramics such as piezoelectric ballasts with a drivable frequency range of from ±lkHz to +2kHz, is advantageous not only for mass production, but also for reduction of the manufacture costs. The two newly developed ballasts can be adjusted by the inductance values matched with the piezoelectric coupler. The inductances used for the ballasts are between 2.0mH and 3.0mH and the optimal value thereof vary dependant on the lamp type, power consumption, etc. However, once an optimal inductance is determined, the lighting efficiency remains almost constant. The ballasts show a lighting efficiency of 6 lm/W to 8 lm/W higher than that of a commercially traded electronic ballast with the highest lighting efficiency, which means a reduction in power consumption of about 2.5W to 3.2W per a fluorescent lamp. Further, since driving in a wide frequency band other than resonance frequency band of piezoelectric ceramics is enabled, frequency dependency of the lighting efficiency and the input power can be minimized, so that an excellent mass productivity is obtained.

Fig. 9 is a graph showing the voltage and current characteristics at the time of initial ignition of a 20W compact type fluorescent lamp (CFL). As shown in the drawing, after the initial phase of 0.4 second in which only 600V igniting voltage is loaded to the lamp by using non-resonance frequency, tubular current with normal value of 166mA begins to flow and the initial discharging voltage is reduced to tubular voltage with normal value of 83 V, so that a soft start is enabled with a 0.4 second delay.

The driving frequency of 58kHz given here has resulted from a driving in an area about 12kHz apart from the resonance frequency (70.8kHz) of the piezoelectric coupler. If the driving frequency is made to coincide with the resonance frequency of piezoelectric coupler, an immediate ignition is enabled without the above described delay of time, leading to shortening of the life time of a discharging lamp. In contrast, a ballast in accordance with the present invention has a driving frequency apart from the oscillation frequency of the piezoelectric coupler, so that an initial soft start is enabled, leading extension of life time of a discharging lamp.

Fig. 10 is a graph showing the illumination characteristics of a fluorescent lamp (CFL) with a ballast in accordance with the present invention as per change of temperature. Though construction of the ballast used here is basically the same as the non-preheating type ballast in Fig. 3a, those constituents not required for a compact type fluorescent lamp such as power-factor compensation circuit, no-load protection circuit and the like are not applicable thereto. The graph shows results of the tests performed in a constant temperature and constant humidity oven with a temperature range of -40 ^°C to 150 ^°C for the purpose of determining temperature characteristics of the ballast as per the present invention. A whole body of ballast inclusive of the piezoelectric coupler and a fluorescent lamp have been placed in the above oven, and then, tested as to ignitions and illuminations. With the whole body of ballast inclusive of piezoelectric coupler put in the above oven, a normal driving was possible up to 110^°C without change in the illumination intensity, and the ballast was reliable due to its wide igniting frequency band even when the driving frequency was slightly changed by passive components and the temperature characteristics of the piezoelectric coupler. Here, the maximum temperature at which the capacitor among the passive components employed can be used was 105 °C, coinciding with the temperature range in which a conventional ballast can be used. Further, as shown in Fig. 10, tests with piezoelectric coupler placed alone in the above oven have shown stable ignition up to 130 ^°C and a driving frequency change width lower than about 05. kHz at 130 ^°C, which values evidence excellent frequency response characteristics of the piezoelectric coupler with respect to the temperature, Since ballast and lamp are located adjacent to each other in a compact type fluorescent lamp, a construction in which the heat produced by lamp is directly transmitted to the ballast, elements of such ballast require temperature stability higher than those of other ballasts. Although the range of temperatures allowing a normal use differs largely depending on the types of piezoelectric coupler employed, piezoelectric coupler used in the present invention has shown normal on/off characteristics up to 130 ^°C .

Since a discharging lamp produces a considerable amount of heat due to the current flowing through the filaments in the lamp and the characteristics of such lamp itself, and since such heat contributes to enhance the illumination intensity of the lamp, a ballast is driven generally at a temperature between 80 ^°C and 100°C, in particular, inside temperature of a lamp with integrated ballast reaches more than 100 °C due to the close placing of its ballast to the lamp. However, since an instantaneous pre-heating type ballast in accordance with the present invention uses no filaments after a predetermined short period, as short circuited, generation of heat at the lamp surface can be restricted to only that generated when thermo electrons are collided with inner surface of the fluorescent lamp, thereby discharging light. Thus, a substantial reduction of heat generation at the lamp surface as above, enables not only lowering of inner temperature of the ballast, but also, production of a high efficiency ballast by minimizing loss of the electric energy. Another reason for such high efficiency lies in the fact that the energy applied to the thermo electrons at the time of high voltage discharging (in such a ballast) increases higher than in case of an electronic ballast, such that a greater light energy is discharged when the electric energy is converted into light energy, enhancing the light efficiency of the lamp correspondingly.

The low temperature tests show that a normal ignition is possible from about -25 ^°C, Fluorescent lamps using an electronic ballast are generally used for interior lighting devices while limiting the low ignition temperature to 0 ^°C . This is because, as the energy for activating mercury in a low temperature increases, a correspondingly high voltage shall be maintained. In contrast thereto, the ballast in accordance with the present invention, in which ignition voltage varies dependant on the load, maintains a higher voltage in low temperature in comparison to an electronic ballast, enables easy ignition of a fluorescent lamps in low temperature. Accordingly, the non-preheating type ballast is suitable for use in a wider range of temperature, e.g. for back lighting lamps for wide screen outdoor advertisements.

Examples of increase in light efficiencies as above follow below:

Experiment 1 The results for the properties of the non-preheating type ballasts using various discharging lamps as shown in Figs. 3a or 3b with circular plate piezoelectric coupler are shown in Tables la and lb below.

The size of piezoelectric coupler used, being variable depending on type of the discharging lamp used, is indicated in the above Tables as capacitance C of the piezoelectric couplers. In the above Tables, T8, T5, T2 are linear tubular fluorescent lamps with a diameter of 26mm, 16mm, 7mm, respectively, FPL are U-shape fluorescent lamps, compact type fluorescent lamps are fluorescent lamps with integrated ballast (CFL), and high voltage discharging lamps are 50W sodium lamps. For the driving frequency, a non- resonant frequency band has been used varying between 30kHz and 65kHz band dependant on the diameter of the piezoelectric coupler and the inductor employed. A minimum electricity efficiency of 91% has been measured for all discharging lamps measured, in particular, general fluorescent lamp T8 with a relatively large diameter showed a surface temperature of 40 ^°C to 45 °C, which temperature is substantially lower than a surface temperature of 60 °C to 70 °C generated when the lamp is driven by a cathode preheating type electronic ballast. Since these characteristics are due to instant ignition of the ballast, all lamps with the above ballasts show radiant heat lower than that with an electronic ballast.

Ballast in accordance with the present invention showed an electricity efficiency of 91%) for high voltage sodium lamps, while the conventional magnetic ballast showed power efficiency of below 80%. Further, ballast as per the present invention can perform stable ignition by generating an initial ignition voltage of about 3800V without any ignition circuit.

Experiment 2

Table 2 below shows electrical characteristics detected in igniting a 32W linear tubular fluorescent lamp (T8) with a non-preheating type ballast or an instantaneous preheating type ballast equipped with a circular plate form piezoelectric coupler as in Fig. 3a or Fig. 4.

* Luminous flux, light efficiency, and the like vary dependant on types of the lamp and/or experiment conditions. For reference, light efficiency of a standard ballast for linear tubular fluorescent lamp of 32W as used in the above experiment is 70.421m/W.

Experiment 3

Table 3 below represents a comparison of electrical characteristics of the above ballast in Fig. 3 with those of electronic ballast, showing that the former has a higher electric efficiency. Although compact type fluorescent lamp has an input power lower than that of the conventional products, it can achieve illumination intensity same as that of the conventional products with this input power by maximizing the lamp characteristics. In addition, in contrast to a general electronic ballast in which specification of a transformer or choke to be installed as a part thereof shall vary dependant on the discharging lamp or load, the above ballast with piezoelectric coupler controls the initial ignition voltage and lamp voltage automatically in accordance with the load characteristics to be applied.

Input Lamp Lamp Power

Type of

Ballast type Power Voltage current Efficiency Remarks lamp

( ) (V) (mA) (%)

Piezoelectric 49.2 79 582 93.5

FPL 55W

Electronic A 48.7 76 514 80.2

Piezoelectric 14.8 83 166 93.1

Screw type fluorescent Electronic B 16.9 89.5 158 83.7 lamp 20W

Electronic C 17.8 86 188 91.0

Although the present invention has been described above with reference to the embodiment examples and the drawings, the scope of rights of the present invention is not limited thereto, but rather, shall be determined by the claims attached herein after and their equivalents, allowing various modifications and adaptations without departing the spirit of the present invention, as those skilled in the art will understand.

INDUSTRIAL APPLICABILITY

As described above, the present invention enables production of a superb power saving product that achieves a high efficiency over that of a conventional electronic ballast, by using piezoelectric ceramics as an integrated part of the ballast, which is applicable to both a general fluorescent lamp and a high voltage discharging lamp.

Further, the present invention has realized a ballast with higher efficiency by manufacturing a non-preheating type discharging lamp ballast with excellent low temperature ignition characteristics as well as an instantaneous preheating type fluorescent lamp ballast which can prolong life time of a fluorescent lamp and at the same time achieve high efficiency.

Moreover, the present invention allows to minimize the size of piezoelectric ceramics, by driving the ballast in a non-resonance frequency band quite apart from the resonance frequency band of the piezoelectric ceramics, so that a stable driving of a discharging lamp of 20W or more with the size of the piezoelectric ceramics as same as that of a general capacitor is possible. Thus, the present invention provides great advantages in that it allows mass production of reliable ballasts, in difference to a piezoelectric transformer, wherein size of the ceramics shall grow for application in ballasts for general discharging lamp.

In addition, by allowing application of piezoelectric coupler to LCD backlights (CCFL) as well as compact compact type fluorescent lamp (CFL) inverters, the present invention enables a wide range application across the industrial boundaries of piezoelectric coupler, which is far better in terms of both mass productivity and manufacture cost than a piezoelectric transformer currently in use for low power inverters.

Claims

WHAT IS CLAIMED IS:

1. An electronic ballast with piezoelectric ceramics comprising a rectifier for obtaining DC power from AC power; a pulse generator for generating pulse with predetermined frequencies; a frequency setting part for setting said predetermined frequencies; two field effect transistors for switching voltage outputted from said rectifier based on said pulse generated by said pulse generator; an inductor which connects common output of said two field effect transistors to a discharging lamp; and a piezoelectric coupler connected in parallel to said discharging lamp.

2. The electronic ballast as set forth in Claim 1, comprising additionally a time delay part, which disconnects said piezoelectric coupler from said discharging lamp for a predetermined period of time after application of power; and a protection part for reverse electromotive force elimination, which prohibits ignition of said discharging lamp by reducing the oscillation voltage during reverse electromotive force, is generated after application of power.

3. The electronic ballast as set forth in any one of Claim 1 or Claim 2, comprising additionally a reset part, which resets said pulse generating part while it is connected to said pulse generator, if an overload or no load occurs.

4. The electronic ballast as set forth in any one of Claim 1 or Claim 2, comprising additionally a capacitor, which is connected in series to said discharging lamp, while it is connected in parallel to said piezoelectric coupler.

5. The electronic ballast as set forth in any one of Claim 1 or Claim 2, wherein said frequency setting part sets a frequency apart from resonance frequencies generated by combination of said piezoelectric coupler and said inductor.

6. The electronic ballast as set forth in any one of Claim 1 or Claim 2, wherein said piezoelectric coupler is of circular plate form having its resonance mode in planar direction.

7. The electronic ballast as set forth in any one of Claim 1 or Claim 2, wherein said inductor has an inductance of 2mH or more.