Patent Publication Number: US-7902763-B2

Title: Piezoelectric cascade resonant lamp-ignition circuit

Description:
The present invention is a continuous-in-part application of the application that is entitled “HIGH-VOLTAGE LAMP-IGNITION PIEZOELECTRIC OSCILLATOR” (U.S. application Ser. No. 12/025,852) filed Feb. 5, 2008 now U.S. Pat. No. 7,683,554, which is filed presently with the U.S. Patent &amp; Trademark Office, and which is used herein for reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a cascade resonant lamp-ignition circuit, particularly to a piezoelectric cascade resonant lamp-ignition circuit formed via cascading several sets of piezoelectric capacitors to an independent inductor. 
     2. Description of the Related Art 
     The principle of CCFL (Cold Cathode Fluorescent Lamp) is similar to that of the daylight lamp. When a high voltage is input to the electrodes, few electrons impact the electrode at high speed to generate secondary electrons. Then, discharge begins, and electrons collide with mercury atoms, and the mercury atoms radiate ultraviolet ray with a wavelength of 253.7 nm Then, the ultraviolet ray excites the fluorescent powder on the inner tube wall to emit visible light with the correlated color temperature. In addition to be used in display devices, PDA, digital cameras, mobile phones, etc., CCFL is also an indispensable element for backlight modules. 
     With the increasing size of LCD, the number of CCFL of the backlight unit is also increased to maintain the same brightness or even acquire a higher brightness. To achieve brightness uniformity and a long service life, the currents of lamps and the difference of currents should be strictly controlled. In a conventional multi-lamp module, the lamps connect with a traditional coil-type step-up transformer. However, the traditional coil-type step-up transformer has low efficiency and low breakdown voltage. Therefore, the traditional coil-type step-up transformer is an unsafe device because it is likely to be punctured by a high voltage and burned down. Refer to  FIG. 1  for another multi-lamp module. The difference of currents is compensated by the capacitor  110  cascaded to the high-voltage end of the lamp  100 . However, such a design has low efficiency and great leakage current. Further, the capacitor  110  has low breakdown voltage and may explode and cause a fire. 
     SUMMARY OF THE INVENTION 
     The primary objective of the present invention is to provide a piezoelectric cascade resonant lamp-ignition circuit, which uses the intrinsic capacitors of a piezoelectric transformer as piezoelectric capacitors, wherein several sets of the piezoelectric capacitors and a resonant inductor are cascaded to form a resonant lamp-ignition circuit, whereby the lamp-ignition circuit of the present invention has advantages of small leakage current, current balance and high lamp ignition efficiency. 
     Another objective of the present invention is to provide a piezoelectric cascade resonant lamp-ignition circuit, wherein a piezoelectric transformer replaces the capacitors of the traditional lamp-ignition circuits or the coil-type step-up transformers, whereby the present invention has small volume and outstanding electric performance, and whereby the present invention prevents from overheat and malfunction caused by low breakdown voltage, wherefore the present invention has high reliability and superior market competitiveness. 
     A further objective of the present invention is to provide a piezoelectric cascade resonant lamp-ignition circuit, which uses cascade connection to decrease wire length and reduce the final size of the product. 
     To achieve the abovementioned objectives, the present invention replaces the ballast and inverter of the conventional resonant lamp-ignition circuit with a piezoelectric capacitor, which is originally used as the high-power piezoelectric ceramic oscillation plate of the ultrasonic oscillator. The resonant lamp-ignition circuit of the present invention has a step-up ratio varying with the inner impedance of the load; therefore, the present invention is very suitable to drive lamps. When lamps have not lightened yet, the equivalent circuit is in an open-circuit state, and the resonant lamp-ignition circuit of the present invention supplies a very high step-up ratio to instantly ignite the lamps. When the lamps have lightened, the equivalent impedance and the step-up ratio both decrease, and the lamps operate in a steady state. 
     Further, the present invention balances the currents of a plurality of lamps. The present invention uses a fixed frequency to attain a fixed inner impedance of the equivalent circuit of the piezoelectric capacitor of a lamp and make a fixed current flow through the lamp. When the piezoelectric capacitors of the lamps have approximate electric performances, the piezoelectric capacitors also have approximate inner impedances, and the lamps have almost identical currents. Thus are balanced the currents of a plurality of lamps. 
     The present invention integrates several sets of piezoelectric capacitors and independent inductors to form a resonant lamp-ignition circuit. The embodiments of the present invention include a full-bridge double-high-voltage lamp-ignition architecture and a half-bridge single-high-voltage lamp-ignition architecture. 
     Below, the embodiments are described in detail in cooperation with the drawings to make easily understood the objectives, characteristics and functions of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically showing a multi-lamp module using conventional capacitors; 
         FIG. 2  is a diagram schematically showing a piezoelectric cascade resonant lamp-ignition circuit according to one embodiment of the present invention; 
         FIG. 3  is a diagram schematically showing the structure of a piezoelectric capacitor according to one embodiment of the present invention; 
         FIG. 4  is a diagram schematically showing a full-bridge piezoelectric cascade resonant lamp-ignition circuit according to one embodiment of the present invention; 
         FIG. 5  is a diagram schematically showing the application of the present invention to EEFL; 
         FIG. 6  is a diagram schematically showing the application of the present invention to LEDs; 
         FIG. 7  is a diagram schematically showing the application of the present invention to a power saving light bulb; 
         FIG. 8  is a diagram schematically showing another full-bridge piezoelectric cascade resonant lamp-ignition circuit according to one embodiment of the present invention; 
         FIG. 9  is a diagram schematically showing a half-bridge piezoelectric cascade resonant lamp-ignition circuit according to one embodiment of the present invention; and 
         FIG. 10  is a diagram showing the equivalent circuit of a piezoelectric capacitor according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Refer to  FIG. 2  a diagram schematically showing a piezoelectric cascade resonant lamp-ignition circuit according to one embodiment of the present invention. As shown in  FIG. 2 , the lamp-ignition circuit of the present invention comprises several sets of piezoelectric capacitors  10  and  20  and a resonant inductor  40 . Each set of piezoelectric capacitors  10  and  20  are cascaded to a CCFL  30 . All sets of piezoelectric capacitors  10  and  20  are parallel connected, and then all sets of piezoelectric capacitors  10  and  20  are totally cascaded to the resonant inductor  40 . In the present invention, the intrinsic capacitors of the piezoelectric transformer function as the piezoelectric capacitors  10  and  20 . The piezoelectric capacitors  10  and  20  and the resonant inductor  40  are cascaded to form a resonant lamp-ignition circuit containing an inductor and a piezoelectric transformer cascaded to each other. The resonant lamp-ignition circuit is boosted to ignite lamps by adjusting the resonant inductor  40  and the capacitance of the piezoelectric transformer. 
     The piezoelectric capacitors  10  and  20  are structurally similar, and the piezoelectric capacitor  10  is used to exemplify them herein. Refer to  FIG. 3 . In the piezoelectric capacitor  10 , a piezoelectric material is fabricated into a disc-shape piezoelectric substrate  11 , and a conductive paste, such as a silver paste, a copper paste or a nickel paste, is applied onto the upper and lower surfaces of the piezoelectric substrate  11  to form electric conduction layers  12  and  13 . The electric conduction layers  12  and  13  function as the electrodes of the piezoelectric capacitor  10  and conduct current. The piezoelectric substrate  11  and the electric conduction layers  12  and  13  may also be fabricated into a rectangular shape or a square shape. Refer to  FIG. 10  for an equivalent circuit of the piezoelectric capacitor  10  or  20 . The equivalent circuit contains an equivalent resistor R, an equivalent inductor L, and two equivalent capacitors Ca and Cb that respectively represent the mechanical and electric features. Distinct from the traditional capacitors or the coil-type step-up transformers, the piezoelectric capacitor  10  or  20  of the present invention has small leakage current and high breakdown voltage and thus is exempt from the danger of catching a fire. Therefore, the present invention is safe and reliable. The present invention increases the output power by several folds and obviously promotes the lamp-ignition efficiency. Further, the piezoelectric capacitors have a small volume and a small package thickness, and the piezoelectric capacitors, the resonant inductor and the lamps are connected in series. Thus, the final size of the product is reduced. Compared with a parallel connection design, the cascade connection design of the present invention keeps the circuit at a lower temperature and reduce loss. 
     The piezoelectric cascade resonant lamp-ignition circuit of the present invention effectively maintains the balance of lamp currents. When a DC pulse voltage is converted into an AC power to drive the circuit, the piezoelectric capacitor boosts a low voltage to such a high voltage that ignites the lamps. The variation of lamp impedances causes the non-uniformity of lamp currents and then results in uneven brightness and shorter lamp lives. When the present invention drives the resonant lamp-ignition circuit at a fixed frequency, the inner impedance of the equivalent circuit of the piezoelectric capacitor has a fixed value. Thus, a fixed current flows through the lamp. When the piezoelectric capacitors of the lamps have approximate electric performances, the piezoelectric capacitors also have approximate inner impedances, and the lamps have almost identical currents. Thus are balanced the currents of a plurality of lamps. 
     In this embodiment, an auxiliary piezoelectric capacitor  50  may be connected in parallel with the resonant inductor  40  and all the lamps  30  to form a cascade-parallel resonant lamp-ignition circuit. Thereby, in addition to igniting lamps, the present invention adjusts the capacitance to finely tune the output current and optimize the output power. At the moment of lamp ignition, the voltage rises abruptly. After lamps have been ignited, the inner impedance of the load decreases, and the step-up ratio also descends. Therefore, the present invention adjusts the output to reduce power consumption. 
     In the abovementioned embodiment, two piezoelectric capacitors  10  and  20  for each lamp  30  and a single resonant inductor  40  for all the lamps  30  form a half-bridge resonant circuit, which reduces the cost of fabrication and promote the competitiveness of price. Refer to  FIG. 4 . Another resonant inductor  60  is added to the half-bridge resonant circuit to form a full-bridge resonant circuit, which outputs higher power. 
     The present invention applies to a single CCFL, a single EEFL (External Electrode Fluorescent Lamp), a single power saving light bulb, and a single LED (Light Emitting Diode). The present invention also applies to parallel CCFLs, parallel EEFLs, parallel power saving light bulbs, and parallel LEDs. Refer to from  FIG. 5  to  FIG. 7  diagrams respectively schematically showing the applications of the present invention to an EEFL  70 , LEDs  80  and a power saving light bulb  90 . Each of the lamp-ignition circuits shown in  FIGS. 5-7  contains a full-bridge resonant circuit. However, the lamp-ignition circuit containing a half-bridge resonant circuit also applies to the abovementioned cases. 
     The present invention also applies to a large size (such as over 42 in.) backlight plate. A large size backlight plate usually needs longer (such as over 1 m) lamps. However, long lamps have greater brightness difference caused by greater inner capacitance loss thereof. Thus, each long lamp needs independent resonant inductors and independent capacitors to balance capacitance. Refer to  FIG. 8  and  FIG. 9 . In  FIG. 8 , a double high voltage (full-bridge) piezoelectric cascade resonant lamp-ignition circuit  200  is used to exemplify the application of the present invention to a large size backlight plate, and each lamp  30  is cascaded to two piezoelectric capacitors  10  and  20  and two resonant inductors  40  and  60 . In  FIG. 9 , a single high voltage (half-bridge) piezoelectric cascade resonant lamp-ignition circuit  300  is used to exemplify the application of the present invention to a large size backlight plate, and each lamp  30  is cascaded to one piezoelectric capacitor  10  and one resonant inductor  40 . 
     The embodiments described above are only to exemplify the present invention but not to limit the scope of the present invention. Therefore, any equivalent modification or variation according to the spirit of the present invention is to be also included within the scope of the present invention, which is based on the claims stated below.