Abstract:
Circuits and methods employing the inherent capacitance of an electro-luminescent lamp in a resonant circuit to recover the energy otherwise lost in electro-luminescent lamp driver circuits to thereby increase the overall efficiency of such circuits and to lower the noise and increase the life of the electro-luminescent lamp itself.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to electro-luminescent lamp panels and, more particularly, to a resonant circuit for recovering energy stored in these panels that would otherwise be dissipated during the discharge cycle of a drive circuit. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     Electro-luminescent lamps act as capacitors, electrically. These lamps store energy, as do all capacitors, in the form of an electrical voltage charge. In the normal electro-luminescent lamp driver circuit, this charge is dissipated, and therefore lost, during the discharge cycle of operation. 
     Electro-luminescent lamp driver circuits are well known in the prior art, exemplary of which are the Supertex HV803 and the Toko TK659XX. A typical one of these circuits is illustrated in FIG.  1 . In that circuit, components L 1 , S 0 , D 1 , and C 1  constitute a high voltage boost or step-up converter which receives a low voltage (less than 6 volts) and boosts it to between 20 and 100 volts on the capacitor C 1 . Components S 1 , S 2 , S 3 , and S 4  constitute an H-bridge circuit that is used to commutate the high DC voltage on capacitor C 1  into a high AC voltage across an electro-luminescent lamp capacitor (C lamp) that is about twice the DC voltage on capacitor C 1 . This AC voltage charges and discharges the capacitor C lamp, with the energy stored in the capacitor C lamp being dissipated in components S 3  and S 4  of the H-bridge during the discharge cycle. 
     It would be advantageous to recover this energy, existing in the form of charge at high voltage, from the electro-luminescent lamp capactior C_lamp, and reuse it, thus making the entire electro-luminescent lamp driving system more efficient. Accordingly, the present invention is directed to a method utilizing capacitor C_lamp in a resonant circuit to implement the recovery of this otherwise lost energy, which is significant in the case of large electro-luminescent lamps that are driven to higher voltages. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a typical prior art electro-luminescent lamp driver circuit that is unable to recover charge from the electro-luminescent lamp capacitor. 
     FIG. 2 is a schematic diagram of an electro-luminescent lamp driver circuit employing a series resonant circuit. 
     FIG. 3 is a schematic diagram of a first alternative electro-luminescent lamp driver circuit employing a series resonant circuit within an H-bridge circuit. 
     FIG. 4 is a schematic diagram of a second alternative electro-luminescent lamp driver circuit employing a split series resonant circuit within an H-bridge circuit. 
     FIG. 5 is a schematic diagram of an electro-luminescent lamp driver circuit employing a parallel resonant circuit within an H-bridge circuit. 
     FIG. 6 is a schematic diagram of an alternative electro-luminescent lamp driver circuit employing a parallel resonant circuit. 
     FIG. 7 is a schematic diagram of an alternative electro-luminescent lamp driver circuit employing a series resonant circuit. 
     FIG. 8 is a waveform diagram illustrating typical waveforms generated by the electro-luminescent lamp driver circuit of FIG.  2 . 
     FIG. 9 is a waveform diagram illustrating typical waveforms generated by the electro-luminescent lamp driver circuits of FIGS. 3 and 4. 
     FIG. 10 is a waveform diagram illustrating typical waveforms generated by the electro-luminescent lamp driver circuit of FIG.  5 . 
     FIG. 11 is a simplified circuit diagram of an electro-luminescent lamp driver like that of FIG. 7, in which a general AC source is employed in place of the step-up converter/inverter. 
     FIG. 12 is a simplified circuit diagram of an electro-luminescent lamp driver like that of FIG. 6, in which a general AC source is employed in place of the step-up converter/inverter. 
     FIG. 13 is a simplified circuit diagram of an electro-luminescent lamp driver like that of FIG. 3, in which a general DC source is employed in place of the step-up converter. 
     FIG. 14 is a simplified circuit diagram of an electro-luminescent lamp driver like that of FIG. 5, in which a general DC source is employed in place of the step-up converter. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 2, there is shown an electro-luminescent lamp driver circuit employing a series resonant method of charge recovery in which a series resonant circuit formed by capacitors C 1  and C lamp and inductor L 2 . Switches S 1  and S 4  close to connect capacitor C lamp in the resonant circuit, as represented by time t 1  in the waveform diagram of FIG.  8 . Then, when the voltage on capacitor C lamp is decaying after it reaches its resonant peak, switch S 1  opens at time t 2  and switch S 3  closes at time t 3  to quickly complete discharging capacitor C lamp. Next, switch S 4  opens at time t 4 , and switch S 2  closes at time t 5  to initiate charging, by the resonant circuit, of capacitor C lamp in the opposite direction until the voltage on capacitor C lamp begins decaying after it reaches its resonant peak. Switch S 2  opens at time t 6 , and switch S 4  closes at time t 7  to quickly complete discharging capacitor C lamp. At this point, the cycle is repeated with switch S 3  opening at time t 8  and switch S 1  closing at time t 9 . These cycles continue as the step-up power converter, formed by inductor L 1 , switch S 0 , diode D 1 , and capacitor C 1 , pumps energy into the resonant circuit. 
     The merits of the resonant charge recovery electro-luminescent lamp driver circuit of FIG. 2 over prior art electro-luminescent lamp driver circuits are three-fold. First, energy from capacitor C lamp is substantially recovered and transferred to capacitor C 1  through inductor L 2 . Second, the voltage on capacitor C lamp is allowed to rise above the voltage on capacitor C 1 . Third, the waveform of voltage applied to capacitor C lamp is substantially more sinusoidal, which is the preferred waveform for driving an electro-luminescent lamp. 
     There are many alternative ways in which a resonant circuit can be formed with the capacitor C lamp which represents an electro-luminescent lamp panel. FIGS. 3-7 and  11 - 14  illustrate just a few additional examples of the ways in which inductive (L) and capacitive (C) elements can be incorporated around the capacitor C lamp to form a resonant circuit. The present invention lies primarily in the recognition that a resonant circuit can be formed with the capacitive element represented by an electro-luminescent lamp panel and that formation of such a resonant circuit can efficiently drive an electro-luminescent lamp panel. 
     Referring now to FIGS. 3-5, there are shown some alternative ways in which a resonant circuit may be formed. They may employ the same H-bridge switch sequencing employing switches S 1 , S 2 , S 3 , and S 4  as illustrated in FIG.  8  and as described in connection with FIG.  2 . 
     FIG. 3 shows a series resonant circuit formed by inductor L 3  and capacitor C lamp within the H-bridge switches. FIG. 9 illustrates the resulting waveforms of voltage across the capacitor C lamp and voltage on capacitor C 1  of this circuit. This waveform is very similar to the waveform of FIG. 8, except that the transitions are more smooth when changing polarity of the voltage being applied across capacitor C lamp. This is an advatageous effect because a more sinusoidal waveform is preferred for driving an electro-luminescent lamp. This effect may be improved somewhat by refining the timing of switches S 1 , S 2 , S 3 , and S 4 . 
     The circuit of FIG. 4 is the same as that of FIG. 3, except that inductor L 3  has been split into two inductors L 3  and L 4  such that those two inductive elements are symmetrical about the capacitor C lamp. The waveform resulting from this circuit is also represented by FIG.  9 . Therefore, this implementation adds no improvement over the circuit of FIG. 3, unless circuit symmetry is considered important in a particular design. 
     FIG. 5 illustrates a parallel resonant circuit formed by inductor L 5  and capacitor C lamp within the H-bridge switches. FIG. 10 shows the waveform resulting from this circuit. Although this implementation does not show the advantageous feature of the voltage across capacitor C lamp rising above the voltage on capacitor C 1 , as in the previously described waveform diagrams, it does produce a more sinusoidal waveform. 
     FIG. 6 illustrates a parallel resonant circuit formed by inductor L 6  and capacitor C lamp being fed from a drive circuit like that shown in U.S. Pat. No. 5,313,141. It will therefore be appreciated that the resonant circuits of the present invention may be driven by circuits other than the boost and H-bridge circuits described hereinabove. 
     FIG. 7 illustrates a circuit similar to that of FIG. 6, except that the parallel resonant circuit of FIG. 6 has been replaced with a series resonant circuit. 
     FIGS. 11 and 12 are simplified circuit diagrams of electro-luminescent lamp drivers like those of FIGS. 7 and 6, respectively, in which a general AC source is employed in place of the step-up converter/inverters of the earlier figures. 
     FIGS. 13 and 14 are simplified circuit diagrams of electro-luminescent lamp drivers like those of FIGS. 3 and 5, respectively, in which a general DC source is employed in place of the step-up converters of the earlier figures.