Abstract:
An inverter for supplying alternating current to an EL lamp includes a first charging path, a first discharging path, a second charging path, a second discharging path, the paths intersecting at a node, wherein the node is the output of the inverter from which the alternating current flows. The charging paths include inductive boost circuits having a common inductor.

Description:
BACKGROUND 
   This invention relates to inverters for electroluminescent (EL) lamps and, in particular, to an inverter producing lower amplitude current spikes through an EL lamp having one electrode grounded. 
   An EL lamp is essentially a capacitor having a dielectric layer including a phosphor powder and a dielectric layer between two conductive electrodes, one of which is transparent. Because the EL lamp is a capacitor, an alternating current (AC) must be applied to cause the phosphor to glow, otherwise the capacitor charges to the applied voltage and current through the EL lamp ceases. The phosphor particles radiate light in the presence of a strong electric field, using relatively little current. As used herein, an EL “panel” is a single sheet including one or more luminous areas, wherein each luminous area is an EL “lamp.” 
   In portable electronic devices, automotive displays, and other applications where the power source is a low voltage battery, an EL lamp is powered by an inverter that converts direct current into alternating current. In order for an EL lamp to glow sufficiently, a peak-to-peak voltage in excess of about one hundred and twenty volts is necessary. The actual voltage depends on the construction of the lamp and, in particular, the field strength within the phosphor powder. The frequency of the alternating current through an EL lamp affects the life of the lamp, with frequencies between 200 hertz and 1000 hertz being preferred. Ionic migration occurs in the phosphor at frequencies below 200 hertz. Above 1000 hertz, the life of the phosphor is inversely proportional to frequency. 
   An inverter for EL lamps is typically what is known as a “flyback” inverter in which the energy stored in an inductor is supplied to the EL lamp as a small current at high voltage. If one considers a system as including a battery, an inductor, and an EL lamp, the prior art discloses switching one of these elements to obtain an alternating current through the lamp. 
     FIG. 1  is a schematic diagram based upon U.S. Pat. No. 4,527,096 (Kindlmann), in which the EL lamp is switched. When transistor  14  turns on, current flows through inductor  15 , storing energy in the magnetic field generated by the inductor. When transistor  14  shuts off, the magnetic field collapses at a rate determined by the turn-off characteristics of transistor  14 . The voltage across inductor  15  is proportional to the rate at which the field collapses. Thus, a low voltage and large current is converted into a high voltage at a small current. 
   The current pulses are coupled through diode  16  to the DC diagonal of a switching bridge having EL lamp  12  connected across the AC diagonal. The transistors in opposite legs of the bridge conduct alternately to reverse the connections to lamp  12 . The bridge transistors switch at a lower frequency than transistor  14 . The four bridge transistors are high voltage components, adding considerably to the size and cost of the circuit. The circuit is not single ended, i.e. one cannot ground one side of lamp  12 . Inductor  15  discharges directly into lamp  12 , producing current spikes. The bridge circuit disclosed in the Kindlemann patent is also sometimes referred to as an H-bridge output, where the switching transistors form the posts and the EL lamp forms the cross-bar of the H. 
   U.S. Pat. No. 5,436,283 (Sanderson) discloses a variation of the circuit shown in  FIG. 1 . The variation includes a storage capacitor connected across the DC diagonal of the bridge and a constant current source in each of the two upper legs of the bridge. This reduces current spikes but does not provide a single ended output. U.S. Pat. No. 5,686,797 (Sanderson) includes the same disclosure as the &#39;283 patent. 
     FIG. 2  is a diagram taken from U.S. Pat. No. 5,313,141 (Kimball). U.S. Pat. No. 5,668,703 (Rossi et al.) discloses substantially the same circuit, in which the inductor is switched to obtain an alternating current. Inverter  20  is a three terminal device having supply terminal  21 , ground terminal  22 , and high voltage terminal  23 . Within inverter  20 , first switching circuit  25  pumps current pulses through inductor  26  and second switching circuit  27  connects current pulses from inductor  26  to EL lamp  12  through high voltage terminal  23 . 
   Switching circuit  25  includes switches  31  and  32  forming a series circuit with inductor  26  between supply terminal  21  and ground terminal  22 . Switching circuit  27  includes switches  33  and  34  connected between each end of inductor  26  and high voltage terminal  23 . Specifically switch  33  is connected between end  37  of inductor  26  and high voltage terminal  23 . Switch  34  is connected between end  38  of inductor  26  and high voltage terminal  23 . 
   When switches  31  and  34  are closed (conducting) and switch  33  is open (non-conducting), switch  32  opens and closes at a high frequency, producing a series of high voltage pulses that are connected from terminal  38  of inductor  26  through switch  34  to high voltage terminal  23 . When switch  32  opens, the field on inductor  26  collapses, attempting to maintain the current flowing in the same direction as before switch  32  opened. The only current path remaining is through switch  34  to lamp  12 , charging the upper electrode of lamp  12  positively. Diode  35  blocks current from lamp  12  to ground when switch  32  is closed. 
   For the second half of the cycle, switch  32  closes and remains closed, switch  34  opens and remains opened, and switch  33  closes and remains closed. Switch  31  opens and closes at high frequency, producing a series of current pulses through inductor  26 . During this half of the cycle, terminal  37  of inductor  36  is connected through switch  33  to lamp  12 . When switch  31  opens, the collapsing field in inductor  26  tries to maintain the current flowing in the same direction as before switch  31  opened. Since terminal  37  is connected to lamp  12 , this current is drawn from lamp  12 , discharging the upper electrode of lamp  12  and eventually charging the upper electrode negatively. Diode  36  blocks current from lamp  12  to supply terminal  21  when switch  31  is closed. After a given number of high frequency pulses, the upper electrode of lamp  12  is at a peak negative voltage and the cycle ends. 
     FIG. 3  is a functional diagram of a circuit based upon U.S. Pat. No. 5,854,539 (Pace et al.), in which the battery is switched to obtain an alternating current. The circuit operates similarly to the circuit of  FIG. 1 , except that the battery connections are periodically reversed instead of the lamp connections. Inductor  41  dumps current directly into EL lamp  12 , producing undesirable current spikes. Like the circuit shown in  FIG. 1 , one terminal of lamp  12  cannot be grounded. 
   An inverter having a single ended output has several advantages over inverters with bridge type outputs and is very much desired in the market. Unfortunately, the advantages come with a trade-off, viz. an inductor discharges directly into an EL lamp, producing a current spike that causes excessive power consumption, reduced efficiency, and difficulty driving some high impedance EL lamps having an area greater than approximately 15 cm 2 . Specifically, some thin, screen printed EL lamps adapted for backlighting keypads exhibit high impedance. Other lamps do as well, depending upon materials and thicknesses. EL lamps have a nominal capacitance of 0.47 nf per square centimeter. Discharging an inductor directly into a capacitor greater than about 10 nf can cause significant current spikes. Semiconductor components used for implementing an inverter must withstand not only the high voltage from an inductor but the current spike as well. This increases the cost of implementing the inverter as an integrated circuit and restricts the technologies that can be used for making the inverter. 
   In view of the foregoing, it is therefore an object of the invention to provide an inverter having a single ended output and reduced current spikes. 
   Another object of the invention is to provide a single ended inverter that can be implemented in bipolar or CMOS technologies. 
   A further object of the invention is to improve the efficiency of an inverter having an single ended output. 
   SUMMARY OF THE INVENTION 
   The foregoing objects are achieved in the invention in which an inverter for supplying alternating current to an EL lamp includes a first charging path, a first discharging path, a second charging path, a second discharging path, the paths intersecting at a node, wherein the node is the output of the inverter from which the alternating current flows. The charging paths include inductive boost circuits having a common inductor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a schematic diagram of an inverter constructed in accordance with the prior art; 
       FIG. 2  is a schematic diagram of an inverter constructed in accordance with the prior art; 
       FIG. 3  is a schematic diagram of an inverter constructed in accordance with the prior art; 
       FIG. 4  is a block diagram of an inverter constructed in accordance with the invention; 
       FIG. 5  is a schematic of an inverter constructed in accordance with a preferred embodiment of the invention; and 
       FIG. 6  is a chart of the signals at various points in the circuit illustrated in  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 4  is a block diagram of an inverter constructed in accordance with the invention. EL lamp  12  is powered by an inverter including positive high voltage supply  43 , negative high voltage supply  44 , and switches  45 ,  46 ,  47 , and  48 . Switch  46  couples lamp  12  to reference  1 . Switch  48  couples lamp  12  to reference  2 . Reference  1  can be either low voltage supply or common and reference  2  can be either low voltage supply or common, independently of reference  1 . Thus,  FIG. 4  represents any one of four combinations of the circuit. The charging and discharging paths have common node  49 , wherein the node is a single ended output terminal for the inverter. 
   Switch  45  closes to charge lamp  12  from supply  43  and switch  46  closes to discharge lamp  12 . Switch  47  closes to charge lamp  12  from supply  44  and switch  48  closes to discharge lamp  12 . No two switches are closed simultaniously. The operation of the switches is controlled by suitable logic, not shown. Using a bi-directional semiconductor switch for one of switches  46  and  48 , the other of switches  46  and  48  can be eliminated. However, implementation is simpler if directional current paths are used. Thus, two discharge paths are shown for the preferred embodiment. 
   Supplies  43  and  44  can be any known circuit for inverting low voltage DC, e.g. 3–15 volts, to high voltage DC, e.g. 50–160 volts. Separate supplies provide some advantages but not including cost. In a preferred embodiment of the invention, the high voltage supplies share some components. 
     FIG. 5  is a schematic of an inverter constructed in accordance with a preferred embodiment of the invention. Rail  51  is connected to a source of low voltage DC, such as a battery. Rail  52  is common. Transistor  53 , inductor  54 , and transistor  55  are coupled in series between rail  51  and rail  52 . Diode  61  couples one end of inductor  54  to capacitor  61 . Diode  63  couples the other end of inductor  54  to capacitor  64 . 
   As thus configured, transistor  53 , inductor  54 , transistor  55 , diode  61 , and capacitor  62  constitute a negative high voltage supply. Similarly, transistor  53 , inductor  54 , transistor  55 , diode  63 , and capacitor  64  constitute a positive high voltage supply. To generate a positive voltage, transistor  53  conducts, as indicated in  FIG. 6  by signal “Y”, while transistor  55  is pulsed, as indicated by signal “X”. The result is a series of positive, high voltage output pulses through diode  63  that charge capacitor  64  positively. To generate a negative voltage, transistor  55  conducts while transistor  53  is pulsed, producing a series of negative, high voltage output pulses through diode  61  that charge capacitor  62  positively. Diodes  61  and  62  are oppositely poled, thereby providing opposite polarity voltages. 
   The positive voltage on capacitor  64  is coupled to lamp  12  through transistor  71  and resistor  72  during a first interval, represented by curve  73  in  FIG. 6 . Transistor  71  conducts while signal “A” ( FIG. 6 ) is high. Current spikes are minimized or absorbed by capacitor  64  and resistor  72 . Lamp  12  is then discharged through diode  74 , transistor  75 , and resistor  76  during a second interval, represented by line  78  in  FIG. 6 . As also indicated by  FIG. 6 , discharge pulse “B” is high for a period longer than the time required to discharge lamp  12  substantially to common. 
   In theory, lamp  12  will never discharge to zero volts through a resistor of finite resistance. What is of interest here is the practical, not the theoretical. Lamp  12  is discharged to a sufficiently low voltage that reversing the polarity of the voltage applied to lamp  12  will not cause excessive current. As noted above, the discharge circuit can be referenced to either supply voltage, if there are two, and common or to supply or common. Thus, the residual voltage on lamp  12  can be as much as the absolute magnitude of the supply voltage plus some voltage, e.g. a residual voltage of ±20 volts. 
   The negative voltage on capacitor  62  is coupled to lamp  12  through transistor  81  and resistor  82  during the next interval, represented by curve  83  in  FIG. 6 . Transistor  81  conducts while signal “C” ( FIG. 6 ) is low. Current spikes are minimized or absorbed by capacitor  62  and resistor  82 . Lamp  12  is then discharged through diode  84 , transistor  85 , and resistor  86  during the next interval, represented by line  88  in  FIG. 6 . As also indicated by  FIG. 6 , discharge pulse “D” is low for a period longer than the time required to discharge lamp  12  substantially to rail  51 . As shown by  FIG. 6 , transistors  71  and  81  conduct alternately to produce an alternating current through lamp  12  and are periodically simultaneously non-conducting twice each cycle of the alternating current, allowing lamp  12  to discharge. 
   It is known in the art to discharge EL lamps at two different rates for noise reduction; see U.S. Pat. No. 5,789,870 (Remson). Transistor  91  and  92  provide optional, reduced resistance current paths for increasing the discharge rate after lamp  12  has discharged somewhat. Control signal B′ (not shown) begins after pulse B and ends with pulse B. Similarly, control signal D′ (not shown) begins after pulse D and ends with pulse D. The result is a gradual discharge, represented by the dashed lines in waveform V,  FIG. 6 , followed by a more rapid discharge. 
   The invention thus provides an inverter having a single ended output and reduced current spikes, thereby improving the efficiency of the inverter. The inverter can be implemented in bipolar or CMOS technologies. 
   Having thus described the invention, it will be apparent to those of skill in the art that various modifications can be made within the scope of the invention. For example, the discharge current paths for a two stage discharge can have the same or different impedances; i.e., the paths do not have to be matched. The reduction in resistance comes from the paths being parallel. Alternatively, one could bypass resistors  76  and  86  with transistors driven by signals B′ and D′, respectively, to obtain the same effect. Plural transistors can be used where a single transistor is illustrated. That is, for example, two or more transistors can be used in parallel to increase current capacity, or in series to increase voltage capacity, where a single transistor is shown. This is frequently done when implementing a circuit in integrated circuit form. The same technique is often used with passive components also.