Abstract:
A driver for powering an EL lamp includes a boost circuit for converting low voltage DC to high voltage DC and an inverter for converting direct current to alternating current. The driver also includes at least one transistor for discharging,the EL lamp and a generator for producing a ramp voltage that is coupled to the base of the transistor. The ramp voltage causes the discharge current through the EL lamp to be sinusoidal, without harmonics. The discharge current does not cause the EL lamp to produce sound.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to an electroluminescent (EL) lamp and, in particular, to a driver for powering an EL lamp without causing the lamp to produce sound. 
         [0002]    An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer may include a phosphor powder or there may be a separate layer of phosphor powder adjacent the dielectric layer. The phosphor powder radiates light in the presence of a strong electric field, using very little current. Because an EL lamp is a capacitor, alternating current must be applied to the electrodes to cause the phosphor to glow, otherwise the capacitor charges to the applied voltage, the current through the EL lamp ceases, and the lamp stops producing light. 
         [0003]    In portable electronic devices, automotive displays, and other applications where the power source is a low voltage battery, an EL lamp is powered by a driver that converts low voltage direct current into high voltage 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. 
         [0004]    The prior art discloses several types of drivers that include a boost circuit having an inductor in series with a switching transistor. Current through the inductor causes energy to be stored in a magnetic field around the inductor. When the current is abruptly shut off, the induced magnetic field collapses, producing a pulse of high voltage. The voltage across the inductor is proportional to L•δi/δt. Thus, a low voltage at high current is converted into a high voltage at low current. The voltage on a storage capacitor, or the EL lamp itself, is pumped up by a series of pulses from the boost circuit. 
         [0005]    As understood by those skilled in the art, an electrical circuit is physical, not mathematical. In other words, reference to a half cycle is not intended to have mathematical precision. Transistors take time to turn on and turn off, causing transition periods than make periods slightly different from exactly one half period or one quarter period. Nevertheless, in the vernacular of the art, a period is referred to as one half cycle or one quarter cycle. 
         [0006]    As used herein, “ground” does not necessarily mean a connection to earth, merely a connection to electrical common or to a current return path. 
         [0007]    As used herein, a bridge is a circuit having four arms with two pairs of arms connected in parallel between a first pair of terminals. In each pair of arms, the arms are connected in series. The junctions of the arms in each series pair are a second pair of terminals. With unidirectional current elements in the arms and alternate arms conducting simultaneously, a bridge has a DC diagonal across one pair of terminals and an AC diagonal across the second pair of terminals. 
         [0008]    The direct current produced by the boost circuit must be converted into an alternating current in order to power an EL lamp. U.S. Pat. No. 4,210,848 (Suzuki et al.) and U.S. Pat. No. 4,527,096 (Kindlmann) disclose a switching bridge, known as an H-bridge, to alternate the current through an EL lamp. The bridge changes the polarity of the current through the lamp at a low frequency (200-1000 hertz). The H-bridge has an AC diagonal, coupled to an EL lamp, and a DC diagonal, coupled to a boost circuit. The bridge operates like a double pole, double throw switch, as illustrated in  FIG. 1 , to produce an alternating current through the EL lamp. 
         [0009]    When an EL lamp is lit, the front and rear electrodes are oppositely charged and, therefore, are electrostatically attracted. Each cycle of the AC from a driver causes a slight but audible movement in the lamp. Thus, an EL lamp acts like an electrostatic speaker. It is known that a sinusoidal voltage causes substantially no noise to be emitted by an EL lamp. It is also known that the loudness of the noise depends upon a number of variables, e.g. the peak voltage, the thickness of the lamp, the way in which the lamp is mounted, and the size of the cavity in which the lamp is mounted. Because the energy stored in a capacitor is proportional to the square of the voltage (J=½ CV 2 ), reducing lamp voltage will reduce noise but at the expense of brightness. An otherwise identical but thinner lamp requires less energy to vibrate and a lower voltage will produce as loud a sound as made by a thicker lamp at a higher voltage. Increasing the discharge time dissipates energy over a longer time and little or no sound is produced. 
         [0010]    One can easily provide a sinusoidal voltage for an EL lamp if the size of the power supply does not matter. A driver including an LC circuit resonant at low frequency requires an inductor and a capacitor that are physically quite large; too large, for example, for a cellular telephone. Thus, the problem is how to provide a compact driver, suitable for implementing in a single integrated circuit, and how to obtain a sinusoidal voltage from a boost circuit. In either case, the efficiency of the inverter must not be impaired. 
         [0011]    In the prior art, U.S. Pat. No. 5,293,098 (Brownell) discloses powering an EL lamp by way of a transformer coupled to a sine wave generator and a power amplifier. U.S. Pat. No. 5,789,870 (Remson) discloses discharging an EL lamp slowly, then more rapidly to avoid the production of audible noise. The contents of the Remson patent are incorporated herein by reference. U.S. Pat. No. 6,038,153 (Andersson et al.) discloses discharging an EL lamp with a series of pulses to approximate a sine wave. U.S. Pat. No. 5,886,475 (Horiuchi et al.) discloses using a ramp voltage to generate timing signals for discharging an EL lamp. With the exception of the Brownell patent, the prior art uses digital signals to approximate a sine wave. As such, there is always some harmonic distortion. See, for example, the data sheet for an SM8146A driver by Seiko NPC Corporation, which discusses harmonic distortion in a pulsed discharge. An analog driver for an EL lamp, if such were commercially available, would be too large for many applications and be relatively expensive due to the high voltages needed by an EL lamp. 
         [0012]    In view of the foregoing, it is therefore an object of the invention to a sinusoidal discharge for an EL lamp. 
         [0013]    Another object of the invention is to provide an essentially analog discharge in a digital circuit. 
         [0014]    A further object of the invention is to eliminate noise from an EL lamp by providing a sinusoidal discharge of the lamp. 
       SUMMARY OF THE INVENTION 
       [0015]    The foregoing objects are achieved in this invention in which a driver for powering an EL lamp includes a boost circuit for converting low voltage DC to high voltage DC and an inverter for converting direct current to alternating current. The driver also includes at least one transistor for discharging the EL lamp and a generator for producing a ramp voltage that is coupled to the base of the transistor. The ramp voltage causes the discharge current through the EL lamp to be sinusoidal, without harmonics. The discharge current does not cause the EL lamp to produce noise. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  illustrates the operation of an H-bridge inverter; 
           [0017]      FIG. 2  is a schematic of a commercially available inverter having an H-bridge output; 
           [0018]      FIG. 3  is a schematic of an inverter constructed in accordance with a preferred embodiment of the invention for an H-bridge output; and 
           [0019]      FIG. 4  is a partial schematic of an inverter constructed in accordance with a preferred embodiment of the invention for a single ended output. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    As note above, an electroluminescent lamp requires an alternating current for operation. When a direct current source is all that is available, alternately reversing the connections of an EL lamp and a source of direct current will provide an alternating current. As illustrated in  FIG. 1 , the terminals of EL lamp  11  are coupled to respective poles of double pole, double throw (DPDT) switch  12  through resistors  13  and  14 . The throws of switch  12  are connected to capacitor  16 , which stores high voltage DC from a suitable source, not shown. When switch  12  is closed to the left, current flows from the upper electrode of lamp  11  to the lower electrode and flows in the opposite direction when the switch is closed to the right. 
         [0021]      FIG. 2  is a schematic of a commercially available driver that includes the electronic counterpart to a DPDT switch. Driver  20  includes inductor  21  and switching transistor  22  operating in a well known boost configuration to charge capacitor  23  to a high, positive voltage with high frequency pulses. EL lamp  24  is connected to the AC diagonal of a bridge including SCR  25 , SCR  26 , switching transistor  27 , and switching transistor  28 . Capacitor  23  is connected across the DC diagonal of the bridge. SCR  25  and transistor  28  conduct simultaneously to pass current in a first direction through EL lamp  24 . SCR  26  and transistor  27  conduct simultaneously to pass current in a second direction through EL lamp  24 , alternating with SCR  25  and transistor  28  at low frequency. EL lamp  24  is discharged by charging to the opposite polarity. There is no separate discharge period. EL lamp  24  is essentially short circuited to the voltage on capacitor  23  when polarity is initially reversed. 
         [0022]      FIG. 3  is a schematic of an inverter constructed in accordance with a preferred embodiment of the invention for an H-bridge output. The circuit operates in almost the same manner as the circuit of  FIG. 2 , except that the voltage applied to the low side switches, transistors  27  and  28 , are not pulses but a ramp voltage provided by ramp generators  31  and  32 . The duration of the ramp portion is substantially one quarter cycle of the low frequency drive signal. 
         [0023]    There are many known ways to generate a ramp voltage. A simple way is to charge a capacitor from a constant current source. This produces a linear (straight) ramp. A non-linear (curved) ramp is produced by charging a capacitor through a series resistor. Whether the curve is convex upward or concave upward depends upon the particular circuit. 
         [0024]    When a linear ramp voltage is applied to the base of transistor  27 , the transistor becomes slightly conductive and discharge (i.e. charging to the opposite polarity) takes place slowly. As the ramp voltage increases, the rate of discharge increases. The result is a sinusoidal waveform and, despite being an essentially digital circuit, the discharge is analog, i.e. not discontinuous, and is not a simulation or an approximation of a sinusoidal waveform. There are no harmonics. 
         [0025]    The voltage on the base of transistor  27  is a ramp for one quarter cycle, then steady for one quarter cycle, while EL lamp  24  charges to the opposite polarity. At the end of the first quarter cycle, EL lamp  24  is substantially discharged. Charging for the second quarter cycle then occurs as in the prior art. The third quarter cycle of the alternating current begins with a ramp voltage from ramp generator  32  applied to transistor  28 , which is slightly conductive initially. During the fourth quarter cycle, EL lamp  24  charges to the opposite polarity, as in the prior art. Thus, the low side switches conduct for half cycles each but are fully conductive for only a quarter cycle. 
         [0026]    Transistors that are partially conducting dissipate more power than transistors that are fully conducting. One can provide a non-linear ramp to reduce dissipation. Specifically, if the ramp voltage is convex upward, e.g. from the voltage across a capacitor being charged through a resistor, discharge is slightly more rapid initially and the low side switches dissipate less power. Obviously, if the ramp voltage were concave upward, the situation is reversed. If dissipation is not a factor, e.g. for small area lamps, one can tune the discharge for optimum results such as for timing or for whatever other criterion might apply. A linear ramp is preferred. 
         [0027]      FIG. 4  is a partial schematic of an inverter constructed in accordance with a preferred embodiment of the invention for a boost circuit having a single ended output. In this embodiment, the discharging circuit (“new”) is coupled to output  41  of the charging circuit (“old”). A circuit similar to the charging circuit is described in U.S. Pat. No. 6,320,323 (Buell et al.), the contents of which are incorporated herein by reference, and in U.S. Pat. No. 5,313,141 (Kimball). The charging circuit combines the functions of boost and inverter. 
         [0028]    Charging circuit  40  includes transistors  43  and  44  having inductor  45  connected in series between the transistors and the series circuit is connected between DC voltage source  46  and ground. The junction between transistor  43  and inductor  45  is coupled through diode  51  and SCR  53  to output  41 . The junction between inductor  45  and transistor  44  is coupled through diode  52  and SCR  54  to output  41 . The gate of SCR  53  is coupled to ground and the gate of SCR  54  is coupled to the supply voltage. 
         [0029]    Suitable drive signals are applied to the bases of transistors  43  and  44 , whereby transistor  43  is turned on and remains on while transistor  44  is turned on and off at a high frequency. While transistor  44  turns on and off at high frequency, diode  52  is forward biased and a series of positive pulse are applied to lamp  42  through SCR  54 . The voltage on lamp  42  increases incrementally in response to the pulses and a small current flows through lamp  42 . 
         [0030]    After a short period, the operation of transistors  43  and  44  is reversed, i.e., transistor  44  conducts while transistor  43  is turned on and off at a high frequency. During this portion of the operation, inductor  45  produces negative pulses that are coupled through diode  51  and SCR  53  to EL lamp  42 . The negative pulses charge EL lamp  42  in the opposite direction and current flows in the opposite direction through EL lamp  42 . 
         [0031]    After another short period, the operation of transistors  43  and  44  is reversed again. The charging periods are separated by discharge periods, during which either transistor  57  or transistor  58  discharges EL lamp  42 . 
         [0032]    The emitter of transistor  57  is coupled to ramp generator  59  through a bias circuit that includes transistors  61  and  62 . The bias circuit inverts or reverses the polarity of the ramp voltage to bias transistor  57  properly for removing negative charge from EL lamp  41 . The base of transistor  58  is coupled to ramp generator  59 , which biases transistor  58  properly for removing positive charge from lamp  42 . 
         [0033]    Ramp generator  59  operates intermittently, between charging periods, producing one ramp per discharge. The period for discharge is approximately one quarter cycle of the low frequency alternating current produced across lamp  42 . The ramp can be linear or non-linear but linear is preferred. 
         [0034]    The invention thus provides a sinusoidal discharge for an EL lamp that is essentially an analog discharge in a digital circuit. The sinusoidal discharge current substantially eliminates noise from being generated by an EL lamp. There are no harmonics. 
         [0035]    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, although an EL lamp is preferably discharged to ground, an EL lamp can be discharged to a supply voltage. Instead of using the low side switches in an H-bridge, one could add separate discharge circuits, thereby separating the charging circuit from the discharging circuit in an H-bridge. This could be useful for EL lamps having a total area near the limit of the capacity of a driver.