Patent Document

FIELD OF THE INVENTION  
       [0001]     The present invention relates to a device for driving at least two electrolumines-cent lamps.  
       BACKGROUND OF THE INVENTION  
       [0002]     Electroluminescence (EL) is the non-thermal conversion of electrical into luminous energy. Well known electroluminescent devices are light emitting diodes (LEDs), in which light is generated by electron-hole pair recombination near a pn junction. Also known are electroluminescent lamps (EL lamps) which have become a popular light source for backlighting Liquid Crystal Displays.  
         [0003]     In EL Lamps, light is generally generated by impact excitation of a light emitting center (called the activator) by high energy electrons in materials like ZnS:Mn. The electrons gain their high energy from an high electric field. Therefore, this type of electroluminescence is often called high field electroluminescence. Thus, a high voltage is required for driving EL lamps.  
         [0004]     The problem with conventional drivers for EL lamps with low complexity on the high-voltage supply side and with a single high-voltage supply is that it is not possible to independently drive two separate EL lamps in such a way that the luminescent brightness of the two lamps is independently continuously control-lable.  
         [0005]     The currently available integrated dual EL lamp drivers, for example HV839 from Supertex Inc., Sipex SP4490, IMP IMP522, NPC SMS 145 A, only allow each of the two EL lamps to be turned fully on or off. The luminescent brightness of the two lamps, when switched on, can only be controlled in common, as it is also disclosed in U.S. Pat. No. 6,515,522. Another costly implementation, disclosed in U.S. Pat. No. 6,144,164, controls the lamps sequentially.  
       OBJECT OF THE INVENTION  
       [0006]     It is an object of the present invention to provide a device for driving at least two electroluminescent lamps, with which the luminescent brightness of the at least two electroluminescent lamps can be controlled independently and continuously.  
       SUMMARY OF THE INVENTION  
       [0007]     A basic idea underlying the present invention is to control the charging and discharging of EL lamps by the charge waveform. According to the invention, this can be implemented by means of logic circuitry.  
         [0008]     The present invention relates to an electroluminescent lamp driver for controlling the luminescent brightness of at least two EL lamps, wherein the driver is adapted to receive at least two setpoint values corresponding to luminescent brightness of the EL lamps and to create charge waveforms in accordance with the received setpoint values for charging or discharging the EL lamps. Typically, the charge waveforms are created depending on the desired luminescent brightness as determined by a setpoint value. This can be done either in open or closed loop control. For example, in an open loop control implementation, a table can contain various parameters essential for the control of an EL lamp which correspond to different setpoints. Then, if a specific setpoint value is received, the corresponding parameters can be looked up, and depending on these parameters a charge waveform for the EL lamp can be generated.  
         [0009]     Preferably, the driver is adapted to create a first charge waveform for charging a first one of the EL lamps with the highest desired luminescent brightness and a second charge waveform for charging the other EL lamp or lamps with lower desired luminescent brightness.  
         [0010]     Particularly, the first charge waveform fully charges the first one of the EL lamps with the highest desired luminescent brightness and the second charge waveform has essentially the same waveform as the first charge waveform during a time interval which is smaller than the time interval during which the first one of the EL lamps is charged.  
         [0011]     The driver can comprise a controlling device which is adapted to receive on its inputs the setpoint values corresponding to the desired luminescent brightness of the at least two EL lamps and actual values corresponding to the actual luminescent brightness of the at least two EL lamps, to compare the setpoint value and the actual value for each of the at least two lamps, and to drive charging means, discharging means, and H-bridges for the EL lamps in such a way that the setpoint value and the actual value for each of the at least two EL lamps are substantially equal.  
         [0012]     In a preferred embodiment of the driver, the controlling device is adapted so that the luminescent brightness of the EL lamp with the highest luminescent brightness setpoint is set by controlling the output voltage of the charging means, and the luminescent brightness of the EL lamp or lamps with the lower luminescent brightness setpoint or setpoints is set by controlling the time interval during which this EL lamp or these EL lamps is/are charged in parallel to the first EL lamp.  
         [0013]     Furthermore, the controlling device can comprise a controller for each of the at least two EL lamps, wherein each controller receives on its inputs the setpoint value and the actual value of a corresponding one of the at least two EL lamps and is adapted to generate a control signal on its output for controlling the charging and discharging means or a pulse-width modulator for generating a control signal for controlling the charging or discharging of the EL lamp or lamps with the lower luminescent brightness setpoint or setpoints. Preferably, the controllers are Pi controller.  
         [0014]     The charging means can be a high voltage power supply which is able to source current into the EL lamps.  
         [0015]     The power supply should have a high output resistance in order to keep the strain on the EL lamps during charging small.  
         [0016]     Preferably, the power supply is a step-up switching power supply with a control input for setting the duty cycle on the primary side switching.  
         [0017]     The discharging means can be any device which is able to sink current out of the EL lamps.  
         [0018]     Preferably, the device has a control input for determining the amount of sink current.  
         [0019]     Preferably, the device is a resistor, a current sink based on a transistor, or a step-down switching power supply with a charge storage device on the primary side for absorbing energy removed from the EL lamp or lamps. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0021]      FIG. 1  shows a first embodiment of an electroluminescent lamp driver according to invention with an open loop control;  
         [0022]      FIG. 2  shows a second embodiment of the electroluminescent lamp driver according to the invention with a closed loop control; and  
         [0023]      FIG. 3  shows a timing diagram with waveforms of signals of the driver of  FIG. 2 . 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0024]     Preferred embodiments of an EL lamp driver in accordance with the invention will now be described. In the present embodiments, the driver is designed to be used to drive a dual (or multiple) EL lamp.  
         [0025]     In the following description the same and/or equal and/or similar elements can be denoted with the same reference numerals.  
         [0026]      FIG. 1  shows an electroluminescent lamp driver for two EL lamps  28  and  38  with an open loop control. The driver comprises high voltage charging means  10 , discharging means  12 , one H-bridge  20 - 23  and  30 - 33  for each of the two EL lamps  28  and  38 , and a controlling device  40 .  
         [0027]     The controlling device  40  receives input control voltages at two setpoint inputs  41  and  42  in order to control the brightness of the two EL lamps  28  and  38 .  
         [0028]     The input control voltages received at input  41  and  42  correspond to the desired luminance brightness for the EL lamps  28  and  38 , respectively.  
         [0029]     The controlling device  40  generates control signals  44 - 47 ,  28 ′,  29 ,  38 ′, and  39  for controlling the charging and discharging of the two EL lamps  28  and  38  in accordance with the received input control voltages. Particularly, it generates control signals  44  and  45  for controlling the discharging means  12  and charging means  10 , respectively. Further it generates control signals  46  and  47  for the charging switching device  11  and discharging switching device  13 , respectively. The switching devices  20 ,  23  and  21 ,  22  of the H-bridge for EL lamp  28  are controlled by signals  28 ′ and  29 , respectively. The switching devices  30 ,  33  and  31 ,  32  of the H-bridge for EL lamp  38  are controlled by signals  38 ′ and  39 , respectively.  
         [0030]     The controlling device  40  can be implemented by means of a microcontroller programmed to process the received input control voltages and to generate the above mentioned control signals thereof. Particularly, the controlling device  40  generates a charge waveform for the EL lamp with the higher luminescent brightness of the both EL lamps  28  and  38 . If EL lamp  28  is the lamp with the higher brightness, then the control signals  28 ′ and  29  for charging the lamp comprise a charge waveform which fully charges the lamp. The control signals  38 ′ and  39  comprise essentially the same waveform as the control signals  28 ′ and  29 , but merely charge the EL lamp  38  as determined by the input control voltage at setpoint input  42 , i.e. during a time interval which is smaller compared to the charging time interval of the first lamp, i.e. EL lamp  28 . The charging time Interval for the second EL lamp  38  is chosen in order to achieve the desired brightness as determined by the input control voltage. This can be programmed in a memory of the controlling device, e.g. by means of a table comprising values for brightness and the corresponding charge time internals.  
         [0031]      FIG. 2  shows an implementation of the driver according to the invention with a closed loop control. A controlling device comprises setpoint inputs  41  and  42  each of which receives an input control voltage which corresponds to the desired luminance brightness for the EL lamps  28  and  38  respectively. Furthermore, it comprises an input  84  for receiving an actual value of the EL lamp  28  and an input  85  for receiving an actual value of the EL lamp  38 . A comparator  70  of the controlling device compares the values of the two setpoints against each other, and outputs a logical ‘ 0 ’ if the setpoint for EL lamp  28  is higher than the setpoint for EL lamp  38 , and a logical T otherwise.  
         [0032]     Two PI (proportional/integrating) controller  50  and  60  are provided in the controlling device to process the received signals.  
         [0033]     Should the setpoint value at input  41  be higher than at input  42 , then the PI controller  50  of the controlling device receives at its setpoint Input  57  the value of setpoint input  41  through a multiplexer  55  (the multiplexer position is defined by the logical ‘ 0 ’ on comparator output  80 ) and at its actual value input  58  the actual value of the luminescent brightness of EL lamp  28  through a multiplexer  56 .  
         [0034]     The PI controller  60  receives at its setpoint input  67  the value of setpoint input  42  through a multiplexer  65 , and at its actual value input  68  the actual value of the luminescent brightness of EL lamp  38  through a multiplexer  66 .  
         [0035]     If the setpoint value at input  41  is lower than at input  42 , then the inputs of the two PI controllers  50  and  60  are exchanged between each other through the action of the multiplexers  55 ,  56 ,  65 ,  66 .  
         [0036]     In this embodiment, the controllers  50  and  60  are PI controllers only for illustrative purposes, they could be replaced by any other kind of closed loop controller which brings the difference between its setpoint input and actual value input to substantially zero through its controlling action.  
         [0037]     PI controller  50  drives the control inputs of charging means  10  and discharging means  12 . Charging means  10  can be any high voltage power supply which is able to source current into the EL lamps  28  and  38  and with a control input which determines the output voltage or amount of sourcing current of the supply, but preferably a power supply with a relatively high output resistance in order to keep the strain on the EL lamps during charge time small. An example is a step-up switching power supply where the control input sets the duty cycle of the primary side switching.  
         [0038]     Discharging means  12  can be any device which is able to sink current out of the EL lamps, and preferably, but not necessarily, with a control input which determines the amount of sinking current Examples are a resistor, or a current sink based on a transistor, or the mentioned switching power supply operated in reverse (step-down) direction with a charge storage device on the primary side to absorb the energy removed from the EL lamp.  
         [0039]     Switching devices  11  and  13  alternately source current to or sink current from the supply node  14 . The switching devices  11  and  13  are controlled by a sequencer  71  of the controlling device through the control lines  46  and  47 . An example of the waveforms of the charge and discharge control lines  46  and  47  respectively, and of the supply node  14 , are shown in the timing diagram in  FIG. 3 . The numbers in the timing diagram correspond to the control line numbers in  FIG. 2 , the signals in  FIG. 3  are active if they are logically ‘high’.  
         [0040]     PI controller  60  drives the control input of a pulse-width modulator  72  of the controlling device. Pulse-width modulator  72  accepts at its inputs a control signal  69  from PI controller  60  and a charging control signal  46 . The pulse width at the output  83  of pulse-width modulator  72  is substantially proportional to the input voltage of the output  69  of PI controller  60 . The minimum pulse width at output  83  is substantially 0 seconds for the minimum voltage on control line  69 , and the maximum pulse width is substantially equal to the pulse width of the charging control signal  46  for the maximum voltage on control line  69 . An example of the timing of output signal  83  is shown in  FIG. 3 .  
         [0041]     Sequencer  71 , together with an inverting gate  73 , logical OR gates  74 ,  75 , and logical AND gates  76 ,  77 ,  78 ,  79  determines whether the EL lamps  28  and  38  are either charged in the (arbitrarily defined) positive or negative direction, or if they are not charged at all. Control lines  81  and  82  determine the charging direction. A logical ‘high’ on line  81  enables the positive charging of both EL lamps  28 ,  38 , while a logical ‘high’ on line  82  enables the negative charging of both EL lamps  28 ,  38 .  
         [0042]     If the setpoint value of input  41  is higher than the setpoint value of input  42 , then control lines  81  and  82  are logically connected to line  28 ′ and line  29  respectively through AND gates  76  and  77 . One of the lines  28 ′ or  29  is then always logically ‘high’, which implies that the luminescent brightness of EL lamp  28  is wholly controlled by the PI controller  50 . The lines  38 ′ and  39  are only logically ‘high’ if the control lines  81  and  82  are respectively logically ‘high’ and the output  83  of pulse-width modulator  72  is logically ‘high’, which implies that the luminescent brightness of EL lamp  38  is wholly controlled by the PI controller  60 .  
         [0043]     If the setpoint value of input  41  is lower than the setpoint value of input  42 , the reverse is true, and EL lamp  28  is controlled by PI controller  60 , and EL lamp  38  is controlled by PI controller  50 .  
         [0044]     If line  28  is logically ‘high’ and line  29  is logically ‘low’, then switching means  20  and  23  are on, switching means  22  and  21  are off, and EL lamp  28  is charged in the positive direction. If discharge means  12  is active and switching device  13  is on, then EL lamps  28  and  38  are discharged through diodes  24  and  27 , or  25  and  26  depending on the polarity of their charge, respectively  34  and  37 , or  35  and  36 . Waveforms  128  and  138  in  FIG. 3  respectively are examples for the voltage across EL lamps  28  and  38  respectively for the case where the setpoint on input  41  is higher than the setpoint on input  42 .  
         [0045]     Difference amplifiers  91  and  93 , and rectifiers  92  and  94 , respectively determine the actual value of the luminance brightness of EL lamps  28  and  38  respectively. In the shown example, the rectified voltage differences across the EL lamps  28  and  38  are used as actual values of the luminance brightness, however it is clear that other quantities can be used as actual value, for example the averaged absolute current, the absolute peak voltage amplitude, or the real luminescent brightness measured with a light sensitive electronic device, such as a photodiode.  
         [0046]     It is clear that this implementation is only shown to illustrate the idea and that other implementations are possible, for example most of the sequencing, controlling and signal measurement and routing items  70 ,  55 ,  56 ,  50 ,  65 ,  66 ,  60 ,  71 ,  72 ,  73 ,  74 ,  75 ,  76 ,  77 ,  78 ,  79 ,  91 ,  92 ,  93 ,  94  could be implemented by means of a single microcontroller containing integrated peripheral blocks like analog to digital converters, pulse-width modulators and pulse generators.  
         [0047]     It is also apparent that the number of individually driven EL lamps can be expanded from two to an arbitrary number by controlling the EL lamp with the highest luminescent brightness setpoint with the closed-loop controller which controls the charging and discharging means, and the rest of the EL lamps each by a closed-loop controller which controls the charge time of the respective EL lamp by pulse-width modulation.  
       REFERENCE NUMERALS  
       [0000]    
       
           10  charging means  
           11  switching device  
           12  discharging means  
           13  switching device  
           14  supply node  
           20 - 23  switching device  
           24 - 27  diode  
           28  EL lamp  
           28 ′, 29  control line  
           30 - 33  switching device  
           34 - 37  diode  
           38  EL lamp  
           38 ′, 39  control line  
           40  controlling device  
           41  setpoint input  
           42  setpoint input  
           44  discharging means control signal  
           45  charging means control signal  
           46  charging switching device control signal  
           47  discharging switching device control signal  
           50  PI controller  
           51  subtractor  
           52  amplifier  
           53  integrator  
           54  adder  
           55 , 56  multiplexer  
           57  setpoint input of PI controller  50   
           58  actual value input of PI controller  50   
           59  output of PI controller  50   
           60  PI controller  
           61  subtractor  
           62  amplifier  
           63  integrator  
           64  adder  
           67  setpoint input of PI controller  60   
           68  actual value input of PI controller  60   
           69  output from PI controller  60   
           70  comparator  
           71  sequencer  
           72  pulse-width modulator  
           80  comparator output  
           83  output of the pulse-width modulator  72   
           84  node with actual value of the EL lamp  28   
           85  node with actual value of the EL lamp  38   
           91 , 93  difference amplifier  
           92 , 94  rectifier  
           128  waveform of the voltage over El lamp  28   
           138  waveform of the voltage over El lamp  38

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