Abstract:
It is presented a lighting device comprising: at least one alternating current source configured to provide alternating current of at least a first and a second frequency, at least one light source, at least one impedance unit connected to the light source, affecting a first current from the at least one alternating current source to flow through the at least one light source, wherein an impedance of the impedance unit is configured to be frequency controlled, such that when the alternating current is of the first frequency the first current is relatively high and when the alternating current is of the second frequency the first current is relatively low. A corresponding display device, television device and method are also presented.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to lighting devices, and more particularly to controlling light intensity of light emitting diodes. 
       BACKGROUND OF THE INVENTION 
       [0002]    Light Emitting Diodes (LED&#39;s) can be used for many purposes. One such purpose is to provide backlighting for Liquid Crystal Display (LCD) TVs. With other TV technologies, light is often generated as part of the image rendering. For example in Cathode Ray Tube (CRT) TVs, electrons are shot on a fluorescent screen to render a video image to the user, whereby light is generated in the same process as the video image is rendered. Rendering of images using LCD&#39;s in LCD TV&#39;s however, does not produce light inherently and requires either reflected light from the room or, more commonly, a light source for the user to be able to view the video image with sufficient light intensity. 
         [0003]    In the prior art, it is known to use LED&#39;s or fluorescent lamps as backlights for LCD-TV&#39;s. 
         [0004]    Using LED&#39;s in backlights frequently leads to complex, matrix structures with active switches to drive and control these LED&#39;s. In particular, when features like scanning, dimming and local highlighting are implemented the topology becomes even more complex. In practice, large areas of printed circuit boards (PCB&#39;s) are needed to connect all these devices. This mounts to a problem of large costs that can make the backlight too expensive. Therefore, a solution is required for a simple and inexpensive control of LED&#39;s. 
         [0005]    Using fluorescent lamps in backlights there is a problem that the backlight requires one inverter (power source) for each fluorescent lamp. As inverters are quite costly, there is a desire to reduce the number of required inverters. 
       SUMMARY OF THE INVENTION 
       [0006]    In view of the above, an objective of the invention is to solve or at least reduce the problems discussed above. 
         [0007]    Generally, the above objectives are achieved by the attached independent patent claims. 
         [0008]    A first aspect of the invention is a lighting device comprising: at least one alternating current source configured to provide alternating current of at least a first and a second frequency, at least one light source, at least one impedance unit connected to the light source, affecting a first current from the at least one alternating current source to flow through the at least one light source, wherein an impedance of the impedance unit is configured to be frequency controlled, such that when the alternating current is of the first frequency the first current is relatively high and when the alternating current is of the second frequency the first current is relatively low. This first aspect provides a simple way of controlling light intensity of light sources, which may, for example, form part of a backlight of LCD displays. Costs are reduced compared to prior art solutions for light intensity controls, which are complex and/or expensive. 
         [0009]    The lighting device may comprise a first light emitting diode string comprising at least one light source comprising a light emitting diode arranged to allow a first current to flow in a first direction, and a second light emitting diode string comprising at least one light source comprising a light emitting diode arranged to allow a second current to flow in a second direction, the second direction differing from the first direction. With two LED strings, current can flow in both directions in the LED device, allowing for a simpler assembly. 
         [0010]    The first light emitting diode string may be connected in parallel with the impedance unit, and the second light emitting diode string may be connected in parallel with the impedance unit. A parallel arrangement allows the use of only one impedance unit to control light intensity for an entire LED device. 
         [0011]    The lighting device may comprise a plurality of light emitting diode devices, wherein each of the light emitting diode devices comprises at least one light source and at least one impedance unit, the light emitting diode devices being connected in series forming a light emitting diode device strip, wherein the light emitting diode strip may be connected to at least one of the at least one alternating current source. A series of LED devices may advantageously be connected in series, allowing for efficient production and simple assembly into an environment where the lighting device will be used. 
         [0012]    A plurality of the light emitting diode device strips may be connected in parallel. With a plurality of LED device strips connected in parallel, a single current source may drive all LED device strips. 
         [0013]    The impedance of the impedance unit of all light emitting diode devices may be the same within a fault tolerance for any frequency which can be generated by the alternating current source, and one alternating current source may be arranged to provide alternating current to all of the light emitting diode device strips. Having the same impedance (within a fault tolerance) for all impedance units for any frequency, all LED devices can be controlled simultaneously and will behave similarly. Moreover, having the same specifications for all LED devices will make production simpler and more economical. 
         [0014]    The impedance may differ between impedance units of light emitting diode devices within each light emitting diode strip, and one alternating current source may be arranged to provide alternating current to all of the light emitting diode device strips. With differing impedances, individual control may be achieved by means of shifting the frequency. 
         [0015]    The impedance may differ between impedance units of light emitting diode devices within each light emitting diode strip, and one alternating current source may be arranged to provide alternating current to each the light emitting diode device strip. Having a current source for each strip provides a refined control over light intensity in each LED device. 
         [0016]    In each of the plurality of light emitting diode strips, the impedance units of light emitting diode devices in corresponding positions of each strip may have the same impedances within a fault tolerance for any frequency which can be generated by the alternating current source. By dimensioning impedance units in corresponding positions to have the same impedance at any frequency, the light intensity of corresponding LED devices may be controlled simultaneously. If the LED strips are aligned in parallel, this allows a scanning effect to be produced with ease. 
         [0017]    The light emitting diode device strip may be implemented on a printed circuit board. Using a PCB simplifies production and makes it economical. 
         [0018]    Each of the at least one light sources may be a fluorescent lamp. Fluorescent lamps also benefit from more efficient control, reducing the number of inverters required. 
         [0019]    The lighting device may comprise a plurality of multi-lamp drivers, wherein each multi-lamp driver may comprise an alternating power source, a plurality of impedance units, the multi-lamp driver may be configured to provide power to a plurality of fluorescent lamps. 
         [0020]    The impedance unit may comprise a first capacitor connected in parallel to an inductor. This is a simple circuit which allows for frequency controlled impedance. 
         [0021]    The impedance unit may further comprise a second capacitor connected serially with the inductor. Connecting this second capacitor prevents direct current to flow through the impedance unit. 
         [0022]    The lighting device may be in the form of a backlight for a liquid crystal display television. It is very useful to be able to control backlight, while still being able to produce this backlight with good economy. 
         [0023]    A second aspect of the invention is a display device comprising a liquid crystal display and a lighting device according to the first aspect of the invention. 
         [0024]    A third aspect of the invention is a television device comprising a display device according to the second aspect of the invention. 
         [0025]    A fourth aspect of the invention is a method for controlling light intensity of a lighting device, the method comprising the steps of: arranging at least one alternating current source configured to provide alternating current of at least a first and a second frequency, connecting at least one light source, connecting at least one impedance unit connected to the light source, affecting a first current from the at least one alternating current source to flow through the at least one light source, controlling an impedance of the impedance unit using frequency control, such that when the alternating current is of the first frequency the first current is relatively high and when the alternating current is of the second frequency the first current is relatively low. 
         [0026]    Other objectives, features and advantages of the present invention will appear from the following detailed disclosure, from the attached dependent claims as well as from the drawings. 
         [0027]    Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to “a/an/the [element, device, component, means, step, etc]” are to be interpreted openly as referring to at least one instance of the element, device, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0028]    Embodiments of the present invention will now be de-scribed in more detail, reference being made to the enclosed drawings, in which: 
           [0029]      FIG. 1  shows an exemplary impedance unit according to an embodiment of the present invention. 
           [0030]      FIG. 2  shows a LED unit with an associated control unit according to an embodiment of the present invention. 
           [0031]      FIG. 3  is a diagram of how current through LED&#39;s are affected by frequency in the LED unit shown in  FIG. 2 . 
           [0032]      FIGS. 4   a  and  4   b  show two different arrangements of LED units such as the LED unit of  FIG. 2 . 
           [0033]      FIG. 5  shows two LED units arranged to allow DC controlled color shifts according to an embodiment of the present invention. 
           [0034]      FIGS. 6A and 6B  show embodiments of the present invention using fluorescent lamps. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0035]      FIG. 1  shows an exemplary impedance unit  106  according to an embodiment of the present invention. The impedance unit  106  consists of a first capacitor  112  and an inductor  111  connected in parallel. Optionally, a second capacitor  110  is connected serially to the inductor  111  to block DC current through the impedance unit  106 . As is known in the art per se, when the impedance unit  106  is connected to an alternating current (AC), the impedance of the circuit varies as a function of the frequency of the alternating current. With a circuit such as the one shown here, the impedance unit has a particular frequency where its impedance reaches a peak, which frequency is called the resonance frequency, or the high impedance frequency of the impedance unit. The resonance frequency depends on the capacitances and inductances of the capacitors  110 ,  112  and the inductor  111 . Although a simple LC-circuit is shown here, it is a mere example to allow a man skilled in the art to implement or use the invention. Consequently, the invention is not limited to an impedance unit of this type and may be any type of circuit with a frequency controlled impedance. 
         [0036]    With reference to  FIG. 2 , an impedance unit  206 , such as the impedance unit  106  of  FIG. 1 , is connected in parallel with at least two LED strings. A first LED string, made up of LED&#39;s  205   a  and  205   b , emit light when a current flows downwards and blocks current when it flows upwards. On the other hand, LED&#39;s  205   c  and  205   d , of a second LED string, are arranged in the opposite direction, emitting light when the current flows upwards and blocks downward current. The LED&#39;s  205   a - d,  connected to the impedance unit  206  make up an LED unit  208 . Control unit  201  is a source of an alternating current (AC), or alternating voltage, and controls frequency, amplitude and DC offset of this alternating current through the LED unit  208 . With further reference to  FIG. 3 , when the frequency of the current is close to a resonance frequency of the impedance unit  206 , F res , the impedance of the impedance unit  206  is relatively high. The current then flows relatively easy through the LED&#39;s  205   a - d,  leading to a relatively higher current. As can be seen in  FIG. 3 , the current peaks at the frequency F res    303 , where F res  is the resonance frequency of the impedance unit  206 , with a value of I max    302 . In other words, the LED&#39;s have their strongest light intensity when the frequency of the AC is F res . Thus, by controlling the frequency of the alternating current, the light intensity of the LED unit  208  is controlled, using only simple and inexpensive components. Although not shown in  FIG. 2 , there may be one impedance unit for each LED string, where each impedance unit is connected in series with each LED string. 
         [0037]    As can be seen in  FIG. 4   a , a plurality of LED units  422   a ,  422   b , . . . ,  422   z  are connected serially with a control unit  421   a.  These components together may all be combined on a Printed Circuit Board (PCB) strip  420   a . Correspondingly, a second PCB strip  420   b  comprises a control unit  421   b  and LED units  423   a ,  423   b , . . . ,  423   z . An arbitrary number of PCB strips, including  420   z , comprising a control unit  421   z  and LED units  429   a ,  429   b , . . . ,  429   z , may thus be combined to form a backlight for a LCD TV. The PCB strips may be arranged horizontally, vertically, radially, diagonally or in any other suitable fashion. It is to be noted that each PCB strip can house an arbitrary number of LED units. 
         [0038]    If the resonance frequencies of each LED unit in each PCB strip are configured to differ from each other, a matrix is effectively created, allowing two-dimensional control over light intensity. The light intensity of an entire PCB strip is effected by the amplitude of the AC for the PCB strip in question. The band-pass characteristics of the LED units in a strip may optionally overlap to suit a particular light output demands for the backlight. For instance, this may be needed in case a smooth transition from one zone to another is needed. 
         [0039]      FIG. 4   b  shows a simplified arrangement for only dimming and scanning. Here only one control unit is required, thus reducing cost. A first PCB strip  430   a  then comprises LED units  432   a ,  432   b , . . . ,  432   z . A second PCB strip  430   b  comprises LED units  433   a ,  433   b , . . . ,  433   z , while a last PCB strip  430   z  of an arbitrary number of PCB strips, comprises LED units  439   a ,  439   b , . . . ,  439   z . With this arrangement, one control unit  431  provides a current for all PCB strips, whereby the current cannot be controlled for an individual strip. On the other hand, by controlling the frequency of the current, light intensity can be controlled for different LED units within each light strip. In one embodiment, the resonance frequencies of LED units in the same position of each strip are chosen to be the same (within a given fault tolerance, such as 1, 5 or 10%). For example,  432   a ,  433   a  and  439   a  are chosen to have the same resonance frequency,  432   b ,  433   b  and  439   b  are chosen to have the same resonance frequency, etc. This allows simultaneous control over corresponding LED units, leading to an ability to perform effects such as scanning (horizontal line of light). Additionally, by changing the amplitude of the current, light intensity for all LED units are affected simultaneously, in other words dimming of all LED units. In another embodiment, all LED units are chosen to have the same resonance frequency. While this configuration allows less control, it may be a configuration which is more cost effective to produce. Although the PCB strips in  FIG. 4   b  are connected in parallel, another possible configuration is cascading the PCB strips, or a combination of cascade and parallel connections. 
         [0040]    Hitherto it has only been mentioned that the control unit can control amplitude and frequency of the alternating current it produces. With the addition of direct current (DC) shift, the control unit can also control color balance.  FIG. 5  shows a first and a second LED unit  508 ,  518 , connected to a first and second control unit  501 / 511 , respectively, and having a first and a second impedance unit  506 / 516 , respectively. The first LED unit  508  has red LED&#39;s  505   a ,  505   b  in one current direction and blue LED&#39;s  505   c ,  505   d  in an opposite current direction. The second LED unit has only green LED&#39;s  515   a - d.  If the first control unit applies a DC shift downwards, the red LED&#39;s will produce slightly more light intensity. Correspondingly, a DC shift in the opposite direction will produce more blue light. For the second LED unit  518 , any shift in DC from zero will result in an increased intensity of green. Accordingly, color balance can be controlled efficiently by means of a DC shift of these two LED units. As is easily realized by a man skilled in the art, other the configuration of the colored LED&#39;s can be adjusted, while still providing a DC controllable color balance. For example, LED&#39;s with the colors red, green, blue and white may be used, or other colors may be used, such as including amber color in the configuration. 
         [0041]      FIG. 6   a  shows an embodiment in which the invention is used in conjunction with fluorescent lamps. While fluorescent lamps are used in this example, any light source supporting bi-directional current can be used, such as light bulbs. Control unit  601 , also known as an inverter, is a source of an alternating current or alternating voltage. In this embodiment, there are three impedance units  606   a - c,  such as impedance unit  106  of  FIG. 1 , whose impedance depends on the frequency of the voltage provided, as explained in conjunction with  FIG. 3  above. The control unit  601  and the impedance units  606   a - c  are part of a multi-lamp driver  609 . As the name implies, the multi-lamp driver is capable of driving a number of lamps, in this example three lamps  607   a - c,  whose light intensity depends on the impedance of the respective connected impedance unit  606   a - c,  which in turn then depends on the frequency of the voltage from the control unit  601 . It is thus possible to design a multi-lamp driver  609  with appropriate frequency characteristics to drive the connected fluorescent lamps  607   a - c,  in a similar fashion to what is described above in conjunction with LEDs. It is to be noted that any number of lights is within scope of the present invention. In a situation where a duty cycle of the lamps  607   a - c  is about 33% or less, an arrangement such as the one shown in  FIG. 6   a  only needs one inverter  601  to drive all three lamps. In a traditional arrangement, each lamp is connected to a separate inverter. Consequently, the arrangement shown in  FIG. 6   a  reduces the need of inverters to one third compared to a traditional arrangement, reducing cost and availability. 
         [0042]      FIG. 6   b  shows an embodiment where a plurality of multi-lamp drivers  609   a - c  are employed. In this example, each multi-lamp driver  609   a - c  drives three lamps. Multi-lamp driver  609   a  drives lamps  607   a - c;  multi-lamp driver  609   b  driver lamps  607   d - f  and multi-lamp driver  609   c  driver lamps  607   g - i.  Note that for reasons of clarity, the full electrical circuit is not illustrated here. All multi-lamp drivers  609   a - c  are controlled by backlight control unit  640 . The backlight control unit  640  can also use vertical synchronization, optical feedback from the backlight and/or temperature feedback from the backlight as input to consider. The output from backlight control unit  640  is frequency control provided to the multi-lamp drivers. The layout of the lamps  607   a - i  is such that each row has lamps from each of the multi-lamp drivers  609   a - c.  For example, in the first row of lamps, lamp  607   a  is connected to multi-lamp-driver  609   a ; lamp  607   d  is connected to multi-lamp-driver  609   b  and lamp  607   g  is connected to multi-lamp-driver  609   c.    
         [0043]    The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.