Patent Publication Number: US-9414460-B2

Title: Methods and systems for controlling LEDs

Description:
FIELD OF THE INVENTION 
     The invention relates to the field of lighting and/or displaying. More specifically it relates to methods and systems for measuring and/or taking into account aging of a lighting element or variation of a lighting element as function of environmental parameters. 
     BACKGROUND OF THE INVENTION 
     The use of lighting elements, such as for example LED devices, is widely spread. One known problem of lighting elements is that they are often subject to aging effects. Aging effects refer to effects, such as for example a decrease in emission intensity, of lighting elements as function of the operation time. Furthermore, it is known that environmental conditions also can influence the performance of lighting elements, such as LED devices. 
     A plurality of techniques are known for compensating for aging effects. European patent application EP 2334144 describes a method of determining ageing characteristics of a LED whereby a current stress pulse is applied. The LED thereby is monitored to determine when the thermal heating induced by the current stress pulse has been dissipated to a desired level. The operational characteristics of the LED are then measured before applying a next stressing pulse. The technique presented typically is a test technique. 
     U.S. Pat. No. 5,859,658 describes an apparatus for compensating LED printbars for aging wherein changes in the slopes of the forward voltage drop versus forward current characteristics of the LEDs are determined and wherein the drive currents of the LEDs are changed as function of the changes in the slope so as to compensate for changes in the LED light output. 
     Nevertheless, there is still need for a good techniques for compensating for aging effects for LEDs. 
     SUMMARY OF THE INVENTION 
     It is an object of embodiments of the present invention to provide good methods and systems for measuring and/or taking into account aging of one or more LEDs or environmental effects influencing the operation of the one or more LEDs, the methods and systems being accurately applicable during normal driving operation of the one or more LEDs. 
     It is an advantage of embodiments according to the present invention that aging effects of a LED or environmental effects can be taken into account, during normal driving operation. 
     It is an advantage of embodiments of the present invention that the information about aging or environmental effects can be used for compensating performance deterioration of the LEDs. It is an advantage of embodiments according to the present invention that aging of LEDs or environmental effects influencing the performance of the LED can be compensated so that these effects are not visible on the performance of the LED, even not when the operation time of the LED becomes long—e.g. even not over the lifetime of the LED. 
     The above objective is accomplished by a method and device according to the present invention. 
     In a first aspect, the present invention relates to an electronic device, the electronic device comprising at least one LED, a driving unit for applying a driving algorithm for driving the LED during normal operation, and a measurement unit for determining a forward voltage of the LED by imposing a test current to the LED, the measurement unit being programmed for determining test current characteristics taking into account said driving algorithm. 
     The measurement unit may be programmed for dynamically adjusting the test current as function of the driving algorithm. The measurement unit may be configured for measuring the forward voltage differentially. 
     The measuring unit may comprise a differential amplifier for measuring the forward voltage differentially. 
     The measuring unit may comprise a differential amplifier, a variable current source and an ADC converter, for determining a forward voltage of the LED. 
     The electronic device may comprise a plurality of different LEDs, wherein a single differential amplifier is configured for measuring the forward voltage differentially for different LEDs. 
     The differential amplifier may be configured for selecting an input for each LED of a set of LEDs of an RGB LED configuration in the electronic device. 
     The device may comprise a controller for measuring the forward voltage over different LEDs of a set of LEDs in a sequential manner. 
     The electronic device furthermore may comprise an offset compensation means for compensating for an offset of the differential amplifier. 
     The offset compensation means may be a hardware (HW) offset compensation means or software (SW) offset compensation means. 
     The electronic device furthermore may comprise an adjustment means for adjusting the driving of the LED for compensating for the determined effects due to aging and/or environmental effects. 
     The differential amplifier may comprise continuous time switched capacitors with integrated offset compensation. 
     The measurement unit may comprise a differential amplifier circuit comprising a first and second input, the first and the second input of the differential amplifier circuit being connected such that the forward voltage over the LED can be determined from the output of the differential amplifier. 
     The present invention also relates to a method for measuring and/or compensating environmental effects or aging effects on a LED, the method comprising determining a forward voltage of the LED and using the forward voltage of the LED device for determining environmental effects and/or aging effects. 
     Determining the forward voltage may comprise differentially measuring the voltage in relation to the voltage where the LED is connected to. 
     The method may comprise compensating for a change in properties of the LED device related to environmental effects and/or aging effects. 
     Determining environmental effects may comprise determining an ambient temperature. 
     The method may comprise distinguishing between different effects based on the rate of change of the performance of the LED device. 
     The method may comprise distinguishing between different effects by taking into account different performance characteristics. 
     The method may comprise compensating for each LED of one or more RGB-LED devices. 
     The present invention also relates to a controller being programmed for performing a method for measuring and/or compensating environmental effects or aging effects on a LED as described above. 
     In another aspect, the present invention also relates to a measurement unit for use with an electronic device comprising at least one LED and a driving circuit, the measurement unit being programmed for determining a forward voltage of the LED by imposing a test current to the at least one LED and being programmed for determining a test current characteristic taking into account said driving algorithm. 
     The measurement unit may be programmed for dynamically adjusting the test current as function of the driving algorithm. 
     The measurement unit may comprise a differential amplifier for measuring the forward voltage differentially. 
     The measurement unit may comprise a differential amplifier, a variable current source and an ADC converter, for determining a forward voltage of the LED. 
     The measurement unit may comprise a single differential amplifier being configured for measuring the forward voltage differentially for different LEDs. 
     The differential amplifier may be configured for selecting an input for each LED of a set of LEDs of an RGB LED configuration in the electronic device. 
     The measurement unit may comprise an offset compensation means for compensating for an offset of the differential amplifier. The offset compensation means is a hardware (HW) offset compensation means or software (SW) offset compensation means. 
     The measurement unit may be configured for providing an output signal to an adjustment means for adjusting the driving of the LED for compensating for the determined effects due to aging and/or environmental effects. 
     The differential amplifier may comprise continuous time switched capacitors with integrated offset compensation. 
     The measurement unit may comprise a differential amplifier circuit comprising a first and second input, the first and the second input of the differential amplifier circuit being connected such that the forward voltage over the LED can be determined from the output of the differential amplifier. 
     The present invention also relates to a computer program product and a data carrier comprising a set of instructions for, when executed on a computer, performing a method for measuring and/or compensating environmental effects or aging effects on a LED. 
     In a further aspect, the present invention may relate to a method for measuring and/or compensating environmental effects or aging effects on a LED, the method comprising determining a forward voltage of the LED and e.g. using the forward voltage of the LED device for determining an environmental effects and/or aging effects. 
     In one embodiment, determining the forward voltage may be performed by differentially measuring the voltage in relation to the voltage where the LED is connected to. 
     The method may comprise compensating for a change in properties of the LED device related to environmental effects and/or aging effects. The environmental effect may be an ambient temperature. Distinguishing between different effects may e.g. be performed based on the rate of change of the performance of the LED device, on different performance characteristics taken into account or on other aspects of the performance of the LED. 
     The method may comprise compensating for each LED of one or more RGB-LED devices. 
     The present invention also may relate to an electronic device, the electronic device comprising at least one LED and a measurement unit for determining a forward voltage of the LED. The measurement unit may be adapted for measuring the forward voltage. The measurement unit may be adapted for measuring the forward voltage differentially, e.g. using a differential amplifier. In one embodiment, the measuring unit may comprise a differential amplifier, a variable current source and an ADC converter. A single differential amplifier may be used for different LEDs, e.g. for different LEDs of an RGB-LED device. The differential amplifier may be arranged for selecting an input for each LED of a set of LEDs, e.g. in RGB-LEDs. The measurement of the forward voltage over different LEDs of a set of LEDs may be performed sequentially. The electronic device furthermore may comprise an offset compensation means for compensating for an offset of the differential amplifier. Such an offset compensation means may be a hardware (HW) offset compensation means or software (SW) offset compensation means. The system furthermore may comprise an adjustment means for adjusting the driving of the LED for compensating for the determined effects due to aging and/or environmental effects. Such an adjustment means may make use of predetermined algorithms, look up tables or other means for determining the adjustment required for compensating the LED performance for the aging and/or environmental effects 
     Particular and preferred aspects of the invention are set out in the above and below description and examples. Features from one embodiment may be combined with features of other embodiments as appropriate and not merely as explicitly set out. 
     These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic overview of different components and subcomponents, and their interaction, of a system according to an embodiment of the present invention. 
         FIG. 2  is a schematic overview of different components, and their interaction, of a system according to an embodiment of the present invention. 
         FIG. 3  illustrates an example of a configuration wherein the forward voltage of a LED can be measured, according to an embodiment of the present invention. 
     
    
    
     The drawings are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. 
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     The terms first, second and the like in the description, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein. 
     Moreover, the terms top, under and the like in the description are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein. 
     It is to be noticed that the term “comprising” should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression “a device comprising means A and B” should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments. 
     Similarly it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the invention requires more features than are expressly recited as essential. 
     Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. 
     Where in embodiments of the present invention reference is made to a driving algorithm, reference may be made to the driving conditions for driving one or more LEDs. Such driving conditions typically may comprise driving times and driving values used for driving the one or more LEDs. 
     According to embodiments of the present invention, a LED forward voltage is measured, and this may be used for compensating performance effects of the LED due to aging and/or due to environmental factors. It is an advantage of embodiments according to the present invention that the forward voltage of LEDs can be used for the determination of the ambient temperature and that consequently, the influence of environmental effects such as the temperature on the LED properties can be compensated for. 
     In a first aspect, the present invention relates to an electronic device comprising one or more LEDs and a driving unit for driving the one or more LEDs according to a driving algorithm—which may be different over time and between different LEDs—during normal operation. The device also comprises a measurement unit for determining a forward voltage of the LED by imposing a test current to the LED, the measurement unit being programmed for determining test current characteristics taking into account said driving algorithm. 
     By way of illustration, an exemplary embodiment of such an electronic device  100  is shown in  FIG. 1 , where the LED configuration  120 —driven using a driving unit  110 —interfaces with the measurement unit  130  through a parallel interface. This interface enables the measurement unit to select a LED, set a current through the selected LED, and measure the forward voltage over the selected LED. The measurement unit  120  on its turn interfaces with the driving means  110  for adjusting the driving means such that during operation mode aging effects are taken into account. In the exemplary embodiment of  FIG. 2  the subcomponents of an embodiment of the current invention, as well as their interaction, are shown. The driving unit  110  drives the individual LEDs of the LED configuration. In the exemplary embodiment a first LED  121  and a second LED  122  are shown. The LED configuration interfaces with the measurement unit  130 . The selector  131  of the measurement unit  130  can select the interface of one of the LEDs. Once selected, the variable current source  132  can set a current through the selected LED. In embodiments of the current invention the measurement current through the LED is smaller, e.g. smaller than 50% of, e.g. smaller than 25% of, e.g. smaller than 10%, e.g. smaller than 5% of the current during normal operating mode. The measurement only takes place during a limited amount of time in order not to disturb the normal operating mode at max. 16-bit resolution. Through a differential amplifier with switched capacitors, also referred to as differential amplifier circuit  133 , the voltage over the selected LED is amplified and fed to an AD convertor system  134  which converts the signal and feeds it to a processing means  135 . The measured voltage can be converted into the forward voltage of the LED by the processing means  135 . Based on the measured forward voltage, the processing means  135  can determine an adjustment for adjusting the driving unit  110  such that aging effects of the LED are taken into account during operating mode. 
     Further by way of illustration, the present invention not being limited thereto, an exemplary device is further described below, embodiments of the present invention not being limited thereto. The forward voltage of an LED can in one example be differential measured in relation to the voltage where the LED is connected to. The following figure is showing such kind of concept. The differential amplifier comprises continuous time switched capacitors with integrated offset compensation.  FIG. 3  illustrates one example of a device comprising a LED and comprising a measurement unit for determining an effect of an aging and/or environmental factor. 
     The exemplary embodiment of  FIG. 3  shows in more detail a possible implementation of the measurement unit. A battery  301  is used to power the LED  121 . In between the battery  301  and the LED  121  is a protection diode  351 . An input protection capacity  352  protects against pulses in an automotive environment The selector  131  can select between different input channels HVIO 0   304 , HVIO 1   305 , HVIO 2   306 , HVIO 3   307 . 
     The differential amplifier  310  with switched capacitors  133  connects with VS  303  and with HVIO 0   304  (since HVIO 0  is selected by the selector  132 ). The selector  132  typically may comprise a source enable element for enabling the current source. 
     More particularly, in one example, capacitor C 2   332  connects VS with the first input of the differential amplifier. Capacitor C 3   333  connects HVIO 0  with the second input of the differential amplifier. A switch CP 1   311  between VS and HVIO 0  is used for offset scaling. Capacitor C 1   331  connects the first input of the differential amplifier with the output of the differential amplifier, provided the switch CP 2   321  is closed. When switch CP 1   313  is closed, capacitor C 1   331  connects the first input of the differential amplifier with a reference voltage Vref. When switch CP 1   312  is closed the first input of the differential amplifier is connected with the output of the differential amplifier. Capacitor C 4   334  interconnect the second input of the differential amplifier with a reference voltage Vref. When switch CP 1   314  is closed, the second input of the differential amplifier is directly connected with a reference voltage Vref. 
     In  FIG. 3 , the differential amplifier circuit thus comprises a first capacitor  331 , comprising a first and a second node, the first node connected to the first input of the amplifier circuit, the second node connected to a first switch  321  that also connects to the output of the differential amplifier. 
     The differential amplifier circuit comprises a second capacitor  332 , comprising a first and a second node, the second node connected with the first input of the differential amplifier, the first node being the first input of the differential amplifier circuit. 
     The differential amplifier circuit comprises a third capacitor  333 , comprising a first and a second node, the second node connected with the second input of the differential amplifier, the second node being the second input of the amplifier circuit, 
     The differential amplifier circuit a fourth capacitor  334 , comprising a first and a second node, the first node connected with the second input of the differential amplifier, the second node connected with a reference voltage. 
     The differential amplifier circuit a first switch  311  that, when closed, interconnects the first node of the second capacitor and the first node of the third capacitor. 
     The differential amplifier circuit a second switch  312  that, when closed, interconnects the first input of the differential amplifier with the output of the differential amplifier, 
     The differential amplifier circuit a third switch  313  that, when closed, interconnects the second node of the first capacitor  331  with the reference voltage. 
     The differential amplifier circuit a fourth switch  314  that, when closed, interconnects the second node of the differential amplifier with the reference voltage. 
     The differential amplifier circuit a fifth switch  321  that, when closed, interconnects the second node of the first capacitor  331  with the output of the differential amplifier. 
     The output signal of the differential amplifier circuit is outputted to the ADC converter system  134 , which comprises an AD channel selector and the actual AD converter. 
     It can be seen in  FIG. 3  that the LED is connected to voltage source VS. Voltage source VS corresponds with a voltage similar to the battery voltage V bat  but with a polarity protection V protection   _   diode  included: VS=V bat −V protection   _   diode    
     The forward voltage of the LED is measured between VS and the LED itself. The current through the LED typically is adjusted and limited via the internal current source. The internal current source is variable. Because of that, the forward voltage of the LED can be calculated as in the following:
 
 U   LED =( U   VS   −U   HVIOx )
 
wherein U LED  is the forward voltage of the LED, U VS  is the voltage over the voltage source VS U HVIOx  is the voltage at the internal current source connected to HVIO.
 
Differential Amplifier
 
     The internal current source is calibrated during final test. Therefore, the adjusted current of the current source is well-known. (U VS −U HVIOx ) is not known and must be internally measured. The measurement is done via an internal differential amplifier and the ADC. 
     The calculation of the differential amplifier looks like in the following where the Cs are considered as resistors to keep it as simple as possible: 
               U   diff_out     =           (       C   ⁢           ⁢   1     +     C   ⁢           ⁢   2       )     *     (         U   HVIOx     *   C   ⁢           ⁢   4     +       U   VREF     *   C   ⁢           ⁢   3       )         C   ⁢           ⁢   2   *     (       C   ⁢           ⁢   3     +     C   ⁢           ⁢   4       )         -         U   VS     *   C   ⁢           ⁢   1       C   ⁢           ⁢   2               
The given formula can be simplified for C1=C4 and C2=C3:
 
     
       
         
           
             
               U 
               diff_out 
             
             = 
             
               
                 
                   ( 
                   
                     
                       U 
                       HVIOx 
                     
                     - 
                     
                       U 
                       VS 
                     
                   
                   ) 
                 
                 ⁢ 
                 
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                 
               
               + 
               
                 U 
                 VREF 
               
             
           
         
       
     
     In case, the difference between U VS  and U HVIOx  is 0 then there remains the common mode voltage U VREF . C1/C2 can be considered as gain-factor of the differential amplifier. 
     EXAMPLE CALCULATION FOR THE ACCURACY OF THE LED FORWARD VOLTAGE 
     The gain-factor of the differential amplifier is in the present example given by factor 1/5. Furthermore, U VREF  is 1.25V. As a result, the inputs of the differential amplifier can have a difference of 0 . . . 6.25V (maximum: U VREF /gain). The internal ADC reference voltage is given by 1.5V for getting the highest accuracy over the complete output range of the differential amplifier (0 . . . 1.25V→UVREF). 
     In this case, the resolution of the measurable LED forward voltage is ˜7.33 mV per ADC digit where the ADC has a resolution of 10 Bit (5*1.5V/1023→ADCreference_voltage/(ADCresolution*gain)). Let&#39;s assume a temperature coefficient of ˜−3.7 mV for the LED. It allows a maximum measurable temperature resolution of ˜2-3 K. 
     Result 
     The forward voltage of an RGB-LED can be differential measured. Therefore, a single differential amplifier, a variable current source and an ADC must be available. The input of the differential amplifier can be selected for each LED of the RGB-LED (see  FIG. 1 ). Therefore, the measurement of the forward voltage over the three LEDs must be done sequentially. Alternatively, different differential amplifiers could be used for example. The offset of the differential amplifier can be compensated by HW and/or SW where the differential amplifier with SC-Technology can fully handle the offset compensation in HW. The total resolution obtained in some devices is ˜7.33 mV per ADC digit. Moreover, the maximum difference between VS and HVIOx (protection resistor+LED) can be 6.25V. 
     In one aspect, the present invention also relates to a measurement unit as described above, while describing the electronic device. Such a measurement unit therefore does not need to be part of the electronic device, but at least is suitable for determining an aging or environmental effect of one or more LEDs in an electronic device comprising a driving unit. The measurement unit thus may be configured for co-operating with the electronic device. Specific features of the measurement unit may correspond with features of the measurement unit described with reference to the first aspect. 
     In yet another aspect, the present invention also relates to a controller being programmed for performing a method for measuring and/or compensating environmental effects or aging effects on a LED. According to embodiments of the present invention, the controller is adapted for performing the method for measuring and/or compensating as described above. The controller may comprise software and/or hardware components for controlling the measuring and/or compensating of environmental or aging effects on the LED. 
     In yet a further aspect, the present invention also relates to a method for measuring and/or compensating environmental effects or aging effects on a LED implemented as a computer implemented invention in a processor and to corresponding processors, e.g. a controller. One configuration of such a processor may for example include at least one programmable computing component coupled to a memory subsystem that includes at least one form of memory, e.g., RAM, ROM, and so forth. It is to be noted that the computing component or computing components may be a general purpose, or a special purpose computing component, and may be for inclusion in a device, e.g., a chip that has other components that perform other functions. Thus, one or more aspects of the present invention can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations of them. For example, each of the method steps may be a computer implemented step. Thus, while a processor as such is prior art, a system that includes the instructions to implement aspects of the method for measuring and/or compensating environmental effects or aging effects on a LED is not prior art. 
     The present invention thus also includes a computer program product which provides the functionality of any of the methods according to the present invention when executed on a computing device. 
     In another aspect, the present invention relates to a data carrier for carrying a computer program product for measuring and/or compensating environmental effects or aging effects on a LED. Such a data carrier may comprise a computer program product tangibly embodied thereon and may carry machine-readable code for execution by a programmable processor. The present invention thus relates to a carrier medium carrying a computer program product that, when executed on computing means, provides instructions for executing any of the methods as described above. The term “carrier medium” refers to any medium that participates in providing instructions to a processor for execution. Such a medium may take many forms, including but not limited to, non-volatile media, and transmission media. Non-volatile media includes, for example, optical or magnetic disks, such as a storage device which is part of mass storage. Common forms of computer readable media include, a CD-ROM, a DVD, a flexible disk or floppy disk, a tape, a memory chip or cartridge or any other medium from which a computer can read. Various forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to a processor for execution. The computer program product can also be transmitted via a carrier wave in a network, such as a LAN, a WAN or the Internet. Transmission media can take the form of acoustic or light waves, such as those generated during radio wave and infrared data communications. Transmission media include coaxial cables, copper wire and fibre optics, including the wires that comprise a bus within a computer.