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WO2008120133A2 - Procédé et dispositif pour piloter un système de del - Google Patents

Procédé et dispositif pour piloter un système de del Download PDF

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Publication number
WO2008120133A2
WO2008120133A2 PCT/IB2008/051097 IB2008051097W WO2008120133A2 WO 2008120133 A2 WO2008120133 A2 WO 2008120133A2 IB 2008051097 W IB2008051097 W IB 2008051097W WO 2008120133 A2 WO2008120133 A2 WO 2008120133A2
Authority
WO
WIPO (PCT)
Prior art keywords
led
representing
node
light output
leds
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
PCT/IB2008/051097
Other languages
English (en)
Other versions
WO2008120133A3 (fr
Inventor
Petrus J. Bremer
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Koninklijke Philips NV
Original Assignee
Koninklijke Philips Electronics NV
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Koninklijke Philips Electronics NV filed Critical Koninklijke Philips Electronics NV
Publication of WO2008120133A2 publication Critical patent/WO2008120133A2/fr
Publication of WO2008120133A3 publication Critical patent/WO2008120133A3/fr
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

Links

Classifications

    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B45/00—Circuit arrangements for operating light-emitting diodes [LED]
    • H05B45/20—Controlling the colour of the light
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B45/00—Circuit arrangements for operating light-emitting diodes [LED]
    • H05B45/10—Controlling the intensity of the light
    • H05B45/18—Controlling the intensity of the light using temperature feedback

Definitions

  • the present invention relates in general to the control of LEDs.
  • a particular field of application of LEDs is as backlight in LCD panels, such as for instance in LCD TVs, and the present invention will be specifically explained for this application, but it is to be noted that the present invention is not restricted to this application.
  • An LCD display panel of transmission type needs to have a backlight, i.e. a light source placed behind the LCD as seen from a viewer. It is possible to implement a backlight with one light source only.
  • Backlight embodiments have been developed on the basis of a plurality of elongated light sources, typically gas discharge lamps, placed in a horizontal orientation above each other; such embodiments have an advantage that it is possible to give different light magnitude to different portions of the panel in order to enhance contrast differences.
  • gas discharge lamps it is also possible to use a plurality of LEDs.
  • the LEDs are typically arranged in horizontal strips, and the backlight comprises a plurality of such strips arranged above each other.
  • the LEDs of one strip can be driven in parallel, but it is also possible to individually drive the individual LEDs such as to individually set the light output magnitude of the individual LEDs in order to enhance the contrast in the corresponding portions of the LCD panel.
  • This facility is also indicated as two-dimensional dimming/boosting, and is executed on the basis of the content of a video signal. Since this is known per se to persons skilled in the art, a further explanation can be omitted here.
  • LED control it is desirable to control the light output. For instance, in the above example, if analysis of the video signal indicates that the video content does not change, the light output of the backlight LEDs should not change.
  • the light output of a LED is not necessarily constant when the drive signal (LED current) is maintained constant.
  • the light output of a LED depends on temperature: the higher its junction temperature, the lower its light output.
  • the rate at which the light output decreases with rising temperature depends, inter alia, on LED type. For instance, for a white LED, rising the temperature from 20 0 C to 70 0 C may cause a reduction of light output of 20%. This is already undesirable in a single LED system, although it will probably go unnoticed to most users. In a multi LED system, the effect is more noticeable, because the user will see different LEDs simultaneously having different light outputs where they should have the same light output.
  • a disadvantage of these prior art solutions is that the number of sensors needed is proportional to the number of LEDs, and these sensors, together with the required measurement hardware and interfacing hardware, add to the costs and complexity of the system, be it a backlight system or an illumination system. Even if it would be possible to have a plurality of LEDs share one sensor, the costs would still be high.
  • An object of the present invention is to eliminate or at least reduce the above- mentioned problems.
  • the present invention aims to provide a method and device for driving a LED system in which temperature-dependent light output is compensated without the necessity of having sensors and measurement hardware.
  • the temperature of the LEDs is not measured, but instead the temperature of the individual LEDs is calculated/predicted/estimated on the basis of a thermal model of the LED system, taking into account the individual powers supplied to the individual LEDs, and individual control signals for the individual LEDs are adapted on the basis of the thus calculated/predicted/estimated individual temperatures.
  • the thermal model may be implemented in hardware, but may conveniently be implemented in software.
  • US-6.970.811 discloses a circuit for driving a single LED, comprising an ambient temperature sensor, and comprising a hardware model modeling the behavior of the LED light output in response to the drive signals.
  • this solution is not suitable in the case of a system comprising multiple LEDs.
  • Figure 1 schematically illustrates a backlight system
  • Figure 2 is a graph illustrating a relationship between the temperature of an
  • Figure 3 is a block diagram schematically illustrating model-based control of a LED system according to the present invention.
  • Figure 4 is a block diagram schematically illustrating an electronic circuit as a hardware implementation of a portion of an LED control circuit according to the present invention.
  • FIG. 1 schematically illustrates a backlight system 1 , comprising a rectangular matrix 2 of backlight LEDs mounted on a backplate 3.
  • the LEDs will be indicated by the reference numeral 10; if it is intended to indicate a specific individual LED, the subscripts X and Y will be used, indicating the horizontal and vertical position of such LED in the rectangular matrix.
  • the system 1 further comprises a controller 30, which determines the amount of light each LED 10 should emit.
  • the controller 30 may have an input 31 for receiving a video signal, and may be designed for determining dimming or boosting of individual LEDs on the basis of the video content, as is known per se.
  • the controller 30 For outputting control signals for the LED control, the controller 30 has a corresponding number of outputs, which together will be indicated as "output 32".
  • the system 1 may comprise a corresponding LED driver 20, which receives the corresponding control signal from the controller 30 and generates a suitable drive signal for the LED; in figure 1, this is illustrated for one LED only, for sake of simplicity. Likewise, for sake of simplicity, a power source and power lines to the LEDs are not shown in the figure.
  • the suitable drive signal for the LED may be a block- shaped LED current, suitably switched ON/OFF at a certain switching frequency, wherein the duty cycle determines the average LED current and thus the dimming or boosting level.
  • the control signal from the controller 30 may be a signal indicating the required duty cycle, or it may be a block-shaped signal itself, in which case the LED driver 20 simply switches the LED current ON and OFF at the instances dictated by the block-shaped control signal.
  • Figure 2 is a graph illustrating the basic problem underlying the present invention.
  • the horizontal axis of figure 2 represents temperature T (degrees Centigrade), the vertical axis of figure 2 represents relative light output RLO (in percent) of an LED.
  • RLO in percent
  • the figure shows that the light output is inversely proportional to the temperature: with increasing temperature, the light output decreases.
  • room temperature is taken as a reference value.
  • Different types of LEDs may have different temperature dependencies.
  • the light output decreases with 10% for every 25 0 C temperature rise.
  • the controller 30 Assume that it is intended for the controller 30 to drive a certain LED to obtain a certain constant light output. Assume that, to this end, the controller generates a constant control signal. Because of being ON, the LED dissipates some energy and its temperature is rising; consequently, its light output is dropping. If the LED is operated at a higher duty cycle to achieve a higher light output (boosting), the temperature rise is higher therefore the relative decrease of the light output is higher. A complicating factor is that this active LED will also cause a temperature rise of the adjacent LEDs. Even if such adjacent LED is operated at a relatively low (average) power, its temperature will not only be determined by its own relatively low energy dissipation but will also be determined by the heat generated by its neighbors.
  • the temperature of a LED does not only depend on its current power setting and the history of its power setting, but also on the power settings and the histories of the power settings of its neighbors.
  • each LED with an associated temperature measuring device, be it a direct temperature sensor or a sensor for measuring junction resistance.
  • the controller 30 would receive individual measuring signals for each individual LED, enabling the controller 30 to individually adapt its control signals for the individual LEDs to compensate for the temperature effect.
  • this is a very costly solution which, although it undoubtedly works, is not proposed by the present invention.
  • the system 1 comprises a model 100 representing the thermal behavior of the LED matrix 2. This is illustrated in figure 3.
  • the model which receives the output signals 32 from the controller 30 as input signals, and which produces output signals 199 representing the temperatures of the LEDs, may be implemented in hardware or in software; in the following, a hardware implementation will be explained with reference to figure 4.
  • an LED is represented as a node in an electrical circuit.
  • a heat flow in the system 1 may be represented by a current flowing in the circuit.
  • Heat resistance may be represented by a resistor
  • heat capacitance may be represented by a capacitor.
  • Heat input to a LED may be represented by a supply of current from a current source to the corresponding circuit node
  • heat output from a LED may be represented by a current being drawn from the corresponding circuit node by a current source. Then, the resulting voltage at a circuit node represents the resulting temperature of the corresponding LED.
  • Figure 4 is a circuit diagram illustrating one cell 101 of the hardware model, the cell 101 corresponding to one LED 10 ⁇ .
  • a node 103 ⁇ represents a portion of the backplate 3 corresponding with the LED 10 ⁇ ; the voltage V(103 ⁇ ; ⁇ ) at this node represents the temperature of said portion of the backplate 3.
  • the heat capacity of this backplate portion is represented by a capacitor 131 connecting this node 103 ⁇ to mass.
  • the heat exchanging contact of this backplate portion with the adjacent backplate portions is represented by resistors 141, 142, 143, 144 which connect the node 103 ⁇ , ⁇ to adjacent nodes 103 ⁇ , ⁇ .i, 103 ⁇ , ⁇ + i, 103 ⁇ .i , ⁇ , 103 ⁇ + i , ⁇ , respectively, which adjacent nodes are shown in dotted lines in figure 4.
  • the backplate portion may loose heat through radiation or convection, which is represented by a current source 151 connecting this node 103 ⁇ to mass.
  • the current drawn by this current source 151 may be considered to be fixed. If more accuracy is needed, this current source 151 may be a controllable current source, controlled by the voltage at node 103 ⁇ , representing a heat loss which is proportional to the temperature difference between the backplate portion and its surroundings, but this is not shown in the figure.
  • a node 110 ⁇ , ⁇ represents the LED 10 ⁇ , ⁇ ; the voltage
  • V(110 ⁇ ; ⁇ ) at this node represents the temperature of the LED 10 ⁇ .
  • the heat capacity of the LED 10 ⁇ is represented by a capacitor 132 connecting this node 110 ⁇ to mass.
  • the LED 10 ⁇ is in heat exchanging contact with the corresponding backplate portion; this is represented by a resistor 145 which connect the node 103 ⁇ to the node 110 ⁇ .
  • the LED may loose heat through radiation or convection, which is represented by a current source 152 connecting this node 110 ⁇ to mass. Again, in first approximation, the current drawn by this current source 152 may be considered to be fixed.
  • this current source 152 may be a controllable current source, controlled by the voltage at node 110 ⁇ , representing a heat loss which is proportional to the temperature difference between the LED and its surroundings, but this is not shown in the figure. Last, but not least, the LED is heated due to dissipation of energy from its operational current. This is represented by a current source 153, connecting this node 110 ⁇ , ⁇ to a supply voltage source Vs. This current source 153 is a controllable current source, controlled by the control signal 32 or a signal derived therefrom.
  • model 100 may be implemented in discrete components but may also be implemented in an integrated circuit. However, instead of a hardware implementation, the model may also be implemented in software and executed by a processor or specialized hardware like, for instance, microcontroller, microprocessor or FGPA.
  • the resistance values and capacitor values may be stored as fixed parameters in a memory.
  • the voltage values and current source values may, after each calculation step, be stored in a random access memory and may, before each calculation step, be read from this random access memory. It is further noted that the model 100 and the controller 30 may be integrated into one unit.
  • V(103 ⁇ , ⁇ ) NEW V(103X,Y)OLD + ⁇ V x , ⁇ , with
  • the controller 30 On the basis of the output signals 199 from the model 100, the controller 30 knows the momentary temperature T ⁇ ; ⁇ of each individual LED 10 ⁇ . Using the information in the memory 40, the controller 30 calculates the relative light output RLO ⁇ of each individual LED 10 ⁇ , and adapts its control signals 32 ⁇ ; ⁇ in a corresponding manner to compensate for the temperature effect. For instance, assume that, based on the video content in a video signal received at input 31, the controller 30 calculates that a particular LED 10 ⁇ should produce a light output of 60% of its nominal light output. Assume further that the output signals 199 from the model 100 predict/estimate for this particular LED 10 ⁇ a momentary temperature of 75 0 C. Assume further that the relationship graphically represented by figure 2 applies for this particular LED 10 ⁇ .
  • the present invention proposes to use a model 100.
  • the model comprises: a first node 110 ⁇ representing the LED; a first capacitive component 131 connected to said first node, representing heat capacitance of the LED; a second node 103 ⁇ representing a backplate portion corresponding with the LED; a second capacitive component 132 connected to said second node, representing heat capacitance of said backplate portion; a resistive component 145 between said two nodes, representing heat resistance of a thermal coupling between the LED and the backplate; - resistive components 141, 142, 143, 144 between said second node 103 ⁇ and adjacent second nodes, representing heat resistances of thermal couplings between the backplate portion and adjacent backplate portions; a first flow source component 153 connected to said first node, representing heat dissipation of the LED.
  • the model 100 for each LED 10 ⁇ , further preferably comprises: a second flow source component 152 connected to said first node 110 ⁇ , ⁇ , representing heat loss through radiation or convection of the LED 10 ⁇ ; - a third flow source component 151 connected to said second node 103 ⁇ , representing heat loss through radiation or convection of said backplate portion.
  • the calculation can be modified by adapting the parameters of the model, for instance the current sources 151, 152, 153. For instance, if it appears in reality that the temperature at the sensor location increases more than the model predicts, then it is possible to increase the current value of current source 153.
  • a computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope. In the above, the present invention has been explained with reference to block diagrams, which illustrate functional blocks of the device according to the present invention.
  • one or more of these functional blocks may be implemented in hardware, where the function of such functional block is performed by individual hardware components, but it is also possible that one or more of these functional blocks are implemented in software, so that the function of such functional block is performed by one or more program lines of a computer program or a programmable device such as a microprocessor, microcontroller, digital signal processor, etc.

Landscapes

  • Circuit Arrangement For Electric Light Sources In General (AREA)
  • Led Devices (AREA)

Abstract

Selon l'invention, pour estimer des températures de DEL d'une matrice (2), un modèle (100) est utilisé. Pour chaque DEL (10xy), le modèle comprend : un premier nœud (110xy) représentant la DEL; un premier composant capacitif (131) connecté audit premier nœud, représentant une capacité thermique de la DEL; un second nœud (103x, y) représentant une partie de plaque de fond correspondant à la DEL; un second composant capacitif (132) connecté audit second nœud, représentant une capacité thermique de ladite partie de plaque de fond; un composant résistif (145) entre lesdits deux nœuds, représentant une résistance thermique d'un couplage thermique entre la DEL et la plaque de fond; des composants résistifs (141, 142, 143, 144) entre ledit second nœud (103x, y) et des seconds nœuds adjacents, représentant des résistances thermiques de couplages thermiques entre la partie de plaque de fond et des parties de plaque de fond adjacentes; un premier composant de source d'écoulement (153) connecté audit premier nœud, représentant la dissipation thermique de la DEL.
PCT/IB2008/051097 2007-03-29 2008-03-25 Procédé et dispositif pour piloter un système de del Ceased WO2008120133A2 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
EP07105163 2007-03-29
EP07105163.5 2007-03-29

Publications (2)

Publication Number Publication Date
WO2008120133A2 true WO2008120133A2 (fr) 2008-10-09
WO2008120133A3 WO2008120133A3 (fr) 2009-01-29

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PCT/IB2008/051097 Ceased WO2008120133A2 (fr) 2007-03-29 2008-03-25 Procédé et dispositif pour piloter un système de del

Country Status (2)

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TW (1) TW200904239A (fr)
WO (1) WO2008120133A2 (fr)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP2407007A1 (fr) * 2009-03-09 2012-01-18 Koninklijke Philips Electronics N.V. Système et appareil pour commander une intensité de lumière émise de matrices de diodes électroluminescentes
CN107071984A (zh) * 2017-05-25 2017-08-18 上海地铁第运营有限公司 一种手持终端设备电源自适应动态管理系统及方法

Family Cites Families (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6349023B1 (en) * 2000-02-24 2002-02-19 Robotic Vision Systems, Inc. Power control system for illumination array
KR101190214B1 (ko) * 2004-07-23 2012-10-16 코닌클리즈케 필립스 일렉트로닉스 엔.브이. 고체 상태 조명 장치에서 온도에 우선 순위를 두고 컬러를 제어하기 위한 시스템
EP1808050B1 (fr) * 2004-10-22 2022-12-07 Signify Holding B.V. Procede de commande d'un dispositif d'eclairage a diodes
EP1891837A2 (fr) * 2005-05-27 2008-02-27 Koninklijke Philips Electronics N.V. Controle d'un systeme de semi-conducteurs emettant de la lumiere de differentes couleurs
JP2008543012A (ja) * 2005-06-03 2008-11-27 コーニンクレッカ フィリップス エレクトロニクス エヌ ヴィ Ｌｅｄ照明灯を制御するためのシステム及び方法
WO2007019663A1 (fr) * 2005-08-17 2007-02-22 Tir Technology Lp Systeme de luminaire a commande numerique
JP4757585B2 (ja) * 2005-09-21 2011-08-24 Ｎｅｃ液晶テクノロジー株式会社 光源ユニット及び照明装置

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP2407007A1 (fr) * 2009-03-09 2012-01-18 Koninklijke Philips Electronics N.V. Système et appareil pour commander une intensité de lumière émise de matrices de diodes électroluminescentes
CN102349351A (zh) * 2009-03-09 2012-02-08 皇家飞利浦电子股份有限公司 用于控制发光二极管阵列的光强度输出的系统和设备
JP2012519977A (ja) * 2009-03-09 2012-08-30 コーニンクレッカ フィリップス エレクトロニクス エヌ ヴィ 発光ダイオードアレイの光強度出力を制御するためのシステム及び装置
CN107071984A (zh) * 2017-05-25 2017-08-18 上海地铁第运营有限公司 一种手持终端设备电源自适应动态管理系统及方法

Also Published As

Publication number Publication date
TW200904239A (en) 2009-01-16
WO2008120133A3 (fr) 2009-01-29

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