EP0952757A2 - Circuit for controlling the light intensity of current controlled LEDS for display illumination - Google Patents

Circuit for controlling the light intensity of current controlled LEDS for display illumination Download PDF

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Publication number
EP0952757A2
EP0952757A2 EP99107236A EP99107236A EP0952757A2 EP 0952757 A2 EP0952757 A2 EP 0952757A2 EP 99107236 A EP99107236 A EP 99107236A EP 99107236 A EP99107236 A EP 99107236A EP 0952757 A2 EP0952757 A2 EP 0952757A2
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EP
European Patent Office
Prior art keywords
light
emitting diodes
control unit
degradation
memory
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Granted
Application number
EP99107236A
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German (de)
French (fr)
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EP0952757B1 (en
EP0952757A3 (en
Inventor
Dieter Dipl-Ing. Klostermeier (Fh)
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Siemens AG
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Mannesmann VDO AG
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    • 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]
    • H05B45/50—Circuit arrangements for operating light-emitting diodes [LED] responsive to malfunctions or undesirable behaviour of LEDs; responsive to LED life; Protective circuits
    • H05B45/58—Circuit arrangements for operating light-emitting diodes [LED] responsive to malfunctions or undesirable behaviour of LEDs; responsive to LED life; Protective circuits involving end of life detection of LEDs
    • 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
    • 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
    • H05B47/00—Circuit arrangements for operating light sources in general, i.e. where the type of light source is not relevant
    • H05B47/20—Responsive to malfunctions or to light source life; for protection
    • 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
    • H05B47/00—Circuit arrangements for operating light sources in general, i.e. where the type of light source is not relevant
    • H05B47/20—Responsive to malfunctions or to light source life; for protection
    • H05B47/28—Circuit arrangements for protecting against abnormal temperature

Definitions

  • the invention relates to a circuit arrangement for adjusting the brightness of current-controlled light-emitting diodes for illuminating a display, in particular for illuminating displays, buttons and switches in vehicles.
  • the proposed circuit arrangement can be used both for controlling active display elements and for backlighting or lighting.
  • Active display elements can e.g. Segment displays or status display fields of an instrument cluster in the dashboard of a vehicle.
  • the invention is based on an arrangement according to DE 196 02 891 A1.
  • An arrangement for adjusting the brightness of a current- or voltage-controlled lamp for illuminating a display is described there, in which, at the end of the display manufacturing process, a correction factor is determined depending on the brightness of the lamp, by means of which the control voltage or the control current of the lamp is set becomes.
  • the correction factor is determined and set with the aid of a light measuring device, which measures the brightness emitted by the illuminant, and with the use of an external test computer which can be connected to the display control unit via a diagnostic interface.
  • the measured brightness value is compared with a table value stored in a memory belonging to the display is deposited.
  • the correction factor can also be taken from a map which characterizes the luminous characteristics of the illuminant.
  • the correction factor taken from a table or a map is then used to influence the control of the illuminant by a change in current or voltage in such a way that the measured brightness value is adapted to the brightness value specified in the memory as the setpoint value.
  • the setting of the control of the lamp made in this way should be retained for the service life of the lamp.
  • control unit can also record and register information about the electrical power and / or the operating temperature of the light-emitting diodes, so that the degradation value is determined in order to set the control current for the light-emitting diodes in accordance with the data for the degradation behavior stored in the memory not only as a function of the registered operating time, but also as a function of the electrical power and / or the operating temperature of the light-emitting diodes.
  • the inclusion of the electrical power in the determination of the degradation value carried out by the control unit for setting the control current for the light-emitting diodes should actually only be dispensed with if this is done by the control unit via the input interface recorded signal for the desired brightness of the light emitting diodes is relatively constant, that is, if a change in the brightness of the light emitting diodes, once set, is not expected or is not intended at all during the period of use of the display.
  • the inclusion of the operating temperature of the light-emitting diodes is therefore a very sensible option because aging, i.e. the degradation behavior of the light-emitting diodes is temperature-dependent to a considerable degree and in many, in particular the automotive applications for the proposed circuit arrangement, operation in a wide temperature range with frequent temperature fluctuations must be expected.
  • the solution found can be designed in such a way that the data representing the degradation behavior of the light-emitting diodes is not only stored in the memory in the form of individual discrete and therefore relatively less punctual values, but also in the form of a curve taken from a data sheet or by means of a formulaic connection or by Parameters for a specific, but not detailed, algorithm describing the degradation behavior, whereby the degradation value to be determined by the control unit can reproduce the actual degradation behavior of the light-emitting diodes in even finer coordination.
  • This way an under-compensation or possibly associated with the proposed tracking of the control current can be achieved Overcompensation of the degradation behavior for the light-emitting diodes can be avoided even better.
  • the main advantage of the solution found is that a constant brightness of the light-emitting diodes can be ensured for a reasonable period of use of the display without the need for external measuring or testing equipment.
  • the control current of the light-emitting diodes is determined on the basis of a Degradation value tracked that the brightness of the light emitting diodes remains constant for a user of the display in spite of their physically determined and thus unavoidable degradation behavior.
  • the control unit updates the degradation value at reasonable time intervals on the basis of data stored in a memory, which characterize the degradation behavior.
  • the update takes place in that the control unit registers the operating time and preferably also the electrical power with which the light-emitting diodes have already been operated as the operating parameters of the light-emitting diodes, and then uses the data stored in the memory, which characterize the degradation behavior, to determine what the degradation value is typically results from the registered operating parameters.
  • This update of the degradation value will only be carried out in the price at reasonable intervals, in order not to unduly burden the control unit with calculations.
  • the control unit Until the next update, the control unit preferably stores the determined degradation value in a memory assigned to the control unit and carries out all the brightness settings for the light-emitting diodes desired until the next update, based on this currently applicable degradation value, by the control unit setting the control current for the light-emitting diodes taking this degradation value into account. Since the degradation value indicates to what extent the brightness of the light-emitting diodes has already decreased compared to their initial brightness, the control current for the light-emitting diodes is tracked in inverse proportion to the degradation value. A prerequisite for a successful application of the proposed circuit arrangement is, of course, that the control current is initially set in such a way that its amount can be tracked without overdriving the light-emitting diodes.
  • the working range for the control current of the light-emitting diodes must therefore be selected such that the limit range of the light-emitting diodes is not reached as far as possible by tracking the control current during the planned period of use of the display.
  • the proposed Circuit arrangement makes a burn-in, which intercepts the initially strong degradation, unnecessary for the light-emitting diodes.
  • the acquisition and registration of the information about the operating parameters required for the determination of the current degradation value ie the information about the operating time, the electrical power and / or the operating temperature
  • the price therefore generally does not require any sensor sensors that work independently, because the control unit, through suitable programming, both the duration of the current flow to the light-emitting diodes and an indication of the amount of the control current can register.
  • the control unit can easily calculate the electrical power of the light emitting diodes as soon as it can refer to the amount of the control current. Since the control current is in many cases provided directly by the control unit, the control unit only needs to register in the memory associated with it how high the control current made available by it in a specific time unit was by storing a corresponding indication. As soon as the control unit logs the behavior of the control current it provides at its output, essential information on the operating parameters is available in order to determine the current degradation value with the data on the degradation behavior of the light-emitting diodes stored in the memory.
  • thermosensor is required to obtain an indication of the operating temperature of the light-emitting diodes, this indication then being fed to the control unit for further processing.
  • the data to be stored in the memory connected to the control unit for setting the control current of the light-emitting diodes can be stored during the manufacturing process of the display to be illuminated with the light-emitting diodes. All that is required is to transfer the information from the manufacturer of the light-emitting diodes into the memory of the circuit arrangement in the form of suitable data.
  • This data can be very easily matched to the exact specification of the light-emitting diodes by an appropriate program selection, so that optimal data is always used for each display. This also simplifies the manufacturing process for a generic display because a selection of the light-emitting diodes used in the manufacturing process of the display, which is dependent on the component specification, can be omitted.
  • the proposed solution will be briefly explained again using a simplified block diagram.
  • the figure shows a display 1 with light-emitting diodes, the control current of which is set by a control unit 2.
  • the control is to be carried out by a pulse width modulated signal (PWM).
  • PWM pulse width modulated signal
  • the control could of course also be carried out analogously with a continuously running current.
  • the control unit 2 receives, via an input interface 6, a signal which is a measure of the desired brightness of the light-emitting diodes.
  • the signal picked up via the input interface 6 can be tapped by a dimmer with which the desired brightness for the light-emitting diodes is set by the user of the display 1.
  • this signal can also be a default value, which e.g. is transmitted from another control device to the device containing display 1 via a CAN data bus.
  • the control unit 2 sets the control current taking into account a degradation value.
  • the control unit 2 detects and registers the operating time of the light-emitting diodes and preferably also their electrical power and - if necessary - their operating temperature. If information on the two operating parameters operating time and electrical power is not recorded directly by the control unit 2, an operating hours counter 3 can be used as a means for recording the operating time, which determines the current flow duration, and the electrical power can also be recorded using a suitable measuring device 4 will. Furthermore, the control unit 2 can be supplied with the signal of a temperature sensor 5 arranged in the vicinity of the light-emitting diodes. The information about the detected operating parameters is fed to the control unit, where it is continuously accumulated and updated and in one of the control unit 2 assigned memory 7 filed.
  • the control unit 2 now uses the current information about at least one of the detected operating parameters to check which degradation value is assigned to them on the basis of the data stored in the memory 7 about the degradation behavior of the light-emitting diodes, and then makes this degradation value available for a corresponding setting of the control current.
  • the control unit provides the control current directly at its output or via suitable drivers is not significant for the present invention.
  • the degradation value will not be updated continuously, but only at reasonable time intervals, e.g. depending on the load on the LEDs, or possibly daily, or depending on brightness reductions that the human eye perceives as different brightnesses in the respective operating environment of the display can be.
  • the memory 7, in which the data for the operating parameters operating time, electrical power or operating temperature for collecting the degradation value is collected, is to be designed as a non-volatile memory (EEPROM or flash). It is expedient in this memory 7 also to store the data for the degradation behavior of the light-emitting diodes, which can be given by table values, data sheet curves or parameters for algorithms to be executed by the control unit 2. Which data are to be stored advantageously in which form will have to be decided on a case-by-case basis depending on the application and the available capacity of the memory 7.

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  • Control Of Indicators Other Than Cathode Ray Tubes (AREA)
  • Control Of El Displays (AREA)
  • Circuit Arrangement For Electric Light Sources In General (AREA)

Abstract

The circuit has a control unit (2) that receives (6) a signal corresponding to the desired intensity and sets the LED (1) control current taking into account a degradation value. The control unit determines the degradation value by recording the operating duration of the diode and places a corresponding value in a memory (7) containing degradation information. The control unit determines the degradation value from the stored information and the measured operating duration and updates it at appropriate time intervals.

Description

Die Erfindung betrifft eine Schaltungsanordnung zur Einstellung der Helligkeit stromgesteuerter Leuchtdioden zur Beleuchtung einer Anzeige, insbesondere zur Beleuchtung von Displays, Tasten und Schaltern in Fahrzeugen. Die vorgeschlagene Schaltungsanordnung kann sowohl zur Ansteuerung aktiver Anzeigeelemente als auch bei Hinterleuchtungen oder Beleuchtungen angewendet werden. Aktive Anzeigeelemente können z.B. Segmentanzeigen oder Status-Anzeigefelder eines Kombiinstruments im Armaturenbrett eines Fahrzeugs sein.The invention relates to a circuit arrangement for adjusting the brightness of current-controlled light-emitting diodes for illuminating a display, in particular for illuminating displays, buttons and switches in vehicles. The proposed circuit arrangement can be used both for controlling active display elements and for backlighting or lighting. Active display elements can e.g. Segment displays or status display fields of an instrument cluster in the dashboard of a vehicle.

Die Erfindung geht von einer Anordnung gemäß DE 196 02 891 A1 aus. Dort ist eine Anordnung zur Einstellung der Helligkeit eines strom- oder spannungsgesteuerten Leuchtmittels zur Beleuchtung einer Anzeige beschrieben, bei der am Ende des Herstellungsprozesses der Anzeige in Abhängigkeit von der Helligkeit des Leuchtmittels ein Korrekturfaktor bestimmt wird, mittels welchem die Steuerspannung oder der Steuerstrom des Leuchtmittels eingestellt wird. Die Bestimmung und Einstellung des Korrekturfaktors erfolgt mit Hilfe eines Lichtmeßgerätes, das die von dem Leuchtmittel ausgesandte Helligkeit mißt, sowie unter Verwendung eines über eine Diagnoseschnittstelle an das Steuergerät der Anzeige anschließbaren externen Prüfrechners. Dabei wird der gemessene Helligkeitswert mit einem Tabellenwert verglichen, der in einem zur Anzeige gehörenden Speicher hinterlegt ist. Um bei der Wahl des Korrekturfaktors nicht nur auf einige diskrete Tabellenwerte beschränkt zu sein, kann der Korrekturfaktor auch einem Kennfeld, das die Leuchtcharakteristik des Leuchtmittels kennzeichnet, entnommen werden. Der einer Tabelle oder einem Kennfeld entnommene Korrekturfaktor wird dann dazu benutzt, die Ansteuerung des Leuchtmittels durch eine Strom- oder Spannungsänderung derart zu beeinflussen, daß der gemessene Helligkeitwert an den im Speicher als Sollwertvorgabe festgelegten Helligkeitswert angepaßt wird. Die auf diese Weise vorgenommene Einstellung der Ansteuerung des Leuchtmittels soll gemäß DE 196 02 891 A1 für die Gebrauchsdauer des Leuchtmittels erhalten bleiben.The invention is based on an arrangement according to DE 196 02 891 A1. An arrangement for adjusting the brightness of a current- or voltage-controlled lamp for illuminating a display is described there, in which, at the end of the display manufacturing process, a correction factor is determined depending on the brightness of the lamp, by means of which the control voltage or the control current of the lamp is set becomes. The correction factor is determined and set with the aid of a light measuring device, which measures the brightness emitted by the illuminant, and with the use of an external test computer which can be connected to the display control unit via a diagnostic interface. The measured brightness value is compared with a table value stored in a memory belonging to the display is deposited. In order not to be limited to just a few discrete table values when selecting the correction factor, the correction factor can also be taken from a map which characterizes the luminous characteristics of the illuminant. The correction factor taken from a table or a map is then used to influence the control of the illuminant by a change in current or voltage in such a way that the measured brightness value is adapted to the brightness value specified in the memory as the setpoint value. According to DE 196 02 891 A1, the setting of the control of the lamp made in this way should be retained for the service life of the lamp.

Werden als Leuchtmittel aus einem Halbleiterwerkstoff gefertigte Leuchtdioden verwendet, reicht eine einmalige Einstellung des Helligkeitswertes jedoch nicht aus und auch die Möglichkeit für eine nur gelegentliche Nachbesserung der im Herstellungsprozeß der Anzeige vorgenommenen Einstellung wird den Ansprüchen des Marktes nicht gerecht, denn Leuchtdioden büßen im Laufe der Zeit selbst dann an Helligkeit ein, wenn sie während der Gebrauchsdauer der Anzeige gleichbleibend mit einem konstanten Strom angesteuert werden.If light-emitting diodes made of a semiconductor material are used as illuminants, a one-off adjustment of the brightness value is not sufficient, however, and the possibility of only occasional reworking of the setting made in the manufacturing process of the display does not meet the demands of the market, because light-emitting diodes become penalized over time even if they are constantly controlled with a constant current during the period of use of the display.

Es ist nun die Aufgabe der vorliegenden Erfindung, eine gattungsgemäße Schaltungsanordnung zur Einstellung der Helligkeit stromgesteuerter Leuchtdioden zur Beleuchtung einer Anzeige derart weiter zu entwickeln, daß eine gleichbleibende Helligkeit der Leuchtdioden während einer angemessenen Gebrauchsdauer der Anzeige sichergestellt wird, und zwar ohne Zuhilfenahme von externen Meßmitteln oder Prüfmitteln.It is the object of the present invention to further develop a generic circuit arrangement for adjusting the brightness of current-controlled light-emitting diodes for illuminating a display in such a way that a constant brightness of the light-emitting diodes is ensured during an appropriate period of use of the display, without the aid of external measuring means or Test equipment.

Die Aufgabe wird durch die Merkmale des ersten Anspruchs gelöst. Die abhängigen Ansprüche zeigen vorteilhafte Weiterbildungen der gefundenen Lösung.The object is achieved by the features of the first claim. The dependent claims show advantageous developments of the solution found.

Die Lösung ist dadurch gekennzeichnet,

  • a) daß die Schaltungsanordnung eine Steuereinheit enthält, die über eine Eingabeschnittstelle ein Signal aufnimmt, das ein Maß für die gewünschte Helligkeit der Leuchtdioden ist,
  • b) daß die Steuereinheit den Steuerstrom der Leuchtdioden unter Berücksichtigung eines Degradationswertes derart einstellt, daß die Leuchtdioden trotz ihres Degradationsverhaltens die gewünschte Helligkeit während einer angemessenen Gebrauchsdauer der Anzeige in gleichbleibender Intensität erzeugen, wobei die Steuereinheit den Degradationswert dadurch ermittelt,
  • c) daß die Steuereinheit zumindest die Betriebsdauer der Leuchtdioden registriert und eine der Betriebsdauer entsprechende Angabe in einem mit der Steuereinheit verbundenen Speicher hinterlegt, in dem auch Daten über das Degradationsverhalten der Leuchtdioden gespeichert sind,
  • d) und daß die Steuereinheit den Degradationswert zur Einstellung des Steuerstromes für die Leuchtdioden in Abhängigkeit von deren registrierter Betriebsdauer nach Maßgabe der im Speicher hinterlegten Daten für das Degradationsverhalten ermittelt und in sinnvollen Zeitabständen aktualisiert.
The solution is characterized by
  • a) that the circuit arrangement contains a control unit which receives a signal via an input interface which is a measure of the desired brightness of the light-emitting diodes,
  • b) that the control unit adjusts the control current of the light-emitting diodes taking into account a degradation value in such a way that the light-emitting diodes, despite their degradation behavior, produce the desired brightness in a constant intensity during a reasonable period of use of the display, the control unit thereby determining the degradation value,
  • c) that the control unit registers at least the operating time of the light-emitting diodes and stores an indication corresponding to the operating time in a memory connected to the control unit, in which data on the degradation behavior of the light-emitting diodes are also stored,
  • d) and that the control unit determines the degradation value for setting the control current for the light-emitting diodes as a function of their registered operating time in accordance with the data stored in the memory for the degradation behavior and updates them at reasonable time intervals.

Als Weiterbildung der gefundenen Lösung können von der Steuereinheit auch Angaben über die elektrische Leistung und/oder die Betriebstemperatur der Leuchtdioden erfaßt und registriert werden, so daß die Ermittlung des Degradationswertes zur Einstellung des Steuerstromes für die Leuchtdioden nach Maßgabe der im Speicher hinterlegten Daten für das Degradationsverhalten nicht nur in Abhängigkeit von der registrierten Betriebsdauer, sondern zusätzlich auch in Abhängigkeit von der elektrischen Leistung und/oder der Betriebstemperatur der Leuchtdioden erfolgen kann.As a further development of the solution found, the control unit can also record and register information about the electrical power and / or the operating temperature of the light-emitting diodes, so that the degradation value is determined in order to set the control current for the light-emitting diodes in accordance with the data for the degradation behavior stored in the memory not only as a function of the registered operating time, but also as a function of the electrical power and / or the operating temperature of the light-emitting diodes.

Auf die Einbeziehung der elektrischen Leistung in die von der Steuereinheit durchgeführte Ermittlung des Degradationswertes zur Einstellung des Steuerstromes für die Leuchtdioden sollte eigentlich nur dann verzichtet werden, wenn das von der Steuereinheit über die Eingabeschnittstelle aufgenommene Signal für die gewünschte Helligkeit der Leuchtdioden relativ konstant ist, also wenn während der Gebrauchsdauer der Anzeige eine Veränderung der einmal eingestellten Helligkeit der Leuchtdioden nicht zu erwarten oder gar nicht vorgesehen ist.The inclusion of the electrical power in the determination of the degradation value carried out by the control unit for setting the control current for the light-emitting diodes should actually only be dispensed with if this is done by the control unit via the input interface recorded signal for the desired brightness of the light emitting diodes is relatively constant, that is, if a change in the brightness of the light emitting diodes, once set, is not expected or is not intended at all during the period of use of the display.

Die Einbeziehung der Betriebstemperatur der Leuchtdioden ist deshalb eine durchaus sinnvolle Option, weil die Alterung, d.h. das Degradationsverhalten der Leuchtdioden in beachtlichem Maße temperaturabhängig ist und bei vielen, insbesondere den automotiven Anwendungen für die vorgeschlagene Schaltungsanordnung mit einem Betrieb in einem weiten Temperaturbereich bei häufigen Temperaturschwankungen gerechnet werden muß.The inclusion of the operating temperature of the light-emitting diodes is therefore a very sensible option because aging, i.e. the degradation behavior of the light-emitting diodes is temperature-dependent to a considerable degree and in many, in particular the automotive applications for the proposed circuit arrangement, operation in a wide temperature range with frequent temperature fluctuations must be expected.

Ferner kann man die gefundene Lösung derart ausgestalten, daß die das Degradationsverhalten der Leuchtdioden wiedergebenden Daten nicht nur in Form einzelner diskreter und damit relativ weniger punktueller Werte im Speicher hinterlegt werden, sondern in Form einer einem Datenblatt entnommenen Kurve oder durch einen formelmäßigen Zusammenhang bzw. durch Parameter für einen bestimmten, hier aber nicht näher ausgeführten, das Degradationsverhalten beschreibenden Algorithmus, wodurch der von der Steuereinheit zu ermittelnde Degradationswert in noch feinerer Abstimmung das tatsächliche Degradationsverhalten der Leuchtdioden wiedergeben kann Auf diese Weise kann eine eventuell mit der vorgeschlagenen Nachführung des Steuerstromes einhergehende Unterkompensation oder Überkompensation des Degradationsverhaltens für die Leuchtdioden noch besser vermieden werden.Furthermore, the solution found can be designed in such a way that the data representing the degradation behavior of the light-emitting diodes is not only stored in the memory in the form of individual discrete and therefore relatively less punctual values, but also in the form of a curve taken from a data sheet or by means of a formulaic connection or by Parameters for a specific, but not detailed, algorithm describing the degradation behavior, whereby the degradation value to be determined by the control unit can reproduce the actual degradation behavior of the light-emitting diodes in even finer coordination.This way, an under-compensation or possibly associated with the proposed tracking of the control current can be achieved Overcompensation of the degradation behavior for the light-emitting diodes can be avoided even better.

Der wesentliche Vorteil der gefundenen Lösung besteht darin, daß eine gleichbleibende Helligkeit der Leuchtdioden während einer angemessenen Gebrauchsdauer der Anzeige sichergestellt werden kann, ohne daß dafür externe Meßmittel oder Prüfmittel benötigt werden. In einer offenen Steuerkette wird der Steuerstrom der Leuchtdioden derart anhand eines Degradationswertes nachgeführt, daß die Helligkeit der Leuchtdioden für einen Anwender der Anzeige trotz ihres physikalisch bedingten und damit unvermeidbaren Degradationsverhaltens konstant bleibt. Dabei wird der Degradationswert von der Steuereinheit in sinnvollen Zeitabständen anhand von in einem Speicher hinterlegten Daten, die das Degradationsverhalten kennzeichnen, aktualisiert. Die Aktualisierung erfolgt dadurch, daß die Steuereinheit als Betriebsparameter der Leuchtdioden deren Betriebsdauer und vorzugsweise auch die elektrische Leistung registriert, mit der die Leuchtdioden bereits betrieben wurden, und dann anhand der in dem Speicher hinterlegten Daten, die das Degradationsverhalten kennzeichnen, ermittelt, welcher Degradationswert sich typischerweise aufgrund der registrierten Betriebsparameter aktuell ergibt. Diese Aktualisierung des Degradationswertes wird man in der Preis nur in sinnvollen Zeitabständen durchführen, um die Steuereinheit nicht über Gebühr mit Berechnungen zu belasten. Bis zur nächstfolgenden Aktualisierung speichert die Steuereinheit den ermittelten Degradationswert vorzugsweise in einem der Steuereinheit zugeordneten Speicher und führt alle bis zur nächsten Aktualisierung gewünschten Helligkeitseinstellungen für die Leuchtdioden anhand dieses aktuell geltenden Degradationswertes durch, indem die Steuereinheit den Steuerstrom für die Leuchtdioden unter Berücksichtigung dieses Degradationswertes einstellt. Da der Degradationswert angibt, in welchem Maße die Helligkeit der Leuchtdioden im Vergleich zu ihrer Anfangshelligkeit bereits nachgelassen hat, wird der Steuerstrom für die Leuchtdioden umgekehrt proportional zum Degradationswert nachgeführt. Voraussetzung für eine erfolgreiche Anwendung der vorgeschlagenen Schaltungsanordnung ist natürlich, daß der Steuerstrom anfänglich derart eingestellt wird, daß eine Nachführung seines Betrages möglich ist, ohne die Leuchtdioden zu übersteuern. Der Arbeitsbereich für den Steuerstrom der Leuchtdioden muß also so gewählt werden, daß der Grenzbereich der Leuchtdioden durch die Nachführung des Steuerstromes während der geplanten Gebrauchsdauer der Anzeige möglichst nicht erreicht wird. Im übrigen ist es auch von Vorteil, daß die vorgeschlagene Schaltungsanordnung einen die anfänglich starke Degradation abfangenden Burn-in für die Leuchtdioden entbehrlich macht.The main advantage of the solution found is that a constant brightness of the light-emitting diodes can be ensured for a reasonable period of use of the display without the need for external measuring or testing equipment. In an open control chain, the control current of the light-emitting diodes is determined on the basis of a Degradation value tracked that the brightness of the light emitting diodes remains constant for a user of the display in spite of their physically determined and thus unavoidable degradation behavior. The control unit updates the degradation value at reasonable time intervals on the basis of data stored in a memory, which characterize the degradation behavior. The update takes place in that the control unit registers the operating time and preferably also the electrical power with which the light-emitting diodes have already been operated as the operating parameters of the light-emitting diodes, and then uses the data stored in the memory, which characterize the degradation behavior, to determine what the degradation value is typically results from the registered operating parameters. This update of the degradation value will only be carried out in the price at reasonable intervals, in order not to unduly burden the control unit with calculations. Until the next update, the control unit preferably stores the determined degradation value in a memory assigned to the control unit and carries out all the brightness settings for the light-emitting diodes desired until the next update, based on this currently applicable degradation value, by the control unit setting the control current for the light-emitting diodes taking this degradation value into account. Since the degradation value indicates to what extent the brightness of the light-emitting diodes has already decreased compared to their initial brightness, the control current for the light-emitting diodes is tracked in inverse proportion to the degradation value. A prerequisite for a successful application of the proposed circuit arrangement is, of course, that the control current is initially set in such a way that its amount can be tracked without overdriving the light-emitting diodes. The working range for the control current of the light-emitting diodes must therefore be selected such that the limit range of the light-emitting diodes is not reached as far as possible by tracking the control current during the planned period of use of the display. In addition, it is also advantageous that the proposed Circuit arrangement makes a burn-in, which intercepts the initially strong degradation, unnecessary for the light-emitting diodes.

Die Erfassung und Registrierung der Angaben über die für die Ermittlung des jeweils aktuellen Degradationswertes benötigten Betriebsparameter, d.h. die Angaben über die Betriebsdauer, die elektrische Leistung und/oder der Betriebstemperatur, kann mit Mitteln erfolgen, die in einem mit einer gattungsgemäßen Anzeige ausgestatteten Gerät bereits zur Verfügung stehen, so daß die Erfassung und Registrierung der erforderlichen Angaben praktisch kostenneutral erfolgt. Auch dieses ist für die Realisierung der gefundenen Lösung ein erheblicher Vorteil. In der Preis werden zur Erfassung der Angaben über die Betriebsdauer und die elektrische Leistung der Leuchtdioden deshalb in der Regel keine eigenständig arbeitenden sensorischen Meßmittel benötigt, weil die Steuereinheit durch eine geeignete Programmierung sowohl die Dauer des Stromflusses zu den Leuchtdioden als auch eine Angabe über den Betrag des Steuerstromes registrieren kann. Weil in den meisten Anwendungsfällen die Betriebsspannung der Leuchtdioden weitgehend konstant ist, kann die Steuereinheit auf einfache Weise die elektrische Leistung der Leuchtdioden errechnen, sobald sie auf eine Angabe zum Betrag des Steuerstromes zurückgreifen kann. Da der Steuerstrom in vielen Fällen direkt von der Steuereinheit bereitgestellt wird, braucht die Steuereinheit lediglich durch Speicherung einer entsprechenden Angabe in dem ihr zugeordneten Speicher zu registrieren, wie hoch der von ihr in einer bestimmten Zeiteinheit zur Verfügung gestellte Steuerstrom war. Sobald die Steuereinheit das Verhalten des von ihr an ihrem Ausgang zur Verfügung gestellten Steuerstromes protokolliert, stehen wesentliche Angaben zu den Betriebsparametern zur Verfügung, um mit ihnen anhand der im Speicher hinterlegten Daten zum Degradationsverhalten der Leuchtdioden den aktuellen Degradationswert zu ermitteln. Als wirklich zusätzliches Meßmittel zu den bereits vorhandenen Komponenten für die vorgeschlagene Schaltungsanordnung wird lediglich ein in der Nähe der Leuchtdioden angeordneter Temperaturfühler benötigt, um eine Angabe zu der Betriebstemperatur der Leuchtdioden zu gewinnen, wobei dann diese Angabe der Steuereinheit zur weiteren Verarbeitung zuzuführen ist.The acquisition and registration of the information about the operating parameters required for the determination of the current degradation value, ie the information about the operating time, the electrical power and / or the operating temperature, can be carried out using means that are already provided in a device equipped with a generic display Are available so that the collection and registration of the required information is practically cost-neutral. This is also a considerable advantage for the implementation of the solution found. In order to record the information on the operating time and the electrical power of the light-emitting diodes, the price therefore generally does not require any sensor sensors that work independently, because the control unit, through suitable programming, both the duration of the current flow to the light-emitting diodes and an indication of the amount of the control current can register. Because in most applications the operating voltage of the light emitting diodes is largely constant, the control unit can easily calculate the electrical power of the light emitting diodes as soon as it can refer to the amount of the control current. Since the control current is in many cases provided directly by the control unit, the control unit only needs to register in the memory associated with it how high the control current made available by it in a specific time unit was by storing a corresponding indication. As soon as the control unit logs the behavior of the control current it provides at its output, essential information on the operating parameters is available in order to determine the current degradation value with the data on the degradation behavior of the light-emitting diodes stored in the memory. As a really additional measuring device for the already existing components for the proposed circuit arrangement, only one in the vicinity of the light-emitting diodes is used arranged temperature sensor is required to obtain an indication of the operating temperature of the light-emitting diodes, this indication then being fed to the control unit for further processing.

Die in dem mit der Steuereinheit verbundenen Speicher zu hinterlegenden Daten für die Einstellung des Steuerstromes der Leuchtdioden können beim Herstellungsprozeß der mit den Leuchtdioden zu beleuchtenden Anzeige gespeichert werden. Dafür braucht man nur die Angaben des Herstellers der Leuchtdioden in Form geeigneter Daten in den Speicher der Schaltungsanordnung zu übertragen. Diese Daten lassen sich sehr einfach durch eine entsprechende Programmwahl auf die genaue Spezifikation der verwendeten Leuchtdioden abstimmen, so daß für jede Anzeige stets optimale Daten zur Anwendung gebracht werden. Dadurch wird auch der Herstellungsprozeß für eine gattungsgemäße Anzeige vereinfacht, weil eine von der bauteilemäßigen Spezifikation abhängige Selektion der im Herstellungsprozeß der Anzeige verwendeten Leuchtdioden entfallen kann.The data to be stored in the memory connected to the control unit for setting the control current of the light-emitting diodes can be stored during the manufacturing process of the display to be illuminated with the light-emitting diodes. All that is required is to transfer the information from the manufacturer of the light-emitting diodes into the memory of the circuit arrangement in the form of suitable data. This data can be very easily matched to the exact specification of the light-emitting diodes by an appropriate program selection, so that optimal data is always used for each display. This also simplifies the manufacturing process for a generic display because a selection of the light-emitting diodes used in the manufacturing process of the display, which is dependent on the component specification, can be omitted.

Die fortlaufende Nachführung des Steuerstromes der Leuchtdioden bewirkt nun eine gleichbleibende Helligkeit der Beleuchtung der Anzeige, was ein bedeutsames Qualitätsmerkmal der gesamten Anzeige ist. Auch kann durch diese Maßnahme erreicht werden, daß ein einmal eingestelltes harmonisches Abstimmungverhältnis der Helligkeit aller in einem Fahrzeug angeordneten Anzeigen für eine angemessene Gebrauchsdauer, die in der Regel mehrere Jahre beträgt, konstant bleibt. Disharmonien infolge eines unterschiedlichen Degradationsverhaltens in der Helligkeit der Leuchtdioden werden vermieden. Disharmonien könnten auch zwischen Anzeigen unterschiedlicher, einem Anwender aber gleichzeitig präsentierter Geräte entstehen, wenn eine Anzeige durch Leuchtdioden und eine andere durch ein Leuchtmittel beleuchtet ist, das nicht oder nicht in dem Maße an Helligkeit verliert wie Leuchtdioden.The continuous tracking of the control current of the light-emitting diodes now brings about a constant brightness of the illumination of the display, which is a significant quality feature of the entire display. It can also be achieved by this measure that a harmonious coordination ratio of the brightness of all the displays arranged in a vehicle, once set, remains constant for an appropriate period of use, which is generally several years. Disharmonies due to a different degradation behavior in the brightness of the LEDs are avoided. Disharmonies could also arise between displays of different devices, but presented to a user at the same time, if one display is illuminated by light-emitting diodes and another is illuminated by a light source that does not or does not lose as much brightness as light-emitting diodes.

Anhand eines vereinfachten Blockschaltbildes soll die vorgeschlagene Lösung nochmals kurz erläutert werden. Die Figur zeigt eine Anzeige 1 mit Leuchtdioden, deren Steuerstrom durch eine Steuereinheit 2 eingestellt wird. Im vorliegenden Fall soll die Ansteuerung durch ein Pulsweiten moduliertes Signal (PWM) erfolgen. Alternativ könnte die Ansteuerung natürlich auch analog mit einem kontinuierlich verlaufenden Strom erfolgen.The proposed solution will be briefly explained again using a simplified block diagram. The figure shows a display 1 with light-emitting diodes, the control current of which is set by a control unit 2. In the present case, the control is to be carried out by a pulse width modulated signal (PWM). Alternatively, the control could of course also be carried out analogously with a continuously running current.

Die Steuereinheit 2 nimmt über eine Eingabeschnittstelle 6 ein Signal auf, das ein Maß für die gewünschte Helligkeit der Leuchtdioden ist. Das über die Eingabeschnittstelle 6 aufgenommene Signal kann von einem Dimmer abgegriffen sein, mit dem die gewünschte Helligkeit für die Leuchtdioden vom Anwender der Anzeige 1 eingestellt wird. Es kann sich bei diesem Signal aber auch ohne weiteres um einen Vorgabewert handeln, der z.B. über einen CAN-Datenbus von einem anderen Steuergerät an das die Anzeige 1 enthaltende Gerät übertragen wird. Um auf diese Vorgabe für die Helligkeit im Sinne der Zielsetzung für diese Erfindung angemessen zu reagieren, stellt die Steuereinheit 2 den Steuerstrom unter Berücksichtigung eines Degradationswertes ein.The control unit 2 receives, via an input interface 6, a signal which is a measure of the desired brightness of the light-emitting diodes. The signal picked up via the input interface 6 can be tapped by a dimmer with which the desired brightness for the light-emitting diodes is set by the user of the display 1. However, this signal can also be a default value, which e.g. is transmitted from another control device to the device containing display 1 via a CAN data bus. In order to react appropriately to this specification for the brightness in the sense of the objective for this invention, the control unit 2 sets the control current taking into account a degradation value.

Zur Ermittlung des Degradationswertes erfaßt und registriert die Steuereinheit 2 die Betriebsdauer der Leuchtdioden sowie vorzugsweise auch deren elektrische Leistung und - wenn erforderlich - deren Betriebstemperatur. Sofern Angaben zu den beiden Betriebsparameter Betriebsdauer und elektrische Leistung nicht unmittelbar von der Steuereinheit 2 aufgezeichnet werden, kann als Mittel für die Erfassung der Betriebsdauer ein Betriebsstundenzähler 3 verwendet werden, der die Stromflußdauer feststellt, und die elektrische Leistung kann ebenfalls mit einer geeigneten Meßeinrichtung 4 erfaßt werden. Ferner kann der Steuereinheit 2 das Signal eines in der Nähe der Leuchtdioden angeordneten Temperaturfühlers 5 zugeführt werden. Die Angaben über die erfaßten Betriebsparameter werden der Steuereinheit zugeführt und dort fortlaufend kumuliert und aktualisiert und in einem der Steuereinheit 2 zugeordneten Speicher 7 abgelegt. Die Steuereinheit 2 prüft nun anhand der aktuellen Angaben über mindestens einen der erfaßten Betriebsparameter, welcher Degradationswert ihnen aufgrund der im Speicher 7 hinterlegten Daten zum Degradationsverhalten der Leuchtdioden zugeordnet ist und stellt diesen Degradationswert dann für eine entsprechende Einstellung des Steuerstromes zur Verfügung. Ob die Steuereinheit den Steuerstrom direkt an ihrem Ausgang oder über geeignete Treiber bereitstellt, ist für die hier vorliegende Erfindung nicht erheblich. In der Praxis wird man den Degradationswert nicht ständig, sondern nur in sinnvollen zeitlichen Abständen aktualisieren, so z.B. in Abhängigkeit von der Beanspruchung der Leuchtdioden eventuell täglich oder in Abhängigkeit von Helligkeitsminderungen, die in der jeweiligen Betriebsumgebung der Anzeige vom menschlichen Auge überhaupt als unterschiedliche Helligkeiten wahrgenommen werden können.To determine the degradation value, the control unit 2 detects and registers the operating time of the light-emitting diodes and preferably also their electrical power and - if necessary - their operating temperature. If information on the two operating parameters operating time and electrical power is not recorded directly by the control unit 2, an operating hours counter 3 can be used as a means for recording the operating time, which determines the current flow duration, and the electrical power can also be recorded using a suitable measuring device 4 will. Furthermore, the control unit 2 can be supplied with the signal of a temperature sensor 5 arranged in the vicinity of the light-emitting diodes. The information about the detected operating parameters is fed to the control unit, where it is continuously accumulated and updated and in one of the control unit 2 assigned memory 7 filed. The control unit 2 now uses the current information about at least one of the detected operating parameters to check which degradation value is assigned to them on the basis of the data stored in the memory 7 about the degradation behavior of the light-emitting diodes, and then makes this degradation value available for a corresponding setting of the control current. Whether the control unit provides the control current directly at its output or via suitable drivers is not significant for the present invention. In practice, the degradation value will not be updated continuously, but only at reasonable time intervals, e.g. depending on the load on the LEDs, or possibly daily, or depending on brightness reductions that the human eye perceives as different brightnesses in the respective operating environment of the display can be.

Falls die Angaben für die Betriebsdauer aufzeigen, daß die Leuchtdioden beispielsweise bereits 1000 Stunden betrieben worden sind, schaut die Steuereinheit 2 in dem Speicher 7 nach, welcher Degradationswert gemäß den Angaben des Herstellers der Leuchtdioden dieser Betriebsdauer zugeordnet ist. Um die zwischenzeitlich erfolgte Helligkeitsabnahme auszugleichen, muß der Betrag des einzustellenden Steuerstromes mit dem Kehrwert des Degradationswertes multipliziert werden. Wird beispielsweise festgestellt, daß die Helligkeit der Leuchtdioden aufgrund der Betriebsdauer typischerweise bereits auf das 0,7-fache der Anfangshelligkeit gesunken ist, erhöht die Steuereinheit 2 nun an ihrem Ausgang das Tastverhältnis des PWM-Signals auf den Kehrwert des ermittelten Degradationswertes, was in diesem Beispiel 1/0,7 = 1,43 wäre.If the information for the operating time shows that the light-emitting diodes have, for example, already been operated for 1000 hours, the control unit 2 looks in the memory 7 to see which degradation value is assigned to this operating time in accordance with the information from the manufacturer of the light-emitting diodes. To compensate for the decrease in brightness that has occurred in the meantime, the amount of the control current to be set must be multiplied by the reciprocal of the degradation value. If, for example, it is found that the brightness of the light-emitting diodes has typically already dropped to 0.7 times the initial brightness due to the operating time, the control unit 2 now increases at its output the pulse duty factor of the PWM signal to the reciprocal of the determined degradation value, which in this Example 1 / 0.7 = 1.43 would be.

Der Speicher 7, in dem die Daten für die Betriebsparameter Betriebsdauer, elektrische Leistung oder Betriebstemperatur zur Ermittlung des Degradationswertes gesammelt werden, ist als nichtflüchtiger Speicher (EEPROM oder Flash) auszubilden. Es ist zweckmäßig, in diesem Speicher 7 auch die Daten für das Degradationsverhalten der Leuchtdioden zu hinterlegen, die durch Tabellenwerte, Datenblattkurven oder Parameter für von der Steuereinheit 2 auszuführende Algorithmen gegeben sein können. Welche Daten in welcher Form vorteilhaft zu hinterlegen sind, wird fallweise abhängig von der Anwendung und der zur Verfügung stehenden Kapazität des Speichers 7 zu entscheiden sein.The memory 7, in which the data for the operating parameters operating time, electrical power or operating temperature for collecting the degradation value is collected, is to be designed as a non-volatile memory (EEPROM or flash). It is expedient in this memory 7 also to store the data for the degradation behavior of the light-emitting diodes, which can be given by table values, data sheet curves or parameters for algorithms to be executed by the control unit 2. Which data are to be stored advantageously in which form will have to be decided on a case-by-case basis depending on the application and the available capacity of the memory 7.

Claims (3)

Schaltungsanordnung zur Einstellung der Helligkeit stromgesteuerter Leuchtdioden zur Beleuchtung einer Anzeige (1),
dadurch gekennzeichnet, a) daß die Schaltungsanordnung eine Steuereinheit (2) enthält, die über eine Eingabeschnittstelle (6) ein Signal aufnimmt, das ein Maß für die gewünschte Helligkeit der Leuchtdioden ist, b) daß die Steuereinheit (2) den Steuerstrom der Leuchtdioden unter Berücksichtigung eines Degradationswertes derart einstellt, daß die Leuchtdioden trotz ihres Degradationsverhaltens die gewünschte Helligkeit während einer angemessenen Gebrauchsdauer der Anzeige (1) in gleichbleibender Intensität erzeugen, wobei die Steuereinheit (2) den Degradationswert dadurch ermittelt, c) daß die Steuereinheit (2) zumindest die Betriebsdauer der Leuchtdioden registriert und eine der Betriebsdauer entsprechende Angabe in einem mit der Steuereinheit (2) verbundenen Speicher (7) hinterlegt, in dem auch Daten über das Degradationsverhalten der Leuchtdioden gespeichert sind, d) und daß die Steuereinheit (2) den Degradationswert zur Einstellung des Steuerstromes für die Leuchtdioden in Abhängigkeit von deren registrierter Betriebsdauer nach Maßgabe der im Speicher (7) hinterlegten Daten für das Degradationsverhalten ermittelt und in sinnvollen Zeitabständen aktualisiert. Circuit arrangement for setting the brightness of current-controlled light-emitting diodes for illuminating a display (1),
characterized, a) that the circuit arrangement contains a control unit (2) which receives a signal via an input interface (6) which is a measure of the desired brightness of the light-emitting diodes, b) that the control unit (2) sets the control current of the light-emitting diodes taking into account a degradation value in such a way that the light-emitting diodes produce the desired brightness in a constant intensity during an appropriate period of use of the display (1), despite their degradation behavior, the control unit (2) determining the degradation value determined by c) that the control unit (2) registers at least the operating time of the light-emitting diodes and stores an indication corresponding to the operating time in a memory (7) connected to the control unit (2), in which data on the degradation behavior of the light-emitting diodes are also stored, d) and that the control unit (2) determines the degradation value for setting the control current for the light-emitting diodes as a function of their registered operating time in accordance with the data stored in the memory (7) for the degradation behavior and updates them at reasonable time intervals. Schaltungsanordnung nach Anspruch 2, dadurch gekennzeichnet, daß von der Steuereinheit (2) auch Angaben über die elektrische Leistung und/oder die Betriebstemperatur der Leuchtdioden erfaßt und registriert werden, so daß die Ermittlung des Degradationswertes nach Maßgabe der im Speicher (7) hinterlegten Daten für das Degradationsverhalten zusätzlich zur registrierten Betriebsdauer auch in Abhängigkeit von der elektrischen Leistung und/oder der Betriebstemperatur der Leuchtdioden erfolgen kann.Circuit arrangement according to Claim 2, characterized in that the control unit (2) also records and records information about the electrical power and / or the operating temperature of the light-emitting diodes, so that the degradation value is determined in accordance with the data stored in the memory (7) for the degradation behavior in addition to the registered operating time can also take place as a function of the electrical power and / or the operating temperature of the light-emitting diodes. Schaltungsanordnung nach einem der vorangegangenen Ansprüche, dadurch gekennzeichnet, daß die Daten für das Degradationsverhalten der Leuchtdioden anhand von Datenblattkurven oder durch einen formelmäßigen Zusammenhang im Speicher (7) hinterlegt werden.Circuit arrangement according to one of the preceding claims, characterized in that the data for the degradation behavior of the light-emitting diodes are stored in the memory (7) on the basis of data sheet curves or by means of a formula.
EP99107236A 1998-04-25 1999-04-14 Circuit for controlling the light intensity of current controlled LEDS for display illumination Expired - Lifetime EP0952757B1 (en)

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EP0952757B1 (en) 2004-08-04
EP0952757A3 (en) 2001-03-07
DE19818621A1 (en) 1999-10-28

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