EP1075706A1 - Maintien de l'intensite lumineuse de diodes electroluminescentes (led) - Google Patents

Maintien de l'intensite lumineuse de diodes electroluminescentes (led)

Info

Publication number
EP1075706A1
EP1075706A1 EP98918770A EP98918770A EP1075706A1 EP 1075706 A1 EP1075706 A1 EP 1075706A1 EP 98918770 A EP98918770 A EP 98918770A EP 98918770 A EP98918770 A EP 98918770A EP 1075706 A1 EP1075706 A1 EP 1075706A1
Authority
EP
European Patent Office
Prior art keywords
led
pulses
adjusting
set forth
luminous output
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.)
Withdrawn
Application number
EP98918770A
Other languages
German (de)
English (en)
Other versions
EP1075706A4 (fr
Inventor
Peter A. Hochstein
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.)
Individual
Original Assignee
Individual
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 Individual filed Critical Individual
Priority claimed from PCT/US1998/008432 external-priority patent/WO1999056303A1/fr
Publication of EP1075706A1 publication Critical patent/EP1075706A1/fr
Publication of EP1075706A4 publication Critical patent/EP1075706A4/fr
Withdrawn legal-status Critical Current

Links

Classifications

    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B20/00—Energy efficient lighting technologies, e.g. halogen lamps or gas discharge lamps

Definitions

  • the subject invention relates to light emitting diodes (LEDs) .
  • LED light emitting diode
  • LEDs also exhibit a non- permanent or recoverable diminution of luminous output with increasing temperature. Typically, a loss of approximately one percent (1 %) of intensity with every one degree Centigrade (1°C) increase in temperature is observed in certain commercially available LEDs.
  • any optical sensor used to monitor the powered LED must be shielded from ambient light, which would be added to LED luminous output, and "confuse" the feedback control system.
  • ambient light which would be added to LED luminous output, and "confuse" the feedback control system.
  • the influence of extraneous, ambient light is of particular concern in outdoor applications where the light level might change substantially over time.
  • the sensor could be closely coupled to one LED in an array of LEDs and be fully shielded from extraneous light, but isolating one emitter (LED) in a closely packed array is difficult if all operating variables (such as temperature) are to be identical.
  • any linear current control system is intrinsically dissipative and inefficient.
  • the linear or regulating control element in such circuits necessarily acts as a resistive element to reduce current flow to the LED(S) when less light output is required.
  • the linear control element effectively reduces its resistance to current.
  • Such current control elements are transistors of various types, which must dissipate the controlled current multiplied by the voltage drop across the control element as heat. That is, power not utilized by the LED(S) is dissipated as heat when less current through the LED is indicated,
  • a circuit for maintaining the luminous intensity of a light emitting diode including at least one light emitting diode (LED)for producing an luminous intensity.
  • a power supply electrically connected to the LED for supplying pulses of electrical energy to the LED.
  • the power supply includes a switching device responsive to the luminous intensity signal for adjusting the electrical energy supplied by the pulses per unit of time to adjust the average of the current passing through the LED to maintain the luminous intensity of the LED at a predetermined level.
  • the invention also includes a method of maintaining the luminous intensity of a light emitting diode (LED) comprising the steps of supplying pulses of electrical energy from an adjustable power supply to an LED for establishing electrical current passing through the LED; sensing a condition proportional to the luminous intensity of the LED; and adjusting the electrical energy supplied by the pulses per unit of time to adjust the average of the current passing through the LED to maintain the luminous intensity of the LED at a predetermined level.
  • a light emitting diode comprising the steps of supplying pulses of electrical energy from an adjustable power supply to an LED for establishing electrical current passing through the LED; sensing a condition proportional to the luminous intensity of the LED; and adjusting the electrical energy supplied by the pulses per unit of time to adjust the average of the current passing through the LED to maintain the luminous intensity of the LED at a predetermined level.
  • the present invention will compensate for the diminution of light output from LED signals due to temperature, either as operating temperature varies and/or to compensate for diminution of light output due to permanent temperature induced degradation, i.e., aging.
  • the subject invention increases the average current through the LED to compensate for a loss of luminous output, and vice versa.
  • Figure 1 is a schematic view of a first embodiment
  • Figure 2 is a schematic view of a second embodiment
  • Figure 3 is a graph showing variation in the width of the electrical pulses
  • Figure 4 is a graph showing variation in the frequency of the electrical pulses
  • Figure 5 is a schematic view of a third embodiment
  • Figure 6 is a graph showing variation in the sinusodial wave form of the third embodiment.
  • FIG. 1 a first embodiment of a circuit for maintaining the luminous intensity of a light emitting diode is shown schematically in Figure 1
  • FIG. 2 a second embodiment is shown in Figure 2
  • Figure 5 a third embodiment is shown in Figure 5.
  • each embodiment there is included an array of light emitting diodes 12, each of which is hereinafter referred to as an LED.
  • the LEDs are mounted on a circuit board 14 as is well known in the art.
  • the invention includes to at least one LED but normally comprises a plurality of LEDs electrically connected in series and/or parallel on a circuit board 14.
  • An adjustable power supply 16 is electrically connected via a lead 18 to the LED array for adjusting the average current passing through the LEDs
  • the power supply 16 is connected via a lead 20 to a source of electrical power, d.c. power in the embodiments of Figures 1 and 2, and a.c. power in embodiment of Figure 3.
  • the adjustable power supply 16 may adjust voltage or current, but in either case it is the average current passing through the LEDs that controls the luminous output of the LEDs.
  • Such power supplies include a means for switching and may be adjustable in response to a signal from a sensor. Even in cases where a pulse width modulated power supply 16 is employed, changing the pulse width or the pulse rate (frequency) as a function of operating temperature will change the average current through the LED array, and thus the average luminous output.
  • the power supply 16 includes a switching device responsive to the luminous intensity signal for adjusting the electrical energy supplied by the pulses per unit of time to adjust the average of the current passing through the LED 12 to maintain the luminous intensity of the LED 12 at a predetermined level.
  • Both embodiments include a sensor 22 or 24 electrically connected via a lead 26 to the power supply 16 for sensing a condition proportional to the luminous intensity of the LEDs and for sending a signal to the power supply 16 to increase the average current passing through the LEDs to maintain the luminous intensity of the LEDs at a predetermined level.
  • the sensor 22 includes means for sensing changes in luminous output of the LED array.
  • the sensor 22 also includes means 28 for differentiating ambient light from the luminous output of the LED array for measuring the actual luminous output of the LED without the influence of ambient light.
  • the light sensing modulator 22 includes a light sensing transducer which is coupled to one or more of the LEDs in the array to measure the actual light output of the LED array under all operating conditions. The sensitivity of the light detector 22 to ambient light is minimized by shielding or close coupling of the sensor 22 to the LEDs. More specifically, a collimator or tube 28 could be used to block out ambient light so that the light sensor 22 only sees the luminous output of the LEDs.
  • synchronous detection could be employed to differentiate between ambient light and the LED output plus ambient light.
  • the differential signal may then be employed to modulate the LED array average current to keep the output luminous intensity essentially constant.
  • Such closed loop control with the proper feedback time constants, will assure an essentially constant luminous output irrespective of operating temperature.
  • the senor 24 includes means for sensing changes in temperature of the LEDs.
  • a temperature sensitive element such as a thermistor, a thermocouple, a temperature sensing semiconductor, or the like, is used to program the voltage or current output of the power supply 16 to provide more average current passing through the LEDs in response to temperature rise.
  • the transfer function or gain or rate at which the average operating current passing through the LEDs is increased as a function of temperature is based upon a predetermined LED behavior model.
  • This model establishes the necessary increase in the average operating current through the LEDs as a function of operating temperature of the LEDs in order to keep the luminous output of the LED array essentially constant at a predetermined level.
  • the sensor 22 includes a predetermined temperature model to establish the increase in the current passing through the LED array as a function of the operating temperature of the LED array integrated with the predetermined temperature model. This model may be preprogrammed into a chip.
  • the switching device of the power supply 16 may include means for adjusting the electrical energy supplied by said pulses per unit of time by adjusting the width of said pulses as illustrated in Figure 3.
  • the switching device includes means for adjusting the electrical energy supplied by the pulses per unit of time by adjusting the frequency of the pulses as illustrated in Figure 4.
  • the switching device includes means for adjusting the electrical energy supplied by the pulses per unit of time by adjusting the phase of the pulses within an a.c. sinusodial wave form.
  • the invention includes a method of maintaining the luminous intensity of a light emitting diode (LED) comprising the steps of supplying pulses of electrical energy from an adjustable power supply 16 to an LED 12 for establishing electrical current passing through the LED 12; sensing 22,24 a condition proportional to the luminous intensity of the LED 12; and adjusting the electrical energy supplied by the pulses per unit of time to adjust the average of the current passing through the LED 12 to maintain the luminous intensity of the LED 12 at a predetermined level.
  • LED light emitting diode
  • the sensing of a condition is further defined as sensing changes in temperature of the LED. This step may be further perfected by establishing a predetermined temperature model and increasing -7-
  • the current passing through the LED as a function of the operating temperature of the LED integrated with the predetermined temperature model.
  • the sensing of a condition is further defined as sensing changes in luminous output of the LED.
  • This step may be further defined as differentiating ambient light from the luminous output of the LED for measuring the actual luminous output of the LED without the influence of ambient light.
  • the present invention relates to a new method of maintaining an essentially constant luminous output from an LED array, irrespective of operating temperature. Unlike the proposed method in the cited reference, using linear regulation of the LED current, the present invention uses pulse width modulation or frequency variation, or a combination thereof, of a power source to control the average current through the LED(s).
  • switch mode operation of power supplies is very efficient. It is also widely recognized that control of power supply output voltage or output current is most efficiently accomplished by varying the pulse width or frequency of the switched waveform. Normally, d.c. power supplies filter the switched output voltage to produce a constant, relatively ripple free output. LED arrays, particularly those used in outdoor environments such as message boards, traffic signals and automotive tail lights are subject to severe temperature excursions. As discussed, the higher temperatures diminish the luminous output of the LEDs if they are operated at constant current.
  • the primary purpose of the present invention is to increase the average current through the LED array with increasing temperature, by adjusting the pulse width or frequency of LED switch mode power supply.
  • switch mode supplies include any power source 16 that is turned on and off at a frequency consistent with the other operating parameters of the system. Typically, the switching frequency would extend from 60 Hz to over 50 KHz.
  • phase controlled a.c. power supplies as illustrated in Figures 5 and 6, is also explicitly included as a suitable power supply. While not generally considered switch mode in the narrowest sense, phase controlled supplies will provide very efficient, variable pulse width, variable average current to the LED array. In other words, for the purpose of this invention, phase controlled power supplies are considered to be a variant of switch mode supplies.
  • Two sensor means are contemplated by the present invention: Light sensing 22 the output of the LED array or a representative LED in that array, or temperature 24 sensing of the LED array. Either type of sensor can be used to modulate the average current through the LED array to maintain essentially constant luminous output, irrespective of operating temperature.
  • the basic feedback control system is configured to sense the light output from one or more LEDs, with a light sensitive transducer 22 such as a photodiode that will program or modulate the average output current of the switch mode power supply.
  • a filter circuit 28 for the light sensing transducer may be necessary to accommodate the pulsing light output from the LED array.
  • the pulsing current delivered by the power supply could be filtered to essentially d.c, making the transducer filter unnecessary.
  • the average current delivered to the LED is varied to compensate for a change in LED luminous output. This change in output may be due to permanent degradation and / or temperature induced diminution.
  • the light sensing transducer 22 will compensate for the aggregate light loss and maintain the luminous output essentially constant at a predetermined level. Accordingly, a filter is included for filtering the output of the power supply 16 for averaging the luminous intensity of the LED.
  • Fig. 2 shows a LED array, feedback control system that will maintain an LED array at a nominal constant luminous output by sensing the operating temperature of the array.
  • a temperature sensing transducer 24 such as a thermistor, semiconductor device or thermocouple is used to program or modulate the average current of a switch mode power supply that drives the
  • Temperature compensation of the LEDs is easier to implement than optical feedback because ambient light no longer presents any interference. Also, temperature changes are relatively slow so that operating the LEDs in pulsed mode will not require temperature sensing transducer filtering. Of course, long term degradation of LED luminous output cannot be compensated for by simple temperature compensation schemes.
  • I and 2 is shown as d.c, the switch mode supply could easily be operated on a.c. by using common rectifier means.
  • Fig. 3 shows the well known, constant or fixed frequency, variable pulse width modulation of average drive current.
  • the LEDs deliver more lumens per average current (mA), so that a lower average current or pulse width is necessary to maintain a prescribed light output.
  • the pulse width of the switch mode power supply is increased, thereby increasing the average current, thus maintaining the prescribed light output.
  • Fig. 4 shows the adjustment of average current using a fixed pulse width, variable frequency modulation scheme. Functionally, the result of either form of switch mode modulation is the same, in that the average current to the LED array is varied according to a sensed parameter, i.e., either light or temperature.
  • Temperature or luminous output of the LEDs varied or modulated in response to a measured process parameter: Temperature or luminous output of the LEDs.
  • an LED array is powered by a phase angle modulated, full wave, rectified a.c. controller similar to traditional triac or silicon controlled rectifier "light dimmers".
  • a.c. controller similar to traditional triac or silicon controlled rectifier "light dimmers”.
  • the width of the output pulses is controlled by either a light detector or temperature sensor.
  • the circuit of Fig 5 employs full wave rectification for efficient flicker free performance of the LED array.
  • Fig. 6 Shown in Fig. 6 are the phase controlled wave forms that could be expected for hot and cold LED operating environments. Naturally, the pulsing output could be filtered if necessary to provide d.c. operation of the LED array if desired.
  • the transfer function of the feedback control systems for the present invention are device specific and would be engineered for particular families of LEDs. That is, in the case of temperature compensated LED arrays, the actual diminution of luminous output per degree of temperature increase would be used program the correct increase in average LED current. In the case of optical sensing, the feedback loop is essentially closed, and only loop gain and response time need be set.

Landscapes

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

Abstract

L'invention concerne un circuit permettant de maintenir l'intensité lumineuse d'une diode électroluminescente (LED) (12) comportant au moins une diode électroluminescente (LED) (12) destinée à produire une intensité lumineuse; un capteur (22, 24) pour détecter un état proportionnel à l'intensité lumineuse de la diode LED (12) et pour produire un signal d'intensité lumineuse; et une alimentation en énergie (16) électriquement connectée à la diode LED (12) pour amener des impulsions d'énergie électrique à la diode LED (12). L'alimentation en énergie (16) comporte un dispositif de commutation sensible au signal d'intensité lumineuse, de façon à réguler l'énergie électrique amenée par les impulsions par unité de temps pour réguler le courant moyen passant dans la diode LED (12) et, ainsi, maintenir l'intensité lumineuse de la diode LED (12) à un niveau prédéterminé. Selon une réalisation, le capteur (22) comporte un moyen pour détecter les changements de température de fonctionnement de la diode LED (12). Selon une seconde réalisation, le capteur (24) comporte un moyen (28) permettant de détecter les changements de sortie lumineuse de la diode LED (12). L'énergie électrique amenée par les impulsions par unité de temps est régulée (1) par modification de la fréquence, (2) par modification de la largeur des impulsions, (3) par une combinaison de fréquence et de largeur, ou (4) par réglage de la phase des impulsions à l'intérieur d'une forme d'onde sinusoïdale de courant alternatif.
EP98918770A 1998-04-27 1998-04-27 Maintien de l'intensite lumineuse de diodes electroluminescentes (led) Withdrawn EP1075706A4 (fr)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
PCT/US1998/008432 WO1999056303A1 (fr) 1997-01-10 1998-04-27 Maintien de l'intensite lumineuse de diodes electroluminescentes (led)

Publications (2)

Publication Number Publication Date
EP1075706A1 true EP1075706A1 (fr) 2001-02-14
EP1075706A4 EP1075706A4 (fr) 2001-07-18

Family

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Family Applications (1)

Application Number Title Priority Date Filing Date
EP98918770A Withdrawn EP1075706A4 (fr) 1998-04-27 1998-04-27 Maintien de l'intensite lumineuse de diodes electroluminescentes (led)

Country Status (2)

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EP (1) EP1075706A4 (fr)
CA (1) CA2328439A1 (fr)

Families Citing this family (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
BRPI0715803A2 (pt) * 2006-08-21 2013-07-16 Tir Technology Lp aparelho e mÉtodo para compensar ondulaÇço em uma corrente de conversor, e, fonte de luz
RU2453078C2 (ru) 2006-12-11 2012-06-10 Конинклейке Филипс Электроникс Н.В. Способ и устройство для цифрового управления устройством освещения
RU2470496C2 (ru) 2006-12-11 2012-12-20 Конинклейке Филипс Электроникс Н.В. Система и способ управления осветительными приборами
CA2708984C (fr) * 2006-12-12 2017-11-07 Tir Technology Lp Systeme et procede permettant de commander un eclairage
US7592757B2 (en) * 2007-03-29 2009-09-22 Magna International Inc. System and method for dimming one or more light source
CN101731023A (zh) * 2007-05-16 2010-06-09 托尼·迈耶 恒光输出照明器系统

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4117499A (en) * 1976-02-25 1978-09-26 Rollei-Werke Franke & Heidecke Photographic camera with automatic exposure control
US4843283A (en) * 1987-08-24 1989-06-27 Chen Jack Y C Infrared ray detector control illumination system
EP0572009A1 (fr) * 1992-05-27 1993-12-01 Spacelabs Medical, Inc. Système de balayage pour clavier à action fugitive

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4117499A (en) * 1976-02-25 1978-09-26 Rollei-Werke Franke & Heidecke Photographic camera with automatic exposure control
US4843283A (en) * 1987-08-24 1989-06-27 Chen Jack Y C Infrared ray detector control illumination system
EP0572009A1 (fr) * 1992-05-27 1993-12-01 Spacelabs Medical, Inc. Système de balayage pour clavier à action fugitive

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See also references of WO9956303A1 *

Also Published As

Publication number Publication date
CA2328439A1 (fr) 1999-11-04
EP1075706A4 (fr) 2001-07-18

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