United States Patent
`
`[19]
`
`[11] Patent Number:
`
`5,783,909
`
`Hochstein
`[45] Date of Patent: Jul. 21, 1998
`
`
`
`US005783909A
`
`[54] MAINTAINING LED LUMINOUS INTENSITY
`
`[75]
`
`Inventor: Peter A. Hochstein. Troy. Mich.
`
`Primary Examiner—Benny T. Lee
`Assistant Examiner—Haissa Philogene
`Attorney, Agent, or Fima—Howard & Howard
`
`[73] Assignee: Relume Corporation. Troy. Mich.
`
`[57]
`
`ABSTRACT
`
`[21] Appl. No.2 781,688
`
`[22] Filed:
`
`Jan. 10, 1997
`
`Int. Cl.6 ..................................................... H0513 37/02
`[51]
`[52] US. Cl. ....................... 315/159; 315/158; 315/169.3;
`315/224: 315/307; 363/89
`[58] Field of Search ..................................... 315/307. 224.
`315/1693. 154. 155. 158. 159; 250/214 R.
`214.1. 214 AL. 214 C; 363/80. 89. 126.
`26. 36. 41
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`A circuit for maintaining the luminous intensity of a light
`emitting diode (LED) (12) comprising at least one light
`emitting diode (LED) (12) for producing an luminous inten-
`sity; a sensor (22.24) for sensing a condition proportional to
`the luminous intensity of the LED (12) and for producing a
`luminous intensity signal; a power supply (16) electrically
`connected to the LED (12) for supplying pulses of electrical
`energy to the LED (12); and wherein the power supply (16)
`includes a switching device responsive to the luminous
`intensity signal for adjusting the eleclrical 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.
`In one instance. the sensor (22) includes means for sensing
`changes in the operating temperature of the LED (12). In a
`second instance. the sensor (24) includes means (28) for
`sensing changes in luminous output of the LED (12). The
`electrical energy supplied by the pulses per unit of time are
`adjusted by anyone of (1) varying the frequency. (2) varying
`the width of the pulses. (3) a combination of frequency and
`width. or (4) adjusting the phase of the pulses within an ac.
`sinusoidal wave form.
`
`1,662,348
`2,503,574
`3,919,546
`4,587,459
`5,661,645
`
`3/1928 Snicker.
`4/1950 Allen ......................................... 40/132
`11/1975 Lutus
`..
`5/1986 Blake .......
`8/1997 Hochstein .
`
`
`
`.
`
`. ...... 363/89
`
`01‘HER PUBLICATIONS
`
`Hewlett Packard Application Brief I—012 Temperature Com—
`pensation Circuit for Constant LED Intensity. May 1995.
`
`18 Claims, 3 Drawing Sheets
`
`14
`
`12
`
`20
`
`
`
` SWITCH MODE
`
`POWER SUPPLY
`
`
`0-0-
`POWER
`
`22
`
` 26
`
`FILTER
`
`28
`
`HTC, Exhibit 1008
`
`HTC, Exhibit 1008
`
`

`

`US. Patent
`
`Jul. 21, 1998
`
`Sheet 1 of 3
`
`5,783,909
`
`14
`
`12
`
`
`
`20
`
`
`SWITCH MODE
`POWER SUPPLY
`
`0-0-
`POWER
`
`22
`
`
`
`FILTER
`
`26
`
`28
`
`14
`
`12
`
`2o
`
`
`
`
`
`SWITCH MODE
`POWER SUPPLY
`
`0.0.
`POWER
`
`FIG-2
`
`HTC, Exhibit 1008
`
`HTC, Exhibit 1008
`
`

`

`US. Patent
`
`Jul. 21, 1993
`
`Sheet 2 of 3
`
`5,783,909
`
`FIG - 3A
`
`cu RENT
`
`TORLEDS
`
`FIG - 3B
`
`fi%fifi§§f
`
`FIG - 4A
`
`CURRENT
`“DLEDS
`
`FIG - 43
`
`CURRENT
`TOLEDs
`
`l I COLDLEDs
`
`HME4——.>
`
`HOTLEDs
`
`'HME ——4p>
`
`WME-——.>
`
`'HME-—-I>
`
`COLDLEDs
`
`HOTLEDs
`
`HTC, Exhibit 1008
`
`HTC, Exhibit 1008
`
`

`

`US. Patent
`
`Jul. 21, 1998
`
`Sheet 3 of 3
`
`5,783,909
`
`14
`
` 16
`
`12
`
`20
`
` FULL WAVE
`
`PHASE CONTROL
`
`POWER SUPPLY
`
`A.C.
`POWER
`
`22\
`
`5------------------------------
`
`FILTER
`
`i -------.
`
`....................
`
`~
`
`-- 26
`
`FIG - 5
`
`28 """"""
`
`LED DRIVE
`CURRENT
`
`CURRENT
`TO LED
`
`INPUT
`VOLTAGE
`
`‘\
`
`
`
`COLD LEDs
`
`
`
`INPUT
`VOLTAGE
`
`‘\
`
`CURRENT
`T0 LED
`
`LED DRIVE
`CURRENT
`
`HOT LEDs
`
`HTC, Exhibit 1008
`
`HTC, Exhibit 1008
`
`

`

`5.783.909
`
`1
`MAINTAINING LED LUMINOUS INTENSITY
`
`TECHNICAL FIELD
`
`The subject invention relates to light emitting diodes
`(LEDs).
`
`BACKGROUND OF THE INVENTION
`
`The increasing use of light emitting diode (LED) signals
`in a variety of outdoor environments has presented some
`serious deficiencies in the LED technology. Traffic signals.
`outdoor message boards. railroad crossing signs. and similar
`safety critical signals have been converted from incandes-
`cent lamps to LED designs to take advantage of the energy
`savings and long service life provided by LED devices. In
`some situations. however. the LED devices have not per-
`formed satisfactorily and. in fact. can present a safety hazard
`due to diminution of luminous output.
`It is well known that the luminous output of LED devices
`degrades with time and temperature. Degradation is gener-
`ally a linear function of time whereas degradation is an
`exponential function of temperature. At elevated
`temperature. circa 85° C.. certain LEDs exhibit a permanent
`degradation or loss of luminous output of approximately
`forty percent (40%) in twenty thousand (20.000) hours of
`use. This degradation must be factored into the design of
`safety critical signals so that. at the end of specified service
`life. some minimum luminous intensity is still available.
`Another. less appreciated fact is that most LEDs also
`exhibit a non-permanent or recoverable diminution of lumi-
`nous 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.
`The non permanent temperature induced diminution of
`LED luminous output has only recently been formally
`acknowledged and is clearly set forth in Hewlett Packard
`Application Brief 1-012 (document 5963-7544E 5/95).
`One solution to this temperature induced diminution is
`suggested by the cited 1-012 reference. This document
`recommends linearly controlling the current through an LED
`by means of a linear feedback control system. The sensing
`element in this feedback control circuit is a photodiode that
`monitors the luminous output of the illuminated LED. While
`this proposed solution is theoretically workable. practical
`application of this principle is very diflicult and inefficient.
`Firstly. 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. 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 emit-
`ter (LED) in a closely packed array is difficult if all operating
`variables (such as temperature) are to be identical.
`Secondly. 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. When a larger current through the LED
`(S) is dictated by the control circuit.
`the linear control
`element effectively reduces its resistance to current. Typi-
`cally such current control elements are transistors of various
`types. which mu st dissipate the controlled current multiplied
`by the voltage drop across the control element as heat. That
`
`2
`
`is. power not utilized by the LED(S) is dissipated as heat
`when less current through the LED is indicated.
`
`SUMMARY OF THE INVENTION AND
`ADVANTAGES
`
`A circuit for maintaining the luminous intensity of a light
`emitting diode including at least one light emitting diode
`(LED) for producing a luminous intensity. A sensor for
`sensing a condition proportional to the luminous intensity of
`the LED and for producing a luminous intensity signal. A
`power supply electrically connected to the LED for supply-
`ing 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) compris-
`ing the steps of supplying pulses of electrical energy from an
`adjustable power supply to an LED for establishing electri-
`cal 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.
`In all embodiments. the subject invention increases the
`average current through the LED to compensate for a loss of
`luminous output. and vice versa.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`Other advantages of the present invention will be readily
`appreciated as the same becomes better understood by
`reference to the following detailed description when con-
`sidered in connection with the accompanying drawings
`wherein:
`'
`
`FIG. 1 is a schematic view of a first embodiment;
`FIG. 2 is a schematic view of a second embodiment;
`FIG. 3 is a graph showing variation in the width of the
`electrical pulses;
`FIG. 4 is a graph showing variation in the frequency of the
`electrical pulses;
`FIG. 5 is a schematic view of a third embodiment; and
`FIG. 6 is a graph showing variation in the sinusodial wave
`form of the third embodiment.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`
`5
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`Referring to the Figures. wherein like numerals indicate
`like or corresponding parts throughout the several views. a
`first embodiment of a circuit for maintaining the luminous
`intensity of a light emitting diode is shown schematically in
`FIG. 1. a second embodiment is shown in FIG. 2. and a third
`embodiment is shown in FIG. 5.
`
`65
`
`In 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
`
`HTC, Exhibit 1008
`
`HTC, Exhibit 1008
`
`

`

`3
`
`5.783.909
`
`4
`
`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 12. The power supply 16
`is connected via a lead 20 to a source of electrical power. dc.
`power in the embodiments of FIGS. 1 and 2. and ac. power
`in embodiment of FIG. 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 predeter-
`mined level.
`In the embodiment of FIG. 1. the sensor 22 includes
`means for sensing changes in luminous output of the LED
`array. The sensor 22 also includes means 28 for differenti-
`ating 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.
`Alternatively. in the pulsed LED signal. synchronous detec—
`tion could be employed to diflerentiate 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 essen-
`tially constant. Such closed loop control. with the propa
`feedback time constants. will assure an essentially constant
`luminous output irrespective of operating temperature.
`In the embodiment of FIG. 2. the sensor 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 behavior 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. Accordingly.
`
`the sensor 22 includes a predetermined temperature behavior
`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 predeter-
`mined temperature behavior model. This behavior model
`may be pre-programmed 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 FIG. 3. On the other hand. 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 FIG. 4. In the
`embodiment of FIG. 5. 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
`ac. sinusodial wave form.
`Therefore 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.
`In the first embodiment. 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 the current
`passing through the LED as a function of the operating
`temperature of the LED integrated with the predetermined
`temperature model.
`In the second embodiment. the sensing of a condition is
`further defined as sensing changes in luminous output of the
`LED. This step may be further defined as difi‘erentiating
`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 main-
`taining an essentially constant luminous output fi'om an LED
`array.
`irrespective of operating temperature. Unlike the
`proposed method in the cited reference. using linear regu-
`lation 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).
`It is well known that switch mode operation of power
`supplies is very eflicient. 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. dc. power
`supplies filter the switched output voltage to produce a
`constant. relatively ripple free output.
`LED arrays. particularly those used in outdoor environ-
`ments 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.
`It will be appreciated that such a switch mode power
`supply can take many forms. Within the scope of the present
`
`10
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`15
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`20
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`25
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`3O
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`35
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`45
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`50
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`55
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`65
`
`HTC, Exhibit 1008
`
`HTC, Exhibit 1008
`
`

`

`5
`
`6
`
`5.783.909
`
`invention. switch mode supplies include any power source
`16 that is turned on and 01f 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. The use of traditional phase controlled a.c. power
`supplies as illustrated in FIGS. 5 and 6. is also explicitly
`included as a suitable power supply. While not generally
`considered switch mode in the narrowest sense. phase con-
`trolled 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 inven-
`tion: 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.
`As shown in FIG. I. 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. The use of a filter circuit 28 for
`the light sensing transducer may be necessary to accommo-
`date the pulsing light output from the LED array. Of course.
`the pulsing current delivered by the power supply could be
`filtered to essentially d.c.. making the transducer filter
`unnecessary. In either case. 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 diminu-
`tion. The light sensing transducer 22 will compensate for the
`aggregate light loss and maintain the luminous output essen—
`tially 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 LED array. 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.
`
`More sophisticated temperature compensation topologies
`that do take into account the permanent ‘time at temperature”
`degradation mechanisms plus the instantaneous diminution
`of luminous output due to temperature are indeed possible.
`Such a comprehensive compensation approach using tem-
`perature sensors could be implemented if the degradation
`mechanisms of the LEDs were carefully modeled. In such a
`comprehensive case. the average current drive to the LED
`array would still varied according to mathematical models
`describing the time-temperature induced variation in lumi-
`nous output and the equation describing the instantaneous
`diminution with temperature.
`
`While the power input to the switch mode power supply
`in both FIGS. I and 2 is shown as d.c.. the switch mode
`
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`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.
`When cold. 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. In order
`to maintain essentially this same light output at higher
`temperatures. 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 tem-
`perature.
`
`For applications where a.c. powered LED arrays are
`suitable. direct phase control of the a.c. line is also feasible.
`and is similar to fixed frequency. pulse width modulation.
`Once again. the average current to the LED array is varied
`or modulated in response to a measured process parameter:
`Temperature or luminous output of the LEDs.
`As shown in FIG. 5.. 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 dim-
`mers”. Of course instead of a control potentiometer to vary
`the phase angle for changing the average output current. the
`width of the output pulses is controlled by either a light
`detector or temperature sensor. Unlike most common ac.
`power controllers the circuit of FIG. 5. employs full wave
`rectification for eflicient flicker free performance of the LED
`array.
`
`Shown in FIG. 6 are the phase controlled wave forms that
`could be expected for hot and cold LED operating environ-
`ments. 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.
`It is important to note that either the light sensing or
`temperature sensing feedback control scheme is viable only
`if the LEDs incorporate adequate heat rejection. LEDs that
`are not adequately heat sinked could exhibit destructive
`thermal runaway if the drive current is not limited.
`The invention has been described in an illustrative
`manner. and it is to be understood that the terminology
`which has been used is intended to be in the nature of words
`of description rather than of limitation. Obviously. many
`modifications and variations of the present invention are
`possible in light of the above teachings. It is. therefore. to be
`understood that within the scope of the appended claims.
`wherein reference numerals are merely for convenience and
`are not to be in any way limiting. the invention may be
`practiced otherwise than as specifically described.
`What is claimed is:
`
`65
`
`1. A circuit for maintaining the luminous output of a light
`emitting diode. said circuit comprising:
`at least one light emitting diode (LED) (12) for producing
`a luminous output;
`
`HTC, Exhibit 1008
`
`HTC, Exhibit 1008
`
`

`

`5.783.909
`
`7
`
`a sensor (22.24) for sensing a condition proportional to
`said luminous output of said LED ( 12) and for produc—
`ing a luminous output signal;
`a power supply (16) electrically connected to said LED
`(12) for supplying ONIOFF pulses of electrical energy
`to produce the luminous output of said LED (12); and
`said power supply (16) including a switching device
`responsive to said luminous output signal for adjusting
`the electrical energy supplied by said pulses per unit of
`time to adjust
`the average of said current passing
`through said LED (12) to maintain the luminous output
`of said LED (12) at a predetermined level.
`2. A circuit as set forth in claim 1 wherein said sensor (22)
`includes means for sensing changes in temperature of said
`LED (12).
`3. A circuit as set forth in claim 2 wherein said sensor (22)
`includes a predetermined temperature behavior model to
`establish the increase in said current passing through said
`LED (12) as a function of the operating temperature of said
`LED (12) integrated with said predetermined temperature
`behavior model.
`4. A circuit as set forth in claim 1 wherein said sensor (24)
`includes means (28) for sensing changes in luminous output
`of said LED (12).
`5. Acircuit as set forth in claim 4 wherein said sensor (24)
`includes means (30) for diflerentiating ambient light from
`the luminous output of said LED (12) for measuring the
`actual luminous output of said LED (12).
`.
`6. A circuit as set forth in claim 1 wherein said switching
`device includes means for adjusting the electrical energy
`supplied by said pulses per unit of time by adjusting the
`frequency of said pulses.
`7. A circuit as set forth in claim 1 wherein said switching
`device includes means for adjusting the electrical energy
`supplied by said pulses per unit of time by adjusting the
`width of said pulses.
`8. A circuit as set forth in claim 1 wherein said switching
`device includes means for adjusting the electrical energy
`supplied by said pulses per unit of time by adjusting the
`phase of said pulses within an a.c. sinusoidal wave form.
`9. A circuit as set forth in claim 1 including a filter for
`filtering the electrical energy supplied by said pulses into
`substantially dc. supplied to said LED for producing said
`luminous output.
`
`8
`10. A method of maintaining the luminous output of a
`light emitting diode (LED) comprising the steps of:
`supplying ONIOFF pulses of electrical energy from an
`adjustable power supply (16) for establishing electrical
`current passing through the LED (12);
`sensing (22.24) a condition proportional to the luminous
`output of the LED (12); and
`adjusting the electrical energy supplied by the ON pulses
`per unit of time to adjust the average of the current
`passing through the LED (12) to maintain the luminous
`output of the LED (12) at a predetermined level.
`11. A method as set forth in claim 10 wherein sensing a
`condition is further defined as sensing changes in tempera-
`ture of the LED (12).
`12. A method as set forth in claim 10 further defined as
`
`establishing a predetermined temperature behavior model
`and increasing the current passing through the LED (12) as
`a function of the operating temperature of the LED (12)
`integrated with the predetermined temperature behavior
`model.
`
`13. A method as set forth in claim 10 wherein sensing a
`condition is further defined as sensing (28) changes in
`luminous output of the LED (12).
`14. A method as set forth in claim 13 further defined as
`difierentiating (30) ambient light from the luminous output
`of the LED (12) for measuring the actual luminous output of
`the LED (12) without the influence of ambient light.
`15. A method as set forth in claim 10 further defined as
`adjusting the electrical energy supplied by said pulses per
`unit of time by adjusting the frequency of said pulses.
`16. A method as set forth in claim 10 further defined as
`
`10
`
`15
`
`20
`
`25
`
`30
`
`adjusting the electrical energy supplied by said pulses per
`unit of time by adjusting the width of said pulses.
`17. A method as set forth in claim 10 further defined as
`
`35
`
`adjusting the electrical energy supplied by said pulses per
`unit of time by adjusting the phase of said pulses within an
`a.c. sinusodial wave form.
`18. A method as set forth in claim 10 including filtering
`the output of the power supply for filtering the electrical
`energy supplied by said pulses into substantially d.c. sup-
`plied to the LED for producing said luminous output.
`*****
`
`HTC, Exhibit 1008
`
`HTC, Exhibit 1008
`
`