Method for Driving a Led Based Lighting Device

Abstract

The present invention relates to a lighting device, as well as to a lighting system comprising such a lighting device and an adjustable power source, and also relates to a method of driving such a lighting system. The lighting device comprises at least one LED (1a, 1b), a control device that comprises a measuring means (7, 9) to measure a quantity that is indicative of an electrical resistance of said LED at a predetermined current or voltage, a power supply control means (11) connected to said measuring means (7, 9) and constructed to control an adjustable electrical power supply (3a, 3b) for driving the LED (1a, 1b), said signal being based on said value of said quantity. The electrical resistance of a LED is functionally dependent of the LED's junction temperature, which in turn determines its optical output characteristics. Thus, by measuring the junction temperature indirectly, through measuring of electrical LED characteristics, and mapping them to a temperature, LED output control is possible.

Description

The invention will now be elucidated further, with referral to the drawings, in which non-limiting embodiments of the invention are depicted, and in which:

FIG. 1 schematically illustrates the dependence of light output on junction temperature for a number of LED types;

FIG. 2 schematically shows an embodiment of a lighting system according to the invention;

FIG. 3 schematically shows a time sequence for measuring and driving an LED, according to a method of the invention; and

FIG. 4 schematically shows I,V characteristics for a LED at different junction temperatures.

In FIG. 1, the diagram schematically shows the relative light output I_rel., in arbitrary units, as a function of junction temperature, for four different color LEDs, in this case blue (solid line), green (dashed line), red (dotted line) and amber (dot-and-dash line). It is clearly visible that even a small temperature variation causes a large shift in optical output, which has to be compensated e.g. by adjusting the power to the LED. Note further that the temperature dependence differs for LEDs of different colors. This means that, when different LEDs are used to mix colors, a color shift will occur when the junction temperature shifts. For the example shown, an increase of junction temperature lowers the amber and red contribution much more than the blue and green contribution, thus causing a shift to “cooler” colors.

By means of the invention, knowledge about the junction temperature may be obtained, through measurement of junction resistance or a related quantity. This allows individual correction of the LEDs, and thus correction of color shift.

FIG. 2 schematically shows an embodiment of a lighting system according to the invention. Herein, 1a, 1b, . . . , are light emitting diodes or LEDs, while adjustable current sources are denoted 3a, 3b, . . . . Switching devices 5a, 5b, . . . , can switch the electrical connection of an LED to a measurement voltage source 7 and current meter 9, which is coupled to a control unit 11, which in turn is coupled to the adjustable current sources 3a, 3b, . . . . Note that this measurement is done for one LED at a time. Advantageously, one LED is measured while all other LEDs, if any are present, are switched off or are at least electrically decoupled from said LED. Multiple measuring circuits are possible, each decoupled from the other LEDs.

As an alternative, instead of a measurement current source, a measurement voltage source, that provides a predetermined voltage across the LED, may be used. Herein, a current through the LED is measured by a current meter, instead of the voltage meter.

A third embodiment, not shown here, comprises a driving current source that can be set to a measurement current for the measurement phase and with a switch that allows for monitoring the voltage across the LED.

In FIG. 2, there are shown two LEDs 1a and 1b. It should be noted that any number of LEDs is possible, such as only one LED, but also three or more, for example for mixing colors. In the latter case, it is possible to use for example red, green and blue LEDs, each color receiving its own power, or even each LED receiving its individual electrical power.

In all systems also a series connection of LEDs is possible, especially if the LEDs are of the same kind. The voltage could be measured across one LED exemplarily or also over all LEDs in series, thereby averaging the temperatures of the multiple devices. Individual measurements provide better accuracy, but are more complex.

LED 1b receives electrical power from a current source 3b, since switching device 5b connects the two parts. Current source 3b is adjustable, in order to be able to adjust the optical output of the corresponding LED 1b. Current sources 3a, 3b, . . . , are shown as separate sources, although it is likewise possible to provide one current source which is able to power all desired LEDs with a desired current, e.g. through a voltage divider. Note that it would also be possible to supply electrical power to the LEDs by means of an adjustable voltage source.

Contrarily, with the switching device 5a as shown here, the LED 1a receives a measuring voltage from measuring voltage source 7. This source 7 supplies a measuring voltage Vm to the LED 1a, which causes a measuring current Im to flow through the LED, which current is dependent on Vm and/or the temperature of the junction of the LED. Once Im is given only one of the parameters Vm or T represents a further independent variable. The current is measured with a current meter 9. On the basis of the voltage Vm, which is known, and the measured current, the resistance of the LED, and in particular the junction temperature thereof, may be derived.

The value of the current, or of the resistance, which is in principle corresponding information, is supplied to a control unit 11, depicted only schematically. The control unit may contain information on the dependence on temperature of either the resistance of the LED, or junction, or directly related a quantity such as current through the LED or voltage across the LED. Thereto, the control unit may e.g. comprise a look-up table, or similar circuitry, or may comprise or be connected to a computer or other digital or analogue device that is able to store and provide the relevant data. When the control unit 11 receives a value of a measured current, resistance or voltage, as the case may be, the control unit is able to provide a control unit that will set the correct current, or corresponding voltage, for the relevant LED or LEDs. In this case, measuring of LED 1a will result in the control unit 11 setting current source 3a. Of course, control unit 11 will also be able to control the switching devices 5a, 5b, etc. in order to selectibly measure a desired LED.

A method of measuring and controlling the LEDs will be elucidated in connection with FIG. 3, which schematically shows a time sequence for measuring and driving an LED, according to a method of the invention.

In the diagram, the current I(LED) through the LED is plotted as a function of time t. Initially, i.e. at t<t1, the I(LED) is equal to I_b1, a normal driving current at which the LED gives a desirable output. This current I_b1 is a current which is often, but not necessarily, larger than the “knee current”, or current at the knee voltage of the LED. The knee voltage is, in a linear scale I-V plot, the voltage of the “bend” of the curve, and a kind of lower limit of the forward voltage drop over the LED in any practically useful situation.

At t=t1, the switching device relating to the relevant electrode switches to a measuring position, in which the measuring voltage sources applies a measuring voltage to the LED, resulting in a new current Im to flow through the LED. This current Im is measured. The measurement takes place between time t1 and t2, in order to obtain a reliable value. On the basis of the measured value of Im, and the known value of the measuring voltage, a new value for the current I(LED) is determined by the control unit to be I_b2. This may be brought about e.g. by mapping the current value Im to a junction temperature and subsequently to a value for I(Led) that gives the desired new optical output, by mapping the Im directly to a desired I(LED), etc. As soon as the desired value for I_b2 has been determined, it is set by the control unit, at a time t3.

Note that in the case shown, the new I(LED) is set only some time after determination of the measuring current Im. During the time between t2 and t3, it is e.g. possible to have zero current through the LED, to maintain the measuring current Im until such time that the new I(LED)=I_b2 may be set, or, preferably, to supply again the original I(LED), i.e. I_b1, until the time t3 when I_b2 may be set. The latter measure ensures that the LED may provide output during said time, even when not necessarily the optimum output. Of course, the switching device reconnects the LED to the adjustable current source at a corresponding point in time, such as immediately after determining Im, or only at the time of setting the new I(LED)=I_b2.

It can be seen in FIG. 3 that the measuring current Im is preferably smaller than the normal driving currents I_b1 and I_b2 and the like. Although this is not necessary, a smaller measuring current means that the diode has a higher resistance, which can be measured more precisely.

It is remarked here that the LED control method and system according to the invention require that normal driving of the LED is interrupted. However, in practice a LED is seldom driven continuously, but rather intermittently. It is convenient to measure the LED and calculate a new current in such times of inactivity. However, even in cases in which the LED is driven for a time period that is longer than the desired interval for checking the LED, it is no problem to interrupt operating the LED for a short time, in order to measure the LED and if necessary adjust the I(LED). Most applications do not need a continuous operation of the LED, and interrupting operation of the LED has hardly if any influence on the life span of the LED.

An alternative way of controlling the LED's output, in case the LED is driven by a pulsed current source, would be to change the pulse width and/or frequency, in other words the average electrical power supplied tot the LED. For example, at a certain current level and pulse width and-frequency, a LED has a certain output. If the junction temperature changes, the output also changes, according to a known function. By measuring the temperature change according to the invention, a new input power level can be set in order to obtain the required LED output level. This embodiment, with an adjustable pulsed electrical power source, has an advantage in that other LED characteristics that may be dependent on the absolute level of the current do not change.

In cases where continuous operation of the LED is necessary, it is still possible to apply the control method and system according to the invention. Thereto, it is for example possible to measure the resistance of the LED while in the operative condition. This may be brought about by determining the current through the LED with knowledge of the voltage across the LED. In other words, in practice this comes down to measuring the voltage drop across the LED when the known current I(LED) is supplied to the LED. Note that in most cases this requires a much more precise determination of the resistance, i.e. of the voltage, since in a practical operative condition, the LED has a much smaller resistance than in a condition as described above.

FIG. 4 schematically shows I,V characteristics of an example of a LED at certain junction temperatures. An actual junction temperature may be based on these curves, for example by interpolating a measured current at a predetermined voltage, or vice versa.

Claims

1. Lighting device, comprising at least one light emitting diode (LED) (1a, 1b),a control device that comprises: a measuring means (7, 9) constructed to determine a value of a quantity that is correlated to operation of said LED (1a, 1b),a power supply control means (11) connected to said measuring means (7, 9) and constructed to provide a control signal to an adjustable electrical power supply (3a, 3b) for driving the LED (1a, 1b), said signal being based on said value of said quantity as determined by said measuring means,

2. The lighting device of claim 1, wherein said quantity comprises an electrical current through said LED (1a, 1b) at a predetermined measuring voltage across said LED, and/or a voltage across said LED at a predetermined measuring current through said LED.

3. The lighting device according to claim 2, wherein said measuring means comprises a measurement voltage source (7) for providing said predetermined measurement voltage, and/or a measurement current source for providing said predetermined measuring current.

4. The lighting device according to claim 2, wherein said predetermined measurement voltage is smaller than a forward driving voltage of said LED (1a, 1b), or said predetermined measurement current is smaller than a forward driving current of said LED.

5. The lighting device according to claim 1, wherein the control device comprises a switch (5a, 5b) for selectibly connecting said LED (1a, 1b) to said measuring means (7, 9).

6. The lighting device according to claim 1, wherein the control device (11) comprises an information retrieval means, that contains information on the control signal as a function of the measured value of said quantity.

7. The lighting device of claim 6, wherein the information retrieval means comprises a look-up table.

8. The lighting device of claim 1, comprising at least two LEDs (1a, 1b), wherein said value of said quantity is selectibly measurable by said control device for each of the at least two LEDs.

9. The lighting device of claim 8, wherein each of the at least two LEDs (1a, 1b) is individually drivable by an adjustable electrical power supply (3a, 3b) on the basis of said measured value of said quantity for said LED.

10. A lighting system, comprising a lighting device according to claim 1, and an adjustable electrical power supply (3a, 3b) connected to a LED (1a, 1b) of said lighting device, for supplying electrical energy to drive said LED.

11. The lighting system of claim 10, wherein the adjustable electrical power supply (3a, 3b) is further able to provide a predetermined measuring voltage across said LED (1a, 1b), and/or a predetermined measuring current through said LED, wherein said predetermined measurement voltage is smaller than a forward driving voltage of said LED, or said predetermined measurement current is smaller than a forward driving current of said LED.

12. A method of driving a lighting system according to claim 10,

13. The method of claim 12, wherein measuring said value comprises measuring of an electrical current through said LED (1a, 1b) at a predetermined measuring voltage across said LED, and/or measuring of a voltage across said LED at a predetermined measuring current through said LED.

14. The method of claim or 13, wherein said predetermined measuring voltage is smaller than a voltage across said LED (1a, 1b) in an operating condition of said LED, and/or said predetermined measuring current is smaller than a current through said LED in an operating condition of said LED.