Controlling an arrangement of semiconductors emitting light of distinct colors

The invention relates to a method for controlling an arrangement of light emitting semiconductors emitting light of substantially distinct colors. The invention also relates to a lighting system according to the preamble of claim 6 .

U.S. Pat. No. 6,441,558 discloses a LED luminary system for providing power to LED light sources to generate a desired light color. The system comprises a controller for controlling a supply or power to the LEDs. The controller comprises two parts. The first part measures a temperature of the arrangements of LEDs, it determines a junction temperature of the semiconductors for each distinguished color, and it determines a feed forward junction temperature compensation to provide an intermediate control signal which is supplied to a lumen output module, emitting a wanted output power or lumen output for each color. A second part of said controller comprises a feedback loop, which receives the output of the lumen output module as a set point value. A light output is measured and a measured value is subtracted from the set point value provided by the lumen output module to provide a difference or error signal. The error signal is supplied to a lumen output controller, which adjusts a pulse width modulation (PWM) of power supplied to LEDs of the corresponding distinct color. Thus, the first, feed forward junction temperature dependent part and the second, lumen feedback part are connected in series. With such a controller the output of emitted light is controlled to be identical to a set point value supplied by the lumen output module of the feed forward part.

A controller, which provides feed forward junction temperature compensation only, can be used to compensate for differences of light output and wavelengths shifts due to changes of junction temperature(s).

A controller, which comprises a lumen feedback to control a lumen or light output only to be identical to some set point value, could be used to compensate for changes of light output due to temperature effects and aging of the LEDs.

The prior art controller comprises an algorithm for the feed forward part and the feedback part, which includes many calculation steps. The temperature of the LED arrangement may vary rather fast, and, as a consequence, light output power and wavelengths shift also. Therefore the calculation of such algorithm must be carried out with a high pace, which, in practice, is identical to a pulse width modulation period at which a supply to the LEDs is modulated. To avoid visible flickering in the light output of the module, the pulse width modulation period is usually shorter than 20 milliseconds. As a consequence, a processor for carrying out said calculation must be powerful and therefore will be expensive. A complicating factor is that when using a single light sensitive element to measure the light output of each color, it is required to time shift the on-time for each color. It also requires the use of a minimum on time for each color during each PWM period, so that the combined light output of all colors always contains a fraction of each color. To minimize such fractions and thereby maximizing the control range of light output for each color, the light output for each color must be sensed and evaluated even faster, which requires an even more powerful and expensive processor.

The inventors found that compensating the light output for changes due to aging need not to be carried out with such high pace. In addition, the inventors conceived that an output of a feed forward junction temperature compensating part should not be used as such to provide a set point for the wanted light output.

It is an object of the invention to solve the drawbacks of the prior art as described above and to provide improvements in compensating for changes of light output of emitted light for each distinguished color and wavelengths shifts dependent on changes of junction temperature of the semiconductors and, in combination, compensate for changes of emitted light power due to aging.

The above object of the invention is achieved by providing a method as described in claim 1 .

With the method as claimed, calculations, which are required to compensate for changes of light output due to aging, can be carried out with very long intervals in a range of hundreds or thousands of hours. As a consequence the processor for carrying out all calculations can be less powerful and therefore much cheaper than before. Because of said long intervals a period for sensing and processing emitted light need not to be as short as before and need not to fall within a single PWM period. This allows for using less expensive light sensing elements.

The object of the invention is also achieved by providing a lighting system as described in claim 6 .

The invention will become more gradually apparent from the following exemplary description in connection with the accompanying drawing. In the drawing:

FIG. 1 shows a diagram of a first embodiment of a lighting system according to the invention;

FIG. 2 shows a diagram of a second embodiment of a lighting system according to the invention.

The lighting system shown in FIG. 1 comprises an assembly 2 of an arrangement of light emitting semiconductors, such as diodes (LEDs), and drivers for driving the semiconductors from a power supply. The arrangement of semiconductors comprises semiconductors for emitting light of different distinct colors. As an example, but not limited to that, three different colors can be used, in particularly red, green and blue, which, abbreviated to R, G and B respectively, are used as a suffix to numerals for referring to parts and signals of the system.

At some locations, such as a heat sink, of the arrangement of semiconductors a temperature is measured, which is indicated by dotted arrow 4 . A junction temperature estimator 6 uses a value of the sensed temperature to determine a junction temperature 8 R, 8 G and 8 B of each color. The estimator 6 comprises a thermal model of a luminary containing the arrangement of light emitting semiconductors. The use of such estimator is known per se and therefore a detailed description thereof will be omitted here.

A user interface 10 provides means for a user of the lighting system to set a wanted light output as emitted by the semiconductors of all colors, that is, with wanted intensities of light of each color and, consequently, a wanted ratio of such intensities. To that extent, input provided by the user via interface 10 , is supplied to a calibration matrix 12 . The calibration matrix 12 outputs a nominal value of the wanted intensities, as indicated by numerals 14 R, 14 G and 14 B. The actual output of light is dependent on the junction temperature of the semiconductors. Therefore the estimated junction temperatures 8 R, 8 G and 8 B are supplied to the calibration matrix 12 to compensate the nominal values 14 R, 14 G and 14 B respectively for changes of the junction temperature for the respective distinct color. This allows compensation for wavelength shifts due to changes in junction temperature.

In the drawings, parts, which are identical for each distinct color, are shown only for one distinct color, which is red in the example. A light output calculation unit 16 R receives the junction temperature value 8 R and calculates a light output factor in accordance with, for instance, formula: EXP((T j,R −T ref,R )/T 0R ) is:

T j,R is the estimated junction temperature of the semiconductors emitting red light;

T ref,R is a is a reference temperature at which the output of the red semiconductors is specified;

T 0R is a characteristic value, which can describe a light output (e.g. flux) output of the red semiconductors dependent on junction temperature.

Said formula is known per se and is given as an example only.

A first multiplier 18 R multiplies the nominal value 14 R, received from the calibration matrix 12 and an output from the light output calculation unit 16 R. The output of multiplier 18 R determines a pulse width during which the semiconductors of the corresponding distinct color (red in this case) are supplied with power. By using the light output calculation unit 16 R and using the junction temperature 8 R, changes in the emitted light, by any cause, can be compensated.

A second multiplier 20 R receives an output from the first multiplier 18 R and an output from a divider 22 R. A control unit 24 R receives an output from the second multiplier 20 R and dependent on that it controls the width of pulses during which the semiconductors are to be supplied with power. To that extent, the control unit 24 R supplies a pulse width modulated signal 26 R to the semiconductor and driver assembly 2 .

Light emitted by the semiconductors is indicated by dotted arrow 28 and its light output is measured by a light output measuring unit 30 . The light output measuring unit 30 can comprise a distinct sensor for each distinct color of light emitted by the semiconductors. As an alternative a single sensor can be used in combination with a timing by which each color is measured during different intervals. The light output measuring unit 30 outputs light output values 32 R, 32 G and 32 B for the distinct colors respectively. A light output reference provider 34 R outputs a light output reference 36 R for each distinct color. The divider 22 R divides the light output reference value 36 R by the measured light output value 32 R and it outputs the light output ratio thus calculated to the second multiplier 20 R.

The light output reference values are set for a specific reference junction temperature.

Calculations involved with said operations of the junction temperature estimator 6 , the calibration matrix 12 , the light output calculation unit 16 R, the first multiplier 18 R and the second multiplier 20 R are carried out with a first interval of, for example, twenty milliseconds.

Calculations involved with the operation of the divider 22 R and the operation of the light output measuring unit 30 and light output reference provider 34 R are carried out with a second interval, which is, for example, in a range of 100 to 10000 hours. During the second interval an output from the divider 22 R is retained, so that it can be used by the first multiplier 20 R during each first interval.

By using the light output measuring unit 30 , the light output reference provider 40 R, the divider 22 R and the multiplier 20 R it is possible to compensate for changes in light output caused by aging of the semiconductors. Since aging semiconductors is a slow process compensation may be carried out with said long second intervals, which allows the use of a less powerful processor to carry out calculations during each first interval.

The second embodiment of a lighting system according to the invention shown in FIG. 2 differs from the first embodiment shown in FIG. 1 by that the light output reference provider 34 R is replaced by a light output reference provider 38 R, which is supplied with the junction temperature value 8 R. The light output reference provider 38 R calculates the light output reference value 36 R dependent on the junction temperature value 8 R. While light output reference provider 34 R of the first embodiment was static, light output reference provider 38 R of the second embodiment requires carrying out additional calculations. However, since the additional calculations must be carried out with the long second interval they do no represent a significant load for the processor.

Briefly said, a method for controlling an arrangement of light emitting semiconductors for emitting light of different distinct colors and a lighting system, which is in accordance with that, are provided, in which a junction temperature feed forward control part operating with a short first interval is adjusted dependent on measured light output values with a much longer second interval.

Preferably, adjustments of the temperature dependent control loop by the light output control loop is carried out each time the lighting system is switched on. It need not be carried out completely during a single first interval, but it may span several first intervals.

The second interval can be started when the lighting system is switched on for the first time or with each switching on of the lighting system.