Patent Application
20100060198
The invention relates to a lamp provided with at least two LEDs of substantially the same colour and at least one driver for feeding each of said LEDs. In particular the invention relates to a lamp wherein said LEDs are white light emitting LEDs, for instance blue light emitting diodes provided with red, green and/or yellow emitting phosphors, such as white light emitting InGaN LEDs. The invention furthermore relates to a method for producing a lamp.
Colour temperature is the physical temperature of a heated Planckian radiator. The colour temperature of phosphor LEDs can vary due to small difference in the amounts of phosphor applied to the LED. The consistency of colour temperature of the source is important for uniformity of appearance, especially in white light LED luminaries. When designing a lighting system, colour binning is therefore an important consideration. Colour binning is the process of grouping (white) LED's based on their various characteristics and assigning them a specific bin code. LED's with the same characteristics are placed on a reel. An example of white colour binning within a CIE chromaticity diagram is given in
It is an objective of the invention to provide a LED luminary with a uniform colour temperature. The objective of the proposed idea is to provide a white luminary with adjustable correlated colour temperature (CCT) to avoid visible colour effects between the white LED's.
According to the invention, preferably said at least one driver is adapted to feed said at least two LEDs during operation of the lamp at a different power level such that the largest difference in colour temperature of any pair of said LEDs during operation is less than the largest difference in colour temperature of any pair of said LEDs if being fed at an equal operational power level.
The lamp is preferably provided with means for measuring the temperature of the lamp or a correlated quantity thereof, and processing means arranged to adapt said power levels of said at least two LEDs in dependence of said measurement. Said processing means is preferably arranged to adapt said power level such that a predetermined junction temperature or a correlated quantity thereof is achieved. Said means for measuring the temperature of the lamp are preferably arranged to measure the temperature of each of said at least two LEDs separately, or a correlated quantity thereof, said at least two LEDs are preferably thermally insulated from each other, and each of said at least two LEDs are preferably provided with a separate heat sink. Said means for measuring the temperature are preferably arranged to measure the temperature of the junction of said LEDs or a correlated quantity thereof, such as described in WO 2006/043232. With this temperature information light output will be controlled such that CCT value of each LED will be the same or nearly the same. The control of the individual LEDs is such, that the total light output (in lumen) of the luminaries system will stay constant, or alternatively at a desired dimming level.
According to the preferred method for producing the lamp the colour temperature at a predetermined power level of each of said LEDs is measured, and the driver is adapted in dependence of said measured colour temperature to feed each LED of said lamp during operation of the lamp at a power level, such that the largest difference in colour temperature of any pair of said LEDs during operation is less than the largest difference in colour temperature of any pair of said LEDs if said LEDs are fed at an equal operational power level.
The invention will be further explained by means of a preferred embodiment as shown in the accompanying drawings, wherein:
A LED luminary comprising at least two LED sources is shown in
Each LED source 2 is provided with a separate heat-sink 4, and are thermally insulated from each other LED drivers 5 are configured to provide regulated power to the respective LED light sources 2, wherein the power level of the individual LEDs is controlled by a controller 8.
The luminary further includes temperature sensors 6. Temperature sensors 6 are configured to obtain the junction temperature of the LEDs. Temperature sensors 6 may include a thermistor, or a thermopile, or any silicon based sensor (for instance a NTC resistor). Alternatively one temperature sensor may be employed to measure the case temperature of the LED luminary. The junction temperature of each individual LED may then be estimated by employing a thermal model of the LED light sources and the electrical current input to the LEDs, but also the single measured temperature of the case may be used as a representative of the junction temperature of the individual LEDs. The junction temperature of the LEDs is measured or estimated so as to determine the required power of the LEDs that provide the desired color temperature. Another alternative to estimate the junction temperature is to use the forward voltage drop across the LED, as described in U.S. Pat. No. 7,052,180, as the forward voltage drop across a LED varies approximately linearly with the temperature.
Controller 8 is provided with a microcontroller, digital memory and digital signal processors. As shown in the flow chart of
The CCT value of the white LEDs is changed by changing the junction temperature Tj of the LED. With an increase in junction temperature, the white LED CCT shifts to a higher value, as shown in
By changing the power through the LED, the light output (luminous flux F) as well as the junction temperature Tj will change. The light output is changed by a Pulse Width Modulation based control function with constant forward current Ifmax. Ifmax is equal for all LEDs and is independent from the LED junction temperature Tj. A change of the duty cycle of the PWM signal will change Ifav. The power dissipation in the LED can be described as P=Ifav*Vf, wherein Vf is the forward voltage and is temperature dependent (Tj=Ta+P*Rj-a). If the power is increased the junction temperature will also increase.
The total luminous flux of the system, which is the summation of the luminous flux of each white LED light source in the system, is kept constant, or alteratively, in a luminary which can be dimmed, the total luminous flux will be kept at the desired value. In order to reduce the CCT difference between both LEDs while at the same time maintaining a constant total luminux flux of the luminary, the relevant parameters are changed in opposite directions. This means that the junction temperature of one LED is increased by increasing of the luminous flux of that LED. For the other LED the junction temperature is decreased by decreasing the luminous flux of that LED, as shown in the example of
For this example, the initial CCT values at a certain junction temperature as measured in the factory for both white LEDs are stored in memory:
The total luminous flux is F1+F2=2F.
However, the CCT1 and CCT2 values of both white LEDs have to be more or less equal in order to avoid visible colour errors. This is achieved by changing the individual power levels of LED1 and LED2. At the same time, the two different power levels through LED1 and LED2 have to be chosen such, that the total luminous flux of the luminaries remains constant (F1+F2=2F). As shown in
These two constraints force the controller in accordance with the algorithm of
Another example is given in
Although the invention is described herein by way of a preferred embodiment as an example, the man skilled in the art will appreciate that many modifications and variations are possible within the scope of the invention.
