At present, in architectural and entertainment lighting applications more and more solid state lighting based on Light Emitting Diodes (LED) is used. LEDs or LED units have several advantages over incandescent lighting, such as higher power to light conversion efficiency, faster and more precise lighting intensity and color control. In order to achieve this precise control of intensity and color from very dim to very bright light output, it is necessary to have accurate control of the forward current flowing through the LEDs.
In order to provide said forward current through the LED or LEDs, a converter (or a regulator such as a linear regulator) can be used. Examples of such converters are Buck, Boost or Buck-Boost converters. Such converters are also referred to as switch mode current sources. Such current sources enable the provision of a substantially constant current to the LED unit. When such an LED unit comprises LEDs of different color, the resulting color provided by the LED unit can be modified by changing the intensity of the different LEDs of the unit. This is, in general, done by changing the duty cycles of the different LEDs. Operating the LEDs at a duty cycle less than 100%, can be achieved by selectively (over time) providing a current to the LEDs, i.e. providing the LEDs with current pulses rather than with a continuous current. By appropriate selection of the duty cycle a required color and intensity can be provided. In order to provide a high resolution with respect to the intensity or color of the light source, a precise control of the current pulses is required to enable high-resolution LED lighting color or white mixing control.
In practice, a current source will not instantaneously provide an appropriate current but may need some time to reach the current set point, especially in the case of switch mode current sources. As such, when an LED unit is controlled to operate at a certain duty cycle, in order to generate a required intensity and/or color, the color or intensity that is actually obtained may be different from the required values because the actual current or current profile through the LEDs does not correspond to the required or expected values. This effect may occur when a current through the LED is turned on as well as when the current is turned off. In practice, turning the current through an LED on or off can be realized by opening or closing a low impedance connection parallel to the LED thereby redirecting the current either through the LED or through the low impedance connection. Opening or closing the connection can e.g. be realized using a FET or a MOSFET. It can further be noted that a mismatch between a required characteristic and an actual characteristic may also be due to aging or thermal influences.
Due to the mismatch between the required and the actual characteristic, the contrast that can be obtained with respect to e.g. color or intensity, is reduced. This can be understood as follows: In practice, the contrast with respect to e.g. the intensity of an LED can be represented by the minimal intensity that can be provided. Due to the transient behavior of the converter powering the LED or e.g. manufacturing tolerances affecting the LED characteristics, a large spread can be observed between different LEDs of the same product line. Therefore, in order to ensure that all LEDs of the same product line perform in the same way, the minimum intensity may need to be set comparatively high in order to ensure substantially the same behavior of different LEDs. As such, tolerances and transient behavior may affect the contrast available for the product line.
Furthermore, in the case of switch mode current sources, the internal switch mode control frequency is, in general, independent of the pulse turn-on or turn-off moment. This means that for short pulses, under about 5 times the length of the switcher cycle, the current pulse may have an uncertain start that leads to large differences in actual current output.
It may be acknowledged that precise current control may be achieved in the current state of the art by using special components with low temperature drift and high accuracy, thereby alleviating or mitigating some of the effects mentioned. Such an approach is however rather expensive and therefore not preferred.
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