In a LED lighting device, a plurality of LEDs may be applied. In a LED lighting device which is either designed for switching on and off, or designed for dimming applications, LEDs of different types may be combined to obtain a light output having a predetermined color at steady state operating conditions. As an example, when combining InGaN type LEDs with AlInGaP type LEDs, an efficient LED lighting device in a low correlated color temperature, CCT, range (2,500-3,000 K) can be made.
It is known that the luminous flux output, also referred to as light flux output, light output, or lumen output, of a LED changes as a function of its junction temperature. When the junction temperature increases, the luminous flux output decreases. The phenomenon will be referred to as luminous flux output degradation.
When using different LED types in a lighting device, a problem arises when the LEDs of one type show different luminous flux output degradation as a function of their junction temperature than LEDs of another type. Different luminous flux output degradations may result in different proportions of luminous flux output from the different LED types in the total light output of the LED lighting device, and consequently, when the LEDs of different type emit light of different color, this may lead to the lighting device emitting a different color of light at different junction temperatures of the LEDs. This is undesirable.
Solutions to this problem usually propose a feedback loop with a temperature sensor and a micro-processor to control an electric quantity of the power supply to at least one or some of the LEDs to maintain the color of the light output by the lighting device within a predetermined range by keeping the ratio of the luminous flux output from the different types of LEDs substantially constant at different junction temperatures, as measured by the temperature sensor.