Thermal management in color variable multi-chip led modules

One of the main advantages of LED technology is the possibility to build efficient color variable lighting systems. In order to apply these systems in illumination applications in high power, color variable LED modules are developed. These modules contain different LED colors, e.g. red, green and blue dice, which can be combined to generate colored and white light. For optimal color mixing, the dice of the different colors are mounted close to each other, resulting in an increased thermal load of the LED dice in the module. One of the key issues in color variable LED applications is to maintain a defined color point over life and over all flux levels. The relevant optical properties of LEDs, like luminous flux and wavelength, depend on the junction temperature of the LED. One of the ways to maintain a stable color point is to compensate for these temperature dependencies. In order to be able to perform this compensation, one has to know the junction temperature of the different dice under all loads and the relation between the luminous flux and the wavelength and this junction temperature. In this paper we present a thermal design method of a multi-chip LED module that is able to handle an increasing thermal load up to 20 Watt. Further, we present a compact model to estimate the junction temperature of the different dice at an arbitrary load. This model is used in the color control system to calculate the junction temperatures under load and adjust the loads of the different dice accordingly to maintain a defined color point. The model is validated with transient tester measurements. Application of this model results in a considerable improvement in color stability