Thermal Design of the High Brightness Thin LCDs

Often, Medical Monitors have a special cabinet with closure to avoid liquids to hamper the monitor functionality and for cleaning requirements but limiting the airflow. The thermal management of the electronics and LCD panel should be designed with more accuracy and it is a limiting factor of the monitor specification. Introducing LED backlight in Medical Monitors not only allows a mercury free system (Green product) but provides the capability to match the white colour among similar Medical Monitors. For thermal matters of the LCD backlights, one of the main differences between LEDs and CCFLs is that, the latter ones have an optimal operational temperature: CCFLs (and HCFLs as well) work better when they are hot. On the other hand, heating produces permanent damage to the electrodes, phosphor and gas mixture in the CCFL tube. This effectively reduces the lamp efficacy. For LEDs the situation is just the opposite: for higher efficacy, the light source temperature should be lower, which goes along with the increasing of the lifetime of the LEDs. Moreover, red, red/orange and amber emitters are more sensitive to increases of junction temperature than other colors. Also, the wavelength of the emitter radiation shifts to higher wavelengths when the junction temperature rises. Red, Red/Orange and Amber emitters are the most sensitive to this effect. Another difference is the construction of the light sources. The main heat transfer in CCFLs occurs via its glass tube, which is a light emitting surface as well. This makes difficult the realization of an efficient cooling without blocking the emitted light. Additionally, glass is famous for its low mechanical resistance against thermal stress. In case of LED, the heat transfer and light emitting surfaces are different. That makes easy the light and heat extracting designs. In this paper, we report on the thermal design of a Medical Monitor based on LED backlight with brightness of 1000 cd/m2. The research has been carried out with the aim to define the optimum backlight architecture for an 18” size monochrome display. The design is based on an edge-lit configuration, with two or one row of high power LEDs, and a passive cooling by means of heat sinks or cooling plates