In LCD backlighting, multicolour LEDs are used to create a white light with a colour temperature of 6500K. This requires precise colour matching throughout the display and to achieve this precision, the thermal and optical design must be optimised for each system.
Temperature effects on LEDs
The dominant wavelength, luminosity and forward voltage of the LED are all dependent on the junction temperature. Because the colour and brightness properties are sensitive to temperature, it is essential to have control over the thermal performance of the LED lighting system.
In addition to changes in light intensity, the dominant wavelength or radiated colour of the LED will drift slightly with temperature. Even though the drift is slight, a large junction temperature change results in a noticeable change in the colour temperature of the backlight.
The simplest method of colour compensation is to use junction temperature feedback, while the most complex method is to use colour sensor feedback.
The temperature control method first determines the LED junction temperature by measuring the board temperature and the LED forward voltage and current. Using the vendor’s temperature characteristic plots, the colour and brightness properties of the LEDs are approximated and refreshed to be consistent with the new temperature.
To achieve high colour accuracy, the mixed colour point of the LED is recalculated using the new LED properties from the straight line approximations.
There are mixed-signal microcontroller available which allow the design to measure the LED parameters, calculate a mixed colour point and generate drive signals.
Colour sensor feedback
Colour sensing feedback achieves greater accuracy by using a colour sensor to measure the light radiated by the LEDs. The light is measured using photodetectors tuned to wavelengths that correspond to red, green, blue and wideband (ambient) visible colour spectra.
The sensor communicates the colour spectra data back to the mixed-signal device which processes the information with its MCU. If the colour or the intensity level measured is incorrect, the processor corrects for the error by changing the intensity level of each LED in the system.
Not only is the placement of the sensor important to colour uniformity, but the temperature rise for each LED in the backlight should be the same.
The main issue with the colour sensor method is that as the temperature of an LED increases the luminous intensity degrades.
Naturally, as the sensor reports back a lower intensity level, the processor will try to increase the intensity of each LED. This is accomplished by driving the LEDs harder, but as the power dissipated increases then more heat is generated by the LED.
As the heat generated increases, the junction temperature of the intensity degrades and the processor drives the LEDs harder until they reach their thermal limit and prematurely fail. This thermal runaway problem happens in sensor feedback systems that do not monitor or manage heat.
A reliable LED protection technique is to design an electromechanical system with a controlled temperature rise for a given power dissipation.
One way of controlling temperature rise is to use high thermal conductivity heat spreader materials. Metal core PCB and natural graphite heat spreaders are proven thermal management materials that will, when used properly, control temperature rise and maintain the temperature uniformity of a solid state lighting system.
Thermal management planning starts with modelling a system with a thermal resistance network and ensuring the temperature rise of the system is within the specifications of the LEDs. To generate a thermal resistance network for a design, first consider the source of the heat – in this case the LED.
Next consider the materials between the heat source, the ambient (analogous to earth ground in electrical systems) and finally, the ambient temperature itself. In thermal models, the power dissipation of the LED is modelled as a current source, the thermal resistance is modelled as a resistor and the ambient temperature is modelled as a voltage source.
The LED junction temperature will be lower if the thermal impedance is smaller and likewise with a lower ambient temperature. To maximise the useful ambient temperature range for a given power dissipation, the total thermal resistance from junction-to-ambient must be minimised.
The values for the thermal impedances, with the exception of that for convection, vary widely depending on the material or component supplier. This can be as much as 2.6-18°C/W depending on the LED manufacturer.
Designers attempting to create high power LED systems are well served by understanding the thermal problems of LEDs.
Smart thermal management will increase the operating temperature range and thermal monitoring will maintain the accuracy of LED products. And thermal monitoring of these systems with programmable mixed signal controllers has its advantages over conventional circuits.
Patrick Prendergast is senior applications engineer at Cypress Semiconductor