The use of thermal feedback reduces the temperature rise of the LED

Date：2012-09-15

Using LED high brightness lighting requirements using manufacturers allow maximum current drive, but the average life expectancy of LEDs is highly dependent on temperature. Operating temperature rose by only 10 ° C can make life shortened by half. This situation forced designers to adjust the current must be reduced, sacrificing brightness to extend the service life. If required LED working under high ambient temperatures, the need to further reduce the current to minimize the environment to the chip temperature rise, in order to ensure the service life. However, due to the presence of the upper temperature limit, this will reduce the brightness of the illumination of the medium and low ambient temperature range. Essentially by lowering the brightness to achieve the work of high-temperature environments. Figure 1 shows the LED driving circuit that uses a thermistor control operational amplifier (op amp), the temperature rise of the LED circuit board to reduce the driving current.

    LED array current is adjusted by the voltage of the detected current detection resistor R7, and is used as a feedback control of the controller, for example: TPS40211. The operational amplifier circuit (R9), to inject a current to the feedback node (FB) regulating current in order to reduce, or to increase the regulation current pour its current. FB node voltage is kept constant 0.26V. R7 voltage must be reduced to improve the voltage of the op-amp output (TP1), to obtain the compensation, so as to reduce the LED current. When the operational amplifier output just as 0.26V, the injection current is zero, while the LED adjustment will not be affected.

    Thermistor RT1 is a negative temperature coefficient (NTC) devices. 25 ℃ It has a nominal resistance of 10K ohms, but the increase at -40 ℃ to 300K ohms or more, and at 100 ℃ are reduced to less than 1K ohm, and is in a nonlinear way. Resistors R8 and R10 5V bias voltage close FB voltage reduction, the value of R9 is to control the speed of the current reduced with the high temperature variations. The use of better adjusting the bias voltage is very important, because the accuracy of the circuit is subject to the influence of the bias tolerance. Resistors R9 must be placed as close to the current mode boost controllers placed, the purpose is to minimize the noise sensitivity. Use thermal epoxy the thermistor RT1 as close as possible to the central LED on the PWB connection.

    Figure 2 shows the data obtained under various temperature conditions. Only LED and thermistor in this temperature range. The thermistor detects the temperature is plotted as a graph, compared with the ambient temperature. We will calculate the obtained LED chip temperature is also plotted, which is equal to the temperature of the circuit board coupled to each LED of the power multiplied by the junction to the chassis (8 ° C / W) Thermal impedance. We can see that under high ambient temperature conditions, the operational amplifier circuit to reduce the LED current, the LED chip temperature close to the temperature of the LED circuit board. In this case, the temperature of the LED circuit board close to the ambient temperature, since the LED current is almost zero. This can be achieved LED chip temperature stability did not change. The RT1 nonlinear highest temperature LED current rapidly changing. The TP1 temperature "control voltage" also been drawn into the diagram, and well matched with the expected value.

    Summary

    High temperature environment, using the drive LED brightness, high power LED causes degradation and shorten the life of the thermal feedback circuit in this case it will be very useful. It can reduce the LED current, thus reducing the power consumption of the LED, and ultimately reduce the temperature rise of the LED. Reduced with the temperature rise due to LED brightness, so this method in some applications requiring constant brightness may not be practical. However, this circuit can extend the effective life of the LED in extreme environments.

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