How to solve the LED heat problem? Use of thermal feedback control LED temperature rise

LED high brightness lighting requires the use of vendors to allow the maximum current drive, but the average life expectancy of the LED is highly dependent on temperature. Operating temperature rose to 10 ° C only can it shorten the life span in half. This situation has forced designers to adjust the current must be reduced to the expense of brightness to prolong the service life. If the LED in high ambient temperatures, you must further reduce the current to minimize the environment to the chip temperature rise, to ensure that the service life. However, due to the existence of the upper temperature limit, doing so will reduce the brightness of the lighting in low ambient temperature range. In essence, we have by reducing the brightness to achieve the high-temperature environments work. Figure 1 shows a LED driver circuit uses a thermistor to control the operational amplifier (op amp), reduce the drive current in the LED circuit board temperature rise.

LED array current detection current detection resistor R7 voltage regulation, and used as the feedback control of the controller, for example: TPS40211. The op amp circuit (R9), to reduce the adjusted current injected into the feedback node (FB) a current, or poured into the current to increase the regulated current. FB node voltage remains constant 0.26V. Improve the voltage of the op amp output (TP1), by reducing the R7 voltage to be compensated, thereby reducing the LED current. When the op amp output just as 0.26V, the injection current is zero, and LED regulator will not be affected.

Thermistor RT1 is a negative temperature coefficient (NTC) devices. 25 ° C and nominal resistance of 10K ohms, but increased to more than 300K ohms at -40 ° C at 100 ° C down to below 1K ohm, and is in a nonlinear way. Resistor R8 and R10 reduce the 5V bias voltage close to the FB voltage, the value of R9 to control the speed of the current decreases with the high temperature changes Better regulate the bias voltage is very important, because the accuracy of the circuit by the bias tolerance limits. Resistor R9 must be as close to current-mode booster controller placed the aim is to minimize the noise sensitivity. Using thermal epoxy, thermistor RT1 as much as possible close to the central LED PWB connection.

The only LED and the thermistor in the temperature range. Detected by the temperature of the thermistor is plotted as graphs, compared with the ambient temperature. We will calculate the temperature of the LED chip is also plotted, equal to the power of the circuit board temperature coupled with each LED to the chassis thermal impedance (8 ° C / W) multiplied by the node. We can see that high ambient temperature conditions, the operational amplifier circuit will reduce the LED current, the LED chip temperature close to the temperature of the LED circuit board. In this case, the LED board temperature close to ambient temperature, because the LED current is almost zero. In this way, you can achieve no change in the temperature stability of the LED chip. The RT1 nonlinear LED current rapid change in the maximum temperature. TP1 temperature control voltage is also drawn into the map, and well matched with the predicted value.

Summary

High temperature environment, user-driven high-power LED will LED brightness degradation and shorten the life of this case, the thermal feedback circuit 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. LED brightness is reduced with temperature rise, so this method in some applications require a constant brightness may not be practical. However, this circuit can extend the useful life of the LED in extreme environments.