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How to Solve The LED Heat Problem?LED Temperature Feedback Control Using Heat

The use of high brightness LED lighting manufacturers require the use of the maximum allowable current drive , but the average life of LED is highly dependent on the operating temperature . Operating temperature rose by only 10 ° C can make life shortened by half. This situation forces the designer to adjust the current must be reduced , sacrificing brightness to prolong life. If the required LED in higher ambient temperature, the current must be minimized to further reduce the chip temperature environment to guarantee the life . However, due to the presence of the upper temperature limit , doing so will reduce the low ambient temperature range of illumination . Essentially, we have achieved by reducing the brightness temperature environments.

Detecting the current through the LED array voltage of the current detecting resistor R7 is adjusted and used as feedback control of the controller , e.g. : TPS40211 like. Operational amplifier circuit ( including R9), the feedback node (FB) injected a current regulator to reduce current or sink current to increase its current regulation . 0.26V FB node voltage is maintained constant . Improving the operational amplifier output (TP1) of the voltage that must be compensated by reducing the voltage R7 , reducing the LED current. When the op-amp output exactly as 0.26V , the injection current is zero , and LED regulator unaffected.

Thermistor RT1 is a negative temperature coefficient (NTC) devices. 25 ° C under its nominal resistance of 10K ohms, but the increase in the -40 ° C down to 300K ohms or more, and then at 100 ° C under reduced to 1K ohm , and is based on a nonlinear way . Resistor R8 and R10 to 5V bias voltage is lowered close to the FB voltage , and the value of R9 control current with changes in temperature decreases the speed . Use is preferably adjustable bias voltage is very important because the accuracy is affected by the bias circuit tolerance . Resistor R9 must be placed as close to the current mode boost controller is placed , the purpose is to minimize the noise sensitivity . Using thermal epoxy , the thermistor RT1 as close to the center LED is connected on the PWB .

LED and thermistor only work in this temperature range. Thermistor temperature detected graphed , compared with the ambient temperature. We will calculate the temperature of the LED chip plotted , which is equal to the temperature of the circuit board with the power of each LED is multiplied by the thermal resistance from junction to case (8 ° C / W). We can see that under high ambient temperature conditions , the operational amplifier circuit will reduce the LED current, and LED chip temperature close to the temperature of the LED circuit board . In this case , LED board temperature close to the ambient temperature , since the LED current is almost zero. Thus , the temperature stability of the LED chip can be realized without change. RT1 nonlinearity is the reason for the highest temperature LED current is rapidly changing . TP1 temperature " control voltage " is also plotted , and well matched with the expected value .

Summary

High-temperature environments , the use of high-power LED driver LED brightness degradation will shorten the life of this case thermal feedback circuit will be very useful. It can reduce the LED current , thereby reducing power consumption of the LED , and ultimately reduce the temperature rise of LED . Brightness of the LED with temperature decrease , thus requires constant luminance in some applications this method may not be practical . However , this circuit can effectively extend the service life of LED in extreme environments.


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