It is very troublesome to measure the temperature of high-power LED lamp beads. Both thermal resistance and junction temperature will affect the temperature of high-power LED beads. It is very difficult to measure the thermal resistance of high-power LED.
1. principle
The conductivity of semiconductor materials is heat sensitive, and changing the temperature can significantly change the number of carriers in the semiconductor. The band gap usually decreases with the increase of temperature, and has a good linear relationship with the change of temperature above room temperature. It can be considered that the forward voltage drop of semiconductor devices has a linear relationship with junction temperature: Δ VF = k Δ TJ (k: coefficient of forward voltage drop changing with temperature). From the formula (1) and its derivation, we can see that the thermal resistance of high-power LED (from node to environment) is: rth ja =δVF/ (. As long as the change of forward voltage drop Vf of LED is monitored, the K value can be obtained and the thermal resistance can be calculated.
2. Measuring system
The thermal resistance test system requires that the temperature control accuracy of the thermostat used in the test is 65438 0℃ and the voltage accuracy is 65438±0mv. In the figure, R 1 is a shunt resistor, and R2 is used to adjust the current flowing through the LED. By adjusting the resistors R 1, R2 and the output of the constant current source itself, the current flowing through the led can be accurately controlled, so that the current flowing through the LED remains constant during the whole test.
3. Measure the temperature coefficient k (1) during the test: a. Put the LED in a thermostat with the temperature of Ta for enough time to reach thermal balance, at this time TJ1= Ta; B. Quickly measure VF 1 of LED with low current (the heat influence generated by it can be ignored, such as if' =10ma); C. place the LED with the temperature of ta' (ta' >: Ta) in the thermostat for a long time enough to achieve thermal balance, tj2 = Ta '；; D, repeat step 2 to measure Vf2；; E. calculate k: k = (vf2-Vf1)/(tj2-TJ1) = (vf2-VF1)/(Ta'-ta) (2) measure the change of the input electric power when it is heated: a. put the LED in a temperature of ta. C. measure that temperature (highest point) ts of the LED radiator; D. cut off the input power supply, and immediately (< < 10ms) proceed to step b of (1) to measure Vf3. (3) Data processing: △ VF = VF3-VF 1, take PD = 0.65 * IF * VF to calculate: RTHJA = △ VF/(k * PD).
rth sa =(Ts-Ta)/Pd =(Ts-Ta)/(0.65 * IF * Vf)rth js = rth ja-rth sa
The reasons for the death of high-power LED lamp beads are as follows:
1. Static electricity damages the LED chip, making the PN junction of the LED chip invalid, increasing the leakage current and turning it into resistance.
2. The internal connection leads of LED lamps are disconnected, which leads to the LED lamps' death due to no current flowing, which will affect the normal work of other LED lamps, because the working voltage of LED lamps is relatively low (the working voltage of red, yellow and orange LEDs is 1.8V-2.2V, and the working voltage of blue, green and white LEDs is 2.8-3.2V), and generally series and parallel connections are adopted. The more LED lights are connected in series, the greater the impact. As long as one of the LED lights is open, the whole string of LED lights in the series circuit will not light up, which shows that this situation is much more serious than the first situation.
There are many reasons for the death of high-power LED bulbs, which cannot be listed one by one. Dead lights may occur in all aspects of packaging, application and use. How to improve the quality of LED products is a problem that packaging enterprises and application enterprises should attach great importance to and seriously study. From the selection of chips and brackets to LED packaging, the whole process should be operated according to ISO2000 quality system. Only in this way can the quality of LED products be comprehensively improved, and long life and high reliability can be achieved. In the applied circuit design, selecting varistor and PPTC components to improve the protection circuit, increasing the number of parallel circuits, adopting constant current switching power supply and increasing temperature protection are all effective measures to improve the reliability of LED products. As long as the packaging application enterprises operate in strict accordance with ISO2000 quality system, the quality of LED products will definitely reach a new level.