LED light bar bulb junction temperature measurement

The blister bulb made of LED strips can realize 360 ​​degree full angle light source without adding a lens, which makes people feel the return to traditional incandescent lamps. LED light bar lights have many application advantages, and there is a big whirlwind in the market, which is quickly accepted by users. The LED light bar lamp combines the traditional tungsten filament bulb manufacturing technology with the emerging LED technology, and uses the glass bubble inflation technology to seal the LED light bar in the glass bulb and fill the mixed gas inside to make it dissipate heat. In order to reduce the LED junction temperature, reduce light decay, and extend the life.

The junction temperature of LED strips under working conditions is the main factor affecting various performance indicators, and is also a key factor that seriously affects LED light decay and service life. These parameters are extremely important lighting quality evaluation indicators for general lighting. A consensus has been reached in the lighting industry.

The LED light bar is sealed in the glass bulb filled with the mixed gas. Only the two guide wires of the positive and negative electrodes are connected to the driving power supply outside the sealed glass bulb. What method is used to test the junction temperature of the LED light bar lamp? Measurements commonly include pin temperature measurement, infrared imaging, and voltage methods. Obviously, the pin method cannot measure the temperature because the thermocouple cannot be bonded to the LED strip in the closed glass bulb; the infrared imaging method cannot detect the temperature of the surface of the LED strip through the glass casing; The law can solve the problem of testing. Recently, the company's LEDT-300B full-lamp thermal management system using the voltage method as the test principle has measured and analyzed an LED light bar bulb manufactured by a certain company, and has obtained many interesting data to share with you. see picture 1.

Test principle: voltage method, first make the K line calibration of the sample LED strip, obtain the corresponding relationship between the temperature T and the Vf value of the light bar; then use the self-contained driver to illuminate the sample light to be tested as the heating source, after lighting the bulb, The change of junction temperature with respect to time was continuously measured in real time, and a continuous curve was drawn on the computer interface, and the reference point temperature and ambient temperature curve of a thermocouple bonded to the glass bulb were drawn at the same time.

From the above figure, we can see that the LED junction temperature in the bulb gradually rises after the bulb is turned on (the first period), and the temperature reading is 122 °C when it reaches a steady state. Continue the test. At this time, artificially destroy the bubble-sealing port, let the mixed gas in the shell be released, and return the internal pressure to the atmospheric pressure. In the figure, the junction temperature curve starts to rise (the second time period) and gradually reaches the new stable platform. The junction temperature reached 160 ° C and the junction temperature was clearly too high. As can be seen from the above figure, the ambient temperature is basically above and below 26 °C throughout the experiment, while the reference point temperature of the glass bulb surface is maintained at about 40 °C under both test conditions.

From this experiment, you can see the following aspects:

1. When the ambient temperature is 26 °C, the junction temperature after normal lighting is 122 °C. When the ambient temperature rises to 40 °C or higher, the junction temperature may be as high as 140 °C or higher. Is the junction temperature high or low? ? What is the impact on light decay and longevity? We believe that this may depend on the following aspects: the product designed by the LED strips allows junction temperature; the data obtained by light decay, life test (ie life test).

2. In the second period, when the pumping (charge) gas pipe is destroyed and the mixed gas in the lamp housing is replaced by air, the junction temperature rises to 160 ° C, and the temperature rises by nearly 40 ° C relative to the first period, and the glass shell surface at this time The reference point temperature remains substantially unchanged from the first stage state.

The data proves that a mixed gas with a certain pressure is filled in the glass bulb, and the gas molecules are used to form a heat exchange between the LED light bar and the glass shell, and the effect of lowering the junction temperature is very effective and very significant;

3. The temperature change at the reference point on the surface of the glass envelope is not sensitive because of the large distance between the heating element and the measuring point and the temperature rise of the second stage LED strip. Therefore, the relationship of the temperature difference is not reflected; nor can the temperature of the glass shell be measured by hand or other methods to correctly reflect the heat dissipation effect of the gas.

In order to make LED lighting products enter the civilian field, it is a universally accepted, affordable and reliable lighting product that is popular among the people. The most effective way to achieve high light efficiency, low light decay and long life is to know the junction temperature. In the state of the bulb, the driving performance, heat dissipation conditions and process are comprehensively designed and comprehensively evaluated. Only when the LED is operated at a reasonable temperature can a reasonable cost and a reasonable life be achieved.

The junction temperature of the LED in the lamp is the most concerned issue for the R&D personnel. The LEDT-300B whole lamp thermal management system provides a way to understand the LED junction temperature of the light bar bulb, and can directly measure the brightness of the LED junction temperature Tj. The most effective means. The ability to correctly know the temperature of the most core and most essential LED strips in the bulb, and to see the graph of the temperature curve of the bulb strip Tj in real time, can comprehensively evaluate the overall performance of the product. The junction temperature curve of the instrument is real-time and continuous, which reminds us immediately that the whole lamp junction temperature measurement method can be used as an auxiliary means of design, process, manufacturing, quality control, for example:

1. Does the drive match its load? The output current directly affects the junction temperature of the bulb. The external drive power supply regulates the input current supplied to the LED strip, increases or decreases the current, observes the corresponding curve displayed by the computer, and finally reaches the reasonable junction temperature target value determined by the design. .

2. The contribution of the mixed gas in the shell to the reduction of the junction temperature, the composition of the mixed gas, the ratio, and the pressure on the heat dissipation effect. By changing the pressure and type of gas through the glass exhaust pipe, the junction temperature curve in the measurement will change immediately to reflect the test effect, which will promote the development of new products, and can quickly evaluate the design change to reduce the junction temperature in the shortest time. Effect.

3. Which LED strips are used for the best photoelectric effect of the product, and the relationship between light decay and lifetime is most reasonable.

4. Change the input voltage of the LED light bar bulb, for example, 10% rise or fall, simulate the fluctuation of the mains voltage. At this time, the corresponding curve of the junction temperature also reflects the safety state of the product when the voltage fluctuates. (See Figure 2)

5. Use the external force to break the glass bulb, observe the display screen, and you can see the rapid change of the junction temperature, so that it stops at the heat balance line. In this way we can know the safety status of the product in special circumstances.

6. The new process and new material application will quickly evaluate the influence of LED junction temperature and heat dissipation.

7. In the manufacturing process, important quality control points, such as whether the inflation is normal, whether there is air leakage, etc., carry out on-line inspection to ensure the implementation of the process, and improve the pass rate of the semi-finished products flowing into the assembly line.

The junction temperature of the above products in various states can be measured by using LEDT-300B. In the same way, various other types of whole lamps can be analyzed in various ways. The company has carried out junction temperature analysis on LED street lamps, spotlights, straight tube lamps, etc., due to space limitations, will be re-described in future articles.