Silanex's structural material and thermal analysis laboratory can get the precise TJ under the whole-light working condition, helping customers to confirm service time of lamps. Thermal efficiency is the most effective way to improve whole-lamp's thermal performance by reducing system thermal resistance to convert condutive efficiency into power efficiency and improve whole-lamp's service life and reliability.
The meaning of whole-lamp TJ test
1. We can test directly inside the LED lamps P-N junction temperature through TM-one, which is unable to be obtained by thermocouples or infrared thermal imager.
2. Test the TJ with LED build-in driving power in the actual work condition in order to restore real TJ of application state, avoiding influencing TJ assessment due to the external power source.
3. Evaluate thermal conductivity performance of whole-lamp parts material and structure by test TJ parameters.
1. Calibration of LED source
Use basic features of semiconductor: TJ rises and forward voltage drops; under specific current, the temperature relation between LED positive pressure and LED chip is a linear. As long as we test two or more Vf value of temperature point, we could confirm the relation slope between LED voltage and temperature, that is, voltage temperature coefficient K value, unit mV/℃.
K value can be obtained by formula
Linear relationship between LED joint temperature and voltage:
(Vf2-Vf1) / (Tj2—Tj1)=K
Put LED lighting into heating box and heat LED with ladder method in condition of no electricity. If set heating steps as 40℃, 70℃ and 100sheshiddu5, it demonstrates LED has arrived heat balance of certain setting point when LED forward voltage dropping to stability. It can be confirmed that temperature T is equal to TJ. Temperature T is LED light source temperature sensitive point bonding to thermocouple and we can begin to test Vf. Input a set of measuring current If to LED and makes the current as small as possible on the premise of meeting measuring requirements (such as 1 mA) and measure the forward voltage dropping Vf under the temperature. Repeat it until finish all the temperature set point measurement. With the measurement of LED forward voltage (Vf1-Vfn) and setting point measurement, we can obtain the relation curve between forward voltage Vf and TJ. As long as get the LED Vf value, we can read the corresponding TJ value in the following picture.
2. TJ test of whole-lamp
Under normal working position and condition with rated voltage and frequency, From LED lighting to heat balance, power of the drive instantly, input a measuring current If (such as 1mA) under K-coefficient calibration from measuring port and obtain a set of forward voltage Vf under continuous temperature changes. Read a set of TJ value above to get the continuous curve of LED lighting from lit to test closure.
The test is all based on international standards
1. EIA/JESD51-1 www.jedec.org/standards-documents/results/JESD51-1
2. IES LM-85
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