If the junction temperature is TJ, the ambient temperature is TA, and the power consumption of the LED is PD, the relationship between RJA and TJ, TA, and PD is:
RJA=(TJ-TA)/PD (1)
The unit of PD in the formula is W. The relationship between PD and LED forward voltage drop VF and LED forward current IF is:
PD=VF×IF (2)
If the temperature TC of the LED thermal pad has been measured, then equation (1) can be written as:
RJA=(TJ-TC)/PD+(TC-TA)/PD
Then RJC=(TJ-TC)/PD (3)
RBA=(TC-TC)/PD (4)
In the heat dissipation calculation, when the high power LED is selected, the RJC value can be found from the data data; when the forward current IF of the LED is determined, the PD can be calculated according to the VF of the LED; if the temperature of the TC has been measured, Then, according to the formula (3), TJ can be obtained.
Before measuring TC, first make a test board (select a certain PCB, determine a certain area), solder the LED, input IF current, etc. After stabilization, measure the cooling pad temperature TC of the LED with a K-type thermocouple point thermometer.
In equation (4), TC and TA can be measured, and PD can be found, and the RBA value can be calculated. If TJ is calculated, substituting (1) can be used to find RJA.
This method of testing and calculating the TJ is based on the use of certain PCBs and a certain heat dissipation area. If the calculated TJ is less than the required (or equal to) TJmax, then the selected PCB and area are considered appropriate; if the calculated TJ is greater than the required TJmax, the PCB with better heat dissipation performance is replaced, or the heat dissipation area of ​​the PCB is increased.
In addition, if the RJC value of the selected LED is too large, it is also possible to replace the high-power LED with better performance and smaller RJC value so that the calculated TJ ≤ TJmax is satisfied. This is illustrated in the calculation example.
Edit: Sophy
Rice Mill,Rice Mill Machine,Small Rice Milling Machine,Rice Mill Equipment
Hunan Furui Mechanical and Electrical Equipment Manufacturing Co., Ltd. , http://www.frcornthresher.com