How to measure the junction temperature of LED lamp beads?
The junction temperature seems to be a temperature measurement problem, but the junction temperature to be measured is inside the LED, and it is impossible to put a thermometer or thermocouple into the PN junction to measure its temperature. Of course, its case temperature can still be measured with a thermocouple, and then based on the given thermal resistance Rjc (junction to case), its junction temperature can be calculated. , But after installing the radiator, the problem becomes more complicated. Because the LED is usually soldered to the aluminum substrate, and the aluminum substrate is mounted on the radiator, if only the temperature of the radiator shell can be measured, a lot of thermal resistance values must be known to calculate the junction temperature. Including Rjc (junction to case), Rcm (case to aluminum substrate, in fact, it should also include the thermal resistance of the film printed plate), Rms (aluminum substrate to radiator), Rsa (radiator to air), of which there is only one Inaccurate data will affect the accuracy of the test. Figure 3 shows a schematic diagram of the various thermal resistances from the LED to the heat sink. It incorporates a lot of thermal resistance, making its accuracy even more limited. In other words, the accuracy of estimating the junction temperature from the measured surface temperature of the heat sink is even worse.
Fortunately, there is an indirect method of measuring temperature, which is to measure voltage. So which voltage is the junction temperature related to? What about this relationship?
We must first start with the volt-ampere characteristics of the LED.
4. Temperature coefficient of LED volt-ampere characteristics
We know that LED is a semiconductor diode, it has a volt-ampere characteristic like all diodes, and like all semiconductor diodes, this volt-ampere characteristic has a temperature characteristic. Its characteristic is that when the temperature rises, the volt-ampere characteristic shifts to the left. Figure 4 shows the temperature characteristics of the volt-ampere characteristics of the LED.
Assuming that the LED is supplied with a constant current of Io, when the junction temperature is T1, the voltage is V1, and when the junction temperature rises to T2, the entire volt-ampere characteristic shifts to the left, the current Io remains unchanged, and the voltage becomes V2. These two voltage differences are removed by temperature, and the temperature coefficient can be obtained, expressed in mV/oC. For ordinary silicon diodes, this temperature coefficient is approximately -2mV/oC. But most of the LEDs are not made of silicon materials, so its temperature coefficient has to be measured separately. Fortunately, most of the LED manufacturers' data sheets give their temperature coefficients. For example, for Cree's XLamp7090XR-E high-power LED, its temperature coefficient is -4mV/oC. It is 2 times larger than ordinary silicon diodes. As for the American Bridgelux LED Array (BXRA), more detailed data is given.
However, the scope of the data they gave is too broad, so that it loses the value of use.
In any case, as long as the temperature coefficient of the LED is known, it is easy to calculate the junction temperature of the LED from the measurement of the forward voltage of the LED.
5. How to measure the junction temperature of LED in detail.
Now take Cree's XLamp7090XR-E as an example. To illustrate how to specifically measure the junction temperature of the LED. It is required that the LED has been installed in the heat sink, and a constant current driver is used as the power source. At the same time, lead out the two wires connected to the LED. Before powering on, connect the voltmeter to the output terminals (the positive and negative terminals of the LED), and then turn on the power supply. Before the LED has heated up, immediately read the reading of the voltmeter, which is equivalent to the value of V1, and wait. At least 1 hour, when it has reached thermal equilibrium, measure again, the voltage across the LED is equivalent to V2. Subtract these two values to get the difference. After being removed by 4mV, the junction temperature can be obtained. In fact, most of the LEDs are connected in series and connected in parallel. This does not matter. At this time, the voltage difference is contributed by many LEDs connected in series. Therefore, divide the voltage difference by the number of LEDs connected in series and then divide it by 4mV. , You can get its junction temperature. For example, the LED is 10 strings and 2 parallel, the voltage measured for the first time is 33V, the voltage measured after the second thermal balance is 30V, and the voltage difference is 3V. This number must be divided by the number of LEDs connected in series (10) to get 0.3V, and then divided by 4mV to get 75 degrees. Assuming that the ambient temperature before turning on is 20 degrees, the junction temperature at this time should be 95 degrees.
The junction temperature obtained by this method is definitely much more accurate than using a thermocouple to measure the temperature of the radiator and then to calculate the junction temperature.
