LED Junction temperature measurement using forward voltage method

LED performance highly dependence to junction temperature. Therefore, LED junction temperature is one of the important parameter in LED. Until now, there are two kinds of measurement method which can be used for determining LED Junction temperature, Forward voltage method and Shift wavelength method. This article will discuss about LED junction measurement temperature using forward voltage method.

Forward voltage method based on the temperature dependence of diode forward voltage. Since LED is diode, this dependence also occurs at LED. The simple explanation of this dependence is the difference temperature will result different forward voltage, although current flows are same. For example, let’s say LED junction temperature is 20 Celsius and current is 20 mA, the forward voltage 2 Volt. Then when the junction temperature increase, let’s say 40 Celsius, the forward voltage will decrease, may be 1.96 volt. This kind of relation can be used for determining junction temperature.

This method contains two steps, calibration measurement and actual measurement. The function of calibration measurement is to get the reference data. For calibration measurement, we need several equipments: oven, pulse generator, oscilloscope, thermocouple, resistor (if using pulse voltage generator).

LED is put and heated inside oven until certain temperature. Then it is injected by pulsed current using pulse generator. The current must be same with the operating current of the LED. Then, measure forward voltage of LED. This measurement is repeated again with different temperature by increasing set point temperature of oven. It can be done by choosing several temperatures, such as: 20, 40, 60,..,120 Celsius. By this way we will get the relation between temperature and forward voltage.

There are several important things that we have to consider:

1.        Duty cycle from pulse generator must be very low, such as 0.1%. By using very low duty cycle, so heat dissipated from LED can be neglected, thus oven temperature can be assumed is same with LED Junction temperature. Finally, we can establish relation between Junction temperature and forward voltage.
2.    Thermocouple is used to make sure the oven temperature. Thermocouple must be put close to the LED. 
3. To make sure that LED temperature is same with oven temperature, LED must be waited inside oven for while after oven has achieved set point temperature. It can be 30 minutes or longer. 
4. If there are differences between set point temperature and temperature shown by thermocouple, so choose the temperature which is shown by thermocouple. It is because thermocouple is put close to LED, while temperature sensor at oven system may be at the other place.

The data from calibration measurement can be drawn as a graph. This graph is usually very linear. We can do the linear regression.

Next step is actual measurement. At this measurement, LED is put at the operating place and supplied by operating current. It may be DC current. When the LED is supplied by DC current, its voltage will decrease and achieve stable condition after a while. So, at this measurement, LED must be waited until it achieves stable condition. Approximately it will take 2 minutes. After that we can measure the forward voltage of LED by voltmeter or multimeter.

This forward voltage can be used to determine junction temperature measurement. Data from calibration measurement is used as reference. For example, the voltage from actual measurement is 1.7 Volt. Then, we can search what temperature shown when the forward voltage is 1.7 Volt.

How to measure pulsed current on electrical circuit??

In some cases, pulsed current generator is needed, such as of LED performance test. In that case, LED need to be injected by pulsed current. Actually, there are some company that produce and sell pulse current generator but it is usually very expensive. To solve this problem we can use pulsed voltage generator instead of it which are cheaper than pulsed current generator.

By using pulsed voltage generator, we may be confused how to measure the current. Pulse current is difficult to measure because of two reasons. First, it is pulsed, so we cannot measure the current by ampere meter or clamp ampere since those are for constant current. Second, it is current which cannot be measured by oscilloscope. So, how to measure it??

To measure pulsed current on electrical circuit, we need a known resistant (such as resistor) to be attached serial on the circuit. Then, activate the circuit and measure voltage at the resistor by oscilloscope. By this method, voltage can be measured and resistance is already known. As known that currents at serial connection have same value, Then by simple equation that current value equals voltage divided by resistance (I=V/R)current can be calculated easily.

Also we can control the current by controlling voltage of pulse generator. If we can’t control the voltage we may use potentiometer. These have the same principle, (I=V/R). When we control voltage of pulse generator it means we change V. And the other hand if we use potentiometer, it means we change the resistance(R).

In using pulsed generator, be careful to set the current (it means set up voltage and selection of the resistor). If current exceed generator limit (see the specification of generator), it may cause damage to it. If it happens, fuse inside generator will be cut. But if fuse is already broken, damage may occur in generator component.