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.

Fiber optic

Fiber optic is often used in telecommunication network. Comparing with electrical cable, signal transmission by fiber optic is faster because it uses light as a signal. Besides that, it is also immune to electromagnetic interference, transmits longer distance, and higher bandwidth data rates. How can fiber optic be a path for light?

Light can travel from edge of fiber optic to another edge. The principle of fiber optic is total internal reflection. Before discuss about total internal reflection, let us review about light refraction. Light will be refracted when light through different medium which their refraction index value is different. For example, if light is emitted from inside the water with certain angle, then it will be refracted when comes out from water to the air. The phenomenon of light refraction is the angles are different. If refraction index of medium where the light comes from higher than refraction index of medium where light goes on, the angle will be bigger. If the initial angle is enlarged, there will be a critical angle, that is the angle when the light is not transmitted to the second medium. Moreover, if the angle is still enlarged, the light will be reflected as total internal reflection.

Fiber optic consists of core and cladding. Core has higher refraction index than cladding, then if light travel inside core with the angle more than critical angle, so there will be total internal reflection. By this phenomenon light can travel from one edge to another edge. 

White LED

Light Emitting Diode or LED is one of the light sources. Basically, LED is diode which consists of two types semiconductor, those are n-type and p-type. These semiconductors are connected at a junction. As a diode, LED can be a conductor if forward current flow through the LED.  How can LED emit light??

Electrons are majority charge carrier at n-type semiconductor, while holes are majority charge carrier at p-type semiconductor. When forward voltage is applied to the LED holes and electrons are pushed to the junction, then they meet followed emitting light. This light depends on band gap energy between conductance and valence band.

Basically, LED can emit only one color, but now white LED which consist many colors has been introduced to be applied in technology such as monitor. Now, White LED has begun to replace incandescent lamp or fluorescent lamp as a lighting lamp. There are two primary ways to produce white LED, RGB system and Phosphor-based system.

In RGB System, three basic colors (Red, Green, and Blue) are mixed resulting white LED. The second way is similar with fluorescent lamp which use phosphor to create white light. How can Phosphor create white light??

Stokes Shift is the key how to produce white light in Phosphor based system. Light from junction could be blue light or UV light then the energy from this wavelength will be is absorbed by phosphor. Then phosphor emits the light with lower energy than it absorbs.

LED has many advantages compared with another light source such as incandescent and fluorescent. Long life, shock resistance, compact, high efficiency are the advantages. However, the problem that challenging now is how to maintain junction temperature. LED performance largely depends on the junction temperature. Increasing junction temperature will decrease LED performance. So that thermal management in LED is very important.  

Photometry Quantities

Photometry is defined as a science that study about light which is perceived by human eyes. It is different with radiometry which measure all of radiant energy including visible light. Photometry focus radiant energy which is sensitive to human eyes, that is wavelength range from 380 nm to 750 nm.

IEC said light is energy radiation which can be evaluated visually. Gigahertz-Optic defined light as part of electromagnetic spectrum radiation which can be seen. The point of these definitions is same but in different words. There are several parameters in Photometry: Luminous flux, Luminous Intensity, illuminance and Luminance.

Luminous flux is the total power electromagnetic radiation emitted by source which the spectral is can be sensed by human eye. In other words, it is on in range of visible light (380 nm -750 nm). Unit of this parameter is Lumen. 

Luminous Intensity is total luminous flux per unit solid angle that emitted in certain angle. Unit of this parameter is Lumen/steradian or Candela (cd).

Illumninance is the total luminous flux which is received by a certain surface area. Unit of this parameter is Lumen/m² or Lux.

Luminance is total luminous intensity which is received by a certain surface area. Unit of this parameter is candela/m².

These parameters are similar with radiometry. The difference is photometry measure only electromagnetic radiation than can be sensed by human eye.

Source of light emits both of visible light and non visible light. The ratio of power in visible light range to the total radiant power is luminous efficacy which has unit Lumen/Watt.

Luminous efficiency is also known to determine the amount of visible light power in unit watt which emitted by total radiant power (also) in unit watt. This can be represented in percent (%)

LED Junction Temperature measurement

Light Emitting Diode or LED has begun to be applied as light source replacing other source light such as incandescent or fluorescent lamp. LED has many advantages such as high efficiency, shock resistance, and long life time. However, LED performance depends on the temperature thus thermal management in LED technology is very important.

Talking about thermal management, the important parameter in LED technology is Junction temperature, but the problem is how to measure it. LED junction is very small and inside the encapsulation, therefore thermocouple or other temperature sensor couldn’t be used to measure it.

There are several methods to measure LED junction temperature, those are forward voltage and shift wavelength method. Both of them based on the dependence temperature, it means the difference temperature causes change of forward voltage and wavelength. LED junction measurement by these methods consists of calibration and actual measurement.

In calibration measurement, LED (including Junction) is heated until certain temperature then injected by pulse current with very low duty cycle. LED junction temperature is assumed same with that certain temperature because heat dissipated from LED is very small due to low duty cycle. Then forward voltage or wavelength is measured. Next, this measurement is repeated at different temperature. By this measurement the relation between forward voltage/wavelength and temperature is got and can be used as reference in actual measurement

In actual measurement, LED is operated in normal condition (in ambient temperature) and current (constant). Junction temperature can be determined by measuring the forward voltage/Wave length. Then data from calibration measurement is used as reference to determined junction temperature.