Phase and Temperature Change

Phase change of substance can be obtained by heating or cooling. When we heat the substance the temperature will rise until melting point or boil temperature. For example, when we heat ice at below 0^oC (let’s say -5^oC), it will not melt immediately, but temperature will rise until melting point temperature (0^oC) first. Then, ice will melt while the temperature stays at  0^oC. After all of water melt, then temperature will rise again until boiled temperature (100 ^oC). Water temperature will stays at 100 ^oC, when water is boiled. Then water phase will change to vapor. After all become vapor the temperature will increase again.

For simplifying these phenomena:

•           Rising temperature until melting point temperature. The phase is solid
•           Phase change from solid to liquid. This phenomenon doesn’t change temperature
•           Rising temperature from melting point temperature to boiling point temperature. The phase is liquid
•           Phase change from liquid to gas. This phenomenon doesn’t change temperature.
•           Rising temperature of gas.

The explanation above is heating process. Cooling process is similar but the opposite.

For calculate heat needed to removed, the general formula are:

•           Every Rising temperature: Q = m.c.∆T
•       Where Q: heat, m: specific heat of the substance, ∆T: temperature differences
•          Change phase from solid to liquid: Q=m.L
•       Where L: heat fusion of the substance
•          Change phase from liquid to gas: Q = m.U 
•       Where U: heat of vaporization

These conditions are approach, in actual we may find that water will evaporate although the temperature is not at boiling temperature. Also the other phenomena such as melting.

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.