Glossary - thermal

During conduction, heat is transferred through the vibration of molecules in a substance. As something gets warmer, it begins to increase the vibration and movement of the molecules that it consists of. In solids, particles are closely packed together and are in direct contact. Since they are close to each other, particles that vibrate near the point of contact will pass on the vibrations to other particles nearby. The vibrations then spread throughout the object. Conduction works similarly with liquids and gases, but since the density of the particles is less, there will be less efficient energy transfer between the molecules. Conduction occurs when two things at different temperatures touch each other and energy directly transfers from the material in the hotter item to the material in the colder item at the point of contact.

Convection is the flow of heat through a bulk, macroscopic movement of matter from a hot region to a cool region, as opposed to the microscopic transfer of heat between atoms involved with conduction. Suppose we consider heating up a local region of air. As this air heats, the molecules spread out, causing this region to become less dense than the surrounding, unheated air. Being less dense than the surrounding cooler air means the hot air will subsequently rise due to buoyant forces - this movement of hot air into a cooler region is then said to transfer heat by convection.

Junction temperature is the highest temperature of the actual semiconductor temperature and the temperature of the part's exterior. The difference is equal to the amount of heat transferred from the junction to case multiplied by the junction-to-case thermal resistance. As junction temperature is the highest temperature within the LED, it represents figure-of-merit when predicting LED lifetime. There are three things which can affect the junction temperature of LED’s: drive current, thermal path and ambient temperature. In general, the higher the drive current, the greater the heat generated at the junction. From the thermal point of view, junction temperature is affected by many factors such as cooling system, environment, interface material, etc. It is important to maintain junction temperature of LED at lowest possible value. The word "junction" refers to the p-n junction inside the semiconductor die. You can find the maximum recommended value for each LED product in the data sheet.

A very innovative and possibly the most effective way of cooling LEDs is by using ‘SynJet™Fanless Air Cooling™’ from Nuventix. The Nuventix developed SynJet module is a new air-based synthetic jet cooling technology that takes advantage of turbulent pulses of air generated from an electromagnetic actuator. SynJets operate almost silently with extremely high reliability. The SynJet airflow is a series of high velocity pulses. The intake and exhausting cycle is repeated 50...70 times per second depending on the SynJet creating the feeling of constant airflow. The net effect is highly turbulent, high velocity airflow that is very effective at removing heat from the surface of a Heatsink.An oscillating diaphragm creates pulses of high velocity turbulent air flow.The high velocity flow ‘entrains’ or pulls air in its wake increasing overall air flow by as much as 5 times .The turbulent air flow improves the heat transfer out of the heat sink, while the entrained air sweeps the hot air out of the system, thus cooling more efficiently

A factor used in lighting calculations that compensates for the change in light output of a fluorescent lamp due to a change in bulb wall temperature. It is applied when the lamp-ballast combination under consideration is different from that used in the photometric tests.

Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation. The mechanism is that bodies with a temperature above absolute zero have atoms or molecules with kinetic energies which are changing, and these changes result in charge-acceleration and/or dipole oscillation of the charges that compose the atoms. This motion of charges produces electromagnetic radiation in the usual way. Examples of thermal radiation include the visible light and infrared light emitted by an incandescent light bulb, the infrared radiation emitted by animals and detectable with an infrared camera, and the cosmic microwave background radiation. Thermal radiation is different from thermal convection and thermal conduction--a person near a raging bonfire feels radiant heating from the fire, even if the surrounding air is very cold. Sunlight is thermal radiation generated by the hot plasma of the Sun.

Thermal resistance is the ratio between the temperature difference and the power. It shows how good the heat transfer between the materials/components is. The junction temperature will be lower if the thermal impedance is smaller and likewise, with a lower ambient temperature. To maximize the useful ambient temperature range for a given power dissipation, the total thermal resistance from junction to ambient must be minimized. The values for the thermal resistance vary widely depending on the material or component supplier. For example, RJC will range from 2.6 °C/W to 18 °C/W, depending on the LED manufacturer. The thermal interface material’s (TIM) thermal resistance will also vary depending on the type of material selected. Common TIMs are epoxy, thermal grease, pressure sensitive adhesive and solder. Power LEDs are often mounted on metal-core printed circuit boards (MCPCB), which will be attached to a heat sink. Heat conducted through the MCPCB and heat sink is dissipated by convection and radiation. In the package design, the surface flatness and quality of each component, applied mounting pressure, contact area, the type of interface material and its thickness are all important parameters to thermal resistance design.