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The insider's guide to thermal management of LED bulbs

High power light-emitting diodes (LEDs) can use 350 milliwatts or more in a single LED. Most of the electricity in an LED becomes heat rather than light (about 70% heat and 30% light). If this heat is not removed, the LEDs run at high temperatures, which not only lowers their efficiency, but also makes the LED less reliable. Thus, thermal management of high power LEDs is a crucial area of research and development. It is necessary to limit the junction temperature to a value that will guarantee the desired LED lifetime.


The insider's guide to thermal management of LED bulbs

In order to maintain a low junction temperature to keep good performance of an LED, every method of removing heat from LEDs should be considered. Conduction, convection, and radiation are the three means of heat transfer. Typically, LEDs are encapsulated in a transparent resin, which is a poor thermal conductor. Nearly all heat produced is conducted through the back side of the chip. Heat is generated from the PN junction by electrical energy that was not converted to useful light, and conducted to outside ambience through a long path, from junction to solder point, solder point to board, and board to the heat sink and then to the atmosphere. 

 

Heat Conduction

Flip chip – The concept is similar to flip-chip in package configuration widely used in the silicon integrated circuit industry. Briefly speaking, the LED die is assembled face down on the sub-mount, which is usually silicon or ceramic, acting as the heat spreader and supporting substrate. The flip-chip joint can be eutectic, high-lead, lead-free solder or gold stub. The primary source of light comes from the back side of the LED chip, and there is usually a built-in reflective layer between the light emitter and the solder joints to reflect up the light which is emitted downward. Several companies have adopted flip-chip packages for their high-power LED, achieving bout 60% reduction in the thermal resistance of the LED while keeping its thermal reliability.

The thermal conductivity of the material that the heat sink is made from directly affects the dissipation efficiency through conduction. Normally this is aluminum, but copper as the solutions which offer better thermal transfer than aluminum with a lower weight plus the advantage for flat-sheet heat sinks. Copper is considered an exotic cooling solution and does come at a higher production cost.

For heat transfer between LED sources over 15 Watt and LED coolers, it is recommended to use a high thermal conductive interface material (TIM) which will create a thermal resistance over the interface lower than 0.2K/W

 

Heat Dissipation

After conduction, the heat dissipation starts to work and transfer produced heat to outside ambience. Typically, the fan in the housing of LED headlight bulb plays an important role to achieve heat dissipation work and the spinning rate determines the performance of heat dissipation. However, some manufactures replace the fan with copper wire for further heat conduction. To some extent, it saves manufacturing cost and simplifies the construction of LED bulb, but it’s for sure to affect the reliability and longer life span of LED bulb. After all, the fan is more powerful than static heat conduction of copper wires.

 

 

New technology:

High temperature is still a significant threat to affect the stability of LED bulbs, and continuously keeping headlights on at night time for a long time is certain to lead headlight bulbs to be under negative condition. We couldn’t measure the temperature level manually and we couldn’t tell whether headlight bulbs are reaching the limit or not. To resolve this issue, the Temperature Sensor Protection System (TSPS) emerges.

TSPS is the chip installed between copper board and fan. The function of it is to detect the temperature in time and automatically adjusts the brightness of headlights. For example, when temperature going high up to preset warning level of 194 ℉, the brightness will be decreased to 60% automatically. After proper temperature comes back, the brightness will recover to normal level fast.

Category: FAQs

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