Brief Analysis of The Heat Dissipation Design of LED Lamps

LED, also known as Light Emitting Diode, is a semiconductor component.

Since 1962, when General Electric in the United States developed the world's first practical red LED, LEDs have transitioned into the full-color era. The basic principle of LED light emission involves a P-type semiconductor containing holes and an N-type semiconductor containing electrons, which together form a PN junction diode. When a forward bias is applied to the PN diode, and current flows through it, electrons and holes move toward the junction surface. At the junction, they recombine and release energy in the form of light (as shown in the figure below).

LED, originally a monochromatic light source, has evolved with improved efficiency and the advent of blue LEDs, expanding its applications from low-power indicators to high-power uses like backlight modules and lighting. Known as the 21st century's new light source, LEDs offer unmatched advantages: high efficiency, long life, energy savings, durability, and eco-friendliness without mercury. As global energy conservation and carbon reduction efforts rise, along with government policies like the U.S. "Energy Independence and Security Act" (2007) phasing out incandescent lamps and Japan's "Energy Basic Plan" (2010) targeting carbon reduction, lighting—a major energy consumer—has become a focus for replacement. These trends and LED's unique properties have driven rapid industry growth, attracting significant investment in all stages of LED production.

LEDs, like all electronic components, will generate heat and temperature rise during use or operation. If the heat dissipation problem is ignored, the LED will burn out prematurely due to high temperature. The design of LED lamps is more complex than traditional lamps, including optics, mechanisms, electronics and heat dissipation. Among them, "heat dissipation" is particularly important, because the current conversion rate of high-power LED lamps is only 20% converted into light, and the remaining 80% is converted into heat. If the heat cannot be exported from the lamp, the claimed life of the LED light source of 50,000 hours will not be achieved. At the same time, the heat will affect the luminous efficiency of the LED, resulting in serious light decay and lamp damage.

Heat dissipation design of LED lamps

The luminous efficiency and life of LED are closely related to the operating temperature, showing an inverse relationship. The figure below is the LED life report released by CREE in the United States. For every 10°C drop in temperature, the life will be extended by 2 times and the luminous flux will increase by 3%-8%.

The advancement of high-power LED technology has brought significant challenges to thermal management and heat dissipation design in LED lamps. High temperatures not only reduce brightness but also accelerate the degradation of lamp bodies and packaging materials, especially when exceeding 100°C. Thus, beyond the heat dissipation technology within the LED packaging itself, the thermal conductivity and heat dissipation design of LED lamps are critical to ensuring their longevity.

For outdoor lighting, LED heat dissipation design is more complex and varied compared to other LED applications, such as backlight panels or automotive lighting. This is due to the harsher operating conditions, including temperature fluctuations, dust, humidity, and other environmental factors. For instance, LED streetlights must withstand outdoor environments over prolonged periods while meeting safety standards (e.g., UL, CE). Additionally, they must overcome rigorous reliability tests, addressing issues such as optical stability (e.g., light decay), dust infiltration, bird droppings, airborne particles, and water vapor infiltration caused by waterproofing and dustproofing challenges.

Lighting design

In terms of lamp design, the LED chip, LED chip substrate, chip packaging, circuit design, system circuit board, heat sink fins and lamp housing all test the R&D capabilities of the upstream, midstream and downstream of the LED industry. Traditional LEDs used for indicator lights are mostly cannonball-shaped structures, which are encapsulated with insulating epoxy resin on all sides. Therefore, the heat energy generated by the LED grain is mainly dissipated by conduction from the two metal wires below to the system circuit board. However, when LED entered the lighting field, high-power LEDs above 1W became the mainstream. In order to increase the heat conduction area, LEDs for lighting purposes adopted flat-panel packaging, so that the LED chip substrate and the system circuit board can have a larger bonding area.

Currently, the most common LED chip substrate is a ceramic substrate, which has good heat dissipation and low expansion coefficient, which reduces deformation caused by thermal stress. In addition, it also has the advantages of heat resistance, moisture resistance, and insulation. Therefore, ceramic substrates have become a common heat dissipation material for high-power LED chip substrates. Ceramic substrates are currently divided into three categories: (1) aluminum oxide (Al2O3), (2) low-temperature co-fired ceramics (LTCC), and (3) aluminum nitride (AlN). Among them, AlN has the best thermal conductivity, but the technical threshold is the highest. Therefore, AlN is mostly used in LED products above 3W, while Al2O3 is used in the range of 1W-3W. LTCC is suitable for large-size and high-power LED products and small-size and low-power LED products. Take the Cree XLamp LED series as an example, which uses a ceramic base to optimize heat dissipation capabilities.

Led packaging

In terms of packaging, the chip and the LED heat dissipation substrate can be connected by wire bonding, eutectic or flip chip. Wire bonding is to connect the LED chip and the chip substrate through metal wires. The heat generated by the chip can only be conducted through the wires. The heat dissipation efficiency is limited by the material and slender geometric shape of the wires, so the heat dissipation efficiency is greatly limited. In comparison, the eutectic and flip chip bonding methods greatly reduce the wire length and increase the wire cross-sectional area, thereby improving the heat dissipation conduction capacity.

Line improvement

In terms of circuit improvement, some manufacturers have launched high-voltage LED products. The principle is to connect many low-power LEDs in series to obtain high-voltage, low-current products. High-voltage LEDs are mostly used in space-constrained lighting products such as bulbs, tubes, and projection lamps, which can reduce the difficulty of controlling circuit layout. Compared with ordinary LEDs, high-voltage LEDs have a smaller driving current and generate relatively less heat, which can avoid falling into the vicious cycle of "temperature rise → impedance drop → current increase → heat energy increase → temperature rise", and can design LED lamps with better system stability.

About System Circuit Board

After introducing the LED chip substrate, the next thing to mention is the system circuit board, which also has an important role in transferring heat. The LED chip is connected to the system circuit board through welding, and the heat energy generated by the chip is also transferred from the chip substrate to the system circuit board. Currently, the commonly used one is the metal core substrate (Metal Core PCB; MCPCB) with a high thermal conductivity coefficient. Although it has been mentioned above that the ceramic substrate has good thermal conductivity, due to the larger area of the system circuit board, considering the cost factors and the weight of the lamp, the ceramic substrate will be abandoned and the MCPCB will be used as the system circuit board instead. MCPCB consists of 3 layers, from top to bottom, they are the conductive circuit layer, the high thermal conductivity insulation layer and the metal substrate. The material of the high thermal conductivity insulation layer must be carefully selected. If a material with a high expansion coefficient is used, the insulation layer is prone to expand at high temperature and produce cracks and voids, which will allow air to enter the MCPCB, forming additional thermal impedance and reducing the efficiency of thermal conductivity. Some manufacturers will spray ceramic heat dissipation paint between the thermal conductive insulation layer and the metal substrate to improve the insulation impedance of the insulation layer, save the material cost of multiple layers of thermal conductive adhesive and enhance the heat dissipation capacity of the MCPCB; the bottom metal substrate is mostly made of aluminum alloy, which uses the better heat dissipation characteristics of aluminum alloy to achieve the purpose of heat conduction.

The back end of the system circuit board is combined with a heat dissipation system for heat dissipation. The heat dissipation system can be divided into active heat dissipation and passive heat dissipation. Active heat dissipation includes fan forced heat dissipation and magnetic jet heat dissipation. Passive heat dissipation includes natural convection heat dissipation and loop heat pipe heat dissipation. They will be introduced one by one below:

1. Fan forced cooling:

Fan-forced cooling, as the name implies, uses fans to generate air convection to direct hot air out of the lamp body for heat dissipation. Using fans to force heat dissipation can effectively discharge heat. Fans are used for forced heat dissipation in computers, air conditioners, and cars. Currently, Xinyuansheng Technology's S01 Glory Series LED street light series uses fan-forced cooling technology.

2. Electromagnetic jet cooling:

Electromagnetic jet cooling does not use fan blades to generate airflow. Its structure is a hollow cavity with a thin film. It uses an electromagnetic or piezoelectric driver to oscillate the film at a frequency of 100 to 200 times per second, causing the film to oscillate up and down. As the film moves up and down, air will flow into the hollow cavity and then be ejected. The airflow after ejection will drive the surrounding air to produce vortexes, thereby enhancing the air convection capacity. It has been used in GE 27W Energy Smart LED bulbs.

3. Natural convection cooling:

Natural convection cooling is through direct contact between the heat sink (such as heat sink fins, lamp housing, system circuit board, etc.) and the air. The air around the heat sink absorbs heat and becomes hot air. Then the hot air rises and the cold air falls, which naturally drives the air to produce convection and achieve the cooling effect. With the introduction of high-power lamp products, the use of natural convection cooling requires a larger cooling surface area, so cooling fins are born. Most of them are installed on the back of the lamp to provide a larger cooling area and enhance the effect of convection cooling. Yangquan Optoelectronics' LED ceiling lights use fin natural cooling technology.

Although the use of heat sink fins increases the heat dissipation effect, it also increases the overall weight and cost of the lamp, and increases the risk of safe hanging of pole-type lamps. In addition, LED lamps often face problems such as dust accumulation. Once used for a long time, too much dirt and dust accumulate on the heat sink fins, which will weaken the heat dissipation ability. In contrast, some manufacturers choose to design the heat sink fins in the same direction as the light-emitting surface of the lamp (downward heat dissipation), completely avoiding the problem of dust accumulation. Many LED street lights produced by Xinyuansheng Technology on the market (for example: S02 Orra Series, S06 Fudo Series) adopt a downward natural heat dissipation design.

4. Loop heat pipe cooling:

This heat dissipation method is to dissipate heat through a circulating heat pipe. The two ends of the loop pipe are the system circuit board (heat source) and the radiator. The inside of the loop pipe is filled with working fluid and is equipped with an evaporator. Its working principle is: when the system circuit board transmits heat energy, the working fluid at the heat source absorbs the heat and is converted into gas through the evaporator. Using the fast movement of gas, the heat at the heat source can be quickly transferred to the lamp housing or radiator. Therefore, the heat dissipation of the loop heat pipe only solves the problem of heat conduction and cannot effectively achieve the "heat dissipation" function.

In lamp design, the heat sink fins and outer lamp housing are exposed to the air, so they are often anodized to avoid oxidation. In recent years, some manufacturers have introduced soft ceramic heat sink paint to replace the anodizing process, and claim that its thermal resistance is close to that of metal and can achieve the effect of accelerated heat conduction. However, its effectiveness is unknown and needs to be used and shared by colleagues in the industry.

So far, the above discussions have all been about heat conduction and convection. Currently, some manufacturers claim that their ceramic heat dissipation substrates can use far-infrared radiation to dissipate heat for long-distance heat transfer, and claim that they can be used to replace the thermally conductive metal (heat dissipation fins, metal lamp housing) at the back end of the LED chip substrate to achieve successful heat dissipation and reduce the weight of the lamp. If the heat dissipation performance of this technology is really as the manufacturer claims, it will also bring significant progress to the heat dissipation design of LED lamps.

In conclusion

The heat dissipation design of some commercially available LED lamps tends to overlook some details, such as neglecting the uniformity of heat conduction, that is, the temperature distribution of the heat dissipation fins is seriously uneven, resulting in a limited or even no heat dissipation effect on some of the fins. Some design errors can bring dangers, especially since LED street lamps are usually installed on poles 8-12 meters high. If the center of gravity of the radiator is not well designed, it may lead to excessive weight and wind resistance, increasing the danger, and may cause serious accidents when encountering typhoons or earthquakes.

Rice Lighting, as a practitioner in the high-tech energy-saving lighting industry, must recognize that good LED lamp heat dissipation (including heat conduction, temperature uniformity, heat exchange, etc.) must be based on complex basic hard theories of thermal transfer. The performance of LED street lamps must be taken seriously. It must be supported by scientific data and theories, with a complete system design method as a guideline and a good process as the basis. Finally, the test report of a third-party impartial unit must be used as the basis. Only in this way can the vicious cycle of light decay and hindering the development of the industry be avoided again and again.

Therefore, how to prudently evaluate the quality, R&D and process capabilities of their own products while competing for the market, and how to design the heat dissipation of LED lamps to withstand the test of time and the environment are all key points that the industry must strictly regulate itself.

Thanks for your time for reading :)

---END---