Analysis of the heat dissipation design of LED lamps (Figure)

The design of LED luminaires is more complex than traditional luminaires, including optical, mechanical, electronic and thermal, etc., where heat dissipation is especially important. Because only 20% of the conversion rate of high-power LED lamps will be converted into light, the remaining 80% will be converted into heat. If the heat cannot be exported to the lamp, it will not reach the 50,000-hour life claimed by the LED source, and the heat. It will affect the luminous efficiency of the LED, resulting in severe light decay and damage to the lamp. This paper analyzes the heat dissipation design from LED chip, LED chip substrate, chip package, circuit design, system circuit board, heat sink fin to lamp housing, and points out the existing problems. It is called for LED lighting designers to attach great importance to the heat dissipation design of LED lamps, and scientifically and reasonably do the heat dissipation design of LED lamps.

LEDLED Lamp Thermal Design Preface LED, also known as Light Emitting Diode, is a semiconductor component. Since the United States General Electric Company developed the world's first practical red LED in 1962, LED has entered the era of full color. The LED itself is a monochromatic light source. Nowadays, with the improvement of light efficiency and the appearance of blue LED, its application has gradually diversified. From the previous low-power power indicator light, it has evolved into high-power applications such as LED backlight module and LED lighting. LED is known as the new light source for lighting in the 21st century. It has the advantages of high efficiency, long life, energy saving, non-breakable, environmentally friendly and mercury-free. It is incomparable with traditional light sources such as energy saving, carbon reduction and environmental protection. The government's successive declaration of energy policy has made lighting, which accounts for a large proportion of domestic electricity consumption, one of the projects that encourage elimination. Like all electronic components, LEDs generate heat and temperature rise during their operation. If the heat dissipation problem is neglected, it will result in early burning of the LED due to high temperature. The design of LED luminaires is more complex than traditional luminaires, including optics, mechanics, electronics and heat dissipation, where heat dissipation is especially important. Because only 20% of the conversion rate of high-power LED lamps will be converted into light, the remaining 80% will be converted into heat. If the heat cannot be exported to the lamp, it will not reach the 50,000-hour life claimed by the LED source, and the heat. It will affect the luminous efficiency of the LED, resulting in severe light decay and damage to the lamp.

LED luminaire thermal design LED luminous efficiency and lifetime are closely related to working temperature, showing an inverse relationship. For the LED life report released by CREE in the United States, the temperature will be extended by 2 times for every 10 °C temperature drop and the luminous flux is due to the development of high-power LED technology. LED lamps face the severe challenges of thermal management and heat dissipation design, because the temperature rise will not only cause the brightness to drop, but also accelerate the deterioration of the lamp body and packaging materials when the temperature exceeds 100 degrees Celsius. Therefore, in addition to the heat dissipation technology of the LED package component itself, the heat dissipation and heat conduction design of the LED lamp is the biggest key to maintaining the life of the lamp.

The primary heat dissipation design of the LED package is determined by the process of the LED production stage. The general flow diagram of the heat dissipation design of the LED package is given, which is mainly composed of the thermal design inside the chip and the thermal design of the package. In this way, satisfactory LED heat conduction and heat dissipation effects can be obtained through scientific and rational design and typical packaging. A typical LED package structure is shown. It can be seen from the figure that since the package lens material is almost non-thermally conductive, its function is to distribute and take out the light output of the chip, and the heat of the chip is mainly discharged by the internal heat sink and then radiated through the external heat sink, so the LED package is The secondary heat dissipation design is based on the requirements and conditions of its use, and the high heat generated by the chip is effectively exported and conducted to the heat sink through the scientific design of the internal heat sink.

It is described as: Calculate the thermal resistance and junction temperature, to see if it can meet the heat dissipation requirements of the LED. If the heat dissipation requirement can be met, the result will be directly output. If the heat dissipation requirement of the LED cannot be met, the heat sink design should be performed, and then the design can be satisfied. The heat dissipation requirements of the LEDs require an optimized design for the next step. If not, the heat sink design needs to be re-designed until it meets the requirements.

The schematic diagram of the thermal resistance network of the LED secondary heat dissipation design is as shown. In the dotted line frame, the heat dissipation of the LED package is mainly caused by the heat generated by the LED chip PD, which is transmitted outward through the internal thermal resistance Rc, and is diffused outward by the outer casing and the package lens, and the thermal resistance is RTP. The heat conduction process is expressed as follows: The internal heat sink of the LED transfers heat to the metal circuit board through the bonding layer. The thermal resistance between the internal heat sink and the metal circuit board is Rc-b, and then the circuit board transmits the heat to the heat sink through the bonding layer. The thermal resistance is Rb. -s, the heat sink radiates heat through the thermal resistance Rs-a to the air (in the figure: the junction temperature of the T LED chip, the temperature of the heat sink inside the Tc, the temperature of the Ts heat sink, the temperature of the Ta ring i).

By analyzing the scheme and mechanism of LED secondary heat dissipation, it can be seen that the main factors affecting LED heat dissipation include chip substrate, heat dissipation substrate (system circuit board), temperature equalization plate, bonding layer and heat sink.

The specific analysis is as follows: At present, the common LED chip substrate is a ceramic substrate, which has good heat dissipation, low expansion coefficient and the like, reduces variations due to thermal stress, and secondly has the advantages of heat resistance, moisture resistance, insulation, etc., so the ceramic substrate becomes A common heat sink material for LED chip substrates for high power lighting.

Ceramic substrates are currently divided into three categories: (1) alumina (Al23), 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 for LED products of 3W or more, while Al2O3 is used for the range of 1W ~ 3W, and LTCC is suitable for LED products of large size, high power, small size and low power. Take the CreeXLampLED series as an example, using a ceramic pedestal to optimize heat dissipation.

Table 1 Classification, Expansibility, and Thermal Conductivity of the Heat Dissipation Substrate Introduce the zero system ft (WWK type connects the chip and the LED heat dissipation substrate, and the wire is connected by the metal wire to the LED chip and the chip substrate, and the heat generated by the chip can only be used by The conduction of the wire is limited by the material of the wire and the elongated geometry. Therefore, the heat dissipation performance is limited. In contrast, the eutectic and flip chip bonding methods greatly reduce the wire length and increase the wire cut. Area, improve heat transfer capacity. As shown.

Wire-wound packages (left) and flip-chip packages (right) system boards play a significant role in transferring heat. The LED chip is connected to the system board by soldering, and the thermal energy generated by the chip is also transmitted from the chip substrate to the system board. Currently, a metal core substrate (MetalCorePCB; MCPCB) having a high thermal conductivity is commonly used, although the foregoing The thermal conductivity of the ceramic substrate is good, but due to the large area of ​​the system board, considering the cost factors and the weight of the lamp, the ceramic substrate will be discarded and the MCPCB will be used as the system board. The MCPCB is composed of a three-layer structure, which is a conductive circuit layer, a high thermal conductive insulating layer and a metal substrate from top to bottom. The material of the high thermal conductive insulating layer must be carefully selected. If a material with a high expansion coefficient is used, the insulating layer is easily exposed to high temperatures. The expansion causes cracks and voids, which in turn causes air to enter the MCPCB, forming additional thermal impedance and reducing the efficiency of heat conduction. Some manufacturers will spray ceramic heat-dissipating paint between the thermal conductive insulation layer and the metal base to improve the insulation resistance of the insulation layer. It saves the material cost of the multilayer thermal conductive adhesive and enhances the heat dissipation capability of the MCPCB; the bottommost metal substrate is mostly made of aluminum alloy, and the heat dissipation characteristics of the aluminum alloy are utilized to achieve the purpose of heat conduction.

The rear end of the system 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 cooling and magnetic jet cooling. Passive cooling includes natural convection cooling and loop heat pipe cooling. Will be introduced below.

2.1 Fan forced cooling fan forced cooling As the name suggests, the air is convected by the fan, and the hot air is led out to the outside of the lamp for heat dissipation. The forced cooling of the fan can effectively discharge the heat, and the fan is forced in the computer, air-conditioning and automobile. Cooling. At present, some manufacturers' LED street lamps use fan forced cooling technology, but there are few.

2.2 Electromagnetic jet heat dissipation Electromagnetic jet heat dissipation does not use fan blades to generate airflow. The structure is a hollow body with a film that oscillates the film at a frequency of 100 to 200 times per second using an electromagnetic or piezoelectric actuator to promote the film. Oscillation up and down, as the film moves up and down, the air will flow into the hollow cavity and then eject. The airflow after the jet will drive the surrounding air to generate eddy currents and enhance the air convection capability. It has been applied to LED bulbs.

2.3 Natural convection heat dissipation Natural convection heat dissipation is through direct contact with air through heat sinks (eg, heat sink fins, lamp housings, system boards, etc.). The air around the radiator becomes hot air by absorbing heat, and then the hot air rises. When the cold air drops, it will naturally lead to convection of the air to achieve the effect of heat dissipation. With the introduction of high-power luminaire products, the use of natural convection heat dissipation requires a large heat dissipation surface area, so the heat dissipation fins are born, most of which are installed on the back of the lamp to provide a larger heat dissipation area and enhance the effect of convection heat dissipation. At present, most manufacturers of high-power LED lamps use finned natural cooling technology.

Although the use of heat-dissipating fins increases the heat dissipation effect, it also increases the overall weight and cost of the luminaire, and increases the risk of safety suspension of the pole-type luminaires. In addition, LED lamps often face problems such as dust accumulation and long-term use. Excessive dirt and dust accumulate in the heat dissipation fins, which will weaken the heat dissipation capability. In contrast, some manufacturers choose to design the heat sink fins in the same direction as the light emitting surface of the lamps (downward heat dissipation), completely avoiding the problem of dust accumulation.

2.4 Loop heat pipe heat dissipation This heat dissipation method is to dissipate heat through a circulating heat pipe. The two ends of the circuit pipe are the system circuit board (heat source) and the heat sink. The inside of the circuit pipe is filled with working fluid and equipped with an evaporator. The working principle is as follows: when the heat is transmitted from the system board, the working fluid at the heat source absorbs heat, and then is converted into gas by the evaporator. With the rapid movement of the gas, the heat at the heat source can be quickly transmitted to the lamp housing or dissipate heat. Therefore, the heat dissipation of the loop heat pipe only solves the heat conduction problem and cannot effectively achieve the heat dissipation function.

In the design of the luminaire, the heat-dissipating fins and the outer lamp housing are exposed to the air. In order to avoid oxidation and more anode treatment, in recent years, some manufacturers have introduced soft ceramic heat-dissipating paint to replace the anode treatment program, and claimed that the thermal resistance value is close to metal. To achieve the effect of accelerating heat conduction, its effectiveness is unknown, and it is still to be shared and shared by industry colleagues.

So far, the above discussion is about two ways of heat conduction and heat convection. At present, some manufacturers claim that their ceramic heat-dissipating substrate can use the far-infrared type to dissipate heat radiation for long-distance heat transfer, and claim to be used to replace the LED chip substrate. The heat-conducting metal at the back end (heat-dissipating fins, metal lamp housing) achieves the effect of successful heat dissipation and light weight reduction. If the heat dissipation performance of this technology is as stated by the manufacturer, it will bring significant progress to the heat dissipation design of LED lamps.

Some problems in the heat dissipation design of LED lamps Some of the heat dissipation designs of commercially available LED lamps are easy to ignore some details, such as ignoring the temperature uniformity of heat conduction, that is, the temperature distribution of the heat dissipation fins is seriously uneven, resulting in a limited part of the heat dissipation effect of the fins. Even did not play a cooling effect. Some design errors can be dangerous. In particular, LED street lights are usually installed at a pole height of 8 to 12 meters. If the center of gravity of the radiator is poorly designed, the weight and wind resistance may be too large, and the danger may increase when a typhoon is encountered. Or an earthquake can cause serious accidents.

4 Conclusions Because the main technical indicators such as the efficacy and longevity of high-power LEDs are related to the working junction temperature of the chip, it is particularly important to study the heat dissipation design of one of the key technologies of high-power LED lighting design. Especially for the high-power LED lighting, mainly from the primary and secondary heat dissipation of the LED chip package, to the thermal design of the LED lamp, to study the technical solutions for the secondary and secondary heat dissipation of the high-power LED package. And the heat dissipation design that should be applied to high-power LED lighting fixtures to meet the heat dissipation requirements of high-power LED lighting fixtures. How to carefully evaluate the quality of its products while striving for the market, scientifically and reasonably do a good job in the heat dissipation design of LED lamps can withstand the time and environmental test, which is the key point that designers must strictly control.