1. Radiator

The radiator is a key component of LED lamps. Its shape, volume and heat dissipation surface area should be designed just right. If the radiator is too small, the working temperature of the LED lamp will increase, which will affect the luminous efficiency and life. If the radiator is too large, the Consumable materials increase the cost and weight of the product, which reduces the competitiveness of the product. Designing the right heat sink for LED lamps is critical. The design of the radiator has the following parts:

1) Clarify the power that LED lamps need to dissipate. Some parameters used to design the heat sink are: the specific heat of the metal, the thermal conductivity of the metal, the thermal resistance of the chip, the thermal resistance of the radiator, and the thermal resistance of the surrounding air.

2) Determine the type of heat dissipation used. The types of heat dissipation include: natural convection heat dissipation, strong wind cooling, heat pipe heat dissipation, and jet cooling and other heat dissipation methods. From the cost comparison, natural convection heat dissipation is the lowest cost, strong wind cooling is medium, heat pipe heat dissipation cost is high, and jet cooling cost is the highest.
3) Determine the maximum allowable working temperature of LED lamps (ambient temperature plus allowable temperature rise of lamps).

4) Calculate the volume and heat dissipation area of ​​the radiator, and determine the shape of the radiator.
5) Combine the radiator and the LED lamp into a complete lamp, and power on for more than 8 hours. Check the temperature of the lamp at a room temperature of 39~40℃ to see if it meets the heat dissipation requirements to check whether the calculation is correct. If it does not meet the conditions of use , the parameters need to be recalculated and adjusted.

6) The sealing of the radiator and the lampshade should be waterproof and dustproof. An anti-aging rubber pad or a silicone rubber pad should be placed between the lampshade and the radiator, and fastened with stainless steel bolts to achieve waterproof and dustproof sealing.

The working conditions and heat dissipation of the LED are not only directly related to the luminous efficiency of the LED in actual work, but also related to the service life of the LED. And because the lamps used outdoors should have a certain level of dustproof and waterproof function (IP), good IP protection often hinders the heat dissipation of the LED. In this respect, it is also the case that China has the most unqualified and unreasonable situations when LED is applied to general lighting fixtures. The unqualified and unreasonable situations of LED lighting fixtures currently produced in China are basically:

1) A radiator is used for the LED, but the design of the terminal and the radiator of the LED connection cannot reach the IP45 level and above, and cannot meet the requirements of the GB 7000.5/IEC 6598-2-3 standard.
2) Use the ordinary lamp shell and use matrix LEDs in the light-emitting surface of the lamp. Although this design can meet the IP test, due to the lack of ventilation in the lamp, the temperature of the inner cavity of the lamp will rise to 50°C when the lamp is working. ~80℃, under such high working conditions, the luminous efficiency of LED is impossible to be high, and the service life of LED will also be greatly reduced.

3) The instrument fan is used in the lamp to dissipate the heat of the LED and the radiator. The air inlet is designed under the lamp to avoid the entry of rainwater, and the air outlet is designed around the downward LED light source. This can also effectively prevent the entry of rainwater. In addition, the radiator and the LED (light source cavity) are not in the same cavity. If this design is done well, it can pass the IP test requirements of the lamp smoothly. This solution not only solves the problem of heat dissipation of LEDs, but also meets the requirements of IP level. But this seemingly good design actually has obvious unreasonable situations. Because in the use of most outdoor lamps in China, the amount of flying dust in the air is relatively large, sometimes reaching a large amount (such as sandstorms). Half a year), the gap of the internal radiator will be filled with dust, which will greatly reduce the effect of the radiator, and finally make the life of the LED significantly shortened due to the high operating temperature. The disadvantage of this scheme is that it cannot be used well for a long time.

To take into account the heat dissipation and IP protection of LEDs in lamps, a more reasonable design guideline is: use heat pipes in key heat dissipation positions. The thermal conductivity of the heat pipe can reach 8~12 times that of the copper plate of the same thickness. Although the price is high, if it is used in key parts, it will play a multiplier effect on the heat dissipation of the LED.

The shell of the lamp is designed as a diffuser. At present, most of the LED outdoor lamp shells are made of aluminum. Directly using the outside of the lamp shell as a radiator can not only meet the requirements of IP protection level, but also obtain a large heat dissipation area. In addition, the radiator formed by the housing of the lamp can be washed by natural wind and rain when there is falling dust, so as to ensure the continuous effectiveness of the radiator.

  1. High power LED heat pipe radiator

Heat pipe is a phase-change heat transfer element with particularly high thermal conductivity developed in the 1960s, which can transmit a large amount of heat over a long distance through its small cross-sectional area without external power. It transfers heat through the evaporation and condensation of the working medium in the fully enclosed vacuum tube shell. A series of advantages such as temperature control.

The heat pipe can obtain a large heat transfer rate with a small temperature difference, has a simple structure, and has the characteristics of one-way heat conduction. In particular, due to the unique mechanism of the heat pipe, the heat exchange between the cold and hot fluids is carried out outside the pipe, which can facilitate heat transfer enhancement. In addition, since the inside of the heat pipe is generally evacuated, the working medium is easily boiled and evaporated, and the heat pipe starts very quickly.

In 1984, Cotter first proposed the concept of miniature heat pipes for electronic cooling. After 1993, Mallik and others integrated micro heat pipes with semiconductors to solve the problem of heat dissipation, and applied the technology to notebook computers and other fields. Now heat pipes have been used in space technology, electronics, metallurgy, power, petroleum, chemical industry, etc. It has been widely used in various industries. The loop heat pipe is composed of thin film resistance heater, evaporator, liquid accumulator, steam pipeline, liquid return pipeline, condenser, DC power supply, data acquisition and other parts, as shown in Figure 1. Among them, there is a group of capillary structures inside the evaporator [the structure of the evaporator and the arrangement of the internal wire mesh capillary structure are shown in Figure 1(b) A-A section Figure 7, the basic working principle is that the capillary structure itself can move the liquid upwards Suction, so that the capillary structure is filled with working fluid.

Figure 1 - Schematic of the loop heat pipe
Figure 1 – Schematic of the loop heat pipe

When the evaporator is heated, the capillary structure is also heated, and the liquid in the capillary structure will evaporate into gas, and pass the steam channel along the steam section to the condensing section, while taking away heat; in the condensing section, the gas is It condenses into a liquid and releases heat; the capillary force of the capillary structure makes the liquid flow back to the liquid reservoir along the return section and reach the capillary structure. In this way, a working fluid circulation and heat transfer process are formed. The function of the accumulator is mainly to accommodate the liquid in the steam section and the condensation section when it is started, and to prevent the liquid from returning too late to cause the evaporator to dry up during operation.

When the heat pipe has no working fluid, the function of the heat pipe is equivalent to a copper plate radiator. When the heat flow density is large, the copper plate can no longer take away the heat in time, and the heat accumulates on the film resistor, resulting in the temperature of the film resistor in a short time. sharply increased to 200°C
about. In the test, in order to protect the thin film resistor, no longer test was done, which just shows that the loop heat pipe transfers heat by phase change, and its thermal conductivity is better than that of the copper plate.

The junction temperature of the LED device is mainly determined by the ambient temperature of the device and the thermal resistance from the PN junction of the device to the environment. Thermal resistance is the ratio of the temperature difference between the two ends of the thermal conduction medium to the power passing through the heat flow, in units of °C/W or K/W. The total thermal resistance of the high-power LED device plus the external thermal village is (2):

Since the high-power LED package is generally surface-mounted, in order to closely connect the LED device to the wall of the heat pipe evaporator, a layer of thermally conductive insulating glue should be applied between the LED device and the wall of the heat pipe evaporator. The total thermal resistance of the high-power LED device plus the external thermal lining can be expressed as the sum of several thermal resistances on the thermal path from the PN junction to the external environment (3):

In the formula, Rj-s is the thermal resistance of the LED device, that is, the thermal resistance between the PN junction and the heat liner; Rttach is the thermal resistance of the thermal insulation adhesive between the heat liner and the heat pipe evaporator wall; Rheat-pie is the heat pipe to the environment (generally thermal resistance between air). (4) can be obtained from formula (2) and formula (3):

Since the LED junction temperature directly affects the luminous flux, light quality and life of the LED, it is generally stipulated that the LED junction temperature should not be higher than 125°C, namely (5):

The thermal resistance of the loop heat pipe is defined as (6):

In the formula: Rth is the thermal resistance, Te is the temperature of the evaporator, Tcc is the average temperature of the condensation section, and Q is the heat transfer amount of the loop heat pipe.
All the internal performance indicators of the heat pipe are finally reflected in the thermal resistance of the heat pipe. As the power increases, the thermal resistance of the loop gradually decreases and tends to be flat. The thermal resistance at 100W is about 0.21K/w. The thermal resistance of the heat sink can only reach about 0.5K/w.

With the increase of the input power of the thin film resistor, the temperature of the thin film resistor, the temperature of the evaporator and the inlet temperature of the condenser rise almost linearly. When the input power is 85W, the temperature of the evaporator wall is 82.6 ℃, and the temperature of the thin film resistor is 87.5°C, and the inlet temperature of the condenser is 80°C. At this temperature, most electronic chips are still in normal working condition, which reflects the excellent performance of the loop heat pipe in heat transfer.

Compared with conventional heat sinks, the loop heat pipe radiator has the advantages of small size and high heat dissipation efficiency, and can quickly and effectively dissipate heat from high-power LED arrays and other high-power density devices; the miniaturization and low-cost of the loop heat pipe The key to the application of LED is also a subject that needs further research.