Application of heat pipe in high power LED lighting

High-power LEDs are characterized by energy saving, long life and long life. Especially in recent years, the rapid development of high-power LEDs is a concrete manifestation of this advantage. However, the heat dissipation problem of high-power LEDs still severely restricts the development of high-power LEDs. Therefore, changing the heat dissipation mode of high-power LEDs and dissipating the heat of LEDs as much as possible is of increasing importance to the development of high-power LEDs.

Keywords: high power LED heat pipe technology application advantages

LED (Light Emitting Diode), a solid-state semiconductor device that converts electricity directly into light. The heart of the LED is a semiconductor wafer. One end of the wafer is attached to a holder, one end is the negative pole, and the other end is connected to the positive pole of the power supply, so that the entire wafer is encapsulated by epoxy resin. The semiconductor wafer consists of two parts, one part is a P-type semiconductor, in which the hole dominates, and the other end is an N-type semiconductor, which is mainly electrons here. But when the two semiconductors are connected, they form a "PN junction" between them. When a current is applied to the wafer through the wire, the electrons are pushed toward the P region where the electrons recombine with the holes and then emit energy in the form of photons.

High-power LEDs are important applications for outdoor lighting, and their superiority is reflected in:

1. LED as a point source, if the design is reasonable, it can largely solve the traditional spherical light source directly, and must rely on light reflection to solve the problem of secondary light extraction and light loss;

2. The uniformity of the light-irradiated surface is controllable, and theoretically it can be completely uniform in the target area;

3. The color temperature is optional, so the application of LEDs with different color temperatures in different occasions is an important way to improve efficiency and reduce costs;

4. The space for technological progress is still very large. The development of high thermal conductivity metal materials will effectively improve the light efficiency of LEDs. For example, CREE can achieve a chip efficacy of 130-150 lm/W. The application of the key technology of the heat balance heat dissipation structure, under the premise of maintaining low cost and passive heat dissipation mode, can effectively reduce the thermal resistance, effectively reduce the PN junction temperature, and effectively reduce the PN junction temperature by using a high thermal conductivity medium and a new device/lamp overall structure. The junction operates within the allowable operating temperature to maintain the maximum amount of photon output.

I. Overview of heat pipes

1. The heat pipe is composed of a shell, a wick that acts as a capillary, and a working medium that transfers heat.

　　 The wick is firmly attached to the inner wall of the envelope and is infiltrated by the working fluid. The heat pipe itself forms a closed system of high vacuum, the structure of which is shown in Figure 1:

The working principle is: the heat generated by the high-power LED, the temperature of the liquid working medium rises by the heat conduction to the tube wall of the evaporation section and the immersion liquid of the working fluid; when it rises to a certain temperature, the liquid surface evaporates until it reaches The saturated vapor pressure, at which time the heat is transferred to the steam in the form of latent heat, and the saturated vapor pressure rises as the temperature of the liquid rises. Under the action of the pressure difference, the steam flows to the condensation section where the pressure and temperature are low, and condenses at the gas-liquid interface of the condensation section to emit latent heat. The released heat is transferred from the gas-liquid interface to the heat sink outside the tube through the heat conduction of the wick and the tube wall.

2. Characteristics of heat pipe technology:

(1) Excellent thermal conductivity The inside of the heat pipe mainly relies on the vapor and liquid phases of the working liquid to change heat, and the thermal resistance is small, so it has a high thermal conductivity.

(2) Irreversibility Due to the existence of temperature difference, heat is irreversible during transmission.