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Why do LED lights Generate Heat and the Lighting Calculation Formula

Author: Apogeeweb Date: 28 Feb 2018  1705

how does an led work

Warm hints: The word in this article is about 2500 and  reading time is about 12 minutes.

Introduction

This paper is main about the reason LED lights generate heat and the lighting calculation formula. LED light bulb has a hot market, the price of big brand in the online store dazzling, expensive also has precious reason, cheap also has cheap market. Today, one of the shortcomings of the LED bulb: heat. What about the LED light bulb? LED will generate heat, and many people think that LED is a cold light source, which is only the light principle of LED.

How Does LED Heat Dissipation Looks Like?

Catalog

Introduction

I Does an LED Produce Heat

II The Reason Caused LED Generate Heat

III Solution of Heat Dissipation of LED Bulb

IV Matters of Heat Dissipation

V Lighting Calculation Formula

Ⅵ Recommended Reading


I Does an LED Produce Heat

Integrated current injection efficiency, radiant emitting quantum efficiency and chip external light extraction efficiency will probably only convert 30-40% input energy into light energy. The remaining 60-70% energy will mainly be transformed into thermal energy in the form of lattice vibration with non radiation recombination.

The increase of the chip temperature will enhance the non radiation recombination and further weaken the luminous efficiency. Because people subjectively think that the high power LED has no heat, in fact it does. A lot of heat, so that there is a problem in the use of the process. With many people who first use high power LED, they do not know how to solve the heat problem effectively, which makes the reliability of the product the main problem.

If we explained the phenomenon that LED produce heat according to its form,it can be said: the LED lamp is a solid state light source. The core is the luminescent two level tube (LED). Under the forward voltage, the LED obtains energy from the power. Under the driving of the electric field, the electric field of PN junction is overcome, and the N area is transferred to the P area. These electrons are combined with the holes in the P region. Because the free electrons drifted into the P region are higher than the energy of valence electrons in the P region, the electrons return to the low-energy state when combined, the excess energy is released in the form of photons, and the wavelength of the emitted photons is related to the energy difference. The common dipole, in the combination of electron hole pairs, is that the emission spectrum is not within the range of visible light because of the factor of energy differential.

Why do LED lights Generate Heat and the Lighting Calculation Formula

The LED lamp produces heat during work, depending on the overall luminous efficiency. Under the action of electric energy, the radiation of the electron and hole is combined to generate electroluminescence. The photon radiated near the P-N junction needs to go through the semiconductor medium and the packaging medium of the chip itself. Combined calculation of current injection efficiency, radiation emitting quantum efficiency and chip external light extraction efficiency, only about 30-40% of the input energy is converted to light energy. The remaining 60-70% energy is mainly transformed into the thermal energy in the form of lattice vibration. The increase of the chip temperature will enhance the non radiation combination and further weaken the luminous efficiency, so the heat control of the LED lamp becomes the focus of the LED technology.

 

II The Reason Caused LED Generate Heat

Specifically, the production of LED junction temperature is caused by two factors:

The internal quantum efficiency is not high, that is, when electrons and holes are combined, no photon can be generated at all 100%. It is usually referred to as "current leakage", which reduces the recombination rate of carriers in PN region. Leakage current multiplied by voltage is the power of this part, that is to say, it is converted to heat energy, but this part is not the main component, because the internal photon efficiency is close to 90% now.

Why do LED lights Generate Heat and the Lighting Calculation Formula

The internal generated photon can not be all emitted to the outside of the chip, and finally converted to heat. This part is the main reason, because at present, the external quantum efficiency is only about 30%, and most of them are converted into heat.

As mentioned before, though the light efficiency of incandescent lamp is very low, only about 151m/W, it almost transforms all electric energy into light energy and radiates out, because most of the radiant energy is infrared, so the light efficiency is very low, but it eliminates the problem of heat dissipation.

 

III Solution of Heat Dissipation of LED Bulb

To solve the heat dissipation of LED bulb, it is mainly from two aspects. Before and after encapsulation, it can be understood that LED chip heat dissipation and LED light bulb heat dissipation. The heat dissipation of LED chip is mainly related to the selection of substrate and circuit and technology. This article is not expound for the time being. This article mainly introduces the heat dissipation of LED bulbs, because any LED will be made into lamps and lanterns, so the heat generated by LED chips will always be dispersed into the air through the shell of lamps and lanterns. Lei Yusi lighting that if the heat is not good, because the heat capacity of the LED chip is very small, will accumulate a little heat makes the chip junction temperature increases rapidly, if a long period of time to work in a high temperature state, it will quickly shorten the life. However, the heat can actually guide the chip to the outside air, through many ways. Specifically, the heat generated by the LED chip comes out of its metal cooling block, then passes through the solder to the PCB of the aluminum substrate, and then passes through the thermal conductive adhesive to the aluminum radiator. So the heat dissipation of LED hotel engineering lamps and lanterns in fact includes two parts: heat conduction and heat dissipation.However, the size of LED lamp cooling according to power and place, also have different choices. Now there are several kinds of heat dissipation methods:

  • Aluminum fin: This is the most common way of heat dissipation, with aluminum fin as part of the shell to increase the area of heat dissipation.

  • Heat conduction plastic shell: fill the heat conduction material in the plastic shell injection, increase the heat conduction and heat dissipation of the plastic shell.

  • Air: air cooling fluid mechanics fluid mechanics using shell shape, create convection air, this is the lowest cost way to strengthen the heat.

  • The lamp shell with long life and high efficiency fan fan strengthen heat: low cost, good effect. However, it is a little trouble to change the fan, and it is not suitable for outdoor. This kind of design is rare.

  • Heat conduction tube heat dissipation technology: heat pipe using heat conduction tube technology, the heat from the LED chip to the shell fin fin. This is a common design in large lamps, such as street lamps.

The surface radiation heat treatment lamp surface radiation treatment is simple: apply radiation coating, heat can be taken away from the lamp surface with radiation.

Why do LED lights Generate Heat and the Lighting Calculation Formula

 

IV Matters of Heat Dissipation

LED lights need to solve the following problems of heat dissipation:

  • 1. The chip is bound to the epitaxial layer

  • 2. Epitaxial layer to package substrate

  • 3. Encapsulate the substrate to the external cooling device

These three links constitute the heat conduction channel of LED lamp. Any problems in the heat conduction channel will cause the LED junction temperature to be too high, which will cause the phenomenon of LED light decay, until the product is scrapped early.

The heat conduction square search formula of the LED node to the surrounding environment can be divided into three types:

  • 1. Conduction. Heat is passed directly through the adjacent atoms, so a low interfacial thermal resistance material should be used.

  • 2. Convection. Heat transfers through the flow of objects such as air and water to the radiator, and then from the radiator to the surrounding environment.

  • 3. Radiation. Heat is passed through the liquid, gas and vacuum by electromagnetic waves, so high radiation material is needed.

In order to achieve better heat transfer effect, the three links need to take appropriate measures.

 

V Lighting Calculation Formula

Under the forward voltage, LED gains energy from the power source. Under the driving of electric field, the electric field of PN junction is overcome, from N Qu Yueqian to P region, and these electrons are combined with the holes in P area. As the free electrons drift to the P region have the energy higher than the valence electrons of the P region, the electrons return to the low energy state, and the excess energy is released in the form of photons. The wavelength of the photon is related to the energy difference Eg. It can be seen that the luminescence region is mainly near the PN junction, and the luminescence is the result of the combination of electrons and holes to release energy. A semiconductor dipole, the electron will encounter resistance in all the way that enters the semiconductor area and leaves the semiconductor area. From a simple perspective, the physical structure of semiconductor diodes is, in principle, the physical structure of semiconductor diodes. The electrons emitted from the source and negative electrodes are equal to the number of electrons coming back to the positive electrode. The common dipole, in the combination of electron hole pairs, is that the emission spectrum is not within the range of visible light due to the factor of differential Eg.

In the path of the dipole, the electrons consume the power because of the existence of the resistance. The power consumed is in accordance with the basic laws of electronics:

P=I2R=I2(RN++RP)+IVTH

In this formula: RN is the body resistance of the N region; VTH is the open voltage of PN junction; RP is the body resistance of the P region

The heat generated by the consumed power is: 

Q=Pt

In this formula,t is the time for the electric power of the dipole.

In essence, LED is still a semiconductor diode. Therefore, when LED is working forward, its working process is in line with the above description. The electric power it consumes is:

PLED=ULED&TImes; ILED

In the formula: ULED is the forward voltage at both ends of the LED light source;ILED is the current that flows through the LED.

The electrical power consumed is converted to heat release: in this formula: t is the power time.

Q=PLED&TImes; t

In fact, the energy released by electrons when combined with holes in the P region is not directly supplied by the external power source, but because the electron level in the N region is higher than that in the P region when it has no external electric field. The energy level of the electron is higher than Eg. When it reaches the P area, it will release so much energy when it is combined with the hole to become the valence electron in the P region. The size of the Eg is determined by the material itself and has nothing to do with the external electric field. The effect of an external power supply on the electron is only to drive it to be directed and overcome the PN junction.

The heat production of LED has nothing to do with the light effect; there is not a few per cent of the electric power to produce light, and the rest of the electrical power of a few percent produces a heat relationship. Through understanding the concept of heat generation, thermal resistance and junction temperature of large power LED, deriving the theoretical formula and measuring the thermal resistance, we can study the actual package design, evaluation and product application of high-power LED. It is the thermal management is a key problem in the LED products of the luminous efficiency is not high at the present stage, fundamentally improve the luminous efficiency in order to reduce the heat generated is a fundamental solution to the need to develop the technical aspects of chip manufacturing, packaging and LED application products progress.

 

LED Lighting Circuit Protection

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