Analysis on the Basic Principle of LED Heat Generation

Based on the current injection efficiency, radiation luminescence quantum efficiency and external light extraction efficiency of the chip, only about 30-40% of the input electric energy is converted into light energy, and the remaining 60-70% of the energy is mainly converted into heat energy in the form of lattice vibration generated by non radiation recombination.

The increase of chip temperature will enhance non radiative recombination and further weaken the luminous efficiency. Because people subjectively think that high-power LED has no heat, in fact, it does. So much heat that problems occur during use. In addition, many people who use high-power LED for the first time do not know how to effectively solve the thermal problem, which makes the product reliability become the main problem. So, does the LED generate heat? How much heat can it produce? How much heat does led produce?

Under the forward voltage, the electrons of LED obtain energy from the power supply. Driven by the electric field, they overcome the electric field of PN junction and transition from n region to p region. These electrons compound with the holes in P region. Since the free electrons drifting to the p region have higher energy than the valence electrons in the p region, the electrons return to the low energy state during recombination, and the excess energy is released in the form of photons. The wavelength of the emitted 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 energy released by the recombination of electrons and holes. In a semiconductor diode, electrons will encounter resistance in the whole journey from entering the semiconductor region to leaving the semiconductor region. In principle, the physical structure of the semiconductor diode is simple. In principle, the physical structure of the semiconductor diode is equal to the number of electrons emitted from the negative source and returned to the positive electrode. For ordinary diodes, when electron hole pair recombination occurs, the photon spectrum released is not in the visible range due to the energy level difference eg.

On the way inside the diode, electrons will consume power due to the existence of resistance. The power consumed complies with the basic laws of Electronics:


Where: RN is the body resistance of n area

Vth is the on voltage of PN junction

RP is the body resistance of P area

The power consumed and the heat generated are:


Where: t is the power on time of the diode.

In essence, LED is still a semiconductor diode. Therefore, when the LED works in the forward direction, its working process conforms to the above description. The electric power consumed is:


Where: uled is the forward voltage at both ends of LED light source:

Iled is the current flowing through the LED

These consumed electric power is converted into heat release:

Q=PLED × t

Where: t is the power on time

In fact, the energy released when the electron recombines with the hole in the p region is not directly provided by the external power supply, but because when the electron is in the N region and there is no external electric field, its energy level is eg higher than the valence electron energy level in the p region. When it reaches the p region and recombines with holes to become valence electrons in the p region, it will release so much energy. The size of EG is determined by the material itself and has nothing to do with the external electric field. The effect of external power supply on electron is only to push it to make directional movement and overcome the effect of PN junction.

The heat production of LED has nothing to do with light effect; There is no relationship that a few percent of the electric power produces light and the remaining few percent produces heat. Through the understanding of the concepts of thermal generation, thermal resistance and junction temperature of high-power LED, the derivation of theoretical formula and thermal resistance measurement, we can study the actual packaging design, evaluation and product application of high-power LED. It should be noted that heat management is a key issue at this stage when the luminous efficiency of LED products is not high. Fundamentally improving the luminous efficiency to reduce the generation of heat energy is a drastic move, which requires technological progress in all links of chip manufacturing, LED packaging and application product development.

Analysis on the Basic Principle of LED Heat Generation 1

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