In recent years, along with the deepening of the research of semiconductor luminescence material, high power LED light source manufacturing technology progress and the development of new materials (nitride crystal and phosphor powder), and application of all colors of ultra-high brightness LED has made big progress, this makes the LED application field across to efficient lighting source market becomes possible.However, the development of LED lighting system is greatly affected by the heat dissipation problem. For high-power LED, the heat dissipation problem has become a bottleneck problem that restricts its development.
Semiconductor refrigeration technology has small volume, not need add refrigerant, simple structure, no noise and the advantages of stable and reliable, with the progress of semiconductor material technology, and the discovery of high thermoelectric conversion material, the use of semiconductor refrigeration technology to solve the heat dissipation problem of LED lighting systems.
This article based on single chip microcomputer AT89C51 as the control core, will be introduced to the semiconductor refrigeration technology LED heat dissipation in the study, adopt the PID algorithm and PWM modulation technique to realize the control of the semiconductor refrigeration piece of input voltage, thus realized the control of the semiconductor refrigeration power, the feasibility of this method is verified by experiment.
1. The cause of LED heat generation and the influence of heat on LED performance
Under the forward voltage, the electron obtains the energy from the power supply. Under the drive of the electric field, the electric field of PN junction is overcome, and the N region jumps to the P region, and these electrons are compounded with the hole in the p-zone.Because the free electron in the P area has the energy of the valence electron in the P region, the electron returns to the lower energy state when the electron is recomposed, and the excess energy is released in the form of photons.However, only 30%~40% of the photons are converted to light energy, while 60%~70% of the other photons are converted to heat energy in the form of point vibrations.
Because of leds are semiconductor luminescent devices, semiconductor devices change with temperature, and their inherent characteristics will change significantly.The increase of LED temperature will lead to the change and attenuation of device performance.This change is mainly reflected in the following three aspects:
1) Reduce the external quantum efficiency of LED;
2) Shorten LED life;
3) The main wavelength of the LED emitting light is offset, which causes the color of the light source to shift. The high power LED generally uses over 1W power input, and the heat generated quite high, so it is urgent to solve the heat dissipation problem.
2.Principle of semiconductor refrigeration
Semiconductor refrigeration is also called electronic cooling, or thermoelectric refrigeration, developed from the '50s between the refrigeration technology and semiconductor technology on the edge of the discipline, and the compression refrigeration and absorption refrigeration and called the world three big refrigeration way.Semiconductor refrigerating unit is the basic device of thermocouple for, namely an N type semiconductor and a P type semiconductor connection into a thermocouple after applying a direct current, the interface will produce temperature difference and heat transfer.
Several pairs of semiconductor thermocouples are connected in series on the circuit, and the heat transfer is connected in parallel, thus forming a common cooling thermoelectric reactor.By means of heat exchangers and other means of heat transfer, is the hot end of the thermopile constant heat and keep a certain temperature, and put the thermopile cold end in the work environment to endothermic cooling, this is the principle of semiconductor refrigeration.
This article uses the semiconductor refrigeration because compared with other refrigeration system, no mechanical part, no turning refrigerants, no pollution, high reliability, long service life and easy to control, both power and volume will be smaller, very suitable for application in LED’s limited working space.
3.Overall design system
LED radiator control system consists of temperature setting module, reset module, display module, temperature acquisition module, control circuit module and refrigeration module.The system uses microprocessor as the control core, and the temperature acquisition module is used to collect the real-time temperature of the controlled object, and set the cooling starting temperature and the strong refrigeration temperature with the temperature setting module communication.
The unprocessed programming can be realized in C language. When the real-time temperature of the collection is less than the cooling startup temperature, there is no PWM modulated wave output and the refrigeration module is idle.When the real-time temperature of the collection is larger than that of the cooling startup temperature but less than the strong refrigeration temperature, the PWM modulated wave with a certain amount of air ratio is output, and the refrigeration module starts the low power refrigeration mode.When the real-time temperature of the acquisition is greater than that of the strong refrigeration temperature, the PWM modulated wave with a certain amount of air ratio will be output, and the refrigeration module can start the high power refrigeration mode.
4.Hardware circuit design and component selection
The system consists of temperature setting, temperature acquisition, PWM control circuit and auxiliary circuit (reset circuit and display circuit).The scheme adopts low price and high performance AT89C51 as the main control chip to realize the logic control function of the whole system.A high-precision temperature sensor DS18B20 with single line communication is adopted to realize real-time temperature acquisition of controlled object LED chip.
At the same time, the 4 x 3 input keyboard was designed, the temperature of the cooling startup and the strong refrigeration temperature were input from the keyboard.Design of PWM control circuit, realize the semiconductor refrigeration piece of TEC  the working voltage of the control, thus realize the semiconductor refrigeration piece TEC refrigeration power of control, in order to achieve LED chip in time heat dissipation effect.
PWM. Control circuit consists of a photoelectric coupler and a Cuk circuit .In this control circuit, the photoelectric coupler can effectively suppress the noise of the ground loop, eliminate the interference, and improve the anti-interference ability of the whole system.The photoelectric coupler isolates the input terminal (single chip computer AT89C51) and the output end (semiconductor refrigeration chip TEC) electrical isolation, avoids the accidental injury of the main control chip AT89C51, and effectively protects the single-chip microcomputer AT89C51.