Taking the single chip AT89C51 as the control core, the semiconductor refrigeration technology is introduced into the LED heat dissipation research. The PID algorithm and PWM modulation technology are used to control the input voltage of the semiconductor refrigeration chip, and then the control of the semiconductor refrigeration power is realized. The feasibility of this method.
With the rapid development of LED technology, LED has entered the market of general lighting. However, the development of LED lighting systems is largely affected by thermal issues. For high-power LEDs, heat dissipation has become a bottleneck restricting its development. The semiconductor refrigeration technology has the advantages of small size, no need to add refrigerant, simple structure, no noise, and stability. With the advancement of semiconductor material technology and the discovery of high thermoelectric conversion materials, semiconductor refrigeration technology is used to solve the LED lighting system. The heat dissipation problem will have a very realistic meaning.
1 LED heat generation and the effect of heat on LED performance
At the forward voltage, the electrons receive energy from the power source. Under the drive of the electric field, the electric field of the PN junction is overcome, and the N region transitions to the P region. These electrons recombine with the holes in the P region. Since the free electrons drifting to the P region have higher energy than the P region valence electrons, the electrons return to the low energy state when recombined, and the excess energy is released in the form of photons. However, only 30% to 40% of the released photons are converted into light energy, and the remaining 60% to 70% are converted into heat energy in the form of point vibration.
Since the LED is a semiconductor light-emitting device, and the semiconductor device changes itself with changes in temperature, the inherent characteristics thereof change significantly. An increase in the junction temperature of the LED results in a change in device performance and attenuation. This change is mainly reflected in the following three aspects: (1) reducing the external quantum efficiency of the LED; (2) shortening the life of the LED; (3) causing the main wavelength of the LED to emit light to shift, thereby causing the color of the light source to shift. High-power LEDs generally use more than 1W of electric power input, which generates a lot of heat, and solving the heat dissipation problem is a top priority.
2 semiconductor refrigeration principle
Semiconductor refrigeration, also known as electronic refrigeration, or thermoelectric cooling, is a discipline that has developed from the 1950s on the edge of refrigeration technology and semiconductor technology. It is also known as the world's three major refrigeration methods with compression refrigeration and absorption refrigeration. The basic device of a semiconductor refrigerator is a thermocouple pair, that is, an N-type semiconductor and a P-type semiconductor are connected into a thermocouple (see Figure 1). After the DC current is applied, temperature difference and heat transfer occur at the interface. . A pair of pairs of semiconductor thermocouples are connected in series on the circuit, and the heat transfer is parallel, thus forming a common refrigeration thermopile. By means of various heat transfer means such as heat exchangers, the hot end of the thermopile continuously dissipates heat and maintains a certain temperature, and the cold end of the thermopile is placed in the working environment to absorb heat and cool down, which is the principle of semiconductor refrigeration.
This article uses semiconductor refrigeration because it has no mechanical rotating parts, no refrigerant, no pollution, high reliability, long life and easy control, and can be made small in volume and power compared with other refrigeration systems. It is very suitable for LED limited. Application in the workspace.
3 system overall design
The LED heat dissipation control system consists of a temperature setting module, a reset module, a display module, a temperature acquisition module, a control circuit module [2] and a refrigeration module. The overall block diagram of the system is shown in Figure 1. The system uses a microprocessor as the control core, communicates with the temperature acquisition module to collect the real-time temperature of the controlled object, and communicates with the temperature setting module to set the cooling start temperature and the strong cooling temperature. Unprocessed programming can be realized by C language. When the real-time temperature collected is less than the cooling start temperature, there is no PWM modulation wave [1,6] output, and the cooling module is idle; when the collected real-time temperature is greater than the cooling start temperature but less than mandatory When the temperature is cold, the PWM modulation wave with a certain duty ratio is output, and the refrigeration module starts the low-power cooling mode; when the real-time temperature collected is greater than the strong cooling temperature, the PWM modulation wave with a certain duty ratio is output, and the refrigeration module starts the high-power Cooling method.
4 hardware circuit design and its component selection
The system is mainly composed of temperature setting, temperature acquisition, PWM control circuit and auxiliary circuit (reset circuit and display circuit). This scheme adopts low-cost and high-performance AT89C51 as the main control chip to realize the logic control function of the whole system. The high-precision temperature sensor DS18B20 with single-wire communication realizes the real-time temperature collection of the LED chip of the controlled object. At the same time, 4&TImes is designed. 3 input keyboard, cooling start temperature and strong cooling temperature are input by the keyboard; PWM control circuit is designed to realize the control of the working voltage of the semiconductor refrigeration chip TEC[5], thereby realizing the control of the cooling power of the semiconductor refrigeration sheet TEC, Achieve the effect of timely cooling of the LED chip.
4.1 main control chip AT89C51
The main control chip of the system is a single-chip AT89C51. The single-chip AT89C51 is a low-voltage, high-performance processor produced by American ATMEL Company, which provides a flexible and inexpensive solution for the embedded control system. The AT89C51 microcontroller contains 4KB of Flash memory, which can repeatedly erase 1000 times, 128 bytes of RAM, four parallel 8-bit bidirectional I/Os and two 16-bit programmable timers. In addition, the main control chip AT89C51 uses a crystal oscillator with a frequency of 12MHz, so that the system runs a machine cycle, which is conducive to the programming of the program. The main function of the single-chip AT89C51: read the set cooling start power and forced power from the keyboard circuit, read the real-time collected LED chip operating temperature from the temperature sensor DS18B20, compare the two to the optocoupler output PWM modulation wave through C language programming The temperature collected by the DS18B20 in real time is output to the LCD display.
4.2 keyboard circuit
The system uses 4 & TImes; 3 keyboard, contains 0 to 9 a total of 10 number keys, a "OK" button and a "clear" button. The operation flow is as follows: input 2 sets of set temperature, press "OK", input the set temperature to a storage unit in the user-defined area of ​​AT89C51, as the starting temperature of the semiconductor refrigeration chip. Then, similarly, the 2-digit temperature is input again as the strong cooling temperature of the semiconductor refrigerating sheet. The working principle of the keyboard: I/O port P1.0~P1.3 acts as the row selection line, and P1.5~P1.7 (external pull-up resistor to +5V power supply) acts as the column selection line. P1.0~P1.3 is set to low potential during initialization, P1.5~P1.7 is set to high potential and wait for the button. When a key is pressed, the corresponding column selection line level is forcibly pulled low, and the corresponding line code and column code are read, and the number of the button can be determined.
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