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, stability and reliability. 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 driving 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 during recombination, 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 systems 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. Compared with other refrigeration systems, the volume and power can be made very small, which 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. Using C language for unprocessed programming, when the real-time temperature collected is less than the cooling start temperature, there is no PWM modulation wave [1,6] output, the cooling module is idle; when the real-time temperature collected 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 program 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, it designs 4 ×3 input keyboard, the cooling start temperature and the strong cooling temperature are input by the keyboard; the PWM control circuit is designed to realize the control of the working voltage of the semiconductor cooling sheet TEC[5], thereby realizing the control of the cooling power of the semiconductor cooling 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 a 4 × 3 keyboard [4], including 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 cooling sheet. 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 low when initializing, P1.5~P1.7 is set high and wait for the button. When a key is pressed, the corresponding column selection line level is forcibly pulled down. When the corresponding line code and column code are read, the number of the button can be determined.
4.3 temperature acquisition circuit
The system uses the digital temperature sensor DS18B20.DS18B20 produced by DALLAS in the United States. It is a temperature measurement chip that communicates with a single-chip microcomputer using only one signal line (1-Wire). It can measure (to meet the temperature measurement requirements of the system). The temperature can be realized by programming. 9 is the digital temperature output. The measurement accuracy is because the DS18B20 exhibits a large leakage current when the temperature is higher. The communication with the AT89C51 may crash, so the external power supply mode is used. The biggest feature of the DS18B20 is the single-bus transfer mode, so it has strict timing requirements for reading and writing data bits. Timing includes: initialization timing, read timing, and write timing. Each command and data transmission starts from the start-up write sequence of the MCU. If the DS18B20 is required to send back data, after the write sequence is executed, the MCU needs to start the read sequence to complete the data reception, and the data and command transmission are all in the first place.
4.4 PWM control circuit
The PWM. control circuit consists of a photocoupler and a Cuk circuit [3]. In this control circuit, the optocoupler can effectively suppress the noise of the ground loop, eliminate ground interference, and improve the anti-interference ability of the whole system; the optocoupler electrically connects the input terminal (single chip AT89C51) and the output terminal (semiconductor refrigeration chip TEC) Isolation avoids accidental damage to the main control chip AT89C51, effectively protecting the AT89C51 microcontroller. In addition, the control circuit also uses a photocoupler to form a switching circuit, which saves the use of the switching device. The function of the Cuk DC chopper circuit is to convert the +15V external power supply into a regulated voltage DC, that is, the voltage at the output of the Cuk circuit (the operating voltage of the semiconductor cooling fin TEC) is adjustable. The adjustable voltage between the output terminals OUT+ and OUT- is controlled by the turn-off frequency between Q1 and Q2. The Cuk circuit is selected in this control circuit, because the Cuk chopper circuit has a significant advantage, that is, its input supply current and output load current are continuous, and the pulsation is small, which is beneficial to ensure that the semiconductor refrigeration chip TEC is in good working. status.
Due to limited space, only the PWM control circuit is briefly introduced. When the PWM control signal is low, the transistor T1 is in the off state, and the current of the LED in the photocoupler is approximately zero, between the output terminals Q1 and Q2. The resistance is very large, which is equivalent to the switch "off"; when the PWM wave control signal is high level, the transistor T1 is in an on state, the light emitting diode in the photocoupler emits light, and the resistance between the output terminals Q1 and Q2 is small, equivalent to The switch is "on". As can be seen from the above, when the real-time temperature collected by the DS18B20 is less than the cooling start temperature, when the PWM input of the photocoupler has no signal input, the photocoupler is inactive, and the OUT+ terminal in Figure 5 There is no output voltage at the OUT- terminal, that is, the semiconductor cooling chip is in an idle state; when the real-time temperature collected by the DS18B20 is greater than the cooling start temperature, the PWM input terminal of the photocoupler has a signal input, and the OUT+ terminal and the OUT- terminal in FIG. 5 have The output voltage. The PWM modulation wave is used to control the on and off of both ends of Q1 and Q2, so that the operating voltage of the semiconductor refrigeration chip TEC can be controlled, and the heat dissipation power of the semiconductor refrigeration chip TEC can be controlled. The OUT+ terminal and the OUT- terminal in FIG. 5 are respectively connected to the input terminal lines of the semiconductor cooling fin TEC. According to the relationship between the output voltage of the CUK circuit and the power supply voltage, the relationship between the PWM wave duty ratio and the input voltage of the semiconductor cooling chip TEC can be obtained:
Where D is the duty cycle of the PWM wave,
For the operating voltage of the semiconductor cooling sheet TEC, E is the voltage of the power supply (E=15V in this circuit). It can be seen from the above equation that the operating voltage of the semiconductor cooling fin TEC can be controlled by controlling the duty ratio of the PWM wave.
5 Conclusion
In this paper, some low-cost and relatively high-performance components are selected. Different power cooling is performed for different operating temperatures of LED chips, which saves power resources to a certain extent. Compared with the traditional heat dissipation scheme, this scheme has the advantages of good controllability and good cooling effect, and has very practical significance for solving the heat dissipation problem of high-power LED lighting system.
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