Solution To Heat Dissipation Problem Of High Power LED Lighting System
- Jul 11, 2018 -

With single chip microcomputer AT89C51 as the control core, will be introduced to the semiconductor refrigeration technology LED heat dissipation in the study, the PID algorithm and PWM modulation technology to achieve 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. 

With the rapid development of LED technology, LED has entered the general lighting market. (LED High Bay Light)

However, the development of LED lighting system is greatly affected by the heat dissipation problem. (LED High Bay Light)

For high-power LED, the heat dissipation problem has become a bottleneck restricting its development. (LED High Bay Light)

And semiconductor refrigeration technology has small volume, no need to 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, will has very realistic significance. (LED High Bay Light)


1. Causes of LED heat generation and its effect on LED performance


LED at positive voltage, the electrons get energy from the power source. Driven by the electric field, they overcome the electric field of PN junction and jump from N area to P area. (LED High Bay Light)

Since the free electrons drifting to the P region have energy higher than the valence electrons in the P region, the electrons return to a low energy state when they are composite, and the excess energy is released as photons. (LED High Bay Light)

However, only 30 to 40 percent of the photons released are converted to light energy, while the remaining 60 to 70 percent are converted to heat energy in the form of point vibration. (LED High Bay Light)


LED is a semiconductor luminescent device, and the semiconductor device itself changes with the change of temperature, so its inherent characteristics will change significantly. (LED High Bay Light)

The increase of LED junction temperature will lead to the change and attenuation of device performance. (LED High Bay Light)

This change is mainly reflected in the following three aspects:

Parallel processing reduces the life of the LED;

The main wavelength of light emitted by leds is offset, causing the color of the light source to shift.

High-power LED usually USES more than 1W of electric power input, which generates a lot of heat. Therefore, it is urgent to solve its heat dissipation problem.


Principle of semiconductor refrigeration


Semiconductor refrigeration is also called electronic cooling, or thermoelectric refrigeration, developed from the '50 s with a door 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. (LED High Bay Light)

Semiconductor refrigerating unit is the basic device of thermocouple for, namely an n-type semiconductor and a p-type semiconductor connection into a thermocouple (as shown in figure 1), after applying a direct current, the interface can produce temperature difference and heat transfer. (LED High Bay Light)

Several pairs of semiconductor thermocouples are connected in series on the circuit, and the heat transfer is in parallel, thus forming a common cooling thermoelectric reactor. (LED High Bay Light)

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. (LED High Bay Light)




This article USES the semiconductor refrigeration is because compared with other refrigeration system, no mechanical part, no turning refrigerants, no pollution, high reliability, long service life and easy to control, and power can do small volume, very suitable for application in LED limited working space. (LED High Bay Light)


3. Overall design scheme of the system


LED heat dissipation control system consists of temperature setting module, reset module, display module, temperature acquisition module, control circuit module [2] and refrigeration module. The overall block diagram of the system is shown in figure 1. (LED High Bay Light)

The system takes the 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 starting temperature and the strong cooling temperature. (LED High Bay Light)

Unprocessed programming can be realized by using C language. When the real-time temperature collected is lower than the starting temperature of refrigeration, there is no PWM modulation wave output [1,6], and the refrigeration module is idle. (LED High Bay Light)

When the real-time temperature collected is greater than the starting temperature of refrigeration but less than the strong cooling temperature, the PWM modulation wave with a certain space ratio is output. (LED High Bay Light)

When the collected real time temperature is greater than the strong refrigeration temperature, the PWM modulation wave with a certain space ratio is output, and the refrigeration module starts the high-power refrigeration mode. (LED High Bay Light)


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).

This scheme adopts low price 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 is used to collect the real time temperature of the controlled object LED chip.

At the same time, the 4 x 3 input keyboard was designed. (LED High Bay Light)

Design of PWM control circuit, realize the semiconductor refrigeration piece of TEC [5] the working voltage of the control, thus realize the semiconductor refrigeration piece TEC refrigeration power of control, in order to achieve in time for the LED chip heat dissipation effect.


4.1 main control chip AT89C51


Master control chip of the system chosen is single chip microcomputer AT89C51 single chip microcomputer AT89C51 is the ATMEL company production of low voltage, high performance processor, for embedded control system provides a high flexibility of cheap solution.

MCU AT89C51 contains 4KB Flash memory, which can be repeatedly wiped 1000 times, 128 bytes of RAM, four parallel 8-bit two-way I/O and two 16-bit programmable timers. (LED High Bay Light)

In addition, the main control chip AT89C51 USES the crystal vibration with the frequency of 12MHz, so that the system runs a machine cycle, which is conducive to programming. (LED High Bay Light)

Single-chip computer AT89C51 is main functions: read from keyboard circuit set start cooling power and enforcement power, from the collection of temperature sensor DS18B20 read in real time operating temperature of the LED chip, through the C language programming to the comparison of photoelectric coupler output PWM has affected the DS18B20 temperature real time acquisition and output to the LCD display. (LED High Bay Light)


4.2 keyboard circuit


The system USES a 4-by-3 keyboard and contains 10 numeric keys, a "ok" key and a "clear" key.

The operation process is as follows: input 2-bit set temperature, press "ok" and input the set temperature to a storage unit in the user-defined area of AT89C51 as the starting temperature of the semiconductor refrigeration unit. (LED High Bay Light)

Then, by the same logic, enter the two-bit temperature again as the strong cooling temperature of the semiconductor refrigeration unit.

How the keyboard works: I/O port P1.0~P1.3 ACTS as the line selection, and P1.5~P1.7 (external pull resistance to +5V power supply) ACTS as the line selection. (LED High Bay Light)

P1.0~P1.3 set low potential, P1.5~P1.7 set high potential and wait for the key. (LED High Bay Light)

When a key is pressed down, the corresponding line selection level of the corresponding column is forced to be lowered, and the corresponding line code and column code are read, then the key number can be determined. (LED High Bay Light)




4.3 temperature acquisition circuit


The system adopts the DALLAS company's production of digital temperature sensor DS18B20. The DS18B20 is a use only one signal line (1 - Wire), communicate with single chip microcomputer temperature measurement chip can be measured (meet the requirements of the system of measuring temperature), the temperature between the use of programming can achieve 9 for output, digital temperature because the temperature is higher than for accuracy of measurement, DS18B20 showed a larger leakage current, and single chip microcomputer AT89C51 communication may occur collapse, it adopts the model of external power source supply. (LED High Bay Light)

The biggest characteristic of DS18B20 is the single-bus transmission mode, so it has strict time sequence requirements for reading and writing data bits. (LED High Bay Light)

The timing sequence includes: initializing the timing sequence, reading timing sequence, and writing timing sequence. (LED High Bay Light)

Every commands and data transmission from the microcontroller start write timing starts, if requirements DS18B20 the echo data, after writing timing, single chip microcomputer to start reading sequence data reception, data transmission and command are a prior position.




4.4PWM control circuit


The control circuit consists of a photoelectric coupler and a Cuk circuit [3].

In this control circuit, the photoelectric coupler can effectively suppress the noise of the ground loop, eliminate the ground interference, and improve the anti-interference ability of the whole system. (LED High Bay Light)

Photoelectric coupler to input (single chip microcomputer AT89C51) and output (TEC) to the semiconductor refrigeration electrical isolation, avoiding the main control chip AT89C51 is accidental damage, effectively protect the single chip microcomputer AT89C51. In addition, also used in the control circuit of photoelectric coupler switch circuit, save the use of switching devices. (LED High Bay Light)

The function of Cuk dc chopper circuit is to convert the external power supply of +15V into the direct current of adjustable voltage, that is, the voltage of the output end of Cuk circuit (the working voltage of semiconductor refrigeration chip TEC) is adjustable.

The adjustable voltage between OUT+ and OUT- is controlled by the switching frequency between Q1 and Q2. (LED High Bay Light)

Choose Cuk circuit in the control circuit, because Cuk chopper circuit has an obvious advantages, its input output power source current and load current is continuous, and the pulse is very small, can ensure the semiconductor refrigeration piece of TEC in good working condition. (LED High Bay Light)


Under the limited to the space is limited, only to the introduction of PWM control circuit is simple: when the PWM control signal for low electricity at ordinary times, transistor T1 in by the state, the current leds in the photoelectric coupler is near to zero, the output end of Q1 and Q2 between the resistance being very high, equivalent to switch "disconnect"; (LED High Bay Light)

When the PWM wave to control signal for high electricity at ordinary times, transistor T1 in conduction state, light-emitting diodes (leds in the photoelectric coupler, output between Q1 and Q2 resistance is small, equivalent to switch conduction. The mentioned above shows that when the DS18B20 collecting the real-time temperature of less than start cooling temperature, the PWM input end of the photoelectric coupler when no signal input, photoelectric coupler is in not working condition, in figure 5 OUT + side and OUT - no output voltage, the semiconductor refrigeration piece of idle;

When the real-time temperature collected by DS18B20 is greater than the starting temperature of refrigeration, the PWM input end of the photocoupler has signal input, and the OUT+ end and OUT- end in figure 5 have output voltage.

By controlling the on-off between Q1 and Q2 through PWM modulated wave, the working voltage of semiconductor refrigeration chip TEC can be controlled, and the cooling power of semiconductor refrigeration chip TEC can be controlled. (LED High Bay Light)

In figure 5, the OUT+ end and the OUT- end are respectively connected to the input terminal line of semiconductor refrigeration chip TEC.

According to the relationship between the output voltage of CUK circuit and the voltage of the power supply, the relationship between the PWM wave duty ratio and the TEC input voltage of the semiconductor refrigeration chip can be obtained: (LED High Bay Light)


Where D is the duty ratio of PWM wave, (LED High Bay Light)


Is the working voltage of semiconductor refrigeration chip TEC, and E is the voltage of power supply (in this circuit, E=15V).

As can be seen from the above equation, controlling the duty ratio of PWM wave can control the working voltage of semiconductor refrigeration chip TEC.




5 conclusion


In this paper, some components with low cost and relatively high performance are selected to conduct different power refrigeration for different operating temperatures of LED chips, which can save power resources to a certain extent.

Compared with the traditional cooling scheme, this scheme has the advantages of good controllability and good cooling effect, which is of great practical significance for solving the heat dissipation problem of high-power LED lighting system.