“Environmental issues have become the most important issue in the economic development of various countries, so energy-saving LED lighting has become the “new favorite” of the lighting industry. Because LEDs have higher luminous efficiency and lower manufacturing costs, their application prospects and markets are very broad. However, the heat dissipation problem of high-power LED lights limits the development of the LED lighting industry. If the heat dissipation problem is not solved, the temperature of the LED lights will rise, resulting in a decrease in luminous efficiency and a shortened service life. This article proposes methods and technologies to reduce the temperature rise and temperature control of high-power LED lamps from two aspects of the design of lamps and drivers, which effectively reduces and limits the temperature rise of high-power LED lamps.
Environmental issues have become the most important issue in the economic development of various countries, so energy-saving LED lighting has become the “new favorite” of the lighting industry. Because LEDs have higher luminous efficiency and lower manufacturing costs, their application prospects and markets are very broad. However, the heat dissipation problem of high-power LED lights limits the development of the LED lighting industry. If the heat dissipation problem is not solved, the temperature of the LED lights will rise, resulting in a decrease in luminous efficiency and a shortened service life. This article proposes methods and technologies to reduce the temperature rise and temperature control of high-power LED lamps from two aspects of the design of lamps and drivers, which effectively reduces and limits the temperature rise of high-power LED lamps.
1. Reduce temperature rise
At present, the heat dissipation methods of LED lights mainly include natural convection heat dissipation, additional fan forced heat dissipation, heat pipe and loop heat pipe heat dissipation, etc.
1.1 The power supply is separated from the lamp body
As the power supply itself generates a certain amount of heat, the heat on the LED lamp increases. At the same time, the integrated design of the power supply and the lamp makes the overall heating of the LED lamp uneven. These factors will cause fatigue and early failure of the lamp, which will affect its life. Figure 1 is the temperature curve of the LED lamp with the working time. In the figure, T1 is the temperature where the power supply is placed, T2 is the temperature far away from the power supply, and T3 is the center temperature of the lamp body. It can be seen from the figure that with the increase of working time, T1 in Figure 1 (a) is much larger than T2 and T3; in Figure 1 (b), the two curves of T1 and T2 coincide, and T3 is slightly larger than T1 and T2. It can be seen that after the power supply is separated, the temperature distribution of the whole lamp is very uniform.
1.2 Choose high-quality LED modules
The choice of LED modules also plays a key role in reducing temperature rise. Choosing LED lamp beads encapsulated by a material with high thermal conductivity and consistent material can improve the internal thermal diffusivity. The metal substrate with high thermal conductivity and high heat dissipation is used as the wick board to make the temperature distribution of the heat sink uniform, so as to maximize the heat dissipation effect.
1.3 Increase the heat dissipation area
There is easily a gap at the interface between the aluminum substrate and the heat sink, and the thermal conductivity of air is very small, only about 0.03W/m·K, so it can be enlarged by coating the contact surface with colloidal thermal grease with higher thermal conductivity. Actual contact area. At the same time, the heat dissipation area of the heat sink is increased, and the structure of the heat sink is deformed to facilitate heat dissipation.
2. Temperature control system
When the heat generated by the LED lamp working at the rated power exceeds its heat dissipation capacity, this article also uses temperature control technology to limit the temperature rise while strengthening the heat dissipation. When the temperature is high, the temperature control system starts to work, and the output of the driver is appropriately reduced to achieve the purpose of limiting and reducing the temperature rise; when the temperature drops, the original working state is restored. Choose the following 2 ways to drive LED lights in this article.
2.1 Constant current drive
This solution realizes the temperature control of the LED lamp by controlling the output current of the driver. Figure 2 is a block diagram of a constant current driver driving an LED lamp. The driver outputs to the LED module. The heat generated on the LED module is conducted to the wick board through a good thermal conductivity material, and finally dissipated to the atmosphere through the heat sink. When the external heat dissipation environment is bad, the temperature of the LED module will reach the temperature set by the temperature control system. After receiving the feedback information, the driver reduces the output to achieve the purpose of limiting and reducing the temperature of the LED module. Figure 3 is a schematic diagram of a constant current power supply for power supply control of the LED lamp. The power supply index is: 220V AC input, current 1.2-1.7A adjustable, voltage self-adapting (36～39V). The part in the thin dashed line box on the left in Figure 3 is the control circuit, in which W1 is an adjustable resistor; NTC is a negative temperature coefficient thermistor; Kt is a normally open temperature relay with a closing temperature of 56°C and an automatic opening temperature of 45℃; Rx is matching resistance. The part in the thick dashed line frame on the right side of Figure 3 is the LED module part. The temperature relay and thermistor are installed on the LED module and are in close contact with the module to feed back the temperature information of the LED to the control circuit. Kt is in the disconnected state at normal temperature, and only W1 in the control circuit plays a control role at this time, and the total current at normal temperature is set to be 1.60A. When the temperature of the relay rises to 56°C, Kt is automatically closed, and the entire control circuit starts to work to reduce the output of the constant current power supply; when the temperature drops to 45°C, Kt is automatically disconnected and the power supply is rated output. This process can be shown in Figure 4, where r is the temperature of Kt and Rntc is the resistance of NTC.
The relationship between the resistance of the control circuit and the total output current is listed in Table 1, where R is the equivalent resistance of the control circuit. After testing in a thermostat, one set of data is recorded every 2°C to obtain the temperature-resistance curve of the NTC thermistor as shown in Figure 5.
In this scheme, the drive power supply controls the output current by receiving the feedback temperature information. According to the relationship between the temperature and the resistance of the NTC in Figure 5, as long as the relationship between the output current and the total resistance is found (as shown in Figure 6), Then the appropriate resistance matching can find the relationship between the temperature and the output current of the driver.
Combined with Table 1, when working at room temperature, the total resistance value is 5.7kll, W1 in Figure 3 can be set to 5.7kQ, when the LED lamp bead temperature r ≥ 56 ℃, because of K. Closed, the output of the constant current power supply decreases. At this time, the total resistance of the control network is 3kll. After calculation, the Rx value is 3.6 kQ.
2.2 Constant voltage drive
This solution realizes the control of LED lights by controlling the output voltage of the driver. The overall framework is similar to the constant current drive. The difference is that the solution uses a constant voltage drive and the temperature control system circuit is different.
Figure 7 is the wiring diagram of the temperature control of the constant voltage driver. The Trim terminal is used to adjust the output of the power supply. The part in the dotted line frame on the left is the control circuit, in which: PTC is a positive temperature coefficient temperature-sensitive resistor; R1, R2, and Rx are common resistors, which are matched with the PTC temperature-sensitive resistor to adjust the output voltage of the driver; Kt is a normally closed type The temperature relay has an open temperature of 60°C and an automatic closing temperature of 48%. The part in the dashed box on the right is the LED module part. Kt and PTC are installed on the LED module and are in close contact with the module. K at room temperature. In the closed state, the rated output of the driver is controlled in the control circuit at this time, and the rated total voltage of the LED module at room temperature is 24V. When the relay temperature rises to 60℃, K.
It is automatically disconnected and the entire control circuit works, thereby reducing the output of the constant voltage power supply. When the temperature drops to 48°C, the temperature relay is automatically closed and the power supply is output normally. After testing, drive V is obtained. The relationship between the total resistance scale connected between the terminal and the Trim terminal and the output voltage U of the driver is shown in Table 2. It can be seen that as the resistance increases, the output voltage shows a decreasing trend. When the temperature reaches 60°C, Figure 7 controls the temperature relay K in the circuit. Disconnect, at this time, as long as the resistance is matched properly, we can get the set output voltage. The calculation method of each resistance value is the same as above, and no specific calculation is made here.
3. The actual test results of the pilot drive
This research carried out the research and development of high-power LED street lamps and LED projection lamps and drivers. Figure 8 shows the LED street lamp sample lamp and its constant current driver. The lamp body adopts an integrated design. The measured AC current input to the driver at room temperature is 270mA. The lamp runs well for a long time. Its total luminous flux is 3408lm. When the temperature is controlled, The output current is reduced to 87% of normal temperature. Figure 9 shows the LED projection lamp sample lamp and its constant voltage driver. The measured AC current input to the driver at room temperature is 140mA, and the total luminous flux is 1011lm. When the temperature is controlled, the voltage is reduced to 90% of the room temperature.
In the LED lighting process, the constant voltage driver provides a constant voltage to the LED lamp, and when the temperature rises, the PN junction voltage V of the LED lamp will drop at a speed of about -2mV/°C, so that the current flowing through the LED lamp is rapid Increase, affect its service life; while using a constant current driver to avoid this phenomenon. Therefore, it is generally recommended to use a constant current driver to drive LED lights.
The above-mentioned solution in this article effectively reduces the temperature rise of high-power LED lamps. Once the temperature rises above the set control temperature, the driver will reduce the output. Without affecting the use, the luminous flux and power of the LED lamp will be appropriately reduced. It avoids LED light decay and shortened service life due to overheating. The temperature control scheme has shown many advantages in the research process, and it is believed that it will be applied on a large scale in the near future, and LED lighting will also be improved to a greater extent.