Design of Over Temperature Protection Circuit for Reducing LED Light Attenuation

introduction

LED lighting lamps have the advantages of energy saving, high efficiency, environmental protection and long service life. LED lighting lamps will gradually replace incandescent lamps and fluorescent lamps. If LED lighting lamps go to the world, it will become a massive product. Obviously, continuously improving the input power and luminous efficiency of LED is the only way to become a general lighting mode.

For LED lamps, especially high-power LED street lamps, if the thermal design is not well done and the LED node temperature is high, it will cause reversible light failure and unrecoverable permanent light failure, affecting the performance and service life of LED lamps. In order to increase the reliability of LED lamps, the performance and reliability of driving power supply need to be improved. A large number of practices show that the basic reason why LED can not increase input power is that led will release a lot of heat in the working process, resulting in the rapid rise of die junction temperature. The higher the input power, the greater the heating effect. The increase of temperature will lead to the change and attenuation of device performance, or even failure. In this paper, the over temperature protection circuit of LED driving circuit is designed, so as to reduce the light attenuation of LED and improve the service life of LED.

1. Principle of led over temperature protection circuit

1.1 integrated circuits

Mbi1801 is a driving IC for immediate switch and is also a driving IC specially designed for high-power LED. Mbi1801 provides a constant current output channel and high output current capability. Mbi1801 current can be set through an external resistance (rext), and the current output range is from 50 Ma to 1200 Ma to control the luminous brightness of LED. In addition, it can also control the brightness of led by inputting PWM signal. To ensure the reliability of application products, mbi1801 has built-in temperature sensor and thermal protection (TP) function. The temperature sensor can detect the temperature state of mbi1801; When the temperature of mbi1801 exceeds 165 ℃, the overheat protection function will turn off the current to prevent the temperature of the driver from being too high. Mbi1801 has added heat dissipation capacity to the to-265 package to safely handle high output current.

Chip features: the constant current output value is not affected by the load voltage at the output end; Large constant current output range: 1.2 A; Using an external resistor, the current output value can be adjusted; Built in overheating protection device; Operating voltage: 5 V; "Lead free and environment-friendly" packaging with heat sink.

Product application: high brightness LED lighting; Infrared LED camera.

1.2 circuit working principle

Figure 1 shows mbi1801 with led over temperature protection circuit.

Mbi1801 needs an external resistor (rext) to determine the LED preset current. The resistor to be used by mbi1801 is replaced by a potentiometer Pot1. The nominal value of the potentiometer is calculated as follows:

In some common designs, rext often uses two resistors in series to divide the voltage, so as to obtain U1 in the over temperature protection circuit. At this time, U1 is a fixed value. Once the resistance value of R2 is determined, the over temperature protection circuit can only set one over temperature protection point. In order to improve the adaptability of the circuit to the external environment, the over temperature protection point can be set to an adjustable temperature, Therefore, a potentiometer is selected here to replace two resistors in series.

The negative temperature coefficient sensor RT can be connected to the LED board to sense the temperature of the LED. When the temperature of the LED increases, the resistance of the thermistor will decrease, and the U2 voltage will also increase. When the voltage of U2 exceeds the voltage value of U1, the diode VD2 is turned on and the potential of U1 will be pulled up. At this time, the calculation formula of LED current is:

In equations (1), (2) and (3), uvd2 is the forward voltage, iled is the LED current, and ur-ext is the voltage of mbi1801 R-EXT pin.

The LED current will begin to drop until the temperature is balanced, and vice versa.

The voltage of U1 can be set through the potentiometer. Once the voltage is determined, the starting voltage of the line is also determined.

The function of diode VD2 is when U2

2 design process

2.1 define the resistance change curve of thermistor with negative temperature coefficient

Negative temperature coefficient thermistor is referred to as NTC for short. Service temperature range: - 40 ℃ 125 ℃. The resistance decreases as the temperature increases.

The variation curve of 100 K Ω negative temperature coefficient thermistor value versus temperature is shown in Figure 2.

2.2 determining potentiometer Pot1

The LED is connected in 4 series and 4 parallel. Its input voltage is 13.5 V and the LED current is 1.2 A. since the voltage ur-ext of mbi801 R-EXT pin is equal to 1.251 V, the potentiometer used by mbi801 is

Therefore, 1 K Ω potentiometer is selected here.

2.3 determining resistance R2

Before determining R2, the voltage U1 in an over-temperature circuit must be determined, and the potentiometer can be adjusted to make its voltage value 1.025 v. if the temperature of the over-temperature protection point is 85 ℃, when the temperature exceeds 85 ℃, the LED over-temperature protection circuit starts to work and the LED current begins to decrease. From the change curve of negative temperature coefficient resistance value versus temperature in Figure 2, it can be found that the resistance value of the corresponding thermistor at 85 ℃ is 7.92 K Ω. U2 = U1, the resistance value of R2 is:

R2 = 2 K Ω can be selected.

Similarly, after the resistance value of R2 is determined, the formula of U1 in overtemperature protection circuit can be obtained from formula (4):

When the ambient temperature of the LED changes, such as the ambient temperature is high, it is necessary to reduce the temperature of the over temperature protection point, which can be realized by adjusting the voltage of the potentiometer. If the temperature of the protection point of the over temperature protection circuit is reduced to 80 ℃, at this time, the resistance value of the corresponding thermistor on the change curve of the resistance value of the negative temperature coefficient to the temperature is 10 K Ω. The resistance value can be substituted into the formula to calculate the voltage of U1 as follows:

Adjust the potentiometer to make its voltage value 0.85 v.

3 test results

The LED adopts the connection mode of 4 series and 4 parallel, and its input voltage is 13.5 v. select the protection point temperature of the over temperature protection circuit to 80 ℃, the potentiometer Pot1 = 1 K Ω, and adjust the middle tap voltage of the potentiometer to U1 = 0.85 v. see Table 1 for the temperature change of the LED before the over temperature protection circuit is not added

After mbi1801 is equipped with led over temperature protection circuit, the temperature change of LED circuit board is shown in Table 2. The data of overtemperature protection circuit elements are as follows: the LED adopts the connection mode of 4 series and 4 parallel, and its input voltage is 13.5 V, UDD = 5.1 V, R2 = 2 K Ω, potentiometer Pot1 = 10 K Ω. Diode VD2 is scd32

It can be seen from table 2 that the LED current is basically maintained at a certain level before the over temperature protection point is 80 ℃. When the temperature of the LED circuit board exceeds 80 ℃, the current begins to decrease. After 20 min, the temperature of the LED circuit board reaches the equilibrium state and the temperature hardly increases.

Special attention: the protection point of over temperature protection line cannot be set within the temperature point during normal operation, otherwise the protection line will start early and affect the brightness of LED. It is recommended to set the protection point at 10 ℃ higher than the normal working temperature to avoid misoperation.

4 Summary

The over temperature protection circuit designed in this paper has the advantages of simple structure and reliable operation, and can set over temperature protection points for different ambient temperatures, which not only reduces light attenuation, but also improves the adaptability of the circuit to the environment.

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