Temperature compensation principle of high-power LED

introduction

When the ambient temperature of the LED is higher than the safe operating point temperature, the forward current of the LED will exceed the safe zone, greatly reducing the life of the LED or even damaging it. The solution is to use a temperature compensation circuit to continuously reduce the forward current value of the LED to prevent the LED from being damaged due to excessive temperature.

1 High-power LED temperature compensation solution

How to most effectively protect high-power LEDs working under high temperature conditions is a technical problem that needs to be solved urgently, which is crucial to extending the service life of high-power LEDs. There are four main design solutions to solve the overheating problem of high-power LEDs:

Solution 1: Use a suitable heat sink to dissipate the heat generated by the LED chip in time to reduce the junction temperature of the chip. However, this solution is a pre-measurement when designing LED lamps, and sufficient margin must be left for heat dissipation capacity. In actual work, it is difficult to ensure that the ideal and economical effect is achieved.

Solution 2: Use the overheat protection circuit inside the LED driver chip to shut down the output of the LED driver, forcing the LED to turn off and achieve the purpose of cooling. This solution is only an overheat protection measure that the LED driver chip is forced to take in extreme cases, and it cannot guarantee the long-term stable operation of the LED.

Solution 3: Use constant voltage/constant current (CV/CC) LED driver, which is characterized by maintaining constant output current through current control loop when output current reaches the specified value. This solution is a passive indirect overheat protection measure, because the constant voltage/constant current control circuit starts the current loop by detecting the output current, which has no direct relationship with the ambient temperature of the LED, so the output current cannot be adjusted in real time according to the temperature of the LED.

Solution 4: Design a temperature measurement and control system for LED lighting. However, due to the high cost, it is difficult to promote and apply it in the field of LED lighting.

Analysis shows that adding temperature compensation function to LED driver is a simple and easy solution. The basic principle of temperature compensation is that once an abnormal situation occurs and the temperature of LED is too high, the LED driver can automatically reduce the output current value according to the temperature detected by the thermistor to ensure that the LED works within the safe area. This fundamentally solves the problem of LED overheating damage or reduced service life, thereby greatly improving the reliability and safety of LED lamps; when the temperature drops to the safe area, the output current of the LED driver can automatically return to the normal value. Obviously, this LED driver chip with temperature compensation function is an "intelligent" chip with automatic real-time control. It not only represents the development direction of high-end LED drivers, but also has important practical value.

2 Basic principles of temperature compensation for high-power LEDs

The recently introduced temperature-compensated LED driver can satisfactorily solve the above technical problems. Its working characteristics are that when the temperature of the LED is lower than the safe working point temperature, the LED driver works in the constant current area; once the safe working point temperature is exceeded, it immediately enters the temperature compensation area. At this time, the LED driver can not only automatically reduce the output current according to the temperature rise, but also pre-set the safe working point temperature and the slope of the curve through the resistor. The relationship curve between the output current of this high-power LED driver with temperature compensation and the ambient temperature of the LED is shown in Figure 1. This is also a significant feature of high-end LED drivers. In 2009, Si-EN Microelectronics Co., Ltd. of the United States took the lead in the world to launch the SN3352 LED driver with temperature compensation. Its similar products also include the SN3910 HB-LED driver controller with temperature compensation and dimmable light for AC/DC (external power switch tube MOSFET is required), which provides convenient conditions for realizing temperature compensation of high-power LEDs.

The relationship curve with the ambient temperature of the LED is usually unnecessary to worry about whether the brightness of the LED will be significantly reduced as the forward current of the LED decreases. According to the Weber-Fechner law, the subjective brightness perception of the human eye is logarithmically related to the change in objective brightness (equivalent to the change in illuminance on the white paper), and the relationship curve between the two is shown in Figure 2. As can be seen from the figure, when the illuminance on the paper is reduced from 1O001x to 1001x, that is, reduced to 10% of the original, the human eye feels that the brightness has only become 50% darker (the subjective brightness perception of the human eye has dropped from 8 to 4). For example, even if the LED drive current is reduced from 350mA to 175mA, that is, reduced to 50% of the original, this will reduce the objective brightness; but after taking the logarithm, the brightness change perceived by the human eye is not so obvious.

Typical applications of high-power LED drivers with temperature compensation #e# 3 Typical applications of high-power LED drivers with temperature compensation

SN3352 is a high-end LED driver chip with temperature compensation function. It has five functions: constant current drive, temperature compensation, dimmable light, LED open circuit protection and shutdown mode. It can significantly improve the reliability of LED and greatly extend the service life of LED.

SN3352 has an integrated temperature compensation circuit, which is compatible with an external negative temperature coefficient (NTC) thermistor to detect the ambient temperature T of the LED. The NTC thermistor is placed near the LED in the LED lamp. SN3352 can obtain the temperature information of the LED chip in real time by continuously measuring its resistance value RNTC. The R value gradually decreases with the increase of T. When the R value is equal to the resistance value of the temperature compensation starting point setting resistor R, SN3352 begins to reduce the average output current to play a role in temperature compensation. When T is reduced to a safe value, the average current automatically returns to the pre-set constant current value.

The typical application circuit of SN3352 is shown in Figure 3. Input voltage U=+6～40VC is the bypass capacitor at the input end. If the front stage is the 12V AC output by the power transformer, and then the DC voltage is obtained through the rectifier filter, due to the large ripple voltage, the capacity of C should be greater than 200gF. It is recommended to use X5R and X7R series electrolytic capacitors. Ordinary electrolytic capacitors are not suitable for use as decoupling capacitors to avoid affecting the working stability of SN3352.

C2 is the noise elimination capacitor at the RNc terminal.

The LED string is composed of 1oRlW white LED. R is used to set the starting point of temperature compensation. R is an NTC thermistor, and its resistance value at TA: 25℃ is 100kQ. L is an inductance of 47H, and the allowable range is 47~220gH. When the input voltage is high and the output current is small, the inductance needs to be increased to reduce the output ripple and improve the power supply efficiency. The magnetic saturation current of the inductor should be greater than the peak output current of SN3352, and the average current of the inductor should be greater than, o(AvG) value. When o(AvG)=700mA, the magnetic saturation current of the inductor should be greater than 1.2A=350mA, and the magnetic saturation current should be greater than 500mA. The inductor should be as close to SN3352 as possible to reduce the lead resistance. In order to improve the efficiency of the 1MHz driver, the rectifier tube VD must use a Schottky diode with a very short reverse recovery time, low voltage drop, and very small reverse leakage current.

To improve the nonlinearity of the NTC thermistor, a fixed resistor R can be connected in series with R. If the output ripple current needs to be reduced, a bypass capacitor C can be connected in parallel at both ends of the LED string. When C=1gF, the output ripple current can be reduced to about 1/3 of the original. The temperature compensation starting point TH of the LED temperature compensation curve is set by R, and the slope of the curve is determined by the thermal index B and the resistance values ​​of R and R. Once the resistance values ​​of R, R and R are determined, the temperature compensation curve is determined. The recommended resistance range of RTH is 1kQ~100kQ.

For example, the design conditions are B=4485, R=0, RNTc=220kQ, RTH=22.1kQ. The corresponding temperature compensation curve is shown in Figure 4, and R uses a chip thermistor. After selecting the thermal index B, the smaller the resistance of R, the higher the compensation starting point temperature value; when R=ou,-t, the slope depends only on the R value; when R≠0, the greater the total resistance of R and R, the steeper the temperature compensation curve and the greater the slope.

4 Typical applications of lib-LED driver controller with temperature compensation

SN3910 is an AC/DC dual-purpose temperature-compensated dimmable HB-LED (high brightness LED) driver controller suitable for driving HB-LED lighting. The typical application circuit of SN3910 using AC power supply is shown in Figure 5. The AC input voltage U=220V±15%. FU is a 1A/250V fuse tube, and RNTcl is a current-limiting resistor when starting the power supply. C is a line capacitor for suppressing series-mode interference. The rectifier bridge is composed of four 1N4007 type 1A/1000V silicon rectifier tubes. In order to improve the power factor, VD~VD, C, and C are used to form a second-order valley-filled PFC circuit. RNT2 uses a l00kQ (TA=25℃) negative temperature coefficient thermistor. R uses a 1.0Q precision resistor, and an adjustable resistor R (100Q) is connected in parallel at both ends.

The temperature compensation starting point of SN3910 and the slope of output current drop can be set by R and R. The principle of temperature compensation is as follows: First, the reference voltage U of 1.2V is provided by the E pin, and then connected to the ground through the resistor divider RT1I and R. Connect the midpoint of the resistor divider to the LD pin, and set its voltage to the relationship:

From the analysis of equations (1) and (2), we can see that when the ambient temperature T increases, the resistance value of R decreases rapidly.

Once U<0.24V, the temperature compensation function is activated, and the output peak current is immediately reduced through the internal circuit of SN3910, and the average current is also reduced accordingly, thus achieving the purpose of temperature compensation.