LED driver design based on junction temperature protection

With the advancement of LED epitaxial materials, chip processes and packaging technologies, the luminous efficiency of LEDs has been continuously improved, making it possible for LED light sources to replace traditional light sources. In theory, LEDs have advantages such as long life and high efficiency, but in some practical applications, they leave people with the impression of large light decay and short life, which greatly affects the popularization and promotion of semiconductor lighting. The main reason is the problem of LED driving power supply.

There is a prerequisite for LED to have a long life and high efficiency, that is, suitable working conditions. The main factor affecting the life and luminous efficiency is the working junction temperature of the LED. The test data provided by mainstream LED manufacturers show that the luminous efficiency of LED is almost inversely proportional to the junction temperature, and the life decreases almost exponentially with the increase of junction temperature. Therefore, controlling the junction temperature within a certain range is the key to ensuring the life and luminous efficiency of LED. In addition to heat dissipation measures, it is very necessary to include the junction temperature in the control parameters of the driving power supply to control the junction temperature within a certain range. This paper discusses the measurement method of LED junction temperature, and proposes to measure the junction temperature of the LED light source in real time through a single-chip microcomputer, and generate a PWM signal to adjust the output power of the power supply when the junction temperature exceeds the set value. The paper gives the power supply schematic diagram based on junction temperature protection composed of LM3404 and PIC12F675 and the single-chip microcomputer program flowchart. The experiment shows that the LED driver based on junction temperature protection can effectively improve the reliability of LED lamps.

1 Detection of LED junction temperature

The junction temperature of LED refers to the temperature of PN junction. It is difficult to actually measure the junction temperature of LED, but it can be measured indirectly based on the temperature characteristics of LED.

The volt-ampere characteristics of LEDs are similar to those of ordinary diodes. The typical volt-ampere characteristics of blue LEDs used for white light illumination are shown in Figure 1.

Figure 1 LED's volt-ampere characteristics

The LED's volt-ampere characteristics, like other diodes, have a negative temperature coefficient, that is, the I/V curve shifts to the left when the junction temperature rises, as shown in the figure below.

Figure 2 Temperature characteristics of volt-ampere characteristics

Generally, for every 1°C increase in the junction temperature of an LED, the I/V curve will shift 1.5~4mV to the left. If the applied voltage is constant, then the current will obviously increase, and the increase in current will only make its junction temperature rise higher, and even lead to a vicious cycle. Therefore, the current LED driver power supply is generally designed to be a constant current power supply.

According to the rule that the I/V curve shifts to the left as the junction temperature increases, the LED junction temperature can be inferred by measuring the forward voltage of the LED under constant current power supply.

In practical applications, it is often not necessary to determine the exact value of the LED junction temperature. In this case, the estimated value of the junction temperature of the LED light source of the entire lamp can be determined by experimental methods. Take a 12W downlight as an example. The light source part consists of 4 parallel and 6 series of medium-power LEDs. The circuit connection form is as follows:

Figure 3 LED light source circuit connection diagram

The experimental steps to determine the relationship between forward voltage and junction temperature are: 1) Place the light source in a constant temperature box; 2) Set the temperature of the constant temperature box; 3) After the temperature in the constant temperature box is fully balanced and stable, connect a constant current source to both ends of the light source; 4) Quickly measure the forward voltage of the light source and record it; 5) Repeat the above steps 1) to (4), and measure more data from the constant temperature box from low to high.

According to the above steps, the 12W downlight light source was measured three times, and the data are as follows:

Table 1 Measurement data of LED forward voltage drop and junction temperature

As can be seen from Table 1, the consistency and regularity of the measurement data are very obvious.

Because the test time is short, the temperature of the thermostat can be set to be approximately equal to the junction temperature of the LED light source during measurement. Under the condition of 600mA constant current, it is not difficult to derive the relationship between the forward voltage and junction temperature of the light source module through mathematical methods. Using Excel tools, with temperature as the X-axis and the average value as the Y-axis, a (X,Y) scatter plot is generated. Selecting the linear regression analysis type can generate the following trend chart and formula.

Figure 4 Trend chart generated by Excel

It can be seen that the relationship between the forward voltage and junction temperature of a light source composed of 4 parallel and 6 series medium-power LEDs when driven by a constant current of 600mA is:

Vf = -0.0207Tj+ 20.332 (1)

Tj= 982.22-48.31Vf (2)

Where Vf is the forward voltage drop of the LED light source, and Tj is the junction temperature. It should be noted that although LED products of different specifications from different manufacturers all conform to the above trend, the specific data are somewhat different. Therefore, the specifications and models need to be retested after changing the manufacturer.

2 Introduction to LM3404

With the development of LED lighting applications, domestic and foreign manufacturers have launched many devices for driving LEDs. Among them, the LM3404 and series products launched by National Semiconductor Corporation of the United States are a constant current driver chip that is very suitable for small and medium-power LED light sources.

The LM3404 has a built-in MOS switch tube, a maximum output current of 1A, and an efficiency of up to 95%. This chip uses an 8-pin SOIC package, one of which can use a pulse width modulation (PWM) input signal to control the brightness of the LED.

In addition, this chip can provide current detection function with feedback voltage as low as 0.2V. The input voltage is 6~42V, and its internal circuit structure is shown in Figure 5.

Figure 5 LM3404 internal circuit structure diagram

Pin definition:

SW: internal MOS tube output terminal, generally requires an external inductor and a Schottky diode;

BOOT: internal MOS tube startup pin, generally connected to the SW terminal with a 10nF capacitor;

DIM: PWM dimming input terminal, by inputting PWM signals with different duty cycles, the output average power can be adjusted;

GND: ground terminal;

CS: feedback pin, used to set the constant current value;

RON: online control terminal, this pin can be grounded to stop the chip from working and put it in a low power consumption state;

VCC: power supply pin, this terminal provides a 7V voltage from the inside of the chip, and a filter capacitor is connected to the ground when used;

VIN: input terminal, the voltage range is 6~42V, and the range for LM3404H is 6~75V.

The application of LM3404 is very simple. A typical application of LM3404 is shown in Figure 6.

Figure 6 LM3404 typical application circuit diagram

In the figure, Rsns is the sampling resistor, which can be determined according to the design constant current value; Ron is generally selected as a resistor of about 100k; it can determine the switching frequency; L1 is the output inductor, which can be determined according to the design ripple and switching frequency and other parameters. 3 LED power supply design based on junction temperature protection

The key to the LED drive circuit based on junction temperature protection lies in the junction temperature detection and how to protect it. According to the above relationship between junction temperature and LED forward voltage, the junction temperature can be determined by measuring the forward voltage of the LED light source. However, the ripple of the general LED constant current drive circuit is large. In order to avoid false protection, the detection circuit must filter the measured value. On the other hand, when the junction temperature exceeds the set value, the protection measures, such as reducing the power of the light source and degrading the operation of the entire lamp, are more reasonable solutions. Using a low-power microcontroller with analog input, the detection data can be digitally filtered, and the LED light source power can be adjusted through PWM output control drive, which can simplify the design of the detection circuit and control circuit.

Microchip's PIC12F675 is a low-power online programmable microcontroller with programmable 4-channel analog input and 10-bit resolution analog-to-digital conversion. It has a built-in watchdog, 4MHz oscillator, 128-byte EEPROM, single-byte instruction system, and 8-pin package. It is a simple, practical, and cost-effective microcontroller. The forward voltage of the LED light source is sampled and connected to the analog input of PIC12F675. After AD conversion, gross errors are removed, and the average of multiple data is taken as the basis for junction temperature judgment, the PWM signal is output to control the constant current driver chip to achieve the effect of adjusting the output power.

In addition, the open circuit judgment can also be performed based on the measured value, thereby simplifying the open circuit protection circuit.

Taking the downlight composed of 4 parallel and 6 series medium-power LED chips as an example, the design constant current value is 600mA, and the junction temperature protection point is about 80℃. According to formula (1), the light source voltage protection point is 18.68V, that is, when the voltage at both ends of the light source is lower than 18.68V, the LED junction temperature will exceed 80℃, and the driver should take protective measures at this time. The schematic diagram of the LED power supply circuit based on junction temperature protection composed of LM3404 and PIC12F675 is shown in Figure 7.

Figure 7 Schematic diagram of LED power supply based on junction temperature protection

In the schematic diagram, CX1, L1, and L2 form an input EMC filter circuit, which outputs 24V DC after AC/DC conversion. This part is omitted for applications such as battery-powered emergency lighting, solar lighting, and vehicle-mounted lighting. R1, LM3404, C4, D1, L3, and R7 form a typical constant current drive circuit. For a light source module composed of 4 parallel and 6 series LED medium-power chips, the sampling resistor is 0.39Ω. R2, R3, R4, and LM431 form a voltage stabilization circuit to provide a stable 5V power supply and internal AD conversion voltage reference for PIC12F675.

The output of LM3404 is divided by R5 and R6 and input to the analog port AN2 of PIC12F675. PIC12F675 obtains the forward voltage of the LED light source through internal AD conversion and calculation, generates a PWM signal according to the set value program, and adjusts its output power by connecting to the DIM terminal of LM3404 through the GP4 pin.

PIC12F675 initially sets GP4 to output a high level. If the LED forward voltage is measured to be within a reasonable range, the high level output is maintained to allow the LM3404 to work normally. If the LED forward voltage gradually decreases and is lower than the set value of 18.68V, a PWM signal is output at the GP4 pin, and its duty cycle can be reduced in sequence until the LED forward voltage is lower than the set value. When the LED forward voltage is measured to be very high, the output can be determined to be open, and the PIC12F675 can output a low level to turn off the output of the LM3404. It

should be pointed out that the output voltage sampling includes the current sampling voltage of about 0.23V used for the LM3404 constant current control, which should be adjusted in the calculation program of the PIC12F675.

The program flowchart of the PIC12F675 is shown in Figure 8.

Figure 8 MCU program flowchart

4 Conclusion

For a 12W downlight composed of 4 parallel and 6 series of medium-power LEDs, in the test using the above driving scheme, when hot air is blown to the heat dissipation shell or the light source is out of contact with the heat dissipation shell, the LED light source will quickly dim, and the temperature of the light source substrate will drop accordingly, effectively protecting the light source itself. When the lamp is restored to normal, the brightness of the LED light source will also quickly return to normal.

In actual applications, there are many reasons why the junction temperature exceeds the set value, such as harsh environment, heat sink contact problems, or fan stop under forced air cooling conditions. The increase in junction temperature will cause the forward voltage of the LED light source to drop, especially when the light source is composed of multiple LEDs in series, the drop is very obvious.

By detecting the forward voltage of the LED light source, indirectly measuring the junction temperature, and using the single-chip microcomputer to adjust the power of the LED light source, the reliability and life of the overall lamp can be greatly improved. In addition, since the LED power supply based on junction temperature protection is controlled by a single-chip microcomputer, it is easy to expand other functions. For example, as a street lamp, it can be programmed to reduce power in the second half of the night, thereby further saving energy and extending the life of the lamp; adding other sensors can achieve on-demand lighting; adding remote communication modules can enable the lamps to form an intelligent control network, etc.