Analysis of problems related to LED life and luminous efficiency

Analysis of problems related to LED life and luminous efficiency

Long life and high efficiency are prerequisites, that is, suitable working conditions. The main factor that affects the lifetime and luminous efficiency is the operating junction temperature of the LED. The test data provided by mainstream LED manufacturers shows that the luminous efficiency of LEDs is almost inversely proportional to the junction temperature, and the lifetime decreases exponentially as the junction temperature increases. Therefore, controlling the junction temperature within a certain range is the key to ensuring the lifetime and luminous efficiency of the LED. In addition to heat dissipation measures, the means to control the junction temperature within a certain range is very necessary to incorporate the junction temperature into the control parameters of the drive power supply.

Long life and high efficiency are prerequisites, that is, suitable working conditions. The main factor that affects the lifetime and luminous efficiency is the operating junction temperature of the LED. The test data provided by mainstream LED manufacturers shows that the luminous efficiency of LEDs is almost inversely proportional to the junction temperature, and the lifetime decreases exponentially as the junction temperature increases. Therefore, controlling the junction temperature within a certain range is the key to ensuring the lifetime and luminous efficiency of the LED. In addition to heat dissipation measures, the means to control the junction temperature within a certain range is very necessary to incorporate the junction temperature into the control parameters of the drive power supply.

1 LED junction temperature detection

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

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

Analysis of problems related to LED life and luminous efficiency

Figure 1 LED’s volt-ampere characteristics

Like other diodes, the volt-ampere characteristics of LEDs have the characteristics of a negative temperature coefficient, that is, when the junction temperature rises, the I/V curve shifts to the left, as shown in the figure below.

Analysis of problems related to LED life and luminous efficiency

Figure 2 Temperature characteristics of volt-ampere characteristics

Generally, when the junction temperature of LED rises by 1°C, the I/V curve will shift to the left by 1.5~4mV. If the applied voltage is constant, then obviously the current will increase, and the increase in current will only make its junction temperature rise more. High, and even lead to a vicious circle. Therefore, the current LED driving power supply is generally designed for constant current power supply.

According to the law that the I/V curve shifts to the left with the increase in junction temperature, in the case of constant current power supply, the LED junction temperature can be estimated by measuring the forward voltage of the LED.

In practical applications, it is often not necessary to determine a particularly accurate value of the LED junction temperature. At this time, an experimental method can be used to determine the estimated value of the LED light source junction temperature of the overall lamp. Taking a 12W downlight as an example, the light source part is composed of 4 parallel 6 strings of medium power LEDs, and the circuit connection form is as follows:

Analysis of problems related to LED life and luminous efficiency

Figure 3 LED light source circuit connection diagram

The test steps to determine the relationship between forward voltage and junction temperature are: 1) Put the light source in the thermostat; 2) Set the temperature of the thermostat; 3) After the temperature in the thermostat is fully balanced and stable, connect to both ends of the light source Constant current source; 4) Quickly measure and record the forward voltage of the light source; 5) Repeat the above steps 1)~(4), the temperature of the incubator is from low to high, and more data are measured.

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

Analysis of problems related to LED life and luminous efficiency

Table 1 LED forward voltage drop and junction temperature measurement data

It can be seen from Table 1 that the consistency and regularity of the measurement data are obvious.

Due to the short test time, the setting temperature of the incubator can be approximately equal to the junction temperature of the LED light source during the measurement. In the case of a constant current of 600mA, it is not difficult to obtain the relationship between the forward voltage of the light source module and the junction temperature through mathematical methods. Use Excel tool to generate (X, Y) scatter plot with temperature as X axis and average value as Y axis. Select the linear regression analysis type to generate the following trend chart and formula.

Analysis of problems related to LED life and luminous efficiency

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 6 strings of mid-power LEDs when driven at a constant current of 600 mA 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 the LED products of different manufacturers with different specifications are in line with the above trends, the specific data are somewhat different, so the specifications and models need to be re-tested after changing the manufacturer.

2 Introduction to LM3404

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

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

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

Analysis of problems related to LED life and luminous efficiency

Figure 5 LM3404 internal circuit structure diagram

Pin definition:

SW: The output terminal of the internal MOS tube, generally requires an external Inductor and a Schottky diode;

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

DIM: PWM dimming input terminal, by inputting PWM signals with different duty cycles, the average output 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 and be in a low power consumption state;

VCC: Power supply pin, this terminal provides a 7V voltage from the chip, and connect a filter capacitor to the ground during application;

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

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

Analysis of problems related to LED life and luminous efficiency

Figure 6 LM3404 typical application circuit diagram

In the figure, Rsns is the sampling resistance, which can be determined according to the design constant current value; Ron generally selects a resistance of about 100k; it can determine the switching frequency; L1 is the output inductance, 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 is junction temperature detection and how to protect it. According to the above-mentioned 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 relatively large. In order to avoid false protection, the detection circuit must check the measured value. Perform filtering. 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 operation of the entire lamp, is a more reasonable solution. The use of a low-power MCU with analog input can digitally filter the detection data, and adjust the power of the LED light source through the PWM output control drive, which can simplify the design of the detection circuit and the control circuit.

Microchip’s PIC12F675 has a programmable 4-channel analog input, 10-bit resolution analog-to-digital conversion low-power online programmable microcontroller, with built-in watchdog, 4MHz oscillator, 128-byte EEPROM, single-byte instruction system , 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 terminal of the PIC12F675. After AD conversion, gross errors are removed, the average value of multiple data is taken as the basis for judging the junction temperature, and the PWM signal is output to control the constant current drive chip. To achieve the effect of adjusting the output power.

In addition, the open circuit can also be judged based on the measured value, which also simplifies the open circuit protection circuit.

Still taking the light source part of the downlight composed of 4 parallel 6 strings of 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 18.68. V, that is, when the voltage across the light source is lower than 18.68V, the junction temperature of the LED will exceed 80°C. At this time, the driver should take protective measures. 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 LED power supply schematic diagram based on junction temperature protection

Analysis of problems related to LED life and luminous efficiency

In the schematic diagram, CX1, L1, and L2 form the input EMC filter circuit, which outputs 24V DC after AC/DC conversion. For battery-powered emergency lighting, solar lighting, and automotive lighting, this part is omitted. R1, LM3404, C4, D1, L3, and R7 form a typical constant current drive circuit. For a light source module composed of a 4-parallel 6-string LED mid-power chip, the sampling resistance is 0.39Ω. R2, R3, R4 and LM431 form a voltage regulator circuit to provide a stable 5V power supply and internal AD conversion voltage reference for the PIC12F675.

The output of LM3404 is divided by R5 and R6 and then input to the analog port AN2 of PIC12F675. PIC12F675 gets the forward voltage of the LED light source through internal AD conversion and calculation. The PWM signal is generated according to the set value program, and it is connected to the DIM of LM3404 through the GP4 pin. End to adjust its output power.

PIC12F675 initially sets GP4 to output high level. If the measured LED forward voltage is within a reasonable range, maintain the high level output to make LM3404 work normally; if the LED forward voltage gradually decreases and is lower than the set value 18.68V, then The PWM signal is output on the GP4 pin, and its duty cycle can be sequentially reduced until the LED forward voltage is lower than the set value. When the measured LED forward voltage is high, it can be judged that the output is open, and the PIC12F675 can output a low level to close the output of the LM3404.

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

The block diagram of PIC12F675 is shown in the figure

Analysis of problems related to LED life and luminous efficiency

Figure 8 Block diagram of the MCU program

4 Conclusion

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 middle of the night to further save energy and extend the life of the lamp; add other sensors to achieve on-demand lighting; add a remote communication module to make the lamps form an intelligent control network and so on.

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