“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 lifespan decreases exponentially with the increase of the junction temperature. Therefore, controlling the junction temperature within a certain range is the key to ensure LED life and luminous efficiency. In addition to heat dissipation measures, it is necessary to incorporate the junction temperature into the control parameters of the driving power supply to control the junction temperature within a certain range.

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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 lifespan decreases exponentially with the increase of the junction temperature. Therefore, controlling the junction temperature within a certain range is the key to ensure LED life and luminous efficiency. In addition to heat dissipation measures, it is necessary to incorporate the junction temperature into the control parameters of the driving power supply to control the junction temperature within a certain range.

1 Detection of LED junction temperature

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 indirectly measured according to the temperature characteristics of the LED.

The volt-ampere characteristics of LEDs are similar to ordinary diodes. Typical volt-ampere characteristics of blue LEDs for white lighting are shown in Figure 1.

Figure 1 Volt-Ampere Characteristics of LEDs

Like other diodes, the volt-ampere characteristics of LEDs have 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.

Figure 2 Temperature characteristics of volt-ampere characteristics

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

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

In practical applications, it is often not necessary to determine the particularly precise value of the LED junction temperature. At this time, the estimated value of the junction temperature of the LED light source of the whole lamp can be determined experimentally. Taking a 12W downlight as an example, the light source part consists of 4 parallel 6 strings of medium power LEDs, and its circuit connection is as follows:

Figure 3 LED light source circuit connection diagram

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

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

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

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

Due to the short test time, the temperature set in the thermostat during measurement can be approximately equal to the junction temperature of the LED light source. 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. Using Excel tools, take the temperature as the X-axis and the average value as the Y-axis to generate a (X,Y) scatter chart, and select the linear regression analysis type to 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 the junction temperature of a light source composed of 4 parallel 6 strings of 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 with different specifications from different manufacturers conform to the above trends, there are certain differences in specific data. Therefore, the specifications and models need to be re-tested after changing manufacturers.

2 LM3404 Introduction

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

The LM3404 has a built-in MOS switch, the maximum output current is 1A, and the efficiency is as high as 95%. This chip adopts 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, the chip can provide current sensing with feedback voltages 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: the output terminal of the internal MOS tube, generally an external Inductor and a Schottky diode are required;

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

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

GND: ground terminal;

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

RON: On-line control terminal, the grounding of this pin can make the chip stop working and be in a low power consumption state;

VCC: power supply pin, this terminal is provided with a 7V voltage inside the chip, and a filter capacitor is connected to the ground during application;

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

LM3404 application is very simple, a typical application using LM3404 is shown in Figure 6.

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 uses a resistance of about 100k; the switching frequency can be determined; L1 is the output inductance, which can be determined according to the design ripple and switching frequency and other parameters.

3 Design of LED power supply based on junction temperature protection

The key to the LED driver circuit based on junction temperature protection lies in junction temperature detection and how to protect it. According to the relationship between the above junction temperature and the forward voltage of the LED, the junction temperature can be determined by measuring the forward voltage of the LED light source, but generally the ripple of the LED constant current drive circuit is large. In order to avoid false protection, the detection circuit must filter. 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 single-chip microcomputer with analog input, the detection data can be digitally filtered, and the power of the LED light source can be adjusted through the PWM output control drive, which can simplify the design of the detection circuit and the control circuit.

Microchip’s PIC12F675 is a low-power online programmable microcontroller with programmable 4-channel analog input, 10-bit resolution analog-to-digital conversion, built-in watchdog, 4MHz oscillator, 128-byte EEPROM, and a 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 then connected to the analog input terminal of the PIC12F675. After AD conversion, the gross error is removed, and 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 driver chip. To achieve the effect of adjusting the output power.

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

Still taking the downlight whose light source part is composed of 4 parallel and 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°C. 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 LED junction temperature will exceed 80°C, and the driver should take protective measures at this time. The schematic diagram of 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 is converted to 24V DC after AC/DC conversion. If it is used for battery-powered emergency lighting, solar lighting, and vehicle lighting, this part is omitted. R1, LM3404, C4, D1, L3, R7 form a typical constant current drive circuit. For the light source module composed of 4 parallel 6 series LED medium power chips, the sampling resistance is 0.39Ω. R2, R3, R4 and LM431 form a voltage regulator circuit, which provides a stable 5V power supply and a voltage reference for internal AD conversion for the PIC12F675.

The output of LM3404 is divided by R5 and R6 and then input to the analog port AN2 of PIC12F675. PIC12F675 is converted by internal AD, calculates and obtains the forward voltage of the LED light source, generates a PWM signal according to the set value program, and connects to the DIM of LM3404 through the GP4 pin terminal 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 becomes lower and lower than the set value of 18.68V, then The PWM signal is output on the GP4 pin, and its duty cycle can be decreased in turn until the forward voltage of the LED is lower than the set value. When the measured LED forward voltage is 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 LM3404 constant current control, which should be adjusted in the calculation program of PIC12F675.

The block diagram of the PIC12F675 is shown in Figure 8.

Figure 8 MCU block diagram

4 Conclusion

Because the LED power supply based on junction temperature protection is controlled by a single chip, it is easy to expand other functions. For example, as a street lamp, it can be programmed to reduce power operation in the second half of the night, thereby further saving energy and extending the life of lamps; adding other sensors can realize on-demand lighting; adding remote communication modules can make lamps form an intelligent control network, etc.

Link to this article：Analysis of problems related to LED life and luminous efficiency