Long service life and high efficiency of LED are based on appropriate working conditions. The main factor affecting the lifetime and luminous efficiency is the working junction temperature of 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 in a certain range is the key to ensure the LED life and luminous efficiency. In addition to heat dissipation measures, it is very necessary to include the junction temperature into the control parameters of the driving power supply.
1. LED junction temperature detection
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 according to 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 LED for white lighting are shown in Figure 1.
Fig. 1 volt ampere characteristics of LED
The volt ampere characteristics of LED, like other diodes, have the characteristics of negative temperature coefficient, that is, when the junction temperature rises, the I / V curve shifts to the left, as shown in the figure below.
Fig. 2 temperature characteristics of volt ampere characteristics
Generally, when the junction temperature of LED increases by 1 ° C, the I / V curve will shift to the left by 1.5 ~ 4MV. If the applied voltage is constant, it is obvious that the current will increase. The increase of current will only make its junction temperature rise higher, and even lead to a vicious circle. Therefore, at present, LED driving power supply is generally designed as constant current power supply.
According to the law that the I / V curve moves to the left with the increase of junction temperature, under the condition of constant current power supply, the LED junction temperature can be calculated by measuring the forward voltage of LED.
In practical application, it is often not necessary to determine the particularly accurate value of LED junction temperature. At this time, the estimated value of LED junction temperature of the whole lamp can be determined by experimental method. Taking a 12W downlight as an example, the light source part is composed of 4 parallel 6 series medium power LEDs, and its circuit connection form is as follows:
Fig. 3 circuit connection diagram of LED light source
The test steps to determine the relationship between forward voltage and junction temperature are as follows: 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 constant current source at 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) ~ (4), the temperature of the incubator changes from low to high, and multi-point 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 setting temperature of the incubator during measurement can be approximately equal to the junction temperature of the LED light source. Under the condition of 600mA constant current, it is not difficult to obtain the relationship between the forward voltage of the light source module and the junction temperature by mathematical method. Using Excel tool, take the temperature as the x-axis and the average value as the y-axis to generate (x, y) scatter diagram. If the linear regression analysis type is selected, the following trend diagram and formula can be generated.
Figure 4 trend chart generated by Excel
It can be seen that when a light source composed of 4 parallel 6 series medium power LEDs is driven by 600mA constant current, the relationship between its forward voltage and junction temperature is as follows:
Vf = -0.0207Tj+ 20.332 (1)
Tj= 982.22-48.31Vf (2)
Where VF is the forward voltage drop of LED light source and TJ is the junction temperature. It should be noted that although LED products of different specifications from different manufacturers comply with the above trend, the specific data are different. Therefore, the specifications and models need to be retested after changing manufacturers.
2 lm3404 introduction
With the development of LED lighting applications, manufacturers at home and abroad have introduced many devices for driving LED. Among them, lm3404 and its series products launched by national semiconductor company of the United States are a constant current drive chip very suitable for small and medium-power LED light sources.
Lm3404 has built-in MOS switch, the maximum output current is 1a, and the efficiency is up to 95%. This chip is packaged with 8-pin SOIC, one of which can use pulse width modulation (PWM) input signal to control the brightness of LED.
In addition, the chip can provide current detection 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
Sw: output terminal of internal MOS tube, generally connected with an inductor and a Schottky diode;
Boot: the start pin of internal MOS tube is generally connected with SW terminal with a 10nf capacitor;
Dim: PWM dimming input. The average output power can be adjusted by inputting PWM signals with different duty cycles;
GND: grounding terminal;
CS: feedback pin, used to set the constant current value;
Ron: on-line control terminal. The pin is grounded to stop the chip and keep it in a low power state;
VCC: power supply pin, which provides a 7V voltage from the chip, and is connected to a filter capacitor to the ground during application;
Vin: input terminal, voltage range 6 ~ 42V, for lm3404h, range 6 ~ 75V
The application of lm3404 is very simple. A typical application of lm3404 is shown in Figure 6.
Figure 6 typical application circuit diagram of lm3404
In the figure, RSNs is the sampling resistance, which can be determined according to the design constant current value; Ron generally adopts 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, switching frequency and other parameters.
3 design of LED power supply based on junction temperature protection
The key of LED driving circuit based on junction temperature protection is 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 LED light source. However, generally, the ripple of LED constant current driving 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 whole lamp, are a more reasonable scheme. Using a low-power single chip microcomputer with analog input, the detection data can be digitally filtered, and the LED light source power can be driven and adjusted through PWM output control, which can simplify the design of detection circuit and control circuit.
Pic12f675 of microchip company has a low-power online programmable single chip microcomputer with programmable 4-channel analog input and 10 bit resolution analog-to-digital conversion. It has built-in watchdog, 4MHz oscillator, 128 byte EEPROM, single byte instruction system and 8-pin package. It is a simple, practical and cost-effective single chip microcomputer. The forward voltage of the LED light source is sampled and connected to the analog input of pic12f675. After AD conversion, the gross error is removed, the mean value of multiple data is taken as the junction temperature judgment basis, and the PWM signal is output to control the constant current drive chip, so as to achieve the effect of adjusting the output power.
In addition, the open circuit can be judged according to the measured value, which also simplifies the open circuit protection circuit.
Still take 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 protective measures shall be taken for the drive. The schematic diagram of LED power supply circuit based on junction temperature protection composed of lm3404 and pic12f675 is shown in Figure 7.
Fig. 7 electrical 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 and output 24V DC through AC / DC conversion. This part is omitted when battery powered emergency lighting, solar lighting, vehicle lighting and other applications. R1, lm3404, C4, D1, L3 and R7 form a typical constant current driving circuit. For the light source module composed of 4 parallel and 6 series LED medium power chips, the sampling resistance is 0.39 Ω. R2, R3, R4 and lm431 form a voltage stabilizing circuit to provide stable 5V power supply and internal AD conversion voltage reference for pic12f675.
The output of lm3404 is input to analog port an2 of pic12f675 after being divided by R5 and R6. Pic12f675 obtains the forward voltage of LED light source through internal AD conversion and calculation, generates PWM signal according to the set value program, and connects it to dim terminal of lm3404 through gp4 pin to adjust its output power.
Pic12f675 initially sets the gp4 output high level. If the measured led forward voltage is within a reasonable range, maintain the high level output to make the lm3404 work normally; If the led forward voltage gradually decreases and is lower than the set value of 18.68v, the PWM signal is output at gp4 pin, and its duty cycle can be reduced successively until the led forward voltage is lower than the set value. When the measured led forward voltage is very high, it can be determined that the output is open. Pic12f675 can output low level and close the output of lm3404.
It should be noted that the output voltage sampling includes the current sampling voltage for lm3404 constant current control, which is about 0.23V, which should be adjusted in the calculation program of pic12f675.
The program block diagram of pic12f675 is shown in Figure 8
Fig. 8 program block diagram of single chip microcomputer
Because the LED power supply based on junction temperature protection is controlled by 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 so as to further save energy and prolong the life of lamps and lanterns. Add other sensors to realize on-demand lighting; By adding the remote communication module, the lamps can form an intelligent control network, etc.