Introduction: Compared with other light sources, high-power LEDs can cause serious heat dissipation problems, mainly because LEDs do not dissipate heat through infrared radiation. In general, the power consumption for driving LEDs is 75% to 85%, which is ultimately converted into thermal energy. Excessive heat reduces the light output of the LEDs and produces color cast, which accelerates LED aging. Therefore, thermal management is one of the most important aspects of LED system design. LED system manufacturers are addressing this challenge by seeking optimized heat sinks, high-efficiency printed circuit boards, and high thermal conductivity enclosures. However, engineers need to change their philosophy. Thermal management is not a patent for mechanical designers, and electronic engineers can also design thermal management. Practice has proved that thermal management through the circuit to achieve temperature compensation is an economical and reliable method.
In general, the high-power LED product specification will indicate the maximum allowable output current (Figure 1) for different ambient temperatures (or LED solder joint temperatures). When the ambient temperature is lower than the safe temperature point, the maximum allowable current of the output remains unchanged; when it is higher than the safe temperature point, the maximum allowable current of the output decreases as the ambient temperature increases, the so-called derating curve. In order to ensure that the performance life of the LED is not affected, it is necessary to ensure that the LED works in the safety zone covered by the derating curve and the horizontal and vertical axes.
Figure 1 LED derating curve
However, most LED luminaire manufacturers currently design the LED drive current as a constant current source that does not change with temperature. Therefore, when the temperature around the LED is higher than the safe temperature point, the operating current is not in the safe zone, which will result in LED The life is much lower than the value of the specification and even directly damaged. The temperature around the LED is too high, which is caused by the LED itself. There are two ways to solve this problem.
One way is to use a heat-dissipating device with better thermal conductivity to reduce the thermal resistance of the LED chip to the environment, and to control the internal temperature of the LED not to be much higher than the ambient temperature, but this requires a high cost. In addition, the inevitable problem is that when the heat sink is used for a period of time, dust is deposited on the heat sink of the lamp body casing, and the dielectric layer of the aluminum alloy base copper plate is connected to the copper layer and the aluminum substrate is degummed, which will result in greater thermal resistance. The amplitude rises, resulting in a decrease in overall heat dissipation performance. Another way is to make the LED work at the margin of the safe zone, so that it can meet the output current at the safe temperature point, the output power works at the rated state and is constant, and the output current is proportionally decreased at a higher temperature than the safe temperature point to ensure negative compensation. LED life, this is the meaning of temperature compensation.
Digital temperature sensor with driver for temperature compensation
Some lighting products require some intelligent control, such as the application of some advanced street lights, which often use a single-chip microcomputer to monitor and control the entire system. At this time, the original MCU control system can be used to add temperature compensation function. Even in harsh environments, such as summer exposure, the temperature inside the system can be well controlled.
Figure 2 Temperature compensation system using a digital temperature sensor.
Figure 2 is a schematic illustration of such a system driving a single LED string. The temperature detection part adopts the high-precision digital temperature sensor SN1086 produced by Yann Microelectronics Co., Ltd. The SN1086 can simultaneously detect the temperature of the chip itself, which is equivalent to indirectly detecting the PCB temperature and detecting the temperature of the remote three-stage tube. If the triode is combined with the LED The temperature of the aluminum substrate can be detected by soldering on an aluminum substrate. The SN1086 performs the analog-to-digital conversion of the two detected temperatures through the high-precision Delta-Sigma ADC inside the chip, and communicates the digital result of the temperature with the microcontroller through the SDA data line and the SCL clock line of the I2C bus. When the MCU receives the temperature result of the aluminum substrate, it compares with the preset safe temperature point threshold. When the temperature is too high, the temperature compensation program is started, and the output current of the LED driver is proportionally reduced by PWM1. The MCU monitors the temperature of the PCB at the same time. When the temperature is too high, the fan is controlled by the PWM2 signal line to dissipate heat from the PCB, so that the temperature of the components on the board, especially the electrolytic capacitor, is not too high.
This kind of system control greatly enhances the stability of the system and guarantees the service life of the whole system. It has been proved that the internal temperature of the system is well controlled, but the hardware cost is high, which is suitable for applications in the middle and high-end fields.
DC-DC buck LED driver for temperature compensation
If the temperature compensation function can be integrated inside the chip, this will greatly reduce the cost of use and space. SN3352 is a chip designed for this purpose. SN3352 is a step-down DC-DC constant current chip with an operating voltage range of 6~40V and an output current of 700mA. It has excellent constant current performance when temperature compensation is not started. It is suitable for driving series connection. 1W or 3W LED lights. The SN3352 has a dimming function that can be dimmed by changing the analog voltage on the ADJI pin or by applying a PWM signal to this pin. The SN3352 integrates the temperature compensation circuit of the patented technology of the company. The temperature compensation function requires an external resistor R th to set the temperature compensation point T th for temperature compensation and a negative temperature coefficient thermistor R for detecting temperature. Ntc works with.
The formula for the output current is as follows:
When Rntc>Rth, the temperature compensation is not started, the output current remains unchanged, and the size is determined by setting the current resistance Rs and the ADJI pin voltage:
Among them: VADJI is the voltage of the ADJI pin of the dimming pin, the unit is V, the dimming range is 0.3V~1.2V, and the voltage when floating is 1.2V;
When Rntc
among them:
Result of output compensation current
In addition, the SN3352 also has a cascading function. The ADJO pin of each chip is connected to the ADJI pin of the next-level chip, and the voltage with temperature compensation information is output from the ADJO pin of the previous stage chip to the next-level chip. ADJI pin. Each ADJO pin can drive up to 5 ADJI pins. Therefore, only one thermistor is needed to allow the entire system to share the temperature compensation function. When the temperature compensation is started, all the LEDs connected to the SN3352 system will drop as the temperature rises.
Another SN3910 with temperature compensation function is mainly used for step-down DC-DC constant current chip in high voltage field, full voltage range input, external high voltage MOS tube, output current up to 700mA, and the chip works in constant off time mode. Has excellent line voltage regulation. This chip is mainly used in fluorescent lamp solutions and other direct access solutions. Yann Microelectronics will provide application circuits, BOMs and layout recommendations according to different customer application plans and temperature compensation requirements to shorten the time to market.
Linear constant current LED driver for temperature compensation
Another LED linear constant current source driver with temperature compensation is the SN3118. Its output current can be programmed by an external resistor for low current LED applications from 20mA to 200mA. SN3118 working voltage 6V~30V, the matching between the four branch currents is less than 5%, the maximum current capacity of each channel is 175mA, and there is no EMI problem during operation. The circuit also uses a common resistor and a negative temperature coefficient thermistor to achieve temperature compensation. When the thermistor resistance drops to the normal resistance value, the temperature compensation starts.
Summary of this article
The temperature compensation function takes into account the LED life and output power with its low cost and high reliability, and does not affect the performance and life of the LED due to the harsh environment or the abnormality and aging of the heat sink.矽æ©å¾®ç”µå's LED driver IC products cover a wide range of applications, with advanced temperature compensation technology, can be customized for customers.
Figure 5 SN3910 driver LED typical application diagram
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