LED controllable constant current source drive system design scheme

LED as the third generation of lighting light source, has the advantages of low operating voltage, low power consumption, high luminous efficiency, long life, etc. LED is a non-linear device, when the LED on, as long as the LED voltage changes slightly, will make the current is too large leading to LED device heat damage. LED's operating characteristics of the quality of the power supply is very dependent on the degree of power supply, so the realization of a high-quality power supply is important to improve the quality of LED lighting, power utilization, extend the service life of LED. Power supply is of great significance to improve the lighting quality of LEDs, power utilization, and extend the service life of LEDs. The stability of the power supply mainly depends on the LED drive circuit design, constant current source drive is the best LED drive mode, the use of constant current source drive, LED current will not be affected by the voltage, ambient temperature changes, as well as the LED parameters of the dispersion, so as to keep the current constant, and give full play to the LED's various excellent characteristics. Currently widely used constant current source has two forms: one is a linear power supply to improve the constant current source, the other is a switching power supply constant current source. Linear power supply to improve the type of constant current source of linear loss, the scope of application is small; switching power supply type constant current source of poor reliability, adapt to the small range, and high cost. Therefore, economical and practical, reliable performance of CNC constant current source has been widely used. In this paper, for the small power LED in the existing lighting system in the drive mode of some of the deficiencies, the design of a highly efficient drive system, proposed a corresponding new type of drive system.

  1 LED characteristics

  1.1 LED volt-ampere characteristics

  The mathematical model of LED volt-ampere characteristics can be expressed as:

  Where V is the LED starting voltage; RS indicates the slope of the volt-ampere curve; IF indicates the LED forward current; T indicates the ambient temperature; △VF/△T is the temperature coefficient of the LED forward voltage, which has a typical value of -2V/°C for most LEDs. From the mathematical model of the LED, in a certain ambient temperature conditions LED in the forward conduction after the forward voltage of the small changes will cause a large change in the LED current.

 1.2 LED temperature characteristics

  The size of the LED forward current is with the temperature changes, white LED operating current is generally about 200mA, when the ambient temperature once more than 50 ℃, white LED allowable forward current will be reduced and not up to the normal luminous brightness of the required operating current, in this case, if you still apply high current, it is easy to make the white LED aging.

  1.3 LED optical properties

  The luminous flux of the light source refers to the visible light energy passing through the 4π stereo angle per unit time. The relationship between white LED current and luminous flux is shown in Figure 1, with the increase of current, the luminous flux of the LED increases nonlinearly and gradually tends to saturation. The reason is mainly because with the increase in current and time, the internal temperature of the high-power LED rises, which occurs in the P/N junction region of the carrier composite probability decreases, resulting in a reduction in the LED luminous efficiency.

  2 system program selection and comparison

  2.1 Core controller selection

  Controller using the more common STC series microcontroller STC89C52, a high-performance 8-bit microprocessor with 8K-byte flash programmable erasable read-only memory (FPEROM-FalshProgrammableandErasableReadOnlyMemory). The device is fabricated using high-density non-volatile memory manufacturing technology and is compatible with the industry-standard MCS-51 instruction set and output pins. With the combination of a versatile 8-bit CPU and flash memory in a single chip, STC's STC89C52 is a highly efficient microcontroller that provides a flexible and inexpensive solution for many embedded control systems.

 2.2 Clock function module selection

  Program 1 uses the DS1302 clock chip. This chip has a small size, few pins, and is very easy to operate. The disadvantage is that the use of external backup battery and external crystal oscillator, the hardware line is more complex, the cost is higher.

  Program 2 uses DS12C887 clock chip. This chip, relatively large size, internal integration of rechargeable lithium batteries, but also integrated 32.768kHz standard crystal, can effectively maintain the continuity of time, very convenient to use, but expensive.

  Scheme 3 uses the timer of the microcontroller (crystal 11.0592M) to design the clock. The time is displayed on the 1602 LCD, and the hours, minutes and seconds of the clock are adjusted with a separate keyboard, and the timer can be set. Low cost, do not need to enable other chips and peripheral circuits in, but the program is more complex.

  Considering the cost-effective and circuit optimization issues, so choose program 3.

2.4 Constant current source module selection

  Scheme 1 uses a microcontroller to generate PWM signals, output to the Darlington tube, through the filter to eliminate ripple, to achieve the function of constant current source. Using PWM pulse to achieve constant current source can simplify the hardware circuit, easy to control and regulate, but the program is difficult to ensure the accuracy, to adapt to the design of the precision requirements of the higher technical difficulty, and the program is difficult to adapt to the application of a large range of current regulation, ripple and stability and other factors, it is difficult to achieve.

  Program 2 by the operational V / I conversion circuit constitutes a constant current circuit. Constant current circuit composed of operational amplifiers to get rid of the transistor constant current circuit is limited by the shortcomings of the process parameters. This scheme can realize 0~5V/0~500mA V/I conversion, and the conversion accuracy is high. If the input is controlled by a microcontroller with a digital potentiometer, a numerical control constant current source can also be easily realized.

  Program 3 through the specialized constant current / constant voltage chip LT1769 and a simple control line to achieve voltage-controlled current source program. This constant voltage chip has a high degree of integration, the use of the control system hardware and software have become relatively simple advantages. But the disadvantage is that the program is not flexible enough to achieve; due to the chip precision is not high, the device performance is limited to the performance of this dedicated chip performance indicators allowed. So this design is generally only suitable for precision requirements are not high, but the integration and portability requirements of high occasions, it has been proved that this is not to do the ideal CNC current source to realize the program.

  In view of the arguments and comparisons, the final choice of program 2.

 2.5 D / A converter selection

  For D / A converter, I use a very common 8-bit D / A converter DAC0832, its conversion time of 1 μs, the operating voltage of +5V ~ +15V, the reference voltage of ± 10V, and the microprocessor interface is fully compatible with the low price, simple interface, conversion control is easy, etc., in the application of microcontroller systems are widely used. Its D/A converter consists of 8-bit input latch, 8-bit DAC register, 8-bit D/A conversion circuit and conversion control circuit.

  3 Hardware circuit design

  3.1 System power supply circuit

  The power supply utilizes positive voltage integrated regulator LM7812 and negative voltage integrated regulator LM7912 to provide symmetrical positive/negative 12V regulated output, which is supplied to the op-amp, and then regulated to 5V output by LM7805, which is supplied to the microcontroller.

3.2 LED Driver Circuit

  The circuit can easily realize the voltage/current conversion. Operational amplifier U1A constitutes a comparator, U1C constitutes a voltage follower, playing the role of negative feedback. Input signal Vi compared with the feedback signal Vf, the output voltage V1 at the output of the comparator U1A, V1 control the output voltage V2 of the op-amp U1B, thus changing the output current IL of the transistor Q1, and the output IL affects the feedback voltage Vf, to reach the purpose of tracking the input voltage Vi. The output current IL is calculated as IL=Vf/R13, because the negative feedback makes Vi=Vf, so IL=Vi/R13. If R13 takes the value of 10Ω, the V/I conversion of 0~5V/0~500mA can be realized; if the performance parameters of the selected device are stable, and the amplification of the op-amps UA1 and UA2 is large enough, the conversion accuracy is higher. the voltage of V13 is controlled by the MCU. With the D / A output control, can easily realize the digital constant current source output.

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