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Basic Knowledge Of LED Lighting Design
Aug 01, 2017

Basic knowledge of LED lighting design

Light-emitting diode (LED) in medium and small size portable products, such as application received a large number of the backlight of the screen, as it has to further improve the luminous performance and cost optimization, in recent years has entered the field of general lighting, such as building lighting, street lighting, landscape lighting, signs, lights, and the lighting inside the house, just application.

LED lighting design, on the other hand, also brought the engineering community, including Chinese engineers, challenge, because it not only LED lighting has a very wide range of application scope, the application of power levels, types and power supply topology can be driven by the power supply, etc., also each are not identical. Engineers urgently need to systematically learn and learn more about the basics of LED lighting design. Because of this, various semiconductor's director of product application Bernie Mr Weir recent special writing relevant training materials, for the engineers to teach basic design knowledge, content involves the general requirements of the LED drive, power supply topology, power factor correction, power conversion efficiency and drive standard, as well as other problems such as reliability and service life, convenient they better introduction to design and improve, to better serve the LED lighting market. Limited to space, this article is a summary of the training materials.

General requirements of LED drives

The driving LED faces many challenges, such as the forward voltage varies with temperature and current, while the LED forward voltage of different individuals, different batches and different suppliers will also differ. In addition, leds' "color points" will also drift with current and temperature changes.

In addition, multiple leds are used in the application, which involves the arrangement of multiple leds. Various kinds of arrangement, the preferred drive series single series of LED, because this way no matter how change forward voltage, output voltage (Vout) how to "drift", provide excellent current matching performance. Of course, users can also use parallel, series-parallel combinations, cross-linking, and other arrangements to use the "matched" LED forward voltage application and gain other advantages. In a cross connection, if one of the leds is out of the way, only one LED in the circuit will double the drive current, thus minimizing the impact on the entire circuit.

The arrangement of LED and the specification of LED light determine the basic drive requirements. The main function of LED drive is to limit the current of the LED under certain working conditions, regardless of input and output voltage. LED drive basic working circuit schematic diagram, in which the so-called "isolation" said ac line voltage and the LED (i.e., the input and output) between the electrical connection in the physical, the most commonly used is to use transformer to electrical isolation, and the "isolation" no use high-frequency transformer to electrical isolation.

It is worth mentioning that in LED lighting design, AC - DC power converter with constant current drive circuit can use different configurations: the two parts 1 integral configuration, or both together, in which both are located in lighting lamps and lanterns, the advantage of this configuration including optimizing efficiency and easy installation, etc.; 2 distributed configuration, which is both alone, simplifies security considerations and increases flexibility.

According to different application requirements, the LED driver can adopt constant voltage output, which is to output the voltage of the clamp in a certain current range. It can also be used with constant current output, and the output design can strictly limit the current. The constant current constant pressure output is also possible, that is, the constant output power is provided, and the current is determined by the positive voltage of the LED.

Overall, LED lighting design needs to consider the following factors:

Output power: involves the scope of LED forward voltage, electric current and LED arrangement, etc

Power supply: AC - DC power supply, DC - DC power supply, directly driven by AC power

Functional requirements: dimmer requirement, dimming method (analog, digital or multistage), lighting control 

Other requirements: energy efficiency, power factor, size, cost, troubleshooting (features) of protection, to conform to standards and reliability etc.

The topology of LED power supply

With AC - DC power supply of LED lighting applications, the construction of the power conversion module includes diode, switch (FET), such as resistance and capacitance and inductance discrete component is used to perform their respective functions, and pulse width modulation (PWM) voltage regulator is used to control the power conversion. Circuits are usually joined the transformer isolation type AC - DC power conversion including the flyback, normal shock and half bridge topology, which is less than 30 W flyback topology standard selection of low and medium power applications, whereas half bridge structure is best suited to provide higher energy/power density. In the case of the transformer in the isolation structure, the size of its size is related to the switching frequency, and most isolated LED drives adopt "electronic" transformer basically.

In the application of LED lighting with DC - DC power supply, the LED driving mode can be used with resistance type, linear voltage stabilizer and switch stabilizer, etc., basic application schematic. Resistance type drive mode, adjust and LED series in the current sense resistor can control the LED forward current, the drive mode is easy to design, low cost, and no electromagnetic compatibility problem, disadvantage is dependent on the voltage, need to screen LED, and lower efficiency. Linear regulator is also easy to design and no EMC problems, also support the current steady flow and over-current protection, and provide external current set point, is lack of power dissipation problem, and the input voltage is always higher than the forward voltage, and the efficiency is not high. The switch stabilizer controls the opening and closing of the switch (FET) through the PWM control module to control the flow of current.