Operating principles of LED driver

A device that converts other forms of energy into electrical energy is called a driver.

Generators can convert mechanical energy, chemical energy, etc. into electrical energy, and dry batteries can convert chemical energy into electrical energy.

The battery itself is not charged. There are positive and negative charges at the two poles of the battery. The voltage is generated by the positive and negative charges (the current is formed by the directional movement of the charge under the action of the voltage). The charge is a conductive ion widely existing in the conductor. To generate the current, you only need to add the voltage.

When the two poles of the battery are connected with conductors, the charge is released to generate current. When the charge is dissipated, dry battery is called driver. Through transformer and rectifier, the device that changes alternating current into direct current is called rectifier power supply. An electronic device that can provide a signal is called a signal source.

LED driver is constant current source, general switch type driver is constant voltage source.

LED has two requirements for driver. First, the output voltage should be greater than the conduction voltage of the LED. Secondly, the working current should be stable and should not be greater than the rated current of the LED. When the working current of the LED exceeds the rated current, the LED will quickly appear aging damage. Therefore, the drivers used by LED must have constant current function.

In the design of switching driver for LED, it is necessary to determine the LED current, and then determine the driver voltage according to the number of lamp beads in series.

Generally speaking, LED driver has these following important parameters.

1. Voltage

The general working voltage of LED driver is 3.0 ~ 3.6V. Some of them have lower operating voltage, such as 2.0, 2.5, 2.7V, and others are 1.2V. The commonly used operating voltage is 5V, 12V, 24V, and there are a few 15V or 28V voltage sources for special purposes.

2. Current

Most of the embedded electronic products work current is less than 300 mA, so the driver of 30 ~ 300mA accounts for a large proportion in variety and quantity.

3. Size

Nowadays, the portable products are all using SMD devices, including SO package, SOT-23 package, μ MAX package, SC-70 package with the smallest package size and the latest SMD package, which make the space occupied by driver smaller and smaller.

4. Protective measures

The new driver has perfect protection measures, including output over-current limit, overheating protection, over-voltage protection, short-circuit protection and battery polarity reversal protection.

5. Power consumption

The static current of the new driver is small, generally from dozens of μ a to hundreds of μ a.

The static current of a few linear regulators with low power consumption is only 1.1 μ a. In addition, many power IC has the function of power off control terminal (controlled by battery). When the power is off, the power consumption of IC is about 1 μ a.

Because it can make part of the circuit do not work, can greatly save power.

For example, on a wireless communication device, the receiving circuit can be turned off when the transmission state is in progress, and the display circuit can be turned off when the signal is not received, etc.

6. Output

Many portable electronic products have single-chip microcomputer. When the output voltage drops due to overheating or low battery voltage, the driver has a driver working state signal sent to the single-chip microcomputer to reset the single-chip microcomputer. This signal can also be used to indicate the working state of the driver. When the battery voltage is low, there is led display.

7. Voltage

The general output voltage accuracy is between ± 2% and 4%, and the accuracy of many new drivers can reach ± 0.5%～1%. And the temperature coefficient of output voltage is small, generally ± 0.3 to ± 0.5mv / ℃, and some of them can reach the level of ± 0.1mv / ℃.

The linear adjustment rate is generally 0.05% ～ 0.1% / V, some can reach 0.01% / V; the load adjustment rate is generally 0.3 ～ 0.5% / MA, and some can reach 0.01% / MA.

8. Power supply

LED can not directly use the driver as the traditional light source, so it needs the driver circuit to convert the driver into DC current to work. The type and structure of LED driving circuit is related to the type of driver, which is usually divided into DC driver and AC driver.

DC driver refers to various dry cells, batteries and solar cells that can directly provide DC current.

AC driver is one of the most valuable driver methods for LED lighting applications. It is a problem that must be solved in the popularization and application of semiconductor lighting. When AC driver is applied to LED drive, it generally needs to go through step-down, rectification, filtering, voltage stabilization (or current stabilization) to convert AC driver to DC driver, and then provide appropriate working current for led through appropriate driving circuit It has higher conversion efficiency, smaller volume and lower cost. In addition, security isolation should be addressed. Considering the influence on power grid, electromagnetic interference and power factor should be solved. For small and medium power LED, the best circuit structure is isolated single ended flyback converter. For high power applications, bridge converter circuits should be used.

9. Life time

LED chip and driver are installed together, the general space is narrow and the heat dissipation condition is poor. How to ensure the quality and life of LED driver should be considered from the beginning of design, so as to avoid the LED power failure quickly. It can be said that the life of LED driver is the key to the development of LED.

The factors that affect the life of LED driver include environment, temperature, input voltage and quality of electrolytic capacitor.

10. Switching times

Most drivers are equipped with capacitor input rectifier circuit. When the driver is connected, surge current will be generated, which will lead to fatigue of switch contacts, increase of contact resistance and adsorption problems. In theory, during the expected life of the driver, the number of on and off of the switch is about 10000.

11. Isolation and non isolation

In LED lighting market, there are two types of driver,isolation and non isolation driver.

The non isolated design is limited to dual insulated products, such as alternative products for light bulbs, where the LED and the entire product are integrated and sealed in a non-conductive plastic, so there is no risk of electric shock to the user.

Secondary products are isolated and relatively expensive, but they are essential where users can reach the LED and output wiring.

LED driver with isolation transformer or electrical isolation means that led can be directly touched by hand without electric shock.

Although the LED driver without isolation transformer can still achieve partial mechanical insulation with the help of protective shell, the LED can not be directly contacted when it's working.

Due to the advantages of environmental protection, long life and high photoelectric efficiency, LED has been widely used in various industries in recent years.

At present, LED driver is widely used in street lamp, tunnel lamp, LED floor tile, LED point light source, led grid lamp, LED indoor lamp, led ceiling lamp, lawn lamp, curtain wall lamp, led wall washing lamp, led plant lamp, aquarium lamp, etc.

The information plane display includes LED display screen, display board, dynamic billboard, simulation animation, stadium, indicator light and internal reading light in the carriage, brake light, tail lamp, turn signal lamp, side lamp, explosion-proof lamp, miner's lamp in mining production, etc.

We believe that in the future of rapid development of science and technology, more and more advanced technologies will be applied to LED industry for the benefit of human.