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LED Power

LED power is constant current source, general switching power supply is constant voltage source.

How it works: there are two requirements for the supply of LED power. First, the output voltage should be larger than derived voltage; second, the working current is required to be stable and cannot be greater than the rated current of LED. When the working current of the LED exceeds the rated current, the LED will soon be damaged. Therefore, the LED power must have a constant current function.

In the design of LED switching power supply, first determine the LED current, and then according to the number of beads used in series, determine the power supply voltage. The main working parameter is current and the auxiliary parameter is voltage.

Voltage


The general working voltage of LED power supply is 3.0 ~ 3.6v. Some operating voltages are lower, such as 2.0, 2.5, 2.7V, etc. There are also some 1.2v common operating voltages of 5V, 12V, 24V, and a few 15V or 28V, which are special purpose voltage sources.

Current


There are a few milliamps to a few amps, but because the working current of most embedded electronic products is less than 300mA, the power sources of 30-300ma account for a large proportion in the variety and quantity.

size


The development of portable products is SMD LED devices, such as SO encapsulation, sot-23 encapsulation, micron MAX encapsulation and the smallest sc-70 and the latest SMD encapsulation, making the power take up less and less space.

Measures


The new power has perfect protection measures, including: output overcurrent limit, overheat protection, overvoltage protection, short circuit protection and battery polarity reverse protection.

Power function


The static current of the new power is small, usually dozens of μA to hundreds of μA.

Static current of individual micro-power linear regulators is only 1.1 A. In addition, many power supply IC has power off control function (with the battery to control), when the power off state of IC itself power consumption at 1 μA.

Because it can make a part of the circuit does not work, it can greatly save electricity.

For example, on a wireless communication device, the receiving circuit can be turned off while sending; The display circuit can be turned off when no signal is received.

The output


Many portable electronic products have SCM. The low power due to overheating or the battery voltage and the output voltage drop in a certain percentage, the power supply has a power signal to microcontroller working status, to reset the MCU, use this signal is used as the power supply working state indicating (when the battery is low voltage, LED display).

Voltage


The general output voltage accuracy is between ±2 ~ 4%, there are a lot of new power supply accuracy can be up to ±0.5 ~ 1%;
And the output voltage temperature coefficient is small, generally is + / - 0.3 ~ + 0.5 mV / ℃, and some can reach the level of + / - 0.1 mV / ℃.

The linear adjustment rate is generally 0.05% ~ 0.1%/V, and some can reach 0.01%/V. Load adjustment rate is generally 0.3 ~ 0.5%/mA, some up to 0.01%/mA.

The power supply


Step-up DC/DC converter has high efficiency, but large ripple and noise voltage; Linear regulators have low efficiency, but the noise is minimal, the combination of the two output power supply IC can better solve the problem of efficiency and noise. For example, the digital circuit uses booster DC/DC converter power supply, while the noise sensitive circuit uses LDO power supply, such as max710/711, max1705/1706 and so on.

Another example is the charge pump +LDO composition, output stabilized charge pump supply IC, such as MAX868, it can output 0 ~ -2vin adjustable stable voltage, and can provide 30mA current; MAX1673 stabilized charge pump power supply IC output and VIN the same negative pressure, the output current can be up to 120mA.

classification


1. Power supply voltage: high voltage ac85-265, low voltage 1.5-36v
2. Power supply mode: constant voltage source and constant current source.
3. According to the input and output of the isolation relationship: isolated power supply and non-isolated power supply.
4, the relationship between the input voltage and the output voltage can be divided into: boost type, buck type, voltage type
The LED cannot use the power supply directly like the traditional light source, and the drive circuit needs to convert the power supply to dc current to work. The type and structure of LED drive circuit are related to the type of power supply, which is usually divided into DC power supply and ac power supply.

Dc power supply


Refers to the direct current can provide a variety of dry cells, batteries and solar cells, and so on. According to the supply voltage, it can be divided into the following forms.

Low voltage drive low voltage drive is to drive the LED with a voltage lower than the voltage drop of the LED positive guide, such as a common dry battery or nickel-chromium/nickel-metal hydride battery, whose normal supply voltage is 0.8 ~ 1.65v. The low voltage drive LED needs to raise the voltage enough to make the LED on. This is a common use case for LED low-power lighting devices, such as LED flashlights, LED emergency lights, energy-saving lamps, etc. Limited by the capacity of a single battery, it generally does not need a lot of power, but requires the lowest cost and relatively high conversion efficiency. In addition, considering the possibility of working with a no. 5 battery with a minimum volume, the best technical solution is a charge pump booster converter.

Transient voltage drive transient voltage drive refers to the voltage value of the power supply to the LED changes around the voltage drop of the LED tube. Sometimes this voltage may be slightly higher than the voltage drops of the LED tube, and sometimes it may be slightly lower than the voltage drops of the LED tube. For example, the voltage of a lithium battery or two lead-acid batteries in series is above 4V when fully charged and below 3V when the battery is running out. Typical applications powered by this type of power supply include LED mining lamps, etc. The power conversion circuit of LED driven by transition voltage has to solve the problem of voltage boost and voltage reduction. In order to work with a lithium battery, it also needs to have the smallest volume and the lowest cost. In general, the power is not large, the most cost-effective circuit structure is the reverse polarity charge pump converter.

High voltage drive high voltage drive refers to the voltage value of the LED power supply is always higher than the LED tube voltage drop, such as 6V, 12V, 24V battery, typical applications are solar lawn lamp, solar garden lamp, motor vehicle lighting system. High voltage drive LED to solve the problem of voltage reduction, because the high voltage drive is generally powered by ordinary batteries, will use relatively large power (such as vehicle lighting and signal light), should have as low as possible cost. The optimal circuit structure of the converter is a series switching buck circuit. Mains ac power supply (driver), which is a most valuable way of power supply for LED lighting applications and semiconductor lighting must solve the problem of good, to popularize and apply ac power supply (mains drive) is applied to the LED driver, typically after decompression, rectifier, filter, voltage (or steady flow), the ac power is converted to dc power supply, and then through the appropriate driver circuit for LED to provide suitable working current, but also have high conversion efficiency, small volume and low cost. In addition, security isolation should be addressed. In consideration of the impact on the power grid, electromagnetic interference and power factor problems should be solved. For small and medium power LED, the best circuit structure is isolated single - end fly back converter. For high-power applications, a bridge converter circuit should be used.

USE


LED power supply is widely used in street lamps, tunnel lamps, LED floor tiles, LED point light sources, LED grille lights, LED indoor lights, LED ceiling lights, buildings, roads and Bridges, square construction facilities, lawn lights, curtain wall lights, LED Wall Moudle, table lamps, hotel stadium LED plant lights and aquatic lamps, etc.

The information platform display includes LED display screen, display board, dynamic billboard, simulation animation, sports stadium, indicator light inside the carriage and internal reading light, brake light outside the car, taillight, turn signal, side light, explosion-proof lamp, mining lamp, etc.
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