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LED switching power supply introduction
2019-10-24 23:16:22
 LED switching power supply introduction
Devices that convert other forms of energy into electrical energy are called power supplies.
A generator can convert mechanical energy, chemical energy, etc into electrical energy, and a dry battery can convert chemical energy into electrical energy.
The battery itself is not charged, its two poles have positive and negative charge, from the positive and negative charge generated voltage (current is the charge under the action of voltage directional movement and formed), charge is a widely existing conductive ions in the conductor, to generate current only need to add voltage.
When the two electrodes of the battery are connected to the conductor, the charge is released in order to generate a current. When the charge is dissipated, the dry battery is called a power supply. The device that changes alternating current into direct current through transformers and rectifiers is called rectified power supply. An electronic device that provides a signal is called a signal source.
LED power supply is constant current source, general switching power supply is constant voltage power supply.
Working principle: LED has two requirements for power supply: first, the output voltage is required to be >LED switching voltage; second, the working current is required to be stable and not larger than the rated current of LED. When the working current of LED exceeds the rated current, the LED will be damaged quickly. Therefore, the power supply used by LED must have constant current function.
In the design of switching power supply for LED, first determine the current of LED, and then determine the power voltage according to the number of series beads used. The current is the main working parameter and the voltage is the auxiliary parameter. Its block diagram is shown as follows:
The new power supply has perfect protection measures, including: output overcurrent limit, overheat protection, overvoltage protection, short circuit protection and battery polarity reverse protection.
Power function
New sources of electricity typically have small static currents, ranging from tens to hundreds of microns.
Individual micropower linear regulators have A static current of only 1.1 microns. In addition, many IC's have the ability to turn off the power supply (controlled by A battery) and consume about 1 mua when the power is turned off.
Because it can make part of the circuit does not work, can save electricity greatly.
For example, on a wireless communication device, the receiving circuit can be closed in the sending state; The display circuit can be closed when the signal is not received.
The output
With single chip microcomputer in many portable electronic products, 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). [1]

The accuracy of general output voltage is between ±2 ~ 4%, and the accuracy of many new power sources is up to ±0.5 ~ ±1%.
And the output voltage temperature coefficient is small, generally ±0.3 ~ ±0.5mV/℃, and some can reach the level of ±0.1mV/℃.
Linear adjustment rate is generally 0.05% ~ 0.1%/V, some up to 0.01%/V; Load adjustment rate is generally 0.3 ~ 0.5%/mA, some up to 0.01%/mA.
Switching power supply
The efficiency of booster DC/DC converter is high, but the ripple and noise voltage is large, low voltage difference; The linear voltage regulator is low in efficiency, but the noise is the least. For example, the digital circuit USES booster DC/DC converter power, and the noise-sensitive circuit USES LDO power, such as max710/711, max1705/1706 and so on.
Another example is the charge pump +LDO composition, output voltage stabilized charge pump power IC, such as MAX868, it can output 0 ~ 2VIN adjustable stable voltage, and can provide 30mA current; MAX1673 stabilized charge pump power IC output and VIN the same negative pressure, output current up to 120mA.
1. Press the voltage point of power supply: high voltage ac80-277, low voltage 1.5-36v
2, according to the power supply: constant voltage source and constant current source.
3, according to the input and output isolation relationship: isolated power supply and non-isolated power supply.
4. The relationship between input voltage and output voltage is divided into step-up voltage type, step-down voltage type, up-down voltage type and so on
Unlike the traditional light source, the LED cannot directly use the power supply. It needs the drive circuit to change the power supply to dc current to work. The type and structure of LED driving circuit are related to the type of power supply, which is usually divided into dc power supply and ac power supply. [2]

Dc power supply
Refers to the direct current can provide a variety of dry batteries, batteries and solar cells, according to the supply voltage can be divided into the following forms.
Low voltage drive low voltage drive means to drive the LED with a voltage lower than the forward guide voltage drop, such as a common dry battery or ni-cr/ni-mh battery, whose normal supply voltage is 0.8 ~ 1.65V. The low voltage driving LED needs to increase the voltage enough to make the LED turn on. For LED such low-power lighting devices, this is a common use case, such as LED flashlight, LED emergency lamp, energy-saving table lamp and so on. Due to the limitation of single battery capacity, it does not need a lot of power, but requires the lowest cost and relatively high conversion efficiency. In addition, given the possibility of working with a no. 5 battery and the minimum volume, the best technical solution is a charge pump booster.
Transition voltage drive transition voltage drive means that the voltage value of the power supply to the LED varies around the LED tube voltage drop, which may be slightly higher than the LED tube voltage drop or slightly lower than the LED tube voltage drop. For example, a lithium battery or two lead-acid batteries in series have a voltage of more than 4V at full charge and less than 3V when the battery is running out. Typical applications of this type of power supply include LED mining lamps. The transition voltage drives the LED's power conversion circuit to solve both the booster and the buck problem. To work with a lithium battery, it needs to be as small as possible and as cheap as possible. In general, the power is not large, and the most cost-effective circuit structure is 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 lights, solar yard lights, motor vehicle lighting system. High-voltage drive LED to solve the buck problem, because the high-voltage drive is generally powered by ordinary batteries, will use relatively large power (such as motor vehicle lighting and signal lighting), should have as low as possible cost. The optimal circuit structure of the converter is 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 is 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. Security isolation should also 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 - end flyback converter. For high power applications, bridge switching circuits should be used