The basic principle and specific method of power supply driving LED display_LED display power supply

The basic principle and specific method of power supply driving LED display_LED display power supply

by:XY Screens     2021-08-25
There are various forms of the original power supply, but no matter which kind of power supply, generally can not directly supply power to the LED display. Therefore, to use LED as a lighting source, the problem of power conversion must be solved first. LED is actually a current-driven low-voltage unidirectional conductive device. The LED driver should have features such as DC control, high efficiency, PWM dimming, overvoltage protection, load disconnection, small size, and ease of use. The following items must be paid attention to when designing a power converter for LED power supply.  ①Because the LED is a unidirectional conductive device, it is necessary to use a direct current or a unidirectional pulse current to power the LED.  ②Because the LED is an LED lighting device with a PN junction structure, it has a barrier electromotive force, which forms the turn-on threshold voltage, so the voltage applied to the LED must exceed this threshold voltage before the LED can be fully turned on. The threshold voltage of a high-power LED is generally above 2.5V, and the voltage drop of the LED is 3 to 4V during normal operation.  ③The current and voltage characteristics of LED are non-linear. Because the current flowing through the LED is equal to the electromotive force of the power supply minus the barrier electromotive force of the LED and then divided by the total resistance of the loop (the sum of the internal resistance of the power supply, the lead resistance, and the resistance of the LED body), the current flowing through the LED It is not proportional to the voltage applied across the LED.  ④Since the PN junction of the LED has a negative temperature coefficient, the barrier electromotive force of the LED will decrease when the temperature rises. Therefore, the LED cannot be directly powered by a voltage source, and current limiting measures must be taken, otherwise as the temperature of the LED increases, the current will become larger and larger, which will damage the LED.  ⑤The ratio of the current flowing through the LED to the luminous flux of the LED is also non-linear. The luminous flux of the LED increases with the increase of the current flowing through the LED, but it is not proportional. The later the luminous flux increases the less. Therefore, the LED should be made to work under a current value with a relatively high luminous efficiency.   In addition, LED is also the same as other light sources, and the electric power it can withstand is limited. If the electric power applied to the LED exceeds a certain value, the LED may be damaged. Due to the difference in production process and material characteristics, the barrier electromotive force and the internal resistance of the LED of the same model are not exactly the same, which leads to inconsistent voltage drop during LED operation, and the LED barrier electromotive force has a negative temperature coefficient , So LEDs cannot be used directly in parallel. Voltage driving method 1. Low voltage driving LED    low voltage driving refers to driving the LED with a voltage lower than the forward voltage drop of the LED. Between 0.8 and 1.65V. When driving an LED with a low voltage, the voltage needs to be increased to a voltage value that is sufficient to turn on the LED. For low-power lighting devices such as LEDs, low-voltage driving is a common use case, such as LED flashlights, LED emergency lights , Energy-saving desk lamps, etc. Due to the limitation of single-cell battery capacity, low-voltage drive power generally does not require a lot of power, but requires the lowest cost and relatively high conversion efficiency. Considering that sometimes it may be necessary to work with a AA battery, it is necessary It has the smallest volume. The best technical solution is to use a capacitive boost converter.  2. Transition voltage drive LED   Transition voltage drive means that the voltage value of the power supply to the LED changes near the LED voltage drop. This voltage may sometimes be slightly higher than the LED voltage drop, and sometimes it may be slightly lower than the LED voltage drop. For example, a power supply composed of a lithium battery or two lead-acid batteries connected in series, the voltage of the battery is above 4V when the battery is fully charged, and the voltage is below 3V when the battery is discharged, the typical application is LED miner's lamp.   Transition voltage driving LED power conversion circuit not only solves the boost problem, but also solves the buck problem. In order to work with a lithium battery, it also needs to have the smallest possible volume and the lowest possible cost. Under normal circumstances, its power is not large, and the most cost-effective circuit structure is an inductive boost and buck converter.  3. High voltage drive LED   High voltage drive means that the voltage value of the power supply to the LED is always higher than the voltage drop of the LED. Common power supplies are 6V, 12V, 24V batteries. Typical applications of this method are solar lawn lights, solar garden lights, and automotive lighting systems. High-voltage driving LEDs must solve the problem of voltage reduction. Because high-voltage driving is generally powered by ordinary batteries, relatively large power will be used, such as vehicle lighting and signal lights, so the cost should be as low as possible. The best circuit structure of the converter is an inductive buck converter.  4. Mains driving LED  Using mains power to drive LED is the most practical driving method, and it is also a problem that must be solved to promote the application of LED in the field of lighting. In order to solve the problem of step-down and rectification, the use of city power to drive the LED must have a relatively high conversion efficiency, a smaller volume and a lower cost, and the problem of safety isolation should also be solved. Taking into account its impact on the power grid, electromagnetic interference and power factor issues must also be resolved. For medium and low-power LEDs, the best circuit structure is an isolated single-ended flyback converter: for high-power applications, a bridge converter circuit should be used.
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