The basic principles and specific methods of LED driving power supply

There are various forms of raw power supply, but no matter what kind of power supply, it generally cannot directly power LEDs. Therefore, to use LEDs as lighting sources, the power conversion problem must be solved first. LED is actually a current-driven low-voltage unidirectional conductive device. The LED driver should have characteristics 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 to power LEDs.

　　① Since LED is a unidirectional conductive device, it is necessary to use direct current or unidirectional pulse current to power the LED.

　　② Since LED is a semiconductor device with a PN junction structure, it has a barrier electromotive force, which forms a conduction threshold voltage. Therefore, the voltage value applied to the LED must exceed this threshold voltage for the LED to fully conduct. The threshold voltage of a high-power LED is generally above 2.5V, and the voltage drop of the LED is 3-4V during normal operation.

　　③The current and voltage characteristics of LED are nonlinear. Because the current flowing through the LED is numerically equal to the electromotive force of the power supply minus the electromotive force of the LED barrier divided by the total resistance of the circuit (the sum of the power supply internal resistance, lead resistance, and LED body resistance), the current flowing through the LED is not proportional to the voltage applied across the LED.

　　④ Since 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 powered directly by a voltage source, and current limiting measures must be taken, otherwise as the temperature of the LED increases during operation, the current will become larger and larger, thus damaging the LED.

　　⑤ The ratio of the current flowing through the LED to the luminous flux of the LED is also nonlinear. The luminous flux of the LED increases with the increase of the current flowing through the LED, but it is not proportional. The luminous flux increases less and less as the current increases. Therefore, the LED should be operated at a current value with a relatively high luminous efficiency.

　　In addition, like other light sources, LEDs are limited in the amount of power they can withstand. If the power applied to an LED exceeds a certain value, the LED may be damaged. Due to differences in production processes and material properties, the barrier electromotive force and internal resistance of the same model of LEDs are not exactly the same, which leads to inconsistent voltage drops when the LEDs are working. In addition, the LED barrier electromotive force has a negative temperature coefficient, so LEDs cannot be used directly in parallel.

　　There are four situations when using the original power supply to power LEDs: low voltage drive, transition voltage drive, high voltage drive, and AC drive. Different situations require different solutions for the implementation of power converter technology. The following briefly introduces the above-mentioned methods of driving LEDs with power supplies .

　　1. Low voltage LED driver

　　Low voltage drive means driving the LED with a voltage lower than the forward voltage drop of the LED. For example, if an ordinary dry cell or nickel-chromium/nickel-metal hydride battery is used to drive the LED, the normal supply voltage is between 0.8 and 1.65V. When driving the LED with a low voltage, the voltage needs to be increased to a voltage value sufficient to turn on the LED. For low-power lighting devices such as LEDs, low voltage drive is a common use case, such as LED flashlights, LED emergency lights, energy-saving desk lamps, etc. Due to the limitation of the capacity of a single battery, the low-voltage drive power supply generally does not require a lot of power, but requires the lowest cost and relatively high conversion efficiency. Considering that it may sometimes need to work with a No. 5 battery, it also needs to have the smallest volume. The best technical solution is to use a capacitive boost converter.

　　2. Transition voltage driving LED

　　Transition voltage drive means that the voltage value of the power supply to the LED changes around the LED voltage drop. This voltage may be slightly higher than the LED voltage drop sometimes, and may be slightly lower than the LED voltage drop sometimes. For example, a power supply composed of a lithium battery or two lead-acid batteries in series has a voltage of more than 4V when the battery is fully charged, and a voltage of less than 3V when the battery is about to be discharged. The typical application is LED mining lamps.

　　The power conversion circuit of the transition voltage driving LED needs to solve both the boost and buck problems. In order to work with a lithium battery, it also needs to be as small as possible and as low cost as possible. Generally, its power is not large, and the circuit structure with the highest cost performance is the inductive boost and buck converter.

　　3. High voltage LED driver

　　High voltage drive means that the voltage value of the power supply for LED is always higher than the voltage drop of LED. Common power supplies include 6V, 12V, and 24V batteries . Typical applications of this method include solar lawn lights, solar garden lights, and lighting systems for motor vehicles. High voltage drive of LEDs requires solving the problem of voltage reduction. Since high voltage drive is generally powered by ordinary batteries, it will use relatively large power, such as motor vehicle lighting and signal lights, so the cost should be as low as possible. The best circuit structure of the converter is the inductive buck converter.

　　4. Mains power drives LED

　　Using AC 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 lighting field. Using AC power to drive LED requires solving the problems of voltage reduction and rectification, and also requires relatively high conversion efficiency, small size and low cost, and should also solve the problem of safety isolation. Considering its impact on the power grid, electromagnetic interference and power factor problems must also be solved. For medium and low power LEDs, the best circuit structure is an isolated single-ended flyback converter: for high-power applications, a bridge conversion circuit should be used.