Introduction to LED driver
LEDs are characteristic-sensitive semiconductor devices with negative temperature characteristics. Therefore, it needs to be stabilized and protected during the application process, which leads to the concept of driver. LED devices have almost harsh requirements for driving power. Unlike ordinary incandescent bulbs, LEDs can be directly connected to a 220V AC power supply.
Function of LED driver
According to the power rules of the power grid and the characteristic requirements of the LED driver power supply, the following points should be considered when selecting and designing the LED driver power supply:
High reliability: especially like the driver of LED street lights. Maintenance is difficult and costly in high altitude areas.
High efficiency: The luminous efficiency of LEDs decreases with increasing temperature, so heat dissipation is very important, especially when a power supply is installed in the bulb. LED is an energy-saving product with high driving power efficiency, low power consumption and low heat generation in the lamp, which helps to reduce the temperature rise of the lamp and delay the light attenuation of the LED.
High power factor: The power factor is the requirement of the power grid on the load. Generally, there are no mandatory indicators for electrical appliances below 70 watts. Although the power factor of a single low-power electrical appliance is very low, it has little impact on the power grid. However, if the lights are turned on at night, similar loads will be too concentrated, which will cause serious loads on the grid. It is said that for LED driver of 30 to 40 watts, there may be certain index requirements for power factor in the near future.
LED driver principle
The relationship curve between forward voltage drop (VF) and forward current (IF). It can be seen from the curve that when the forward voltage exceeds a certain threshold (approximately 2V) (usually called the on-voltage), it can be approximately considered that IF and VF are proportional. See the table below for the electrical characteristics of current major super bright LEDs. It can be seen from the table that the highest IF of current super bright LEDs can reach 1A, while VF is usually 2 to 4V.
Since the light characteristics of the LED are usually described as a function of current rather than a function of voltage, that is, the relationship curve between luminous flux (φV) and IF, the use of a constant current source driver can better control the brightness. In addition, the forward voltage drop of the LED has a relatively large range (up to 1V or higher). As can be seen from the VF-IF curve in the above figure, a small change in VF will result in a large change in IF, resulting in greater brightness and large changes.
The relationship curve between LED temperature and luminous flux (φV). The figure below shows that luminous flux is inversely proportional to temperature. The luminous flux at 85°C is half of the luminous flux at 25°C, and the luminous output at 40°C is 1.8 times of the luminous flux at 25°C. Temperature changes also have a certain effect on the wavelength of the LED. Therefore, good heat dissipation is a guarantee to ensure that the LED maintains a constant brightness.
Therefore, using a constant voltage source to drive cannot guarantee the consistency of LED brightness, and affects the reliability, life and light attenuation of the LED. Therefore, super bright LEDs are usually driven by a constant current source.
Post time: Sep-03-2021