Why do LED tubes have to require constant current?
The output of the LED lamp drive power supply is mostly a constant current source that can change the voltage with the change of the forward voltage drop of the LED. Since LED is a characteristic-sensitive semiconductor device and has negative temperature characteristics, it needs to be stabilized and protected during the application process, resulting in the concept of driving. The LED lamp driving power supply converts the power supply into a specific voltage The current is used to drive the voltage converter of the LED light. Under normal circumstances: the input of the LED lamp drive power supply includes high-voltage power frequency AC (ie mains), low-voltage DC, high-voltage DC, low-voltage and high-frequency AC (such as the output of an electronic transformer), etc. . Why do LED tubes have to require constant current?
When using LED as lighting (LED tube, LED bulb), display or other equipment or backlight, it needs to be driven by constant current.
There are three main reasons:
1. Avoid the drive current exceeding the maximum rated value and affect its reliability.
2. Obtain the expected brightness requirements, and ensure the consistency of the brightness and chromaticity of each LED
3. It can effectively avoid lightning strikes, power grid surges, over-current, over-voltage protection, and improve the life of the LED.
The basic constant current source circuit is mainly composed of an input stage and an output stage. The input stage provides a reference current, and the output stage outputs the required constant current.
①The basic principles of forming a constant current source circuit:
The constant current source circuit is the basis to be able to provide a stable current to ensure the stable operation of other circuits. That is, the constant current source circuit is required to output a constant current, so the device as the output stage should have the volt-ampere characteristic of the saturated output current. This can be achieved by using BJTs or MOSFETs operating in a saturated output current state. In order to ensure that the current of the output transistor is stable, two conditions must be met: a) its input voltage must be stable-the input stage needs to be a constant voltage source; b) the output resistance of the output transistor should be as large as possible (preferably infinite)- The output stage needs to be a constant current source.
②Requirements for input-level devices:
Because the input stage needs to be a constant voltage source, a device with voltage saturation volt-ampere characteristics can be used as the input stage. General pn junction diodes have this characteristic-exponentially rising volt-ampere characteristics; in addition, diodes formed by short-circuiting the source-drain of an enhanced MOSFET also have similar volt-ampere characteristics-parabolic rise The volt-ampere characteristics.
