LED drive power classification(1)

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LED drive power classification(1)

By drive mode

(1) constant current type

A. the output current of the constant current drive circuit is constant, while the output DC voltage varies in a certain range with the size of the load resistance value. If the load resistance value is small, the output voltage will be low. The larger the load resistance value is, the higher the output voltage will be;

B. the constant current circuit is not afraid of load short circuit, but it is strictly prohibited to open the load completely.

C. the constant current drive circuit is ideal to drive LED, but the price is relatively high.

D. pay attention to the maximum withstand current and voltage value used, which limits the use of LED;

(2) stabilized voltage type:

A. when the parameters of the voltage stabilizing circuit are determined, the output voltage is fixed, while the output current changes with the increase or decrease of the load;

B. the voltage stabilizing circuit is not afraid of load open circuit, but it is strictly prohibited to short circuit the load completely.

C. drive led by voltage stabilizing driving circuit, and add appropriate resistance for each string to make each string of LED display brightness average;

D. the brightness will be affected by the voltage change from the rectifier.

(3) pulse drive

Many LED applications require dimming, such as LED backlight or building lighting dimming. The dimming function can be realized by adjusting the brightness and contrast of the LED. Simply reducing the current of the device may be able to adjust the LED's light emission, but making the LED work under the rated current will cause many adverse consequences, such as color difference problems. Instead of simple current regulation, a pulse width modulation (PWM) controller is integrated in the LED driver. The PWM signal is not directly used to control the LED, but to control a switch, such as a MOSFET, to provide the required current to the LED. The PWM controller usually works at a fixed frequency and adjusts the pulse width to match the required duty cycle. At present, most LED chips use PWM to control LED lighting. In order to ensure that people will not feel obvious flicker, the frequency of PWM pulse must be greater than 100Hz. The main advantage of PWM control is that the dimming current of PWM is more accurate and the color difference of LED is reduced to the greatest extent.

(4) AC drive

According to different applications, AC drivers can be divided into three types: step-down type, step-up type and converter. The difference between AC driver and DC driver is that in addition to rectifying and filtering the input AC power, there are still problems of individual isolation and non isolation in terms of safety.

AC input driver is mainly used for modified lamps: for ten par (parabolic aluminum reflector, a common lamp on professional stage), standard lamps, etc., they operate under 100V, 120V or 230V AC input; for MR16 lamps, they need to work under 12V AC input. Due to some complex problems, such as the dimming ability of standard three terminal bidirectional thyristor switch or front edge rear edge dimmer, and the compatibility with electronic transformer (generating 12V AC when MR16 lamp works from AC line voltage) (i.e. flicker free operation), the field of AC input driver is more complex than that of DC input driver.

AC power supply (city power drive) is applied to LED drive, generally through step-down, rectification, filtering, voltage stabilization (or current stabilization) and other links, so that the AC power supply is converted into DC power supply, and then suitable driving circuit is used to provide suitable working current for LED, with high conversion efficiency, small volume and low cost, while solving the problem of safety isolation. Considering the influence on the power grid, the problems of electromagnetic interference and power factor should be solved. For small and medium power LED, the best circuit structure is isolated single ended flyback converter; for high-power applications, bridge converter should be used.