LED drive power supply is a power converter that converts the power supply into a specific voltage and current to drive LED lighting. Generally, the input of LED drive power supply includes high-voltage power frequency AC (i.e. mains power), low-voltage DC, high-voltage DC, low-voltage high-frequency AC (e.g. output of electronic transformer), etc.

Classification of LED driving power supply

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 within a certain range with the size of the load resistance. The smaller the load resistance, the lower the output voltage. The larger the load resistance, the higher the output voltage;

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 for driving LED, but the price is relatively high.

D. Pay attention to the maximum withstand current and voltage value used, which limits the number of LEDs used;

(2) Regulated type:

A. After the parameters in the voltage stabilizing circuit are determined, the output voltage is fixed, while the output current changes with the load;

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

C. The LED is driven by the regulated driving circuit. Each string of LED needs to be added with appropriate resistance to make the display brightness of each string of LED average;

D. Brightness will be affected by the voltage change caused by rectification.

Classification of LED driving power supply

(3) Pulse drive

Many LED applications require dimming functions, such as LED backlight or building lighting dimming. Dimming function can be realized by adjusting the brightness and contrast of LED. Simply reducing the current of the device may be able to adjust the LED light, but letting the LED work below the rated current will cause many adverse consequences, such as color difference. The method to replace simple current regulation is to integrate pulse width modulation (PWM) controller in 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 on 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 do 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 chromatic aberration of LED is minimized.

(4) AC drive

According to different applications, AC drives can also be divided into three types: buck type, boost type and converter. The difference between AC driver and DC driver is that in addition to rectifying and filtering the input AC converter, there is also a problem of isolation and non-isolation from the perspective of safety.

AC input driver is mainly used for modified lamps: for 10 PAR (Parabolic Aluminum Reflector, a common lamp on professional stage) lamps, standard bulbs, etc., they operate under 100V, 120V or 230V AC input; For MR16 lamp, it needs to work under 12V AC input. Due to some complex problems, such as the dimming ability of the standard three-terminal bidirectional thyristor switch or the front and rear edge dimmer, as well as the compatibility with the electronic transformer (generating 12V AC when the MR16 lamp works from the AC line voltage) (i.e. no flicker operation), the AC input driver involves more complex fields than the DC input driver.

AC power supply (mains drive) is applied to LED drive. Generally, it needs to go through the steps of voltage reduction, rectification, filtering, voltage stabilization (or current stabilization) to convert AC power supply to DC power supply, and then provide appropriate working current for LED through suitable drive circuit. It also needs to have high conversion efficiency, small volume and low cost, and solve the problem of safety isolation at the same time. In consideration of the impact on the power grid, electromagnetic interference and power factor should also 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.

By circuit structure

(1) Resistance and capacitance voltage reduction method: voltage reduction through capacitance. When flashing, due to the effect of charging and discharging, the instantaneous current through LED is very large, which is easy to damage the chip. It is easy to be affected by the voltage fluctuation of the power grid, and the power efficiency and reliability are low.

(2) Resistance voltage reduction method: voltage reduction through resistance is greatly disturbed by the change of grid voltage, and it is not easy to make a stabilized power supply. The voltage reduction resistance consumes a large part of energy, so the power supply efficiency of this power supply method is very low, and the reliability of the system is also low.

(3) Conventional transformer step-down mode: the power supply is small in size, heavy in weight, low in power efficiency, generally only 45%~60%, so it is rarely used, and its reliability is not high.

(4) Step-down mode of electronic transformer: the power efficiency is low, the voltage range is not wide, generally 180~240V, and the ripple interference is large.

(5) RCC step-down mode switching power supply: the range of voltage stabilization is relatively wide, the power efficiency is relatively high, and it can generally achieve 70% - 80%, and the application is also wide. Because the oscillation frequency of this control mode is discontinuous, the switching frequency is not easy to control, the load voltage ripple coefficient is also relatively large, and the adaptability of abnormal load is poor.

(6) PWM control mode switching power supply: It is mainly composed of four parts: input rectification and filtering part, output rectification and filtering part, PWM voltage stabilization control part and switching energy conversion part. The basic working principle of PWM switching voltage stabilization is that in the case of changes in input voltage, internal parameters and external load, the control circuit carries out closed-loop feedback through the difference between the controlled signal and the reference signal, and adjusts the pulse width of the switching device of the main circuit to make the output voltage or current of the switching power supply stable (that is, the corresponding regulated power supply or constant current power supply). The power supply efficiency is very high, generally 80%~90%, and the output voltage and current are stable. Generally, this circuit has perfect protection measures and belongs to high reliability power supply.

Classification of power supply installation location

The drive power supply can be divided into external power supply and internal power supply according to the installation position.

(1) External power supply

As the name implies, the external power supply is installed outside. Generally, if the voltage is relatively high and there is a danger to human safety, an external power supply is required. The difference from the built-in power supply is that the power supply is equipped with a shell, and the common one is street lamp.

(2) Built-in power supply

The power supply is installed in the lamp. Generally, the voltage is relatively low, from 12v to 24v, and there is no safety hazard to people. This common bulb lamp.