Classification of LED power supply

LED power supply is a power converter that converts the power supply into a specific voltage and current to drive the LED to emit light. Under normal circumstances: the input of the LED drive power includes high-voltage power frequency AC (ie mains), low-voltage DC, high-voltage DC, low-voltage and high-voltage Frequency AC (such as the output of an electronic transformer), etc.

Classification of LED power supply

According to the driving method

(1) Constant current type

a. The output current of the constant current drive circuit is constant, but 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 output The higher the voltage;

b. The constant current circuit is not afraid of the load short circuit, but it is strictly forbidden to open the load completely.

c. It is ideal for a constant current drive circuit to drive LEDs, 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

a. When the various parameters in the voltage regulator circuit are determined, the output voltage is fixed, but the output current changes with the increase or decrease of the load;

b. The voltage regulator circuit is not afraid of load opening, but it is strictly forbidden to short-circuit the load completely.

c. The LED is driven by a voltage-stabilizing drive circuit, and each string needs to be added with suitable resistance to make each string of LEDs show an average brightness;

d. The brightness will be affected by the voltage change from rectification.

(3) Pulse drive

Many LED applications require dimming functions, such as LED backlighting or architectural 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 light emission, but letting the LED work under the condition of lower than the rated current will cause many undesirable consequences, such as chromatic aberration. An alternative to simple current adjustment is to integrate a pulse width modulation (PWM) controller 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. Most current LED chips use PWM to control LED light emission. In order to ensure that people will not feel obvious flicker, the frequency of the PWM pulse must be greater than 100HZ. The main advantage of PWM control is that the dimming current through PWM is more accurate, which minimizes the color difference when the LED emits light.

(4) AC drive

According to different applications, AC drives can also be divided into three types: step-down, step-up, and converter. The difference between an AC drive and a DC drive, in addition to the need to rectify and filter the input AC, there is also a problem of isolation and non-isolation from a safety point of view.

AC input drivers are mainly used for retrofit lamps: For ten PAR (Parabolic Aluminum Reflector, a common lamp on professional stage) lamps, standard bulbs, etc., they are under 100V, 120V or 230V AC input Operation; For MR16 lamp, it needs to work under 12V AC input. Due to some complex problems, such as the dimming ability of standard triacs or leading edge and trailing edge dimmers, and compatibility with electronic transformers (from AC line voltage to generate 12V AC when MR16 lamps work) The problem of performance (that is, flicker-free operation), therefore, compared with the DC input driver, the field involved in the AC input driver is more complicated.

AC power supply (mains drive) is applied to LED drive, generally through steps such as step-down, rectification, filtering, voltage stabilization (or current stabilization), etc., to convert AC power to DC power, and then provide suitable LEDs through a suitable drive circuit The working current must have high conversion efficiency, small size and low cost, and at the same time solve the problem of safety isolation. Taking into account the impact on the power grid, electromagnetic interference and power factor issues must also be resolved. For low- and medium-power LEDs, the best circuit structure is an isolated single-ended flyback converter circuit; for high-power applications, a bridge converter circuit should be used.

According to circuit structure

(1) Resistor and capacitor step-down method: the capacitor step-down, when used in flashing, due to the effect of charging and discharging, the instantaneous current through the LED is extremely large, which is easy to damage the chip. It is susceptible to the influence of power grid voltage fluctuations, and the power supply has low efficiency and low reliability.

(2) Resistive step-down method: The resistance step-down is greatly affected by the changes in the grid voltage, and it is not easy to make a regulated power supply. The step-down resistor consumes a large part of the energy, so the power supply efficiency of this power supply method is very low. , And the reliability of the system is low.

(3) Conventional transformer step-down method: the power supply is small in size, heavy in weight and low in power efficiency, generally only 45% to 60%, so it is generally rarely used and has low reliability.

(4) Electronic transformer step-down method: the power supply efficiency is low, the voltage range is not wide, generally 180~240V, and the ripple interference is large.

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

(6) PWM control mode switching power supply: mainly composed of four parts, input rectification filtering part, output rectification filtering part, PWM voltage stabilization control part, and switching energy conversion part. The basic working principle of PWM switching regulator is that when the input voltage, internal parameters and external load change, the control circuit performs closed-loop feedback through the difference between the controlled signal and the reference signal to adjust the pulse width of the main circuit switching device. 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 extremely high, generally 80% to 90% can be achieved, and the output voltage and current are stable. Generally, this kind of circuit has perfect protection measures, which is a high-reliability power supply.

LED power supply can be divided into the external power supply and built-in power supply according to the installation position.

(1) External power supply

As the name suggests, the external power supply is to install the power supply outside. Generally, the voltage is relatively high, which is a safety hazard to people, and an external power supply is required. The difference with the built-in power supply is that the power supply has a shell, and street lights are common ones.

(2) Built-in power supply

The power supply is installed inside. Generally, the voltage is relatively low, from 12v to 24v, which poses no safety hazards to people. This common one has bulb lights.