OFweek share: LED drive design knowledge

The LED driving power supply is a voltage converter that converts the power supply into a specific voltage and current to drive the LED to emit light. Generally, the input of the LED driving power source includes high-voltage power frequency AC (ie, mains), low-voltage DC, high-voltage DC, and low voltage. Frequency communication (such as the output of an electronic transformer). The output of the LED driver is mostly a constant current source that changes the voltage as the LED's forward voltage drop changes.

According to the power consumption rules of the power grid and the characteristics of the LED drive power supply, the following points should be considered when selecting and designing the LED drive power supply:

1. High reliability is especially like the driving power of LED street lamps. It is installed at high altitude, it is not convenient for maintenance, and the maintenance cost is also large.

2. High-efficiency LEDs are energy-saving products, and the efficiency of driving power supplies is high. It is especially important to install the LED driver power supply in the luminaire for the power supply. Since the luminous efficiency of the LED decreases as the temperature of the LED increases, the heat dissipation of the LED is very important. The power supply has high efficiency, its power consumption is small, and the heat generated in the lamp is small, which reduces the temperature rise of the lamp. It is beneficial to delay the light decay of LEDs.

3. High power factor The power factor is the load requirement of the grid. Generally, there are no mandatory indicators for electrical appliances below 70 watts. Although the power factor of a single power consumer with low power has little effect on the power grid, at night, everyone lights up, and the similar load is too concentrated, which will cause serious pollution to the power grid. For 30 watts to 40 watts of LED driver power, it is said that in the near future, there may be certain indicators for power factors.

4. There are two types of driving methods: one is a constant voltage source for multiple constant current sources, and each constant current source supplies power to each LED separately. In this way, the combination is flexible, and all the LED faults do not affect the work of other LEDs, but the cost will be slightly higher. The other is direct constant current supply, with LEDs running in series or in parallel. Its advantage is that the cost is lower, but the flexibility is poor, and it is necessary to solve a certain LED failure without affecting the operation of other LEDs. These two forms coexist for a while. Multi-channel constant current output power supply mode will be better in terms of cost and performance. Perhaps it is the mainstream direction in the future.

5. The ability of surge protection LED to resist surge is relatively poor, especially the ability to resist reverse voltage. It is also important to strengthen protection in this area. Some LED lights are installed outdoors, such as LED street lights. Due to the start of the grid load and the induction of lightning strikes, various surges will be invaded from the grid system, and some surges will cause LED damage. Therefore, the LED driver power supply must have the ability to suppress the intrusion of surges and protect the LEDs from damage.

6. Protection function power supply In addition to the conventional protection function, it is better to increase the LED temperature negative feedback in the constant current output to prevent the LED temperature from being too high.

7. For the protection aspect of the lamp, the power supply structure should be waterproof and moisture-proof, and the outer casing should be light-resistant.

8. The life of the drive power should be matched to the life of the LED.

9. To comply with safety and electromagnetic compatibility requirements.

With the increasing use of LEDs, the performance of LED drive power will be more and more suitable for LED requirements.

1, according to the driving method can be divided into two categories:

(1) Constant current type:

a, the current output of the constant current drive circuit is constant, and the output DC voltage varies within a certain range according to the magnitude of the load resistance. The load resistance is small, the output voltage is low, and the load resistance is larger. The higher the voltage;

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

c, constant current drive circuit to drive LED is more ideal, but relatively high price.

d, should pay attention to the maximum withstand current and voltage value used, which limits the number of LEDs used;

(2) Voltage regulation:

a. When the parameters in the voltage stabilizing circuit are determined, the output voltage is fixed, and the output current changes as the load increases or decreases;

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

c, driving the LED with a voltage-stabilized driving circuit, each string needs to add a suitable resistor to make each string of LED display brightness average;

d. The brightness is affected by the rectified voltage change.

2, according to the circuit structure

(1) Resistor and capacitor step-down mode: By means of capacitor step-down, when flashing, due to the effect of charge and discharge, the instantaneous current through the LED is extremely large, which easily damages the chip. It is susceptible to grid voltage fluctuations, low power efficiency and low reliability.

(2) Resistor step-down mode: through the resistor step-down, the interference caused by the grid voltage change is large, it is not easy to make a regulated power supply, and the step-down resistor consumes a large part of the energy, so this power supply mode has low power efficiency. And the reliability of the system is also low.

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

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

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

(6) PWM control mode switching power supply: mainly composed of four parts, input rectification and filtering part, output rectification and filtering part, PWM voltage regulation control part, switching energy conversion part. The basic working principle of PWM switching regulator is that in the case of input voltage, internal parameters and external load changes, the control circuit performs closed-loop feedback by the difference between the control signal and the reference signal to adjust the pulse width of the main circuit switching device. The output voltage or current of the switching power supply is stabilized (ie, the corresponding regulated power supply or constant current power supply). The power supply efficiency is extremely high, generally 80% to 90%, and the output voltage and current are stable. Generally, such circuits have perfect protection measures and are highly reliable power supplies.

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