led constant current drive power
Posted by sere on March 13th, 2020
LED constant current drive power is a voltage converter that converts the power supply into a specific voltage and current to drive LEDs. In general, the input of LED drive power includes high-voltage power frequency AC (that is, mains power), low-voltage DC, and high-voltage DC. , Low-voltage high-frequency AC (such as the output of electronic transformers) and so on. The output of LED driving power is mostly a constant current source that can change the voltage with the change of the LED forward voltage drop.
Reasons for using constant current
1) LED is a non-linear device, small changes in forward voltage will cause great changes in forward current!
For example: 20mA LED at 3.3V, powered by three dry batteries, the new battery voltage can reach 4.5V, the current exceeds 100mA, which is 5 times larger, and it is easy to burn.
2) For a 1W high-power LED, if the forward voltage changes by 10% (from 3.4V to 3.1V), it will cause the forward current to decrease from 350mA to 100mA, a change of 3.5 times.
3) LED light output is proportional to forward current
Figure 1: If the forward current is reduced from 350mA to 100mA, its light output will be reduced by as much as 70%.
4) LED is a semiconductor diode, its volt-ampere characteristics change with temperature (-2mV / oC)
As shown in figure two: if a constant voltage power supply is used: the temperature increases to 85 degrees, the forward current increases from 20 mA to 35-37 mA, and the brightness does not increase. When the temperature decreases to -40 degrees, the forward current decreases from 20 mA to 8-10 mA. Since the brightness decreases, it cannot be powered by a constant voltage source, but must be powered by a constant current source.
1.DC input-DC output (DC / DC)
1.1 According to the input-output voltage relationship, it can be divided into the following types:
a.Boost constant current drive
b. Step-down constant current drive
c. Buck-boost constant current drive
e. Double inductive
2. AC input DC output AC / DC
2.1 Classification of AC / DC constant current source
a. Non-isolated type (in compliance with safety requirements in specific cases)
b.Isolated (in compliance with safety requirements)
According to the power consumption rules of the power grid and the characteristics of the LED constant current drive power supply, the following points must be considered when selecting and designing:
1. High reliability Especially like the driving power of LED street lights, installed at high altitude, the maintenance is not convenient, and the maintenance cost is also large.
2. High-efficiency LEDs are energy-saving products, and the efficiency of the drive power must be high. It is especially important for the structure in which the power supply is installed in the luminaire. Because the luminous efficiency of LEDs decreases as the temperature of the LED increases, the heat dissipation of the LED is very important. The efficiency of the power supply is high, its power consumption is small, the heat generation in the lamp is small, and the temperature rise of the lamp is reduced. It is beneficial to delay the light decay of LED.
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 small individual consumer is lower, it has little impact on the power grid, but at night everyone lights up, and similar loads are too concentrated, which will cause serious pollution to the power grid. For 30W ~ 40W LED drive power, it is said that in the near future, there may be certain index requirements for the power factor.
4． There are two driving methods: one is a constant voltage source for multiple constant current sources, and each constant current source supplies power to each LED individually. In this way, the combination is flexible, and LED failures along the way will not affect the work of other LEDs, but the cost will be slightly higher. The other is direct constant current power supply, LEDs run in series or parallel. Its advantage is a little lower cost, but poor flexibility, but also to solve the problem of one LED failure, does not affect the operation of other LEDs. These two forms coexist for a period of time. Multi-channel constant current output power supply mode will be better in terms of cost and performance. Maybe the mainstream direction in the future.
5． Surge protection LED's ability to resist surge is relatively poor, especially its ability to resist reverse voltage. It is also important to strengthen this protection. Some LED lights are installed outdoors, such as LED street lights. Due to the induction of the grid load and the induction of lightning strikes, various surges will be invaded from the grid system, and some surges will cause damage to the LEDs. Therefore, the LED driving power supply must have the ability to suppress the intrusion of the surge and protect the LED from damage.
6. Protection function In addition to the conventional protection function, it is best to add negative feedback of LED temperature to the constant current output to prevent the LED temperature from becoming too high.
7. In terms of protection, the lamp should be installed outside, the power supply structure should be waterproof and moisture-proof, and the shell should be resistant to sunlight.
8. The life of the driving power supply must match the life of the LED.
9. To meet the requirements of safety regulations and electromagnetic compatibility.
With the increasing application of LED, the performance of LED driving power will be more and more suitable for LED requirements.
According to the driving method, it can be divided into two categories:
(1) Constant current:
a. The current output by the constant current drive circuit is constant, but the output DC voltage changes within a certain range with the size of the load resistance. The smaller the load resistance, the lower the output voltage. The higher the voltage;
b. The constant current circuit is not afraid of short circuit of the load, but it is strictly forbidden that the load is completely open.
c. The constant current drive circuit is ideal for driving LEDs, but the price is relatively high.
Should pay attention to the maximum withstand current and voltage value, which limits the number of LEDs used;
(2) Regulated type:
a. After the parameters of the voltage stabilization circuit are determined, the output voltage is fixed, but the output current changes as the load increases or decreases;
b. The voltage stabilization circuit is not afraid of the open circuit of the load, but it is strictly forbidden to completely short the load.
c. Drive the LEDs with a voltage-stabilized driving circuit. Each string needs to be added with a suitable resistor to make each string of LEDs display the average brightness;
d. The brightness will be affected by the voltage change from the rectification.
Classification by circuit structure
(1) Resistor and capacitor step-down method: When the capacitor is stepped down, due to the charge and discharge effect during flashing, the instantaneous current through the LED is extremely large, which easily damages the chip. Susceptible to power grid voltage fluctuations, low power efficiency and low reliability.
(2) Resistance step-down method: The resistance step-down method is greatly affected by the change of 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 this power supply method has low power efficiency , And the reliability of the system is low.
(3) The conventional transformer step-down method: the power supply is small in size, heavy in weight, and the power efficiency is very low, generally only 45% to 60%, so it is rarely used and the reliability is not high.
(4) Step-down method of electronic transformer: The power supply efficiency is low, and 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, and the power supply efficiency is relatively high. Generally, it can achieve 70% to 80%, and it is also widely used. Because the oscillation frequency of this control method is discontinuous, the switching frequency is not easy to control, the load voltage ripple coefficient is relatively large, and the adaptability to abnormal loads is poor.
(6) PWM control mode switching power supply: It is mainly composed of four parts, the input rectification filtering part, the output rectification filtering part, the PWM voltage stabilization control part, and the switching energy conversion part. The basic working principle of PWM switching regulator is that under the condition of input voltage, internal parameters and external load changes, the control circuit performs closed-loop feedback through the difference between the controlled signal and the reference signal, and adjusts 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%, and the output voltage and current are stable. Generally this kind of circuit has perfect protection measures and belongs to high reliability power supply.
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About the Authorsere
Joined: June 6th, 2019
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