Summary of classic circuits for secondary constant current of LED power supply

As the LED market matures, LED lighting has become the main pillar industry of the lighting market, and more and more people are paying attention to LED driving technology. Many practitioners in the LED lighting industry say that LED power supply is a special power supply, which is very different from ordinary power supplies, so many LED lighting manufacturers need professional LED talents. In fact, the characteristics of LED power supply are that it requires constant current and voltage limiting, and it has to work for a long time, so it needs relatively high efficiency support, and some power supplies also have restrictions on structural size and height.

This article summarizes some common methods of secondary constant current in LED lighting power supply , hoping to help novices improve. It is no exaggeration to say that the LED driver power supply will directly determine the reliability and life of LED lights. As power supply engineers, we know that the characteristics of LEDs require constant current drive to ensure uniform brightness and long-term reliable light emission.

First, let’s talk about several popular constant current modes of TL431.

Single TL431 constant current circuit

As shown above, it is a schematic diagram of using a single TL431 constant current. The principle of this circuit is very simple, mainly using the 2.495V reference of 431 to make constant current, and also limiting the voltage drop on the LED, but the advantages and disadvantages are equally obvious.

advantage:

The circuit is simple, with few components and low cost. Because the reference voltage of TL431 is highly accurate, R12 and T13 only need to use high-precision resistors, and the constant current accuracy is relatively high. Disadvantages:

Since TL431 is a 2.5V reference, the loss of the constant current sampling circuit is very large, and it is not suitable for power supply with excessive output current. The fatal flaw of this circuit is that it cannot be unloaded, so it is not suitable for external LED power supply, so we have improved some defects of the circuit below.

Single TL431 constant current improved circuit

As shown above, this is an improved schematic diagram using a single TL431 constant current

Principle: This circuit also uses the 2.495V reference of TL431 to make constant current. The difference from the above circuit is that the voltage of the current sampling circuit is reduced. As long as the values ​​of R12, R13, and R14 are designed in total, the voltage drop on the LED can be limited.

advantage:

电路简单，元器件少，成本低，跟上面电路相比，显著降低了取样电阻的功耗，恒流精度很高，克服了上面的电路不能空载的致命缺陷，当有个别LED击穿时，可以自动调整输出电压

shortcoming:

When the output is unloaded, the output voltage will rise. The rise is determined by the ratio of the current sampling circuit resistance to R12 and R13.

In fact, the real disadvantage of this circuit is that when the voltage drop consistency of a single LED is not high, the constant current point will also change accordingly.

For example, the most common 12-string LED lights have a minimum voltage drop of about 35.5V and a maximum voltage drop of about 37.4V (based on my personal experience, of course the situation may vary from manufacturer to manufacturer), so the constant current accuracy may differ by 5%-8%.

Two TL431 constant current circuits

从图中我们可以看到，左边ZENER可透过Photo限制达恆压效果,但不是保护Shut down而是一直卡着右边ZENER。很难灌350mA到Current sensor。这个电路还有个最大特点是：在某个范围内可以精确的恒压恒流

3 TL431 constant current circuits

In fact, this circuit is just adding a constant voltage circuit to the original circuit.

Transistor constant current solution

First a picture

The principle of this diagram is to control the current in the LED by changing the IB current of the transistor, which also has the disadvantage of large loss .

The advantages of this circuit are that the circuit is relatively simple, the constant current accuracy is extremely high, it is not affected by temperature, and the cost is low. It is a classic circuit used by most manufacturers. You will understand its beauty if you think of it as a reverse proportional operational amplifier.

In fact, there are many changes in the secondary constant current of LED power supply. The circuits we list here may not be complete. We just selected some classic circuits for analysis, so there may be some omissions. You are welcome to supplement them in time.