Summary of LED power supply secondary constant current solution

As LED lighting is getting more and more popular, LED driver power supply, as the life support of LED, is also attracting more and more attention. I have always heard many people say that LED power supply is a special power supply, which is very different from ordinary power supply, so you need to find a professional LED power supply engineer to make LED power supply. This statement casts a mysterious veil over the LED power supply, but as professionals in power supply, we all know that LED power supply is actually nothing special. Its characteristic is that it needs constant current and voltage limitation, and it works under full load for a long time, so the efficiency requirement is relatively high; some power supplies have height requirements due to structural size limitations. Next, I will try to summarize some common methods of secondary constant current for current small and medium power LED lighting power supplies; it may not be very comprehensive or in-depth, but it can finally help some engineers who are new to the industry.

It is no exaggeration to say that the LED driver power supply will directly determine the reliability and life of the LED lamp. As power engineers, we know that the characteristics of LEDs require constant current drive to ensure uniform brightness and long-term reliable light emission. Let's talk about several constant current methods of the more popular TL431.

1. Single TL431 constant current circuit

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

Principle: This circuit is very simple, using the 2.495V reference of 431 for constant current, and also limits the voltage drop on the LED, but the advantages and disadvantages are equally obvious.

Advantages: simple circuit, few components, low cost. Because the reference voltage accuracy of TL431 is high, R12 and T13 only need 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 extremely large, and it is not suitable for power supplies with excessive output current.

The fatal flaw of this circuit is that it cannot be no-loaded, so it is not suitable for external LED power supply.

I believe everyone knows the calculation of the constant current point of this circuit: ID=2.495/(R12//R13)

The power of the sampling resistors R12 and R13 is PR = 2.495 * 2.495 / R13). For a small power supply, this power loss is considerable, so it is not recommended to use this circuit for products with a current greater than 200mA.

2. 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.

Advantages: simple circuit, few components, low cost. Compared with the above circuit, it significantly reduces the power consumption of the sampling resistor, has high constant current accuracy, and overcomes the fatal defect of the above circuit that it cannot be unloaded. When individual LEDs break down, the output voltage can be automatically adjusted.

Disadvantages: When the output is unloaded, the output voltage will rise, and the increase is determined by the ratio of the current sampling circuit resistance to R12, 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 lamp has a minimum voltage drop of about 35.5V and a maximum voltage drop of about 37.4V (personal experience, of course, the situation of different manufacturers will be different), so the constant current accuracy will differ by 5%-8%.

3. Two TL431 constant current circuits

Another major feature of this circuit is that it can accurately maintain constant voltage and current within a certain range.

4. Three TL431 constant current circuits

In fact, this circuit is just a constant voltage circuit added to the circuit in Figure 3.