Analysis of LED power supply constant current circuit

We all know that there is nothing special about LED power supplies. Their only feature is that they require constant current and voltage limiting. Moreover, they work under full load for a long time, so the efficiency requirements are relatively high. Some power supplies have height requirements due to structural size limitations.

Below 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.

Statement: Not all circuits are original. They are posted here to facilitate discussion. If there are any infringement issues, please inform me in time so that they can be deleted in time.

It is no exaggeration to say that the LED driving power supply will directly determine the reliability and life of the LED lamp. As power engineers, we know that the characteristics of LED require constant current driving to ensure its uniform brightness and long-term reliable light emission.

Let's first talk about several popular constant current modes of 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.

advantage:

The circuit is simple, with few components and 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.

shortcoming:

Since TL431 is a 2.5V reference, the loss of the constant current sampling circuit is extremely large and it is not suitable for use as a power supply 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.

You can discuss how to improve the defects first, and I will continue to post the improved circuit tomorrow.

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.

advantage:

The circuit is simple, with few components and 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 no-loaded. When individual LEDs break down, the output voltage can be automatically adjusted.

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.

3. Two TL431 constant current circuits