LED power supply secondary constant current circuit summary

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As the LED market matures, LED lighting has become the main pillar industry in the lighting market, and more and more people are paying attention to LED driving technology. Many practitioners engaged in the LED lighting industry say that LED power supply is a special kind of power supply, which is very different from ordinary power supply, so many LED lighting manufacturers need professional LED talents. In fact, the characteristics of the LED power supply is that it requires constant current limiting voltage, and it has to work for a long time, so it requires relatively high efficiency support, and some power supplies have limitations on the size and height of the structure.
This article summarizes some common methods of secondary constant current in LED lighting power supply, hoping to help newcomers improve. It is no exaggeration to say that the LED driver power supply will directly determine the reliability and life of the LED lamp. As a power supply engineer, we know that the characteristics of LEDs require constant current drive to ensure uniform brightness and long-term reliable illumination.
First, let's talk about several constant current methods of the popular TL431.
Single TL431 constant current circuit

As shown above, it is a schematic diagram using a single TL431 constant current. The principle of this circuit is very simple, mainly using the 431's 2.495V reference for constant current, and also limits the voltage drop across the LED, but the advantages and disadvantages are equally obvious.
advantage:
The circuit is simple, the components are few, and the cost is low. Because the reference voltage precision of the TL431 is high, the R12 and T13 are only high-precision resistors, and the constant current precision is relatively high.
Disadvantages:
Since the TL431 is a 2.5V reference, the loss of the constant current sampling circuit is extremely large, and it is not suitable for a power supply with an excessive output current. The fatal flaw of this circuit is that it can not be idling, so it is not suitable for external LED power supply, so we have improved some defects of the line below.
Single TL431 constant current improved circuit

As shown in the above figure, it is an improved schematic diagram using a single TL431 constant current: this circuit also uses the TL431's 2.495V reference for constant current. The difference from the above circuit is that the voltage of the current sampling circuit is reduced, as long as the total Designing the values ​​of R12, R13, and R14 can limit the pressure drop across the LED:
The circuit is simple, the components are few, and the cost is low. Compared with the above circuit, the power consumption of the sampling resistor is significantly reduced, the constant current precision is high, and the fatal defect that the above circuit cannot be idling is overcome, when individual LED breakdown occurs. The output voltage can be adjusted automatically.
Disadvantages:
When the output is unloaded, the output voltage will rise, and the rising range 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 change accordingly.
For example, the most common 12-string LED lights, the minimum voltage drop is about 35.5V, the highest return to about 37.4V (personal experience, of course, different manufacturers will be different), then the constant current accuracy will be 5% difference - 8%.
Two TL431 constant current circuits

As can be seen from the figure, the left ZENER can limit the constant pressure effect through Photo, but it does not protect Shut down but keeps the right side of ZENER. It is difficult to fill 350mA to Current sensor.
The biggest feature of this circuit is that it can accurately maintain constant current in a certain range.
3 TL431 constant current circuits

In fact, this circuit is based on the original circuit to add a constant voltage circuit.
Transistor constant current scheme (first picture)

The principle of this figure is to control the current in the LED by changing the IB current of the triode, and there is also the disadvantage of large loss.
LM358 constant current circuit

The advantage of this circuit is that the circuit is relatively simple, the constant current precision is extremely high, it is not affected by temperature, and the cost is low. It is a classic circuit used by most manufacturers at present, and you can see it as a reverse proportional operational amplifier. It is.
In fact, the secondary constant current of LED power supply changes a lot. Here, the circuit we listed for you may not be complete, just select some more classic circuits for analysis, so there may be some flaws, welcome everyone in time. Add it.