How does a constant current LED power supply work?
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How does a constant current LED power supply work?

Posted Date: 2024-02-03

Yesterday, I received a picture from a friend. I wanted to know how the constant current LED power supply works. With permission, I posted the circuit diagram as follows:

According to the message, this is a 14W constant current LED driver circuit. When the output voltage is less than 20V, it begins to enter the constant current state, and the current stabilizes between 0.73~0.74, achieving the purpose of constant current; we can see from the picture above that the input voltage Ui=195~265V wide voltage. According to the message, we can get the output voltage Uo=20V; the output current Io=0.7A.

Then I searched the Internet for the specific parameters and recommended circuits of the power chip, and learned that the power efficiency n=86%; the no-load power consumption Po

Line working principle: The line mainly achieves constant current characteristics by detecting the voltage drop formed by the current on R7. In the figure, the reference terminal of LM431 and the cathode K are short-circuited to each other, then the cathode is equivalent to outputting a fixed 2.5V reference voltage. After this voltage is divided by R5, R6 and R8, a reference voltage of about 0.07V is obtained. This voltage is added to the inverting input terminal of the operational amplifier; when the current Io through R7 reaches 0.7A, we can get the voltage Ur7 on R7 = 0.07V according to Ohm's law; when the current is greater than 0.7A, the product enters constant current area, at this time Ur7》0.07V; then the non-inverting input terminal of the amplifier LM321 is larger than the inverting input terminal, the output voltage increases, VD4 is turned on because it is forward biased, and the base of VT is driven by VD4; the chip's The EN/UV terminal voltage is pulled low. When the EN/UV pin current is greater than 115uA, the MOS inside the chip will be disabled, and the current feedback loop can realize constant current control.

It is worth noting that VD1 should use fast recovery diodes instead of ultra-fast recovery diodes when selecting VD1. The recovery time of fast recovery diodes is longer than that of faster recovery diodes to improve the efficiency of the power supply.

Another friend left a message saying that the product sometimes has unstable voltage problems, and almost all the parts could not be repaired. It turned out that there was a problem during PCB wiring, and the hardware engineer routed the power ground wire and signal ground wire. If the ground wire is too close, a large current will flow through the power ground wire, which will be introduced into the signal terminal and affect the output terminal of the chip, thus affecting the voltage stabilization performance of the product.

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