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PC817 photocoupler, a component often used in the feedback loop of switching power supply

baijiahao | 21/05/2021

The photocoupler is an isolated photosensitive device, which is composed of a light-emitting diode and a phototransistor. Because the input is converted into an optical signal by an electric signal, it is excited by electric excitation, and finally the output is converted to obtain an electric signal, so that the input and output Mutual isolation is fully demonstrated in terms of insulation capability and anti-interference. It is widely used in switching power supplies, level conversion, relays, and input and output situations where isolation is required, especially in switching power supplies of about 50W. Many power products Combine the optocoupler PC817 with a precision voltage regulator source to form an output feedback loop.

The PC817 optocoupler has four pins, two inputs (cathode and anode) and two outputs (emitter and collector). When the input is given an electrical signal, it is amplified by the light and the output is turned on to realize the input and output. Complete isolation.

We often see the following two packages, one is DIP-4, the other is SOP-4

The figure below is a schematic diagram of a part of the feedback loop of a flyback switching power supply. In the flyback power supply, many engineers will use optocoupler PC817 and TL431 precision voltage regulator sources as the main components to form a voltage feedback loop. The input of optocoupler PC817 is output by switching power supply. The voltage is the input terminal. When the output voltage fluctuates, the divided sampling voltage formed by R5 and R3 is compared with the 2.5V reference voltage of TL431 to obtain an error voltage, which is used as the PC817 optocoupler input voltage, R33 It is an optocoupler current-limiting resistor, and an input current is obtained under a certain error voltage. Generally, the minimum input current of PC817 is greater than 3mA. In order to ensure reliability, it is better to design greater than 3mA. The feedback current is fed back to the switch through photoelectric coupling. The control terminal of the power IC controls the switch to output a PWM signal and finally maintains the output voltage at a constant value.

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