Common connection methods and working principles of optocoupler isolation

　　Commonly used optocoupler models for feedback include TLP521, PC817, etc. Taking TLP521 as an example, this article introduces the characteristics of this type of optocoupler.

　　The primary side of TLP521 is equivalent to a light-emitting diode. The larger the primary current If, the stronger the light intensity, and the larger the current Ic of the secondary transistor. The ratio of the current Ic of the secondary transistor to the current If of the primary diode is called the current amplification factor of the optocoupler, which varies with temperature and is greatly affected by temperature. The optocoupler used for feedback utilizes the principle that "changes in the primary current will cause changes in the secondary current" to achieve feedback. Therefore, in situations where the ambient temperature changes dramatically, due to the large temperature drift of the amplification factor, feedback should be avoided as much as possible through optocouplers. In addition, when using such optocouplers, attention must be paid to designing peripheral parameters to operate within a relatively wide linear band. Otherwise, the sensitivity of the circuit to operating parameters is too strong, which is not conducive to the stable operation of the circuit.

　　Usually TL431 combined with TLP521 is chosen for feedback. At this point, the working principle of TL431 is equivalent to an internal voltage error amplifier with a reference of 2.5 V, so a compensation network needs to be connected between pin 1 and pin 3.

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