Selection Of Feedback Optocoupler For Switching Power Supply

Optocouplers are very commonly used in switch mode power supplies. They serve as a signal transmission between the primary and secondary sides, and can effectively isolate the components between the primary and secondary sides. Of course, there are many components that can transmit signals and have isolation effects. However, optocouplers are a cost-effective and simple peripheral circuit feature because they need to isolate the primary and secondary sides. Therefore, the voltage resistance of optocouplers in switch mode power supplies generally needs to be 5000V (effective value).
The optocoupler is first divided into linear optocoupler and nonlinear optocoupler. In our switching power supply, the optocoupler used for feedback is linear optocoupler, such as PC817. For PC817, there are A, B, C, and D gears available
These gears are actually classified based on CTR (Transmission Ratio).
CTR is a parameter that describes the control characteristics of the optocoupler, which is the percentage of the Ic current of the secondary side to the input IF current of the primary side.