Principles for selecting linear optocouplers

　　When designing an optocoupler feedback switch power supply, it is necessary to correctly select the model and parameters of the linear optocoupler. The selection principles are as follows:

　　The allowable range for the current transfer ratio (CTR) of an optocoupler is 50% to 200%. This is because when CTR<50%, the LED in the optocoupler requires a larger operating current (IF>5.0mA) to control the duty cycle of the single-chip switch power IC normally, which increases the power consumption of the optocoupler. If CTR is greater than 200%, it is possible to trigger the single-chip switch power supply incorrectly during the startup circuit or when there is a sudden change in load, affecting normal output.

　　② It is recommended to use a linear optocoupler, which is characterized by the ability to linearly adjust the CTR value within a certain range.

　　The 4N XXX series (such as 4N25, 4N26, 4N35) optocouplers produced by Isocom in the UK and Motorola in the US are currently widely used in China. Given that this type of optocoupler exhibits switching characteristics and poor linearity, it is suitable for transmitting digital signals (high and low levels) and therefore not recommended for use in switching power supplies.

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