Principles For Selecting Optocouplers

When designing optocoupler optoelectronic isolation circuits, it is necessary to correctly select the model and parameters of the optocoupler, and the selection principles are as follows:
(1) Due to the fact that optocouplers are unidirectional signal transmission devices, while data transmission in circuits is bidirectional, the size requirements of the circuit board are certain. Combined with the actual requirements of circuit design, it is necessary to choose devices that integrate multiple optocouplers on a single chip;
(2) The allowable range for the current transfer ratio (CTR) of an optocoupler is not less than 500%. Because when CTR<500%, the LED in the optocoupler requires a larger operating current (>5.0 mA) to ensure that the signal does not experience errors during long-distance transmission, which increases the power consumption of the optocoupler;
(3) The transmission speed of optocouplers is also one of the principles that must be followed when selecting optocouplers. If the optocoupler switch speed is too slow, it cannot respond correctly to the input level, which will affect the normal operation of the circuit.
(4) Recommend using linear optocouplers. Its characteristic is that the CTR value can be linearly adjusted within a certain range. In the design, due to the fact that the input and output of the circuit are both high and low level signals, the circuit operates in a nonlinear state. In linear applications, due to the undistorted transmission of signals, suitable static operating points should be set according to the requirements of dynamic operation to ensure that the circuit operates in a linear state.
Usually, devices that integrate multiple optocouplers on a single chip have relatively slow speeds, while faster devices are mostly single channel. A large number of isolation devices require a large board area, which greatly increases the design cost. In design, due to factors such as circuit board size, transmission speed, and design cost, it is not possible to choose single channel optocoupler devices that have a significant advantage in speed