1. The allowable range of current transmission ratio (CTR) of optical coupler is 50% ~ 200%. This is because when the CTR< 50%, the leds in the optical coupling require a larger working current (IF >5.0 mA) to control the proportion of the single-chip switching power supply IC, which increases the power consumption of the optical coupling. If CTR ＞ 200 %, in the startup circuit or when the load is mutated, it is possible to accidentally trigger the single-chip switch, affecting the normal output. If the amplifier circuit is used to drive the optoelectronic coupler, it must be carefully designed to ensure that it can compensate the temperature instability and drift of the coupler.
2. Linear optical coupler
is recommended, which is characterized by the ability of CTR to make linear adjustment in a certain range. When the optocoupler used above works in a linear way, the voltage is controlled at the input end of the optocoupler. The output terminal will produce a proportional voltage for further control of the next circuit. It is a closed loop regulation and control by MCU, and plays a stabilizing role in power output.
In order to completely block the interference signal into the system, not only the signal path must be isolated, but also the input or output circuit must be isolated from the power supply of the system. These circuits use separate isolated power supply respectively. In the case of common mode interference, isolation technology is adopted, namely, the use of a transformer or linear photoelectric coupler, to disconnect the input field and the input field, so that the interference is inhibited without the loop. In the switching power supply, the photoelectric coupler is a very important peripheral device. The designer can make full use of its input and output isolation to design anti-interference for the single chip microcomputer, and control the closed-loop stabilization of the converter.