Optocoupler for Switching Power Supply Applications

　　Optocoupler for switching power supply

　　A switching power supply is a power supply that uses modern power electronics technology to control the on-off time ratio of the switch tube to maintain a stable output voltage. Switching power supplies are widely used in almost all electronic devices due to their small size, light weight and high efficiency.

　　Negative feedback is generally used in the voltage regulation of switching power supplies. The duty cycle of the switching tube is adjusted by sampling and comparing the output voltage to achieve the purpose of voltage regulation.

　　The input end of the power supply is generally high voltage, and the output end is generally low voltage. To ensure safety, it is recommended to isolate the feedback signal at the output end through a linear optocoupler and couple it to the input end to modulate the pulse width.

　　Common linear transistor output optocouplers that can be used in switching power supplies include: TLP785, TLP185, TLP291, PC817, PS2801, etc.

　　Application Principle

　　The output voltage sampling is completed by R3 and R4. The reference electrode of TL431 is connected between R3 and R4. When the output is 5V, the reference electrode of TL431 is 2.5V, and the cathode current is stable. When the power supply voltage changes, such as rising, the reference electrode voltage of TL431 is greater than 2.5V, and the cathode current increases. At the same time, the LED current of the optocoupler increases. Since a linear optocoupler is used, the output current of the optocoupler also increases. The increase in the C-pole current of TOP414G reduces the duty cycle, thereby reducing the output voltage. At the same time, the LED current of the optocoupler decreases. When the output voltage drops below 5V, the reference electrode voltage of TL431 is lower than 2.5V, the cathode current is 0, the optocoupler does not work, and the C of TOP414G has no current. Its duty cycle will increase to increase the output voltage, thereby achieving negative feedback voltage regulation.