The optocoupler of the switching power supply is mainly isolated, providing feedback signals and switching functions. The power supply of the optocoupler in the switching power supply circuit is supplied from the secondary voltage of the high-frequency transformer. When the output voltage is lower than the voltage of the Zener diode, the signal optocoupler is turned on, and the duty ratio is increased to increase the output voltage; Turning off the optocoupler reduces the duty cycle, causing the output voltage to decrease. When the secondary load of the high-frequency transformer is overloaded or the switch circuit is faulty, there is no optocoupler power supply. The optocoupler controls the switch circuit to not start, thus protecting the switch tube from being broken down and burned.
Usually the optocoupler is used with the TL431. The following is part of the circuit of the LED power driver chip (switching power supply chip). The two resistors are sampled in series to the 431R terminal for comparison with an internal comparator. Then, according to the compared signal, the resistance of the 431K terminal (the anode is connected to the photocoupler) to ground is controlled, and then the brightness of the internal LED of the optocoupler is controlled. (The inside of the optocoupler is a light-emitting diode, and one side is a phototransistor) through the intensity of the light. Controlling the resistance of the CE terminal of the other end transistor is also changing the current of the LED power supply driving chip (switching power supply chip) detecting pin (1 pin: voltage feedback pin, adjusting the occupation ratio by connecting the optocoupler to the ground). According to the magnitude of the current, the led power supply driver chip (switching power supply chip) will automatically adjust the duty cycle of the output signal to achieve the purpose of voltage regulation.
The above chip is a highly integrated, high performance PWM + MOSFET tube in one current type offline switching power supply controller. Suitable for all kinds of low-power switching power supplies such as chargers, power adapters, LED drive power supplies, etc. In the DIP8 package, it can output 0~36W without adding a heat sink (plus heat dissipation can make it bigger). The circuit structure is simple and the cost is low. It has perfect protection functions including overvoltage, undervoltage, overtemperature, overload and short circuit protection. Fixed oscillation frequency and frequency hopping function can reduce EMI. The standby power is low, and it enters the skip cycle mode during standby, which meets the requirements of the “Energy Star†waiting machine power consumption standard.
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