4W backup switching regulated power supply

The circuit of the +5V, +12V dual output 4W backup switching regulated power supply composed of TOP221P is shown in the figure. The circuit uses one piece each of TOP221P (ICl) and optocoupler PC817A (IC2). This circuit can continue to supply power after the main power supply is cut off, ensuring that the data in the CPU and real-time calendar clock chip internal RAM of the instrument will not be lost, and can also maintain the working status of the remote control port for a period of time. The input terminal is connected to the DC voltage Ui. The Ui value should depend on the variation range of the AC input voltage u (such as 85" 245V). T is the high-frequency transformer, N1 is the primary coil, N2 is the secondary coil (also known as the output coil), and N3 is the feedback coil. As shown in the figure The black dots represent the same terminal of each coil, also known as the non-inverting terminal. The isolated +5V output is designed to power circuits that require electrical protection, while the non-isolated +12V output can power the PWM controller of the main power supply. In standby mode; once a call is received, the main power supply immediately switches to the normal working state. RTN in the figure is the return terminal (RETURN) of the +5V output, which is the common/ground terminal. This circuit is a single-ended flyback switching regulated power supply. The so-called single-ended means that TOP221P has only one pulse width modulation signal power output terminal - drain D. The flyback type means that when the power switch MOSFET is turned on, the electric energy is stored in the primary coil of the high-frequency transformer; When the MOSFET is turned off, electric energy is output to the secondary. Since the switching frequency is as high as 100kHz, the high-frequency transformer can quickly store and release energy, and continuous output can be obtained after high-frequency rectification and filtering.