A flyback switching power supply is a popular isolated topology widely used in low-to-medium power applications (typically 5-150W) due to its simplicity and cost-effectiveness. Its core components include a switching transistor (usually a MOSFET), a coupled inductor (acting as both energy storage and isolation transformer), a rectifying diode, output capacitors, and a feedback control circuit.  

The primary side of the circuit consists of an input voltage source (AC rectified to DC), a MOSFET (Q1) connected in series with the primary winding of the coupled inductor (T1), and a primary-side control IC. The secondary side includes the secondary winding of T1, a rectifying diode (D1), an output capacitor (Cout), and a voltage feedback network (often an optocoupler and shunt regulator) to regulate the output voltage.  

When the MOSFET is turned on, current flows through the primary winding, storing energy in the inductor’s magnetic field. During this "on-time," the secondary winding’s polarity is reversed, keeping the diode D1 reverse-biased, so no current flows to the output; the load is powered by Cout. When the MOSFET turns off, the magnetic field collapses, reversing the voltage polarity across the windings. This forward-biases D1, allowing the stored energy to transfer to the secondary side, charging Cout and supplying the load.  

Key features of the flyback circuit include its single-switch operation, eliminating the need for a separate transformer reset winding, and its ability to provide multiple isolated outputs by adding extra secondary windings. The turns ratio of the transformer determines the output voltage, while the duty cycle (controlled by the IC) adjusts energy transfer to regulate the output.  

Protection mechanisms are often integrated, such as overcurrent protection (via sensing primary current) and overvoltage protection (via feedback monitoring). However, the flyback topology has limitations, including higher ripple due to discontinuous energy transfer and lower efficiency at high power, making it less suitable for applications requiring ultra-low noise or high power density.  

 the flyback circuit’s simplicity—with fewer components and no need for a separate inductor—makes it ideal for consumer electronics, adapters, and industrial control systems.  

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