The isolated switching power supply circuit that delivers 5V at 2A using the TOP414G is illustrated in the figure. This design is commonly used in low to medium power applications where efficiency and compactness are important. The circuit uses a primary-side control topology, which simplifies the design by eliminating the need for an external feedback resistor network.
C1 serves as the input filter capacitor, smoothing out any voltage fluctuations from the power source. VDz and VD1 form a clamp protection circuit on the primary side, helping to prevent overvoltage conditions that could damage the MOSFET inside the TOP414G. R1 is the control terminal resistor, while C2 acts as a bypass capacitor to stabilize the control signal and reduce noise.
C10 is connected in parallel across the TOP414G's control and ground terminals. Its purpose is to prevent false triggering of the power-off circuit when high-frequency interference is present on the control line. VD2 is the output rectifier diode, and the combination of C3, C4, L, C5, and C6 forms the output filter stage, ensuring a clean and stable DC output. C9 is a noise-canceling capacitor placed across the output to further reduce ripple and noise.
An external error amplifier is implemented using the TL431 shunt regulator. When the output voltage fluctuates, a portion of the voltage is divided by R3 and R4, then compared with the internal reference voltage of the TL431. Based on this comparison, the TL431 generates a control signal that is transmitted through the optical coupler PC817A. This allows the duty cycle of the TOP414G to be adjusted dynamically, resulting in a more stable output voltage.
The gain of the control loop is set by R2, which helps determine how sensitive the system is to changes in the output voltage. The feedback winding voltage is rectified and filtered by VD3 and C7, providing power to the infrared receiving transistor in the PC817A. This ensures reliable communication between the primary and secondary sides of the isolation barrier.
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