The fuse in a switching power supply acts as a safety device, protecting internal components and connected equipment from excessive current caused by short circuits or component failures. When the fuse blows, it indicates a critical fault that must be addressed before replacing the fuse; otherwise, the new fuse will blow immediately. Understanding the common causes of fuse failure is essential for effective troubleshooting.

One of the most frequent causes is a short circuit in the primary side components. The power switch (e.g., MOSFET or IGBT) is particularly vulnerable—if it fails in a shorted state, it allows a large current to flow through the primary winding of the transformer, triggering the fuse. This failure can result from voltage spikes, overheating, or manufacturing defects. To confirm, disconnect the power switch from the circuit and test it with a multimeter; a shorted switch (showing near-zero resistance between drain and source) requires replacement.

Another cause is a faulty rectifier circuit. The bridge rectifier or individual rectifier diodes in the input stage can short, causing a direct path for AC current to flow to ground. Test each diode using the multimeter’s diode mode: a healthy diode should show low resistance in forward bias and high resistance in reverse bias; a shorted diode will show low resistance in both directions.

Capacitor failures in the primary side can also lead to fuse blowing. The main electrolytic capacitor (connected after the rectifier) may short internally due to dielectric breakdown, especially if it is overheated or aged. Additionally, the PFC capacitor, if present, can short, causing a surge in current. Inspect these capacitors for physical damage and test their capacitance and ESR; replace any faulty units.

Less common causes include a shorted power transformer (due to winding insulation failure) or a faulty control IC that sends incorrect signals to the power switch, causing it to stay on continuously. Before replacing the fuse, always identify and repair the underlying fault. Use a fuse of the correct rating (current and voltage) to avoid equipment damage or fire hazards—never use a higher-rated fuse as a temporary solution.

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