1. Input supply voltage too low: The output voltage of a switching power supply generally depends on the input supply voltage. If the input supply voltage is too low, the output voltage of the switching power supply will decrease accordingly. This may be caused by an input supply fault, grid voltage fluctuations, or excessive power line impedance.

2. Excessive output load: The output voltage of a switching power supply is generally affected by its output load. If the output load exceeds the rated load capacity of the switching power supply, the supply may be unable to provide enough current, causing the output voltage to drop.

3. Control circuit fault: The control circuit is the key part of a switching power supply that controls the switching devices. A fault in this circuit may prevent the switching devices from turning on or off properly, affecting output voltage stability and accuracy.

4. Feedback loop problems: Switching power supplies typically use a feedback loop to maintain a stable output voltage. Problems such as excessive deviation, damaged feedback components, or an abnormal regulation circuit may cause the output voltage to be too low.

5. Switching component failure: Switching devices in the power supply, such as switching transistors and diodes, may fail due to aging, damage, or overload, affecting the stability and level of the output voltage.

6. Excessive temperature: A switching power supply generates heat during operation. Poor heat dissipation or excessive ambient temperature may raise its internal temperature, affecting the performance and stability of electronic components and causing a low output voltage.

These are some common reasons for a low switching power supply output voltage. If this problem occurs, check these areas to troubleshoot and repair the fault, or seek professional assistance.