Voltage detection circuit and switching power supply unit

摘要

A switching power supply unit is provided, in which a DC input voltage (Vin) can be detected even if switching operation of the power supply unit is stopped. A switching power supply unit includes: a power supply main section (10) switching a DC input voltage (Vin) inputted from a first power supply (HB) to convert the DC input voltage (Vin) into an AC voltage, and outputting a DC output voltage (Vout) into a second power supply (LB), the DC output voltage (Vout) being obtained by transforming and rectifying the AC voltage; and a voltage detection section (20) having a voltage detection transformer (21), one or more switching elements (24), and detection signal lines (W1,W2). The voltage detection transformer (21) includes a first transformer coil (21A) as a primary winding being intermittently applied with the DC input voltage in response to on/off of the switching element (24), and a second transformer coil (21B) as a secondary winding being connected to the detection signal lines (W1,W2).