Figure 1 is the basic circuit of a buck converter. When the switching element Q1 is turned on, current flows from Vin through the coil L and charges the output smoothing capacitor Co, and the output current Io is supplied to Vin. At this time, the current flowing into the coil L generates a magnetic field, and the electrical energy is converted into magnetic energy and stored. When the switching element Q1 is turned off, the freewheeling diode D is turned on, and the energy stored in L is then released to the output side.