That is, in the discharge and discharge mode shown in FIG. 4, one of the S3 arm and the S4 arm just needs to be in the off state for the first step-up/step-down circuit CNV1, and one of the S2 arm and the S3 arm just needs to be in the off state for the second step-up/step-down circuit CNV2. In other words, when one of the switching elements S2, S3 and one of the switching elements S3, S4 are in the off state, the discharge and discharge mode is set.
Current components respectively flowing through the switching elements S1 to S4 in the drawing are shown to the top left of FIG. 4. As shown in FIG. 4, in the discharge mode in a step-up process, almost no current flows through the switching elements S1 to S4 (≈0), but current flows through the diodes D1 to D4. For this reason, no current flows through the switching element S1, and no current flows though the switching elements S2 to S4 or the switching elements S2 to S4 are set to the off state.
