SWITCH CONTROL DEVICE, SWITCH CONTROL METHOD, AND IN-VEHICLE POWER SUPPLY SYSTEM

A MPU (100) controls a changeover switch, which is an FET switch (12) that connects a main battery (2) and a sub-battery (3) having a low rated voltage. The changeover switch is set to an ON state in a case where a potential difference, which is a difference between a voltage of the main battery and a voltage of the sub-battery, is equal to or greater than a positive first predetermined value. A PWM signal is output to a driver (14) to cause the changeover switch to be in a half-ON state in a case where the potential difference is less than the first predetermined value and is greater than a second predetermined value equal to or less than 0. The changeover switch is set to an OFF state in a case where the potential difference is equal to or less than the second predetermined value.