A controller (3) includes an AC voltage generator (12) that generates first to Nth AC voltages, a DC voltage generator (13) that converts the first to Nth AC voltages into first to Nth DC voltages, respectively, and a driver (14) that turns on and off a switch (1) based on the first to Nth DC voltages. The AC voltage generator (12) includes first to Nth isolation transformers (T1 to TN). The primary windings of the nth and (n+1)th isolation transformers receive an AC source voltage. The nth to first isolation transformers are sequentially connected. The (n+1)th to Nth isolation transformers are sequentially connected. The first to Nth isolation transformers respectively output the first to Nth AC voltages from their respective secondary windings.