When a magnitude of a voltage between a pair of first voltage points in a main circuit exceeds a first clamp value, a first clamp circuit absorbs electrical energy of the main circuit from the pair of first voltage points to clamp the voltage to the first clamp value. When a magnitude of a voltage between a pair of second voltage points in the main circuit falls below a second clamp value, a second clamp circuit injects electrical energy to the main circuit from the pair of second voltage points to clamp the voltage to the second clamp value. A voltage conversion circuit performs voltage conversion between a first clamp voltage defining a first clamp value and a second clamp voltage defining a second clamp value.