To solve the problem in a conventional system that stability varies depending upon comparison accuracy since it compares a voltage value of a certain waveform with a different threshold.

A trapezoidal waveform generated by a control waveform generating circuit 12 is amplified with gains set in amplifiers 13a-13c of each channel. Since the gradient of the edge section of the trapezoidal waveform of the output voltage of each of the amplifiers 13a-13c becomes sharper as the gain becomes higher and the ON voltage is reached earlier in each of power circuits 11a-11c, the output voltage reaches the ON voltage at earlier timing as the gains of the amplifiers 13a-13c become higher when a switch 12a is ON. However, it reaches the ON voltage at slower timing in the case of low gains. As a result, in this example, they are started sequentially in the order of power circuits 11a-11c. Conversely, when the switch 12a is OFF, the output voltage drops to or under the ON voltage at earlier timing as the gains of the amplifiers 13a-13c become lower but it drop to or under the ON voltage with a delay in the case of higher gains. Consequently, in this example, the power circuits terminate in the order of power circuits 11c-11a.