Control method for polyphase step-up converter, and polyphase step-up converter

A polyphase step-up converter comprising:

• a plurality of step-up circuits connected in parallel with each other between an input terminal to which an input voltage is applied and an output terminal from which an output voltage that is supplied to a load circuit is output;
• and
• a controller configured to drive the plurality of step-up circuits with the use of a plurality of driving signals having the same frequency, respectively, wherein
• the controller is configured to, in changing the frequency of each of the plurality of driving signals from a first frequency to a second frequency while N, which is an integer larger than or equal to two, step-up circuits are being driven, determine whether the second frequency is M, which is a natural number smaller than or equal to N, times as high as a resonance frequency of a resonance circuit that is provided between the output terminal and the load circuit, and, when the second frequency is M times as high as the resonance frequency, set a phase difference between the plurality of driving signals to a phase difference that is determined in accordance with a value of M and at which the resonance phenomenon is avoided.