The output current value of a direct-current power source obtained by
low-frequency, minute modulation of the input voltage of a switching
converter is detected in a circuit having an amplification factor
switching function that switches the amplification factor between definite
magnitudes synchronizing with the modulation, and by using a signal
obtained by demodulating in a discriminator circuit the output of this
circuit synchronizing with the modulation to control the switching
converter, the power point of the switching converter can be tracked to
the maximum power point by following the change in state of the direct
current power source.
