PURPOSE:To realize high accurate waveform compensation by judging whether periodic and repetitive occurrence of voltage error is sporadic or not and not accumulating the voltage error when the occurrence is sporadic. CONSTITUTION:The value of evaluation function is compared with a predetermined evaluation coefficient, and when the former is smaller a judgment is made that voltage errors are occurring repetitively. Respective voltage errors are then accumulated in order to operate PWM pulse width U(k) between sampling points k-(k+1) in next(current) period. When the former is larger, a judgment is made that the voltage error is caused sporadically by abrupt variation of load or the like and the voltage error e(k+1-n) at preceding period is removed from accumulation term thus operating the PWM pulse width U(k) through accumulation of other voltage errors.
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