A digitally controlled PWM inverter, modulated by a multi-random technique with a single fixed switching frequency, is proposed for uninterruptible power supplies. Multi-random pulsewidth modulation utilizes a random phase-shift PWM scheme and a random pulse position PWM scheme, both driven at the same switching frequency. The multi-random technique reduces harmonics intensity, and thus reduces undesirable acoustic noise and mechanical vibration. The control scheme is based on instantaneous feedback signals of output voltage and capacitor current. The output voltage is therefore well regulated during sudden load disturbance, as confirmed by simulation results. An experimental prototype was built. When compared with the conventional sinusoidal PWM scheme, experimental results show better spectrum performance, very fast dynamic response, and a great reduction of acoustic noise.
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