This paper proposes new EMI reduction technologies and automatic output voltage ripple cancellation method for the PWM buck converter with voltage-mode or current-mode and the ripple-controlled converters. Genenrally, modfying the clock frequeny is effective to reduce the EMI noise, but it may increase the output ripple substantially. We have developed techniques to cancel the increased ripple by modifying the slope of the saw-tooth signal or current of the ripple injection circuit. The EMI spectrum of the operating frequency is reduced by more than 15dB and the modified large ripple is canceled to the stable level.