EMI Reduction and Output Ripple Improvement of Switching DC-DC Converters with Linear Swept Frequency Modulation

This paper proposes a linear swept frequency modulation for the saw-tooth signal generator to reduce the Electro Magnetic Interference (EMI) noise drastically and maintain a very small stable output voltage ripple in the switching DC-DC converter. In the conventional method, the modulation of the clock frequency reduces the EMI noise dramatically; however, the output ripple is increased largely. The increase of the output ripple is caused by the duty variations of the Pulse Width Modulation (PWM) when the clock frequency is modulated. In the proposed technique, the duty variations of PWM are canceled by the current compensation of the saw-tooth signal generator. The cancelation of the duty variation theoretically leads to improve the frequency modulation and the slope of the saw-tooth signal. As a result, the output ripple is maintained at less than 2mVpp and the spectrum level of the clock frequency is reduced by greater than 28 dB.