2 EMI Noise Caused by Instantaneous Switching

The left side of Figure 2-1 shows the simplified trapezoidal wave with period Tsw, pulse width t₁, rise time t_R, and fall times t_F. Using Fourier analysis, the frequency domain consists of the fundamental frequency and several upper harmonics. The harmonic amplitude envelope is a double-sinc function with corner frequencies of f₁ and f₂, depending on the pulse duration and rise and fall time of the time-domain waveform. Radiated EMI problems often occur in the 50 MHz to 300 MHz range, and increasing rise and fall times shift the f₂ point to lower frequencies, and the high frequencies roll-off more quickly with 40 dB/DEC. In the lower frequency range, the impact of slower rise and fall times is limited.

Figure 2-1 Frequency Spectrum of SW Node Voltage