Have you ever had such an experience? There is still a slight overshoot when testing EMI regardless of the filtering method used. The techniques presented in this article will help you pass EMI testing or simplify your filter design. This technique introduces sideband energy by modulating the power switching frequency and changes the signal radiation characteristics from narrowband noise to wideband noise, effectively attenuating harmonic peaks. It should be noted that the total EMI energy has not decreased, but has been redistributed.
In the case of sinusoidal modulation, there are two variables that can be controlled, namely the modulation frequency (fm) and the magnitude of the change in the power switching frequency (Δf). The modulation index (B) is the ratio of these two variables, namely:
Β=Δf/fm
Figure 1 shows the effect of changing the modulation index with sinusoidal modulation. When B=0, there is no frequency offset, only one line. When B = 1, the frequency characteristic of the signal begins to broaden and the center frequency component drops by 20%. When B = 2, the frequency characteristics are spread wider, and the maximum frequency component is 60% at B = 0. We can use frequency modulation theory to quantify the energy in this spectrum. Carson's Law states that most of the energy is contained in the 2*(Δf+fm) bandwidth.
Figure 1: Modulated power switching frequency to broaden EMI characteristics
Figure 2 shows a larger modulation index, which shows that the peak EMI can be reduced by more than 12dB.
Figure 2: Larger modulation index can further reduce peak EMI
It is important to choose the modulation frequency and frequency offset. First, the modulation frequency should be higher than the EMI receiver bandwidth so that the receiver does not measure sidebands on both sides at the same time. However, if the frequency is too high, the power control loop may not adequately suppress the voltage change, causing the output voltage to change at the same rate. In addition, modulation may produce audible noise in the power supply. So the typical practice is that the modulation frequency should not exceed the receiver bandwidth too much, but outside the audible frequency range. As is apparent from Fig. 2, a change in the height of the operating frequency is preferable. However, it should be noted that this will have an impact on the power supply design, that is, the magnetic components need to be carefully selected for the lowest operating frequency. Because it operates at lower frequencies, the output capacitor also needs to handle larger ripple currents.
Figure 3 compares the measured EMI performance with both frequency modulation and frequency modulation. The modulation index here is 4, and as expected, EMI is reduced by about 8 dB based on the fundamental frequency. The effects of other aspects are also very significant. The harmonics are broadened into the frequency band and the broadening result is related to the number of harmonics, such as the third harmonic can be stretched to three times the fundamental frequency. The broadening process is repeated at higher frequencies, resulting in a significant improvement in the noise floor compared to the fixed frequency case. This technique is therefore not suitable for low noise systems, but many systems can benefit from it, which not only increases the design margin, but also significantly reduces the filter cost of EMI.
Figure 3: Changing the power switching frequency reduces the baseband signal amplitude but increases the noise floor.
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