Have you ever encountered a situation where, no matter which filtering method you tried, there was still a slight overshoot during EMI testing? The techniques discussed in this article can help you pass EMI compliance more easily or simplify your filter design. This approach involves modulating the power switching frequency to introduce sideband energy, transforming narrowband noise into wideband noise. As a result, harmonic peaks are effectively reduced. However, it's important to note that the total EMI energy remains the same—it’s just redistributed across the spectrum.
When using sinusoidal modulation, two key parameters can be adjusted: the modulation frequency (fm) and the change in the switching frequency (Δf). The modulation index (B) is defined as the ratio of these two values:
$$ B = \frac{\Delta f}{f_m} $$
Figure 1 illustrates how changing the modulation index affects the frequency spectrum. When B=0, there is no frequency deviation, and only a single spectral line appears. At B=1, the signal begins to spread out, with the center frequency component dropping by 20%. When B=2, the spectrum becomes even broader, and the peak at B=0 decreases significantly. Using frequency modulation theory, we can estimate the energy distribution. According to Carson’s Law, most of the energy lies within a bandwidth of $ 2(\Delta f + f_m) $.
**Figure 1:** Modulated power switching frequency to broaden EMI characteristics
Figure 2 shows the effect of a larger modulation index, demonstrating that peak EMI can be reduced by more than 12 dB.
**Figure 2:** Larger modulation index can further reduce peak EMI
Selecting the right modulation frequency and frequency offset is crucial. The modulation frequency should be higher than the EMI receiver’s bandwidth so that the receiver doesn’t simultaneously capture sidebands on both sides. However, if the frequency is too high, the power control loop may not adequately suppress voltage fluctuations, leading to output voltage variations at the same rate. Additionally, modulation might generate audible noise in the power supply. Therefore, it's common practice to set the modulation frequency outside the audible range but not too far above the receiver bandwidth.
As shown in Figure 2, adjusting the operating frequency upward is generally beneficial. However, this has implications for the power supply design. Magnetic components must be carefully chosen based on the lowest operating frequency, as lower frequencies require larger inductors. Also, the output capacitor must handle increased ripple currents.
Figure 3 compares EMI performance with and without frequency modulation. Here, the modulation index is set to 4, resulting in an approximate 8 dB reduction in EMI compared to the fundamental frequency. The broadening of harmonics into the frequency band is also significant. For example, the third harmonic spreads out to three times the fundamental frequency. This process repeats at higher frequencies, leading to a noticeable improvement in the noise floor compared to fixed-frequency operation. While this technique isn't ideal for low-noise systems, many applications can benefit from it—enhancing design margins and significantly lowering EMI filter costs.
**Figure 3:** Changing the power switching frequency reduces the baseband signal amplitude but increases the noise floor.
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