Reducing EMI performance by changing the power supply frequency

When measuring EMI performance, do you find that no matter what method you use to filter, there are still problems of several dB exceeding the specification? There is a method that may help you achieve EMI performance requirements or simplify your filter design. This method involves modulating the power switching frequency to introduce sideband energy and change the emission characteristics of narrowband noise to broadband, thereby effectively attenuating harmonic peaks. It is important to note that the overall EMI performance is not reduced, but only redistributed.

With sinusoidal modulation, the two variables you can control are the modulation frequency (fm) and the amount by which you change the power supply switching frequency (Δf). The modulation index (Β) is the ratio of these two variables:

B=Δf/fm

Figure 1 shows the effect of changing the modulation index with a sine wave. When Β=0, there is no frequency shift, just a single spectral line. When Β=1, the frequency signature begins to stretch, and the center frequency component drops by 20%. When Β=2, the signature stretches further, and the maximum frequency component is 60% of the initial state. Frequency modulation theory can be used to quantify the amount of energy in this spectrum. Carson's law states that most of the energy will be contained in the 2 * (Δf + fm) bandwidth.

Figure 1 Modulating the power supply switching frequency extends the EMI signature

Figure 2 shows a larger modulation index and demonstrates that it is possible to reduce peak EMI performance by more than 12dB.

Figure 2 A larger modulation index can further reduce peak EMI performance

Choosing the modulation frequency and the frequency shift are two important aspects. First, the modulation frequency should be higher than the EMI receiver bandwidth so that the receiver does not measure both sidebands at the same time. However, if you choose a frequency that is too high, the power supply control loop may not be able to fully control the change, causing the output voltage to change at the same rate. In addition, this modulation will cause audible noise in the power supply. Therefore, we generally choose a modulation frequency that is not much higher than the receiver bandwidth, but larger than the audible noise range. Obviously, from Figure 2, we can see that a larger change in operating frequency is more desirable. However, it is important to be aware of the impact this will have on the power supply design. That is, choose the magnetic components for the lowest operating frequency. In addition, the output capacitor will need to handle the higher ripple current caused by the lower frequency operation.

Figure 3 compares the EMI performance measurements with and without frequency modulation. The modulation index is 4, and as expected, the EMI performance is reduced by about 8dB at the fundamental frequency. Other aspects are also important. The harmonics are smeared into the frequency band corresponding to their number, that is, the third harmonic extends to three times the fundamental frequency. This situation is repeated at some higher frequencies, which can make the noise floor much higher than the fixed frequency case. Therefore, this method may not be suitable for low noise systems. However, many systems have benefited from this method by increasing the design margin and minimizing the cost of EMI filters.

Figure 3 Changing the power frequency lowers the fundamental frequency but increases the noise floor