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How can the interference from a switching power supply to the power grid be minimized? The voltage interference generated by the switching power supply is typically measured by placing a 50Ω LISN (Line Impedance Stabilization Network) between the power grid and the power supply. Both the German VDE standard and the U.S. FCC standard set limits on conducted disturbances in switching power supplies. These standards are further categorized into Class A and Class B, where Class A applies to industrial and commercial equipment, while Class B is for consumer electronics. Since Class B is designed for residential use, its requirements are more stringent and stricter than those of Class A.
There are several effective methods to reduce grid interference. First, minimizing voltage overshoot can help prevent excessive voltage spikes across the switch, which in turn reduces high-frequency noise. Using diodes with low reverse recovery current, such as silicon carbide (SiC) diodes, can also help lower the level of interference. While adjusting the pulse edge and increasing the gate resistance can reduce the dv/dt (rate of change of voltage), this may lead to higher switching losses and reduced efficiency. Therefore, a balance must be struck between performance and interference reduction.
Second, improving the modulation method can significantly reduce interference. Instead of using fixed-frequency modulation, techniques like random frequency modulation, frequency hopping, or "∑△" (delta-sigma) modulation can spread out the interference across a broader frequency range. This makes it easier to meet electromagnetic compatibility (EMC) standards during testing. Fixed-frequency modulation tends to create concentrated harmonic interference, whereas random modulation disperses it, reducing peak values and making compliance more achievable.
Third, adding an input filter is another key strategy. While methods (1) and (2) focus on reducing the source of interference, adding filters changes the characteristics of the coupling path. A common-mode filter can effectively suppress interference from the power supply to the grid. Without such a filter, the interference levels often exceed the required standards. With the addition of a filter, the interference becomes manageable and meets regulatory requirements. In practice, low-frequency interference is mainly composed of harmonics of the switching frequency, so measurement equipment typically uses a resolution of 200 Hz for the low band and 9 kHz for the 150 kHz–30 MHz range. Shielding measures can also help reduce interference further.
It's important to note that passing EMC tests does not guarantee that a power supply will not cause issues in real-world applications. Improper installation or usage of a certified power supply can still result in significant interference. Additionally, since a switching power supply acts as both a power source and a noise generator, improper coupling with sensitive devices can lead to instability or malfunction. For example, connecting multiple power supplies in parallel might cause system-wide instability due to mismatched coupling channels.
Some systems are highly sensitive to the time-domain waveform of interference. For instance, digital circuits may experience malfunctions if exposed to interference pulses, and these issues depend not only on the amplitude but also on the pulse width. Even if a power supply meets all the standard requirements, the shape of the interference waveform could still cause serious problems. Therefore, users should consider conducting additional tests under actual operating conditions to ensure that the power supply performs well in real environments.
Moreover, some switching components exhibit different transition behaviors when turning on and off. The dv/dt during turn-on is usually higher than during turn-off, leading to greater external interference. The conduction transitions are generally less affected by load conditions. When comparing a randomly modulated power supply with a fixed-frequency one, such as a LGBT-based design, if the drive circuit and pulses are identical, the voltage transitions of the switches will be similar. However, the random modulation approach tends to perform better in terms of frequency domain interference distribution. Despite this, the time-domain waveform remains unchanged, meaning that a non-randomly modulated power supply may still perform better in certain situations.
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