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How can the interference caused by a switching power supply to the power grid be minimized? The interference voltage is typically measured using a LISN (Line Impedance Stabilization Network), which includes a 50Ω resistor placed between the power grid and the power supply. Both the German VDE standard and the US FCC standard define limits for conducted disturbances in switching power supplies, with further classification into Class A and Class B. Class A applies to devices used in industrial, commercial, and office settings, while Class B is designed for consumer-grade equipment. Since Class B is intended for residential use, its requirements are stricter and more demanding.
There are several effective strategies to reduce this kind of interference:
(1) Minimizing voltage overshoot is one of the key approaches. Excessive voltage on the switching components can lead to high-frequency noise that affects the grid. Using diodes with low reverse recovery current, such as silicon carbide diodes, can significantly reduce the level of interference. While adjusting the pulse edge and increasing gate resistance can help lower dv/dt, it may also increase switching losses and reduce overall efficiency. Therefore, a balanced trade-off between performance and efficiency must be considered during design.
(2) Improving the modulation technique is another important method. Instead of fixed-frequency modulation, techniques like random frequency modulation or ∑△ modulation can be applied. These methods spread the low-frequency interference over a broader range rather than concentrating it at harmonic frequencies. This not only helps in passing electromagnetic compatibility (EMC) tests but also ensures better spectral characteristics, making it easier to meet regulatory standards.
(3) Adding an input filter is an effective way to modify the coupling path of the interference. Common-mode filters can significantly reduce the amount of noise injected into the power grid. Without such filters, the interference from the power supply could easily exceed the required limits. When properly implemented, these filters ensure that the power supply meets the necessary standards. In testing, the low-frequency interference is usually found in the harmonics of the switching frequency, so the frequency sweeper should have a resolution of 200 Hz in the low band and 9 kHz in the 150 kHz–30 MHz range. Additionally, shielding measures can further suppress interference and improve system stability.
It's important to note that passing an EMC test does not guarantee that the power supply will not cause interference during actual operation. Improper installation or usage can still result in significant issues. As both a power source and a noise generator, the switching power supply interacts with the surrounding system through various coupling paths. If the coupling characteristics are not well matched with the sensitivity of the receiving device, serious interference can occur. For example, connecting the power supply in parallel might lead to system instability.
Some sensitive devices are particularly affected by the time-domain waveform of the interference. Digital circuits, for instance, may malfunction due to interference pulses, and this is not just dependent on the amplitude but also on the pulse width. Even if a power supply meets all the required standards, its time-domain waveform might still cause more severe interference in certain situations. Therefore, users should go beyond standard EMC testing and evaluate the interference performance under real-world operating conditions.
Moreover, some switching components exhibit different transitions when turning on and off. The dv/dt during turn-on is generally higher than during turn-off, leading to greater external interference. These transitions are largely independent of load conditions. When comparing a randomly modulated power supply with a fixed-frequency one, such as those using LGBT technology, if the drive circuit and pulse parameters are identical, the switch transitions will be similar. However, the random modulation approach tends to perform better in frequency domain tests. Despite this, the time-domain waveform remains unchanged, meaning that a non-randomly modulated power supply might still outperform a fixed-frequency one in certain applications.
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