Talking about the principle of power supply filtering and the method of avoiding interference
A filter capacitor helps reduce the AC impedance of a power supply. This is because real-world power supplies always have internal resistance, and transmission lines also introduce impedance. The decoupling capacitor allows part of the current that changes rapidly to be stored and released directly on the capacitor, thereby reducing voltage fluctuations. DC/DC power supply circuits are known for their low power consumption and high efficiency. They often use an absorption filter composed of capacitors and other components to effectively suppress noise and ensure a stable output.
In DC/DC power supply systems, while they offer advantages in efficiency, they can also introduce noise due to their switching nature. This noise typically appears as ripple and high-frequency noise in the power supply circuit. In high-speed circuit design, especially when working with advanced chips, the power supply’s ripple and noise must be strictly controlled. Usually, the ripple is kept below 1% of the supply voltage, while noise should be limited to 3%-5% of the supply voltage.
Ripple and noise are both forms of fluctuation in the power supply. Ripple refers to low-frequency variations, generally below 5MHz, caused by the switching action of MOSFETs. Noise, on the other hand, consists of high-frequency components, usually above 5MHz, and may include switching noise, white noise, and interference from nearby signals.
To reduce ripple and noise, two common methods are used: absorption filtering and reflection filtering. Absorption filtering involves using capacitors or capacitor networks to completely absorb and eliminate noise. Reflection filtering uses π, T, L, or LC filter circuits. These circuits allow DC to pass through without attenuation but reflect higher frequency noise back to the source, achieving filtering through impedance mismatch. However, this method doesn’t fully eliminate noise, so some interference might still remain in the circuit.
When designing a power supply, it's important to consider not only the filtering technique but also the placement of components. For instance, increasing the number of filter capacitors or adding magnetic beads can help further reduce noise. Magnetic beads are often placed in series with LC filters to enhance the absorption of high-frequency interference.
As shown in the diagram, placing a power supply filter at point A helps reflect interference back to the 3.3V power plane. Additional filter capacitors at point B prevent this interference from affecting other devices connected to the same power rail. This approach ensures a cleaner power delivery and reduces the risk of signal degradation or malfunction in sensitive components.
In more complex designs, combining LC filters with magnetic beads offers the best of both worlds: the reflection capability of LC filters and the absorption ability of magnetic beads. This dual approach enhances overall noise suppression and improves the stability of the power supply system.
By carefully selecting and placing these components, engineers can significantly improve the performance of DC/DC power supplies. Whether it's for a simple consumer electronics device or a high-performance computing system, proper filtering techniques are essential for maintaining clean and stable power delivery.
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