Switching power supply ripple suppression - Database & Sql Blog Articles

SMD Aluminum Electrolytic Capacitor

Chip Ceramic Capacitor

Filter 18.432M

Switching ripple is an inevitable phenomenon, both in theory and practice. To minimize or suppress this ripple, there are several effective methods:

(1) Increasing inductance and output capacitance for better filtering.

According to the basic formula of switching power supplies, the current fluctuation in the inductor is inversely proportional to its inductance value, and the output ripple voltage is inversely proportional to the output capacitance. Therefore, increasing both the inductance and the output capacitance can significantly reduce the ripple. Additionally, the relationship between output ripple and capacitance can be expressed as: vripple = Imax / (Co × f). This shows that a larger output capacitor leads to lower ripple.

In most cases, aluminum electrolytic capacitors are used for their high capacitance values. However, they are not very effective at suppressing high-frequency noise due to their relatively high ESR. To compensate for this, ceramic capacitors are often connected in parallel with electrolytic capacitors, providing better high-frequency performance.

When the power supply is operating, the input voltage remains constant, but the input current varies with the switching cycle. To handle this, a shunt capacitor is typically placed near the switching node (e.g., the switch in a buck converter), allowing it to supply the necessary current during transitions.

While this method is common, it has limitations. Inductors cannot be made too large due to size constraints, and increasing the output capacitance may not always yield significant improvements. Also, raising the switching frequency increases switching losses. Therefore, this approach may not be sufficient for applications with strict ripple requirements. For more detailed information on switching power supply principles, refer to standard design manuals.

(2) Implementing secondary filtering with an LC filter.

An LC filter is highly effective in reducing noise and ripple. By selecting appropriate inductors and capacitors based on the frequency of the ripple you want to eliminate, a low-pass filter can be constructed to significantly reduce the ripple.

The sampling point is usually placed before the LC filter (Pa), which allows for a reduced output voltage. However, the DC resistance of the inductor causes a voltage drop when current flows through it, which can lower the output voltage. This drop depends on the output current.

Alternatively, placing the sampling point after the LC filter (Pb) ensures that the output voltage matches the desired level. However, adding an inductor and capacitor within the system can introduce instability, especially in feedback loops. Stability is a critical consideration in power supply design, and many resources discuss this in detail.

(3) Using an LDO after the switching power supply output.

This is one of the most effective ways to reduce ripple and noise. An LDO maintains a stable output voltage without requiring changes to the original feedback system. However, it is also the most expensive and power-consuming option. All LDOs have a noise rejection ratio, which is typically represented as a frequency vs. decibel curve. For example, the Linear Technology LT3024 demonstrates excellent noise suppression across various frequencies.

PCB layout plays a crucial role in minimizing switching noise, and it's often a complex challenge. Specialized engineers focus on optimizing these layouts. For high-frequency noise, post-filtering can help, but it may not be very effective. Simple techniques include using a capacitor C, RC network, or series inductor across the diode.

(4) Adding a capacitor C or RC network across the diode.

When a diode switches rapidly, parasitic parameters come into play. During reverse recovery, the diode’s equivalent inductance and capacitance form an RC oscillator, causing high-frequency oscillations. To suppress these, a capacitor C or RC buffer circuit is connected in parallel with the diode. The resistor is typically in the range of 10Ω–100Ω, and the capacitor ranges from 4.7pF to 2.2nF.

Choosing the right values for the capacitor and resistor requires trial and error. If not properly selected, it can lead to even worse oscillations. For applications with strict high-frequency noise requirements, soft-switching technology can be employed. There are numerous books and resources dedicated to soft-switching techniques.

(5) Adding an inductor after the diode (EMI filter).

This is another common method to suppress high-frequency noise. Selecting the right inductor for the noise frequency can effectively reduce interference. It’s important to ensure that the inductor’s rated current matches the actual application requirements.