Switching power supply ripple suppression - Database & Sql Blog Articles

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Switching ripple is inevitable, both in theory and practice. There are several effective methods to suppress or reduce it:

1. Increase Inductance and Output Capacitance Filtering

The current fluctuation in an inductor is inversely proportional to its inductance value, while the output ripple is inversely proportional to the output capacitance. Therefore, increasing both can significantly reduce ripple. The formula vripple = Imax / (Co × f) further illustrates that higher output capacitance leads to lower ripple.

Aluminum electrolytic capacitors are commonly used for high-capacity applications, but they have high ESR and are less effective at filtering high-frequency noise. To compensate, ceramic capacitors are often connected in parallel with them. Additionally, when a switching power supply is operating, the input voltage remains constant, but the current varies with the switch. A shunt capacitor is typically placed near the switching node to provide stable current during these transitions.

However, this method has limitations due to physical size constraints. Increasing inductance or capacitance may not yield significant improvements, and higher switching frequencies can increase losses. For strict requirements, more advanced techniques are needed. Refer to design manuals for deeper insights into switching power supply principles.

2. Add a Second-Order LC Filter

An LC filter can effectively reduce noise and ripple. Choosing appropriate inductors and capacitors based on the frequency of the ripple allows you to build a filter that significantly reduces the output ripple. However, placing the sampling point before the filter (Point A) may cause a voltage drop due to the inductor's DC resistance, which affects the output voltage. If the sampling point is after the filter (Point B), the output voltage will be accurate, but adding inductors and capacitors inside the system could lead to instability.

Stability considerations are essential, and detailed analysis is usually required for proper implementation.

3. Use an LDO After the Switching Power Supply Output

This is one of the most effective ways to reduce ripple and noise. It maintains a stable output voltage without altering the original feedback system. However, it’s also the most expensive and power-consuming solution. LDOs have a noise rejection ratio, typically shown as a frequency vs. dB curve. For example, the Linear Technology LT3024 demonstrates excellent noise suppression across various frequencies.

PCB layout plays a crucial role in minimizing ripple and noise. High-frequency components require careful placement to avoid interference. In some cases, simple solutions like adding a capacitor, RC network, or series inductor on the diode can help suppress high-frequency noise, though trial and error is often necessary.

4. Add a Capacitor or RC Network Across the Diode

When a diode switches rapidly, parasitic inductance and capacitance can create an RC oscillator, leading to high-frequency oscillations. To mitigate this, a capacitor or RC buffer is placed in parallel with the diode. Typical values range from 10Ω–100Ω for resistance and 4.7pF–2.2nF for capacitance. Improper selection can worsen the issue, so testing is important.

For more demanding high-frequency noise requirements, soft-switching technology can be employed. Many resources are available on soft-switching techniques.

5. Add an Inductor After the Diode (EMI Filter)

This is another common technique for suppressing high-frequency noise. Selecting an inductor that matches the noise frequency can significantly reduce interference. Ensure the inductor’s rated current meets the actual load requirements to avoid performance issues.