A critical specification for switching power supplies is ripple. Ripple is primarily caused by the switching operation, and its presence can affect the operation of subsequent circuits, especially in ripple-sensitive applications. How can we correctly measure switching power supply ripple? How can we effectively suppress switching power supply ripple to meet the power supply circuit's requirements? These are crucial skills that PCB design engineers must master.

Measuring Switching Power Supply Ripple

To effectively reduce the output ripple of a switching power supply, we must first have a reliable testing method. False waveforms caused by poor testing methods are difficult to correct.

Basic Requirements: Use an oscilloscope with AC coupling, a 20MHz bandwidth limit, and remove the probe's ground lead.

1. AC coupling removes the superimposed DC voltage to obtain an accurate waveform.

2. Enabling the 20MHz bandwidth limit prevents interference from high-frequency noise, which can lead to erroneous measurements. Because high-frequency components have larger amplitudes, they should be removed during measurement.

3. Remove the oscilloscope probe's ground clip and use a ground ring for measurement to reduce interference. Many departments do not have ground rings, so if tolerance allows, use the probe's ground clip directly for measurement. However, this factor should be considered when determining compliance.

Another point is the use of a 50Ω termination. Yokogawa oscilloscope documentation states that a 50Ω termination removes DC components and accurately measures AC components. However, few oscilloscopes come with specialized probes for this purpose. Most use standard 100kΩ to 10MΩ probes, and their impact is currently unknown.

The above are basic precautions when measuring switching ripple. If the oscilloscope probe is not directly touching the output, use twisted-pair or 50Ω coaxial cable.

When measuring high-frequency noise, use the oscilloscope's full passband, generally ranging from several hundred megahertz to a gigahertz. Other aspects are the same as above.

Different companies may have different testing methods. Ultimately, the first thing to do is to understand your test results clearly and the second is to obtain customer approval.

About Oscilloscopes

Some digital oscilloscopes cannot accurately measure ripple due to interference and memory depth issues. In this case, the oscilloscope should be replaced. In this regard, older analog oscilloscopes, although with bandwidths of only tens of megahertz, sometimes perform better than digital oscilloscopes.

Suppressing Ripple in Switching Power Supplies

Switching ripple exists both theoretically and practically. There are five common methods for suppressing or reducing it:

1. Increasing the Inductor and Output Capacitor Filtering

According to the switching power supply formula, the current fluctuation within the inductor is inversely proportional to the inductance value, and the output ripple is inversely proportional to the output capacitance value. Therefore, increasing the inductance and output capacitance values can reduce ripple.

The figure above shows the current waveform within the inductor L of a switching power supply. Its ripple current △I can be calculated using the following formula:

As can be seen, increasing the L value or increasing the switching frequency can reduce the current fluctuation within the inductor.

Similarly, the relationship between output ripple and output capacitance is: ripple = Imax / (Co × f). As can be seen, increasing the output capacitance value can reduce ripple.

Typically, aluminum electrolytic capacitors are used for output capacitance to achieve a large capacitance. However, electrolytic capacitors are not very effective at suppressing high-frequency noise and have a relatively high ESR. Therefore, ceramic capacitors are often connected in parallel to compensate for the shortcomings of the aluminum electrolytic capacitor. Meanwhile, when a switching power supply is operating, the voltage Vin at the input remains constant, but the current varies with the switching operation. In this case, the input power supply cannot provide sufficient current. Typically, a parallel capacitor is added near the current input terminal (near SWITCH in the case of a Buck-type power supply, for example) to provide current.

After applying this countermeasure, the Buck-type switching power supply looks like the following figure:

The above approach has limited effectiveness in reducing ripple. Due to size limitations, the inductor cannot be made very large; increasing the output capacitance beyond a certain value has no significant effect on ripple reduction; and increasing the switching frequency increases switching losses. Therefore, this method is not ideal for demanding applications.

For more information on the principles of switching power supplies, please refer to various switching power supply design manuals.

2. Secondary Filtering: Adding an LC Filter

LC filters significantly suppress noise ripple. Selecting appropriate inductors and capacitors to form a filter circuit based on the desired ripple frequency generally provides excellent ripple reduction.

However, in this case, the sampling point of the feedback comparison voltage must be considered. (As shown in the figure below)

Selecting the sampling point before the LC filter (Pa) will reduce the output voltage. Because every inductor has a DC resistance, when current is output, a voltage drop occurs across the inductor, resulting in a decrease in the power supply's output voltage. This voltage drop also varies with the output current.

Selecting the sampling point after the LC filter (Pb) ensures that the output voltage is the desired voltage. However, this introduces an inductor and a capacitor within the power supply system, potentially causing system instability. System stability is covered in numerous resources, so I won't go into detail here.

3. Connect an LDO filter after the switching power supply output

This is the most effective way to reduce ripple and noise. It maintains a constant output voltage and doesn't require changing the existing feedback system. However, it's also the most cost-effective and power-efficient method.

Every LDO has a performance metric: noise rejection ratio. This is a frequency-dB curve, as shown in the figure on the right for the Linear Technology LT3024.

After passing through the LDO, switching ripple is generally below 10mV.

The figure below compares the ripple before and after the LDO:

Comparing the curve in the upper figure with the waveform on the left, we can see that the LDO effectively suppresses switching ripple at several hundred kHz. However, at higher frequencies, the LDO's performance is less than ideal.

The PCB layout of the switching power supply is also crucial for ripple reduction and is a complex issue. Specialized switching power supply PCB engineers have learned that for high-frequency noise, due to its high frequency and large amplitude, post-stage filtering can provide some benefit, but the effect is not significant. This has been specifically researched, and a simple approach is to add a capacitor C or RC in parallel with the diode, or an inductor in series.

For high-frequency noise, due to its high frequency and large amplitude, post-stage filtering can provide some benefit, but the effect is not significant. This has been specifically researched, and a simple approach is to add a capacitor C or RC in parallel with the diode, or an inductor in series.

4. Adding a Capacitor C or RC in Parallel with the Diode

When a diode is turned on and off at high speed, parasitic parameters must be considered. During the diode's reverse recovery period, the equivalent inductance and equivalent capacitance form an RC oscillator, generating high-frequency oscillation. To suppress this high-frequency oscillation, a capacitor C or RC snubber network is connected in parallel across the diode. The resistor is typically 10Ω-100Ω, and the capacitor is 4.7pF-2.2nF.

The value of the capacitor C or RC in parallel with the diode requires repeated experimentation. Improper selection can even lead to more severe oscillation.

If high-frequency noise requirements are stringent, soft switching technology can be employed. Soft switching is covered in numerous books.

5. Connecting an inductor after a diode (EMI filtering)

This is also a common method for suppressing high-frequency noise. Choosing the appropriate inductor component for the frequency at which the noise is generated can also effectively suppress the noise. It's important to note that the inductor's rated current must meet actual requirements.

Summary

The above summarizes some aspects of switching power supply ripple. Adding some waveforms would be helpful. While this may not be comprehensive, it's sufficient for general applications. Regarding noise suppression, not all methods are applicable in practice. It's important to choose the appropriate method based on your design requirements, such as product size, cost, and development cycle.

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