What Is Ringing? Problems with Switching Power Supplies and Countermeasures

table of contents

• ・What is Ringing?
• ・Causes of Ringing
• ・Countermeasures against Ringing
• ・Understand Principles of Ringing and Take Effective Countermeasures

Ringing is a phenomenon in electrical circuits in which signals are reflected on both sides of a wire, causing the waveform to oscillate. Ringing is also called switching noise because it tends to occur mainly in digital circuits and switching regulators (DC-DC converters).

This article describes how ringing occurs and how to prevent it.

What is Ringing?

Ringing is a phenomenon in electric circuits in which waveforms oscillate due to signal reflection and resonance of reactance components. It occurs when the impedance of the driver (element on the signal output side) is low and that of the receiver (element on the signal receiving side) is high.

Typical examples where ringing is likely to occur are digital circuits where transistors　are used as switches and switching nodes. Switching, the act of turning an electrical circuit on and off, generates high-frequency waves, which can become noise. Ringing is therefore also called switching noise.

Although switching at higher frequencies can achieve higher response speeds, it increases switching noise. Therefore, sufficient countermeasures must be taken to prevent noise from affecting performance.

Here are some examples of ringing at switching nodes. The figure below shows the equivalent circuit of a DC-DC converter. Let’s look at the current path in this case.

In this figure, SW1 on the power supply side is a high-side switch and SW2 on the GND side is a low-side switch. In this configuration, when SW1 is ON (SW2 is OFF), the current flows from the input capacitor through SW1 and the inductor L to the output capacitor.

In contrast, when SW2 is ON (SW1 is OFF), the current flows from SW2 through L to the output capacitor. The current flowing through the input capacitor, SW1, and SW2 (thick red lines in the figure below) changes drastically whenever a switch is turned on or off. As the current changes rapidly in this loop, high-frequency ringing occurs in the loop due to the inductance of the board wiring.

• ・ In red loop, current changes rapidly at SW ON/OFF.
• ・ At SW ON/OFF, high frequency current in the loop causes ringing.
• \( V = L \times \frac{dI}{dt} \)
• e.g.: If 1A is varied for 10ns in 10nH inductance wiring, 1V is generated.

Causes of Ringing

Causes of Ringing

Ringing occurs due to signal reflection when the impedance of the driver (element on the signal output side) is low and that of the receiver (element on the signal receiving side) is high.

For example, if there is a large difference in the characteristic impedances of the wiring, the impedance change will cause signal reflection, which in turn will cause the waveform to oscillate.

As the input impedance of the receiver becomes increasingly higher than the output impedance of the driver, the amplitude and duration of ringing increases.

Ringing can also be caused by parasitic components such as parasitic inductance and parasitic capacitance. Parasitic components are caused by electronic components or the physical structure of the electronic circuit not shown in the circuit diagram. In such cases, ringing can occur even if the circuit itself is not faulty.

For example, when a switching node has a long loop with a sudden change in current, the amplitude of ringing will be high. Ringing can occur if parasitic components are not taken into account during the design of the board layout of the switching node.

• ・ Wiring inductance L is approx. 1nH/mm.
• ・ Rise and fall times of SW MOSFET are several ns.
• \( I = C \times \frac{dV}{dt} \)　\( V = L \times \frac{dI}{dt} \)

Negative Effects of Ringing

Ringing has various negative effects on a product.

First, higher current flows through the circuit. This increases the power consumption of the product and generates heat. This may reduce the performance of the individual devices on the board and shorten their life.

The combined performance degradation of device functions leads to a decrease in the overall performance of the product, including output delays and reduced responsiveness.

Noise added to the power supply can cause the product to malfunction. Noise can also lead to performance problems. For example, it can affect the audio from audio equipment and the video from video equipment.

Countermeasures against Ringing

There are several countermeasures against ringing.

Reduction of Current Loop

The first countermeasure is to reduce the area of the loop in the switching regulator where the current changes rapidly (thick red lines in the figure below). This reduces the parasitic inductance and parasitic capacitance generated on the printed circuit board and in the connecting cables, thereby suppressing noise generation.

• ・ In red loop, current changes rapidly at SW ON/OFF.
• ・ At SW ON/OFF, high frequency current in the loop causes ringing.
• \( V = L \times \frac{dI}{dt} \)
• e.g.: If 1A is varied for 10ns in 10nH inductance wiring, 1V is generated.

Optimizing the board layout has many advantages, such as reducing noise as well as preventing regulation degradation. Design an appropriate layout taking parasitic capacitance and parasitic inductance into account.

▼For more information on the board layout of DC-DC converters, refer to:
https://techweb.rohm.com/product/power-ic/dcdc/2734/

Addition of Snubber Circuit

A snubber circuit absorbs the high-frequency ringing noise generated during switching. An RC snubber circuit consisting of a resistor and a capacitor is most commonly used.

Adding a snubber circuit is a common technique for reducing noise. In the example below, a snubber circuit is added to the switching node, which shunts the ringing noise caused by switching to the GND side.

However, adding a snubber circuit causes current to flow through the capacitors on the snubber circuit, resulting in losses. This additional circuit also increases cost and takes up board area. Before adding a snubber circuit, check the board layout to see whether the current loop area can be reduced.

▼For more information about RC snubber circuits, refer to:
https://techweb.rohm.com/product/nowisee/8289/

Insertion of Resistor at Bootstrap

The high-side switch has a BOOT pin (bootstrap) that provides a gate drive voltage to the high-side MOSFET. Inserting a resistor at the BOOT pin mitigates the noise generated in the area when the high-side MOSFET is turned on (switch is ON).

However, this resistor slows down the switching speed, increasing MOSFET losses.

Use of Decoupling Capacitors

A decoupling capacitor can be added to suppress noise. It temporarily stores electric charge and reduces current variations to suppress noise generation.

When adding a capacitor to reduce noise, it is important to check the noise frequency and select a capacitor with the corresponding impedance frequency response. Adding a capacitor reduces the impedance at the frequency of the noise being generated on the target, thereby reducing the noise amplitude.

▼For details on the effective use of decoupling capacitors, refer to:
https://techweb.rohm.com/product/nowisee/7669/

So far, we have discussed countermeasures against ringing. Of the many countermeasures available, the most recommended is to review the board layout to make the current loop smaller. Revising the board layout can also be effective for resolving other issues, such as unstable device operation.

Other countermeasures, namely the addition of a snubber circuit, the insertion of a resistor at the BOOT pin, and the use of decoupling capacitors, have some drawbacks. For example, they introduce additional losses and occupy board area.

Therefore, depending on the effects of a countermeasure, it may be necessary to consider a compromise between noise level and effectiveness.

Understand Principles of Ringing and Take Effective Countermeasures

This article described the causes and effects of ringing and commonly used countermeasures. Ringing that occurs at switching nodes was used as an example. Ringing can be caused by an impedance difference between the driver and receiver or physical factors not in the circuit diagram. Be sure to address ringing, as it can affect the overall performance of a product.

For details on individual countermeasures against ringing, refer to the relevant articles.