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The causes and solutions of power  supply ripple and noise generation

Article Source：Kinri Energy | Author：Kinri Energy | Issuing Time：2024.04.20
The DC/DC power supply circuit has the advantages of low power consumption and high power, but at the same time, the conversion of the DC/DC circuit is completed through switching, which inevitably leads to the introduction of noise, specifically the ripple and noise of the power supply circuit.



Compared with linear power supplies, switching power supplies (including AC/DC converters, DC/DC converters, AC/DC modules, and DC/DC modules) have the most prominent advantage of high conversion efficiency, generally reaching 80% to 85%, and up to 90% to 97%.

Secondly, switching power supplies use high-frequency transformers instead of bulky power frequency transformers, which not only reduces weight but also reduces volume, making their application range increasingly wide.

But the disadvantage of switch mode power supply is that its switching transistor operates in a high-frequency switching state, and the output ripple and noise voltage are relatively large, generally around 1% of the output voltage (low is about 0.5% of the output voltage). The ripple and noise voltage of the best product is also several tens of mV; The adjustment tube of a linear power supply operates in a linear state, with no ripple voltage, and the output noise voltage is also small. Its unit is μ V.

1.The definition of ripple and noise

Ripple: refers to the fluctuation in the output of a power supply caused by periodic changes in voltage. Usually used to describe the periodic changes in the output waveform of a power supply, especially in DC power supplies where the output voltage is not completely stable and there are certain periodic fluctuations. The low-frequency component in power fluctuations is generally below 5MHz, caused by the switching action of MOSFETs.

Noise: An unexpected, random electrical signal that contains various interferences and random variations introduced in the system. Noise can be caused by instability from electronic components, circuits, external environments, etc. The high-frequency components in power fluctuations are generally higher than 5MHz and more complex, including MOSFET switching noise, white noise, and interference from surrounding signals.

Cause of occurrence

Ripple: Voltage fluctuations mainly caused by factors such as power supply design, switch operation, inductance and capacitance components.

Noise: It can come from various factors such as internal thermal noise, environmental interference, electromagnetic interference, etc. of electronic components.

Periodic

Ripple: It has obvious periodicity and is a regular fluctuation that occurs in the power output.

Noise: It is usually irregular, random, and may not have obvious periodicity.

Common ground

Ripples and noise can both affect the performance of electronic systems, especially for applications that require high voltage stability, such as precision instruments, communication systems, etc.

Influence factor

In some cases, the ripple of the power supply may be considered a special form of noise, especially when the frequency of the ripple is high. In this case, ripple can also be considered as part of the 
noise.

Low frequency ripple is related to the filtering capacitor capacity of the output circuit. Due to the limitation of the volume of the switching power supply, the capacity of the electrolytic capacitor cannot increase unlimited, resulting in residual low-frequency ripple output, which varies with the rectification circuit method.



Common methods for eliminating ripple and noise include absorption filtering and reflection filtering:

Absorption filtering: Absorption filtering consists of capacitors and capacitor circuits, which can completely eliminate noise.

Reflective filtering: composed of π, T, L or LC filtering circuits, its principle is to make the DC signal pass through the filtering circuit without attenuation, while high-frequency ripple and noise are returned to the source end. The impedance mismatch of the filtering circuit was utilized to achieve the filtering effect. Its characteristic is that the filtering circuit cannot completely eliminate noise, and interference still exists in the circuit. There are four types of reflective filtering circuits to choose from, depending on the source impedance and load impedance.

Due to the filtering principle of LC filtering circuit being reflection, interference still exists in the circuit. Therefore, to avoid the impact of these interferences on other circuits, other measures need to be taken:

Increase the filtering capacitance. Both simple capacitors and magnetic beads have the function of completely filtering out noise.



As shown in the above figure, in order to reduce the interference of the input power circuit on the power circuit, a power filtering circuit needs to be placed at point A, which reflects the interference back to the 3.3V power plane. To avoid the impact of these reflected interferences on other 3.3V devices, a dry filtering capacitor needs to be placed at point B.

Connect magnetic beads in series on the LC filtering circuit and use them to absorb power interference. As shown in the following figure:



This not only utilizes the reflection function of the LC filtering circuit, but also fully utilizes the absorption function of magnetic beads for interference such as ripple and noise.

2.If the requirements are stricter, an additional level of LDO can be added

Adding a low voltage differential linear regulator (LDO) after the output of a switching power supply or module power supply can significantly reduce output noise to meet the needs of circuits with special noise requirements. The output noise can reach μ Level V.



Due to the voltage difference (difference between input and output voltage) of LDO being only a few hundred mV, standard voltage can be output when the output of the switching power supply is slightly higher than that of LDO by a few hundred mV, and its loss is not significant.

Summary

Power ripple noise is the fluctuation caused by the periodic variation of power supply voltage in the output of a power supply. This fluctuation can affect the performance of electronic systems connected to the power supply, so it needs to be taken into consideration in power supply design.

Reasons for power ripple generation

The instability of the power supply itself: There are certain imperfections in the design and manufacturing of the power supply, such as the output voltage of the power supply may change due to load changes, temperature fluctuations, or changes in device parameters, resulting in ripple.

Switching operation of switching power supply: In switching power supply, the switching process of switching devices will introduce ripple, especially in situations with significant load changes.
Imperfection of Inductive and Capacitive Components: Inductive and capacitive components themselves have internal resistance and conductivity, which may cause ripple in the output voltage.
Fluctuations in power input voltage: If there are fluctuations in the input voltage of the power supply, it may also introduce ripple in the output.

Solution

Power filter: Use a filter, such as an LC filter, at the output end of the power supply to suppress high-frequency ripple. The combination of inductance and capacitance in the filter can reduce high-frequency noise in the output.

The use of voltage regulators: Voltage regulators can effectively suppress fluctuations in the power output. Linear regulators and switching regulators are commonly used types of regulators that can provide stable output voltage and reduce ripple.

Capacitor decoupling: Connect decoupling capacitors with appropriate capacitance values in parallel at the power output end to smooth out the output voltage. This helps to provide additional current, especially during load transients.

Choose a low ripple power supply: Some power supplies are designed to be more stable, and a power module with low ripple output can be chosen.

Power input filtering: Use input filters to reduce the impact of input voltage fluctuations on the output.

Good PCB layout: In circuit board design, a reasonable layout and connection method can reduce the inductance and impedance of the power circuit, which helps to reduce ripple.

Load balancing of power supply: Ensure uniform load distribution in the design to reduce the ripple generated by the power supply during load changes.
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