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Power-Supply Common Mode Noise
Common-mode noise current is returned to the source through a different path from the normal signal or power path, with chassis or earth ground and adjacent conductors as examples.
 

Noise in an electronic circuit or system consists of random or undesired fluctuations in the electrical signals or voltage source. Noise is often conducted through interconnect cables and conducting metal parts such as brackets, shields, and the chassis. Radiated noise is a form of electromagnetic (EM) interference transmitted through the air by cables and components carrying AC voltages or currents. The radiated coupling can be very local, for example, between a transformer and a nearby wire or printed-circuit-board (PCB) trace, and become conducted noise. Industrial and medical instruments typically operate in noisy environments and are prone to interference from common-mode noise as a result of lack of knowledge or understanding of the inference mechanisms.

Common-mode noise currents often follow a large loop area which then radiate to the environment, adding to the system's EM emissions. This type of noise can also lead to spurious conducted signals within a system which can cause communications errors and malfunctions due to signal disturbances. Sensitive measurement devices can malfunction or misinterpret the noise as data, resulting in erroneous data. AC line transients, such as line surges due to lightning strikes or power switching from motor controls, circuit breakers, or relays, can cause both differential and common-mode disturbances on the AC mains that propagate through the power supply to circuit or system electronic components or is coupled across conductors, resulting in malfunctions or damage to the electronic circuits and systems.

Normal-Mode Noise
Differential-mode noise, also known as normal-mode noise, results from AC voltage disturbances across signal or power lines or current through them. As might be expected when reviewing a wiring diagram or schematic diagram, the noise follows the signal and power paths. Common-mode noise is the AC disturbance from one or more signals or power lines and an external conduction path, such as an earth ground or chassis or other conductive material not intended to conduct power or signal energy. The source of noise may be from the AC mains, the power supply, or even the electronic circuitry being powered. The effects of parasitic impedance are often not obvious, but understanding these effects is essential minimizing them. The impedances along an electronic design's common path may stem from components added to help filter noise.

Common-mode output noise is often overlooked and not specified. Most attention to noise analysis is given to the input electromagnetic-interference (EMI) filter and output differential noise filtering. Yet, there is significant common-mode output noise present due to conductive and radiated coupling to the output. In most applications, this is not an issue since the output is grounded either internally at the power supply or at the end application or significant capacitance to ground is added on the output to earth.

Noise Sources
A typical power supply consists of an AC/DC rectification stage followed by a high-frequency DC/DC stage and control circuitry to regulate the output voltage. Noise from the power supply mainly originates from the switching power semiconductors. Switch-mode power supplies are much more efficient, smaller, and more economical than the linear power supplies they have largely displaced during the past few decades. Power-supply designers have made improvements in reducing noise generated in the power supply from leaking to connected or nearby equipment. However, limiting power-supply noise is still a challenge and common-mode noise is often overlooked, partially due to the lack of specifications defining a requirement for output common mode noise.
These are the noise sources and noise paths possible in a typical switch-mode power supply.
 


The nature of switching power supplies is that these circuits employ high differential voltage (dv/dt) and differential current (di/dt) functions to achieve the high efficiency, reduced size, and cost. With parasitic capacitance as part of a power supply, due to the nature of circuit materials and EM behavior, most power-supply designs contain a natural high-harmonic noise source heavily filtered within the supply, although not perfectly contained within the boundaries of the power supply.

By using a simplified block diagram for a switch-mode power supply, only a few parasitic coupling paths are shown. Depending on the power conversion topology used, the dv/dt and di/dt functions in the power supply can vary greatly. Although filtering reduces the amount of noise present at the output terminals, the amount of noise that conducts through the output cables depends on the load and its impedances, both from differential and common-mode perspectives.

A better understanding of how to control power-supply noise is by studying the voltage between the output power or signals leads, and the chassis or system ground. If the output is shorted to chassis or ground at the power supply, the common-mode voltage is eliminated at that point. Depending on how the circuit or system is configured, common-mode noise could be generated at the system and coupled back to the power-supply ground point. This forms a loop which can also be a source of conducted and radiated EMI and a path for noise currents that can interfere with system performance.

By exploring the source of power-supply noise, it is easier to define requirements for its control and to minimize the noise's impact on a design. Noise can be low-frequency, 50/60Hz line frequency coupled noise, for example, or at the switching frequency of the power supply (typically in the 50-300kHz range). Noise can also exist at high frequency, such as the switching transition of the power-supply active devices, which can occur in the megahertz range.

Understanding a circuit or system's sensitivity to different types of noise can help determine acceptable solutions. For instrumentation systems monitoring low-frequency signals in the low Hz to a few 100Hz and a floating power source, AC mains noise can be a significant problem in high-impedance applications. The relatively small capacitive coupling between the AC mains and the output can be enough to generate 10s of volts of common-mode noise.

In high-impedance applications, where there is little to no capacitance to ground, a small ≈10pF capacitance from an AC input to an output forms a capacitive divider that generates almost 10V rms at 265VAC. This is assuming a capacitance of 270pF from output to chassis/earth ground. While these are very high impedances for most applications, in sensitive high-impedance systems, this can be a challenge. Understanding the noise content and impedance of the noise source will provide good insight into ways to mitigate the impact of the noise.

When making measurements, it is important to be aware of the potential impact a measurement method may have on the results. One measurement method may result in higher readings, due to long leads and loops for high frequencies, for example than another method. In contrast, loading down the noise source because of the probe impedance can lead to lower readings. A choice of measurement method should consider the noise frequencies that are causing a problem and other noise frequencies that are present but only masking the real noise issue.

When power-supply noise includes low- and high-frequency components, it can be advantageous to filter out different frequency ranges to better understand the effects of the different noise signals on circuit or system performance. An investigation of the noise frequencies of interest can be performed with a simple lowpass and highpass filter, which can be made using an RC network. However, for systems with a high-impedance (>1M&937;) requirement between the power supply and the ground, a high-impedance active voltage probe will be needed for studying the different noise figures, to avoid loading down the measurement point.

For a better understanding of the noise source impedance, and what will be needed to filter it, it can be helpful to measure a design's open-circuit voltage and short-circuit current. The open-circuit voltage can be measured from the output return lead (or from the other polarity) and the system or earth ground point. This can be done with an oscilloscope, provided that the test lead is kept suitable short when measuring at supply frequencies of a few Megahertz or higher. This measurement can also be performed with a spectrum analyzer, which can provide more details on the frequencies of the noise. This can be helpful when selecting the proper component values required if additional filtering is needed. It should be remembered that the input port of a spectrum analyzer is usually 50{0} (sometimes 75{0}) which could result in loading down the noise source. Monitoring also with an oscilloscope can reveal how much the noise source is being loaded.

Curbing Common-Mode Noise
Once the noise frequency spectrum, source impedance, and noise frequencies are known, it is possible to make informed decisions how ways to mitigate the noise:

  • Ferrite material, clamp on cores, inductors, and toroids are quite effective at reducing offending currents if the proper material is selected. A number of manufacturers provide ferrite materials, some with excellent characterization data showing impedance verse frequency.
  • For high-frequency noise from 10 to 20MHz and higher, selecting a material with a high resistivity characteristics is often more effective than using materials with reactive characteristics. The resistive element helps dampen and dissipate the energy, while reactive parts will present impedance, but may contribute to resonances depending on the capacitance of the network. However, at lower frequencies, having a higher reactive element may be better since the energy involved could be significant and just providing a high impedance will reduce the current flow and the common-mode noise.
  • Capacitors should be selected with knowledge of their tolerance variations over temperature and applied voltage. Such variations can be significant and result in degraded power-supply performance in terms of noise. Capacitor values should be selected to coincide with the minimum impedance at the frequency of interest.
  • Checks should be made for resonances when adding capacitances and inductances to noise filters. A resonant frequency can be calculated and, depending on the noise level and frequency, it may be useful to measure the voltage if applying the filter components results in worse performance.
  • Capacitive coupling between noisy signals should be minimized, by shielding noisy cables and using separate power and signal lines.
  • The differential voltage, dv/dt, should be minimized, which can be done by grounding the output return to chassis/earth ground near the power supply if possible, and shunting noise to the power supply chassis near the power output.
  • Noise should be decoupled from source to load by adding a common-mode inductor or ferrite cores on the power cables. Ferrite material with a high impedance should be selected for high-frequency noise.

Common-mode noise is present in many power supplies and can cause interference and other performance problems. Power supplies should be selected that provide acceptable common-mode noise performance and any noise source should be shielded from sensitive circuitry to minimize issues.

 

Contact: SL Power Electronics, 6050 King Drive, Ventura, CA 93003 800-235-5929 fax: 805-832-6135 Email:
info@slpower.com Web: http://www.SLPower.com

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