EMC tutorials - Magnetic field shielding

Summary

This video delves into the nuances of electronic circuit shielding, focusing on magnetic field protection. It explores two primary shielding methods: magnetic flux deviation using permeable materials like ferrite, and magnetic field repulsion via eddy currents in conductive materials. The presenter conducts experiments to measure shielding effectiveness, emphasizing the importance of material properties like permeability and conductivity. The discussion highlights how these properties influence shielding at different frequencies, providing practical insights for designing effective magnetic shields.

Takeaways

• 🌐 Magnetic shielding is crucial for protecting electronic circuits from magnetic fields, especially in the near field where magnetic fields can have a more significant impact than electric fields.
• 🛡️ There are two primary methods for magnetic field shielding: magnetic flux deviation using magnetically permeable materials, and magnetic field repulsion using conductive materials that induce eddy currents.
• 🧲 Magnetically permeable materials, like ferrite, draw magnetic field lines through themselves, thereby reducing the field in the space behind the shield.
• 🔩 The effectiveness of magnetic shielding materials is influenced by their thickness and magnetic permeability, with thicker materials generally providing better shielding.
• 💧 Eddy currents, induced in conductive materials by varying magnetic fields, generate their own magnetic fields that oppose and can cancel out the original magnetic field.
• 🔌 The conductivity and permeability of conductive materials are key factors in their ability to shield against magnetic fields, with higher conductivity leading to stronger induced currents and better shielding.
• 📊 Shielding effectiveness can be calculated using the skin depth of a material, which is frequency dependent and affects how deep currents penetrate the material, influencing the shielding performance.
• 📉 As frequency increases, the attenuation (shielding effectiveness) also increases, but not at a constant rate due to the frequency-dependent nature of material properties like permeability.
• 📈 The permeability of materials like steel decreases with increasing frequency, which affects their shielding effectiveness at higher frequencies compared to lower frequencies.
• 🔄 The choice of shielding material and its thickness should be tailored to the specific frequency range and application, as there is no one-size-fits-all solution for magnetic shielding.

Q & A

Why is it important to consider magnetic fields separately from electromagnetic waves for shielding?

Magnetic fields should be considered separately from electromagnetic waves because their effects can differ based on distance and wavelength. In the near field, where you are close to a noise source, the impedance of the electric and magnetic fields are different, which can lead to one being more damaging than the other. Therefore, it's important to create a shield specifically for the type of noise you want to shield against.

What are the two main methods of magnetic field shielding discussed in the script?

The two main methods of magnetic field shielding discussed are magnetic flux deviation using magnetically permeable materials, and magnetic field repulsion by relying on eddy currents induced into conductive materials.

How does magnetically permeable material shield against magnetic fields?

Magnetically permeable materials shield against magnetic fields by providing a path of least resistance or highest permeability for the magnetic field lines to follow. The field lines tend to go through these materials rather than the air around them, closing back to the source. If the material is thick and permeable enough, all field lines that touch it go through it, protecting the space behind it from magnetic fields.

What is the role of eddy currents in magnetic shielding?

Eddy currents play a crucial role in magnetic shielding when using conductive materials. When a variable magnetic field hits the conductive material, it induces currents known as eddy currents. These currents generate their own magnetic field that opposes the original field, ideally leading to a cancellation of the magnetic fields on the other side of the conductive plate.

Why is the thickness of the shielding material important for its effectiveness?

The thickness of the shielding material is important because it affects how much of the magnetic field is blocked. Thicker materials provide a longer path for the magnetic field lines to follow, which can lead to a more effective shielding. Additionally, the shielding effectiveness is related to the skin depth of the material, which is frequency dependent, and every skin depth provides an attenuation of 8.7 decibels.

How does the frequency of the magnetic field affect the shielding effectiveness?

The frequency of the magnetic field affects the shielding effectiveness because the induced currents, which are responsible for creating the opposing magnetic field, are frequency dependent. Higher frequencies can induce stronger currents, leading to better shielding. However, the permeability of the material, which is crucial for the generation of the opposing field, can decrease with increasing frequency, affecting the overall shielding performance.

What is skin depth and how does it relate to magnetic shielding?

Skin depth is a parameter that considers both electrical conductivity and magnetic permeability to calculate how deep currents run through a material. In the context of magnetic shielding, every skin depth of a material provides an attenuation of 8.7 decibels, meaning the thicker the material, the more skin depths it has, and the better the shielding it provides.

How does the permeability of a material affect its shielding effectiveness?

The permeability of a material affects its shielding effectiveness because it determines how strongly the induced current can generate a magnetic field to oppose the original field. Higher permeability allows for stronger opposing fields, leading to better shielding. However, permeability is not constant and decreases with increasing frequency, which can impact the shielding effectiveness at higher frequencies.

What is the significance of the frequency-dependent behavior of permeability in shielding materials?

The frequency-dependent behavior of permeability in shielding materials is significant because it influences the material's effectiveness at different frequencies. At lower frequencies, materials with high permeability like steel can provide better shielding. However, as frequency increases, the permeability of these materials decreases, and the most conductive materials become more effective for shielding.

What practical considerations are there for choosing a shielding material based on the script's discussion?

Practical considerations for choosing a shielding material include the frequency range of the magnetic fields to be shielded against, the permeability and conductivity of the material, the thickness of the material, and the specific application or environment in which the shielding is needed. For example, for audio range frequencies, high permeability materials like steel might be necessary, while for higher frequencies, thinner conductive materials or even conductive paint might suffice.

Outlines

00:00

🛡️ Magnetic Shielding Principles

The video begins with an introduction to electronic circuit shielding, specifically focusing on magnetic field shielding. The presenter discusses the importance of designing effective magnetic shields by understanding the principles of how shields can protect against magnetic fields. The video aims to explore the properties of materials that make them more or less effective at shielding. Two main methods of magnetic field shielding are introduced: magnetic flux deviation using magnetically permeable materials and magnetic field repulsion via eddy currents in conductive materials. The presenter also touches on why magnetic fields should be treated separately from electromagnetic waves, especially in the context of near-field effects where the impedance of electric and magnetic fields can differ significantly.

05:00

🧲 Testing Magnetic Shielding Materials

The presenter sets up a test to evaluate the effectiveness of magnetic shielding materials using two inductors on a plastic frame. One inductor is connected to a power amplifier and signal generator to simulate a noise source, while the other represents a sensitive circuit that needs protection. The initial setup shows voltage being induced in the second inductor due to the magnetic field from the first. The first method tested is the use of magnetically permeable materials, such as ferrite sheets, which are shown to reduce the induced voltage significantly when placed between the coils. The thickness of the material is highlighted as a critical factor in its shielding effectiveness. The video then moves on to discuss the second method, using electrically conductive materials like steel or copper, which rely on induced eddy currents to generate a magnetic field that opposes the noise source's field, potentially canceling it out.

10:02

🔌 Eddy Currents and Shielding Effectiveness

The video delves into the properties that affect the shielding effectiveness of conductive materials: conductivity and permeability. It explains that the induced eddy currents create a magnetic field that opposes the noise source's field, with the effectiveness depending on the strength of these opposing fields. The presenter tests steel and copper sheets, noting that steel provides better shielding at 1 kHz due to its higher permeability, while copper shows minimal effect. However, at 10 kHz, both materials improve shielding, with the effectiveness increasing due to the frequency-dependent nature of induced currents. The concept of skin depth is introduced to explain how the depth to which currents penetrate a material affects shielding, with each skin depth providing an attenuation of 8.7 decibels.

15:04

📊 Frequency Response of Shielding Materials

The final part of the video examines the frequency response of shielding materials using a spectrum analyzer. The presenter normalizes the response and then inserts various materials to observe their effect on the signal. Ferrite sheets show a flat attenuation response across frequencies due to their constant permeability. Copper, on the other hand, demonstrates increasing attenuation with frequency, indicating its effectiveness improves at higher frequencies. Steel initially provides higher attenuation than copper but shows a non-constant increase, highlighting the frequency-dependent nature of its permeability. The video concludes by emphasizing that the choice of material and its thickness is crucial for shielding performance, which varies with the frequency range of interest. The presenter suggests that high permeability materials like steel are suitable for audio frequencies, while at higher frequencies, even thin layers of copper or conductive paint might suffice.

👋 Conclusion and Engagement

In the conclusion, the presenter summarizes the key points discussed in the video, emphasizing that there is no one-size-fits-all solution for magnetic shielding. The choice of material and its application depend on the specific use case and frequency range. The video ends with a call to action for viewers to share their thoughts in the comments and to subscribe for updates on future videos.

Keywords

💡Magnetic Shielding

Magnetic shielding refers to the process of reducing the effect of a magnetic field in a specific area. In the context of the video, it is crucial for protecting sensitive electronic circuits from interference caused by magnetic fields. The script discusses two primary methods for magnetic field shielding: magnetic flux deviation and magnetic field repulsion, both aiming to mitigate the impact of magnetic noise on circuits, such as those near power transformers or switch-mode power supplies.

💡Magnetic Flux Deviation

Magnetic flux deviation is a shielding method that utilizes magnetically permeable materials to alter the path of magnetic field lines, thereby reducing their impact on sensitive areas. The video explains that magnetic field lines naturally follow the path of least resistance, and by introducing a highly permeable material, these lines can be directed away from the protected area. An example given is the use of ferrite sheets, which, when placed between two inductors, significantly reduce the induced voltage in the second inductor, demonstrating effective shielding.

💡Eddy Currents

Eddy currents are circulating currents induced within conductive materials by a changing magnetic field. In the video, it is mentioned that when a conductive material is exposed to a variable magnetic field, it generates eddy currents. These currents, in turn, produce their own magnetic fields that oppose the original field, leading to a shielding effect. The effectiveness of this method depends on the material's conductivity and permeability, as illustrated by the testing of steel and copper sheets in the script.

💡Electrical Conductivity

Electrical conductivity is a measure of a material's ability to conduct an electric current. The video emphasizes the importance of conductivity in the context of magnetic shielding, particularly when using conductive materials to generate eddy currents. Higher conductivity allows for stronger currents, which can lead to more effective shielding. The script contrasts steel and copper, noting that while steel has high permeability, copper's superior conductivity makes it more effective at higher frequencies.

💡Magnetic Permeability

Magnetic permeability is a material's ability to support the formation of magnetic fields within itself. The video discusses how permeability affects the path of magnetic field lines and the effectiveness of magnetic shielding. Materials with high permeability, like ferrite, can redirect field lines, providing a shielding effect. However, the video also notes that permeability is not constant and decreases with increasing frequency, affecting the choice of material for shielding at different frequency ranges.

💡Skin Depth

Skin depth is a concept that describes the depth to which electromagnetic fields penetrate a conductive material. The video explains that skin depth is frequency-dependent and affects the shielding effectiveness of materials. It is used to calculate the attenuation provided by a material, with each skin depth corresponding to an attenuation of 8.7 decibels. The video uses skin depth to compare the shielding effectiveness of different materials at various frequencies.

💡Signal Generator

A signal generator is a device that produces electronic signals of a specific frequency, amplitude, and waveform. In the video, a signal generator is used to simulate a noise source, such as a power transformer, to test the effectiveness of magnetic shielding materials. By connecting the generator to one inductor and observing the induced voltage in a second inductor, the script demonstrates how shielding materials can reduce the impact of the simulated noise.

💡Oscilloscope

An oscilloscope is an electronic test instrument that graphically displays signals, allowing for the analysis of their behavior over time. In the video, an oscilloscope is used to measure the voltage induced in the second inductor, which represents a sensitive circuit that needs protection from magnetic noise. The oscilloscope provides a visual representation of the shielding effectiveness by showing the voltage changes before and after the introduction of shielding materials.

💡Frequency Dependence

Frequency dependence refers to the variation in a property or effect based on the frequency of an applied signal. The video script highlights that both the effectiveness of magnetic shielding and the permeability of materials are frequency-dependent. For instance, the shielding provided by conductive materials improves with increasing frequency due to the stronger eddy currents induced, while the permeability of materials like steel decreases, affecting their shielding effectiveness at high frequencies.

💡Attenuation

Attenuation is the reduction of signal strength, in this case, the reduction of magnetic field strength as it passes through a shielding material. The video discusses how the shielding effectiveness of materials can be quantified by measuring the attenuation they provide. Attenuation is expressed in decibels and is influenced by factors such as the material's conductivity, permeability, and thickness, as well as the frequency of the magnetic field.

Highlights

Exploring electronic circuit shielding with a focus on magnetic field shielding.

Discussing the principles of shielding against magnetic fields and the properties of materials involved.

Testing two main methods of magnetic field shielding: magnetic flux deviation and magnetic field repulsion.

Examining why magnetic fields should be treated separately from electromagnetic waves due to differences in impedance.

The importance of shielding against magnetic fields emitted by power transformers and switch mode power supplies.

Introducing a test setup using two inductors to assess the effectiveness of magnetic shielding materials.

Demonstrating the use of magnetically permeable materials to shield against magnetic fields by diverting field lines.

Measuring the shielding effectiveness of ferrite sheet material and its impact on induced voltage.

Exploring the relationship between material thickness and shielding effectiveness.

Discussing the role of eddy currents in shielding using conductive materials like steel or copper.

Comparing the shielding performance of steel and copper at different frequencies.

Analyzing how skin depth, a parameter considering conductivity and permeability, affects shielding effectiveness.

Observing the frequency-dependent behavior of shielding materials and how it influences their performance.

Highlighting that magnetic permeability is not constant and decreases with increasing frequency.

Providing practical advice on material selection for shielding based on frequency range and application.

Concluding that there is no universal optimum solution for shielding and it depends on specific use cases.

Transcripts

hello and welcome back

so today i want to look at electronic

circuit shielding

and in particular look at how a good

magnetic shield needs to be

designed that of course means looking at

the principles behind how a shield can

shield magnetic fields

so what makes it better or worse at this

by looking at what are the important

properties of the materials

involved so for this i will be testing

and measuring

the two main methods of magnetic field

shielding

so on the one side you have magnetic

flux deviation

using magnetically permeable materials

and

on the other side you have magnetic

field repulsion

by relying on eddy currents induced into

conductive materials

so if you're curious about how this all

works and well

if it works then keep watching

so first of all why should we care about

magnetic fields and why should these be

treated separately

from an electromagnetic wave well the

main issue

involves distance and wavelength so when

you're close to a noise source

so you're in the near field the

impedance of the electric field and the

magnetic field

are different so these two can have

different effects on the circuit

in the sense that one of them can be

more damaging than the other

so it makes sense to create a shield

specifically for the type of noise that

you want to shield against

so for example with a power transformer

working at 50 or 60 hertz

circuitry that is close to it will be

mostly affected by

the magnetic fields emitted by the

transformer you will have almost

negligible amounts of

electric fields being emitted in a

similar way

in modern day switch mode power supplies

the main type of noise

coming from the power inductor or power

transformer will still be

magnetic fields so to keep sensitive

circuitry around these power sources

safe you will need to design a shield

specifically to shield against the

magnetic fields being emitted by

the noise sources so what can be done

about these noise sources how can we

shield against magnetic fields well

before looking into that

let's first look at a test setup that

can assess the effectiveness of

the magnetic shielding material so

the test setup that i will be working

with today involves the usage of two

inductors

placed on this plastic frame and now one

of the inductors is connected to the

power amplifier which is connected to

the signal generator

so this will be working like our noise

source so the noisy circuit or

transformer or whatever and the second

inductor

is connected to the oscilloscope's first

channel and to a termination resistor

so the second inductor represents our

sensitive circuit

which we need to protect from our noise

source so by default when the signal

generator is activated we can see that

there's

a voltage being induced into the second

coil which is displayed on the

oscilloscope so what i will be trying to

do

is add various materials in between the

two coils to work as

a shield to prevent voltage being

induced into the

second inductor so the first shielding

method to look at

is the use of magnetically permeable

materials

so what i drew here is first of all a

noise source

with magnetic field lines closing around

it through

free space so normally you will get

these arcs

going from one side of the noise source

back down to the other

so these field lines will always have to

close through some place

now as with anything in life magnetic

field lines

tend to go through the path of least

resistance

or in this particular case the path of

highest permeability

so one way of expressing how easily

magnetic field lines pass through a

medium

is by its magnetic permeability air

and vacuum and so on have a permeability

of

one but other materials like ferrite or

iron or new metal or whatever

have higher permeabilities

so magnetic field lines will tend to go

through these materials

rather than through the air around so

the way this sort of shielding works

is that it provides a path for the field

lines

other than the air around it through

which these can close back to

the source so in an ideal case

if the material is thick enough and

permeable enough

all the fill lines that touch it go

through it and then close back

so no field lines will go over this

material

so by adding a shield made from this

kind of permeable material

the space behind it will be protected

from the magnetic fields

so it's important to notice that the

field lines are going parallel with

the material in this case so to test

this principle out

i have some ferrite sheet so this is a

material that is commercially available

it's not very cheap but it's available

and

this does not have any sort of

electrical conductivity

so it only has magnetic permeability and

it has about

a value of a hundred so right now if i

insert this into

my inductors so we can see that at the

moment we have

872 millivolts of signal being induced

if i insert this into the two coils

we can see that our induced voltage

decreases to about 408

so we basically have the signal level

that gets into the second inductor

now other than the permeability of the

material

what we are interested in is how thick

the material is

so at the moment i used four pieces of

0.05 millimeter thickness ferrite sheet

but if for example i remove two of the

sheets

so i'm left with only half and we retry

our experiment

we have our initial 856

and we now only drop down to

520 so the thicker the material was

the better the shielding effect now the

other main method of shielding

involves the use of electrically

conductive materials

so things like steel or copper and

the way these work as magnetic shield

is that the field coming from our noise

source

hits the material and if this is a

variable magnetic field so

this will not work at dc it will only

work with oscillating magnetic fields

these fields will induce a current into

the conductive material

so this is what's called an eddy current

and now this current passing through the

conductive material

will generate its own magnetic field so

this one in blue

and the important thing to observe here

is that the two magnetic fields

are going in opposite directions so

on the other side of this conductive

plate we have two magnetic fields

going in opposing directions so in an

ideal world

the two will completely cancel each

other out

now in reality the amount of

shielding provided by your conductive

plate

will be determined by just how strong

the opposing fields you can create

and how close these fields will be to

the initial strength of the

noise source so now there's two

properties that we need to be interested

in

so first of all we care about the

conductivity of this material

the more conductive it is the higher

currents that can pass through it

and on the other side we are interested

in its permeability

because the more permeable the material

is

the stronger magnetic fields that our

induced current can generate

so to test this principle out i have two

sheets of conductive material one is a

bit of steel

so this was from an old soda can and the

other is the psdb

which pipe soldered onto so it doesn't

oxidize so this is roughly 35

micrometers of copper

and this is about 150 micrometers of

steel

so now if i insert this into the coil so

let's start with

the steel we can see that we have

substantial decrease down to about 400

millivolts

and if we insert the copper

we have a decrease so we went from 800

something to 700 something

but it's quite a small decrease so the

sheet of copper is not

that useful at this particular frequency

so right now i'm testing it one

kilohertz

but one of the important things to

observe is how these two materials

behave

if we change the test frequency so with

the exact same setup

i just changed my test frequency to 10

kilohertz and now we have a default

value

when no material is inserted of 888

millivolts being induced into the second

inductor and now if we insert the two

shielding materials so first of all the

iron

we see that we go down to 264 millivolts

and if we try out the copper plate

we go down to 648

so in both cases we're getting better

shielding

at 10 kilohertz than we did at 1

kilohertz

and the main reason for obtaining a

difference here

is that the induced currents the value

of the induced currents

is frequency dependent the higher the

frequency the stronger currents you can

induce

and therefore a better shielding effect

you can get for the same thickness of

material

now to assess just how much shielding

you need the effectiveness of a material

can be calculated based on its skin

depth so this is a parameter that takes

into account both

electrical conductivity and magnetic

permeability

to calculate how deep currents run

through a material

and now from a shielding point of view

where currents are induced from external

magnetic fields

every skin depth of a material will have

an attenuation of 8.7 decibels

so the thicker the material is the more

skin that you have

the better shielding you will be getting

and now of course skin depth is

frequency dependent so we can see that

with increasing frequency

we get better shielding now skin depth

of various materials

is usually characterized in graphs so

you can find this sort of graphs in

various places but this particular graph

is special so i'll be leaving a link to

this in the description of course

because it shows one of the frequency

dependent properties that's not always

shown in

these characterizations and that is

that magnetic permeability is not a

constant

so what we can observe here is that the

material that will achieve the greatest

amount of

shielding for the thinnest amount of

material is steel

because even though it doesn't have

great electrical conductivity

it has a lot of permeability so it's

better than copper for example

but at high frequencies things change a

bit

and the main reason for this is that

although steel has permeability it will

go away at high frequencies so the

permeability is not a constant

it's frequency dependent and it drops

with increasing frequency

so at very high frequencies if you no

longer have

permeability then the best material will

be the most conductive material

and to show of this effect i prepared

the same setup as before so i have my

two coils

and rather than connect them to the

signal generator and oscilloscope i

connected them to the spectrum analyzer

with the tracking generator output

point being that i want to look at the

behavior of the shielding materials over

a wider frequency range so i set the

device to sweep from nine kilohertz

up to one megahertz and now first step

was to normalize the response so we have

a flat response

now by inserting the various materials

we can see how they affect

the signal coming into the second

inductor so first things first

let's start with the ferrite sheet so

all four pieces of these

so if i insert this into the coil

well we can see that we went from -6

decibels down to about -12

and we see that the response is almost

perfectly flat

and this is exactly what we would expect

the permeability of

the ferret sheet is fairly constant so

the attenuation is constant

only at very high frequencies will this

permeability start to drop

and then the attenuation should also

drop but other than that it should be

fairly flat now if we go to

another material go to our copper pcb

we now see our clear frequency dependent

behavior

so as frequency increases so does

attenuation

we start from about minus six decibels

of attenuation so minus six to minus 12

is minus six

and we end up at minus 36 so a total of

30

decibels at one megahertz and we see

that our attenuation

increases at a very constant rate so

there's no bumps in the attenuation

curve it's

a flat line now if we turn to our last

material

our sheet of steel we see that we have

much higher attenuation than with our

copper sheet

that's because the steel is much thicker

but it also

it's far more permeable but another

thing that we notice

is that shielding effectiveness doesn't

constantly

increase it has these bumps so

shielding effectiveness does increase

with frequency but not at a constant

rate

and the main thing that we are seeing

here is that the permeability

of steel is not a constant it

decreases with increasing frequency and

by the time you reach

1 megahertz its permeability is almost

completely gone

so i looked at this in an older video

and there's also proper research done on

the topic

where you can see that the permeability

of iron

decreases with increasing frequency so

it's

definitely not a constant and this

permeability

is highly dependent on the grade of

material that you have

so different materials different

compositions will have

different permeability and this

permeability will behave differently

with frequency

so in the end depending on what you're

trying to do

both the material and its thickness will

have a massive impact on

your shielding performance for example

if you are working in

the audio range then you will need high

permeability materials

so at least steel or if the budget

permits more exotic materials

whereas if you're working in the tens or

hundreds of megahertz range

even a few tens of micrometers of copper

might be

enough or better yet you might just get

away with using

conductive paint so you might not even

need a proper

shielding enclosure but it all depends

on

your particular use case there's no

universal

optimum solution and with that said

hope you got some useful information

after this leave your thoughts in the

comments thank you for watching make

sure to subscribe to be updated on my

videos

and see you next time bye bye