Ferrite bead vs Bypass Capacitor

Thread Starter

selva97

Joined Nov 30, 2020
21
Why we are using ferrite bead and bypass capacitor, if both are for removing noise? Can you explain why we are using both in the circuit and difference between using them, and where we need to use and how to calculate it?
 

Deleted member 115935

Joined Dec 31, 1969
0
Two different but related tasks,

Ferrites are in series with the power
high frequencies , i.e noise is attenuated,
in both directions,
whilst the DC is only mildly attenuated by the resistance of the ferrite.

The capacitance,
is in parallel with the power pin of the chip.
its aim is to provide voltage to the chip when the input voltage dips ,
and to provide the higher current spikes

If the chip did not have the capacitor,
a spike in current, of say 100 ps,
would have to come from the PCB power supply .
the PCB has track impedance as well as the ferrite bead resistance,
so the voltage would drop.
 

DickCappels

Joined Aug 21, 2008
7,985
The ferrite beads I’ve used were around a length of wire and therefore had negligible D.C. losses.

There is not much to calculate. The preferred procedure is to consult the bead manufacturer’s impedance as a function of frequency charts, chose a bead and try it. Repeat if necessary.
 

atferrari

Joined Jan 6, 2004
4,474
The ferrite beads I’ve used were around a length of wire and therefore had negligible D.C. losses.

There is not much to calculate. The preferred procedure is to consult the bead manufacturer’s impedance as a function of frequency charts, chose a bead and try it. Repeat if necessary.
Hola @DickCappels

I recall reading once the Amidon's catalog: incredible variety of mixes/sizes. In actual practice, how do you distinguish them if you stock becomes all mixed up?

Paint them in advance?
 

DickCappels

Joined Aug 21, 2008
7,985
There was one user of this site who was having inconsistent performance with an LED oscillator/driver and was using ferrite beads. He had bought some selected values via eBay and sent some to me to test. It turned out that each envelope had a variety of values in it rather than the values marked on the envelope. An inductance meter or network analyzer are your only hope in a case like that.

Painting first is a great idea considering how much trouble it would be to go through a pile of beads and sorting them out!
 

atferrari

Joined Jan 6, 2004
4,474
There was one user of this site who was having inconsistent performance with an LED oscillator/driver and was using ferrite beads. He had bought some selected values via eBay and sent some to me to test. It turned out that each envelope had a variety of values in it rather than the values marked on the envelope. An inductance meter or network analyzer are your only hope in a case like that.

Painting first is a great idea considering how much trouble it would be to go through a pile of beads and sorting them out!
Thanks Dick!!
 

crutschow

Joined Mar 14, 2008
28,206
There is no "vs" between them as they work together to suppress noise.

A ferrite bead appears as a lossy inductor in series with the wire to limit high frequency noise current through the wire, which acts in conjunction with a capacitor to ground to shunt away noise that gets through the bead, thus suppressing high-frequency noise voltage.

Selecting the right bead and capacitor is often a matter of testing the actual circuit with various values of both to see which best suppresses the noise you have.
 
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Deleted member 115935

Joined Dec 31, 1969
0
Thanks Dick!!

Example

https://www.digikey.co.uk/product-detail/en/würth-elektronik/742792651/732-1593-2-ND/1638997

200 mOhm at DC,
much more than a wire of the same length,

As for painting, dont , it has potential to cause you soldering / de soldering problems ,
just get them from a decent supplier, on a cut tape, and write on the tape what they are.

many others are available,

But its the DC resistance of the Power Deliver System that the parallel capacitor on the chip is aiming to reduce for the spikes.
 

DickCappels

Joined Aug 21, 2008
7,985
I don't see any wires on that chip bead.
As for painting a bead (without integrated wire) a light dusting from a spray paint can is unlikely to complicate one's life.
 

Deleted member 115935

Joined Dec 31, 1969
0
Its a ferrite bead,
used for chip power suppression,
was these not the sort of ferrites you were talking about ?
what have I miss read ?
 

BobaMosfet

Joined Jul 1, 2009
1,898
Two different but related tasks,

Ferrites are in series with the power
high frequencies , i.e noise is attenuated,
in both directions,
whilst the DC is only mildly attenuated by the resistance of the ferrite.

The capacitance,
is in parallel with the power pin of the chip.
its aim is to provide voltage to the chip when the input voltage dips ,
and to provide the higher current spikes

If the chip did not have the capacitor,
a spike in current, of say 100 ps,
would have to come from the PCB power supply .
the PCB has track impedance as well as the ferrite bead resistance,
so the voltage would drop.
capacitor's purpose in bypass mode is to provide CURRENT not voltage. It can provide current faster than the power-supply.
 

BobaMosfet

Joined Jul 1, 2009
1,898
Yes, you are right @BobaMosfet, and as we know , the reason to provide the current is to keep the voltage up,
Yes, we keep the voltage drop across the load at MAX. Without the capacitor, we couldn't deliver enough current, and the voltage would sag. I'm not arguing with you, I'm merely explaining the 'how/why' for people who don't know. People talk about voltage, and frequently forget what they really mean is voltage -drop-.
 

sparky 1

Joined Nov 3, 2018
611
As pointed out the a comparison between bypass capacitor and ferrite bead are different methods.
There is a difference in the location of the bead by function and boundary conditions typically around a wire. The bypass capacitor placed in line close to a gate but not always. The net reduction is that ferrite raises the resistance which works well with higher frequency to absorb transients.
Around sensitive circuits ferrite can be placed between two capacitors to ground which is better seen as a filter network than individual components.

It is common to omit this distinction possibly because of the comparison and contrast only mentality tends to group elements inappropriately.
Where the ferrite is added can sometimes improve noise reduction according to where and how much ferrite is needed
to accomplish that noise reduction. Sometimes we find huge ferrite snapped onto a power cable. In this case it is not being used for signals.
The compare and contrast question creates unneccessay inaccuracies rather than explain the principles that are at work.
 
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