ferrite bead question

[Karamel]

Hi everyone,

I would like to use ferrite bead in my circuit. when I wrote these, I had beed researching something about ferrite bead but I didn't understand anything.


- Ferrite beads are using filters aren't it? (power supply filter)

- Ferrite beads have got some value. Such as, 10r, 1K, 5K. what is it meaning ?

- I would like to use it with stm32F0 microcontroller. Which value should I choose?


Best Regards,
Karamel 

[georges80]

Ferrite bead 'resistance' (actually impedance) is spec'd at a frequency and the datasheet will show curves.

The impedance increases with increasing frequency so that at DC they are essentially 0 ohms and at say 100MHz could be in the 600 ohm (just as an example) range. The idea is to 'resist' high frequency AC components from getting down the power leads to the battery/power supply etc and radiating on those wires (or I/O wires etc etc).

There's more to just sticking some beads on the leads or on pads next to the connectors etc if you want to reduce EMI. Your PCB design, your case design, how the ground paths connect to the case (metal...), will have a larger impact than some beads scattered here and there...

And no, someone can't just tell you what beads will work with 1 particular uC...

cheers,
george.

[T3sl4co1l]

I would like to use ferrite bead in my circuit.

You probably would not, actually.

Ferrite beads are rarely suitable for their claimed purposes.  The problem is they don't give much impedance (until very high frequencies, where they are mostly resistive anyway), and they saturate easily with DC bias.

In almost all cases, you should like to use an inductor instead.  The inductance is higher, the peak impedance is much higher, and it will remain normal up to rated current.  It's also a reminder that you are forming an LC resonant circuit, and need to dampen that circuit with losses: usually from capacitor ESR.

The best places to use ferrite beads are on signal lines, to provide modest attenuation (maybe < 10dB) of high frequency, interfering (RFI) signals.  This is commonly done on signal outputs for reduced emission, signal inputs for improved susceptibility, or some internal signals for filtering or parasitic purposes (occasionally, a ferrite bead is used instead of a gate resistor, for driving MOSFETs).

Tim

[leblanc]

When selecting a ferrite, you typically want to block as much of the high frequency as possible. So you usually aim for high "Impedance @ Frequency". A value of 120Ohm@100MHz basically means that the ferrite will appear as an impedance of at least 120Ohm above 100MHz.

There two other parameters that are important to consider.

Current Rating:
You need to make sure that the current that will go through the ferrite will not exceed the current rating.

DC Resistance (DCR):
You need to make sure that the DC drop across the ferrite will not be too big for your application. If your ferrite has a DCR of 500mOhm you pass 100mA, you get a 50mV drop.

A lot of the time you just need to find a good balance between low DCR and high "Impedance @ Frequency".

occasionally, a ferrite bead is used instead of a gate resistor, for driving MOSFETs

Wouldn't a ferrite cause even more problems with the MOSFET's gate capacitance?

[JohnPen]

I remember a Wireless World FM tuner design back in the late 1970s with an RF stage used a Dual Gate MOSFET.  There were a lot of noise problems generated in the front end and it was solved by placing some ferrite beads on the MOSFET's gate.  We found, using a Rhode and Swartz Spectrum display, that the MOSFET had parasitic oscillations occurring at several hundred Mhz above the FM band which didn't show on the Scopes of the day. Adding a ferrite bead killed the parasitic oscillation and solved the noise problems.  They are very useful for taking out high frequency things you cannot see!
John

[mazurov]

They are very useful for taking out high frequency things you cannot see!

Low-value non-microwave resistors will work here too, likely even better -> http://www.vishay.com/docs/28871/resistorsmicrowaveapp.pdf . They are also quite a bit cheaper.

[T3sl4co1l]

FBs are often superior for gates, because the impedance is lossy in the range where the MOSFET would oscillate (10~100MHz?), serving the same purpose as a gate resistor of larger value (= presumably safer damping).  But because it has a low DCR, and saturates during switching, it allows quite rapid switching edges (limited by the driver, or gate spreading resistance), better than a minimal value resistor, and much better than a comparable value resistor (with regards to the damping).

The waveform shows a slight time delay, corresponding to Phi_sat / Vg(on), before saturation allows it to swing.  This is usually a few nanoseconds, still faster than the RC response you get with a resistor.

One downside is, they get toasty if you're driving big gates fast.  Saturation means maximum core losses.

You don't normally use them for this purpose (or see them used), because component choice is tricky from a design level (FBs aren't rated in saturation current or flux), and such operation isn't usually needed (resistors are slow enough to save on EMC, at cost of efficiency).

Tim

[w2aew]

Have you watched my introductory video on ferrite beads?

[LukeW]

They are very useful for taking out high frequency things you cannot see!

Low-value non-microwave resistors will work here too, likely even better -> http://www.vishay.com/docs/28871/resistorsmicrowaveapp.pdf . They are also quite a bit cheaper.

That's a really interesting appnote.
On a related note, in this system they've mounted the chip resistors upside down to improve their microwave characteristics - with less mismatch, presumably, between the copper microstrip plane and the plane of the resistive film on one side of the ceramic. Similar to that tricky business of side-mounted chip capacitors.

http://wwwphy.princeton.edu/cosmology/mintweb/instrument/backend/channelizer/design/powersplit/

[free_electron]

nobody 'likes' to use ferrite beads. Ferrite beads are used for a purpose !

- Either to keep RF crap out of a sensitive system , or to block RF crap from 'leaking' out of your system and becoming an EMC problem.
- they can be used to dampen ringing caused by steep edges and large current spikes such as in charging/discharging the gate of a mosfet

So the first question you need to answer is : why do you think you need one. it may turn out there is no need at all.

for power rails leaving the board a common mode choke works way better than a single ferrite bead. Same for pools of wires entering and leaving the board. there are special multiwire ferrite beads for that purpose  (or you run the cable harness through a toroid ferrite to accomplish the same thing. )