How to select a good Ferrite Bead

Question

I am building a USB project and I want to use a Ferrite Bead in the USB 5V rail. I am looking for 1206 Ferrite Beads. How to select them properly? Which impedance at 100MHz is a good one?

Answer 1

Ferrite beads are used as components of a low pass filter to limit the amount of high frequency energy that comes out of a circuit or chassis enclosure. For DC to low frequencies you want to select a ferrite that has a current carrying capacity equal to or greater than the load current you expect to pass through it. As part of that selection select also for a DC resistance that is small enough that the voltage drop across that resistance due to the passing load current does not drag down the output voltage beyond a level you will find acceptable.

For the high frequency part of things you need to consider how the cables that exit your system look in terms of their characteristic impedance or termination impedance. This will be the impedance that any emitted RF current coming from your unit will develop a voltage drop across and create interference signals. When you place a ferrite bead in the signal path where it comes out of the unit its impedance at the emitted RF frequency forms an voltage divider with the external cable or termination impedance. You will want to determine what amount of attenuation you need in the emitted RF signal in order to keep the external interference signals down to where they have to be (legally and/or functionally required). Select the ferrite impedance so that the voltage divider effect I described gives you the amount of attenuation required.

It should be obvious now why it can be so useful to keep the external cable and/or termination impedance as low as possible. It it gets too high the ferrite bead impedance has to get extremely high at the RF frequency in question to achieve significant attenuation.

The need for RF attenuation on signals exiting from a product enclosure may be needed even if not expected. If your device has an oscillator, a microcontroller or other other logic with fast rise / fall times then you will have potential of RF generated inside the product. This could even be from high frequency switching power supply circuit.

The agency approval needed for ability to sell your product in any particular country or region will specify the legal emission levels that are allowed. Since I do not know your product, how it is designed or what the local legal requirements are I would have a hard time to recommend a particular filter for you. My hope was that my answer would give you some ideas of how you can evaluate for yourself.

Many times engineers or circuit designers are looking for easy methods to determine an answer to the question "Will my product pass certification"? The only way to make sure is to have it tested. Do be aware that even at a certified testing site there can be some variance on what is seen and observed for any particular test run that is repeated at another time with the same equipment. For this reason it is the best strategy to have the emissions limits for your product be well below the legal limits to allow for variance to still be acceptable.

It is possible to use an air core coil connected to a scope probe as a sniffer probe in and around your product and circuit board to get some idea of what you may be up against. Keep in mind that such measurements are mostly useful only as a relative comparison between an initial condition and a later time after you have tried some change. More on this below.

For folks that are relatively new to the requirements for agency testing and certification I recommend using the services of a special lab from the outset. (Note that a lab is generally required anyway because most smaller companies do not have the necessary equipment and environment to perform certification scans. Some larger companies do have the resources and equipment and will elect to self certify).

For the first timer going through a test sequence on a product at a lab can be an incredible learning experience if you go with your product and work side by side with the engineers at the lab. You will learn a lot about what the actual requirements for your product classification are and how the testing is performed. If there are issues you may also learn about some of the mitigation strategies that are used. With that experience under your belt you'll be able to setup some simple tools that you can use in your lab to check your product in a relative way to see if you have hot spots and emissions leakages. These tools allow you to make relative comparisons and not the absolute ones like would be required for actual certification testing and reporting.

Rather than try to recommend any specific tools I would suggest searching out some books on agency testing and the processes used.