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The Many Benefits of FeedThru Capacitors

Three-terminal capacitor filters (also called FeedThru capacitors) are being widely accepted in designs ranging from satellites and cell phones to automobiles and SMART grid controllers. The list of viable applications is growing rapidly as these capacitors evolve into smaller packages that can handle more current at higher temperatures.

FeedThru capacitor filters appeal to design engineers not only because of their small size and light weight, but also because of their high reliability, simplicity of implementation, and low cost. The filter can be a single three-terminal device or a four-element array, 10-terminal one. These miniature devices act as broadband filters and allow designers an exceptionally wide filter response across frequency spectrums — typically 300MHz wide. FeedThru capacitors can be chosen for filtering frequencies as low as ~75MHz or as high as many gigahertz.

FeedThru capacitor filters can replace the various value capacitors placed in parallel on Vcc lines. Designers sometimes use multiple capacitors with different values that combine frequency responses to create a broadband filtering effect. The problem with this approach is that the effective parallel inductance of the various capacitors is still quite high relative to the parallel inductance of FeedThru capacitor filters. Also, series inductances resulting from the additional traces and pads tend to affect the combined frequency of all the capacitors.

It is not uncommon, as part of a redesign, to replace at least two of the parallel capacitors (possibly three, depending on the frequency range) with a FeedThru capacitor filter. Replacing three parts with one improves reliability; simplifies layout and routing; decreases weight, size, and assembly cost; and can increase electrical efficiency.

It is also possible to replace some discrete LC T filters with a single FeedThru. In this case, the capacitor still offers the previously stated physical and electrical advantages, along with a frequency response that tends to be more repeatable than LC T filters and quite stable over temperature.

Recently, AEC Q200-qualified FeedThru capacitors were introduced to address strict electromagenetic interference concerns in drive-by-wire automotive systems. As auto use expands, so will the need for 150°C-rated FeedThrus. Additionally, high-reliability FeedThru capacitors are used in a variety of spacecraft, because of the dramatic size and weight reduction they provide on a systems level.

FeedThru capacitor filters are a simple, low-cost way to provide broadband filtering with a single component. The trend is for larger values of parallel capacitance to be integrated to make FeedThru capacitors more applicable to low-frequency filtering. The current rating of FeedThru capacitor filters is also increasing, both within each case size and with the introduction of larger case sizes.

4 comments on “The Many Benefits of FeedThru Capacitors

  1. Houngbo_Hospice
    August 9, 2011

    Hi Ron,

    Thank you for this blog as it reminds me many things about my Electronics Engineering courses. 

    FeedThru capicitors filters seem to be well suited to high frequency applications than low frequency applications and there is the need to add other capacitors in order to use them for low frequency applications. My question is: Do low frequency FeedThru and high frequency FeedThru cost the same thing? I suspect that low frequency FeedThru might be more expensive. Am I right?

  2. Clairvoyant
    August 9, 2011

    I have had experience with feed-thru capacitors myself and have seen their advantages. They are very handy is providing filtering for signals that are feeding into an RF isolated section of a product.

  3. FLYINGSCOT
    August 10, 2011

    Great article. I agree there are many benefits of feedthru caps.  In many applications they offer the best compromise between filter performance, board space and cost.  Just make sure to understand the electrical specs of the device chosen to make sure it can deliver what is expected in terms of system performance.  If it does not there are many other types of in-line filters (including active solutions) that might do the job.

  4. Barbara Jorgensen
    August 12, 2011

    Ron is traveling, but was kind enough to ask me to post his reponse below:

     

    If you want to create a low frequency feedthru a ferrite material is sometimes formed around the feedthru  input & output lines. This is expensive and does add cost. Traditionally this has been the method to get a low frequency response.  


    Another way to accomplish this – a new metod – (at least to some extent) is to use internal electrodes with maximized inductance. The limitation to that approach is that it can not get to the very low frequencies – its probably got a practical limitation of 20 mhz today but I could see that extending to 10 mhz in the fairly near future. This method essentially is a balance beem – we could get to < 10 mhz but feedthru current gets limited so my bet is there will be series of different feedthru current capable parts with different low frequency response cut offs.  

     

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