What are feed through capacitors?

Feed through capacitors are used in DC powered RF systems. The systems might high side RF frequencies to be dealt with which might affect the DC supplies. The purpose of this feed through capacitors is to eliminate the RF frequencies affect on DC supply. Feed through capacitors act as local power banks in many cases and help to localize the power supply. We see many connectors with feed through capacitors inbuilt. This helps for the system to pass critical EMI/EMC certifications that are desired for the system. These feed through capacitors are similar to normal capacitors in which they have ESR. The below diagram shows the representation of feed through capacitor.

We need to remember that feed through capacitors are 3-terminal components unlike the normal capacitors which have 2 terminals. In feed through capacitors one terminal is connected to input and other to output. The other terminal is connected to ground. 

We all that a capacitor at a self-resonant frequency helps ground all the foreign signals that enter into the board at that resonant frequency. Feed through capacitor is nothing different from that. We all know that there are several ceramic capacitors available for this purpose but their noise rejection band-width is limited compared to feed through capacitors. So, a feed through capacitor is a crucial component in EMI/EMC certification. 

The main advantage of the feed through capacitors that help in filtering is that they have low parallel inductance and high series inductance. This helps in better noise rejection and capacitor performance. Based on the characteristic curves of feed through capacitors, some say that their performance is similar to low pass filter. Feed through capacitors are available in different form factors starting from a simple SMD type to the bulky ones which are fit onto metal cases.

The most important parameter that helps feed through capacitors perform these desired functionalities is their construction. A resin material and a dielectric material powder combination provides this rugged performance of these capacitors. Some of the current rating of feed through capacitors can go beyond 200A as well.

Example Feed through capacitors available in the market:

1. NFM18PS105R0J3D - SMD type feed through capacitor from Murata

2. NFE61PT472C1H9L - Block type feed through capacitor from Murata

Applications where commonly see these type of capacitors used:

1. Automotive
2. Telephone exchanges
3. Base stations
4. Power supplies
5. Electrical machines
6. Power feed through - For high power applications
7. Instrumentation feed through - For ADC related low current applications
8. RF feed through - For high frequency RF applications