capacitors - RFI suppression

[bernardg]

Could anyone help me to understand how the capacitors aid in the RFI suppression?

reference of notes, technical papers or websites would be very helpful.

Thanks in advance.
bernie

 

[dpc]

Capacitors have a circuit impedance that is inversely proportional to the frequency of the signal. At dc, they appear as an open circuit. As the frequency increases, the capacitor impedance goes down, allowing more current to flow through it.

So they are helpful in some instances in diverting unwanted high frequency components of a signal or power output to ground within a device to prevent this high frequency component from being radiated as RFI once it leaves the enclosure or circuit board. Sizing and physical placement are crucial to their effectiveness in this application.

 

[fuzzydiode]

This can be a very complex subject with that appears to have many contradictions because of the broad definition of "RFI" and many other not so obvious factors. But generally, capacitors can be viewed as the previous post explains. Their impedance is inversely proportional to the frequency of the voltage applied across the device. For most “RFI” applications the inductance and resistance of the circuit are also critical factors. A capacitor is usually applied as part of a filter network where it is in series with resistance or inductance. The series resistance or inductance presents a higher impendence compared to the capacitor at high input frequencies which causes the “RFI” is attenuated. The “quiet” (reduced RFI) portion of the circuit is the voltage across the capacitor. The series resistance or inductance does not have to be a physical component, many times it is wiring inductance or resistance, but the capacitor always needs the mismatch of impedances created by another circuit component that reduces power transfer at the higher frequencies. One thing to be aware of is that capacitors themselves are basically a series circuit consisting of resistance inductance and “of course” capacitance. These parasitic components limit the frequency at which the capacitor can be effective. Generally, when the input frequency increases to a point where the reactive impendence of the capacitance is equal to the reactive impendence of the inductance from the device leads and other circuit connections, the capacitor is at the series resonant point and the impedance of the part is at the minimum possible. For higher frequencies, the part/network starts to look inductive (ie impedance increases with increasing frequency). The capacitor can also have a parallel resonant point with other circuit components at which point the network looks like an extremely high impendence. These resonant points can often trouble points in power systems.

General rules of thumb for “RFI” capacitors are:
1. Place the device as close as possible to the point of interest.
2. Provide a impedance for the capacitor to work against (ie resistance/inductance)
3. Lower capacitance ESR (Effective Series Resistance) is usually good from a filtering standpoint but can cause problems in some applications like with switching power supplies.
4. Pay attention to voltage ratings and dielectric characteristics at various voltages

That is all I can think of at the moment. Henry Ott has a good book that explains this material.

http://www.hottconsultants.com/

Most capacitor manufactures also have application notes also.

Regards,
RP.

 

[BrianR]

Capacitor impedance is Xc=1/(omega*f) at low frequencies, however at relatively low frequencies the internal inductance and lead inductance of the capacitor, Xl=omega*f, starts to become significant and the capacitor impedance is actually much higher than an Xc calculation would suggest.

The aim of a capacitor in RFI suppression is to control the resonant frequency of various circuit elements. In some cases like relay contacts this is to prevent high dV/dt from developing causing sparks that generate interference as well as damage the contacts. In other cases like a power line filter, a series of L-C elements with carefully selected characteristics create a filter that offers a low imeadance to the power line frequency but a high impedance to other frequencies.

A very good book on this topic is "Noise Reduction Techniques in Electronic Systems" by Henry W Ott.