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Reducing Ferrite Coil Capacitance

May 15th Fact-of-the-Day

It generally is best to keep coil capacitance as low as reasonably possible in most ferrite core applications. That tends to be true whether a coil is used as an RF choke, a transformer winding, or part of a balun (where coils typically function as both RF chokes and transformer windings). There is more than one reason. A coil functions as a capacitor, rather than as an inductor, above its self-resonant frequency and coil capacitance lowers the self-resonant frequency. Coil capacitance increases the RF current circulating in a coil, which increases losses and lowers the Q. Several things can be done to reduce coil capacitance. Coil turns should be spaced equally from each other if a coil requires less than one layer. A physical gap should be left between the two ends of a toroid coil winding. Some ferrites are electrical conductors. Coil windings can be spaced from conducting ferrite to reduce coil capacitance. That will reduce magnetic-coupling into the core and increase magnetic losses, but the self-resonant frequency generally will be higher. ©2005 Martek International All rights reserved.

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