Does Q-Factor depends on distributed capacitance?

[albert22]

As far as I know Q-factor for an inductance (coil) is defined as Q=wL/R.Or Q = fo/Δf for a resonant circuit. But somebody is saying that a coil with less distibuted capacitance has greater Q.
Is it possible ?
Regards

 

[HMS1021]

It is true. You write Q=ωL/R, the addition of ωC will reduce ωL. Also if there is a lossy dielectric R may be increased. Either of these reduce Q.

 

[albert22]

Sorry HMS1021, I dont understand how ωC will reduce ωL if it does no appear in Q=ωL/R. For a resonant circuit I understand that if I add an extra capacitance Cd it will change the resonance frequency and then the Q of the whole circuit. But according to the definition for the Q of an inductor only ωL and R are taken into account.
Thanks for your answer.

 

[HMS1021]

Q=ωL/R is for a ideal inductor with just resistance in the wire. In the real world there are additional terms such as capacitance between windings, windings to core capacitance if used, and between terminals. If PCB mounted the mounting pads are shunt Cs to ground. As the inductance increases so do the undesired distributed C effects with the effect the inductors useful maximum frequency is lower.

It's these distributed caps that cause inductors to have self resonate frequencies specified on their data sheet. If you use the inductor well below this frequency the C values are small enough they can be ignored and the inductor treated as a ideal inductor.

You asked about distributed capacitance, if it was just ωL there would not be a question.

Similarly caps have self resonate specifications due to their leads and electrode inductance. They also have a spec called ESL.