FERRITE CORE INDUCTORS 267 



them in an entirely new class as core materials for inductors. To exploit 

 these advantages properly it is necessary to reconsider some of the 

 fundamental aspects of coil design, since some of the assumptions that 

 applied to powdered core coils are no longer valid, and others have to be 

 modified. 



DISSIPATION FACTOR 



Legg has shown that the core loss characteristics of metallic magnetic 

 materials can be described by three constants, defined as eddy current 

 (e), hysteresis (a), and residual (c), coefficients, respectively.^ The total 

 increment of resistance {Rm), due to core losses is given by 



Rm = efifL + ayiBmjL + c/^/L (1) 



where ^ = effective permeability of the core structure 

 / = frequency in c.p.s. 

 L = inductance in henrys 

 Bm = max. flux density in gausses 



This expression is valid for practical applications so long as the flux 

 density is low. 



Measurements on ferrite cores indicate that the same formula can be 

 used, with appropriate constants, at least for frequencies up to 200 or 

 300 kc, and, again, for low flux density applications. However, the empha- 

 sis on the terms becomes quite different. The residual loss, as its name 

 implies, is usually of negligible importance in metallic magnetic ma- 

 terials, but in ferrites because of the very low value of the eddy current 

 constant, the residual loss often emerges as a controlling factor. 



Beside the core losses the other factors in an inductor that contribute 

 to the total measured resistance are the dc resistance of the winding, ac 

 losses in the winding and the parasitic capacitances within the structure. 

 The latter two, especially the capacitance effects, have always had to be 

 taken into account in higher frequency work, but their contribution to 

 the total loss becomes a matter of first order importance when feriites 

 are used since they are no longer small in comparison with the eddy 

 current loss in the core. 



The objective in the design of inductors for wave filter and similar 

 applications is to provide a specified inductance with as low an associated 

 resistance as practicable. The quahty factor, Q, the ratio of reactance to 

 resistance, is the traditional measure of the extent to which this has 



