Core Selection
Designing With Magnetics Powder Cores Wound Coil Dimensions Wound coil dimensions are listed for 70% winding factor, as these are the largest dimensions necessary for packaging the wound coil. These dimensions are attainable. As a 70% winding factor (no residual hole) yields the same overall coil dimensions as a 100% (unity) winding factor (no interstices). Coil dimensions for coils wound to 40% winding factor can be estimated as follows.
where:
OD40% = 0.5(ODcore + OD70%) ODcore = core OD after finish OD70% = wound coil OD
where:
HT40% = 0.45(HTcore + HT70%) HTcore = core OD after finish HT70% = wound coil OD
For other winding factors, OD dimension can be approximated by:
where:
Temperature Rise Calculation Heat to be dissipated in a magnetic component arises from energy losses due to DC resistance in the coil (I2R); AC copper losses, if high frequency AC current is present; and AC core losses, if AC current is present. (DC current does not result in any core losses, regardless of the magnitude.) Heat dissipation and temperature rise (∆T) depend on many factors, so there is no simple way to predict ∆T precisely. But the following formula is useful for approximating ∆T for a component in still air.
Surface Area is that of the wound component. In this catalog, surface area is presented in two ways: 1.
2.
Unwound, coated core Wound core, assuming 40% winding factor
2-7
MAGNETICS
rx=radius at unique winding factor x=winding factor ODx=2rx