Interwinding capacitance or coupling capacitance (Cww) is the capacitance between the primary and secondary windings of the coil. When the windings are wound tighter together by twisting to improve the leakage inductance, then the coupling capacitance is increased.
Calculate the distance (d) between the bifilar wires wound on the core
Calculate wire diameter:
Example calculations are run using HPN wire insulation. Refer to Chapter 5 for TPN and QPN formulas. Results should be acquired for all three types of wire insulation to insure the best isolation voltage between windings.
dW = 0.127602 – 7.507e-3 (x) + 1.546e-4 (x2 ) –1.107e-6 (x3 )
dw = d (optimum- due to winding
dw = 0.0043 in.
where: AWG = 39 HPN
Determine the length of wire on the core for each winding
The length of wire is the wire length of each turn around the toroid multiplied by the number of turns in the winding.
L/T = [ (OD – ID) + 2H ]
Wire Length = lwire = L/T (N)
L/T = [(0.115 – 0.067) + 2(0.095)] = 0.238 in/T.
Where: OD = 0.115 in. ID = 0.067 in.
H = 0.095 in. N = 26 turns
Wire Length = lwire = L/T (N) = 0.238 (26) = 6.188 in.
Calculate the surface area of the wire:
The surface area of the wire is one half of the wire circumference, which is that portion of the wire facing the secondary winding wire surface, multiplied by the length of the winding wire.
Aw = [dw (π ) lwire ] / 2
Aw = [(0.0043) (3.14159] / 2) (6.188) = 0.04189 in2
Calculate Coupling Capacitance
Cww = [0.224 (1.87) (0.04189) (6.188)] / 2 (0.0043) = 12.60 pf.
* The K factor is an estimated value based on the fact that the wire is coated with polyurethane (K=3.5) and the wires are separated by air (K-1.0). The equivalent K factor is estimated as the geometric mean of these two values, or:
( SILICONE BUFFER – K=3.5 )
Cww = [0.224 (3.5) (0.04189) (6.188)[ / 2 (0.0043) = 23.58 pf.