Core Selection
Inductor Core Selection Procedure (e) Increase the number of turns by dividing the initial number of turns (from step 4(a)) by the per unit initial value of permeability. This will yield an inductance close to the required value after steps 4 (b), (c) and (d) are repeated.
Only two parameters of the design application must be known to select a core for a current-limited inductor; inductance required with DC bias and the DC current. Use the following procedure to determine the core size and number of turns. 1.
Compute the product of LI2 where: L = inductance required with DC bias (mH) I = DC current (A)
2.
Locate the LI2 value on the Core Selector Chart (page 2-3, 2-4, & 2-5). Follow this coordinate to the intersection with the first core size that lies above the diagonal permeability line. This is the smallest core size that can be used.
3.
The permeability line is sectioned into standard available core permeabilities. Selecting the permeability indicated will tend to be the best trade-off between AL and DC bias.
4.
Inductance, core size, and permeability are now known. Calculate the number of turns by using the following procedure: (a)
The inductance factor (AL in nH/T2) for the core is obtained from the core data sheet. Determine the minimum AL by using the worst case negative tolerance (generally -8%). With this information, calculate the number of turns needed to obtain the required inductance from:
Where L is required inductance (碌H) (b)
Calculate the bias in A路T/cm from:
(c)
From the Permeability vs. DC Bias curves (pages 3-18 through 3-20 & 4-39 through 4-41), determine the rolloff in per unit of initial permeability for the previously calculated bias level. Curve fit equations shown in the catalog can simplify this step.
(d)
Multiply the required inductance by the per unit rolloff to find the inductance with bias current applied.
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MAGNETICS
(f)
Iterate steps 4 (b), (c) and (d) if needed to adjust biased inductance up or down until it is satisfactorily close to the target.
5.
Choose the correct wire size using the Wire Table (page 3-28). Duty cycles below 100% allow smaller wire sizes and lower winding factors, but do not allow smaller core sizes.
6.
To calculate winding factor, multiply the number of turns by the wire area found on page 3-28 to find the total wire area. Divide the total wire area by the core window area to obtain the winding factor of the design. Verify that the winding factor is acceptable by referencing the various winding approaches described on page 2-7.
7.
If a significant ripple current will be present, estimate the core losses using the Core Loss Calculation procedure on pages 2-8 through 2-13. If AC core losses will result in too much heating, or efficiency below requirements, then the inductor may be loss-limited rather than saturation-limited. Design options for this core are to consider a larger core, a lower permeability material, a lower loss material or some combination of these three.