Design of Iron Powder Core Filter Inductor 4

H/NI=0.4 π/l or H/NI=1/l

For a ring-shaped magnetic core (many filtering inductors use ring-shaped magnetic cores), due to  diode

l=[(D＋d)/2]×πcm

Where D is the outer diameter and d is the inner diameter, measured in centimeters (cm).

Therefore, H/NI can be obtained by the following method:

H/NI=0.8/(D+d) or H/NI=1/(D+d) 0.5 π

For any specification of annular magnetic core, an H/NI value can be obtained. Multiplying this value by the number of ampere turns is the DC magnetization force, which is:

H=(H/NI) NIOe (or A/cm)

From this, it can be seen that H/NI is inversely proportional to l or (D+d). A large-sized magnetic core has a smaller H/NI value. When the number of ampere turns is constant, using a large-sized magnetic core results in a smaller H value and better constant magnetic permeability. It can also be said that under the condition of a fixed H value, large-sized magnetic cores have a larger number of ampere turns.

Here are a few more formulas: diode

L=N2AL（nH/N2）nH(2)

N=[L (nH)/AL (nH/N2)] 1/2 turns (3)

AL=L（nH）/N2nH/N2(4)

W (energy)=LI2/2J or Ws (5)

The unit of L in this formula is H, and the unit of I is A.

When the unit of L is μ H, then W is μ J or μ Ws. By substituting formula (2) into (6), a relationship can be obtained:

W=LI2/2=N2ALI2/2=(NI)2AL/2(6)

In this formula, if AL takes the value of nH/N2, it is nJ. To obtain a result of μ J, the formula can be changed to:  diode

W=（NI）2AL/2000μJ（6′）

5 Design Methods and Steps

Having familiarized oneself with the above formula, the design can proceed with the DC superposition characteristic curve provided by the magnetic core production unit. Figure 1 shows the curve provided by the American Micro Metal Company for design reference. In practical operation, design calculations can also be made based on the data provided in Table 2.

Due to its high saturation magnetic flux density, good constant magnetic conductivity, and low cost, iron powder cores have been widely used. Therefore, the design methods and steps in this article are based on the iron powder core. As long as the characteristic parameters of other materials are changed, there is not much difference in the rest.  diode

Design basis: The inductance is 100 μ H and the rated current is 3A. At the rated current, the inductance value must be less than 75 μ H. The requirement is for a constant magnetic permeability of 75%.