CHOKE DESIGN STEPS

1. First determine A based on desired inductance and maximum current allowed:

A = 27.5*I*√L

2. Next, calculate NI based on A, using the graph below.

3. Calculate number of turns based on maximum I allowed. For example, in above NI = 350. If I_{max} = .12ma, then N=2917.

4. Next, figure out if this number of turns will fit onto the bobbin.

Above are 2 views of an E-I core (no gap shown). The area marked WH x WL is the area where the wires have to fit. Note that this is also b x w. The bobbin has thickness BT(usually about 2mm). On top of the wires is a top insulation layer and a top clearance (usually about .5mm for both)

So the coil thickness CT = b - BT - .5mm.

and the coil length CL = w - 2BT

The net winding area NWA = CT * CL

Now

d = diameter of wire in mm, and this should be published.

d+.02 = width of wire with insulation

So GA = (d+.02)^{2} = area of wire if you assume wire is square, which it isn't, BTW.

If the wire diameter WITH insulation is published, then you don't need to add the .02mm.

Assuming square wire, you can fit the following number of turns in the net winding area NWA:

N_{N} = NWA / GA

N_{R} is the realistic number of turns that can be fit into the NWA and is determined by skill of winder.

N_{R} = .9 * N_{N} for a skilled winder, .8 or .7 is used for less skilled winder.

EXAMPLE:

You are using EI60 laminations, so from above table

w=30mm and b=10mm. Assuming bobbin thickness of 2mm ->

CL=30 - 4 = 26mm and CT = 10 - 2 - .5 = 7.5mm, so

NWA = 26 * 7.5 = 195mm^{2}

You have wire with diameter .25mm, and you assume an insulation thickness of .02mm, so

GA = .27^{2} = .073mm^{2}

N_{N} = 195/.073 = 2671 turns.

Assuming a semi-skilled winder

N_{R} = .9*2671 = 2404 turns.

Next, figure out the total length of wire. This will allow a calculation of the resistance R of the inductor. The MLT is the mean length of a turn, so multiplying this by the number of turns should give the total length of the wire

L_{W} = MLT * N_{R}

The formula for MLT, for chokes is

MLT = 2/(a+S+4*BT) + π*CT, where S=stack thickness and other variables were described above.

So in the above example, if the stack thickness is say, 25mm,

MLT = 2/(20 + 25 + 4*2) + π*7.5 = 129mm

We can now figure out the resistance of the length of wire:

First, calculate the total length of the wire

L_{W} = MLT x N_{R} = 2404 x 129mm = 310116mm = 310.116m

R_{C} = ρ * L_{W} / πr^{2}, where, from above, r = d/2 = .125mm = .000125m and ρ = resistivity = 1.68 X 10^{-8} Ω-m for copper.

R_{C} = 1.68 x 10^{-8}Ω-m x 310.116m / π(.000125m)^{2} = 106.1 Ω

This is a nearly finished E-I transformer. A bobbin with wire wound around it is fit onto the center leg of the E-piece, then the stack of I pieces (darker brown in photo to the left) is put in place.

The Magnetizing Force or Magnetic Field Strength H is defined as:

(1) H(t) = i(t) * N / ℓ_{MP} where N=number of turns, ℓ_{MP} = mean magnetic length, described below

The flux density B is the Magnetizing force H multiplied by the permeability μ of the core, where μ = μ_{R}*μ_{O}

μ_{R} = relative permeability = 1 for air, much higher than 1 for ferromagnetic materials.

μ_{0} = absolute permeability = 4π*10^{-7}, so

(2) B(t) = μ_{R}*μ_{O}H(t)

Magnetic flux Φ is the flux density B x the cross sectional area A of the core.

(3) Φ(t) = B(t) *A

(4) Φ(t) = μ_{R}*μ_{O}H(t) * A

(5) Φ(t) = μ_{R}*μ_{O}*i(t)*N*A/ℓ_{MP}

Now the induced voltage is proportional to the derivative of the flux Φ(t) and is negated.

V(t) = -N * dΦ(t)/dt

Using equation (5):

V(t) = -N^{2}Aμ_{R}*μ_{O}/ℓ_{MP} * di/dt

Now, this is the familiar inductance equation with:

(6) L = N^{2}Aμ_{R}*μ_{O}/ℓ_{MP}

For any sinusoidal wave,

(7) Φ(t) = Φ_{max}sinwt

As in equation (7), V(t) can be expressed as V(t) = V_{max}*sin(wt), or alternatively

(9) V(t) = -V_{max}*cos(wt)

Comparing (8) and (9), we see that

(10) V_{max} = wNΦ_{max}

From equation (3)

(11) Φ_{max} = A * B_{max}

so, combining (10) and (11)

(12) V_{max} = wNAB_{max} = 2πfNAB_{max}

(13) V

Rearranging (13), can give the turns per volt (TPV)

(14) TPV = N/V_{rms} = 10^{4} / (4.44fNAB_{max})

SPECIAL CASE FOR E-I LAMINATIONS:

For E-I laminations:

(15) TPV = k/A_{EF}, where k is a published number:

50Hz: k=45, GOSS k=40

60Hz: k=37.5, GOSS k=33.3

Rearranging:

k = TPV * A_{EF}

Substituting equation (14) and using A_{EF} for A :

(16) k = 10^{4} / (4.44fNB_{max})

ELECTRICAL ANALOGY

Recalling equation (1), for DC it is

(17) H = NI/ℓ_{MP}

Now, NI is called the magnetomotive force (MMF) or ampere-turns (AT). It is also called the total field. It is analogous to voltage in an electrical circuit. It is what pushes the flux Φ through the core. This can be expressed as:

(18) MMF = Φ * R, where R is called the reluctance.

Figure out what R is:

Rewriting equation (18) => R = MMF / Φ = NI / Φ

Substituting equation (5) gives:

R = NI / (μINA/ℓ_{MP}) where μ = μ_{R}*μ_{O}

(19) R = ℓ_{MP}/μA

So high-permeability materials have low R, so the core presents a low magnetic resistance to magnetic flux, thus very little flux excapes the core into the surrounding air, which has a very high reluctance.

The inverse of reluctance R is called the permeance P and is the inverse of R:

(20) P = μA / ℓ_{MP}

Looking at equation (6), it is easy to deduce that:

(21) L = N^{2}P

THE GAP

Without looking at all the hysteresis curves and permeability curves, suffice it to say that DC current through the core as in a choke or SET output transformer leads quickly to saturation of the core. So an air gap is placed to increase the reluctance. Air has a permeability of 1. The grueling calculations are below.

From equation (19):(22) R

(23) R

Remembering the ohm's law analogy, total R = R

From (18), and remembering that MMF = NI

NI = Φ(R_{g} + R_{c})

= Φ[g/(μ_{O}A_{g}) + ℓ_{MP}/(μ_{R}μ_{O}A_{c}) ]

= Φ/A *[g/μ_{O} + ℓ_{MP}/(μ_{R}μ_{O}) this assumes A_{c}=A_{g}, which is not exactly correct because of fringing.

= B*[g/μ_{O} + ℓ_{MP}/(μ_{R}μ_{O}) from equation (3)

Divide both sides by ℓ_{MP} and remember that H = NI/ℓ_{MP} you get:

H= B * [g/(μ_{O}ℓ_{MP}) + 1/(μ_{R}μ_{O})]

From equation (2) H = B * 1/μ, so......

H = B * 1/μ_{EF}, where

1/μ_{EF} = 1/[g/(μ_{O}ℓ_{MP})+ 1/(μ_{R}μ_{O})]

Things get a little easier for chokes (or transformers) with a gap. A typical μ_{R}μ_{O} for the core may be in the neighborhood of 10,000 while the μ for the gap is 1. For, say, EI66 laminations, a typical gap is .15mm and a typical ℓ_{MP} is maybe 12cm and A=6.8.

So, from equations (22) and (23),

R_{c} = 12/(10000*6.8) = 1.77x10^{-4}

R_{g} = .15/6.8 = 2.2x10^{-3}

So the reluctance R_{g} of the gap is 12 times the reluctance of the core, so is the overwhelming determinant of total reluctance.

EXAMPLE

Assume EI66 lamination with 31mm stack thickness. From chart a=22, so A = 3.1*2.2=6.8cmFrom equation (17):

R

R