I know about the area-product and the Kg criteria, for selecting the core size.
There're also some nice tables, given by Ferroxcube and others, that tells us that a ETD-29 can handle 20-50 W.
For a 45 W design, using area-product formulas recomended by OnSemi, Fairchild,... I get a core requirement of about 0.15 cm^4.
Using Kg formula, requirment is 0.026 cm^5.
ETD-29's area-product is 1.08 cm^4, and Kg is 0.052 cm^5.
Kg is double of that required, and area-product is ten times larger.
It just doesn't make sense to me.
(I'm using 150 mT working flux, and 100 kHz, for Vi=24, Vo=16@3A.)
Now, what if we derive some simple relation?
We know that energy stored in an inductor = 0.5*L*I^2
Using relation: L = N*phi/I = N*Ac*B/I
E = 0.5*N*Ac*B*I
Using relation: H*MPL = N*I, subtitute for N
E = 0.5*B*Ac*H*MPL
Using relation: B = mu*H
E = 0.5*B^2*Ac*MPL/mu
Now, power is energy divided by time, so we can multiply by frequency divided by duty cycle.
P = (f/D)*0.5*B^2*Ac*MPL/mu
Do we agree up to this point?
I get 5 W, far less than the expected.
Ac = 0.761 cm^2
MPL = 7.2 cm
mu = 2000 * pi * 1e-7
D = 0.5
Any comments?
There're also some nice tables, given by Ferroxcube and others, that tells us that a ETD-29 can handle 20-50 W.
For a 45 W design, using area-product formulas recomended by OnSemi, Fairchild,... I get a core requirement of about 0.15 cm^4.
Using Kg formula, requirment is 0.026 cm^5.
ETD-29's area-product is 1.08 cm^4, and Kg is 0.052 cm^5.
Kg is double of that required, and area-product is ten times larger.
It just doesn't make sense to me.
(I'm using 150 mT working flux, and 100 kHz, for Vi=24, Vo=16@3A.)
Now, what if we derive some simple relation?
We know that energy stored in an inductor = 0.5*L*I^2
Using relation: L = N*phi/I = N*Ac*B/I
E = 0.5*N*Ac*B*I
Using relation: H*MPL = N*I, subtitute for N
E = 0.5*B*Ac*H*MPL
Using relation: B = mu*H
E = 0.5*B^2*Ac*MPL/mu
Now, power is energy divided by time, so we can multiply by frequency divided by duty cycle.
P = (f/D)*0.5*B^2*Ac*MPL/mu
Do we agree up to this point?
I get 5 W, far less than the expected.
Ac = 0.761 cm^2
MPL = 7.2 cm
mu = 2000 * pi * 1e-7
D = 0.5
Any comments?
