Hi,
I have a flyback transformer design in ltspice with a 1mH primary coil,
I would like to see how many turns this would require on this transformer:
core size: E375
falco part#: 1831-331-002
cross section area Ac(cm^2) =0.840
magnetic path length (cm) = 6.94
WaAc(cm^4) = 0.856
core volume(cm^3) = 5.830
this info is from:
http://www.falco.com/download/SMPSSelGuide.pdf
Is there a formula to calculate the required number of primary
turns to get about 1mH inductance from this data? Thanks for
any help.
You would not normally need AL values for a ferrite core used in a
flyback, as this is determined by the gap required to store the energy
needed.
L = uo x N^2 x Ae / Lg
L = inductance in henries
uo = permeability of free space = 4 x pi x 10E-7
N = turns count
Ae = cross-sectional area at the gap in meters^2
Lg - length of the gap im meters.
.....................................
You have to determine N based on peak core flux change, limited by
core loss or core saturation at the frequency and duty cycle you are
preparing or able to use. To do this you need more core material
information concerning it's power loss characteristics, whether the
material is ferrite or powdered material.
As powdered material has a distributed gap, it will not normally be
merchandised without reference to it's permeability or the part's AL
value
For ferrite parts, if the frequency is lowish or the part is very
small, the flux density will likely be determined by the saturation
limit.
In this case,
Nmin > V x t / ( Bsat x Ae )
Nmin = minimum turns
V = applied voltage in volts
t = period of applied voltage in seconds
Bsat = saturation flux density in Teslas
Ae = minimum cross-sectional area of the ferrite material in meters^2
( Bsat of ferrite ~ 0.33T @ room temperature )
.........................................................
You seem already to have determined that 1mH of primary inductance is
desirable, by some method or other. Note that depending on the
operating frequency, a certain peak current will be expected in this
primary inductance in order to deliver the required output power.
This is determined by the rough formula
P = L x Ip^2 x f / 2
P = delivered power and all losses in Watts
L = primary inductance in Henries
Ip = peak primary current in Amps
f = pulse repetition rate in Hertz
Please do some reading. The old Unitrode/ Texas Instrument app notes
cover flyback converters and flyback transformers pretty clearly.
RL