Flyback- transformer Design-please urgent

[daniel_p]

Could any body please help me with the following problem:
Flyback transformer in discontineous mode:

Vin"=11-12V Dc
"Ipeak"(primary)=520mA, ton(turn-on time)=5us
"Lp"(primary inductance) is calculated by: Lp=(Vin*ton)/Ipeak[/color]
A EF20 core is used with "Ae"=31mm^2 and "AL"=1200nH.
Wire with "d"=0.38mm is used.

Air-gap calculated by: "Lg"=[0.4*3.14*Lp*Ipeak^2*10^8]/(Ae*Bmax^2)=0.02mm
"Bmax"=2000Gaus selected.
Primary number of turns by: "Np"=(Bmax*Lg)(0.4*3.14*Ipeak)=8 turns
------
Surprisingly the primary current taken from the power supply during the turn-on time is too small, about 100mA, expected 520mA!!
Questions:Why is the current so small?
Why is the inductance not equal to 100uH,it's about 80uH without Air-gap.
(In each case the current is too small for Lp=80uH or 100uH)

I'll be gratefull if any body can help me with this problem!
thank you very much in advance

 

[xu.hob]

PLEASE SEND TO ME: www.magnetic-core.com,

 

[BADFLOPPY]

Please check AN4140 of Fairchild.
Google it & Get it.

https://www.fairchildsemi.com/an/AN/AN-4140.pdf

Thanks

 

[daniel_p]

Hi!
I have a flyback configuration as a part of a more complicated low power design.
in this design I must disconnect the secondary winding from output capacitor
using a switch or a device funktioning as a switch.

Since this switch must be placed between the secondary winding(which has floating voltage levels)
and output capacitor(which is being charged and has no constant voltage), both sides of the switch have
floating potentials, therefore there is no reference potential to activate the switch.


Requirements:
1: an absolute disconnection must exist(no leakage current)
2: Maximum current flowing through this switch is about 1A
3: reverse voltage on the switch(off status) about 900V (difference of the voltage on the
output capacitor and the secondary winding)
4: switch power dissipation must be very low
5: switch must be very fast, some micro seconds
6: rise and fall time for the switch turn on and off must be in nano second

Important: -MOS transistors are not adequate because current leakage through free wheeling diode
makes the absolute disconnection impossible!
-Relays are too slow and bulky


I do appreciate your idea!
I'll be gratefull if any body can help me with this problem!
thank you very much in advance