Help for Ferrite Transformer designe

[kg300]

Hi Friends, I need some help for winding a transformer. I have a 12VDC to 220VAC Inverter, in which EI40 Ferrite core Transformer is used with following specifications,

Pri turns #4-0-4 (center tape)
Sec turns #72
Input volt #12V
Output Volt #300V
Switch Frequency #50khz
Output current #4amp

Now if I use EI42 core in it with same Input and Output ratings, what number of turn on Pri and Sec will be wound?

Thanx

 

[FvM]

Normally, maximum flux rules the number of turns for a power transformer, it should change inverse proportionally to the core area.

EI40 respectively 42 isn't an unequivocal specification, you have to check the core parameters yourself. If the core area is larger
by at least 33 %, you can reduce the number of turns to 3:3:54.

 

[WimRFP]

Hello,

Assuming that a real transformer is used (so no energy storage in an air gap), you can reduce the number of windings inversely proportional with the cross section (to maintain same flux density) as FvM said.

Because the path length for the larger core may be somewhat more (my Ferroxcube data goes to EI40, I don't have EI42), the inductance will reduce somewhat.

In my opinion it is no problem to keep the totally 8 turns for the primary. Make sure you wind the two primary 4-turn windings in between, close to eachother. The actual tap is therefore not in the middle, but at one end. This reduces the leakage inductance between the two primary windings. In my opinion, it is not necessary to twist the two primary winding wires first (before actually wind them).

When the leakage inductance between primary and secondary is of importance, you may wind half the secondary turns first, followed by the primary, and at the end the last half of the secondary winding. If this is not of importance, this is not recommended as it doubles the capacitance between the windings.

Depending on the expected over voltage and safety considerations, you may require certain creepage distance and several layers of kapton (polyimide) tape between the secondary and primary winding (if the windings overlap). In case of double insulation requirements for the transformer, you also need more kapton layers and double creepage distance.

If you don't have EN60950 or other low voltage safety standard, you may search for ECMA 287, this one is free and will guide you to make good judgment on transformer insulation. Ferrite material as used in SMPS power transformers cannot be considered a safety barrier.