Leakage inductance of "short spindle" flyback transformers?

[treez]

Hello,
My boss wants us to design an offline (230VAC) flyback (vout = 30v, pout=40w) switch mode power supply. It will be sited inside a flat screen television, so therefore it will feature a very low profile flyback transformer, and therefore the ‘core spindle’ of this transformer will be very short. Is it true that the leakage inductance is always greater with such transformer profiles?.
(On other power supplies, I have previously always used long spindle transformer cores in the belief that this allowed better coupling of primary and secondary, and thus less leakage)

 

[mtwieg]

Hard to say without pictures to compare. When you say "core spindle" are you talking about the central arm of an EI core pair or something?

For very low profile, planar PCB transformers are often used and they do have substantial leakage, which is why you would want to use resonant or regenerative SMPS topologies.

Or use a normal transformer but shrink it by using a much higher switching frequency.

 

[treez]

thanks, yes, that central arm of eg an EI core pair is what I refer to as the spindle.

 

[mtwieg]

Again without a picture it's hard to say. So long as you are effectively using the available window area and using proper sandwiching of layers, leakage shouldn't be too bad. But at some extreme limit your windings might become flattened into more of a spiral than a solenoid, and at that point you will start to get significant leakage.