Why Is HV Winding Placed Outside and LV Winding Inside on Transformer Core?

Why is the HV winding place outside (further from the core) than the LV winding???....
I read up a post which mentioned introduction of a leakage inductance if the HV winding is placed inside the LV winding (closer to the core). The post also stated that the aforementioned leakage inductance is a function of the separation between the winding turns and transformer core...Can someone elaborate more on this....please?..
Thanks,neteng-1

I can't find much on this.  In the transformers I deal with (small power transformers up to about 100-200W max.) the HV windings generally seem to be closest to the core.  However I have seen all sorts... HV above or below LV (ie both close to the core) is not uncommon, and I seem to recall seeing LV closest but rare.  I work for an electricity distributor so I will ask some of the substation techies how it's done on their big transformers (which will be 5KVA ++).

I looked up a couple custom transformers that we have built here at work.  From what I've seen, the winding with the smallest Gauge wire is on the inside which makes sense as it's easier to wind.In your typical voltage step-down transformer this usually puts the HV winding on the inside. We have some current step-down transformers where the HV winding is heavier and goes on the outside.I've also seen the case with both windings side by side.Leakage inductance is a function of the coupling between the windings. the best coupling is obtained by interspersing the windings. Unfortunately, insulation requirements between windings usually preclude this.

I have over the years worked on transformers  over the whole spectrum of applications, so here are my observations, for what they are worth.  Low power EI transformers for 115 or 230 volts often have the winding's arranged so that a wide range of output voltages can be made available just by changing the secondary winding turns and wire gauge, which means either a side by side arrangement or primary winding closest to the core.  This applies in the case where the secondary has a lower or higher voltage than the primary, assuming the standard mains voltages.  I used to wind custom  output EI mains transformers for widely varying secondary voltages, where we had a range of core sizes, and the associated bobbins pre wound for a 230v primary input.Transformers for power transmission usually have a different core structure, so that all winding's are on different legs of the core, see 3 phase transformers for example for an extreme example. It makes insulation/isolation and cooling simpler for starters, putting aside issues around issues associated with the advantages and disadvantages of the core geometry.I should mention toroidal and planar transformers, but the names are enough, they have difficulties all of there own.Where low leakage inductance is required, the layers are interleaved, as Elizabeth pointed out.  You will often see this in SMPS transformers (fly back is a good example) You can see how these transformers are made by looking at the application notes from the companies that make the controller chip sets, it is quite edifying. With careful design, very high voltages are possible. Special transformer adhesive tape (sort of like sellotape) is available that is very thin, and rated at many KV. It has good thermal properties, allowing the transformer to be dip/vacuum coated and baked after manufacturing.Cheers,Richard

Small 50/60 Hz power transformers often use side-by-side windings wound on a plastic bobbin with a molded-in insulating flange between the windings.