Conventionally, a transformer is constructed of a core formed of magnetic material which will then have two or more coils or windings positioned thereon to form a primary or input winding in a secondary or output winding. The windings are interlinked by the magnetic flux passing through the magnetic circuit formed by the core. The general rule, here, is that the voltage output of the secondary winding will be a proportion of the voltage of the input or primary winding according to a proportion determined by the ratio of the number of turns of secondary winding to the number of turns of the primary winding.
There are certain applications, especially in the peripheral, computer and welding fields that require very low but precise output voltages to be provided while at the same time permitting extremely large current flows.
One commonly used standard magnetic core form is the E-I transformer core. In order to develop low output voltage from the secondary winding of such as transformer, it has often been tried to use a pair of half-turn windings by making a single turn of wire around the center leg of the core and center tapping this wire to ground in order to form two half-turn windings, one on each side of the center-tap. This, however, had the disadvantage in that if the current load on one half-turn of the secondary winding did not match the load on the other half-turn of the secondary winding then the regulation of the transformer was highly inadequate since the leakage reactance of the more heavily loaded half-turn secondary was much larger than the leakage reactance of a secondary winding which consisted of a full-turn.
Due to the leakage reactance in the case of the two one half-turn secondary windings, the voltage across the "loaded" half-turn secondary winding tends to decrease while the voltage across the other half-turn secondary winding tends to increase, thus causing poor voltage regulation.
In an E-I core transformer when it is desired to produce extreme voltage step-down, the secondary winding normally must be at least one-full turn, and any attempt to carry current out of only one half-turn through one window of E-I core, will divert the core magnetic flux to the opposite outer core leg, and this will severely limit the available load current.
If this limitation could be overcome, there would be needed only one-half the length of conductor for a given voltage output, thus reducing the cost of the conductor material and at the same time reducing the operating I.sup.2 R heat loss in the conductor. Thus at a given voltage and load current requirement, a "half-turn secondary " would operate at one half the turns per volt of the normal one turn secondary. Further, this would require only one-half the primary turns that would be required in the normal design, thus reducing its material content and heat loss similarly.
Normally, the price paid is that with the "half-turn" secondary, the core material must operate at twice the flux density, but today with modern power ferrite cores which are designed to carry high flux density, this is no longer a problem.
Even more useful today, with the use of switching inverter applications, the high frequency switching of the switching inverters helps to reduce the actual core flux density operating within the transformer.