General Information on Toroidal Transformers
A toroid is a doughnut-shaped transformer with many magnetic advantages. Its annular shape is generated by revolving a circle around an axis external to the circle.
A coil of insulated wire in a doughnut shape (usually with a core of iron or similar metal) is an example of a toroidal object. These are used as inductors in circuits such as low frequency transmitters and receivers because they possess higher inductance and carry greater current than similarly constructed solenoids. They are also used as transformers in main power supplies. Toroidal coils reduce resistance, due to the larger diameter and smaller number of windings. The magnetic flux in a toroid is confined to the core, preventing its energy from being absorbed by nearby objects.
Advantages of Toroidal Power Transformers
Toroidal transformers offer many advantages over standard laminated power transformers. Toroidals provide quiet, efficient operation with very low stray magnetic fields. Their small size and weight support a package that is easy to design into any application.
The Toroidal Core
At the heart of the toroidal is a highly efficient donut shaped core. To construct the core, grain-oriented silicon-iron is slit to form a ribbon of steel which is then wound, like a very tight clock spring. The result is a core in which all of the molecules are aligned with the direction of flux. Molecules not aligned with the flux direction increase a core's reluctance (the capacity for opposing magnetic induction), degrading performance to the level of common steel when the molecules are 90 degrees out of phase. EI laminated cores, which are stamped from grain-oriented Si-Fe, may have as much as 40% of the total core area perpendicular to the ideal grain direction, with another 40% acting only as a return flux path. This more efficient use of the core material in a toroidal can result in a size and weight reduction of up to 50% (depending on power rating), allowing the design engineer to innovate by exploiting the toroidal's small size, low weight, ease of mounting, and flexible dimensions.
Efficiency
Since toroidal cores are constructed of a continuously wound ribbon, there is virtually no air gap. The windings are evenly wrapped over the entire core allowing the transformer to operate at a higher flux density than in standard transformers. Toroidal transformers can operate at 1.6 to 1.8 Tesla (16,000 to 18,000 Gauss) while EI cores are limited to 1.2 to 1.4 Tesla (12,000 to 14,000 Gauss). The magnetic flux of the windings is oriented in the same direction as the grain-oriented core, thus achieving very high electrical efficiencies. Efficiency is a measure of a transformer's ability to deliver the input power to the load. Efficiency is expressed as a percent by:
% = ( PO / PI ) x 100
where; PO = Output power, PI = Input power, % = Efficiency
Also, standby losses are greatly reduced under no-load operation due to the lower magnetizing currents required by the toroidal core.
Stray Magnetic Fields
The primary cause of leakage flux from any transformer is the air gap. Ideally, a magnetic circuit should have no air gap. In traditional transformers with EI laminations stacked to form the core, the air gap at the junction of the I and the E is the source of most of the leakage flux. This flux strays into the surroundings due to the high reluctance of the air and the concentration of flux in the laminations. For the same reasons, mounting holes and grooves in the laminations also cause a small amount of leakage flux. The tape wound cut-C core is an improvement; but there is still an large air gap causing unwanted stray flux. Since toroidal cores are wound from a continuous ribbon of steel, stray fields from air gaps are eliminated.
In addition, the windings of the toroidal transformer uniformly encase the core in copper. This results in a natural magnetic screening effect which, in combination with the elimination of the air gap, results in an 8:1 reduction of radiated magnetic field over an equivalent rated EI transformer. The windings covering the solid ring core also help reduce magnetostriction -- the main source of acoustic "hum" in standard transformers. Audible noise can be reduced even further by varnish impregnating the toroidal core and/or the copper windings.
Duty Cycle
Significant reductions in transformer size and weight may be realized in many cases where the transformer is loaded intermittently. In such cases, the load is on (tON) for only a small portion of the total period (tCYCLE). The period is much shorter than the thermal time constant of the transformer. To calculate the nominal power rating (VA) of the transformer use the following equation:
PNOM = PLOAD tON / tCYCLE
where; tCYCLE = tON + tOFF
Regulation
The regulation (percentage of voltage drop) may be expressed with the following equation:
