Toroidal Transformers - A Detail Analysis

     In these days of shrinking package size, the transformer is usually the largest component in a given circuit. The geometry of a standard transformer limits how small we can make it as size is proportional to the square root of the total transformer power. However, the familiar E-I core is not the only one that can be used. For instance, using a toroidal, or donut-shaped core makes it possible to reduce the size and weight of a transformer by 20% to 50% compared with conventional cores with-out sacrificing performance. That's possible because core losses in toroids are typically 10% to 20% of the total power loss, with the balance being lost in the windings. That compares with core losses totaling 50% of the total power loss for conventional transformers. Lower core losses provide cooler operating temperatures and low magnetizing current. The toroidal transformer design best uses the high permeability and low-loss characteristics of a modern transformer core. Toroids are commonly used for current and instrument transformers, where low losses are extremely important. Other advantages are higher efficiency lower operating temperature, before regulation, and lower noise. The disadvantage is somewhat higher cost, although improved production techniques are making toroidal transformers more cost competitive.

     The inherent advantages of the toroidal (ring, donut shaped core) transformer, relative to other core configurations, may be summarized generally as (1) nearly ideal magnetic circuit, which results in (2) lower stray magnetic field, (3) less volume and weight, (4) less audible hum, and (5) higher efficiency. Which benefits are of interest in a particular application depend on the type of product and sensitivity of other circuitry to stray magnetic field.

     In an E-I structure it is difficult to align the grain structure of the stamped laminations with the flux path over the entire magnetic path. This inability leads to higher core loss and less efficient operation when compared to toroids. Figure 1 shows a comparison of grain alignment with flux path for a toroidal and E-I laminate.


     Common laminated transformer designs employ a bobbin wound coil placed over a stack of "E" shaped laminations. An "I" shaped stack is butted to the "E", completing the magnetic path. The connection between the E and I is never a perfect junction, giving rise to a discontinuity, or air gap in the magnetic flux path. This gap, having higher reluctance, causes greater radiated magnetic field. In any core with a gap, the properties of the gap are unpredictable, and depend upon pressure and the quality of the surfaces of the gap.

A second feature which gives rise to leakage flux in E-I core transformers is the discontinuity in the windings that surround the flux path. The windings are concentrated in short regions of the laminates, which leaves large portions of the flux path exposed. The abrupt transition from windings to bare laminates creates opportunity for the flux to escape the confinement of the core and form linkage paths outside the transformer. The transitions in the windings can also lead to high leakage inductances for the device.

There is no air gap in the toroidal transformer. The core is tightly wound onto a mandrel, like a clock spring, from a continuous strip of grain oriented electrical steel. Spot welding at the beginning and end prevents loosening. The stresses, which could result in unacceptable core loss, introduced by derailing and winding are relieved by annealing the wound core in a dry nitrogen atmosphere. The result is a stable, predictable core, free from discontinuities, holes, clamps and gaps.


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Figure 2

     In the E-I structure, the magnetic flux is not aligned with the grain of the steel for approximately 25 per cent of the flux path (refer to Figure 1). This misalignment causes higher magnetization losses and reduces the maximum flux density that can be utilized in the core. Higher efficiencies have been made possible by using high grades of grain-oriented steel, which increases flux densities while minimizing losses. However, maximum utilization of these properties occurs only when the flux in the steel is parallel to the grain direction. It can be seen in Figure 1 that the flux in a toroidal core is 100% aligned with the grain of the steel. The typical working flux density of EI laminates is from 1.2 to 1.4 Tesla, whereas toroids typically operate from 1.6 to 1.8 Tesla.

     For a given core cross section, the voltage induced in a winding is directly proportional to the flux density and number of turns. The higher allowable flux density of a toroid requires fewer turns of wire in all windings to achieve the same result.

Comparison of a 960 VA E-I laminate of known characteristics with an equivalent toroid shows the weight and volume of the toroid to be 50% that of the E-I laminate. In an existing product which uses an E-I laminate, it is often possible to replace the E-I with a toroid which has close to the same footprint, but is only 60 per cent as tall. In the case where it is desirable to increase the power supply power rating without increasing its size, the E-I might be replaced with a toroid which is the same size as the E-I, but has 1.5 to 2 times the power capability.

It is true that the empty center hole in a toroidal transformer, which is needed to enable winding, occupies some wasted volume. This wasted volume deficit is overcome by the toroid's volume advantage at roughly 50 VA and above. So, in transformers rated less than 50 VA, reduced size is not a factor, but the other advantages remain.

     Audible hum in transformers is caused by vibration of the windings and core layers due to forces between coil turns and core laminations. Clamps, bands, rivets, and welds cannot bind the entire structure. Varnish penetrates the laminations only partially. Laminations tend to loosen over time, producing increasing hum. The nature of the toroidal transformer's construction helps to dampen acoustic noise. The core is tightly wound in clock spring fashion, spot welded, annealed and coated with epoxy resin.

Audible hum, heard immediately after application of power, may be noticeable in the toroid, and then die down to a quieter level a few seconds after power is applied. This is a result of the toroid's greater inrush current.

Efficiency of a transformer is stated as:
Efficiency = ______________________
     Where Pout is the useful output power delivered to the load, and Pin is the power input to the transformer. The difference between Pin and Pout is consumed by losses in the core and windings. The ideal magnetic circuit of the toroid, and the ability to run at higher flux density than E-I laminates, reduces the number of turns of wire required and/or the core cross sectional area. Either benefit reduces the losses. Toroidal transformers typically are 90 to 95 per cent efficient; whereas E-I laminates have a typical efficiency of less than 90 per cent.


     In recent years, more attention is being given to energy efficiency of electrical equipment, and legislation has been considered which would encourage minimum efficiency standards for all types of electrical products, with lighting and computer equipment being the most prominent. The toroidal transformer will serve as a method for achieving compliance to these new energy efficiency standards.

     A toroidal transformer does a better job of reducing those losses to their minimum practical values than any other production core form, so why not try to use one in your next project?

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