OPTIMIZING LINEAR POWER SUPPLY PERFORMANCE WITH LINE FREQUENCY TOROIDAL TRANSFORMERS

By Gary J. Taggart and Jeffrey D. Goff 

ABSTRACT

We present the conditions in which a toroidal type, line frequency, power transformer can yield reduced stray magnetic field, reduced weight and size, and improved efficiency over an E-I laminate trans­former in single phase linear power supplies. These benefits must be considered in relation to the disad­vantages of slightly higher cost and greater inrush current.

WHY CHOOSE A LINEAR POWER SUPPLY?

Since most modem hybrid digital/analog circuits must be operated from rectified, filtered, power line voltage, a power conversion unit must be utilized. Electronic equipment designers have two primary methods for powering their equipment, switching and linear supplies. If the product's operational and environmental constraints will permit high levels of radiated and conducted EMI, slower closed-loop response to load variations and reduced reliability, then the cost, power density and efficiency of switching supplies become attractive.

Many designs cannot tolerate the characteristics of switching supplies, making linear supplies the only viable alternative. Examples of these high perfor­mance products include high end audio mixers and amps, lighted matrix displays, computers and video processing and display equipment.

A transformer is required to provide voltage lower than that of the power line to the rectification, filter­ing and regulation circuits in a linear supply.

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.

TOROID'S IDEAL MAGNETIC CIRCUIT

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.

FIGURE 1

LOW STRAY FIELD

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. Similarly, in "C" cores, where strip steel is wound into an oblong shape then cut into two identical C shapes, air gaps are present at the junctions where the coils are inserted and the cut faces of the C pieces meet to complete the magnetic circuit. 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 and C core transformers is the discontinuity in the windings which 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 dereeling 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.

Figure 2 compares the stray magnetic field at 100 millimeters distance from an E-I laminate and toroidal transformer of equal power rating.

stray FIELD AT 100 mm FROM CENTER

30    10    80   120   1i0   180   210   240   270   XX)   330 

POLAR ORIENTATION OF TRANSFORMER (DEGREES)

FIGURE 2

If shielding and re-orientation of the E-I power transformer and sensitive devices can improve stray field immunity sufficiently, without undue expense, then a toroidal transformer may not be needed. However, the toroid's substantially lower stray field may mean the difference between acceptable and unacceptable operation of sensitive circuitry.

REDUCED WEIGHT AND SIZE

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 Tesia, 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-1 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

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, which is discussed below.

EFFICIENCY

Efficiency of a transformer is stated as:

Pout

Efficiency =

pin

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 Pour 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.

 INRUSH CURRENT

The characteristics which give the toroidal trans­former advantages also contribute to a disadvantage:high inrush current with initial application of power.

The absence of a gap in the toroidal core means that the maximum possible remanence (residual magne­tization of the core in a particular direction and mag­nitude) can be substantially more pronounced in a toroid when compared to an E-I laminate. This residual magnetism is the mechanism by which old computer cores memories functioned. The core "stores" the static magnetic bias when the power is switched off. If the removal of power occurs at an unfavorable time, the strongest magnetic remanence will be stored in the core. When power is again applied to the primary, the peak inrush current may be as great as Vp-pk divided by Rp, where Vp-pk is the peak primary winding, depending on the power capability of the transformer, and on how strongly the core was magnetized. This inrush current peak occurs for a short time during the first or second half of the power sine wave.

A time delay fuse or circuit breaker with appropriate time delay curve will be needed. A fast blow fuse will not last through more than a few off-on power sequences.

In high power applications, more exotic means may be required to ensure protection that will survive inrush, yet still protect in fault situations. One method involves a relay whose coil is across the switched power line. Prior to application of power, a resistance is present in series with the transformer primary. After power is applied to the relay coil and transformer, the electromechanical relay begins to move from the deenergized to the energized contact position. If the relay takes long enough to operate, then the inrush current has been limited by the series resistor, and the core's magnetic bias has been elim­inated. An example of this circuit is shown in Figure 3.

 

 

HIGHER COST

Toroidal transformers are manufactured individualy, with an operator tending each operation. On the other hand, plastic bobbins of small E-I laminates can be wound up on machines which handle several bobbins at once, and operated nearly unattended.

The process of applying inter winding insulation is also more labor intensive in toroidals. E-I laminate insulation consists of one wrap of adhesive tape or kraft paper, whereas insulation in toroids is applied in an overlapping spiral fashion. This conforms best to the curved surfaces.

The discrepancy in labor is small at power ratings of approximately 500 VA and above. Refer to Figure 4, which illustrates how a toroidal coil is wound.

Magazine Revered to Unwind Wire Onto Core. Core Rotated To Distribute Wire Evenly.

 

FIGURE 4 THREE PHASE TOROIDAL TRANSFORMERS

Employing toroidal construction for three phase transformers does not offer a pure volume advantage over the E-I laminate. The E-I geometry is more of a "natural" for three phase, since each of the three legs of the E portion of the core can be used for the a - b - c phase windings, and flux is then efficiently (except for the aforementioned non alignment of flux to steel grain) linked a to b, b to c and c to a.

Providing a three phase transformer using standard toroidal cores and winding techniques requires three separate transformers. This is inefficient use of vol­ume. In some applications, where a very low profile transformer is required, and the real estate for three transformers is available, a toroidal three phase transformer set is beneficial.

SUMMARY

Toroidal power transformers, at least in North America, have been so seldom used as to be almost unheard of. The equipment for manufacturing them was developed in Europe, where they are more com­mon.

Many engineering students are never introduced to this type of transformer, and learn about them by examining competitor's products, through trade literature and sales contracts.

The North American toroidal transformer market is now growing at a rate of approximately 15 per cent market value per year.

There is a market for both toroidals and E-I lami­nates, and the choice depends on the application. Toroidal transformers offer a set of characteristics, such as: low stray field, smaller size and weight, and higher efficiency, which may be required or desir­able in many products. Awareness of the usefulness of toroidal power transformers by the product and power supply designer can be another resource for optimal product design.