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The pros & cons of toroidal power transformers This article describes how toroidal power transformers can yield lower hum, less weight and size, and improved efficiency compared with conventional E-I laminated transformers. The disadvantages include slightly higher cost and greater inrush currents at switch on. By MICHAEL LARKIN* Why choose a linear power supply? Since most digital/analog circuits cannot operate from rectified, filtered, power line voltage, a step-down conversion unit must be used. Electronic equipment designers have two main methods for powering their equipment: switching and linear supplies. If the product’s operational and environmental constraints permit high levels of radiated and conducted EMI (electro­magnetic interference), slower closed-loop response to load variations and reduced reliability, then the cost, power density and efficiency of a switching supply becomes attractive. By many designs cannot tolerate the characteristics of switching supplies, making linear supplies the only viable alter­native. Examples of these products include high quality audio mixers and amplifiers, lighted matrix displays, and video processing and display equipment. Toroidal transformers are inherently time-consuming and tedious to make. The total length of wire for each winding must first be loaded onto a bobbin which is then wound onto the core, together with inter-layer insulation. 12  Silicon Chip A transformer is required to step down the AC voltage from that of the power line to the rectification, filtering and regu­lation circuits in a linear supply. The inherent advantages of the toroidal (donut-shaped core) transformer, relative to other core configurations, may be summarised generally as a nearly ideal magnetic circuit, which results in lower stray magnetic field, smaller volume and weight, less audible hum and 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 fields. Ideal magnetic circuit The toroidal transformer has a nearly ideal magnetic cir­cuit. In an E-I laminated transformer it is not possible to align the grain structure of the stamped laminations with the flux path over the entire magnetic path. This inability leads to higher core losses and less efficient operation when compared to tor­oids. Fig.1 shows a comparison of grain alignment with flux path for a toroidal and an E-I transformer. Conventional laminated transformer designs employ a bobbin-wound coil placed over a stack of “E” shaped laminations. An “I” shaped stack is butted onto the “E” , completing the magnetic path. The connection between the “E” and “I” is never a perfect junction, causing a discontinuity or air gap in the magnetic flux path. This gap has higher reluctance and so causes a 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 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 on pressure and the quality of the mating surfaces. 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 opportunities 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. By contrast, there is no air gap in the core of a toroidal transformer. The core is tightly wound onto a mandrel, like a clock spring, from a continuous strip of grain-oriented steel. Spot welding at the beginning and ends prevents loosening. The stresses introduced by de-reeling and winding, which could result in unacceptable core losses, are relieved by annealing the wound core in a dry nitrogen atmosphere. The result is a stable, pre­dictable core, free from discontin­ uities, holes, clamps and gaps. Fig.2 compares the stray magnetic field at 100mm from E-I laminated and toroidal transformers of equal power rating. If shielding with a copper strap and careful 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 justified. However, the toroid’s substan­ tially lower stray field may mean the difference between accept­ able and unacceptable operation of sensitive circuitry. Reduced weight and size In the E-I core structure, the magnetic flux is not aligned with the grain of the steel for approximately 25% of the flux path (see Fig.1). This misalignment causes higher magnetisation losses and reduces the maximum flux density that can be utilised in the core. Higher efficiencies have been made possible by using high grades of grain-oriented steel which increases flux densi­ties while minimising loss- Fig.1: comparison of grain alignment with flux path for a toroidal and an E-I transformer. In the toroidal transformer, the grain alignment is always optimum. Fig.2: this graph compares the stray magnetic field at 100mm from E-I laminated and toroidal transformers of equal power rating. If the E-I transformer has a single section bobbin (or no bobbin) it may be possible to fit a copper shielding strap to greatly reduce the stray field. es. However, maximum utilisation of these properties occurs only when the flux in the steel is paral­lel to the grain direction. It can be seen in Fig.1 that the flux in a toroidal core is 100% aligned with the grain of the steel. The typical working flux density of E-I 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 the number of turns. The higher allowable flux density of a toroid thus requires fewer turns of wire in all windings to achieve the same result. Comparison of a conventional 960VA transformer with an equivalent toroid shows the weight and volume of the toroid to be only half. In an existing product which uses an E-I transformer, it is often possible to fit a toroid which has close to the same footprint but is only 60% as tall. In the case where it is desir­able to increase the power supply power rating without increasing its size, the E-I transformer might be replaced with a toroid which is the same size but has 1.5 to 2-times the power rating. It is true that the empty centre hole in a toroidal trans­ former, which is needed to enable winding, occupies some wasted volume. This wasted volume deficit is overcome by the toroid’s December 1995  13 Fig.3: this circuit can be used to substantially reduce the inrush current for a toroidal transformer. The relay coil is energised by the AC input voltage and the delay in the relay operation, which switches R1 out of circuit, is sufficient to reduce the inrush current to a safe value. volume advantage at roughly 50VA and above. So, in transformers rated less than 50VA, the size is not reduced 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 the coil turns and core laminations and due to magnetostriction in the core itself, which is manifest at any gaps in the core. Clamps, bands, rivets and welds cannot bind the entire structure and varnish penetrates the laminations only partially. As a result, laminations tend to loosen over time and produce increasing noise. On the other hand, the nature of the toroidal transformers’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 The efficiency of a transformer is stated as: E = Pout/Pin where Pout is the power delivered to the load and Pin is the power input to the transformer. The difference between Pin and Pout is represented by the losses in the core and windings. The ideal magnetic circuit of the toroid and its 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. Both benefits reduce the losses. Toroidal transformers are typically 90-95% 14  Silicon Chip efficient, whereas E-I laminated types are typically less than 90% efficient. Inrush current The characteristics which give the toroidal transformer advantages also contribute to a disadvantage: high inrush current with the initial application of power. The absence of a gap in the toroidal core means that the maximum possible remanence (residual magnetisation of the core in a particular direction and magnitude) can be substantially more pronounced in a toroid than in an E-I type. The core “stores” the static bias when the power is switched off. If the removal of power occurs at an unfavourable 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 Vpk divided by Rp, where Vpk is the peak primary voltage and Rp is the primary resistance, depending on the power capability of the transformer and how strongly the core was magnetised. To cope with these very high surge currents, a fuse or circuit breaker with an appropriate time delay is needed; a fast blow fuse will not last for more than a few off-on power cycles. In high power applications, more exotic means may be re­ quired to ensure protection that will survive inrush, yet still protect in fault situations. One method involves a relay with its coil 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 transform­ er, the electromechanical relay begins to move from the deener­ gised to the energised 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 Fig.3. Higher cost Toroidal transformers are manufactured individually and have a high labour content. Conversely, the plastic bobbins of small E-I laminated transformers can be wound on machines which handle several bobbins at once and operate nearly unattended. The process of applying inter-winding insulation is also more labour 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 fash­ ion. This conforms best to the curved surfaces. The difference in labour content is reduced for power rat­ings of 500VA and above. Large transformers are usually made in small quantities and become more difficult to wind as the core becomes larger and the wire gauges thicker. 3-phase toroidal transformers Employing toroidal construction for 3-phase transform­ers does not offer a volume advantage over the E-I laminated type. The E-I configuration is more suited to 3-phase transform­ers, since 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 3-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 3-phase transformers is available, a toroidal 3-phase transformer set is bene­fi­cial. Summary The choice between toroidal and E-I laminated transformers depends on the application. Toroidal transformers offer low stray fields, smaller size and weight and higher efficiency, which may be required or desirable in many products. *Michael Larkin is the Managing DiSC rector of Tortech Pty Ltd.