Optimizing Linear Power Supply Performance with Line Frequency Toroidal Transformers

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 cannot operate 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 EM, 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 highi 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 ( I ) 67 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. eE6ent operation when compared to toroids. Figure 1 shows a comparison of grain alignment with flux path for a toroidal and E-I laminate. €4 Laminate Flux Toroid Flux o SIccl Grain b Flux Diredion