AN-4140 Transformer Design Consideration for Offline Flyback Converters Using Fairchild Power Switch ( FPS TM ) 1
For flyback coverters, the transformer is the most important factor that determines the performance such as the efficiency, output regulation and EMI. Contrary to the normal transformer, the flyback transformer is inherently an inductor that provides energy storage, coupling and isolation for the flyback converter. In the general transformer, the current flows in both the primary and secondary winding at the same time. However, in the flyback transformer, the current flows only in the primary winding while the energy in the core is charged and in the secondary winding while the energy in the core is discharged. Usually gap is introduced between the core to increase the energy storage capacity.
HF Transformer with Triaxial Cable Shielding Against Capacitive Current
The ratio of primary inductance LM to stray inductance LS determines the relative bandwidth Br and the pulse response of the transformer. In order to reduce L S , a miniature coaxial cable, whose outside shield serves, e.g., as the primary winding and whose inside conductor serves as the secondary winding, can be used. The relative bandwidth Br of such a transformer is then limited mainly by the mutual capacitance between both windings. When a triaxial (shielded coaxial) cable is used instead of a coaxial cable and when the internal braid is used as a shield and connected so that it has the same instantaneous HF potential with respect to ground as the internal conductor, then the mutual capacitance is practically zero and a shielded HF transformer with a relative bandwidth well over 104can be constructed. This construction, which was developed for precise and fast pulse circuits, can be used whenever a floating pulse or wide-band HF signal source is needed.
A Theoretical and Experimental Study of Transformer Balance
A criterion for describing the degree of balance of transformers is proposed and techniques for measuring balance in accordance with this criterion are described. Such a criterion provides a means for assigning a numerical value to the degree to which a transformer, under a particular set of operating conditions, can convert an unbalanced signal into a balanced signal or suppress unwanted longitudinal signals. It can be applied to any balanced passive system or any grounding arrangement that results in a balanced output. Although it is difficult to make a direct measurement of balance in accordance with this criterion, an indirect technique is proposed which is theoretically rigorous and permits measurements to be made under conditions that closely approximate the anticipated use conditions. These indirect measurements are relatively simple and satisfactory accuracy can be achieved with commercially available test equipment. The theory is verified by experimental data that show excellent agreement between the direct method and the proposed simplified method.