Single-switch Forward Converter Research Articles

Common-mode EMI evaluation of forward converter with various core-reset schemes

Abstract: In switching power converters, electromagnetic emissions can interfere with the normal operation of the converter or other adjacent systems. Among various power converters, forward converters are widely used at low and medium powers that need a transformer core-reset scheme. In this paper, a prediction procedure for conducted commonmode EMI of a single-switch forward converter is presented, and common-mode EMI levels are predicted considering heat-sink parasitic capacitors and main PCB parasitic elements. The accuracy of prediction results is examined via experimental results. In addition, effects of conventional passive core-reset schemes on the conducted EMI are evaluated via experimental results. Along with the passive reset scheme, common-mode EMI of the low-side and high-side active clamps are evaluated. Finally, the EMI comparison between various core-reset techniques is presented.

Conducted electromagnetic interference evaluation of forward converter with symmetric topology and passive filter

Forward converter is popular for isolated switching power supply. Generally, power switching converters are the sources of electromagnetic interference (EMI) because of their high transient voltage and current. In this study, reduction of conducted EMI in the single-switch forward converter is examined using a symmetric topology. At this end, the forward converter topology is modified to achieve a symmetric topology. EMI model of the conventional forward converter and symmetric forward converter are derived taking into account the main parasitic of components and printed circuit tracks. Accuracy of the predicted EMI is verified by the measured EMI for two converters. In addition to the symmetric approach evaluation from the EMI viewpoint, the effect of this approach on the output voltage noise is examined. Finally, the combination of a passive EMI filtering with the symmetric method is utilised for EMC compliance.

Harmonic winding losses in the transformer of a forward pulse width modulated DC–DC converter for continuous conduction mode

This study presents general winding power loss expressions for a high-frequency (HF) three-winding forward transformer operated in continuous conduction mode (CCM), where the source of the transformer excitation is considered to be non-sinusoidal with many significant harmonics. The efficient operation of an isolated converter is dependent upon the performance of the transformer used. The expressions presented in this study take into account the eddy-current losses caused by current harmonics at HFs. The expressions are derived based on Fourier series of the winding current waveforms, and using Dowell's equation, which considers both the skin and proximity effects caused by eddy currents. Harmonic winding loss factors for the three windings are introduced. In addition to the power loss expressions, a step-by-step procedure to design a three-winding transformer used in a single-switch forward converter operated in CCM is presented using the area product (Ap) method.