Half-Bridge Circuit Behavior
by Dennis L. Feucht
Innovatia Laboratories
Of the various forward-switching converter circuits, the half-bridge can be one of the most difficult to design correctly. It is often the optimal choice of converter in a power range between series-forward and full-bridge circuits. It uses the same number of power switches as the series-forward converter, yet provides full-wave flux drive to the transformer, halving its core size. It uses two active switches as a half-bridge and two series capacitors to provide a voltage midway between the power rails. These capacitors can complicate behavior. This article explains a different complication, the transformer secondary currents. Once secondary circuit behavior is understood, yet another problem -- that of transformer-diode resonances -- can be addressed. Despite the problems with the conventional half-bridge, elegant solutions can transform it into one of the most attractive and reliable converter topologies.
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