High efficiency converter using current shaping and synchronous rectification

This paper presents circuit techniques designed to reduce the negative impact of the reverse recovery in rectifiers. The paper describes the power dissipation mechanism due to the reverse recovery of the output rectifier. The author presents four methods of reducing the reverse recovery losses for high voltage output converters 48V and above. These methods can be applied to any application wherein we are facing the reverse recovery losses. The discontinuous-continuous conduction mode with clamped reverse voltage, with application for high voltage and higher frequency operation, are presented first. The second method presented works by reducing the reverse voltage applied to the rectifier during the Trrb period. A further improvement is presented wherein the reduction of the reverse voltage is associated with a reduction of the Irrm by decreasing the current slope at turn off. A current shaping method, which works by forcing the current out of the rectifier before a reverse voltage is applied to it, is presented. This method operates by shaping the current through the output rectifiers using an additional AC voltage source superimposed on the main AC voltage induced in the secondary winding by the primary winding. In this implementation, the reverse recovery losses in the rectifiers are totally eliminated, though the converter operates in continuous conduction mode. Another major advantage of the proposed circuit is the fact that the current reflected in the primary is shaped to a trapezoidal form with a low dI/dt during the turn on of the primary switchers. This allows the completion of the resonant transition to zero voltage across the primary switching elements.