Soft-switching converter DC-to-DC isolated with voltage bidirectional switches on the secondary side of an isolation transformer

A soft-switching dc-to-dc converter comprising

• an isolation transformer with two secondary windings and one primary winding, each secondary winding having one end connected to an output return current path and the other end coupled to an output low-pass filter at a common junction by a separate one of a pair of voltage bidirectional switches, said two secondary windings being wound in opposite sense, such as in a half-bridge converter,
• means for regulation of output voltage from said low-pass filter,
• a pair of current bidirectional switching means connected to said primary winding for convening a dc voltage to a square-wave voltage, said pair of current bidirectional switching means being responsive to control means for alternately turning on each of said pair of current bidirectional switching means at a constant switching frequency with a duty ratio close to 50% and with an adjustable dead time between turning off one of said pair of current bidirectional switching means and turning on the other of said pair of switching means, said dead time being sufficient to assure that voltage across the other of said pair of current bidirectional switching means drops to zero volts before the other of said pair of current bidirectional switching means is turned on, thereby assuring zero volt switching of said pair of current bidirectional switching means for converting a dc voltage to a square-wave voltage, wherein each of said pair of voltage bidirectional switches couples a separate one of said two secondary windings to said low-pass filter at said common junction, thereby delaying turn-on voltage through one of said voltage bidirectional switches that is off while the other of said voltage bidirectional switches that is on is turned off, thereby providing voltage out of phase with current at said common junction until said one voltage bidirectional switch is turned on,
• means for sensing voltage at a common point between said pair of current bidirectional switching means connected to said primary winding, and
• means responsive to said sensing means for enabling alternate turn on of each of said pair of current bidirectional switching means only at zero voltage,
• whereby zero volts switching is preserved for primary current bidirectional switching means and secondary bidirectional voltage switches under all operating conditions from full load current to no-load while said output voltage regulation means maintain output voltage constant.