Synchronous rectifying circuit of an active clamping type with less driving loss and less continuity loss

A synchronous rectifying circuit of an active clamping type for converting an input d.c. voltage into an output d.c. voltage, said synchronous rectifying circuit having a pair of input terminals supplied with the input d.c. voltage and a pair of output terminals for outputting the output d.c. voltage, said synchronous rectifying circuit comprising a main transformer having a primary winding and a secondary winding,a primary circuit connected to the input terminals and the primary winding of said main transformer, a secondary circuit connected to the secondary winding of said main transformer and the output terminals, and a control circuit connected to a preselected one of the output terminals for controlling the primary circuit to produce main and subsidiary control pulse signals,

• said primary circuit comprising:
• a main switching device connected in series to the primary winding of said main transformer to form a primary series connection circuit and operable in response to the main switch control pulse signal to be selectively put into an on-state and an off-state, said primary series connection circuit being connected between said input terminals;
• and
• a first series circuit, connected in parallel to the primary winding of said main transformer, comprising a clamping capacitor and a subsidiary switching device which is operable in response to the subsidiary control pulse signal to be selectively put into an on-state and an off-state, said subsidiary switching devices carrying out reverse operation with said main switching device to clamp a primary reset voltage appearing at the primary winding of said main transformer;
• said secondary circuit including a synchronous rectifier connected to parallel to the secondary winding of said main transformer and comprising:
• a second series circuit, connected in parallel to the secondary winding of said main transformer, comprising first and second synchronous rectifying elements which are operable in synchronism with said main switching device, the first and the second synchronous rectifying elements having first and second control terminals, respectively, the first and the second synchronous rectifying elements being joined together at a node;
• a first impedance circuit inserted between an end of the secondary winding of said main transformer and the first control terminal of said first synchronous rectifying element;
• and
• a second impedance circuit inserted between another end of the secondary winding of said main transformer and the second control terminal of said second synchronous rectifying element.