In a switching DC/DC converter using an insulating transformer Tr, a primary coil and a secondary coil of the transformer Tr, an input choke coil L1, and an output choke coil L3 are wound around a common core and are formed into a single component. A primary circuit of the transformer Tr includes a switching device Q1 connected in series with the primary coil, an input choke coil L1 connected across the terminals of the primary coil, and a series circuit of a clamp capacitor C1 and a switching device Q1' which are connected across the terminals of the primary coil. A secondary circuit of the transformer Tr includes rectifying switching devices Q2 and Q2' connected to the secondary coil, the output choke coil L3, and an output smoothing capacitor C.sub.0. Further, a series circuit of the switching device Q1' and the clamp capacitor C1 may be connected across the terminals of the switching device Q1. The coils are arranged in directions of canceling DC fluxes generated by the coils, so that it is possible to greatly reduce the DC bias magnetization of the core, the volume of the core, and core losses, thereby achieving higher efficiency in the overall apparatus.
A DC conversion apparatus includes a switch that converts a DC voltage of a DC power source to a high-frequency voltage by being turned ON/OFF via a primary winding of a transformer, a synchronous rectifying circuit that performs synchronous rectification of a high-frequency voltage generated at a secondary winding, a smoothing circuit that smoothes a rectified output of the synchronous rectifying circuit by a smoothing reactor and a smoothing capacitor, and outputs a DC voltage, a capacitor that stores via a diode, energy stored in the smoothing rector due to a current backflow of the synchronous rectifying circuit under light load condition, a switch that is turned ON in synchronization with the switch, and returns the energy stored in the capacitor via the transformer to a primary side thereof, and a control circuit that turns ON/OFF the switch.
A DC conversion apparatus includes a switch converting a DC voltage of a DC power source to a high-frequency voltage by being turned ON/OFF via a primary winding of a transformer, a series circuit of a switch and a clamp capacitor being connected to both ends of the primary winding, a synchronous rectifying circuit performing synchronous rectification of a high-frequency voltage generated at a secondary winding, a smoothing circuit smoothing a rectified output of the synchronous rectifying circuit by a smoothing reactor and a smoothing capacitor, a capacitor storing energy stored in the smoothing reactor due to a current backflow of the synchronous rectifying circuit caused under light load condition, a switch being turned ON in synchronization with the switch and returning the energy stored in the capacitor to the clamp capacitor on the primary side via the transformer, and a control circuit turning the switch and the switch ON/OFF states alternately.
A capacitor charging circuit does not include a Zener diode and makes it possible to integrate a large number of circuits and elements within a semiconductor integrated circuit. The capacitor charging circuit includes a flyback transformer having a primary coil, a secondary coil, and a tertiary coil, a switching element that turns the current that flows through the primary coil on and off, a capacitor that is charged by a current produced in the secondary coil through the on-off action of the switching element, and a charge control circuit that controls the on-off action of the switching element. When the switching element is off and a voltage produced at a first end of the tertiary coil is larger than a reference voltage corresponding to the predetermined voltage of the charge voltage of the capacitor, the on-off action of the switching element is stopped.
