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1. A forward type DC-DC converter that converts a voltage of a DC input power supply to a specified DC output voltage insulated from the voltage of the DC input power supply, the DC-DC converter comprising:
a series circuit of a primary side semiconductor switch and a primary winding of a transformer, the series circuit being connected in parallel to the DC input power supply; a rectifying diode that rectifies a voltage across a secondary winding of the transformer; a smoothing filter comprising a DC reactor and a smoothing capacitor, the smoothing filter that smoothes an output voltage of the rectifying diode; a freewheeling diode that freewheels a current in the DC reactor; a first MOSFET connected in inverse parallel to the rectifying diode; and a second MOSFET connected in inverse parallel to the freewheeling diode, a voltage across the smoothing capacitor being served as a DC output voltage, the DC output voltage being controlled by turning-on and -off operations of the first MOSFET and the second MOSFET based on periodical turning-on and -off operations of the primary side semiconductor switch, and an arithmetic circuit in which a turning-on time width of the first MOSFET is calculated by use of a time width within which a current flows in the DC reactor on a secondary side within a turning-off time width of the primary side semiconductor switch in a previous period, the DC output voltage and the voltage across the secondary winding of the transformer, and a turning-on time width of the second MOSFET is calculated by use of a turning-on time width of the primary side semiconductor switch immediately before a turning-on of the second MOSFET, the DC output voltage, and the voltage across the secondary winding of the transformer. |
