Power conversion apparatus

A power conversion apparatus comprising:

• a smoothing capacitor that is connected between a high-potential input terminal and a low-potential input terminal to which input voltage of a direct-current power supply is applied;
• a first primary coil and a second primary coil that configure a primary side of a transformer, one of ends of the first primary coil and one of ends of the second primary coil being connected to a shared center tap that is connected to the high-potential input terminal;
• a secondary coil that configures a secondary side of the transformer, the secondary coil being connected to a load;
• a first switch and a second switch that configure a lower arm of a bridge circuit and are alternately operated at a predetermined switching cycle,
• the first switch having a high-potential side terminal and a low-potential side terminal, the high-potential side terminal being connected to a switch-side end portion that is an end portion of the first primary coil on a side opposite to the center tap, the low-potential side terminal being connected to the low-potential input terminal, and
• the second switch having a high-potential side terminal and a low-potential side terminal, the high-potential side terminal being connected to a switch-side end portion that is an end portion of the first secondary coil on a side opposite to the center tap, the low-potential side terminal being connected to the low-potential input terminal;
• a third switch and a fourth switch that configure an upper arm of the bridge circuit and are alternately operated at the switching cycle,
• the third switch having terminals, one of the terminals being connected to the switch-side end portion of the first primary coil, and
• the fourth switch having terminals, one of the terminals being connected to the switch-side end portion of the second primary coil;
• a clamp capacitor that is connected between the other of the terminals of the third switch and the low-potential input terminal and between the other of the terminals of the fourth switch and the low-potential input terminal;
• and
• a switch controller that calculates a duty ratio that is a ratio of an on-time of each of bridge circuit switches, including the first switch, the second switch, the third switch, and the fourth switch, relative to the switching cycle, and controls operation of the bridge circuit switches, such that at least the fourth switch is turned on during an on-period of the first switch and at least the third switch is turned on during an on-period of the second switch, wherein
• the switch controller includes
• a lower-arm duty ratio calculator that calculates a lower-arm duty ratio that is a duty ratio of each of the first switch and the second switch using a map or a mathematical expression by feed-forward control based on the input voltage,
• an upper-arm duty ratio calculator that outputs a fixed value that is equal to or greater than a maximum value of the lower-arm duty ratio within a variation range of the input voltage as an upper-arm duty ratio that is a duty ratio of each of the third switch and the fourth switch, and
• a pulse width modulation generator that generates a pulse width modulation signal based on output from the lower-arm duty ratio calculator and output from the upper-arm duty ratio calculator, and outputs the pulse width modulation signal to the bridge circuit switches.