Switch drive circuit and switching power supply device using same

A switch drive circuit operable to drive a full-bridge output stage connected to a transformer to alternately switch between a first cycle in which an excitation current in a first direction is supplied to the transformer and a second cycle in which an excitation current in a second direction is supplied to the transformer, the switch drive circuit being operable in:

• a mode in which, in setting a simultaneous OFF time of the full-bridge output stage at a time of shifting to one of the first cycle and the second cycle in accordance with a magnitude of an excitation current flowing in another of the first cycle and the second cycle, the larger the excitation current is, the shorter the simultaneous OFF time is made, and the smaller the excitation current is, the longer the simultaneous OFF time is made;
• and
• a mode in which, in setting a simultaneous OFF time of the full-bridge output stage in accordance with a magnitude of an average excitation current obtained by averaging an excitation current flowing in the first cycle and an excitation current flowing in the second cycle, the larger the average excitation current is, the shorter the simultaneous OFF time is made, and the smaller the average excitation current is, the longer the simultaneous OFF time is made,
• wherein switching between the modes is performed in accordance with a mode switching signal,
• wherein in accordance with a magnitude of an excitation current having flowed in the first cycle in a given period, a simultaneous OFF time in the second cycle in a next period is set, and in accordance with a magnitude of an excitation current having flowed in the second cycle in the given period, the simultaneous OFF time in the first cycle in a period after the next period is set.

A switching power supply device comprising:

• a transformer including a primary winding and a secondary winding which are electromagnetically coupled to each other;
• a full-bridge output stage connected between a power supply terminal to which an input voltage is applied and a first ground terminal;
• a switch drive circuit operable to drive the full-bridge output stage to alternately switch between a first cycle in which an excitation current in a first direction is supplied to the transformer and a second cycle in which an excitation current in a second direction is supplied to the transformer,
• wherein in setting a simultaneous OFF time of the full-bridge output stage in accordance with a magnitude of an excitation current flowing in one of the first cycle and the second cycle, the simultaneous OFF time becoming an operation changing factor in another of the first cycle and the second cycle, the larger the excitation current is, the shorter the simultaneous OFF time is made, and the smaller the excitation current is, the longer the simultaneous OFF time is made,
• the switching power supply device further comprising a rectifying-smoothing unit operable to generate an output voltage from an induction voltage appearing in the secondary winding of the transformer,
• wherein in accordance with a magnitude of an excitation current having flowed in the first cycle in a given period, a simultaneous OFF time in the second cycle in a next period is set, and in accordance with a magnitude of an excitation current having flowed in the second cycle in the given period, the simultaneous OFF time in the first cycle in a period after the next period is set.

A switching power supply device comprising:

• a transformer including a primary winding and a secondary winding which are electromagnetically coupled to each other;
• a full-bridge output stage connected between a power supply terminal to which an input voltage is applied and a first ground terminal;
• a switch drive circuit operable to drive the full-bridge output stage to alternately switch between a first cycle in which an excitation current in a first direction is supplied to the transformer and a second cycle in which an excitation current in a second direction is supplied to the transformer, the switch drive circuit being operable in:
• a mode in which, in setting a simultaneous OFF time of the full-bridge output stage at a time of shifting to one of the first cycle and the second cycle in accordance with a magnitude of an excitation current flowing in another of the first cycle and the second cycle, the larger the excitation current is, the shorter the simultaneous OFF time is made, and the smaller the excitation current is, the longer the simultaneous OFF time is made;
• and
• a mode in which, in setting a simultaneous OFF time of the full-bridge output stage in accordance with a magnitude of an average excitation current obtained by averaging an excitation current flowing in the first cycle and an excitation current flowing in the second cycle, the larger the average excitation current is, the shorter the simultaneous OFF time is made, and the smaller the average excitation current is, the longer the simultaneous OFF time is made,
• wherein switching between the modes is performed in accordance with a mode switching signal,
• the switching power supply device further comprising a rectifying-smoothing unit operable to generate an output voltage from an induction voltage appearing in the secondary winding of the transformer,
• wherein in accordance with a magnitude of an excitation current having flowed in the first cycle in a given period, a simultaneous OFF time in the second cycle in a next period is set, and in accordance with a magnitude of an excitation current having flowed in the second cycle in the given period, the simultaneous OFF time in the first cycle in a period after the next period is set.