Power source switching device and electronic appliance

1. A power source switching device which switches between a first power source and a second power source as a power source for supplying a driving voltage to a load section, the power source switching device comprising:

a first switch circuit provided between the first power source and the load section;

a second switch circuit provided between the second power source and the load section;

a third switch circuit provided between the first power source and the load section in series with the first switch circuit;

a fourth switch circuit provided between the second power source and the load section in series with the second switch circuit;

a first diode provided in parallel with the third switch circuit, the first diode operable to allow a current to flow from the first power source to the load section while preventing a current to flow from the second power source to the first power source;

a second diode provided in parallel with the fourth switch circuit, the second diode operable to allow a current to flow from the second power source to the load section while preventing a current to flow from the first power source to the second power source;

a logic circuit operable to control the third switch circuit and the fourth switch circuit to prevent at least one of the third switch circuit and the fourth switch circuit from turning on while both of the first switch circuit and the second switch circuit are in an on state; and

a control circuit operable to control the first switch circuit, the second switch circuit, and the logic circuit.

2. The power source switching device according to claim 1 , wherein when the control circuit outputs a signal for turning on both of the first switch circuit and the second switch circuit, the logic circuit outputs a signal for turning off both of the third switch circuit and the fourth switch circuit.

3. The power source switching device according to claim 1 , wherein when changing the first switch circuit from the off state to the on state, the logic circuit turns off the fourth switch circuit before turning on the first switch circuit, and when changing the second switch circuit from the off state to the on state, the logic circuit turns off the third switch circuit before turning on the second switch circuit.

4. The power source switching device according to claim 1 , wherein the third switch circuit includes a first FET, the first diode includes a parasitic diode of the first FET, the fourth switch circuit includes a second FET, and the second diode includes a parasitic diode of the second FET.

5. An electronic appliance comprising the power source switching device according to claim 1 .

6. A camera body which can be mounted with an interchangeable lens and includes a load section which is supplied power from at least one of a first power source and a second power source, the camera body comprising:

a first switch circuit provided between the first power source and the load section;

a second switch circuit provided between the second power source and the load section;

a third switch circuit provided between the first power source and the load section in series with the first switch circuit;

a fourth switch circuit provided between the second power source and the load section in series with the second switch circuit;

a first diode provided in parallel with the third switch circuit, the first diode operable to allow a current to flow from the first power source to the load section while preventing a current to flow from the second power source to the first power source;

a second diode that is provided in parallel with the fourth switch circuit, the second diode operable to allow a current to flow from the second power source to the load section while preventing a current to flow from the first power source to the second power source;

a logic circuit operable to control the third switch circuit and the fourth switch circuit to prevent at least one of the third switch circuit and the fourth switch circuit from turning on while both of the first switch circuit and the second switch circuit are in an on state; and

a control circuit operable to control the first switch circuit, the second switch circuit, and the logic circuit.

7. The camera body according to claim 6 , wherein when the control circuit outputs a signal for turning on both of the first switch circuit and the second switch circuit, the logic circuit outputs a signal for turning off both of the third switch circuit and the fourth switch circuit.

8. The camera body according to claim 6 , wherein the first power source is stored in the camera body and the second power source is provided outside the camera body.

9. The power source switching device according to claim 2 , wherein when changing the first switch circuit from the off state to the on state, the logic circuit turns off the fourth switch circuit before turning on the first switch circuit, and when changing the second switch circuit from the off state to the on state, the logic circuit turns off the third switch circuit before turning on the second switch circuit.

10. The camera body according to claim 8 , wherein the second power source is provided in a battery grip which can be mounted to the camera body.

11. The camera body according to claim 8 , wherein the second power source supplies power to the load section via a power cord connected to the camera body.