Power source apparatus supplying power to load

1. A power source apparatus for supplying power to a load, comprising:

a switch;

a controller that:

detects a voltage applied to the switch,

switches a state of the switch from OFF to ON when the voltage is equal to or smaller than a predetermined threshold value;

a first DC power source that applies DC voltage between terminals of the switch; and

a compensating circuit that is connected in parallel to the switch and that comprises a second DC power source and a capacitor,

wherein the controller controls a combined capacitance of parasitic capacitances of the switch and the compensating circuit to be constant by changing a voltage of the second DC power source.

2. The power source apparatus according to claim 1 , wherein the controller switches the state of the switch from OFF to ON by adjusting a duty ratio of the switch.

3. The power source apparatus according to claim 2 ,

wherein the controller:

detects a differentiation value of the voltage based on the voltage, and

controls the duty ratio of the switch based on the differentiation value.

4. The power source apparatus according to claim 3 ,

wherein the controller:

increases the duty ratio of the switch when the differentiation value is a positive value and when the state of the switch is switched from OFF to ON, and

decreases the duty ratio of the switch when the differentiation value is a negative value and when the state of the switch is switched from OFF to ON.

5. The power source apparatus according to claim 2 ,

wherein the controller:

detects input and output power of the power source apparatus,

determines power efficiency based on the input and the output power, and

controls the duty ratio based on the power efficiency.

6. The power source apparatus according to claim 5 , wherein the controller controls the duty radio so that the power efficiency is maximized.

7. The power source apparatus according to claim 1 , wherein the predetermined threshold value is 10% or less than a maximum value of voltage applied between terminals of switching elements of the switch.

8. The power source apparatus according to claim 1 , further comprising:

a plurality of first capacitors connected between the switch and ground,

wherein the controller controls a timing of when the voltage becomes a local minimum or controls the local minimum of the voltage by switching a connection of the plurality of first capacitors.

9. The power source apparatus according to claim 8 , further comprising:

a plurality of second capacitors connected between the switch and the load,

wherein the controller adjusts the timing of when the voltage becomes the local minimum or adjusts the local minimum of the voltage by switching at least one of the connection of the plurality of first capacitors and a connection of the plurality of second capacitors.

10. The power source apparatus according to claim 9 , further comprising:

a plurality of inductors connected between the switch and the load,

wherein the controller controls the timing of when the voltage becomes the local minimum or controls the local minimum of the voltage by switching at least one of the connection of the plurality of first capacitors, the connection of the plurality of second capacitors, and the connection of the plurality of inductors.

11. The power source apparatus according to claim 8 , wherein the controller switches the connection of the plurality of first capacitors based on the duty ratio of the switch.

12. The power source apparatus according to claim 1 , further comprising:

a choke coil that is connected between the first DC power source and one of the terminals of the switch,

wherein the choke coil makes DC current from the first DC power source constant.

13. The power source apparatus according to claim 1 , further comprising:

a thermistor that detects a temperature of the switch; and

a memory that stores a first table that associates the temperature with the voltage of the second DC power source,

wherein the controller controls the voltage of the second DC power source based on the first table so that the combined capacitance becomes constant.

14. The power source apparatus according to claim 13 ,

wherein the memory stores a second table that associates voltages of the first and the second DC power source, and

wherein the controller controls the voltage of the second DC power based on the first and the second table so that the combined capacitance becomes constant.