Semiconductor device

1. A semiconductor device comprising:

a first power semiconductor element;

a second power semiconductor element that is connected in parallel with the first power semiconductor element;

a voltage changing unit that changes a voltage applied to a control terminal of the first power semiconductor element when the second power semiconductor element is turned on;

a detection unit that detects a current flowing in the first power semiconductor element when the voltage changing unit has changed the voltage applied to the control terminal of the first power semiconductor element; and

a temperature estimation unit that estimates a temperature of the first power semiconductor element based on a characteristic of a change of the current of the first power semiconductor element with respect to a change of the voltage applied to the control terminal of the first power semiconductor element.

2. The semiconductor device according to claim 1 , wherein

the first power semiconductor element is an element made of a wide band gap semiconductor.

3. The semiconductor device according to claim 1 , wherein

the second power semiconductor element is an element made of silicon.

4. The semiconductor device according to claim 3 , wherein

the second power semiconductor element includes a plurality of cells connected in parallel,

the second power semiconductor element includes a first terminal in which a current flows and a second terminal in which a current flows, the current flowing in the first terminal being greater than the current flowing in the second terminal depending on a number of the cells connected to each of the first terminal and the second terminal, and

the detection unit detects the current flowing in the second terminal, and detects the current flowing in the first power semiconductor element based on the detected current flowing in the second terminal.

5. The semiconductor device according to claim 1 , wherein

when the temperature estimated by the temperature estimation unit is equal to or higher than a predetermined threshold temperature, the voltage changing unit applies a voltage, by which the current flowing in the first power semiconductor element is zero, to the control terminal so as to shut off the first power semiconductor element.

6. The semiconductor device according to claim 1 , wherein

each of the first power semiconductor element and the second power semiconductor element is turned on and off in accordance with an PWM signal, and

the voltage changing unit begins to change the voltage applied to the control terminal of the first power semiconductor element, when a predetermined time period has elapsed after the PWM signal was turned on, for detecting the temperature of the first power semiconductor element.

7. The semiconductor device according to claim 6 , wherein

the predetermined time period is set to a period equal to or greater than a half of a period of the PWM signal.

8. The semiconductor device according to claim 1 , wherein

each of the first power semiconductor element and the second power semiconductor element is turned on and off in accordance with a PWM signal,

the semiconductor device further comprising

an instruction unit that instructs the voltage changing unit to begin to change the voltage at a time point a predetermined time period before the PWM signal is turned off, wherein

the voltage changing unit begins to change the voltage applied to the control terminal of the first power semiconductor element when the voltage-changing unit is instructed by the instruction unit.

9. The semiconductor device according to claim 1 , wherein

each of the first power semiconductor element and the second power semiconductor element is turned on and off in accordance with a PWM signal, and

the voltage changing unit begins to change the voltage applied to the control terminal of the first power semiconductor element, based on turning off of the PWM signal as a trigger, for detecting the temperature of the first power semiconductor element.

10. The semiconductor device according to claim 9 , wherein

the second power semiconductor element is shut off in synchronization with shutting off of the first power semiconductor element, after detection of the temperature of the first power semiconductor element is completed.

11. The semiconductor device according to claim 1 , wherein

each of the first power semiconductor element and the second power semiconductor element is turned on and off in accordance with a PWM signal, and

the voltage changing unit begins to change the voltage applied to the control terminal of the first power semiconductor element in synchronization with turning on of the PWM signal, for detecting the temperature of the first power semiconductor element.

12. The semiconductor device according to claim 1 , wherein

the first power semiconductor element is provided by of a field effect transistor, and

the second power semiconductor element is provided by an insulated gate bipolar transistor.

13. The semiconductor device according to claim 1 , wherein

the temperature estimation unit estimates the temperature based on at least one of a gradient of the change of the current with respect to the change of the voltage applied and a voltage at which the current is zero, as the characteristic of the change of the current with respect to the change of the voltage applied.