A system including a temperature sensing system and a switching device, comprising: a) a temperature sensor configured to sense a temperature of the switching device; b) a signal converter configured to output a PWM digital signal having a frequency based on a voltage across the temperature sensor by a frequency variable method which receives the voltage across the temperature sensor corresponding to a sensed temperature value when a current flows in the temperature sensor to convert a level of an analog signal into a frequency of the digital signal; c) an isolator configured to be electrically connected to an output terminal of the signal converter to isolate and transmit the digital signal output from the signal converter; d) a controller configured to recognize the temperature from the frequency of the digital signal transmitted through the isolator; and e1) a power supply configured to apply a current to the temperature sensor and supply power to the signal converter; e2) wherein the power supply includes a regulator configured to smooth and output a constant regulated fixed voltage to an output terminal connected to the signal converter and the temperature sensor to allow a substantially constant current flow in an input terminal:of the temperature sensor; and e3) wherein the switching device includes an insulated gate bipolar transistor (IGBT) and the temperature sensor includes one or more diodes integrated at a heat generation portion within the insulated gate bipolar transistor (IGBT); f) wherein the signal converter includes a voltage controlled oscillator having operation characteristics in which the frequency of the output signal varies based on an input voltage; and g) an output of the power supply is connected to a first terminal of a first resistor and a power supply terminal (Vcc) of the signal converter, the other terminal of the first resistor being connected to a first terminal the temperature sensor and to a first terminal of a second resistor the other second terminal of which is connected to an input of the signal converter and is connected through a parallel circuit of a third resistor and a capacitor to ground, wherein the temperature sensor has a second terminal connected to ground, such that the voltage over the one or more diodes represents the junction temperature of the IGBT. a) a temperature sensor configured to sense a temperature of the switching device; b) a signal converter configured to output a PWM digital signal having a frequency based on a voltage across the temperature sensor by a frequency variable method which receives the voltage across the temperature sensor corresponding to a sensed temperature value when a current flows in the temperature sensor to convert a level of an analog signal into a frequency of the digital signal; c) an isolator configured to be electrically connected to an output terminal of the signal converter to isolate and transmit the digital signal output from the signal converter; d) a controller configured to recognize the temperature from the frequency of the digital signal transmitted through the isolator; and e1) a power supply configured to apply a current to the temperature sensor and supply power to the signal converter; e2) wherein the power supply includes a regulator configured to smooth and output a constant regulated fixed voltage to an output terminal connected to the signal converter and the temperature sensor to allow a substantially constant current flow in an input terminal:of the temperature sensor; and e3) wherein the switching device includes an insulated gate bipolar transistor (IGBT) and the temperature sensor includes one or more diodes integrated at a heat generation portion within the insulated gate bipolar transistor (IGBT); f) wherein the signal converter includes a voltage controlled oscillator having operation characteristics in which the frequency of the output signal varies based on an input voltage; and g) an output of the power supply is connected to a first terminal of a first resistor and a power supply terminal (Vcc) of the signal converter, the other terminal of the first resistor being connected to a first terminal the temperature sensor and to a first terminal of a second resistor the other second terminal of which is connected to an input of the signal converter and is connected through a parallel circuit of a third resistor and a capacitor to ground, wherein the temperature sensor has a second terminal connected to ground, such that the voltage over the one or more diodes represents the junction temperature of the IGBT.