Real-time junction temperature monitoring circuit measurement scheme for SiC MOSFET

Silicon carbide equipment or equipment is known for the possibility of replacing traditional silicon equipment in power electronic equipment (especially high-power converter applications) in the near future. 1 Due to the availability of wide band gap, high power density, lower resistance and fast switching frequency, all of these are possible. High-reliability power supply systems require complex, harsh and complicated conditions and environments to work. Most cases experiencing failures are the result of power semiconductor failures. 2 Since temperature levels and changes in semiconductor devices can cause circuit failures, it is recommended to monitor the temperature appropriately, which will ultimately help the next generation of health management systems. A quasi-threshold voltage has been used to extract the junction temperature.

    Quasi-threshold voltage as TSEP

    The threshold voltage (Vth) associated with the MOS structure is the gate voltage responsible for creating a conductive channel in the device and allowing current to flow between the drain and source. Figure 1 shows that since the gate driver voltage (Vgs) is lower than the threshold voltage, the drain current (Id) is completely zero at the starting point of the conduction transition from t0 to t1. It has been observed that when Vgs reaches t1, it moves to Vth. As a result, the value of Id will increase. Here, the concept of quasi-threshold voltage is interpreted as the value of the gate drive voltage corresponding to the time t1 in the turn-on process. 4 When the junction temperature showing the negative temperature coefficient in Figure 1 rises, notice that the amount of t1 decreases. The existing relationship between two important variables (such as threshold voltage and junction temperature) is changed because the voltage on Lss' is observed to change. Due to the parasitic inductance between the Kelvin source of the SiC MOSFET and the power supply Lss', there is the possibility of a sudden increase in the high-side voltage in a synchronous manner, and this parasitic inductance will eventually be reflected by the increase in voltage. Figure 2 shows the equivalent circuit of a four-pin SiC MOSFET.

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    Figure 1: Switching waveforms during conduction

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    Figure 2: Equivalent circuit

    Quasi-Vth measuring circuit and its working principle

    Figure 3 shows the complete process. The accurate quasi-Vth is extracted by a novel method, which depends on the time of the voltage drop on the parasitic inductance when the power supply is turned on between the power supply terminal and the auxiliary power supply terminal. The block diagram in Figure 3 clearly shows the method of measuring current.

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    Figure 3: Block diagram of the measurement circuit

    The circuit shown in this figure consists of three parts:

    Drive part

    Comparison part

    Sample and hold part

    Driving part driving part

    The function of is to measure quasi-Vth by switching to a larger drive resistance. SiC MOSFET is driven by PWM signal isolation generated by TMS320F28335.

    Comparison part

    This part is responsible for converting the analog pulses present in Vss' into logic signals.

    Sample and hold part

    The differential amplifier AMP1 is used to obtain Vgs between the turn-on transient phases.

    It has been noted that the capacitor C usually follows the Vgs of the SiC MOSFET, while the quasi-Vth is maintained by the closed JFET.

    experimental device

    Figure 4 shows the tests that have been completed for the experiment. The experiment consists of a device under test with a double pulse test circuit, freewheeling diode, driver loop and load inductance.

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    Figure 4: The equivalent circuit used in the experiment

    Figure 5 shows the complete setup of the experiment to be performed. For the test equipment, SiC MOSFET and TO-247 have been used. Use the double pulse test board to install the device, and the heat is provided by the J946 temperature controller, which actually controls the closed-loop temperature of the discrete device. Figure 6 shows how to use an infrared camera to capture a strip chip.

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    Figure 5: Complete experiment setup

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    Figure 6: Junction temperature calibration setting

    result

    The results show that the threshold voltage has a linear relationship with the junction temperature. When the junction rises from 36°C to 118°C, the quasi-Vth changes by 0.358V. The load current is also changed from 10 A to 28 A. The result shows that the influence of current change is almost negligible, but the influence of Vds (DC bus voltage) is even greater. Due to the capacitor Cgd, the increase in the DC bus voltage causes the measured value of the quasi-Vth to become smaller, and its value decreases as the voltage increases from 200 V to 600 V.

    in conclusion

    This article introduces a novel measurement circuit to measure the real-time or actual junction temperature of SiC MOSFETs. It can be seen that for the purpose of ordering and processing data or current sensors, no inherently complex algorithms are required. The final result of this experiment shows that there is a good sensitivity linear relationship with the quasi-Vth junction temperature. Under the double pulse test of SiC MOSFET, the temperature coefficient is –4.3mV/°C. The load current is not responsible for this technology, it is directly connected to the DC link voltage and does not affect the linearity and sensitivity factors mentioned above. All data has been carefully collected from real sources.