# Extraction method of stray inductance of SiC MOSFET commutation loop

“Researchers Xie Zongkui, Ke Junji, etc., from the State Key Laboratory of New Energy Power Systems (North China Electric Power University), wrote in the 21st issue of “Journal of Electrotechnical Technology” in 2018, pointing out that in the high-voltage high-frequency based on silicon carbide (SiC) MOSFET devices In the converter, the fast switching transient current change rate di/dt will act on the stray inductance of the commutation loop, causing the SiC MOSFET device to bear greater electrical stress and increasing the system electromagnetic interference.

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**Summary**

Researchers Xie Zongkui, Ke Junji, etc., from the State Key Laboratory of New Energy Power Systems (North China Electric Power University), wrote in the 21st issue of “Journal of Electrotechnical Technology” in 2018, pointing out that in the high-voltage high-frequency based on silicon carbide (SiC) MOSFET devices In the converter, the fast switching transient current change rate di/dt will act on the stray inductance of the commutation loop, causing the SiC MOSFET device to bear greater electrical stress and increasing the system electromagnetic interference. Therefore, the accurate extraction of the stray inductance of the commutation loop is critical for analyzing the switching characteristics of the device.

Therefore, this paper proposes a method for extracting the stray inductance of the commutation loop based on the switching oscillation frequency, which has the advantages of not being limited by the stray resistance, measurement delay and platform size. Finally, the method is compared with different existing stray inductance extraction methods to verify its effectiveness.

Because SiC power devices have the advantages of high current density, fast switching speed, low on-resistance, and high blocking voltage[1,2]has been gradually applied in photovoltaic inverters[3]wind power, electric vehicles[4]and other fields.Compared with silicon IGBTs, SiC MOSFETs switch faster, which makes them more sensitive to stray inductances, which are prone to serious problems during switching, such as switching oscillations[5]Electromagnetic Interference (Electromagnetic Interference, EMI)[6]and additional power loss and electrical stress[7,8].

In order to estimate the electrical stress on the device during the switching process and provide a test reference for practical applications, it is necessary to accurately extract the stray inductance of the SiC MOSFET commutation loop.

At present, the extraction methods of stray inductance can be mainly divided into four categories: analytical method, numerical calculation method, direct measurement method and indirect measurement method.

The analytical method mainly uses the theoretical formula of inductance calculation, and at the same time estimates the stray inductance of the object to be measured according to the relevant parameters of the object to be measured.This method is generally only applicable to very few specific regular conductors or DC busbars, and it is usually difficult to calculate the stray inductance of a commutation circuit composed of multiple conductors and devices with different structures.[9].

Numerical methods mainly rely on finite element analysis or partial element equivalent circuit methods to solve Maxwell’s equations (such as ANSYS Q3D Extractor).This method analyzes and calculates the inductance and capacitance based on the geometry and material information of the measured object[10-12].

However, when the physical structure of the measured object becomes complex, this method not only takes a long time to calculate, but also has poor convergence. In addition, if the current path set during the simulation is different from the actual current path, the simulation result will deviate from the actual value.

The direct measurement method can be divided into two methods: time domain reflectometry and frequency domain impedance measurement.Time Domain Reflectometry is based on transmission line theory and extracts parasitic inductances from reflected signals at delay times[13]but the complex experimental setup, special hardware, and multi-step iterative process limit its scope of application.

The impedance measurement method uses an impedance analyzer to measure the impedance between two terminals of the object to be measured[14-16], but this method is currently limited to single-port measurements between the two terminals of the object to be measured. When other terminals remain in the floating state, the inductive capacitance between the floating terminal and the ground will affect the impedance measurement and introduce measurement errors. In addition, the measurement equipment required for this measurement method is relatively expensive, and the size of the object to be measured is limited.

The existing indirect measurement method mainly solves according to the volt-ampere characteristic relationship of the inductance, and uses the turn-on or turn-off drain-source voltage waveform and the drain current waveform of the corresponding time period to extract the stray inductance.

According to the different methods of mathematically processing the voltage and current waveforms obtained from the measurement, the existing indirect measurement methods can be divided into differential methods.[8,17]and integral method[18-20]. Among them, the differential method has higher requirements on the current waveform, and the experimental waveform usually has high-frequency ripple, which leads to the inability to accurately obtain the current change rate.

Although the integration method overcomes the disadvantage that the differential method is very sensitive to the shape of the current waveform, due to the existence of the stray resistance of the loop, the selection of the integration interval of the integration method will also affect the extraction results of the stray inductance. Additionally, both methods require voltage and current measurements.

In general, current probes are more expensive and have a much lower measurement bandwidth than voltage probes. And this method based on voltage and current waveforms theoretically requires no delay between the voltage and current waveforms. Therefore, these two indirect measurement methods may be suitable for the extraction of the stray inductance of the silicon IGBT device test platform.

However, for SiC MOSFET devices with fast switching speeds, the turn-on and turn-off times are only tens of nanoseconds, and even a few nanoseconds delay between the voltage and current waveforms will have a great impact, and Existing high bandwidth current probes are typically larger in size and more expensive.

In view of the problems existing in the current stray inductance extraction method, this paper proposes a method for extracting the stray inductance of the SiC MOSFET commutation circuit, and based on the switching transient characteristics test platform of SiC power devices, the stray inductance extracted in this paper is extracted. The feasibility of the method was verified. Different from the existing indirect measurement methods, this method solves the commutation loop stray inductance based on the SiC MOSFET switching transient oscillation frequency.

By modeling and analyzing the oscillation phase of the SiC MOSFET switching process, the relationship between the switching oscillation frequency and the loop stray inductance and stray capacitance is derived. Externally inserted discrete capacitors are used to artificially change the switching oscillation frequency, thereby constructing a mathematical equation system including inductance and capacitance parameters, and solving the equation system through the elimination method to calculate the stray inductance of the commutation loop.

Figure 1: Test System Block Diagram

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