Application of the Micsig SigOFiT Optical-Fiber Isolated Probe for the dynamic characterization of silicon carbide (SiC) MOSFETs
Case overview:
Dynamic characterization of SiC MOSFETs allows engineers to determine key parameters such as switching speed and switching losses, thereby optimizing the structure and package of the devices. However, the ultrafast switching operations of SiC MOSFETs place higher demands on the parasitic properties of the measurement setup. Stray inductance and capacitance can affect measurement accuracy, making system optimization and interference control crucial.
Test case
Device under test (DUT): CREE C3M0075120K SiC MOSFET
Test points: Drain-source voltage (Vds) and gate-source voltage (Vgs)
Challenges in testing: Passive standard probes and conventional differential voltage probes introduce excessive parasitic parameters. Due to the very high dv/dt of the SiC MOSFET circuit, the probe's inductance and capacitance interact with the test circuit, resulting in pronounced voltage oscillation or overshoot. Furthermore, the parasitic capacitance generates displacement currents that superimpose unwanted components on the measured current signals, thus reducing the overall accuracy.
Test evaluation
To analyze the performance of SiC MOSFETs, a dynamic switching test was set up. The setup used C3M0075120K SiC MOSFETs together with C4D10120A freewheeling diodes. Gate control was implemented using the UCC21520 driver.
To maintain measurement accuracy:
The drain-source voltage (Vds) and the gate-source voltage (Vgs) were measured using insulated Micsig MOIP200P fiber optic probes, which offer the following
200 MHz bandwidth
o 180 dB common-mode rejection ratio (CMRR)
o Extremely low parasitic capacitance (1 pF)
The drain-source current (Ids) was measured using a Hioki 3276 current clamp (100 MHz bandwidth).
The voltage and current probes were synchronized using a time-aligned calibration circuit.
Order of waveforms (from top to bottom):
Gate-source voltage (Vgs)
Drain-source voltage (Vds)
Drain-source current (Ids)
Test observations
SiC MOSFETs exhibited switching transitions of less than 20ns.
The observed ringing of the waveform resulted mainly from the parasitic inductance of the power loop (a normal characteristic).
EMI in high-speed circuits was effectively suppressed by optical isolation.
Advantages of fiber optic insulated probes
High CMRR (180dB) enables accurate measurements in strong EMI environments.
Ultra-low parasitic capacitance (1pF) reduces displacement current error
Fiber optic transmission prevents interference from ground loops.
Preservation of curve shape accuracy supports precise evaluation of switching losses.
Customer feedback
The MOIP200P probes deliver:
- Strong high-frequency EMI suppression through 180dB CMRR
- Clean Vgs and Vds waveforms that match simulation models
- Reliable data for switching loss calculations
Technological progress compared to conventional approaches
Limitations of conventional measurement:
Parasitic effects
High parasitic capacitances (10-50pF) → artifacts due to displacement currents
Excessive inductance → voltage fluctuations that mask the true switching behavior
EMI sensitivity
Low CMRR (<60dB) → Waveform distortion at high dv/dt
Ground loop coupling → increased risk of equipment damage
Advantages of the SigOFiT Optical-Fiber Isolated Probe:
High-precision measurement
A 1pF parasitic capacitance reduces the current error by a factor of 10-50.
180dB CMRR offers an approximately 1000-fold improvement in EMI suppression.
Progress at the system level
Supports the correlation from component design to system deployment
It facilitates the industry's migration from silicon to wide-bandgap semiconductor platforms.
Conclusion
Academic Reference
L. Zhang, Z. Zhao, R. Jin, et al., “SiC MOSFET Turn-Off Measurement With Air-Core Inductor Design and RC Snubber Correction,” IEEE Transactions on Instrumentation and Measurement, vol. 74, pp. 1-13, 2025, Art no. 1005013, doi: 10.1109/TIM.2025.3545173.
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