Silicon Carbide (SiC) MOSFETs have revolutionized power electronics by enabling faster switching speeds, higher efficiencies, and greater power densities compared to traditional Silicon IGBTs. However, these extremely fast switching transients (high dv/dt) introduce significant challenges during testing and measurement.
Whether it's preventing systematic failures or anticipating and mitigating future risk, functional safety has changed the way engineers think about designing systems. One of the most insidious problems engineers face when characterizing SiC devices is common mode noise.
The dv/dt Problem
When a SiC MOSFET switches, the voltage across it can change by hundreds of volts in just a few nanoseconds. This massive dv/dt injects displacement currents through the parasitic capacitances of your measurement probes. If your differential probe does not have an exceptionally high Common Mode Rejection Ratio (CMRR) at high frequencies, this displacement current manifests as a massive error voltage on your oscilloscope screen.
"What looks like ringing or excessive overshoot on your scope might just be common mode noise bypassing your probe's differential amplifier."
Mitigating the Errors
To accurately measure high-side gate-source voltages (Vgs) or drain-source voltages (Vds) in a half-bridge configuration, engineers must employ specific techniques:
- Optically Isolated Probes: Traditional differential probes often fail at SiC switching speeds. Optically isolated probes physically decouple the measurement tip from the oscilloscope ground, virtually eliminating common mode capacitance.
- Minimizing Ground Loops: Keep your probe connections as short as physically possible. Even a few centimeters of wire can introduce parasitic inductance that rings with the probe's capacitance.
- Coaxial Shunts: For measuring drain current (Id) during fast switching, high-bandwidth coaxial shunts or specialized Rogowski coils are preferred over standard current probes to capture the true high-frequency transient.
Conclusion
At PMT Electronics, our Hardware Design team meticulously accounts for these high-frequency phenomena. By understanding the true source of measurement errors, we ensure that our motor drivers and power modules perform exactly as simulated, providing our clients with reliable, high-efficiency power conversion solutions.
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