Safe gate measurements on SiC and GaN power electronics with optically isolated probes

SiC and GaN power semiconductors enable extremely fast switching edges and high blocking voltages – but place the highest demands on measurement technology. This article shows how you can acquire gate signals safely and reproducibly without endangering the gate driver, oscilloscope, or application.

Why gate measurements are particularly critical for SiC and GaN

Compared to conventional silicon MOSFETs, SiC and GaN devices operate with significantly higher reverse voltages and extremely high dv/dt values. This leads to:

• very fast switching edges in the nanosecond range,
• high common-mode voltages between the power path and the gate driver,
• strong EMC exposure and steep current increases.

Conventional high-voltage probes and standard probes quickly reach their limits here: Capacitive loading, limited bandwidth and insufficient common-mode rejection distort the measurement result – in the worst case, they even endanger the insulation.

Advantages of optically isolated probes such as Micsig SigOFIT

Optically isolated probes galvanically decouple the gate driver stage from the oscilloscope. Systems like the Micsig SigOFIT transmit the signal optically and offer:

• very high common-mode rejection (CMC),
• high bandwidth for fast edges,
• low capacitive load at the gate,
• a defined, safe mass reference.

This allows gate responses to be reliably captured even on high-side switches and fast-switching half-bridges – an important prerequisite for optimizing switching losses, overshoot and EMC behavior.

Typical measurement setup with optically isolated gate probe

The following measurement setup has proven effective for reliable gate measurements on SiC and GaN half-bridges:

• Power stage (half-bridge) with SiC/GaN transistors,
• Gate drivers with a clearly defined reference potential,
• optically isolated gate probe directly at the gate-emitter or gate-source path of the switching transistor,
• Oscilloscope with high sampling rate and sufficient bandwidth,
• Consistent separation of power path and signal acquisition, as well as short connecting lines.

In the eVision webshop you will find suitable oscilloscopes for power electronics and optically isolated probes, such as the Micsig SigOFIT system, as well as other accessories in the category "Power electronics tester".

Typical errors in gate measurements

• Incorrect ground reference: Direct measurement against power ground or strip ground leads to dangerous potential shifts, flashovers and unclear references.
• Excessive probe load: Capacitive load at the gate distorts rise and fall times and can destabilize the switching behavior.
• Large measurement loops: Long cables between probe, device under test and oscilloscope act like antennas and couple in switching edges and EMC interference.
• Insufficient bandwidth: Measurement technology that is too slow "smooths out" critical details such as overshoot, ringing, or parasitic vibrations.
• Lack of separation between control and power levels: Shared grounding and unclear return currents increase the risk of interference.

Best practices for safe and reproducible gate measurements

• Use optically isolated probes with high CMRR and sufficient bandwidth (e.g., Micsig SigOFIT for gate driver applications).
• Position the probe as close as possible to the gate connection point and keep the leads extremely short.
• Define unique grounding points and avoid multiple ground connections.
• Use oscilloscopes with special power electronics functions, e.g. for measuring rise times, overshoot, switching losses and efficiency.
• Start commissioning at reduced voltage and current and gradually increase the operating parameters.
• Document measurement setups, probe settings, and trigger conditions to be able to reproduce results later.

Conclusion

SiC and GaN power electronics unlock new levels of efficiency and performance – but require precise gate control. Optically isolated probes, such as the Micsig SigOFIT solutions, and suitable power electronics oscilloscopes from the eVision webshop enable reliable gate signal acquisition, avoid typical errors, and optimally utilize design reserves.

In the shop category “Power electronics tester” you will find suitable measuring instruments, optically isolated probes and accessories for your next SiC and GaN projects.