Optically isolated measurements on SiC and GaN gate drivers in power electronics

SiC and GaN power semiconductors enable extremely fast switching edges, high blocking voltages, and high switching frequencies. However, these very characteristics place high demands on measurement technology in gate driver environments: gate propagation delays in the tens of nanoseconds range, steep dV/dt values of several tens of kV/µs, and high common-mode voltages overwhelm conventional passive probes.

Optically isolated differential probes fill this gap. They galvanically isolate the measuring electronics from the high-voltage potential, transmit the differential measurement signal via a robust optical path, and simultaneously offer high bandwidth and common-mode rejection. This allows for the reliable characterization of the switching behavior, control, and EMC characteristics of modern SiC and GaN gate drivers.

Typical measurement tasks on SiC/GaN gate drivers

• Measurement of gate voltage waveforms (V _GS ) in high-side and low-side drivers
• Monitoring of overshoot and ringing at gate and source
• Analysis of the propagation delay between driver output and semiconductor
• Evaluation of shutdown behavior and short-circuit protection mechanisms
• dV/dt load and common-mode interference on sense and feedback lines

In practice, optically isolated differential probes from specialist suppliers such as Micsig are frequently used. These combine high bandwidth with generous measuring voltage ranges and are specifically optimized for use in power electronics.

Why optically isolated differential probes?

Compared to conventional differential high-voltage probes, optical probes offer several advantages:

• Galvanic isolation via light path: No electrical return path, therefore very high immunity to common-mode transients.
• High CMRR at high dV/dt: Critical for SiC/GaN switches with very steep edges.
• Slim, compact measuring heads: Easy placement directly on gate driver ICs or isolated high-side drivers.
• Scalable safety: Sensors located close to the power electronics, evaluation unit safely separated in the measuring station.

Especially in laboratory and application environments where gate driver layouts are iteratively optimized, optically isolated probes are a key tool. They reduce measurement errors due to ground loops, decrease the risk of unwanted short circuits, and deliver reproducible measurement results.

Typical measurement setup with optical probes for power electronics

A practical setup in power electronics looks like this, for example:

1. A SiC or GaN power module is connected via an isolated gate driver (e.g.  B. Half-bridge drivers with galvanic isolation).
2. The optically isolated differential probe is connected directly between the gate and source of the high-side or low-side transistor.
3. The probe's evaluation unit is located in the safe low-voltage area next to the oscilloscope.
4. Gate delays, overshoot and switching losses can be analyzed in detail using appropriate bandwidth and filter settings.

Depending on the measurement task, different optical probe variants are used – for example, models with an extended common-mode voltage range for high-voltage intermediate circuits or probes with a particularly high bandwidth for very fast GaN switches. The Micsig portfolio includes optical probes specifically designed for power electronics applications.

Best practices for stable measurements

• Minimize cable routing: Keep conductor loops between probe and gate driver as small as possible to avoid inductive coupling.
• Clearly define reference potentials: Clear separation between power path, control signal ground and measurement technology ground.
• Choose bandwidth consciously: Use only as much bandwidth as necessary to reduce noise and EMC influences.
• Use filtering: For switching frequencies in the range of a few hundred kHz, a slight low-pass filter can significantly smooth out the signal.
• Don't neglect safety: always observe occupational safety regulations, minimum distances and isolation coordinates.

Application areas in power electronics

Optically isolated measurements on SiC and GaN gate drivers are now an integral part of:

• Development and validation of gate driver ICs and modules
• Application laboratories for drive inverters, on-board chargers and DC/DC converters
• Power module characterization and reliability testing
• EMC optimization and layout reviews of highly dynamic half-bridges

These tasks can be covered efficiently and reproducibly with suitable optical probes – for example from the Micsig portfolio for power electronics.

Conclusion

To fully exploit the potential of modern SiC and GaN power semiconductors, measurement technology is needed that can keep pace with the high voltages and steep response times. Optically isolated differential probes provide a safe and precise foundation for this. They protect both the measuring instrument and the user, while simultaneously enabling detailed analysis of gate driver designs in power electronics.