Galvanically safe power electronics measurement with optical probes

Introduction: Galvanically safe power electronics measurement

Power electronics with modern SiC and GaN semiconductors operate at high voltages, extremely fast switching edges, and high dV/dt rates. Conventional probes quickly reach their limits here: they can distort the switching behavior, couple in additional interference, or, in the worst case, even destroy the grounding system of the device under test. A truly galvanically safe measurement therefore becomes a crucial prerequisite for valid results and for the safety of the user and equipment.

Challenges in power electronics

Fast switching edges and parasitic effects

SiC and GaN power semiconductors switch significantly faster than conventional silicon transistors. Rise and fall times in the single-digit nanosecond range are not uncommon. These fast switching edges accentuate parasitic inductances and capacitances in the layout, wiring, and measurement technology. An unsuitable probe can therefore have a much greater impact on the circuit than is typical in the low-frequency range.

EMC, electromagnetic emissions and immunity

High dV/dt and dI/dt values lead to pronounced electromagnetic fields around the circuit. These EMC effects manifest themselves in both emissions and the immunity of the measurement chain. Conducted and induced coupling can distort measurement signals, shift trigger points, or overload sensitive measurement inputs. Suitable, galvanically isolated measurement technology helps to control these effects.

Ground reference and common-mode voltages

In many power electronics applications, important measurement points are not referenced to the protective earth or oscilloscope ground, but are floating relative to earth. High common-mode voltages arise between the gate, source, and drain, or between the DC link and the load. If a conventional, ground-referenced probe is used, equalizing currents can flow through the test leads, potentially damaging both the circuit and the measuring instrument. A properly planned ground reference is therefore essential.

Optical and insulated probes as a key

Operating principle of isolated/optical probes

Isolated or optical probes galvanically isolate the measurement signal from the oscilloscope. Often, the voltage at the measurement point is first converted into an internal, differential low-level signal or directly into an optical signal. The actual transmission to the oscilloscope then takes place via a high-insulation link, for example, using fiber optics. This ensures that the oscilloscope remains at a safe potential while the probe operates in the "hot" environment of the power electronics.

Advantages of fast SiC/GaN switches

Especially with SiC and GaN switches, isolated and optical probes demonstrate their strengths. Galvanic isolation allows for the reliable handling of high common-mode voltages without unnecessarily limiting the bandwidth. Modern optical probes are optimized for very fast edges and offer sufficient bandwidth to accurately reproduce the actual switching behavior, including overshoot and damping. At the same time, the risk of dangerous equalizing currents via ground connections is significantly reduced.

EMC optimization through short, optimized measurement paths

Isolated and optical probes enable short, locally optimized measurement paths directly at the switching node. This reduces the loop area and, consequently, the coupled interference voltage. In combination with adapted shielding and differential signal routing, the EMC load of the measurement can be further reduced. The result is reproducible measurement results that remain reliable even in complex EMC situations.

Best practices for galvanically safe measurements

1. Define the measurement task and limit values

Before selecting a probe, it should be clear what voltages, currents, and frequency components are to be expected. Especially with SiC/GaN designs, a conservative approach to maximum slope and overvoltages is advisable. Only if the probe offers sufficient bandwidth, voltage withstand capability, and common-mode rejection will the measurement remain meaningful and reliable.

2. Plan mass reference and reference potentials

A structured ground reference is the basis of every galvanically safe measurement. Consciously decide which potential should serve as the reference and avoid uncontrolled multiple ground connections. Optical probes simplify this, as the oscilloscope remains electrically decoupled and the reference is defined locally at the probe.

3. Optimize the mechanical placement of the probe

Short cable runs, neat cable twisting, and clear separation of power and signal lines reduce EMC problems. Mount the optical or insulated probe as close as possible to the measurement point without obstructing critical cooling surfaces or insulation. Every additional centimeter of cable increases inductance and thus potential measurement errors.

4. Use bandwidth and filters strategically

A high bandwidth is important for accurately capturing fast switching edges. However, excessively high bandwidths can also reveal unnecessary noise. Utilize the capabilities of modern oscilloscopes and probes to adjust the bandwidth and filters so that relevant signal components remain visible while limiting high-frequency noise. This results in a realistic yet easily interpretable signal image.

Practical checklist for measurements with optical probes

• Analyze the circuit: Determine voltage levels, common-mode voltages, and switching frequencies.
• Select a suitable insulated/optical probe with sufficient bandwidth and insulation strength.
• Consciously define ground reference and reference potentials, avoid multiple groundings.
• Keep the measurement path as short as possible, twist the wires, and minimize loop areas.
• Check the EMC environment and plan for additional shielding measures if necessary.
• Configure the bandwidth, key factor, and filter appropriately on the oscilloscope.
• Document the measurement setup to enable reproducible comparison of measurement results.

Conclusion

Galvanically safe measurements are not a luxury in modern power electronics with SiC and GaN semiconductors, but a fundamental requirement for reliable results. Insulated and optical probes enable the safe handling of high voltages, fast switching edges, and demanding EMC environments. Careful planning of ground reference, measurement path, and probe selection, along with the use of a structured practical checklist, reduces measurement errors and protects both personnel and the measurement system. This forms the basis for the reliable development and optimization of power converters, power supplies, and drive systems.