Oscilloscopes for Measurements on Advanced Power Semiconductors


Dr. Markus Herdin at Rohde & Schwarz

Very short rise times and low losses are two major advantages of power transistors based on SiC and GaN semiconductors. These advantages translate into higher efficiency and power density. At the same time, advanced semiconductors of this kind provide high breakdown voltages, which is necessary for high-power converters. This applies especially in the case of electrical drives and power converters. However, the specified voltage limits for the semiconductors must be strictly followed due to the risk of destruction.

Designers are thus faced with some new challenges:

  • Switching times that are significantly less than 10 ns generally require measurement bandwidths greater than 200 MHz, which is larger than the bandwidth typical high-voltage differential probes offer.
  • In order to optimize switching converters, it is often necessary to simultaneously display and analyze multiple floating signals. This prohibits ground-referenced measurements using passive probes on laboratory oscilloscopes.
  • Measurement voltages range from a few volts in case of gate-source voltages up to 1000 V with similar isolation requirements between different channels.

 

Isolated Channel Oscilloscopes: An Excellent Solution

Specialized probing solutions are available in order to achieve very high common mode rejection but a single probe often costs much more than an oscilloscope. . In addition, the input voltage range of these probes is sometimes limited so that only selected measurements are possible. For many developers of power electronics, this is simply not a viable option.

Advanced oscilloscopes with isolated input channels provide an economical yet effective alternative. The portable R&S Scope Rider is a good example of such an instrument. It has four isolated input channels that can be used to perform floating measurements of voltages up to 1000 V (RMS) – without requiring active differential probes. With 500 MHz bandwidth, it is well suited for measurements on advanced SiC and GaN semiconductors. Moreover, it can be battery-powered for mobile applications. Unlike other oscilloscopes with isolated inputs, the R&S Scope Rider provides analysis capabilities on the level of laboratory oscilloscopes, including an extremely fast acquisition system with an acquisition rate of up to 50,000 signal waveforms per second, flexible trigger functions and a wide range of automatic measurement functions.

 

Figure 1. Measuring the gate source (C1, C2) and output voltage (C3) of a resonant converter. Source: Rohde & Schwarz

 

Figure 2. Measuring the switching cycle of a GaN-based power factor correction (PFC) output stage. Source: Rohde & Schwarz

 

Correct Connections are Essential

Performing high-quality measurements on power electronics require the right test instrument and the right probe as well as careful attention to the correct connection. Passive probes are ideal for measurements with an oscilloscope that has isolated input channels. Here, the contacts with the signal and ground connections must be as short as possible in order to minimize ringing and achieve a high common-mode rejection. Ground springs generally help to ensure optimal contacting when using a passive probe. However, when measuring on high voltages, prefabricated contact points must be used for safety reasons. Normally, small conductor loops are soldered onto the contact points so the passive probes can be connected there. In certain situations, a BNC connector can also be installed as an alternative. BNC adapters can then be used with the probe in order to achieve a nearly ideal connection.

 

Figure 3. Prefabricated conductor loops for safe measurement of hazardous live voltages using passive probes and an oscilloscope with isolated inputs. Source: Rohde & Schwarz

 

Differential probes can generally be connected arbitrarily in the circuit. However, when making measurements with isolated input channels, the exact points where the signal conductor and the ground are connected are important. The unbalanced design used in isolated input stages results in much lower capacitance between the signal path and earth potential compared to what exists between the ground path and earth potential. In order to obtain satisfactory measurement results, the signal path should be connected to the appropriate measurement point.

 

Figure 4. For measurements with isolated input channels, the signal conductor should always be connected to the appropriate measurement point. Source: Rohde & Schwarz

 

The Input Voltage Derating is Important

Converters for electrical drives typical operate with clock frequencies in the range from 10 kHz to 100 kHz. In order to make safe measurements on converters of this kind it is essential to consider the so-called “derating” of the maximum voltage of the measuring system vs. frequency. For every measurement system, the maximum permissible voltage between the input terminals, or one input and the earth potential, decreases with frequency. If this voltage is exceeded, the user can be endangered. The R&S Scope Rider allows measurements with full signal amplitude up to 100 kHz, making the instrument well suited for this application.

 

Figure 5. Derating of the maximum voltage between the oscilloscope input and earth potential in case of the R&S Scope Rider. Clock frequencies up to 100 kHz can be safely measured with the maximum permissible input voltage. Source: Rohde & Schwarz

 

If the DUT is also connected to the electrical installation, the measurement category must be taken into account too. Depending on where the DUT is connected in the electrical installation, the measuring system must tolerate different levels of voltage peaks on the input channel without endangering the user due to flashovers or short circuits. As the location gets closer to the building connection for the electrical installation, higher levels of voltage peaks can occur, thereby increasing the required overload protection. An isolated oscilloscope in category 4 (CAT IV rating) provides the necessary degree of safety. The R&S Scope Rider has overload protection of up to 8000 V and is thus appropriate for 600 V (RMS) in a CAT IV environment or 1000 V (RMS) in a CAT III environment.

 

Figure 6. Measurement categories in line with EN 60664-1. Source: Rohde & Schwarz

 

Additional Functions Simplify Everyday Measurements

Cutting-edge oscilloscopes offer a number of functions that greatly simplify everyday measurements. For power electronics, this includes automatic measurement functions for calculating the apparent, active and reactive power, flexible triggering capabilities to allow selection of specific signal elements in a switching operation, and a measurement data logger or history function for long-term monitoring of selected measured values or signals.

 

Figure 7. Automatic measurement functions calculate the apparent, active and reactive power as well as the power factor from the measured voltage (channel 1) and current (channel 2). Source: Rohde & Schwarz

 

Another useful analysis function is provided by the harmonic analyzer. Current and voltage harmonics are unwanted spurious products of converters. These harmonics must lie within specified limits in order to avoid a negative impact on the quality of the power distribution network. Other important parameters include the phase relationships between the different harmonics as well as the total harmonic distortion (THD).

The R&S Scope Rider provides these measurement functions thanks to its harmonics analysis function. Up to four channels can be simultaneously and automatically evaluated, displayed and monitored with respect to predefined limits if necessary.

 

Figure 8. Harmonics analysis of a three-phase 100 V signal. The blue lines show the current limit value in line with EN 50160. The 13th harmonic and its characteristics are highlighted; the highlighted levels for the three channels lie between –56.5 dB and –51.0 dB. Source: Rohde & Schwarz

 

In order to provide maximum safety for hazardous test situations, the R&S Scope Rider (R&S RTH) can be remotely operated via WLAN to fully decouple test instrument operation from the DUT.

 

Figure 9. In especially hazardous test situations, the R&S Scope Rider can be remotely operated via WLAN. Source: Rohde & Schwarz

 

Summary

Developers have already come to appreciate oscilloscopes with isolated input channels as a powerful yet cost-effective alternative for safely performing measurements on power electronics modules. Thanks to advanced frontend technology, up to 500 MHz bandwidth and powerful signal processing, these oscilloscopes are now also appropriate for measurements on circuits with GaN- or SiC-based semiconductors. The R&S Scope Rider from Rohde & Schwarz is an impressive representative of this class and is also well suited for classic oscilloscope applications due to its wide range of analysis capabilities as well as its uniquely designed isolated input channels.

 

More information:    Source: Bodo's Power Systems, May 2018