Isolated Inputs Are The Key

Advanced semiconductors now support previously unattainable efficiency and power density. But these devices place high demands on test and measurement (T&M) equipment; oscilloscopes with isolated input channels are ideal solutions with wide-ranging analytical abilities. By: Dr. Markus Herdin, Product Manager for Oscilloscopes, Rohde & Schwarz, Munich

Very short rise times and low losses are two major advantages of power transistors based on silicon carbide (SiC) and gallium nitride (GaN) semiconductors. These advantages translate into higher efficiency and power density. At the same time, advanced semiconductors of this kind allow high electrical strength, which is critical when designing 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.

Users are thus faced with some new challenges:

· Switching times that are significantly less than 10 ns generally require measurement bandwidths greater than 200 MHz. This is beyond the performance limit for today's high-voltage probes

· In order to optimize switching converters, it is necessary to simultaneously display and analyze multiple signals on a floating basis

· Measurement of differential signals with a very large common-mode component requires high common-mode rejection

Special probes are too expensive

Large bandwidth and high common-mode rejection can be attained using optically isolated probes, for example. However, special probes of this kind typically cost much more than the actual oscilloscope, and the costs are multiplied if multichannel measurements are required. In addition, the input voltage range of these probes is limited so that only selected measurements are possible. For many developers of power electronics, this is simply not a viable option.

The cost-effective alternative

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 differential measurements on potential differences of up to 1000 V (RMS) – without requiring costly 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.

Fig. 4: For measurements with
isolated input channels, the signal conductor should always be connected to the
appropriate measurement point; source: R&S

Fig. 2: Measuring the switching cycle of a GaN-based power factor correction (PFC) output stage; source: R&S

Performing high-quality measurements on power electronics requires 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 the highest possible common-mode rejection. Ground springs generally help to ensure optimal contacting when using a passive probe. However, when measuring hazardous live voltages, prefabricated contact points must exist 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.