New-Tech Europe Magazine | Jan 2018

Oscilloscopes for measurements on advanced power semiconductors Isolated inputs are the key

Dr. Markus Herdin , Rohde & Schwarz

Advanced now support previously unattainable efficiency and power density. At the same time, however, they place higher demands on T&M equipment used in the development of power electronics. Oscilloscopes with isolated input channels are ideal measuring systems, providing a wide range of analysis capabilities on the level of a laboratory oscilloscope. 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 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 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 multipliedifmultichannelmeasurements 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

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