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High-bandwidth measurement

With switching frequencies of MHz and switching edges lasting less than 10 ns, GHz measurement bandwidths are required to visualise some of the important effects occurring in SiC and GaN converters. Whilst multi-GHz measurement probes exist, they cannot typically be used in this application, as the voltages slew at 100V/ns to many 100s of volts and higher, and there is a lot of electromagnetic noise. On this page we will be adding tips and tricks on high-bandwidth measurement.

Infinity sensor

The name of this sensor stems from its figure-of-eight shape (∞ sensor) and very high bandwidth (300 MHz upwards). It is a magnetic field sensor, and can be thought of as a planar subset of a Rogowski coil. The probe is floating, and has negligible insertion inductance (around 0.2 nH). As a result, its impact on switching performance is negligible, even in high-speed GaN and SiC switching circuits. The pair of coils are connected such that the measurement is immune to currents outside of the sensing region.

Current sensing in GaN circuits

The figure shows the placement of an Infinity sensor in the power loop of a kW GaN bridge leg. For reference, this sensor measures 10×3×1mm (L/W/H). The current return path of the bridge leg lies on the 1st inner layer of the printed circuit board. In contrast to a Rogowski coil, the Infinity sensor does not need to wrap around the conductor, and therefore the current return path can remain unbroken. This helps in achieving low loop inductances (e.g. 2 nH), fast switching edges (100 V/ns), and low switching loss. We have shown that, despite the conductor and the return path being separated by only 100 microns, this magnetic field measurement is highly effective.

Obtaining a high-bandwidth current measurement

The output signal from the Infinity sensor is shown in the upper graph. This is integrated to obtain the current (trace below). By knowing the characteristics of the sensor and voltage probe, post-processing allows an effective measurement bandwidth of GHz to be obtained. Sometimes, the current values are not needed, and instead di/dt is used directly, for example for fast power device protection, or for junction temperature sensing.

References

J. Wang, M. H. Hedayati, D. Liu, S. E. Adami, H. C. P. Dymond, J. J. O. Dalton, and B. H. Stark, “Infinity Sensor: Temperature Sensing in GaN Power Devices using Peak di/dt”, IEEE Energy Conversion Congress and Exposition (ECCE), 2018.

Current sensor samples

We regularly provide samples of our sensors to industry to for evaluation. If you are interested in the Infinity sensor, please contact Bernard Stark for an info pack.

What is a Rogowski coil?

A Rogowski coil consists of a number of series-connected windings, usually wrapped into a loop. The conductor carrying the current that you wish to measure is passed through the coil’s centre. Any change in current causes a change in magnetic field around the conductor, which, in turn, induces a voltage in the coil. This voltage provides the rate of change of current, so it is integrated to provide current. In order to obtain a true absolute current measurement, however, the integration constant needs to be known (i.e. the starting point of the current). This is not a problem in power electronic switching, where the starting-point is usually zero! The measurement is floating (not direct contact to the conductor), and therefore it can be applied to any part of the circuit, even where the potential of the conductor is very high.

What is a coaxial shunt?

A shunt resistor is placed directly into the current path. It is therefore not floating, and is normally only used where circuit potentials are low. The voltage across the shunt provides an absolute measurement of current. A ‘coaxial’ shunt is a resistor that has been designed to have almost no inductance (e.g. 2 nH). As a result, it has little effect on fast switching, and a GHz measurement bandwidth. However, emerging GaN and SiC power electronic circuits switch so fast, that even nH-scale inductances have a significant impact, and other solutions are being developed.

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