Millimetre-Scale 200 MHz Bandwidth Magnetic Field Sensor to Measure Switching Current in SiC Module Terminals With Minimal Insertion Inductance

This paper reports the design method, physical realisation, and experimental validation of a miniature current sensor for use on SiC power module terminals where there is too little space for commercial Rogowski coils, or where their insertion would compromise busbar performance. Current sensing is carried out by inserting a thin, low-inductance copper spacer, between the module terminal and the busbar, in which a mm-scale notch is cut, to house two coplanar anti-series-connected sense coils. The notch shapes the magnetic field to achieve the desired frequency response of the sensor. The design method uses 3D and 2D finite element simulation to model frequency-dependant skin and proximity effects, to optimize the notch location and dimensions, and the coil location in the notch, thereby achieving a flat bandwidth and high gain. The sensor is experimentally validated against a commercial Rogowski coil in double pulse tests at 1200 V and 300 A, albeit with modifications to insert both sensors. Experimental results are also shown to validate the simulations that determined notch position, sensor location, and gain. The sensor is shown to have a 200 MHz bandwidth, an insertion inductance less than 50 pH, and a high immunity to switching currents in nearby terminals and conductors. Finally, the current sensor is demonstrated in use on three parallel modules in a one-megawatt (1200 V, 900 A) inverter with laminated busbars that have been optimized for low inductance leaving no room for commercial Rogowski coils, thus enabling the observation of the individual parallel modules’ switching current.