‘Infinity Gate Sensor’: a Differential Magnetic Field Sensor for Measuring Gate Current of SiC Power Transistors

Abstract:
For silicon power devices, gate current measurement has been shown to provide a means of inferring temperature and degradation, and it is important for active gate driving. However, in silicon carbide circuits, gate current measurement is challenging due to interference from switching-induced noise and the required low insertion impedance. This paper presents a low-cost, miniature magnetic field current sensor with 500 MHz bandwidth, that has been optimised for high noise immunity, to allow the accurate measurement of gate current for fast-switching SiC devices. Experimental results from 800 V, 10 A and 1200 V, 50 A double-pulsed bridge leg circuits switching at 80-100 V/ns show a high correlation with gate current measurements using current sense resistors and a 1 GHz optically-isolated voltage probe. The sensor’s gain is 0.67 V/(A/ns) and its insertion inductance is 3.5 nH at 100 MHz. Magnetic pickup from the adjacent power circuit is seen to contribute less than 1% of the overall measurement, and dv/dt susceptibility is quantified through measurement. The theory behind the operation of the sensor, the design principles, the manufacturing detail, and the signal post-processing requirements are presented, providing the user with an alternative to expensive optically isolated probes, and a method of measuring gate current when the addition of a sense resistor is not viable.