Design Theory and Validation of High-Bandwidth, High-Immunity Twin-Coil Magnetic Current Sensors

Abstract

Wide-bandgap power semiconductors like Gallium Nitride (GaN) and Silicon Carbide (SiC) exhibit high switching speeds, with transient slew rates exceeding 10 A/ns per die. Characterising these devices therefore requires non-invasive current sensors with GHz bandwidth and low insertion impedance. Conventional sensing methods, such as shunt resistors and commercial Rogowski coils, often possess non-negligible insertion impedance and limited bandwidth.
This thesis addresses these challenges by establishing a design theory and validation framework for the Twin-Coil magnetic current sensor architecture. Twin-Coil sensors are a class of magnetic field current sensors employing a pair of oppositely wound sense coils connected in anti-series. This differential configuration rejects any magnetic flux that passes through both coils, while remaining sensitive only to flux components that couples to only one of the coils. This property enables the sensor's sensitivity to arbitrary spatial sources to be engineered through appropriate geometric design. The design method is centred on an analytically derived, 2D sensitivity contour map, which enables the prediction of sensor gain and bandwidth, facilitating optimised sensor placement for a high signal-to-noise ratio (SNR). The design theory is validated through the development of planar and axial sensor prototypes, achieving bandwidths up to 1.17 GHz and confirming the model's accuracy.
The practical viability of the Twin-Coil concept is demonstrated through integration into a compact half-bridge circuit (< 3 nH power loop inductance). Sensor placement guided by the sensitivity map enables accurate measurement of low-side source currents without compromising layout integrity. It also allows the measuring of low-side gate currents with a Signal-to-Noise Ratio (SNR) > 10, despite their proximity to high di/dt aggressors.
To transition from custom designs to readily deployable solutions, this research develops two discrete sensors that integrate the sensor and current path on a single PCB. The V2 Infinity Sensor, designed for source current measurement, offers a bandwidth of 1.2 GHz with an insertion impedance of only ~200 pH, and accuracy comparable to a commercial coaxial shunt. The Infinity Gate Sensor, designed for gate current measurement, maintains a SNR > 10 against source current aggressors positioned just 5 mm away.
The proposed design method provides a systematic approach to designing high bandwidth, high immunity sensors that, in many cases, do not require analysis of the magnetic field. The demonstrated sensors offer a new means of characterising fast-switching power circuits, facilitating advancements in design, control, and diagnostics for future power electronics.

Date of Award	17 Mar 2026
Original language	English
Awarding Institution	University of Bristol
Supervisor	Bernard H Stark (Supervisor) & Saeed Jahdi (Supervisor)