Optimization of senseFET for current sensing in Low Voltage power integrated systems

In Low Voltage (LV) power applications, silicon power MOSFETs are commonly used as high side drivers to control the amount of current flowing through the load (car lighting, body control modules, etc). In those applications, it is common to use diagnostic and safety features that report continuously the health state of the power device and, as a consequence, of the load. Those features are assured by the senseFET structure, which constitutes a loss-less current sensing method with high accuracy. The senseFET is a tiny copy of the main power MOSFET. The two devices are connected in parallel and, ideally, they work at the same operating condition, thus, the integrated system is based on the current mirror principle. For this reason, it is crucial to master the matching between the senseFET and the main MOSFET electrical behavior in terms of current density. In this paper, we explore the intrinsic and extrinsic factors that could affect negatively the matching using 3D-TCAD simulation, electrical characterization and analytical modeling. Therefore, we establish a set of good practices to follow when using a senseFET as a current sensor.