Lossless current sensing technique on MOSFET RDS(on) with improved accuracy

Although the existing current sensing technique on MOSFET RDS(on) (CST) is lossless, its inaccuracy, which is mainly the consequence of the temperature dependence of RDS(on), is the reason why it is not utilised more frequently. To improve accuracy considerably, it is proposed to thermally couple the power MOSFET and the sensing MOSFET. Presented are two methods for minimisation of the current ratio (CR) temperature dependence, i.e. optimisation of resistance for the sensing resistor variant and optimisation of the thermal contact for the CST variant with the virtual ground sensing circuit.