Current Sensing Method Based on MOSFET Junction Temperature Estimation for Low-Voltage Inverters

Abstract

In low-voltage motor-drive inverters, the cost, power loss, and mounting area of the current-sensing circuit are important design factors because of the low DC-link voltage and high phase current. Conventional Hall-effect current sensors provide good accuracy and isolation, but they increase cost and volume, whereas shunt-resistor-based methods are inexpensive but cause additional conduction loss and heat generation. In this paper, a phase-current sensing method that uses the drain-source on-resistance of the MOSFET without an additional current sensor or shunt resistor is proposed. Using the proposed method, we measured the MOSFET drain-source voltage Vds and calculated the current using the junction-temperature-compensated $R_{\mathrm {ds(on)}}$ . For this purpose, a thermal equivalent circuit based on MOSFET conduction and switching losses was constructed, and the junction temperature was estimated with reference to the NTC temperature. In addition, an on-resistance correction model considering the junction temperature, gate-source voltage, and drain current was applied. The proposed algorithm was experimentally verified using a low-voltage PMSM drive inverter. The maximum normalized root-mean-square error (NRMSE) of the estimated current was 1.73% under stationary conditions and 1.27% under driving conditions. The experimental results demonstrate that the proposed method can simplify the current-sensing circuit in low-cost, low-voltage inverters.