Accuracy analysis of UCE(on)-based measurement of the inverter output current at higher motor speeds

This paper presents a new gate driver concept that consists of an integrated measurement of the junction temperature and the load current of a power module during inverter operation. For junction temperature measurement the gate driver identifies the temperature sensitive on-chip internal gate resistor by means of a small identification signal that is modulated onto the positive gate voltage. To determine the current through the power semiconductor the new gate driver also measures the highly current sensitive on-state collector-emitter voltage drop U_CE(on) and compensates the impact of the junction temperature on driver level. This combined measurement of a temperature-sensitive and a current-sensitive parameter enables accurate current measurement with lowest costs. From today's point of view accuracies of 1-5% can be reached with reasonable efforts, but it's hard to predict whether U_CE(on)-based current measurement can ever cope with the stationary accuracy stated in the datasheets of conventional current sensors. However, it is found that the overall accuracy of U_CE(on)-based current measurement profits from very good dynamic properties that become increasingly important in case of high motor speeds and fundamental frequencies. This paper investigates the overall accuracy of a current sensor system to measure a certain load current pointer at a motor speed of 3000 min^-1. In d/q-coordinates the low-pass character of a conventional current sensor is found to be responsible for a deviation of the measured current pointer of up to 3.6%. In contrast, UCE_(on)-based current measurement only deviates 0.4%. In this aspect, the better dynamic properties partially compensate the poorer stationary accuracy of U_CE(on)-based current measurement and makes it interesting for lower performance control tasks in high motor speed drives.