Development of the New Approach for High Side Current Sensing

Abstract

Accurate current sensing is critical in many applications including battery management, motor control, and over-current protection. Current can be sensed by either placing a small shunt resistor between the load and ground or between the battery and the load. High-side current sensing has two major advantages over low-side current sensing: it can detect high load current caused by accidental shorts, and it does not increase the resistance in the ground path. However, it does require interface circuits that can handle input common-mode voltages up to the battery voltage, which can be several tens of volts. The first part of this paper consists of a brief review of the existing approaches and methods of current sensing that widely used nowadays. This is followed by researching and development new schematic solution of the high-side current sensing which doesn’t need usage of HV amplifiers, has variable gain and has small gain error in temperature range from – 40 oC to 125 oC. Finally, suggested solution will be discussed and compared to existed ones. In previous current approach was used for high-side current measurement. However, this approach is not appropriative in mass production. So, the task of this work was to find out or come up with new solution, which would have same or better characteristics then previous one and would result in good yield in mass production. In reviewing the literature, several possible solutions were found. First of them is creating virtual ground for low voltage differential-input-single-output amplifier. Second approach is using fully differential system it comprises two main blocks, a differential amplifier, and an instrumentation amplifier. First approach is simple to be implemented and it doesn’t need matched high voltage transistors. However, one unanticipated finding was that it was not easy to create build in offset, so this approach can’t measure bidirectional current. Second approach system has good noise immunity and it is to create build in offset. At the same time, it is complex because of presence of instrumental amplifier in the second stage. Instrumental amplifier is hard to design and to match. As a result, new approach is proposed. This is mixing of differential-input-differential-output amplifier and virtual ground. This finding supports the ideas of previously described solutions. It has fully differential input, which results in high CMRR and PSRR, furthermore, it doesn’t require matched HV transistors because of virtual ground for amplifier. In addition, this schematic can be easily matched in terms of temperature dependence. However, new solution has a drawback, that is relatively big input current.