Magnetically-coupled current sensors using CMOS split-drain transistors

Abstract

Integrated current sensing circuits intended for Smart-Power and embedded applications featuring galvanic isolation are implemented. They are based on magnetic detection using the CMOS compatible split-drain transistor (MAGFET) that provides a very linear output current versus magnetic field. Two approaches are used to generate the magnetic field. The Coil approach and the Strip approach. In the first one the current to be sensed flows through an integrated coil placed atop the split-drain transistor and produces a relatively strong magnetic coupling enough to cause a detectable current. The second approach features an array of 126-paralleled split-drain transistors along a metal strip intended to carry higher current levels. Both techniques were realized as integrated current sensors built in 0.35 m CMOS technology. The calculated and measured sensitivities were around 1μA/A and 0.75μA/A for the Coil and Strip approaches respectively. For a typical single split-drain bias current of 50μA, the minimum detectable current within 1Hz are 2.8 A/ Hz and 42 A/ Hz for the Coil and Strip approaches respectively. The Strip can carry currents up to 500mA, whereas the flowing current in the Coil is limited to 20mA. Thus, the choice is based on the resolution and sensing current level of the application.

A Simple Drain Current Model for MOS Transistors with the Lorentz Force Effect

“…From [12], the definition of sensitivity of split-drain MagFETs is $\mathrm{normalS}=\frac{\mathrm{sans-serif\Delta }{\mathrm{I}}_{\mathrm{d}}}{{\mathrm{I}}_{\mathrm{d}}.\mathrm{normalB}}={\mathrm{sans-serif\mu }}_{\mathrm{eff}}\mathrm{normalG}\frac{\mathrm{L}}{\mathrm{W}}$ where $\mathrm{sans-serif\Delta }{\mathrm{I}}_{\mathrm{d}}$ is the difference current between two drain terminals and G is the geometrical correction factor. It is evident that the difference current in MagFETs is mainly directly proportional to the strength of the applied magnetic field.…”

“…In [ 9 , 10 , 11 , 12 , 13 ], several analytical or mathematical models have been proposed, but this approach of modelling for split-drain MagFET structures is very complicated. In [ 9 ], the device size is very big, and in [ 10 , 11 ], the analysis is done at very low temperatures impractical for real-life applications.…”

“…In [ 9 ], the device size is very big, and in [ 10 , 11 ], the analysis is done at very low temperatures impractical for real-life applications. Moreover, all of these prior arts [ 9 , 10 , 11 , 12 , 13 ] dealt with MagFET structures only. Therefore, there is no simple model for an MOS transistor as the magnetic sensor with Lorentz force effect, and these presented current models are difficult to use or to be integrated in circuit simulators.…”

A novel 0.7 V high sensitivity complementary differential MAGFET sensor for contactless mechatronic applications

“…Magnetic field lines are thus projecting perpendicular to the plane of the CDMAGFET chips between the magnetic poles above and underneath the test board. The strength of the DC magnetic field can be ramped [28] by adjusting the screw knob up and down vertical to the plane of the CDMAGFET chips. Additional magnetic elements can be placed on the top and underneath the test board near the CDMAGFET chips to tune the variation of the magnetic field strength.…”

Electric current sensors: a review

“…Both sensors were made in CMOS technology. The coil sensor has a noise of 2.8 µA/√Hz@1Hz and a full-scale range of 20 mA, while the strip sensor has a noise of 42 µA/√Hz@1Hz and a range of 500 mA [Castaldo 2009]. These values are promising for future development; however, this device still does not match the parameters of similar Hall current sensors.…”