A MEMS Current Sensor Utilizing NdFeB Magnets

Abstract

The need for a current sensor in a 12V-1V DC-DC converter is discussed. Background topics of current sensing in DC-DC converters and magnetic MEMS are discussed. An iterative design process for a MEMS current sensor utilizing permanent magnets is detailed, which results in two working prototype designs and a working final improved design. The final design is a 5.6 mm X 5.6 mm silicon MEMS DC current sensor that utilizes a miniature NdFeB rare earth magnet attached to a silicon platform suspended by a two silicon torsion springs. An external out-of-plane magnetic field, such as that produced by a nearby DC current, will result in a torque being produced due to the interaction of the external field and the NdFeB magnet, creating a deflection of the platform, which can be sensed capacitively. The variable capacitance is converted into a variable frequency by a CMOS relaxation oscillator. Theoretical predictions of device performance are made and verified with ANSYS simulations. Bulk silicon micromachining processes used to fabricate the silicon MEMS components, cap, and mechanical stop are examined, and resolutions to fabrication issues such as etch uniformity are discussed and demonstrated. Microassembly techniques compatible with the high strength NdFeB magnet are developed and presented. Methods for testing the devices are detailed, and test results showing that the devices perform similarly to predictions are given. A discussion of irregularities in the test results is made, and is attributed to non-uniformity in the fabrication process. The device is also tested as a magnetometer and found to compare favorably in terms of sensitivity to commercially-available Hall-effect magnetometers.