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We report the fabrication and testing of a multilayer flexible screen-printed coil that can be used to generate low-frequency (<1 kHz) magnetic fields for applications such as electrodynamic wireless power transmission (EWPT). Compared to traditional coil-forming methods, screen printing enables rapid fabrication of thin, light weight, conformable, and flexible structures. The flexibility is demonstrated by repeated bending of a stack of up to three layers for a curvature (inverse bending radius) of 53.2 mm −1 . We also demonstrate wireless power transmission of 1 mW of power at a distance of 4 cm between the flexible receiver and transmitter. The effect of the flexibility is observed through a 32% power increase when the receiver is bent concavely and a 20% output power decrease when the stack is bent convexly. It is also seen that the insertion of a 0.2 mm-thick soft-magnetic material sheet increased the magnetic field by 41% from its original value. Mechanical cycling was also performed for the stack. After 10 000 bending cycles, a 16.4% increase in the coil resistance was measured for a 0.33 Hz frequency and 78% increase for a 1 Hz cycling frequency, respectively. Screen printed 3D flexible transmitter coils can enable the incorporation of EWPT for wireless charging in applications such as healthcare and consumer-based electronics.

This paper reports a simple, discrete, ac/dc rectifier architecture intended for magnetic energy harvesters that functions with ac input voltages as low as 390 mVpk. The proposed four NMOS rectifier (4NR) employs cross-connected, saturated-load NMOS inverters. The circuit is completely passive, exhibits good low-voltage performance, consists of only four components for small footprint, and requires no additional blocking diode to prevent power flow from the load. At 2 Vpk input, the 4NR exhibits a power efficiency of 93 % and 400 μW of output power. The 4NR is also successfully connected to an off-the-shelf TI BQ25504 dc/dc converter + battery management chip and shown to charge both capacitor and lithium-ion battery energy storage reservoirs.

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