Instrumentation & Measurement Magazine 23-4 - 69

Fig. 1. Applications of WPT system in biomedical implantable medical devices. (a) Deep brain stimulation for epilepsy [2]. (b) Cochlear implant [3]. (c) Pacemaker
[4]. (d) Retinal prosthesis [5].

enhance the coupling coefficient, and experiments show that
the transmission efficiency can reach 72% with frequency of 24
MHz and the maximum distance of 40 cm. But, this is only a
measurement in a laboratory environment and adding a reflector will increase the size of the system. Capacitors are used to
reduce impedance to optimize transmission efficiency in [12],
and measurement results show that with frequency of 53 MHz,
the transmission efficiency is 75.2% at 1 cm. As distance increases, the efficiency decreases rapidly to 39% at 3 cm. In [13],
the method of using T-type impedance matching for a WPT
system is studied, and the maximum transmission efficiency
can increase to 83%, but 3-coils are used. In a word, the energy
efficiency and output power are difficult to achieve at the same
time. In addition, WPT is usually powering digital circuits,
such as MCU and Digital Signal Processing (DSP), where the
supply voltage of MCU may fluctuate severely with the driving clock of digital circuits switches between Gnd and Vdd,
since the power transmitted wirelessly is limited, and the system may even cut off under wireless charging conditions, since
transient load regulation is also required in a WPT system.
June 2020	

This paper presents a WPT system with load regulation and
optimized antenna design for implantable devices. The first
section highlights the architecture and circuit operation of the
proposed system. The second section shows the experimental
results, including transmission efficiency and dynamic load
measurement. The third section introduces several state-ofthe-art WPT applications in biomedical devices, and the final
section provides conclusions.

Architecture and Circuit Operation
The whole system (Fig. 2) consists of a transmitter-receiver
interface, antenna, match network, rectifier, voltage regulator and dynamic load. The antenna is composed of a primary
coil and secondary coil, which are denoted as L P and L S,
respectively. The secondary coil is followed by a series resonance network for impedance match. A full-wave bridge
rectifier is used to achieve ac to dc conversion. The low dropout linear regulator (LDO) is used to stabilize output voltage
and the dynamic load circuit is setup for load regulation
measurement.

IEEE Instrumentation & Measurement Magazine	69



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