  One of the key design considerations for biomedical implants is to minimize
the system volume while achieving high performance. An attractive approach to
power biomedical implants is to use wireless power transfer (WPT) with
ultrasound. To receive the ultrasonic energy, a piezoelectric transducer is
implemented, which is designed to have the resonance frequency at the frequency
of the ultrasonic wave. To extract the received energy from the transducer, an
energy rectification circuit is typically required. The commonly used rectifier
is called a synchronized switch harvesting on inductor (SSHI) rectifier, which
employs an inductor to increase the energy efficiency. However, the employment
of inductor limits the system miniaturization since it usually needs to be
around 1's mH. The synchronized switch harvesting on capacitors (SSHC)
rectifier was recently proposed to be used on vibration energy harvesting and
this architecture achieves inductor-less fully integrated design. In a
vibration energy harvesting system, the piezoelectric transducer usually has
large inherent capacitance and low resonant frequency. But the piezoelectric
transducer used in ultrasonic WPT typically has very small capacitance and much
higher resonant frequency. In this paper, the performance of a SSHC rectifier
used in ultrasonic WPT is analyzed. The analysis covers system size and
performance. Based on the analysis, this paper presents an important design
consideration on designing a SSHC rectifier to be used in ultrasonic WPT, which
is the allowed maximal ON resistance in the capacitor-to-capacitor charging
loop in function of the transducer capacitance, transducer resonant frequency
and the number of SSHC stages.
