12.8 Wireless power transfer system using primary equalizer for coupling- and load-range extension in bio-implant applications

Wireless power transfer (WPT) in the range of 10 to 100mW [1] is widely used in impiantale medical devices (IMDs) to eliminate the use of a battery. Due to load and coupling variations, the output voltage of the rectifier V
_OUT
is not steady. In [1-2], a 1X/2X reconfigurable rectifier was reported to uphold V
_OUT
when the coupling coils move apart. In [3], V
_OUT
is regulated by the transmitter with feedback through a wire, which defeats the purpose of WPT. In [4], V
_OUT
is regulated by a resonant regulating rectifier as well as an MCU-controlled transmitter with feedback still through a wire. In [5], a reconfigurable resonant regulating (R
³
) rectifier is reported for V
_OUT
regulation, but the load and coupling ranges are limited. In the R
³
rectifier, the 1X/2X rectifier works as a bridge rectifier in 1X mode with V
_OUT
equal to V
_1X
, and as a voltage doubler in 2X mode with V
_OUT
equal to V
_2X
as shown by the dash lines in the HSPICE simulated results in Fig. 12.8.1. V
_OUT
is regulated within [V
_1X
, V
_2X
] by adjusting the duty cycle of mode-switching through the PWM control under the condition that V
_2X
>V
_1X
; otherwise, the PWM control is not stable. This restricts the workable load resistance R
_L
to be larger than the boundary R
_La
. For a smaller coupling factor k, R
_La
is larger (load current is smaller), and thus the coupling range is also limited. In this work, a primary equalizer is introduced to tackle these limitations to extend the load and coupling ranges.