Adaptive Power Converter for Wireless Power Transfer in Biomedical Applications (invited)

Wireless power transfer is a suitable method for powering electronic devices implanted in the human body or attached to the skin, as it prevents the need for inconvenient cables or surgeries to replace batteries. The power received by the device varies due to antenna misalignment, distance from the power source and objects present in the environment that reflect the electromagnetic waves. Moreover, different applications have different power requirements, e.g., a neural stimulator usually requires more power than a temperature sensor. RF power receivers designed to operate at a specific minimum input power do not perform well at different available power levels [1]. Therefore, the power conversion chain must be adaptive to the changing power in order to present better overall efficiency. Since the rectifier is the most inefficient block in the chain [1], it is interesting to not add extra switches to it to keep its efficiency as high as possible. Thus, we can apply two techniques to compensate for the power variations: change the matching network between the antenna and the rectifier [2] (adaptive matching network) and the rectifier's output load [3] (adaptive DC-DC converter). To modulate the rectifier's DC load, a buck-boost converter is proposed. Because this type of converter isolates its input from its output when operating in open-loop DCM (Discontinuous Conduction Mode), it is suitable in applications in which a storage capacitor is employed (since the voltage over it varies considerably) [4]. The designed converter can be dynamically adjusted for 1-μW to 1-mW available input power, meaning that it can supply many types of biomedical devices, and 0.38 to 1.3-V input voltage levels. Its peak efficiency is 76.3% at an input power of 1 μW and 86.3% at 1 mW. This was achieved by using pulse frequency modulation, reconfigurable power switches and adaptively biased zero-current-detection comparator. In order to dynamically select the best switching frequency of the converter, an MPPT (Maximum Power Point Tracking) circuit was designed. It employs a timing scheme that enables the entire circuit to be switched off for a long time, decreasing its average power consumption, and a novel input power estimation circuit that measures the input power indirectly, without sensing the input current and avoiding extra losses. With these characteristics, the presented power converter is suitable for several low-power biomedical devices or devices that require higher instantaneous power but can be duty-cycled.