Biography
Enrollment Date: 2013
Graduation Date：2016
Degree：M.S.
Defense Date：2016.05.31
Advisors：Songping Mai
Department：Institute of Microelectronics，Tsinghua University
Title of Dissertation/Thesis：Research on the Output Stage of Neural Stimulators Based on DC-DC Converter
Abstract:
Over the past decades, with the rapid development of microelectronics and semiconductor industry, with the increasing integration of electronic technology and biotechnology, more and more implantable neural stimulators are used in health detection and treatment of disorders. Since the neural stimulator applies stimulus electronic signal to the human neural tissue through the output stage and power consumption of the output stage occupies the main part of the entire energy consumption of the implant, the circuit design of the output stage is critical. Typically, the output current is constantly changing with stimulus demand and fluctuating widely, the traditional fixed voltage power supply circuits are of low energy efficiency.According to the background of neural stimulator application, this paper introduced the DC-DC converter to the output stage structure, which effectively improved the stimulus efficiency of the output stage, and guarantee the safety of stimulation by current feedback. A detailed analysis of power transistor size in the low load condition for DC-DC conversion efficiency is discussed and the proposed dynamic gate width control technology is adopted to improve efficiency in DC-DC converter when the load current is relatively low. Starting mode circuit is added to ensure that the output voltage can rise slowly when the DC-DC is in power-up. The starting mode circuit has hysteresis and the output voltage can achieve a smooth transition from the starting mode to the normal operating state.In this paper, a neural stimulator output stage with the core of a SEPIC (Single Ended Primary Inductor Converter) converter was completed and taped in CSMC 1um BCD 40 V process. The measured results show that the range of output stimulus voltage is from 0.9 V to 17.8 V，corresponding stimulus current is 0.6 mA-11.8mA with a step of 0.09mA when the load impedance is 1.5 KΩ and the supply voltage is 3.7 V. When the output current is greater than 2.2 mA, the PCE (power conversion efficiency) is greater than 60%. Compared with a conventional current-controlled stimulator, this design can be reduced up to 68% energy loss. In the second version of this design, the current sampling circuit and over current protection circuit are designed and under stimulation to protect the power transistor and inductors from an excessive current. Starting mode circuit is added to ensure that the output voltage can rise slowly to 1.1V when the DC-DC is in power-up. With the gate width control technology, the PCE can be improved up to 20% in the low load condition. The second version was taped in CSMC 0.8um BCD 40V process.