Comparison of SOI power device structures in power converters for high-voltage, low-charge electrostatic microgenerators

An inertial generator based on a moving-plate capacitor can provide energy for medical sensors from low-frequency (human body) motion. The energy exists as a very small charge (4 nC) at high voltage (300 V). An initial proposal for power processing used a carefully scaled lateral power MOSFET-diode pair, which results in a low, but sufficient, energy yield. It was found that increasing the area of the MOSFET to reduce conduction loss is highly detrimental to the energy yield because of capacitive loading of the generator. This paper examines alternative device topologies which may greatly increase the energy yield for a given system size by increasing both the generation efficiency and the conversion efficiency. An insulated gate bipolar transistors (IGBT) and a MOS-triggered thyristor, both based on previous silicon-on-insulator MOSFET designs, are examined for their switching speed and losses using physics-based finite-element simulation. The scaling criteria to achieve optimum system effectiveness are discussed. The small charge available from the generator results in a brief conduction period which does not allow the devices to reach their steady-state carrier distributions. Nevertheless the IGBT, and especially the MOS-triggered thyristor, switch on faster than the MOSFET, run at higher current densities, and provide improved efficiency. This allows the devices to be reduced in area leading to less capacitive loading on the generator. It also allows a reduction in the value and volume of the circuit inductor without a conduction loss penalty. We describe the device behavior in detail for the various phases of the conversion cycle and illustrate device/circuit tradeoffs graphically. Requirements are outlined for the development of power devices for microgenerators in implanted medical sensors.