Dynamics of MEMS electrostatic driving using a photovoltaic source

In this paper we study the potential use of a photovoltaic source as direct drive of electrostatic actuators, mainly for two reasons: first, because the I-V characteristic of a PV array has the shape of a current source limited in voltage and, second, because direct drive with galvanic isolation can be implemented at high voltages without using any dc/dc converter. We have used a standard PSpice model for a PV series array and a PSpice model of an electrostatic actuator. For sake of generality, the model for the PV array is the same one would like to use for the output circuit of a number of opto-couplers connected in series. Main parameters are: irradiance, I-V curve, and voltage, current and movement of the actuator (deflection) as a function of time. The number and surface of individual solar cells can be tailored to drive specific MEMS switches of high pull-in voltage value. As an example, a 14 cells-PV array, with a short circuit density current Isc = 1.2 nA, and open circuit voltage Voc = 8.7 V, would drive an electrostatic actuator having a pull-in voltage of 8.3 V, with three orders of magnitude lower kinetic energy than the equivalent voltage-drive. Both, this reduction in the kinetic energy at the impact, as well as the low hold-on voltage in the photovoltaic actuation reduce the electric field across the dielectric, which may lead to better reliability performance.