A switched-capacitor inverter for driving a highly capacitive load

The paper proposes a switched-capacitor approach to generate a high voltage sinusoid at a frequency of few hundred Hertz for driving a highly capacitive load. The need for such a driver stemmed from an ITS (intelligent transportation systems) project that requires efficient inverters for driving electroluminescent (EL) displays. The EL devices are for outdoor use, to be powered by batteries, and low power consumption is a design requirement. The proposed inverter circuit employs the EL device as the hold capacitor and multiple dc levels are switched in to develop a 'stepped' sinusoid around the capacitor. The large time constant of the load necessitates that switching be done in smaller voltage steps. It also reduces stress on the switches and ordinary low voltage transistors can be used in the circuit implementation. The circuitry takes advantage of the floating nature of the load to employ switching at a frequency higher than the desired signal frequency. This leads to simpler filtering to remove the unwanted noise in the 'stepped' signal. The advantages of the proposed inverter are: power efficiency, no use of bulky inductors and capacitors, digital frequency tuning, and a cost effective low profile design. The circuit operations, configurations, and basic experimental results is presented in the paper.