A galvanic isolator provides a way for transmitting a signal from one electrical circuit to another electrical circuit in a control system when the two electrical circuits may otherwise be electrically isolated from one another. Usually the two electrical circuits operate at different voltages, and thus, may be electrically isolated. For example, consider an application in which a 5V battery powered controller board is configured to control a motor circuit operating at 240V. In this example, the 240V motor circuit may be electrically isolated from the 5V controller circuit, while permitting the 5V controller circuit to send or receive signals from the 240V motor circuit. In this type of application, an isolator may be used to provide voltage and/or noise isolation while still permitting signaling and/or information exchange between the two circuit systems.
Galvanic isolators may be further categorized into opto-isolators, capacitive isolators, magnetic isolators and radio frequency based isolators depending on the technology used to electrically isolate the electrical signal. An opto-isolator may comprise an optical emitter and an optical receiver. Over time, degradation may occur and optical signals emitted from the optical emitter may degrade.
Capacitive isolators may not have the optical degradation issue of the opto-isolators. However, incorporating high voltage capacitor into a semiconductor die may be technically challenging. Capacitors that are fabricated by using conventional semiconductor process may not meet the requirement of high voltage tolerance. For example, most capacitors in integrated circuit are metal oxide semiconductor (designated hereinafter as “MOS”) capacitors or double layer poly-silicon capacitors. Sheet capacitance of MOS capacitors or double layer poly-silicon capacitors may be substantial. However, MOS capacitors may break down under high voltage. One reason may be that distance between the gate and the substrate may be lower than few microns. Overlapping metal layers in semiconductor dies would probably not be recognized as suitable for use in isolation applications because of the low and/or insubstantial capacitance value. In fact, most often, overlapping metal layers in semiconductor dies are considered as related to unwanted parasitic capacitance.
Most isolation capacitors in galvanic isolators may be formed through post processing steps after the semiconductor dies are fabricated. However, the postprocessing involves additional steps and may incur additional cost.