Smart power meters use galvanic isolation to protect internal low-voltage integrated circuits, as well as utility service personnel, from the sensors exposed to the high-voltage mains. In wired metering applications, such as those deployed in high-density residential complexes, isolation also may be used between the controller and the digital data bus.
Other subsystems, especially those that are exposed to high voltages, also must contain isolation circuitry. For instance, galvanic isolation is necessary between an internal smart meter controller IC and a power line communications (PLC) modem. Signal isolation in these systems may be implemented in a number of ways.
Optocouplers are often used in smart meters for signal isolation, but their usage presents design challenges. In particular, optocouplers do not provide a reliable isolation solution for the long term. The performance level of these devices inherently varies over time due to the dielectric material used or isolation and the LED used as the internal signal transmitter. The clarity of the isolation barrier can degrade over time and is subject to a variety of environmental factors such as temperature and humidity. In addition, all LEDs experience a reduction in signal strength with time. These two factors limit the length of time that an optocoupler-based smart meter can reliably remain in service.
Another significant drawback of optocouplers is their limited common mode transient immunity (CMTI). CMTI is a measure of the isolator’s ability to reject fast transient noise signals that are present between the input and the output sides of the isolation barrier. Because of their physical structure, optocouplers tend to have high parasitic input-output capacitance (typically in picofarads).
Download the complete Technical Document from Silicon Labs.
