CMOS digital isolators provide data protection for smart meters

The smart meter market is expected to grow at a double-digit rate each year in the next few years as consumers replace traditional electromechanical meters. Smart meters use the latest integrated circuit (IC) technology to accurately measure and report the amount of electricity consumed. Smart meters are more complex than electromechanical meters, but they pay more attention to the integrity of the measurement data, which directly affects the settlement revenue of utility providers. One of the most effective solutions to ensure data integrity in smart meter design is to use the most advanced digital isolation technology.

Smart meters use galvanic isolation to protect internal low-voltage integrated circuits and also to protect workers from exposure to high-voltage wires. In wired metering applications, such as high-density residential areas, isolators can also be used to isolate the controller and digital data bus, as shown in Figure 1.

Figure 1. Smart meter with digital communication bus

Other subsystems, especially those exposed to high voltages, must also have isolated circuits. For example, galvanic isolation is required between a smart meter controller IC and a power line communication (PLC) modem. Signal isolation in these systems can be achieved in a variety of ways.

Optocouplers are commonly used for signal isolation in smart meters, but their use faces design challenges. Their main disadvantage is that they are limited by the common-mode transient immunity (CMTI) of the optocoupler. CMTI is a measure of isolation capability that represents the ability to suppress fast transient noise signals on both sides of the isolation barrier. Due to the physical structure of optoisolators, they tend to have high parasitic input and output capacitance (generally in the order of pico-farads). High internal parasitic coupling capacitance leads to poor CMTI performance (see Figure 2).

Figure 2 Common-mode transients affect optocoupler signals, causing data errors

Optocoupler suppliers usually advise customers to overdrive the optocoupler LED when the optocoupler is turned on to improve noise immunity, and to reverse bias the LED when the optocoupler is turned off. These measures increase the CMTI of the optocoupler, but also reduce the device life, affect system reliability, and increase maintenance costs.

Another isolation solution used in smart meter applications is isolation transformers, but transformers are generally avoided because they are susceptible to electromagnetic interference (EMI) and corrupt data. Pulse transformers are also not ideal because they require a wide bandwidth, which is extremely important for digital signal transmission.

There are two main reasons why electromagnetic field (EM) immunity is a major concern in meter design. First, there is a high probability that the meter will be installed in a location with complex EM noise. Second, some isolation technologies may be the weakest link in the meter system application. For example, applying an external magnetic field to a transformer-based system will have a negative impact on the integrity of the data, and in fact there have been cases where customers have interrupted the normal operation of meters by applying strong magnets or coils to the instrument. In both cases, external magnetic fields or EM noise will cause erroneous measurement data to be submitted to the controller.

Modern CMOS digital isolators overcome these problems in smart meter applications. Compared with optocouplers, CMOS-based digital isolators provide significantly higher CMTI performance while ensuring longer operating life and high reliability. For example, Silicon Labs' Si84xx CMOS digital isolator series meets the general CMTI specification of 25kV/µs, and the new generation of isolation components is expected to double the performance level of existing products.

In terms of electromagnetic field performance, CMOS digital isolators are significantly superior to other isolation technologies. For example, Si84xx isolators have the highest EMI tolerance among digital isolation components available on the market (>300V/m electric field immunity, and >1000A/m magnetic field immunity). These digital isolators use differential signaling to transmit data across the isolation barrier, and paired narrow passband filtering provides excellent common-mode noise rejection, as shown in Figure 3.

Figure 3. Differential signaling and narrowband digital receivers suppress common-mode noise in digital isolators.

Additionally, the minimal size of CMOS-based isolation components helps prevent the isolator from acting as an antenna for stray magnetic fields, while avoiding the use of transformers allows the system to maintain a high level of magnetic field immunity.

As smart meters become more popular in building the global smart grid market, the installation environment of the meters is sometimes not easy to adapt to local conditions, which increases the possibility of measurement data being corrupted. Any component in the meter can be affected by electrical noise or electromagnetic fields and may become a weak link in the integrity of the entire system. These components have the potential to corrupt the data of the smart meter controller and ultimately make the billing information invalid.

Although optocoupler and transformer isolation technologies are already popular, these solutions have obvious weaknesses that should be paid attention to in electricity meter applications. CMOS digital isolators provide the best solution and excellent electrical noise and external magnetic field rejection. Using CMOS digital isolators in smart meters can ensure that power measurement data accurately reaches the system controller through the isolation barrier.