What are the design considerations when using solid state isolators in PLCs?

Solid state isolators (SSIs) can be an effective tool for providing isolation for programmable logic controllers (PLCs). When using SSIs in PLCs, several important considerations come into play to ensure optimal performance, reliability, and safety.

This article briefly reviews the applications for SSIs in PLCs, looks at safety system architecture options, and reviews the technology options for implementing isolation, with a special focus on the use of SSIs.

PLCs are needed to support the increasing use of networked control across industrial, green energy, and infrastructure applications (Figure 1). Isolation is required to protect both the PLC and the connected devices from electrical faults, noise, and potential damage caused by different voltage levels or ground potential differences. It ensures reliable and safe operation in challenging environments, especially where multiple power sources or noisy signals are present.

Figure 1. PLCs and isolation barriers are needed in a wide variety of industrial and infrastructure applications. (Image: Zero Instruments)

Safety architecture options

There are two common approaches to implementing galvanic isolation in PLCs: group-isolated or channel-to-channel isolated connections (Figure 2). Group isolation is typically used when cost is a major factor and channels can share a common power source. Channel-to-channel isolation is preferred for its robustness and flexibility, especially when dealing with varying power sources or potential noise interference.

Figure 2. Galvanic isolation in PLCs can be implemented using group isolation or channel-to-channel isolation. (Image: Analog Devices)

Choosing the best technology

Galvanic isolation can be achieved using a variety of technologies with different performance characteristics, and designers need to confirm that an SSI is the best choice. Optocouplers can provide high levels of isolation, but they have a limited data rate of about 50 Mb/s. That makes them suitable for low- to moderate-rate applications like I2C but not when high data rates are required. In addition, the LED degrades over time, reducing the performance and limiting the lifetime of optocouplers.

Inductive coupling can provide a higher level of performance. It uses a conventional transformer to send signals and/or power across the isolation barrier. Transformers have a long lifetime and don’t suffer the degradation experienced by LEDs. However, magnetic core materials can experience saturation or hysteresis, which can reduce the consistency and reliability of the connection.

Capacitive isolation can be used to transfer signals or power between electrically isolated circuits using electric fields. It can be useful in applications requiring high-speed data transfer and immunity to magnetic fields. However, it can be sensitive to electric fields and voltage spikes and can be limited in its ability to handle high voltages.

SSIs based on coreless transformers and power semiconductors provide a fourth option. They are available in single-channel and multi-channel designs. Coreless transformer designs, sometimes called magnetic coupling, are compatible with complementary metal-oxide-semiconductor (CMOS) fabrication, simplifying the production of highly integrated solutions. The use of silicon dioxide (SiO2) dielectric provides very thin high-voltage isolation (Figure 3).

Figure 3. Block diagram of a single-channel integrated SSI solution (top) and example of a coreless transformer structure (bottom). (Image: Infineon)

SSI application considerations

Beyond basic performance characteristics like single-channel versus multi-channel designs, there are several application considerations when integrating SSIs and PLCs.

Load capabilities of the SSI, including maximum voltage (AC or DC) and current, plus output switching speed, must match the application requirements.
The input of the SSI must be compatible with the PLC’s output signal. For example, some PLCs have 3.3 V outputs and typical supply currents of 16 mA.
Integrated protective features like overcurrent and over-temperature protection can simplify integration and enhance reliability in industrial environments.
The transmission line termination is important, especially in harsh environments. By inserting a series resistor and making the (apparent) output impedance of the signal source equal to the line impedance, the reflection factor can be zero at the signal source, ensuring clean and accurate switching.
Thermal management can be an important consideration. The heat generated by an SSI depends on several factors, including the heat dissipated by the power semiconductors and the duty cycle. Heatsinking and air flow must be provided to maintain adequate thermal margins.

Summary

Selecting and using SSIs begins with choosing between single-channel and multi-channel architecture and confirming that an SSI based on a CT is the best technology choice based on application requirements. SSI integration considerations include the load characteristics, input compatibility, required protection features, transmission line termination, and thermal management.