What are the advantages of solid-state isolators compared with optical SSRs?

Optically isolated solid-state relays (SSRs) (also called optocouplers or photovoltaic relays) were developed to overcome some of the performance limitations of electromechanical relays. While they can support a larger number of switching cycles, the LED in an optical SSR degrades over time, ultimately limiting the lifetime of the device.

Capacitive and inductive-based isolation technologies were developed to improve the reliability and lifetime limits of optically isolated SSRs. Optically isolated SSRs typically include an internal power switch, while inductive-based SSRs are available with integrated switches for lower-power applications, while others are used to drive external power switches to support higher voltages and currents (Figure 1).

Figure 1. Examples of an optically isolated SSR (left), an inductively isolated SSR with integrated power transistors (center), and an inductively isolated SSR with external power switches (right). (Image: Texas Instruments)

Optical SSR performance

Optical SSRs provide galvanic isolation between the input and output. Adding a discrete MOSFET or IGBT to the output of an optical SSR enables support for higher voltages and currents. The solid-state structure of optical SSRs produces several advantages compared with electromechanical relays:

Faster switching speeds than electromechanical relays.
Elimination of problems like contact bounce, instability, arcing, and noise generation.
The lack of mechanical contacts to wear out significantly improves reliability and lifespan, supporting robust operation in the presence of shock and vibration.

Figure 2. Optical SSRs have an LED on the input side and a phototransistor on the output side to transfer a signal across the isolation barrier. (Image: Renesas)

Coreless transformers for isolation

A more recent development is solid-state isolators (SSI) that provide enhanced performance compared with optically isolated SSRs. Compared to capacitive and inductive isolation in SSRs, SSIs use coreless transformer (CT) technology to provide isolation and signal transfer.

A CT consists of metal spirals and silicon dioxide insulation. The structure is fabricated using standard CMOS manufacturing processes, enabling compact and cost-effective integration and providing a complete electrical isolation barrier up to 5.7 kVrms, or more. SSI-based relays can meet the requirements of UL 1577 and IEC 60747-17.

CT technology supports faster switching speeds compared to optical SSRs. The lack of magnetic core materials eliminates the possibility of core saturation or hysteresis and provides more consistent and reliable performance compared to magnetic isolation.

Like capacitive and inductive-based isolation, CT-based SSIs can be used to develop SSRs with integrated monitoring and protective functions and bidirectional current capabilities. The design example in Figure 3 uses back-to-back MOSFETs to support bidirectional current blocking. It isolates the input gate driver and the grid AC or DC power on the output.

Figure 3. Block diagram of an SSR using a solid-state isolator showing the integrated junction temperature sensor (Tj) and the current sensor (A). (Image: Infineon)

The transient voltage suppressor (TVS) diode across the MOSFETs clamps inductive energy during turn-off. This design includes over-temperature protection using a junction temperature sensor. Overcurrent protection is supported by using a shunt resistor in the power path.

Summary

Optical SSRs are a good technology and were developed to replace mechanical relays. They remain a good choice for many applications, but solid-state isolation offers designers a new option with numerous benefits. CT technology supports faster switching speeds compared to optical SSRs. The lack of magnetic core materials eliminates the possibility of core saturation or hysteresis and provides more consistent and reliable performance compared to magnetic isolation.