How to use SSR for higher reliability isolation and smaller solution size

Relays were used as switches until the invention of the transistor. The ability to safely control high-voltage systems from low-voltage signals, as is the case in isolation resistance monitoring, is necessary for the development of many automotive systems. Although electromechanical relay and contactor technology has improved over the years, it remains challenging for designers to achieve their goals for lifetime reliability and fast switching speeds, as well as low noise, shock vibration, and power consumption.

Relays were used as switches until the invention of the transistor. The ability to safely control high-voltage systems from low-voltage signals, as is the case in isolation resistance monitoring, is necessary for the development of many automotive systems. Although electromechanical relay and contactor technology has improved over the years, it remains challenging for designers to achieve their goals for lifetime reliability and fast switching speeds, as well as low noise, shock vibration, and power consumption.

Capacitive and inductive isolated solid state relays (SSRs) offer performance and cost advantages and are suitable for different levels of isolation (whether basic or reinforced). SSRs also offer advantages over alternative technologies such as electromechanical relays and solid-state photorelays.

Traditional Relay Switch Solutions

Electromechanical relays (EMRs) are common in high voltage switching applications. EMRs use electromagnetic force to mechanically open and close contacts. Due to its mechanical properties, EMRs have extremely low on-resistance. Their contacts are essentially metal-to-metal connections.

EMRs do have trade-offs when it comes to switching speed and reliability. The moving parts inside the relay are a limiting factor, and switching speeds are typically in the 5ms to 15ms range. Over time and use, EMRs can experience failures such as buckling, chattering, and weld closure.

Unlike EMRs, photorelays have no moving parts and provide high isolation voltages. Photorelays are an improvement over traditional EMRs; however, they also have design considerations such as limitations on achievable power transfer and aging of the internal LEDs. Additionally, photorelays require external current-limiting resistors and typically use additional field effect transistors (FETs) to manage the on or off state of the LEDs.

Capacitive and inductive isolation in SSR

TI’s isolated SSRs can be used as switches (with integrated FETs) or as drivers for controlling external FETs. Whether utilizing capacitive or magnetic isolation, TI’s family of isolated SSR products enables designs with basic or reinforced isolation levels. Compared to EMRs, TI’s TPSI2140-Q1 isolated switch and TPSI3050-Q1 isolated driver offer higher reliability and longer life because they do not experience mechanical degradation over time . Therefore, the lifetime reliability of SSR is 10 times that of conventional EMR. TI’s SSR can also switch in microseconds, orders of magnitude faster than EMR.

Since the TPSI3050-Q1 and TPSI2140-Q1 integrate power and signal transmission through a single isolation barrier, there is no need for a secondary bias supply, enabling reduced solution size. Figure 1 illustrates the use of the TPSI2140-Q1 isolation switch in a high-voltage system, eliminating external components such as bias supplies and external control circuits.

How to use SSR for higher reliability isolation and smaller solution size
Figure 1: TPSI2140-Q1 isolation switch reduces solution size in high-voltage systems

SSRs such as the TPSI2140-Q1 and TPSI3050-Q1 also offer advantages over traditional photorelays and optocouplers. TI devices such as the TPSI2140-Q1 and TPSI3050-Q1 are more reliable than photorelays because there is no LED burn-in. No external control circuitry is required as the logic level inputs can directly drive the system. Table 1 qualitatively compares these disconnector technologies.

Electromechanical relays

photorelay

TI SSRs with capacitive and inductive isolation

Insulation Materials

air or epoxy

epoxy or polyimide

polyimide or silica

Dielectric Strength (1s)

≅1VRMS/µm

≅20VRMS/µm

≅20VRMS/µm

≅300VRMS/µm

≅300VRMS/µm

≅500VRMS/µm

Advantage

low resistance

Low electromagnetic interference (EMI) emissions

High speed (µs)

Low power consumption

disadvantage

Low speed (ms)

Mechanical wear, vibration/magnetic immunity

Actinic degradation and partial discharge

Must be designed to limit EMI

Working temperature

-40°C to 85°C

-40°C to 85°C

-40°C to 125°C

Table 1: Comparison of existing switching solutions

Epilogue

TI’s isolated SSRs provide ultra-high dielectric strength at ultra-fast speed, ultra-high operating temperature and ultra-low system cost, and also enable more reliable switching in smaller packages.

The Links:   203DMQ100 ZJ080NA-08A