Compact solid state relays provide isolation for high voltage systems

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Relays have enabled low voltage signals to control high voltage systems since before the invention of the transistor. With very low on-state resistance, the venerable electromechanical relay (EMR) is still used today, but newer solid-state relays (SSR) offer significant advantages in lifetime reliability, speed of switching, power consumption and shock and vibration tolerance. .

Solid-state relays also offer advantages over photorelays (or optical relays), which face achievable power transfer limits and are prone to internal LED degradation. Solid state relays find use in a variety of automotive and industrial applications, including inrush current limiting and insulation resistance monitoring.

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Gate Drivers and Switches

Texas Instruments’ SSR line includes switches with internal FETs as well as gate drivers for use with external power switches, including MOSFETs, IGBTs, silicon carbide (SiC) MOSFETs, and silicon controlled rectifiers . They do not undergo mechanical degradation during their lifetime, which can exceed the lifetimes of MREs by a factor of 10.

The switching times of these SSRs are on the order of microseconds, orders of magnitude faster than those of EMRs. They can transfer both power and signals across their isolation barrier and do not require separate isolated secondary bias supplies. And the devices can be driven by the logic output of a microprocessor, minimizing the need for external control circuitry.

Be aware of one caveat: like all high-speed switches, solid-state relays can generate EMI. Therefore, when using them, follow best EMI design practices, such as minimizing current loops by keeping circuit board traces short.

TI isolation products are available with basic or enhanced levels of isolation using capacitive or magnetic isolation technology. The company’s insulation products use polyimide or silicon dioxide as the insulation material, which have typical dielectric strengths of 300 VRMS/µm and 500VRMS/µm, respectively. In contrast, EMRs and photorelays often use air or epoxy as insulation materials, with dielectric strengths of only around 1 V.RMS/µm and 20VRMS/µm.

Automotive Qualified Products

TI’s new SSR offerings include two automotive-qualified devices: the TPSI3050-Q1, an isolated switch driver with an integrated 10 V gate supply, and the TPSI2140-Q1, a 1400 V, 50 mA isolated switch . Both devices integrate power and signal isolation across a single isolation barrier using a unique approach that improves reliability while reducing solution size and cost compared to electromechanical relays and photo relays semiconductor. Additionally, both can disconnect and connect loads through a single isolation barrier in microseconds, compared to milliseconds for electromechanical relays.

The TPSI3050-Q1 (Fig.1) offers reinforced inductive isolation up to 5 kVRMS and a lifespan 10 times longer than that of electromechanical relays. It reduces solution size by up to 90% compared to mechanical relay solutions by integrating an isolated power supply, digital isolator and gate driver into the IC package.

The TPSI2140-Q1 (Fig.2) provides capacitive base isolation up to 3.75 kVRMS, enabling it to achieve more than 4 times the dielectric breakdown versus time reliability of solid-state photorelays. The SSR reduces solution size by up to 50% compared to traditional solid-state photorelay solutions by integrating a signal FET and resistors, eliminating the need for a reed relay.

The TPSI2140-Q1 can work with a battery monitor such as the BQ79631-Q1 to detect insulation faults in 800 V battery management systems. It allows the use of resistances

The new automotive-qualified devices are the first in a new SSR portfolio that will also include ICs for high-voltage industrial applications.

Design Resources

Texas Instruments offers a variety of resources to help you design with its isolation products which, in addition to solid-state relays, include stand-alone digital isolators, isolated interface ICs, isolated ADCs, and isolated amplifiers. Resources include PSpice models and evaluation boards as well as detailed application notes and reference designs.

One application note, for example, shows how to cascode two TPSI3050 isolated switch drivers to increase gate drive voltage and minimize conduction loss in designs incorporating SiC IGBTs and MOSFETs. TPSI3050 floating secondary power supply allows cascode configuration (Fig.3)which can provide gate drive voltage up to 20V.

Another application note shows a TPSI3050-Q1 used in a precharging circuit to minimize inrush currents in automotive on-board chargers as well as industrial power supplies and power distribution systems. The solid-state approach enabled by the TPSI3050 eliminates the need for bulky, failure-prone electromechanical contactors. A response time of less than 3µs allows the TPSI3050 to quickly turn off a switch in the event of overcurrent or overheating.

TI’s reference designs include design guides, schematics, bills of materials, assembly drawings, Gerber files, PCB layout files, and a calculator to help determine critical component values. A reference design implements an analog front end (AFE) for insulation monitoring (Fig.4). It uses the TPSI2140-Q1 isolated switch and the AMC3330 rugged isolated amplifier, which has a ±1 V input and an integrated DC-DC converter.

The AFE can monitor symmetrical or asymmetrical leakage current and measure the insulation resistance between the positive or negative high voltage supply and protective ground. It requires no external power supply from the hot side.

Other reference designs show the TPSI3050-Q1 isolated switch driver used in a zero-cross switching circuit and in an overtemperature and overcurrent protection application. For each reference design, TI fabricated a fully assembled board for testing and performance validation.

Conclusion

Systems using increasingly higher voltages in automotive and industrial applications require reliable insulation to ensure the safety of personnel and equipment. TI’s new isolated switch and switch driver ICs complement TI’s full line of isolation products to deliver optimal performance and design flexibility. TI provides comprehensive support for its insulation portfolio, from simulation models to reference designs.

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