Development of customized solid-state relay (SSR) solutions
The Si875x Isolated FET Driver Family Reduces Cost and Complexity

Silicon Labs (President Baek Woon-dal) announced the launch of a family of innovative CMOS-based isolated field effect transistor (FET) drivers.

This product family enables developers to replace outdated electromechanical relays (EMRs) and optocoupler-based solid-state relays (SSRs) with application-specific, high-volume FETs. The new Si875x family is the industry's first isolated FET driver designed to deliver power across an integrated CMOS isolation barrier, eliminating the need for an isolated secondary switch-side power supply, reducing system cost and complexity.

When used with discrete FETs, the Si875x drivers provide the best replacement solution for EMRs/SSRs in motor and valve controllers, HVAC relays, battery monitoring, AC mains and communication switches, HEV/EV vehicle charging systems, and other industrial and automotive applications.

Until now, developers have primarily used EMRs and optocoupler-based SSRs for switching applications, but both technologies have limitations: EMRs are expensive, slow, bulky, and noisy. These flaws resulted in double-digit increases in SSR usage, but SSR also had challenges that needed to be addressed.

Optocoupler-based SSRs suffer from inherent limitations, including a short product lifespan due to LED aging, reduced performance and stability in high-temperature environments, and poor noise immunity. Furthermore, the range of integrated FETs available for selection has been limited, forcing a trade-off between performance, cost, and power requirements.

Silicon Labs' CMOS-based Si875x isolated FET drivers offer an excellent alternative for applications utilizing SSRs or EMRs, reducing system cost and power consumption while improving performance. Because the Si875x drivers do not use LEDs or optical components, they provide superior stability over time and temperature. The small footprint of the Si875x drivers offers virtually no switching noise, making them an ideal replacement for bulky EMRs that suffer from electrical switching noise, wear-out issues, and manufacturing limitations.

The Si875x devices drive the FET gate with a turn-on time of 1.1 ms using a 10.3 V supply, utilizing an ultra-low 1 mA input current. Increasing the input current to 10 mA dramatically reduces the turn-on time to 94 µs. A unique power optimization option maximizes turn-on current for fast startup and reduces it by up to 90% for static holding current when the optional external capacitor is discharged. The flexible input voltage range of 2.25 V to 5.5 V allows seamless connection to low-power controllers. Additionally, the Si875x drivers offer an optional Miller Clamp feature to prevent unintended turn-on of the external FET.

The Si875x devices are rated for operation over the full industrial and automotive temperature range (up to +125oC) with 2.5 kVrms of isolation and are designed to meet stringent UL, CSA, VDE, and CQC standards. A variety of inputs allow for digital CMOS pin control (Si8751) or diode emulation (Si8752) to suit the target application, while outputs are available in a variety of configurations to support AC and DC loads.

“Silicon Labs’ Si875x drivers offer a unique combination of robust, reliable CMOS-based isolation technology and a revolutionary ability to transfer power across the isolation barrier, making them ideal replacements for legacy EMRs and optocoupler-based SSRs,” said Ross Sabolcik, vice president of power products at Silicon Labs. “The new Si875x family provides developers with a smooth migration to innovative solid-state switching technology by giving them the choice to use the cost-effective FET that best fits their application requirements.”