Custom solid-state relay solution to replace EMRs and SSRs

22nd September 2016
Lanna Cooper

A CMOS-based isolated FET (Field Effect Transistor) driver family has been introduced by Silicon Labs, enabling developers to use their choice of application-specific, high-volume FETs to replace outmoded electromechanical relays (EMRs) and optocoupler-based SSRs (Solid-State Relays).

The new Si875x family features the industry’s first isolated FET drivers designed to transfer power across an integrated CMOS isolation barrier, eliminating the need for isolated secondary switch-side power supplies and reducing system cost and complexity. When paired with a discrete FET, the Si875x drivers provide a best-in-class EMR / SSR replacement solution for motor and valve controllers, HVAC relays, battery monitoring, AC mains line and communications switches, HEV/EV automotive charging systems, and other industrial and automotive applications.

Developers have traditionally used EMRs and optocoupler-based SSRs in switching applications, and both technologies have limitations. EMRs are costly, slow, bulky and noisy. These drawbacks are driving double-digit growth in SSR usage, but even SSRs pose challenges. Optocoupler-based SSRs have inherent limitations such as shorter lifetimes due to LED aging, reduced performance and stability at higher temperatures, and reduced noise immunity. They also use a limited choice of integrated FETs, further compromising performance, cost and power.

Silicon Labs’ CMOS-based Si875x isolated FET drivers offer a better alternative that reduces system cost and power and enhances performance for applications served by SSRs or EMRs. Since Si875x drivers do not use LEDs or optical components, they provide superior stability over time and temperature. The small-footprint Si875x devices offer completely silent switching, making them an ideal replacement solution for bulky EMRs that are limited by electrical switching noise and wear-out problems, as well as mass production challenges.

The Si875x devices drive FET gates with a nominal 10.3V using a very low 1 mA input with 1.1 ms turn-on time. Increasing the input current to 10mA enables an exceptionally fast turn-on time of 94µs. A unique power optimisation option delivers maximum turn-on current for fast speed and then lowers it by up to 90% for static holding current once the optional external cap is discharged. Flexible 2.25 to 5.5V input side voltages support seamless connection to low-power controllers. The Si875x drivers also feature an optional Miller clamp capability to prevent unintended turn on of the external FET.

The Si875x devices have a 2.5kVrms isolation rating, can operate over full industrial and automotive temperature ranges (up to +125oC), and are designed to meet stringent UL, CSA, VDE, and CQC standards. Versatile inputs provide digital CMOS pin control (Si8751 devices) or diode emulation (Si8752 devices) to best suit the target application, and flexible outputs support AC and DC load configurations.

“With their unique combination of robust, reliable CMOS-based isolation technology and revolutionary capability to transfer power across the isolation barrier, Silicon Labs’ Si875x drivers provide a much-needed replacement solution for antiquated EMRs and optocoupler-based SSRs,” said Ross Sabolcik, Vice President of power products at Silicon Labs. “The new Si875x family gives developers the flexibility to choose a cost-effective FET customised to their application needs, creating an easy migration to state-of-the-art solid-state switching.”

Samples of the Si875x isolated FET drivers are available now with production quantities planned for November 2016. The Si875x devices come in a small SOIC-8 package with industrial (-40 to 105˚C) or automotive (-40 to 125˚C) ambient temperature operating range options. Pricing in 10,000-unit quantities begins at $0.96 for industrial versions and $1.20 for automotive temperature options. The Si8751-KIT (digital input) and Si8752-KIT (LED emulator input) evaluation kits are available now and priced at $39.99 each.

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