Build optimized solid-state relays using CMOS isolation technology

September 23, 2016 // By Graham Prophet
Silicon Labs presents its Si875x isolated FET driver family as a route to reducing cost and complexity of industrial and automotive relay replacement applications. The CMOS-based isolated FET driver family enables developers to use their choice of application-specific, high-volume FETs to replace outmoded electromechanical relays (EMRs) and optocoupler-based solid-state relays (SSRs).

The Si875x family features the 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 an optimized 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 ageing, 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 Si875x devices drive FET gates with a nominal 10.3 V using a 1 mA input with 1.1 msec turn-on time. Increasing the input current to 10 mA enables an exceptionally fast turn-on time of 94 µsec. A power optimization 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 capacitor 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 2.5 kVrms isolation rating, can operate over full industrial and automotive temperature ranges (up to +125C), and are