CMOS-based isolated FET drivers reduce system cost

Article By : Silicon Labs

The Si875x family features isolated FET drivers designed to transfer power across an integrated CMOS isolation barrier.

Silicon Labs has introduced a CMOS-based isolated field effect transistor (FET) driver family that 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 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 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.

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 low 1mA input with 1.1ms 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 2.25V 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.

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˚C to 105˚C) or automotive (-40˚C to 125˚C) ambient temperature operating range options. Pricing in 10,000-unit quantities begins at ₹64.37 ($0.96) for industrial versions and ₹80.47 ($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 ₹2,681.55 ($39.99) each.

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