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Alternative Custom SSR Using CMOS-Based Isolation
Over the last few years, multiple semiconductor suppliers have introduced more advanced CMOS-based isolation products with double-digit growth over traditional optocoupler based isolation. This is especially true in high temperature and high reliability industrial applications. Traditional optocoupler based custom SSRs can address the limitations of integrated FETs but require an additional power supply. Even then, customer SSRs built around an optocoupler cannot address the inherent limitations of using LEDs. Another alternative is now available that enables developers to use their choice of application specific, high volume FETs without the disadvantages of optocoupler based designs as shown in Figure 2.
Figure 2: Custom dc switching SSR with Si8751 CMOS-based isolated FET driver
The Silicon Labs 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. Since Si875x drivers do not use LEDs or optical components, they provide superior stability over time and temperature with up to 125˚C automotive operation. A single Si875x can support either dc or ac load switching with one FET required for dc load switching as shown in Figure 2 or two FETs for ac load switching as shown in Figure 3.
Figure 3: Custom ac switching SSR with Si8752 CMOS-based isolated FET driver
Developers have the option to use a CMOS digital input with the Si8751 (Figure 2) or the diode emulation input of the Si8752 (Figure 3). The Si8752 isolated FET driver makes it easy for developers to migrate from optocoupler-based solutions while efficiently generating a nominal 10.3 V gate drive using only 1 mA of input current. Optional miller clamp inputs are implemented that allow the addition of a capacitor to eliminate the possibility of inductive kickback changing the state of the switch when used in applications with high dV/dt present on the FET's drain. The Si8751 easily interfaces with low-power controllers down to 2.25 V and provides a unique low-power TT mode that provides exceptionally fast turn-on speeds, as fast as 100 µs, while dropping static holding current as much as 90 percent. An optional capacitor is tied to ground using the TT pin to enable this power-saving feature. This approach allows the device to draw more current to initially switch the external FET on quickly yet draw less supply current in the steady state. Total power over time is reduced while maintaining the FET's fast switching speed.
Developers face continual challenges to implement next-generation designs with lower system cost, higher performance and better reliability. Offering a unique combination of robust, reliable CMOS-based isolation technology and advanced capability to transfer power across the isolation barrier, new isolated FET drivers now provide a much-needed replacement solution for antiquated EMRs and optocoupler-based SSRs. CMOS-based isolated FET drivers give developers the flexibility to choose a cost-effective FET customized to their application needs, creating an easy migration to state-of-the-art solid-state switching.
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About the Author
Kevin Kilbane serves as a senior product manager for Silicon Labs' power and isolation products. Mr. Kilbane joined Silicon Labs in 2010. He holds a bachelor's degree in Electrical Engineering from Cornell University.
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