From consumer electronics to automotive applications, optocouplers have done a fine job of isolating components and circuits from high voltages, transients, electrical noise, and other unwanted interferences that can cause failure. Hence, for many isolation requirements in automotive designs, optocouplers are the norm. While optocouplers have adequately served the automotive industry for the last several years, concerns regarding uncertain current transfer ratios, nonlinear transfer functions, and temperature and lifetime effects are growing year by year. Especially since hybrid-vehicle batteries operate at voltages in excess of 600 V, and low-voltage signals power most in-car entertainment, safety, and powertrain systems, the need to galvanically isolate system-critical electronics is far greater today.
Plus, with the pressure to reduce power losses and improve the reliability of the product, along with the difficulty to manage and operate optocouplers at temperatures above 105 °C is driving designers to explore other options.
Seeing the need for a better alternative to optocouplers in the automotive environment, Analog Devices has leveraged its proprietary digital isolation technology and requalified it to meet the reliability and quality needs of emerging electric-hybrid vehicles. Moving away from LEDs and photodiodes of optocouplers, the company has taken its proven iCoupler technology and crafted digital isolators that are AEC-Q100 qualified for +125 °C operation to meet the stringent quality and reliability requirements of automotive systems.
ADI's digital isolators are based on chipscale transformers that exploit on-chip wafer-level processing. In essence, the transformers are planar structures derived from CMOS and gold metal layers. A high breakdown polymide layer underneath the gold layer insulates the top transformer coil from the bottom. CMOS circuits connected to the top coil and bottom coil provide the interface between each transformer and its external signals. According to ADI's product line manager David Krakauer, the iCoupler transformers can support higher data rates and lower power consumption, and are more stable over life as compared to LEDs and photodiodes. By fabricating the transformers directly on-chip, the digital channels can be integrated with each other and other semiconductor functions at low cost, added Krakauer.
These new automotive products provide multiple isolation channels in a variety of channel configurations and data rates up to 25 Mbps. The CMOS-based parts operate with the supply voltage on either side ranging from 3.0 V to 5.5 V, providing compatibility with lower voltage systems as well as enabling voltage translation across the isolation barrier. Unlike optocouplers, which suffer from performance degradation and wear out at high temperatures, digital isolators are relatively insensitive to temperature and demonstrate excellent long-term reliability, claimed ADI. Because these products have a digital interface, there is no need for external signal-conditioning components, and they consume one-tenth to one-sixth the power of optocouplers at comparable signal data rates, affirmed ADI.
As propagation delay is critical in these components. By comparison, digital isolators offer a significant improvement in this spec. The maximum propagation delay for iCoupler digital isolator is given as 32 ns as compared to 100 ns for an optocoupler. Besides automotive qualification, the digital isolators come with the same safety approvals as high-quality optocouplers, namely UL, CSA and VDE.
Designed for electric motor drives and battery management in hybrid-electric vehicles, as well as DC-AC inverters for a variety of auto systems, ADI claims to be the first to produce auto-qualified digital isolators, which are sampling now. The manufacturer has introduced dual, triple and quad channel parts, which are encased in 8-lead narrow-body or 16-lead wide-body SOIC packages.
In late 2006, ADI filed a lawsuit against Silicon Laboratories alleging that Silicon Labs' digital isolators infringe upon multiple U.S. patents addressing physical construction and signaling techniques related to its iCoupler products. However, that lawsuit was recently settled with Silicon Laboratories, a close competitor in this product line. Details were unavailable.