High-voltage isolation, not exactly a new technology, is now being revitalized into a brand-new components domain in the age of electrification drive encompassing high-voltage systems. Take the automotive industry’s shift toward vehicle electrification, where the need for high-voltage isolation leads to fundamental changes in the vehicle architecture and some of the critical components.
CANbus and current sensing are among the more prominent areas impacted by the move to high-voltage systems. Such systems mandate isolation to prevent DC bus voltages and uncontrolled transients from flowing between two points. For example, in hybrid electric vehicles (HEVs), it’s crucial that components operating on the 12-V side are protected from higher-voltage components that run on 48 V or even higher voltages.
High-voltage systems are becoming more prevalent with the increased adoption of electric vehicles (EVs), and with 400 V, 800 V and even higher voltages, isolation becomes even a more critical design consideration. Not surprisingly, therefore, the demand for advanced isolation ICs operating in multiple domains is now apparent in modern electrification designs.
The semiconductor-based isolation solutions offer significant advantages over traditional optocouplers. (Source: onsemi)
So, vehicle OEM and Tier 1 engineers are in the process of developing isolation technology and understanding what it means to the system. For instance, how isolation impacts the handling of high-voltage MOSFETs, IGBTs, gate drivers, and silicon carbide (SiC) devices in automotive and industrial environments.
This article will show the impact of isolation technology on these high-voltage components and how it keeps these devices safe across wide temperature variations with voltage surges. Especially when ever-increasing power densities from raising the wattages create demanding thermal and electrical noise conditions.
Solid-state relays
In this new paradigm for high-voltage designs, the semiconductor-based isolation devices offer significant advantages over the legacy optocoupler solutions. As a result, semiconductor-based isolation products continue to replace the traditional optocouplers due to their higher surge performance, reliability, and easy integration.
For instance, solid-state relays (SSRs) can disconnect and connect loads through a single isolation barrier in microseconds to ensure safer operation of high-voltage automotive systems. On the other hand, electromechanical relays perform the same task in milliseconds.
Solid-state relays, which don’t require any mechanical parts, are typically designed as a simple on-off switch operating when an external control signal is passed to the relay. (Source: Sensata Technologies)
A solid-state relay or SSR, unlike electromechanical relay solutions, integrates the functions of an isolated power supply, digital isolator, and gate driver in a single device. So, an SSR, which integrates power and signal transfer in a single chip, can eliminate at least three components and thus significantly reduce the design size as well as BOM cost.
Take the example of SSR-240A50 and SSR-240D125, the solid-state relays from TE Connectivity which feature low switching acoustical noise and long life. Next, Sensata Technologies offers a range of SSR devices to overcome isolation challenges in automotive and industrial designs.
Other SSR manifestations, such as isolated drivers and switches, also facilitate power and signal isolation across a single barrier. That allows an isolated switch driver to work with a battery-pack monitor to detect insulation faults in 400-V and 800-V battery-management systems (BMS) faster and with higher accuracy than solid-state photorelays.
Compared to traditional solid-state photorelay solutions, isolated switch drivers can significantly reduce the design size by integrating a signal field-effect transistor and resistors. Moreover, these high-voltage isolation devices can eliminate the need for a reed relay.
While SSRs as well as isolated drivers and switches are prominent in reinforcing high-voltage isolation, other components are playing equally important roles. The following section outlines a few of such components and their underlying technology to show how it works for different systems that need to be isolated.
Other high-voltage isolation components
Besides SSTs and isolated drivers and switches associated with this lineup, there are other components that help solve complex isolation challenges as automotive and industrial designs move to higher voltages and currents.
Start with digital isolators, which enable engineers to better protect low-voltage circuitry from high-voltage events in HEV and EV systems and thus eliminate the need to incorporate cooling systems to reduce temperatures to below 125°C. For example, in 48-V HEV systems, where the coexistence of internal combustion engines (ICEs) and battery systems can heat the air around ICs beyond 125°C, digital isolators can be placed in high-temperature areas without increasing BOM or design complexity.
A digital isolator, which combines CMOS and monolithic air core transformers on a single device, supports multiple signal path channels. (Source: Analog Devices Inc.)
When handling CAN communications in automotive designs, engineers can increase in-vehicle signal protection and reach by using digital isolators. These isolation devices also enable engineers to ensure reliability in powertrain and HVAC systems, which require signal transmission across an isolation barrier in HEV and EV subsystems such as starter generators, cooling fans, and traction inverters.
SI8660BD-AS, the automotive-grade digital isolator from Skyworks Solutions, features isolation ratings of 2.5 kV, 3.75 kV and 5 kV, and a selectable fail-safe operating mode to control the default output state during power loss.
Then there is the ADUM1310ARW digital isolator from Analog Devices Inc. (ADI), which handles the design issues like nonlinear transfer functions commonly associated with optocouplers. It also eliminates the need for external drivers and other discrete components while significantly reducing power consumption.
Next, NCID9411R2, a quad-channel digital isolator from onsemi, employs galvanic off−chip capacitor isolation technology to achieve high insulation and noise immunity. That allows it to offer safety reliability of a >0.5-mm insulator barrier, similar to what has historically been offered by optocouplers.
For high-voltage isolation, also worthwhile to mention are isolated comparators, which combine the functions of standard comparators with a galvanic isolation barrier to facilitate ultra-fast isolated bidirectional overcurrent and overvoltage detection in less than 400 ns. Current sensing and insulation monitoring devices are also critical components in designs serving charging stations, high-voltage distribution units, and full-energy storage solutions.
Isolation technology renaissance
Besides EV and HEV systems, high-voltage isolation is an important piece of the electrification recipe in many other industries, including heavy-duty vehicles, specialty transportation and industrial applications. As electrification in these designs moves toward high-wattage power electronics, electrical isolation of the low-voltage side from the high-power system becomes critical.
Here, semiconductor-based isolation devices are vital in interfacing power controllers with high-voltage designs such as EV battery management and charging systems, solar and wind turbine inverters, and industrial motors. In short, high-voltage isolation goes hand in hand with high-density power electronics.
