57 Isolation technologies | Deep insight E-Mobility Engineering | January/February 2025 and it cannot be considered fail-safe. While these devices are accessible to a user, they must be contained within the system. Basic isolation devices are 100% safety tested at 2.5 kVac RMS for a period of one minute and typically have a minimum creepage of 3.2 mm. Reinforced isolation is the highest commercial rating applied to highvoltage systems. One way to meet requirements is to introduce further distance across the isolation barrier such that it can withstand highervoltage testing standards and a longer rated lifetime. This provides two levels of protection, making these devices fail-safe and allowing user access. They are 100% safety tested at 5 kVac RMS for a period of one minute and typically provide a minimum creepage of 6.4 mm. Isolators have several important parameters regardless of the technology used. The creepage and clearance distance, for example, is the shortest distance between two conductive leads across the isolation barrier. The creepage distance is the shortest distance measured between adjacent conductors across the surface of an IC package, whereas clearance distance is measured through the air. Package technology plays an important role in achieving higher measures of creepage and clearance distance by providing various options for engineers. High-quality mould compounds, widebody packages and higher reinforced isolation ratings must complement each other, so the packs don’t break down and arc, says Tim Merkin, senior member of the technical staff at the Kilby Labs of Texas Instruments (TI). Another parameter is currentmode transient immunity (CMTI), which indicates an isolator’s ability to operate reliably in the presence of highspeed transients and it is measured in kilovolts per microsecond or volts per nanosecond. The proliferation of widebandgap semiconductors, such as silicon carbide (SiC) in inverter and DC-DC converter designs, has resulted in higher transient-voltage (dV/dt) edge rates, making the measure of CMTI critical for gauging an isolator’s resilience. High-performance isolators have CMTI ratings that easily reach 100 V/ns, and many are tested in excess of 200 V/ns. A low CMTI isolator operating in a high dV/dt environment can expect to have signal integrity problems, such as pulse jitter, distortion, erratic operation or missing pulse information. Isolation trade-offs are similar at IC and system level. Smaller IC package sizes, higher integration, thermal management and compliance with certification standards often compete against high- and low-voltage circuits, such as voltage and current sensing, powersupply control, digital communications and signal processing. Reliable isolation techniques, materials and controllers allow designers to satisfy this priority. This means isolation needs to cover voltage, current and communication signals, and it needs to assure safe and useful battery voltage. Current isolation protects batteries from overload or high charging rates with an isolated current sensor that monitors the discharge and charge currents. Digital isolators enable safe and robust data communications for the CAN or serial bus. This can be galvanic or optical isolation, and there are design tradeoffs for both. High-voltage systems require additional isolation because more bi-directional signal information is communicated across the isolation barrier. Many analogue and digital circuits have specific bias voltage requirements, where both digital signals and power cross the barrier. An isolated, high-resolution, analogue-to-digital converter (ADC) may require 3.3 V in the same system, whereas an isolated gate driver may require +15 V and -5 V. These requirements necessitate not just the transfer of signals across the isolation barrier, but power as well. Types of isolation There are three main types of galvanic isolation: functional, basic and reinforced. Functional isolation refers to the minimum amount of isolation assigned to a system so that it will function properly without necessarily protecting against electrical shock. One example is proper printed circuit-board (PCB) conductor spacing for a given voltage rating. Basic isolation provides sufficient protection against electrical shock, with a safety rating on a parity with the highest system-level voltage. This is only intended to provide a single level of protection against electrical shock Types of isolation (Image courtesy of Texas Instruments)
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