High-voltage galvanic isolation concerns

There is much to consider when constructing a reliable isolation barrier in a system, including the isolation rating, creepage and clearance distance, CMTI, and EMI.

Functional, basic and reinforced isolation refer to the insulation rating level assigned to an electrical system, as listed in Table 1.

Table 1 Insulation ratings.

Insulator rating	Description
Functional	Insulation necessary for the correct operation of the equipment
Basic	Insulation that provides basic protection against electric shock
Supplementary	Independent insulation applied – in addition to basic insulation – to protect against electric shock in the event of a failure of the basic insulation
Double	Insulation comprising both basic and supplementary insulation
Reinforced	A single insulation system that provides a degree of protection against electric shock equivalent to double insulation

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 of functional isolation 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 at parity with the highest system-level voltage.

Reinforced isolation is the highest commercial rating applied to high-voltage systems. One way to meet reinforced isolation requirements is to introduce further distance across the isolation barrier such that it can withstand higher-voltage testing standards and a longer rated lifetime. For example, in International Electrotechnical Commission (IEC) 60747-17 and IEC 607475-5, the mandatory partial discharge test voltage (V_PD) is held to a higher standard for reinforced isolation compared to basic isolation. Learn more about reinforced isolation in the What is Reinforced Isolation? video.

Certifying a high-voltage system for reinforced isolation begins by selecting isolators compliant with safety and certification testing protocols, as defined by various committees. Underwriters Laboratories (UL) is a global safety certification lab in the United States, but different countries regulate compliance to their local or regional system standards. Thus, isolators intended for global use must comply with various international safety standards.

Table 2 summarizes IEC standard requirements for digital (capacitive and magnetic) isolators and optocouplers.

Table 2 IEC standards for capacitive and magnetic isolators and optocouplers.

Test	IEC 60747-17 capacitive and magnetic isolators		IEC 60747-5-5 optocouplers
Test	Basic isolation	Reinforced isolation	Reinforced isolation only
V_IORM – maximum repetitive peak isolation voltage	AC voltage (bipolar)	AC voltage (bipolar)	AC voltage (bipolar)
V_IOWM – maximum working isolation voltage	AC voltage based on time-dependent dielectric breakdown (TDDB)	AC voltage based on TDDB	Based on partial discharge test
V_PD – partial discharge test voltage	V_TEST = 1.5 × V_IOWM	V_TEST = 1.875 × V_IOWM	V_TEST = 1.875 × V_IOWM
V_IOSM – maximum surge isolation voltage	V_TEST = 1.3 × V_IMP	V_TEST = 1.6 × V_IMP10 kV_PK (minimum)	10 kV_PK (minimum)
Minimum rated lifetime	20 years × 1.2	20 years × 1.5	Not defined
Failure rate over lifetime	1,000 ppm	1 ppm	Not defined
Allowable isolation materials	Silicon dioxide (SiO₂) and thin-film polymer	SiO₂ and thin-film polymer	Not defined

Isolators have several important parameters. The creepage and clearance distance, for example, is the shortest distance between two conductive leads across the isolation barrier. As shown in Figure 3, 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.

Figure 3 Creepage across the surface and clearance through the air across an isolator package.

Package technology plays an important role in achieving higher measures of creepage and clearance distance by providing different options for engineers. High-quality mold compounds, wide-body packages and higher reinforced isolation ratings must complement each other, because higher isolation ratings need wider packages and better mold compounds so that packages don’t cause breakdown and arcing.

Another parameter is CMTI, which indicates an isolator’s ability to operate reliably in the presence of high-speed transients and is measured in kilovolts per microsecond or volts per nanosecond. The proliferation of wide band-gap semiconductors has resulted in higher transient voltage (dV/dt) edge rates, making the measure of CMTI critical for gauging an isolator’s resiliency. High-performance isolators have CMTI ratings easily reaching 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 the IC and system level. Smaller IC package sizes, higher integration, thermal management and compliance with certification standards often compete against the need to reduce EMI and achieve higher efficiency. Selecting isolated components designed to meet all of these needs at the IC level helps facilitate a seamless transition to fully reinforced compliance at the system level.