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How to implement galvanic isolation for power and signal lines in high-voltage systems

■ Reinforced isolation is a single isolation system that provides protection equal to double isolation. The performance and testing requirements for reinforced isolation are more stringent than for basic or supplementary isolation approaches. While optocouplers are widely used for galvanic isolation purposes, they can only be used on signal lines, they tend to be inefficient, can send data in only one direction, and have bandwidth limitations. Optocoupler

bandwidth can be improved by adding LED drive circuitry and amplifiers, but that results in higher costs and increased energy consumption. Using a transformer to provide magnetic isolation can provide an efficient solution for power and high-speed signal lines, but discrete transformers are large and costly. To reliably and more effectively meet the demands of galvanic isolation, designers can turn to integrated capacitive and magnetic solutions that meet the IEC

isolation standards. Capacitive isolators support analog signaling and high-speed bidirectional data transmission, with limited power transfer capability. Integrated magnetic isolation can support bidirectional transmission of high- speed data and the transfer of higher levels of power. Isolation characteristics Isolation voltage, working voltage, and common‐mode transient immunity (CMTI) are three key characteristics of isolators. Isolation voltage specifies the maximum voltage at which the isolator can protect from dangerous voltages for a short time. The working voltage is the long-term voltage at which the isolator is designed to be used. CMTI is the maximum slew rate (frequency) of transients on the common voltage applied between two isolated circuits that can be withstood with no adverse effect on data transmission across the isolation barrier. CMTI is specified in kilovolts per microsecond (kV/ μs) or volts per nanosecond (V/ ns). The capacitance between the isolated ground planes is the path where transient energy can cross the barrier and corrupt the data or waveform. A high CMTI indicates a system that is robust and where the two sides operate within specifications even when exposed to fast transient events. A low CMTI can result in distortion, missing

information, jitter, and other signal integrity problems. A CMTI of 100 V/ns or higher indicates a high- performance isolator. In addition to electrical specifications, isolators must satisfy mechanical requirements related to clearance and creepage distances. Clearance is the distance between adjacent conductors through the air, while creepage is the distance between them across the surface of the package. Various package styles and sizes provide different levels of creepage and clearance performance. The selection of the mold compound and the use of insulator materials with the needed dielectric strength are also factors that determine the isolation rating. The dielectric strengths of commonly used materials are: ■ Air ≈ 1 volt root mean square per micrometer (V RMS /μm) ■ Epoxies ≈ 20 V RMS /μm ■ Silica-filled mold compounds ≈ 100 V RMS /μm ■ Polyimide polymer ≈ 300 V RMS / μm ■ Silicon dioxide (SiO2) ≈ 500 V RMS /μm

Isolation types and choices There are several isolation types to choose from and different international standards governing their use, like IEC 60747-17 for magnetic and capacitive isolation and IEC 60747-5-5 for optocouplers (Table 1). ■ Functional or operational isolation ensures proper

Galvanic isolation technologies Optocouplers use an LED to transmit analog or digital signals through a dielectric insulator to a phototransistor. As mentioned, they are one-way devices. Common insulating materials used in optocouplers include air, epoxy, or mold compounds. Since these materials have relatively low dielectric strengths, a greater physical distance between the LED and phototransistor is required to achieve a given level of isolation. Capacitive isolation uses a SiO2 insulation barrier. SiO2 has a high dielectric strength and is more stable when exposed to moisture of extreme temperatures, compared to most epoxy or mold compounds. Capacitive isolation uses various modulation techniques like on-off keying or phase-shift keying to transmit AC signals across the barrier. Capacitive isolation can be compact and can transmit high- speed signals bidirectionally but has a very limited power transmission capability, typically <100 microwatts (μW). Magnetic isolators can transmit signals and power across the isolation barrier. Some of these isolators can transmit hundreds of milliwatts (mW) of power and can replace a secondary-side bias power supply. Magnetic isolators can use an air core or ferrite core.

system operation but does not protect users from high voltages. It’s not covered by safety regulations.

■ Basic isolation is the simplest form of isolation that’s included in safety regulations. It can protect users from electrical shock, but if it fails, users could still be exposed to a high voltage. ■ Supplementary isolation adds a layer on top of basic isolation. It protects users from high voltages if the basic isolation fails. ■ Double isolation is not a separate type of isolation. It refers to using both basic and supplementary isolation. However, most standards and safety documents refer to double isolation as a type of isolation.

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 V IOWM - maximum working isolation voltage

AC voltage (bipolar)

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 x V IOWM

V IOSM - maximum surge isolation voltage

V TEST = 1.6 V x V IMP 10k V

10 K V

V TEST = 1.3 V IMP

PK (minimum)

Minimum rated lifetime

20 years x 1.2

20 years x 1.5

Not defined

Failure rate over lifetime

1,000 ppm

1 ppm

Not defined

Silicon dioxide (SiO 2 ) and thin- film polymer

Allowable isolation materials

SiO 2 and thin-film polymer

Not defined

Table 1: Testing and operational requirements for reinforced isolation are more demanding than for basic isolation. (Table source: Texas Instruments)

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