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Safety certification of digital isolators

20 Feb 2015  | Alec Makdessian

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These standards are based on a partial discharge test rather than voltage breakdown, and on the isolation withstand voltage. The partial discharge test involves ramping the voltage to the maximum transient over-voltage, VIOTM, for 60 seconds. The voltage is then reduced to 1.5 × VIORM, where VIORM is the maximum repetitive peak isolation voltage for 10 seconds. (If the waveform is sinusoidal, then VIORM is the peak value of VIOWM, which is the maximum working isolation voltage in RMS.) Partial discharge of up to 5pC is allowed. During production, the test time is reduced for 1 second and the test voltage is 1.875 × VIORM. Once again, 5pC is the partial discharge threshold.

To be granted a Reinforced Insulation certification, the VDE0884-10 includes an additional 10kV surge voltage requirement. The various insulation grades are defined in table 1.

Table 1: Definition of Insulation Grades.

Basic Insulation protects against shock as long as the insulation remains intact. However, a redundant (supplementary) system is usually required to protect operators from shock in case one of the insulations fails. Reinforced Insulation is a single insulation system which provides protection equivalent to Double Insulation.

System-level certifications
The IEC also has a series of standards which provides certifications at the system level. The system-level standards most used for applications requiring isolation are:

IEC60950-1: information technology equipment

IEC61010-1: measurement, control and lab equipment

IEC60601-1: medical equipment

In addition to Functional Insulation certification, system-level certifications specify the insulation levels required to protect the user from shock. They include physical dimensions, such as creepage and clearance, environmental conditions, and of course, the ability of the device to withstand specific test voltages.

Creepage and clearance
Creepage is shortest distance along the surface of the insulating material between two conductive parts, i.e., two pins. Clearance is the shortest distance in air between those same two surfaces, and does not need to be line of sight (figure).

Figure: Basic Creepage and clearance for semiconductor packages.

For simple geometries, creepage and clearance may be the same, but normally creepage is larger than clearance. The requirements for creepage can be reduced based on the comparative tracking index (CTI), which is the measure of the electrical breakdown (tracking) properties of the insulating material. A higher CTI allows for smaller creepage. Table 2 shows the material classification according to the CTI index.

Table 2: Comparative Tracking Index.

Another characteristic that determines the creepage requirements is the level of contamination in the environment, specified by the pollution degree. A higher level of pollution will require higher creepage. Pollution degree is classified according to table 3.

Table 3: Pollution Degree Classifications.

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