ADuM3154ARSZ datasheet(21/22 Pages) AD

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Data Sheet

ADuM3154

Surface Tracking

Surface tracking is addressed in electrical safety standards by

setting a minimum surface creepage based on the working

voltage, the environmental conditions, and the properties of the

insulation material. Safety agencies perform characterization

testing on the surface insulation of components that allow the

components to be categorized in different material groups.

Lower material group ratings are more resistant to surface

tracking and, therefore, can provide adequate lifetime with

smaller creepage. The minimum creepage for a given working

voltage and material group is in each system level standard and

is based on the total rms voltage across the isolation, pollution

degree, and material group. The material group and creepage

for the ADuM3154 isolator are detailed in Table 12.

Insulation Wear Out

The lifetime of insulation caused by wear out is determined by

its thickness, the material properties, and the voltage stress

applied. It is important to verify that the product lifetime is

adequate at the application working voltage. The working

voltage supported by an isolator for wear out may not be the

same as the working voltage supported for tracking. It is the

working voltage applicable to tracking that is specified in most

standards.

Testing and modeling have shown that the primary driver of

long-term degradation is displacement current in the polyimide

insulation causing incremental damage. The stress on the

insulation can be broken down into two broad categories, such

as dc stress, which causes very little wear out because there is no

displacement current, and an ac component time varying

voltage stress, which causes wear out.

The ratings in certification documents are usually based on

60 Hz sinusoidal stress, because this reflects isolation from line

voltage. However, many practical applications have

combinations of 60 Hz ac and dc across the barrier, as shown in

Equation 1. Because only the ac portion of the stress causes

wear out, the equation can be rearranged to solve for the ac rms

voltage, as shown in Equation 2. For insulation wear out with

the polyimide materials used in this product, the ac rms voltage

determines the product lifetime.

RMS

(1)

RMS

−

(2)

where:

VRMS is the total rms working voltage.

VAC RMS is the time varying portion of the working voltage.

VDC is the dc offset of the working voltage.

Calculation and Use of Parameters Example

The following is an example that frequently arises in power

conversion applications. Assume that the line voltage on one

side of the isolation is 240 VAC RMS, and a 400 VDC bus voltage is

present on the other side of the isolation barrier. The isolator

material is polyimide. To establish the critical voltages in

determining the creepage clearance and lifetime of a device,

see Figure 17 and the following equations.

TIME

VAC RMS

VRMS

VDC

VPEAK

Figure 17. Critical Voltage Example

The working voltage across the barrier from Equation 1 is

RMS

400

240 +

RMS

VRMS = 466 V

This is the working voltage used together with the material

group and pollution degree when looking up the creepage

required by a system standard.

To determine if the lifetime is adequate, obtain the time varying

portion of the working voltage. The ac rms voltage can be

obtained from Equation 2.

RMS

−

400

466 −

RMS

VAC RMS = 240 VRMS

In this case, the VACRMS is simply the line voltage of 240 VRMS.

This calculation is more relevant when the waveform is not

sinusoidal. The value is compared to the limits for the working

voltage listed in Table 16 for the expected lifetime, under a

60 Hz sine wave, and it is well within the limit for a 50-year

service life.

Note that the dc working voltage limit in Table 16 is set by the

creepage of the package as specified in IEC 60664-1. This value

may differ for specific system level standards.

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