Data Sheet

ADuM4151/ADuM4152/ADuM4153

Rev. A | Page 21 of 22

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 into 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 ADuM4151/ADuM4152/ADuM4153 isolators are

detailed in Table 15.

Insulation Wear Out

The lifetime of insulation due to 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 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 stress 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.

2

2

DC

RMS

AC

RMS

V

V

V





(1)

or

2

2

DC

RMS

RMS

AC

V

V

V





(2)

where:

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 V ac rms, and a 400 V dc 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 19 and the following equations.

TIME

Figure 19. Critical Voltage Example

The working voltage across the barrier from Equation 1 is

2

2

DC

RMS

AC

RMS

V

V

V





2

2

400

240 



RMS

V

The 466 V rms 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.

2

2

DC

RMS

RMS

AC

V

V

V





2

2

400

466 



RMS

AC

V

In this case, the ac rms voltage is simply the line voltage of

240 V rms. 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 19 for the expected lifetime, less than 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 19 is set by the

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

may differ for specific system level standards