ADM2461EBRWZ-R7 datasheet(20/21 Pages) AD

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ADM2461E/ADM2463E

Data Sheet

Rev. 0 | Page 20 of 21

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 to allow the components to be

categorized in different material groups. Lower material group

ratings are more resistant to surface tracking and 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. See Table 5 for

the material group and creepage information for the ADM2461E/

ADM2463E isolated RS-485 transceivers.

Insulation Wear Out

The lifetime of insulation caused by wear out is determined by

the insulation thickness, the material properties, and the voltage

stress applied across the insulation. Ensure 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. The

working voltage applicable to tracking is specified in most

standards.

Testing and modeling show that the primary driver of long-term

degradation is displacement current in the polyimide insulation,

which causes incremental damage. The stress on the insulation

can be divided into broad categories such as dc stress and ac

component, time varying voltage stress. DC stress causes little

wear out because there is no displacement current. AC component,

time varying voltage stress causes wear out.

The ratings in certification documents are typically based on 60 Hz

sinusoidal stress to reflect isolation from the 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 these

products, the ac rms voltage determines the product lifetime.

RMS

AC RMS



(1)

AC RMS

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 example frequently arises in power conversion

applications. Assume that the line voltage on one side of the

isolation barrier is 240 V ac rms and that 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 44, Equation 3, and Equation 4, where VPEAK is the

peak voltage.

TIME

VAC RMS

VRMS

VDC

VPEAK

Figure 44. Critical Voltage Example

For this example, the VRMS from Equation 1 is calculated as follows:

240

400

RMS



= 466 V

(3)

This VRMS value 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 VAC RMS. To

calculate VAC RMS for this example, use Equation 2 as follows:

466

400

AC RMS



= 240 V rms

(4)

In this case, VAC RMS is the line voltage of 240 V rms. This

calculation is more relevant when the waveform is not sinusoidal.

The VAC RMS value is compared to the limits for the working ac

voltage in Table 10 for the expected lifetime (which is less than

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

service life.

Note that the dc working voltage limit is set by the creepage of

the package, as specified in IEC60664-1. This dc value can

differ for specific system level standards.

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