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ADP1031

Data Sheet

Rev. A | Page 34 of 38

INSULATION LIFETIME

All insulation structures eventually break down when subjected to

voltage stress over a sufficiently long period. The rate of insulation

degradation is dependent on the characteristics of the voltage

waveform applied across the insulation as well as on the materials

and material interfaces.

The two types of insulation degradation of primary interest are

breakdown along surfaces exposed to the air and insulation wear

out. Surface breakdown is the phenomenon of surface tracking

and the primary determinant of surface creepage requirements

in system level standards. Insulation wear out is the phenomenon

where charge injection or displacement currents inside the

insulation material cause long-term insulation degradation.

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 allows the components to be

categorized in different material groups. Lower material group

ratings are more resistant to surface tracking. Therefore, lower

material group ratings provide adequate lifetime with smaller

creepage. The minimum creepage for a given working voltage and

material group is determined 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 ADP1031 isolators are shown in Table 4.

Insulation Wear Out

The lifetime of insulation is determined by thickness, 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. The working voltage applicable to tracking 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. This displacement current causes incremental damage

to the insulation. The stress on the insulation can be broken down

into broad categories: dc stress and ac component time varying

voltage stress. DC stress causes very little insulation wear out

because there is no displacement current. AC component time

varying voltage stress causes insulation 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, the ac rms voltage determines the product lifetime.

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 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 75 and

the following equations.

TIME

VAC RMS

VRMS

VDC

VPEAK

Figure 75. Critical Voltage Example

The working voltage across the barrier from Equation 1 is

RMS

400

240 +

RMS

VRMS = 466 V

This VRMS value is the working voltage and is 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. To obtain the ac rms voltage,

use Equation 2.

AC RMS

RMS

−

400

466 −

RMS

VAC RMS = 240 V rms

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 working

voltage in Table 8 for the expected lifetime, which is less than a

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