ADP1032ACPZ-2-R7 datasheet(33/37 Pages) AD

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Data Sheet

ADP1032

Rev. 0 | Page 33 of 37

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 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 ADP1032 isolators are

shown in Table 5.

Insulation Wear Out

The lifetime of insulation caused by wear out is determined by

its 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 causing incremental damage. The stress on the insul-

ation 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 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 is

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

(1)

AC RMS

RMS

−

(2)

where:

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

VRMS is the total rms 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 76 and

the following equations.

TIME

VAC RMS

VRMS

VDC

VPEAK

Figure 76. 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 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

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