ADuM260N/ADuM261N/ADuM262N/ADuM263N Data Sheet
Rev. 0 | Page 22 of 23
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
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 reflects isolation from line
voltages. 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,
Equation 1 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.
22
DCRMSACRMS
VVV
(1)
or
22
DCRMSRMSAC
VVV
(2)
where:
V
AC RMS
is the time varying portion of the working voltage.
V
RMS
is the total rms working voltage.
V
DC
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, a 400 V dc bus voltage is present on
the other side of the isolation barrier, and the isolator material
is polyimide. To establish the critical voltages in determining
the creepage, clearance and lifetime of a device, see Figure 25
and the following equations.
ISOLATION VOLTAG
TIME
V
AC RMS
V
RMS
V
DC
V
PEAK
14998-025
Figure 25. Critical Voltage Example
The working voltage across the barrier from Equation 1 is
22
DCRMSACRMS
VVV
22
400240
RMS
V
V
RMS
= 466 V
This V
RMS
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.
22
DCRMSRMSAC
VVV
22
400466
RMSAC
V
V
AC 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 15 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 15 is set by the
creepage of the package as specified in IEC 60664-1. This value
can differ for specific system level standards.
