Data Sheet
ADuM4120/ADuM4120-1
1k
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.
DISTANCE = 1m
100
10
DISTANCE = 100mm
1
0.1
DISTANCE = 5mm
Testing and modeling show 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.
0.01
1k
10k
100k
1M
10M
100M
MAGNETIC FIELD FREQUENCY (Hz)
Figure 25. Maximum Allowable Current for Various Current to
ADuM4120/ADuM4120-1 Spacings
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.
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.
Two types of insulation degradation are of primary interest:
breakdown along surfaces exposed to 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.
2
V
RMS VAC RMS2 VDC
(1)
or
2
VAC RMS VRMS2 VDC
(2)
Surface Tracking
where:
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 ADuM4120/ADuM4120-1 isolators are shown in Table 4.
V
V
V
RMS is the total rms working voltage.
AC RMS is the time varying portion of the working voltage.
DC is the dc offset of the working voltage.
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 26 and the following equations.
Rev. 0 | Page 15 of 17
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