Data Sheet ADuM4154
Rev. A | Page 21 of 22
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 allow 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 ADuM4154 isolator are detailed in Table 12.
Insulation Wear Out
The lifetime of insulation caused by wear out is determined by
its thickness, the 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. It is the
working voltage applicable to tracking that 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 two 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 shown in Equation 2. For insulation wear out with
the polyimide materials used in this product, the ac rms voltage
determines the product lifetime.
22
DCRMSACRMS
VVV (1)
or
22
DCRMSRMSAC
VVV (2)
where:
V
RMS
is the total rms working voltage.
V
AC RMS
is the time varying portion of the working voltage.
V
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 17 and the following equations.
ISOLATION VOLTAG
TIME
V
AC RMS
V
RMS
V
DC
V
PEAK
12366-017
Figure 17. Critical Voltage Example
The working voltage across the barrier from Equation 1 is
22
DCRMSACRMS
VVV
22
400240
RMS
V
V
RMS
= 466 V
The 466 V rms working voltage 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. The ac rms voltage can be obtained
from Equation 2.
22
DCRMSRMSAC
VVV
22
400466
RMSAC
V
V
AC RMS
= 240 V
In this case, the V
AC RMS
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 the working
voltage listed in Table 16 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 16 is set by the
creepage of the package as specified in IEC 60664-1. This value
may differ for specific system level standards.
