Data Sheet
ADuM220N/ADuM221N/ADuM225N/ADuM226N
Surface Tracking
Calculation and Use of Parameters Example
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
ADuM220N/ADuM221N/ADuM225N/ADuM226N isolators
are presented in Table 9 and Table 10.
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 Table 18 and
Table 19 and the following equations.
V
AC RMS
V
V
V
DC
PEAK
RMS
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. It is the working voltage
TIME
Figure 23. Critical Voltage Example
The working voltage across the barrier from Equation 1 is
2
VRMS = VAC RMS2 + VDC
applicable to tracking that is specified in most standards.
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.
VRMS = 2402 + 4002
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.
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.
VAC RMS = VRMS2 −VDC
2
VAC RMS
AC RMS = 240 V rms
=
4662 − 4002
V
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 the
continuous working voltage in Table 18 and Table 19 for the
expected lifetime, less than a 60 Hz sine wave, and it is well
within the limit for a 50-year service life.
2
VRMS = VAC RMS2 +VDC
(1)
or
VAC RMS = VRMS2 −VDC
(2)
2
Note that the dc working voltage limits in Table 18 and Table 19
are set by the creepage of the package as specified in IEC 60664-1.
These values can differ for specific system level standards.
where:
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.
Rev. A | Page 21 of 23
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