Data Sheet ADuM4135
Rev. B | Page 15 of 17
DC CORRECTNESS AND MAGNETIC FIELD IMMUNITY
The ADuM4135 is resistant to external magnetic fields. The
limitation on the ADuM4135 magnetic field immunity is set by
the condition in which induced voltage in the transformer
receiving coil is sufficiently large to either falsely set or reset the
decoder. The following analysis defines the conditions under
which a false reading condition can occur. The 2.3 V operating
condition of the ADuM4135 is examined because it represents
the most susceptible mode of operation.
100
10
1
0.1
0.01
0.001
1k 10k 100k 1M 10M 100M
MAXIMUM ALLOWABLE MAGNETIC FLUX
DENSITY (kgauss)
MAGNETIC FIELD FREQUENCY (Hz)
13082-029
Figure 25. Maximum Allowable External Magnetic Flux Density
1k
100
10
1
0.1
0.01
1k 10k 100k 1M 10M 100M
MAXIMUM ALLOWABLE CURRENT (kA)
MAGNETIC FIELD FREQUENCY (Hz)
13082-030
DISTANCE = 1m
DISTANCE = 100mm
DISTANCE = 5mm
Figure 26. Maximum Allowable Current for Various
Current-to-ADuM4135 Spacings
INSULATION LIFETIME
All insulation structures eventually break down when subjected
to voltage stress over a sufficiently long period. The rate of insu-
lation 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.
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 ADuM4135 isolator are presented in Table 8.
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
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 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 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.
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.
