ADuM6020/ADuM6028 Data Sheet
Rev. 0 | Page 16 of 18
side. Failure to ensure these steps can cause voltage differentials
between pins, exceeding the absolute maximum ratings specified
in Table 13, thereby leading to latch-up or permanent damage.
THERMAL ANALYSIS
The ADuM6020 and ADuM6028 each consist of three internal
die attached to a split lead frame. For thermal analysis, the die is
treated as a thermal unit, with the highest junction temperature
reflected in the θ
JA
values, shown in Table 8 and Table 9. The value
of θ
JA
is based on measurements taken with the devices mounted
on a JEDEC standard, 4-layer board with fine width traces and
still air. Under normal operating conditions, the ADuM6020 and
ADuM6028 can operate at full load, but at temperatures greater
than 85°C, derating the output current may be needed, as
shown in Figure 3 and Figure 4.
EMI CONSIDERATIONS
The ADuM6020/ADuM6028 dc-to-dc converters must, of
necessity, operate at a high frequency to allow efficient power
transfer through the small transformers. This high frequency
operation creates high frequency currents that can propagate in
circuit board ground and power planes, requiring proper power
supply bypassing at the input and output supply pins (see Figure 25
and Figure 26). Using proper layout, bypassing techniques, and
surface-mount ferrite beads in series with the V
ISO
and GND
ISO
pins, the dc-to-dc converters are designed to provide regulated,
isolated power that is below CISPR22 Class B limits at full load
on a 2-layer PCB with ferrites.
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.
The two types of insulation degradation of primary interest are
breakdown along surfaces exposed to the 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 ADuM6020 and ADuM6028
are presented in Table 6 and Table 7.
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 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
grouped 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 a
60 Hz sinusoidal waveform 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 these products, the ac rms voltage
determines the product lifetime.
(1)
or
(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 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 Figure 27 and
the following equations.
