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Data Sheet

ADN4620/ADN4621

APPLICATIONS INFORMATION

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Rev. 0 | 22 of 24

the applied magnetic field reduces the received pulse from >0.35 V

to 0.295 V. This voltage is still higher than the 0.11 V sensing

threshold of the decoder.

The preceding magnetic flux density values correspond to specific

current magnitudes at given distances from the ADN4620/ADN4621

transformers. Figure 38 expresses these allowable current mag-

nitudes as a function of frequency for selected distances. The

ADN4620/ADN4621 is insensitive to external fields. Only extremely

large, high frequency currents that are close to the component can

potentially be a concern. For the 1 MHz example noted, a 2.64 kA

current must be placed 5 mm from the ADN4620/ADN4621 to affect

component operation.

Figure 38. Maximum Allowable Current for Various Current to ADN4620/

ADN4621 Spacings

In combinations of strong magnetic field and high frequency, any

loops formed by PCB traces can induce sufficiently large error

voltages to trigger the thresholds of succeeding circuitry. Avoid PCB

structures that form loops.

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 insula-

tion 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, which 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 barrier, pollution degree, and material group. The

material group and creepage for ADN4620/ADN4621 are detailed in

Table 4.

Insulation Wear Out

The lifetime of insulation caused by wear out is determined by

the thickness of the insulation, 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 deg-

radation 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 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 type of waveform reflects isolation

from line voltage. However, many practical applications have com-

binations of 60 Hz ac and dc across the isolation 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.

VRMS = VACRMS2+VDC2(1)

VACRMS= VRMS2−VDC2(2)

where:

VRMS is the total rms working voltage.

VAC RMS is the time varying portion of the working voltage.

VDC is the dc offset of the working voltage.

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