ISO121 datasheet(10/18 Pages) TI2

10 / 18 page

ISO120/121

because IMV-induced errors behave like input-referred error

signals. To predict the total IMR, divide the isolation voltage

by the IMR shown in IMR vs Frequency performance curve

and compute the amplifier response to this input-referred

error signal from the data given in the Signal Response vs

Carrier Frequency performance curve. Due to effects of very

high-frequency signals, typical IMV performance can be

achieved only when dV/dT of the isolation mode voltage

falls below 1000V/

µs. For convenience, this is plotted in the

typical performance curves for the ISO120 and ISO121 as a

function of voltage and frequency for sinusoidal voltages.

When dV/dT exceeds 1000V/

µs but falls below 20kV/µs,

performance may be degraded. At rates of change above

20kV/

µs, the amplifier may be damaged, but the barrier

retains its full integrity. Lowering the power supply voltages

below

±15V may decrease the dV/dT to 500V/µs for typical

performance, but the maximum dV/dT of 20kV/

µs remains

unchanged.

Leakage current is determined solely by the impedance of

the 2pF barrier capacitance and is plotted in the Isolation

Leakage Current vs Frequency curve.

ISOLATION VOLTAGE RATINGS

Because a long-term test is impractical in a manufacturing

situation, the generally accepted practice is to perform a

production test at a higher voltage for some shorter time. The

relationship between actual test voltage and the continuous

derated maximum specification is an important one. Histori-

cally, Burr-Brown has chosen a deliberately conservative

one: V

TEST = (2 X ACrms continuous rating) + 1000V for 10

seconds, followed by a test at rated ACrms voltage for one

minute. This choice was appropriate for conditions where

system transients are not well defined.

Recent improvements in high-voltage stress testing have

produced a more meaningful test for determining maximum

permissible voltage ratings, and Burr-Brown has chosen to

apply this new technology in the manufacture and testing of

the ISO120 and ISO121.

Partial Discharge

When an insulation defect such as a void occurs within an

insulation system, the defect will display localized corona or

ionization during exposure to high-voltage stress. This ion-

ization requires a higher applied voltage to start the dis-

charge and lower voltage to maintain it or extinguish it once

started. The higher start voltage is known as the inception

voltage, while the extinction voltage is that level of voltage

stress at which the discharge ceases. Just as the total insula-

tion system has an inception voltage, so do the individual

voids. A voltage will build up across a void until its incep-

tion voltage is reached, at which point the void will ionize,

effectively shorting itself out. This action redistributes elec-

trical charge within the dielectric and is known as partial

discharge. If, as is the case with AC, the applied voltage

gradient across the device continues to rise, another partial

discharge cycle begins. The importance of this phenomenon

is that, if the discharge does not occur, the insulation system

retains its integrity. If the discharge begins, and is allowed

to continue, the action of the ions and electrons within the

defect will eventually degrade any organic insulation system

in which they occur. The measurement of partial discharge

is still useful in rating the devices and providing quality

control of the manufacturing process. Since the ISO120 and

ISO121 do not use organic insulation, partial discharge is

non-destructive.

The inception voltage for these voids tends to be constant, so

that the measurement of total charge being redistributed

within the dielectric is a very good indicator of the size of

the voids and their likelihood of becoming an incipient

failure. The bulk inception voltage, on the other hand, varies

with the insulation system, and the number of ionization

defects and directly establishes the absolute maximum volt-

age (transient) that can be applied across the test device

before destructive partial discharge can begin. Measuring

the bulk extinction voltage provides a lower, more conserva-

tive voltage from which to derive a safe continuous rating.

In production, measuring at a level somewhat below the

expected inception voltage and then derating by a factor

related to expectations about system transients is an

accepted practice.

Partial Discharge Testing

Not only does this test method provide far more qualitative

information about stress-withstand levels than did previous

stress tests, but it provides quantitative measurements from

which quality assurance and control measures can be based.

Tests similar to this test have been used by some manufac-

turers, such as those of high-voltage power distribution

equipment, for some time, but they employed a simple

measurement of RF noise to detect ionization. This method

was not quantitative with regard to energy of the discharge,

and was not sensitive enough for small components such as

isolation amplifiers. Now, however, manufacturers of HV

test equipment have developed means to quantify partial

discharge. VDE, the national standards group in Germany

and an acknowledged leader in high-voltage test standards,

has developed a standard test method to apply this powerful

technique. Use of partial discharge testing is an improved

method for measuring the integrity of an isolation barrier.

To accommodate poorly-defined transients, the part under

test is exposed to voltage that is 1.6 times the continuous-

rated voltage and must display

≤5pC partial discharge level

in a 100% production test.

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