Texas Instruments | ISO72x Digital Isolator Magnetic-Field Immunity (Rev. B) | Application notes | Texas Instruments ISO72x Digital Isolator Magnetic-Field Immunity (Rev. B) Application notes

Texas Instruments ISO72x Digital Isolator Magnetic-Field Immunity (Rev. B) Application notes
Application Report
SLLA181B – January 2006 – Revised August 2018
ISO72x Digital Isolator Magnetic-Field Immunity
Kevin Gingerich .................................................................... High-Performance Linear/Interface Products
ABSTRACT
The purpose of this analysis is to estimate the immunity of the ISO72x family of digital isolators to
magnetic fields.
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Contents
Introduction ...................................................................................................................
Differential Noise Budget ...................................................................................................
emf Calculation ...............................................................................................................
Magnetic Field Sensitivity ...................................................................................................
Conclusions...................................................................................................................
References ...................................................................................................................
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3
4
5
5
List of Figures
....................................................................................................
1
ISO721 Inter-Die Bonds
2
Equivalent Barrier Differential Circuit Schematic ........................................................................ 2
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3
Magnetic Flux Density vs Frequency Magnetic Field Immunities and Test Thresholds ............................ 5
List of Tables
1
Solutions to Equations ...................................................................................................... 4
Trademarks
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1
Introduction
The expected application environment of a digital isolator includes close proximity to large motors and
other magnetic-field-generating equipment. Data errors due to exposure to such fields are a concern. This
is especially so because TI is applying a novel capacitive-coupled isolation barrier.
The ISO72x isolators transfer the data signals differentially across the isolation barrier through two
capacitors formed with a metal top plate and conductive silicon bottom plate on each side of a SiO2
dielectric. The drive circuits reside on one substrate and the capacitors and receiving circuits reside on
another substrate. Figure 1 shows the bond wires between the two substrates that complete the circuit.
The electromotive force (emf) from time-varying magnetic fields in these differential circuits is the subject
of this analysis. A noise budget is developed and Faraday’s Law applied to estimate the magnetic flux
density necessary to exceed the budget. Pulse magnetic field test results per CEI IEC 61000-4-9 offer
empirical validation of the analysis.
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1
Differential Noise Budget
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Figure 1. ISO721 Inter-Die Bonds
2
Differential Noise Budget
Figure 2 shows a basic model of the circuit path across the isolation barrier of the ISO72x. VI is the output
of a CMOS totem-pole driver operating from VCC1 and vn represents differential noise. The isolation
capacitors differentiate vI and vn such that only time-varying and amplitude information is presented to the
receiver inputs. The receiver, operating from VCC2, amplifies the differential signal for level comparison,
latching, validation, and presentation to the output.
Vn
C
+
Via ±
Vi
+
R
Vo
±
C
Figure 2. Equivalent Barrier Differential Circuit Schematic
In the absence of differential noise and during the steady state, there is no differential voltage across the
inputs of the receiver, and the output is the last state latched. When the differential voltage via exceeds the
receiver positive-going or negative-going threshold voltage for a sufficient period, a state change occurs at
the output of the receiving circuitry. Preferably, the state change is from vI and not vn. To prevent state
changes from noise, the input voltage from vn must remain below the positive-going threshold and above
the negative-going threshold of the receiver during the steady state.
2
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emf Calculation
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The lowest positive-going differential input voltage threshold of the ISO72x receiver is 10 mV, and the
highest negative-going threshold is –10 mV. These thresholds apply over its recommended operating
conditions and sinusoidal frequencies up to approximately 30 MHz. Above this frequency, the gain of the
receiver decreases requiring more input voltage to cross the threshold and raising the input threshold
magnitude. Therefore, it can be postulated that no steady-state errors occur if the magnitude of via due to
vn is less than 10 mV for noise frequencies up to 30 MHz. Circuit simulation predicts a 100-MHz minimum
noise threshold magnitude of 16.7 mV.
The input and output voltage of the circuit in the frequency domain is related by the system function H(ω)
such that
where
The requirement is that the magnitude of via be less than the minimum receiver threshold which is
identified as VIT(ω) and |via(ω)|<|VIT(ω)|. Making the substitutions and rearranging gives:
(2)
and
(3)
The value of RC is approximately 43.8 ps, giving sufficient information to calculate differential noise
margins.
Two primary sources of differential noise in the circuit are emf and common-mode-to-differential voltage
conversion. Conservatively, one-half of the differential noise margin is allocated to each source giving the
equation for the emf noise margin as shown in Equation 4.
(4)
3
emf Calculation
An electromotive force is merely a voltage that arises from conductors moving in a magnetic field or from
changing magnetic fields. Faraday’s Law for electromagnetic induction in a nonmoving circuit states that
the emf induced is proportional to the time rate of change of the magnetic flux linked with the circuit. For
stationary circuits, this law may be stated as shown in Equation 5.
where
•
•
E is the electric field intensity along a path length dl.
B is the magnetic flux density normal to the surface ds.
(5)
Assume a B field of B0sinω0t normal to and uniform over the surface. Taking the time partial derivative
gives:
(6)
Of course, the surface integral of ds is simply the area enclosed by the loop, S, giving:
(7)
Each channel of the ISO72x has high-speed and low-speed isolated signals. The high-speed signal
transfers input state changes whereas the low-speed signal transfers low-frequency, internally generated
signals used to keep the input and output in the same state. (The loop nearest to the top of the page in
Figure 1 is the high-speed circuit.) The two circuits are electrically the same but slightly differ in physical
layout.
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3
Magnetic Field Sensitivity
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The differential circuit areas for integration are approximately rectangular, and the centers of the bond
pads used to connect the inter-die wires define the corners. The nominal length of the inter-die bond wires
is 954E-06 m. The distance between differential bond pads are the same on both die and 360E-06 m for
the high-speed circuit and 990E-06 m for the low-speed circuit. This gives areas of 343E-09 m2 for the
high-speed differential loop and 944E-09 m2 for the low-speed circuit. Because the largest area gives the
greatest emf, the low-speed loop area is used.
Substituting this gives emf(t) = –B0ω0944×10–9 cos ω0t and, because the concern is only with magnitudes,
emf becomes:
(8)
4
Magnetic Field Sensitivity
Now, the maximum magnetic flux density can be calculated by bounding the magnitude of the emf with the
emf noise margin or Equation 9
(9)
Solving the inequality for B0 at ω0 gives:
(10)
Table 1 solves Equation 4, Equation 8, and Equation 10 at different noise frequencies (ω = 2πf).
Table 1. Solutions to Equations
f
|VIT|
emf
BO
0.001 MHz
10 mV
72,700 V
12.3E+6 Wb/m2
0.01
10
7,270
122.6E+3
0.1
10
727
1.2E+3
1
10
72.7
12.3E+0
10
10
7.27
122.6E-3
30
10
2.4
13.6E-3
100
16.7
1.2
2.0E-3
For perspective, the magnetic flux density produced by a current element I 0 dl may be interpreted as
shown in Equation 11.
where
•
4πr2 is the surface area of a sphere of radius r positioned at each point along the conductor of length
l.
(11)
If a 0.1-m differential current element is assumed as the source and µ = µ0 and r = 0.1 m, the current
would need to be in excess of ten million amperes to produce a magnetic flux density of 12.3 Wb/m2 and
exceed the 1-MHz limit.
The unshielded ISO721 has successfully passed the Class-5 magnetic field immunity requirements of IEC
61000-4-8 power-frequency fields up to 100 A/m (125.6 × 10-6 Wb/m2) and IEC 61000-4-9 pulsed fields to
1000 A/m (1.256 × 10-3 Wb/m2). The standard defines Class 5 as applying to severe industrial
environments characterized by conductors, bus bars, medium-voltage lines, or high-voltage Iines carrying
tens of kA; ground conductors of the lightning protection system or high structures (like the line towers)
carrying the whole lightning current. Switchyard areas of heavy industrial plants and power stations may
be representative of this environment.
Figure 3 graphically compares the calculated magnetic field immunity thresholds of the ISO721 and
Analog Device’s ADuM1100(1) along with the Class-5 (highest) test levels of IEC 61000-4-8 and IEC
61000-4-9 (2).
4
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Conclusions
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100E+6
ISO721
Magnetic Flux Density (Wb/m2)
100E+3
100E+0
100E-3
ADuM1100
IEC61000-4-8
100E-6
100E-9
IEC61000-4-9
100E-12
100E-15
100E-18
0.001
0.01
0.1
1
10
100
Frequency (MHz)
(1)
Data was taken from Figure 8 of revision E of the ADuM1100 data sheet and converted to comparable units.
(2)
Assumes the permeability of free space and approximates the IEC 61000-4-9 pulse shape as 304 / [(jjω +
78.7 × 103) (jω + 105)].
Figure 3. Magnetic Flux Density vs Frequency
Magnetic Field Immunities and Test Thresholds
5
Conclusions
Magnetic coupling in the differential circuit of the low-speed signal of the ISO72x exceeding the noise
budget requires a magnetic flux density greater than 12.3 Wb/m2 (123 kGauss) at 1 MHz. This would be
the field generated by over ten million amperes in a 0.1-m conductor 0.1 m away from the device. It is
unlikely that this will occur in nature or any manufactured equipment. If it did, it is more likely that
surrounding circuitry would fail before the barrier circuit of the ISO72x.
We currently recommend the ISO7710 from our newest ISO77xx family of digital isolators with improved
isolation ratings.
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References
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•
•
•
Fink, Donald G., Electronic Engineers’ Handbook, McGraw-Hill, 1975
Hayt, William H., Jr., Engineering Electromagnetics, McGraw-Hill, 1974
McGillem, Clare D. and Cooper, George R., Continuous and Discrete Signal and System Analysis,
Holt, Rinehart and Winston, Inc., 1974
Electromagnetic Interference Test Report for the ISO721 HIGH-SPEED DIGITAL ISOLATOR,
Southwest Research Institute, Document no. EMCR 05/019 rev. 00, August 2005
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5
Revision History
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Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from A Revision (February 2006) to B Revision ............................................................................................. Page
•
6
Added the reference to the ISO7710 device in the Conclusions.................................................................... 5
Revision History
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