Magnetic Field Immunity of Digital Capacitive Isolators

Digital capacitive isolators are often used in environments that include large electric motors, generators, and other devices that generate strong electromagnetic fields. Exposure to these magnetic fields can cause potential data corruption issues because the electric potential (EMF) or voltage created by these magnetic fields can interfere with data signal transmission. Because of this potential threat, many digital isolator users require isolators to have high magnetic field immunity (MFI). Many digital isolator technologies claim to have high MFI, but capacitive isolators have a nearly infinite MFI due to their design and internal structure. This article will provide a detailed description of their design.

Basic laws of physics

A current-carrying conductor, such as the power cord of an electric motor, is surrounded by a magnetic field caused by the current flowing through it. The direction of this magnetic field can be easily determined by applying the right-hand rule (see Figure 1). The rule is as follows: Hold the conductor in your right hand with your thumb pointing in the direction of the current, and the fingers around the conductor point in the direction of the magnetic field. Therefore, the plane of the magnetic flux lines is always perpendicular to the current.

Figure 1 shows the magnetic flux density B for DC current. For AC current, the right-hand rule applies in both directions, and both the magnetic field and the AC current vary with the same frequency f: B(f) ~ I(f). The magnetic field (or more precisely the magnetic flux density and its corresponding field strength) decreases with increasing distance from the center axis of the conductor. These relationships can be expressed as:

as well as

where B is the magnetic flux density in volt-seconds per square meter (V•s/m2), μ0 is the magnetic permeability in free space (calculated as 4π × 10–7 V•s/A•m), I is the current in amperes, r is the distance of the conductor in meters, and H is the magnetic field strength in amperes per meter (A/m).

Figure 1 Right-hand rule

When magnetic field lines pass through a nearby conductor loop, they generate an EMF whose strength depends on the area of ​​the loop and the flux density and frequency of the field:

EMF is the electric potential in volts, f is the frequency of the magnetic field, and A is the loop area in square meters (m2).

All isolators have a conductive loop in some shape or form that allows magnetic field lines to pass through and generate EMF. If the strength is large enough, this EMF superimposed on the signal voltage can cause erroneous data transmission. In fact, some isolation technologies are very sensitive to electromagnetic interference. To understand why capacitive isolators are not affected by magnetic fields, we need to examine their internal structure.

Structure of Capacitor Isolator

Capacitive isolators consist of two silicon chips—a transmitter and a receiver (see Figure 2). Data is transmitted across a differential isolation barrier formed by two capacitors, each with a copper top plate and a conductive silicon bottom plate across the silicon dioxide (SiO2) dielectric. The driver output of the transmitter chip is connected to the top plate of the isolation capacitor on the receiver chip via some bond wires. A conductive loop is formed by connecting the bottom plate of the capacitor to the receiver input. Figure 3 shows an equivalent circuit diagram of the isolation barrier, with the loop area marked between the gold bond wires. Obviously, the magnetic field passing through this loop will generate an EMF, which represents the input voltage noise Vn1 of the RC network below. The second differential noise component we often encounter, Vn2, is caused by the conversion of common-mode noise to differential noise. The two noise components together make up the combined noise Vn. If only the EMF effect is considered, Vn can be conservatively divided into two:

Figure 2 Simplified diagram of the internal structure of the capacitor isolator

Figure 3 Equivalent circuit structure diagram of the isolation layer

To trigger the receiver, the output of the RC network must provide a differential input voltage, VID, that exceeds the receiver input threshold. Whether a false trigger occurs or not depends on the gain response, G(f), of the RC network.

Converting the differential network to a single-ended network (see Figure 4) simplifies the derivation of G(f), but requires that C′1 = 2C1, R′1 = R1/2, C′2 = 2C2, and R′2 = R2/2.

Figure 4 Single-ended RC network

Table 1 Current and magnetic field values ​​of a conductor 0.1m away from a capacitive isolator

Using equations 2 and 3 we also get the EMF, magnetic field strength (H), and the corresponding current (I) in the conductor (here assuming a future isolator of 0.1 m).

From the extremely high values ​​listed in Table 1, it is clear that neither 5 mA of low frequency current nor 500 A at 100 MHz can stop this isolator from functioning properly. The reason for this almost infinite MFI is the location of the isolation capacitors. If these capacitors were located on the transmitter chip, any EMF generated in the bond wires would be able to affect the undisturbed receiver inputs.

Obviously, such high MFI values ​​are impossible to test realistically. The data sheets of capacitive isolators state only a modest value of 1000 A/m for practical testing. However, unshielded capacitive isolators easily pass the Class 5 MFI requirements of the IEC61000-4-8 and IEC61000-4-9 standards. These standards describe the application of up to 100 A/m power frequency electromagnetic fields and 1000 A/m pulsed electromagnetic fields, respectively. Class 5 specifies harsh industrial environments with many conductors, buses or medium and high voltage lines, all carrying tens of kiloamperes of current. Also included are many lightning protection systems and grounding conductors of tall building structures (e.g., pylons, etc.) that carry the full lightning current. Heavy industrial plants and outdoor power distribution installations in power stations are also representative of this environment.

Figure 6 compares the calculated MFI threshold for capacitive isolators with the Class 5 (highest) test level of IEC 61000-4-8 and IEC 61000-4-9.

Figure 6 MFI test threshold

in conclusion

Magnetic coupling that exceeds the noise budget of the capacitive isolator differential circuit requires a magnetic flux density greater than 11.7 V•s/m2 (117 kilogauss) at 1MHz. This would require more than 5 million amps of current in a conductor 0.1m from the device to generate such a field. This is impossible to exist in nature or in any manufactured device. If it did exist, the designer could assume that the surrounding circuits would fail before the isolation barrier failed.