EMC - EV

Electromagnetic Compatibility for Electric Vehicles

Shielding Effectiveness

The generic shielding effectiveness requirement is 40 dB for magnetic field, electric field, and plane waves. Depending on the application the frequency range can start from 10 Hz going up to GHz.

To predict shielding effectiveness (SE) of a metal sheet the following factors are summed:  Absorption Loss (A), Reflection Loss (R), re-Reflection Correction Factor (C).  SE = A + R – C (see MIL-HDBK-419A).












Absorption loss depends on material thickness, permeability, electrical conductivity, and the frequency of the incident wave.  It is the same for all electromagnetic waves.

Reflection loss depends on the distance of the EMI source to the material (different for electric, magnetic, and plane waves), material electrical conductivity, and the frequency of the incident wave.

Sources:
Christian Rosu

Automotive BCI Test Limits

The Bulk Current Injection (BCI) test method simulates a field-to-wire coupling from nearby low frequencies radiated fields induced onto a test harness small relative to wavelength.
The coupling from BCI probe will increase with test frequency when the cable is electrically short, and then flatten out when the cable approaches and exceeds a half-wavelength in length.

The transducers (RF current transformers) inject current into both sides of the test harness, therefore both DUT and Load Simulator are subject to test. RF radiation from load simulator cables is possible, but it can be reduced placing 20cm of clip-on split-ferrite RF suppressers close to the transducer.

The BCI common-mode current injected in the test harness simulates an illuminating RF field.
To simulate conducted differential-mode disturbances the BCI induced current is injected in individul conductors.

Using the Substitution Method the actual current injected in test cables can vary from what was initially  calibrated, being less likely to over-test but more real-life representative.

To ensure the repeatability of test results, the cable under test must be centered within BCI current probe, the test set-up must be consistent, especially cable routing, placement of the clamp, and proximity to metal structures.


BCI Calibration Levels per MIL STD 416F CS114:


BCI Probe Insertion Loss per MIL STD 416F CS114:


RF Immunity Ratio mA versus V/m per MIL STD 416F:


Ford RI 112 (BCI) Calibration Limits requirements per FMC1278:



CAN Bus Noise Tolerance

The data is carried on the CAN bus as a voltage difference between the two signal lines. If both lines are at the same voltage, the signal is a recessive bit. If the CAN_H line is higher than the CAN_L line by 0.9V, the signal line is a dominant bit.

Immunity to Ground Noise

The CAN bus does not use the ground as reference point for these two signal lines. Therefore the CAN bus transmissions lines are immune to any ground noise typically present in automotive applications.

Immunity to Electromagnetic Filed

The signals on the two CAN lines will both be subject to the same electromagnetic filed level. Therefore no differences in voltages between the two lines should become relevant under electromagnetic interference.

Using Twisted Pair Wires for Differential Signal Lines

Bad connectors are almost guaranteed to present an impedance discontinuity, and hence will cause reflections. Transmission line stubs of any length are also a source of reflections, longer the stub, the worse the impact of the reflections on lower data rate signals. Reflections are bad because they can cause destructive interference that can corrupt any transmitted data.

Christian Rosu

LISN (Line Impedance Stabilization Network) or AN (Artificial Network)

Purpose of the LISN:
1. Provide well defined RF impedance to the DUT.
2. The 1μF & 50μH filter isolates the noise that is put on the power lines from the DUT from feeding back to the power supply.
3. Provide a low impedance path for the noise to be measured at the output port of the LISN coupling the interference voltage generated by DUT via 0.1μF to the analyzer or receiver.

The role of the LISN is to isolate the DM current and CM current from the power supply, and to minimize the impact of the CM current by returning it to its sources. The wire harness inductance for large systems (aircraft) is 50μH whereas for small systems (automotive) is 5μH. However, the LISN selection criteria should be based on the frequencies of the measurements required.

     

Types of LISN

  1. V-LISN: Unsymmetrical emissions (line-to-ground)
  2. Delta-LISN: Symmetric emissions (line-to-line)
  3. T-LISN: Asymmetric emissions (mid point line-to-line)

 

There are two types of V-LISN with different impedances.

  • 5 µH inductance (CISPR 16-1-2, CISPR 25, ISO 7637, SAE J1113-41, DO160) are normally used to measure equipment for vehicles, boats and aircrafts connected to on-boards mains with DC or 400 Hz.
  • 50 µH according to CISPR 16-1-2, MIL STD 461 and ANSI C63.4 is intended to operate at mains frequencies of 50 Hz or 60 Hz.

The T-LISN measures the asymmetric disturbance voltage (common mode voltage) and provides it to an EMI Receiver. It is normally used for measuring telecommunication and data transmission equipment connected to symmetrical lines as e.g. twisted pairs.

CISPR-25 (Ed 3.0)
A network inserted in the supply lead or signal/load lead of apparatus to be tested which provides, in a given frequency range, a specified load impedance for the measurement of disturbance voltages and which may isolate the apparatus from the supply or signal sources/loads in that frequency range.
CISPR-25 (Ed 3.0) & ISO-11452-2:2004 & ISO-11452-4
The AN impedance ZPB (tolerance ± 20 %) in the measurement frequency range of 0.1 MHz to 100 MHz it is measured between the terminals P and B with a 50 Ω load on the measurement port and with terminals A and B short-circuited.

  

The 1μF capacitor is populated in CISPR-25 LISN; R=1Kohm.


ISO 7637-2:2011 & ISO-11452-2:2004 & ISO 7637-2:2004
The artificial network is used as a reference standard in the laboratory in place of the impedance of the vehicle wiring harness in order to determine the behavior of electrical/electronic devices.
ISO 7637-2:2011 & ISO 7637-2:2004
The resulting values of impedance ZPB, measured between the terminals P and B while terminals A and B are short-circuited, are given in figure below as a function of frequency assuming ideal electric components. In reality, the impedance of an artificial network shall not deviate more than 10 % from the given curve.

  

No 1μF populated in ISO 7637-2 LISN; R =50 ohm, C is function of voltage.


Sample setup: CISPR-25 require separate LISN for B+ and GND lines.

Christian Rosu

Electric Field Shielding

Types of Electromagnetic Coupling: Conducted, Radiation, Magnetic Field, Electric Filed

Electric Field Coupling

The EF lines start on positive charge and end on negative charge from higher voltage conductors to lower voltage conductors. Any two conductors at different potentials (voltages) have electric field lines between them. EF shields are connected to “ground” to maximize their effectiveness.

 

     The electric field lines are passing through ungrounded metallic planes. 

 

Grounding a copper enclosure does not increase or decrease its shielding property but it reduces the crosstalk within the product itself. The ungrounded shield allows coupling signals from circuits within the shielded enclosure. If the device is connected via external cable to another module the ungrounded shield can serve to capacitively couple signals from outside the enclosure. 

Shielding Effectiveness

Shielding Effectiveness S = A + R + B (dB)
   A = Absorption Loss
   R = Reflection Loss
   B = Correction Factor (for multiple reflections in the shield)

Electromagnetic Filed Shielding (Far Field)
Assuming an electromagnetic wave that propagates perpendicular to the shield surface:

Absorption Loss A = 131.4 * t * SQRT (f * relative permeability * conductivity) dB
   • increase due to the skin effect
   • is the primary contributor to the shielding effectiveness at high frequencies

   t = thicknes of the shield in meters
   f = frequency

Reflection Loss R = 168 - (10 * log (relative permeability * f / conductivity)) dB
   • decrease with the frequency
   • is the primary contributor to the shielding effectiveness at low frequencies

• For sources with high voltages the dominant near-field is an electrical field.
• For sources with high currents the dominant near-field is a magnetic field.

Electric Field Shielding (Near Field)
Reflection Loss R = 322 - (10 * log (conductivity / relative permeability * f^3 * r^2))
   r = distance between the source and the shield
   electric near-field reflection loss =< far-field reflection loss

Magnetic Field Shielding (Near Field)
Reflection Loss R = 14.57 - (10 * log (conductivity * f * r^2 / relative permeability))
• reflection loss decreases for decreasing frequencies, and is lower than the reflection loss for the plane wave reflection.
• reflection losses are usually negligible for lower frequencies and absorption losses are small for low frequencies too.

Magnetic Field (MF) shielding methods:
• Deviation of the magnetic flux with high permeability material.
• The shorted tuned method, which consists in the generation of opposing fluxes that cancel the magnetic field in the area of interest.

Using magnetic material as shield:
• The permeability of a magnetic material decreases by increasing the frequency (depends only on the material).
• The permeability of a magnetic material decreases by increasing the MF strength (depends on the material and the section of the magnetic circuit).

The steel is a better magnetic field shield at low frequencies than good conductors like aluminium or copper. However at high frequencies, good conductors provide better magnetic shielding.

Shielding Effectiveness

  • For non-magnetic material increases with the frequency, therefore, it is recommended to calculate the attenuation for the lowest frequency of interest.
  • For magnetic materials may reduce due to the decrease of the permeability with the frequency.

EMI Control Techniques

EMI suppression involves grounding, shielding, and filtering.

1. Grounding

An ideal ground plane is a zero-potential, zero impedance body that can be used as a reference for all signals in associated circuitry, and to which any undesired current can be transferred for the elimination of its effects.

The multiple-point grounding minimizes ground lead lengths. The ground plane might be a ground wire that is carried throughout the system or a large conductive body.

Bonding

The physical implementation for grounding is done through bonding of a low-impedance path between two metal surfaces to make a structure homogeneous with respect to the flow of electrical currents, thus avoiding the development of potentials between the metallic parts, since such potentials may result in EMI.

  • provide protection from electrical shock
  • power circuit current return paths
  • antenna ground plane connections
  • minimize the potential difference between the devices
  • can carry large fault current
  • direct bond is a metal-tometal contact between the elements connected
  • indirect bond is a contact through the use of conductive jumpers

Bond Quality

The dc resistance Rdc = length of the bond / (conductivity * cross-sectional area)

The ac resistance Rac =  length of the bond / (conductivity * width of the bond * skin depth)

Bonding effectiveness can be expressed as the difference (in dB) between the induced voltages on an equipment case with and without the bond straps.

2. Shielding

The purpose of shielding is to confine radiated energy to a specific region or to prevent radiated energy from entering a specific region. Shields may be in the form of partitions and boxes as well as in the form of cable and connector shields.

Shield types:

  • solid
  • nonsolid (e.g., screen)
  • braid, as is used on cables.

Shielding Effectiveness SE = 10*log(10) * (incident power density / transmitted power density)

  • incident power density is the power density at a measuring point before a shield is installed and the
  • transmitted power is the power density at the same point after the shield is in place
  • electric field strength SE = 20*log(10) * (Ei / Et)
  • magnetic field strength SE = 20*log(10) * (Hi / Ht)

3. Filtering
An electrical filter is a network of lumped or distributed constant resistors, inductors, and capacitors that offers comparatively little opposition to certain frequencies, while blocking the passage of other frequencies. Filters are used to substantially reduce the levels of conducted interference.

Insertion Loss IL = 20*log(10) * (V1 / V2)

  • V1 is the output voltage of a signal source with the filter in the circuit
  • V2 is the output voltage of the signal source without the use of the filter

Low-pass filters IL = 10*log(10) * (1 + F^2) dB

  • F = PI*f*R*C for capacitive filter (f = frequency)
  • F = PI*f*L/R for inductive filter (f = frequency)

Lumped System is an electrical circuit with passive elements (e.g. R, L, C) that are constant.
For example, the current at a capacitor with capacity C is i(t) = C * (dv(t) / dt)

A lumped element size is much smaller than the wavelength of the applied voltages and currents. In this case wave propagation effects may be neglected.

Distributed System is an electrical circuit with passive elements (e.g. R, L, C) where the  inductance, capacity and resistance are not constant but functions of time and space length. This leads to partial derivatives of i(t,x) and v(t,x) in t (time) and x (position).