EMC FLEX BLOGA site dedicated to Automotive EMC Testing for Electronic Modules

Shielding Effectiveness

12. July 2015 21:34 in

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

12. July 2015 03:52 in

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).

WPT System Example

10. July 2015 18:34 by streng in

1) Off-board components

2) Primary Device

3) Secondary Device

4) On-board power components

5) EV supply equipment

6) WPT vehicle power supply circuit

7) Supply equipment communication controller

8) Electric vehicle communication controller

9) Portable EV supply equipment

10) CB & RCD or RCBO

11) Plug & socket-outlet

12) RESS or traction battery

13) Electric load

a) Wireless power transfer

b) Communication according to IEC 61980-2

EMC re-validation driven by manufacturing plant or production process changes

10. July 2015 08:32 in

Depending on the automotive OEM a re-validation may be mandatory whenever an electronic controller production is moved from one manufacturing plant to another (e.g. Mexico to China). This refer to a situation with unchanged design, parts vendors, and even identical manufacturing process flow.

The EMC/EMI related advantages when using SMT pick and place machines are:

• lower the resistance and inductance at the connection ensuring fewer unwanted RF signal effects, a better and more predictable high-frequency performance.
• better EMC performance (lower radiated emissions) due to the smaller radiation loop area (because of the smaller package) and the smaller lead inductance.

However, by using a different type/model of SMT pick and place machine a full EMC/EMI validation of your product might be necessary. The reason for re-validation is that even a small amount of inductance may change the self-resonant frequency of operation significantly, making the capacitor ineffective for optimal or desired performance. A capacitor remains capacitive up to its self-resonant frequency. Above self-resonance, the capacitor starts to appear as an inductor due to lead length and trace inductance. Inductance minimizes the ability of the capacitor to decouple or remove RF energy that exists between power and ground.

The self-resonant frequency of surface mount (SMT) capacitors is always higher, although interconnect inductance may obviate this benefit. Interconnect inductance includes routed traces and the bond wires internal to a component package. Depending on the type of product being designed, as well as on the frequency of operation, a change of inductance in the picohenry range may be too much to tolerate.

SMT 0805 1.0 uF capacitor (lead inductance L = 1 nH) the self-resonance occurs at 5 MHz.
SMT 0805 0.01 uF capacitor (lead inductance L = 1 nH) the self-resonance occurs at 50 MHz.
SMT 0805 100 pF capacitor (lead inductance L = 1 nH) the self-resonance occurs at 503 MHz.

In my EMC laboratory testing experience I've seen such changes in radiated emissions or conducted emissions performance from modules using the same hardware and software design built in different manufacturing plants.

References: Printed Circuit Board Design Techniques for EMC Compliance 2nd Ed. (Mark I. Montrose)

WPT Transfer Power Classess

9. July 2015 11:51 in

Magnetic Field WPT Input Power Classes (drawn from the grid)

 Class MF-WPT1 MF-WPT2 MF-WPT3 MF-WPT4 Power P =< 3.7 3.7 < P =< 7.7 7.7 < P =< 22 P >

Interoperability is the state of the supply device and the EV enabling wireless power transfer via magnetic field in a safe and efficient manner. Power transfer is allowed to occur only when interoperability is established.

Interoperability Power Classes

 Primary Device SecondaryDevice MF-WPT 1 2 3 4 1 Required Desired * * 2 Desired Required * * 3 * * Required * 4 * * * Required * Under Consideration