electromagnetic compatibility

Electromagnetic CompatibilityVesa Linja-aho – 2012-07-19

Me

Senior Lecturer in Automotive Electronics at Metropolia UAS

Background: M.Sc. in Electromagnetics and Circuit Theory, Helsinki University of Technology

Worked as researcher, lecturer and journalist. Interested in Open Educational Resources and

Electric Work Safety.

2012-07-19 2Vesa Linja-aho

Metropolia UAS

In Finland, we have two kinds of institutions of higher education University: Bachelor (3 yr), Master (2 yr), PhD

(3-4 yr) University of Applied Sciences (UAS): Bachelor

(4 yr) and Master (2 yr) Metropolia is the largest UAS in Finland Automotive engineering education:

Formula Student –team Electric Raceabout (ERA)


http://www.metropolia-motorsport.fi/




http://www.raceabout.fi/era/



Studying in Finland

Metropolia unfortunately does not have automotive engineering degree programmes in English, unfortunately.

But we have other interesting programmes: http://www.metropolia.fi/en/

In Finland, we do not have tuition fees (some universities charge a tuition fee from non-EEA citizens, but not Metropolia).

The application time is at spring.


http://www.metropolia.fi/en/

http://www.metropolia.fi/en/

EMC = Electromagnetic Compatibility

All electric and electronic devices should be designed so that they will accept any normal interference won’t interfere other devices.


Everyday Examples?


Everyday Examples

The ”GSM song” FM radio crackling when your neighbor drills a

hole on his wall. FM radio clicking when fluorescent tubes start.


http://www.youtube.com/watch?v=emxPVUM3y54

http://www.youtube.com/watch?v=emxPVUM3y54

Not-so-everyday Examples

Electric wheelchair turns in the water when a police officer pushes talk button in the police boat radio.

Piezoelectric cigarette lighter opens barrier at a parking lot.

Starting a welding transformer causes the central computer to crash (in another building).

The roof and central locking system react to cellphone.


Electromagnetic Interference (EMI)

Natural interference: cosmic radiation and thunderstorm.

Technical interference: Electrostatic discharge (ESD) Digital circuits Changes in the mains voltage Wireless communication


Coupling Mechanisms

Conducting Capacitive coupling (via electric field) Inductive coupling (via magnetic field) Electromagnetic waves


Coupling Mechanisms


How to Fight EMI?

Prevent the emergence of interference Cut the path of propagation Improve the tolerance for the interference


Countermeasures in Practice

Layout design and position of the wires Symmetrical transmission Filtering Using digital electronics Using optical transmission


Brief Physical Background

An electric charge creates an electric field. Electric current or changing electric field creates

a magnetic field. Changing magnetix field induces a voltage into a

wire.


Capacitive coupling = Coupling via Electric FieldCountermeasures Metallic enclosing Increasing the distance between wires Positioning the wires near the ground plane Decreasing the impedance level


Inductive Coupling = Coupling via Magnetic FieldCountermeasures Layout design Decreasing the impedance level


EMC is not a Separate Matter

The risky way: design the product and fix the EMC issues afterwards.

The safe way: keep EMC in mind during the whole design process


Common Mode vs. Differential Signal

Common mode signal together with the ground plane causes a large loop between circuits.


Three-level Protection

Layout design Interface filtering Metallic enclosing


Do we need protection?

In simple non-critical devices, the layout design is often enough. Especially, if there are no cables to/from the

device. About 90 % of post-design EMC-problems are

caused by poor layout design!


Good layout design

Split the system in parts Think the ground plane as a large current

conducting path. Choose grounding points well and minimize the

grounding impedance. Remember that every conducting part can carry

interfering currents!

Vesa Linja-aho 212012-07-19

Splitting the system in parts

Decide which parts are critical vs. non-critical. Place the parts which are neither sensitive nor

sources for interference, into separate locations even on their own circuit boards.

For example, linear power supplies, non-clocked logic circuits and power amplifiers are usually immune to interference.


Grounding

The ground plane is non-ideal. The correct grounding style depends on the circuit.

Single-point grounding is common in switching-mode power supplies. It prevents interfering voltages caused by currents through common impedances. But: on large frequencies, the wires act as

transmission lines! Multi-point grounding. Works well on large

frequencies. Each circuit has its own ground, and the grounds are interconnected with short wires.


Rule of thumb for grounding

< 1 MHz: Single-point grounding > 10 MHz: Multi-point grounding


Using a ground plane

If you use a multilayer PCB, using a ground plane is possible and recommended.

With RF circuits and fast digital circuits using the ground plane is practically mandatory.

What is the purpose of the large ground plane?


The purpose of the ground plane

The main purpose is to provide low grounding impedance.

The secondary purpose is to act as a shield.


Crosstalk

Two wires on the PCB are – unfortunately – connected to each other capacitively and inductively.

In practice, there is no cross-talk if the distance between wires is larger than 1 cm.

Dropping the impedance and using the ground plane will reduce crosstalk.


Design Checklist

Avoid long wires on the PCB board. Sensitive and interfering components should not

be placed near each other. Do not place sensitive parts near the edges of

the ground plane. Split the circuit in parts very carefully.


Interfaces and filtering

Most devices are interconnected to other devices via a cable.


Ferrite chokes

Very common in data cables. Adds series inductance to cable. Effective on frequencies 1-1000 MHz

(approximately). Disadvantage: relatively low attenuation (10-20

dB). Advantage: easy to add afterwards. Ferrite choke attenuates also fast transients

caused by ESD.


Mains filtering

Example: an electric shock from computer chassis.

Maximum leak current 0,5 mA. (EN 60601-1-1)


Cables

Coaxial cable Twisted pair Shielded twisted pair

The shield should be connected evenly – avoid the pig tail mistake!


Switches

Switches cause disturbance in two ways: Arcing Bouncing

Use RC snubber


Relays

RC snubber + protective diode


Electric motors

Motors cause strong magnetic fields. If the motor has commutator or brushes, the

arcing causes wide-spectrum RF interference. Inverter-driven permanent magnet machines are

more EMC-safe.


ESD protection

Have you ever destroyed anything with ESD. What precautions you should take when handling

and installing an expensive graphics card to your computer?

MOSFET components are the most sensitive part to ESD.

The main method for ESD protection is protective diodes.


Legislation and Standards

Harmonized by European Union EMC-directive 2004/108/EY Radio and telecommunications equipment

1999/5/EC Automotive EMC-directive 2004/104/EC. European standards and national laws.


There are Plenty of Standards

Many device types have their own standard. If there is no standard available, the general

standard is to be applied.


EMC Testing

EMI immunity EMI emissions


EMC Testing Case Example: Electric Raceabout

Name surname 42

www.metropolia.fi/en/www.facebook.com/[email protected]

Thank you!

http://www.facebook.com/MetropoliaAMK

electromagnetic compatibility

Education

point grounding

magnetic field

ground plane

ground plane

electric field

19 vesa linja

19 vesa linja

19 vesa linja