industrial solutions and services your success is our goal emc hardening for apparatus and special...

Industrial Solutions and ServicesYour Success is Our Goal

EMC Hardening for apparatus and special product EMC problems

Diethard E. C. Moehr, I&S CTF EMC

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Common Technical Functions

Aims of EMC - EMC Marketing

The aim of EMC is to ensure that equipment and systems do not affect their surroundings to more than an acceptable extent and that they themselves can not be affected by interference from their surroundings.

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EMC Areas-1

ImmunityImmunityEmissionEmission

EMCEMCEMCEMC

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in complete devices

EMC requirements fordevices

in complete devices

EMC requirements fordevices

on PCB Level

PCB EMC requirement _

on PCB Level

PCB EMC requirement _

EMCEMCEMCEMC

EMC Areas-2

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EmissionEmission ImmunityImmunity

EMCEMCEMCEMC

conducted radiated

power mains

interconnectcabledata, signal

H-field E-field ESD RFI BURST SURGE

others

IEC 61000 - 4parts 2-6

Various 77A and CISPR standards

EMC Areas-3

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EMC / EMI always requires an emitter (source), a coupling medium and a receptor (receiver)

EMI sourcesEMI sourcesEMI sourcesEMI sources

• TV & radio transmitters

• Radar, Navigation• ITE & telecom equipment• local oscillators of receivers• power lines• switches• motors, engine ignition• computers and peripherals• fluorescent lights• electric heaters• arc welders• diathermy devices• natural sources: ESD noise,

lightning, galactic noise

• TV & radio transmitters

• Radar, Navigation• ITE & telecom equipment• local oscillators of receivers• power lines• switches• motors, engine ignition• computers and peripherals• fluorescent lights• electric heaters• arc welders• diathermy devices• natural sources: ESD noise,

lightning, galactic noise

Coupling mechanismCoupling mechanismCoupling mechanismCoupling mechanism

• Conductive

• inductive / capacitive

• radiation

• Conductive

• inductive / capacitive

• radiation

EMI receptorsEMI receptorsEMI receptorsEMI receptors

• computers & peripherals

• analog sensors and amplifiers

• industrial public controlsystems

• radio & TV receivers

• safety equipment

• military equipment

• humans (biological risks and hazards caused by

electromagnetic fields)

• computers & peripherals

• analog sensors and amplifiers

• industrial public controlsystems

• radio & TV receivers

• safety equipment

• military equipment

• humans (biological risks and hazards caused by

electromagnetic fields)

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Two pulses in time and frequency domain

-20dB/Decade

-40dB/Decade

t1 > t2 and tr1 > tr2

A B

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Shielded rooms; e. g. computer centreProblem: Emission Immunity

Emission of thecomputer system

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Shield Rooms

Transmitter;e. g. radio,television,intercom

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EMI in a usual small computer system

INTRASystem EMC

CRTSJ

PRINTER

Mainframe Workstation

CC

RE RS

RE RS

RE RS

CE CSCS

CE

CSCS

CE

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PCB - EMI Emitters & Receptors

EmittersEmittersEmittersEmitters

• clock oscillators and clock signal lines

• data signal lines

• relays

• switched mode power supplies

• line drivers

• high voltage parts

(supplies)

• video and deflection amplifiers

• all fast data signal lines

• clock oscillators and clock signal lines

• data signal lines

• relays

• switched mode power supplies

• line drivers

• high voltage parts

(supplies)

• video and deflection amplifiers

• all fast data signal lines

ReceptorsReceptorsReceptorsReceptors

• analog amplifiers

• disc read heads

• write / read amplifiers

• logic circuits

(microprocessor, buffers, drivers)

• analog amplifiers

• disc read heads

• write / read amplifiers

• logic circuits

(microprocessor, buffers, drivers)

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EMC shielding

F1 - F4 Filters shielded cables non shielded cables

metal ormetalized box

complete integral shielding of every boxand all cables(compartment shielding)

plastic box

F1 F1

F2

F2

F3

F4F3

PCB2

PCB3PCB1

PS

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Integral shielding - Compartment shielding

disadvantagesdisadvantagesdisadvantagesdisadvantages

• not always possible

• design problems

• does not help against self-jamming

• not always possible

• design problems

• does not help against self-jamming

disadvantagesdisadvantagesdisadvantagesdisadvantages

• each sub-assembly (PCB, cables,

connectors) has to be shielded

• quite expensive

• each sub-assembly (PCB, cables,

connectors) has to be shielded

• quite expensive

advantagesadvantagesadvantagesadvantages

• internal shielding is not necessary

• cheap solution in the shielded box

• internal shielding is not necessary

• cheap solution in the shielded box

advantagesadvantagesadvantagesadvantages

• design of the complete device (housing cheaper)

• no self-jamming problems between

the sub-assemblies

• design of the complete device (housing cheaper)

• no self-jamming problems between

the sub-assemblies

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PCB Layout Guidelines

PCB with one connector - CASE 1

low frequency logic

medium frequency logic

high frequency logic

connector

Note: Use separate supplies for the logic families

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digital logic

interfaces lines drivers

analog logic

analog signal driver / receivers

connector

Note: Prevent supply layer overlaps (VCC / GND) between the analog and digital area.

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Principle of a star supply system for PCBs

fast digital circuits

slow digital circuits

analogcircuits

interfaces and line driver circuits

+5V

OV

connector

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high

frequency

logic

low

frequency

logic

medium

frequency

logic

connector

Note:

• Use a “star” supply system for VCC / GND.

• Prevent loops in VCC / GND.

• Prevent loops in clock signal and data lines.

• Keep supply voltage paths for drivers short.

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PCB with several connectors

Note:

• Assign logic (interface logic)to the connectors.

• Sort logic according to its purpose.

• Separate logic families.

• Use separate supply systems.low freq. logic

high freq.logic

low freq. logic

mediumfreq.logic

mediumfreq.logic

high freq.logic

connector M1

connector E1

connector M2

connector E2

I / O circuits for interface 1

Interface 1 Interface 2

mother board

edge connectors

I / O circuits for interface 1

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b

ld

d = thickness of the lineb = width of the linel = length of the linef = frequency

b = 1mm d = 0,03mm b = 3mm d = 0,03mm b = 10mm d = 0,03mm

f 10 30 100 300 l/mm 30 100 300 l/mm 100 300

10Hz 6m 17m 57m 172m 6m 19m 57m 6m 17m

100Hz 6m 17m 57m 172m 6m 19m 57m 6m 17m

1kHz 6m 17m 57m 172m 6m 19m 57m 6m 17m

10kHz 6m 17m 58m 174m 6m 20m 61m 7m 24m

100kHz 7m 24m 92m 311m 14m 62m 225m 44m 174m

1MHz 44m 173m 727m 2,6 131m 590m 2,18 440m 1,73

10MHz 437m 1,72 7,25 25,8 1,31 5,89 21,8 4,4 17,3

100MHz 4,37 17,2 72,5 258 13,1 58,9 218 44 173

1GHz 43,7 172 725 2,58k 131 589 2,18k 440 1,73k

Dependence of line impedance on length, widthand thickness of the line considering the pulse frequency

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SummarySummarySummarySummary

• Use different areas for high, medium and low frequency logic.

• Use a “star” supply voltage system for VCC and GND.

• Use different areas for digital and analog logic.

• Assign interface logic to the connectors and keep connection as short as possible.

• Avoid parallel trace runs for interface signals and clock signals (cross talk problem).

• Consider cross talk between high, medium and low frequency signal lines.

• Use different areas for high, medium and low frequency logic.

• Use a “star” supply voltage system for VCC and GND.

• Use different areas for digital and analog logic.

• Assign interface logic to the connectors and keep connection as short as possible.

• Avoid parallel trace runs for interface signals and clock signals (cross talk problem).

• Consider cross talk between high, medium and low frequency signal lines.

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PCB Multilayer Guidelines - CASE 1

IL - Interconnect Layer

Advantage

• easy to repair

Disadvantage

• radiated emission from IL located outside of the PCB

IL 1

IL 2

VCC

GND

1

4

3

2

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Note:

Direction priority in IL 1 and IL 2 to avoid cross talk.

Advantage

• no radiated emission from signal and data lines

Disadvantages

• not easy to repair

• problems with very fast signals in the inner layers

VCC

GND

IL 1

IL 2

1

4

3

2

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SH - Shield Layer

IL - Interconnect Layer

GND - Ground

VCC - Supply LayerSH

IL

VCC

GND

IL

IL

IL

IL

IL

SH

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Location of decoupling capacitances

decoupling capacitance DC useless due to R and L in the current path

possible solution withlow R and L

optimal position of DC

Attn.: multi layer

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Decoupling capacitances -the wrong and the right way

IC

+

-

+5V

OV

IC

IC

OV +5V

Decoupling capacitances

wrong long paths connecting the pins

introduce L and C

rightshort paths connecting the pins

minimize L and C

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• EMC on PCB’s Part 1

•Generally you should take note of the following:Generally you should take note of the following:•Generally you should take note of the following:Generally you should take note of the following:

• avoid ground loops on the PCB

• avoid VCC loops on the PCB

• use a filter to avoid noise from the PCB being reradiated from supply data cables

• use completely shielded oscillators only

• connect the housing of used oscillators to the 0 volts of the PCB, locate a 0 volt area under oscillators to avoid other wires passing this area

• keep all clock traces as short as possible, the faster the signal is the shorter the trace has to be

• use decoupling caps for any chip (value 100 nF)

• traces from the decoupling capacitance's to the supply pins of the chips have to be as short as possible

• avoid ground loops on the PCB

• avoid VCC loops on the PCB

• use a filter to avoid noise from the PCB being reradiated from supply data cables

• use completely shielded oscillators only

• connect the housing of used oscillators to the 0 volts of the PCB, locate a 0 volt area under oscillators to avoid other wires passing this area

• keep all clock traces as short as possible, the faster the signal is the shorter the trace has to be

• use decoupling caps for any chip (value 100 nF)

• traces from the decoupling capacitance's to the supply pins of the chips have to be as short as possible

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• EMC on PCB’s Part 2

• the faster the logic is the shorter the distance between the supply plug and the fast logic has to be

• use a “star” power (VCC / ground) supply, for instance, to separate analog and digital devices

• use a separate VCC / ground path for chips known to produce most of the noise

• don’t connect 0 volt and the shield

• ensure the possibility of placing a capacitor between 0 volts and the shield in case of ESD or radiation problems

• if using multi layer PCB’s take care to have shielding layers on the top and on the bottom

• put interconnecting layers inside

• ensure that the VCC / ground traces are as wide as possible in order to create low HF resistance and inductance

• terminate all inputs of gates to ensure better immunity of the device

• the faster the logic is the shorter the distance between the supply plug and the fast logic has to be

• use a “star” power (VCC / ground) supply, for instance, to separate analog and digital devices

• use a separate VCC / ground path for chips known to produce most of the noise

• don’t connect 0 volt and the shield

• ensure the possibility of placing a capacitor between 0 volts and the shield in case of ESD or radiation problems

• if using multi layer PCB’s take care to have shielding layers on the top and on the bottom

• put interconnecting layers inside

• ensure that the VCC / ground traces are as wide as possible in order to create low HF resistance and inductance

• terminate all inputs of gates to ensure better immunity of the device

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EMC on PCB’s Part 3

• avoid overshoots on any signal because they produce a broad frequency spectrum

• ensure that rising and falling slopes of used clocks are as slow as possible

• terminate all metal parts of the housing to the shield potential (not to 0 volts)

• don’t forget spark over problems caused by ESD discharges from persons and devices touching the surface (pay special attention to LED’s)

• use an electrolyte cap. (at least 10 µF) at the point where the supply voltage reaches every PCB (no tantalum cap)

• use the slowest system frequency possible

• especially for keyboards (and similar units): use the slowest scanning frequency possible

• use level dependent interrupters instead of slope dependent interrupts

• avoid overshoots on any signal because they produce a broad frequency spectrum

• ensure that rising and falling slopes of used clocks are as slow as possible

• terminate all metal parts of the housing to the shield potential (not to 0 volts)

• don’t forget spark over problems caused by ESD discharges from persons and devices touching the surface (pay special attention to LED’s)

• use an electrolyte cap. (at least 10 µF) at the point where the supply voltage reaches every PCB (no tantalum cap)

• use the slowest system frequency possible

• especially for keyboards (and similar units): use the slowest scanning frequency possible

• use level dependent interrupters instead of slope dependent interrupts

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EMC on PCB’s Part 4

• use a watchdog timer for monitoring the system

• fill unused ROM space with a restart command

• be careful with spike dependent units (flip flops and latches), level triggering, if possible

• use a watchdog timer for monitoring the system

• fill unused ROM space with a restart command

• be careful with spike dependent units (flip flops and latches), level triggering, if possible

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EMC Guidelines for complete systems

Complete electronic system, e.g. example computer system:Complete electronic system, e.g. example computer system:Complete electronic system, e.g. example computer system:Complete electronic system, e.g. example computer system:

Question: What has to be done to meet the EMC requirements in order to get FCC / CE approval?

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Main questions to be solved:Main questions to be solved:Main questions to be solved:Main questions to be solved:

• Which shielding system is necessary

(complete or compartment shielding)?

• Which filtering is necessary for the mains?

• Which cable shielding is necessary for internal system cables?

• Which connectors have to be used for internal system cables?

• What EMI hardening is necessary for other I/O ports

(telephone lines, etc.) ?

• How to keep EMI hardening cheap?

• Which shielding system is necessary

(complete or compartment shielding)?

• Which filtering is necessary for the mains?

• Which cable shielding is necessary for internal system cables?

• Which connectors have to be used for internal system cables?

• What EMI hardening is necessary for other I/O ports

(telephone lines, etc.) ?

• How to keep EMI hardening cheap?

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Situation:Situation:Situation:Situation:

Reason:Reason:Reason:Reason:

Solution:Solution:Solution:Solution:

Choosing a filter by the manufacturer’s data sheet.

• filter performance from a manufacturer’s data sheet

• attenuation in the required frequency band (10 kHz - 30 MHz) looks high enough

• purchasing dept. orders 1000 filters

• filter is installed in the device and the system is taken to the EMI test site for compliance testing and

• the system fails the test because the filter does not work as promised

• filters are measured by the manufacturers acc. MIL STD 220 in a 50 / 50 Ohm system (50 Ohm source impedance / 50 Ohm load)

• real installation conditions vary widely (for instance for mains filters 1 Ohm / 103 Ohm not unusual)

• check any filter in the real installation condition before ordering

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What to do and what not to do in filter mounting: What to do and what not to do in filter mounting: What to do and what not to do in filter mounting: What to do and what not to do in filter mounting:

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What to do and what not to at cable entries: What to do and what not to at cable entries: What to do and what not to at cable entries: What to do and what not to at cable entries:

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The performance of pigtail connections 1: The performance of pigtail connections 1: The performance of pigtail connections 1: The performance of pigtail connections 1:

Example: coaxial cable (braided shield) with two pigtails to connect the shield via a connector pin to GND

2 x 10 cm wire (~4 inches) with 1 µH /mZT = 10 mohm at 1 MHz 1 m longZTP = jL = 0,6 ohm at 1 MHz

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The performance of pigtail connections 2:The performance of pigtail connections 2:The performance of pigtail connections 2:The performance of pigtail connections 2:

Voltage drop along the shield for:

I = 1 A (Ohm’s law)

U = I x (ZT + ZTP )

U = 1 A x (10-2 + 1,2 ) = 1,21 V

120 times the voltage drop as without pigtails.

Voltage drop along the shield for:

I = 1 A (Ohm’s law)

U = I x (ZT + ZTP )

U = 1 A x (10-2 + 1,2 ) = 1,21 V

120 times the voltage drop as without pigtails.

Conclusion:Conclusion:Conclusion:Conclusion:

• This shield is not an effective shield.

• Transfer impedance of the shield ZT can

be as low as possible, according to the

pigtails the whole impedance is too

high.

• Emission and susceptibility problems

can be expected.

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Shielded cables and the performance of pigtails: Shielded cables and the performance of pigtails: Shielded cables and the performance of pigtails: Shielded cables and the performance of pigtails:

Transfer impedance ZT and shielding effectiveness as of the cable:

in db

for our example ZT = 1,21

as = 32 dB

Good coaxial cables have as 70 dB ( ZT ≤ 15 m )

Cable manufacturers are able to measure the as and ZT of a cable according to MIL STD 220.

Transfer impedance ZT and shielding effectiveness as of the cable:

in db

for our example ZT = 1,21

as = 32 dB

Good coaxial cables have as 70 dB ( ZT ≤ 15 m )

Cable manufacturers are able to measure the as and ZT of a cable according to MIL STD 220.

as = 20 lg as = 20 lg 50

ZT

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Summary 1Summary 1::Summary 1Summary 1::

• All metal surfaces which are expected to have a good metal conducting contact to each other have to be protected against corrosion (for instance Sn coating on steel or aluminium surfaces).

• Metal or metalized surfaces where contact may arise during operation or normal service of a device should never be floated but connected to GND or safety potential where available.

• Connections between metal surfaces have to be solid; no accidental connections allowed; enforced contacts recommended.

• Shields of cables have to be connected 360° with metal or metalized connectors.

• All metal surfaces which are expected to have a good metal conducting contact to each other have to be protected against corrosion (for instance Sn coating on steel or aluminium surfaces).

• Metal or metalized surfaces where contact may arise during operation or normal service of a device should never be floated but connected to GND or safety potential where available.

• Connections between metal surfaces have to be solid; no accidental connections allowed; enforced contacts recommended.

• Shields of cables have to be connected 360° with metal or metalized connectors.

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• Bodies of connectors have to be connected with metal or metalized PCB frame if available; with connections between logic GND and the shield potential take special care.

• Main filters have to be connected with the metal or metalized housing of the device at the place of cable entry in a solid way.

• Anti-corrosive coatings, paint, Al2O3 and chromate coatings are not conductive even if they sometimes look like metal (remember to be aware of ESD problems).

• Moving parts such as doors have to be contacted by finger stock systems every 10 - 20 cm depending on the highest frequency.

• Sometimes additional shielding is required for special functional assemblies in the devices (turners, video amplifiers).

• Bodies of connectors have to be connected with metal or metalized PCB frame if available; with connections between logic GND and the shield potential take special care.

• Main filters have to be connected with the metal or metalized housing of the device at the place of cable entry in a solid way.

• Anti-corrosive coatings, paint, Al2O3 and chromate coatings are not conductive even if they sometimes look like metal (remember to be aware of ESD problems).

• Moving parts such as doors have to be contacted by finger stock systems every 10 - 20 cm depending on the highest frequency.

• Sometimes additional shielding is required for special functional assemblies in the devices (turners, video amplifiers).

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• Radiation sources cannot be placed behind slots for ventilation; also pay attention to signal lines behind slots.

• Prevent ESD from sparking directly into PCBs.

• In case of a plastic housing connect the shielding of cables also via metal or metalized connectors to metal or metalized PCB frames where available and via these frames to logic ground or better safety ground.

• If possible always ensure the separation of logic ground (GND) and safety ground (G/Y wire), only one conductive connection in a complete system is allowed.

• Always remember: HF current always takes the path of lowest impedance; this path can often be unusual; in case of different path in parallel the HF current uses only the path with the lowest impedance.

• Radiation sources cannot be placed behind slots for ventilation; also pay attention to signal lines behind slots.

• Prevent ESD from sparking directly into PCBs.

• In case of a plastic housing connect the shielding of cables also via metal or metalized connectors to metal or metalized PCB frames where available and via these frames to logic ground or better safety ground.

• If possible always ensure the separation of logic ground (GND) and safety ground (G/Y wire), only one conductive connection in a complete system is allowed.

• Always remember: HF current always takes the path of lowest impedance; this path can often be unusual; in case of different path in parallel the HF current uses only the path with the lowest impedance.


Thank you very much for your attention.

Contact:

Diethard MoehrI&S CTF EMCSchuhstraße 60D-91052 ErlangenPhone: +49 (9131) 7-27210

Mail: [email protected]

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