Solutions for EMC Issues - cache.freescale.comcache.freescale.com/files/training/doc/ftf/2014/FTF-SDS-F0091.pdfSolutions for EMC Issues in Automotive System Transmission Lines

External Use

TM

Solutions for EMC Issues in Automotive System Transmission Lines

A P R . 1 0 . 2 0 1 4

Todd H. Hubing

Michelin Professor of Vehicle Electronics

Clemson University

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External Use 1

EMC Requirements and Key Design Considerations

Radiated

Emissions

Radiated

Susceptibility

Transient

Immunity

Electrostatic

Discharge

Bulk Current

Injection

• 1 HF GND

• Risetime Control

• Filtered I/O

• Adequate Decoupling

• Balance Control

• 1 HF GND

• Filtered I/O


• Balance Control

• LF Current Path Control

• Chassis GND on board

• Filtered I/O


• LF Current Path Control


• Filtered I/O


• 1 HF GND


• Filtered I/O


• Balance Control

Designing a product that is guaranteed to meet all of these

requirements is relatively easy.

Fixing a non-compliant product can be difficult and costly.

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External Use 2

Automotive EMI Sources, Victims and Antennas

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External Use 3

Agenda

1. Control your transition times

2. Recognize where currents flow

3. Recognize the 4 (not 2) possible coupling mechanisms

4. Identify your antennas

5. Identify your sources

6. Don’t rely on EMC design guidelines

7. Don’t gap your ground planes

8. There are no shielded enclosures in the automotive world

9. Provide adequate power bus decoupling

10. Provide adequate transient protection

10 rules for ensuring you meet your automotive EMC requirements

the first time your product is tested.

Hour 1

Hour 2

Hour 3

Hour 4

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External Use 4

First Rule of Automotive EMC Design

To guarantee that your design will meet its EMC requirements the first time,

you MUST:

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External Use 5

RLCG Parameters

If the length of each section of the RLCG lumped model is small relative to a wavelength (e.g. n<<l/8), the electrical behavior of the

model is the same as the electrical behavior of the transmission line.

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External Use 6

Propagation Velocity

1 1v

LC

Propagation velocity (m/sec)

Inductance per unit length (H/m)

Capacitance per unit length (F/m) This term is

independent of the

geometry

Determined by the dielectric material

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External Use 7

Propagation Delay (Electrical Length)

PDtv

Propagation Delay (sec)

RS

Z0

lS1

RL

The propagation delay is the amount of time required for a signal to propagate from one point to another point (total distance, ) on the

transmission line.

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External Use 8

Characteristic Impedance

0

R j L LZ

G j C C

Characteristic Impedance (ohms)

RS

Z0

lS1

RL

The characteristic impedance is the ratio of the voltage to the current in a

signal traveling in one direction down the transmission line.

Low-Loss

Approximation

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External Use 9

Attenuation

R j L G j C j

Attenuation in dB/m

RS

Z0

lS1

RL

x

0V V e

x 0m

0

x 1m

0

V e20 log

V e

20 log e

8.7

Low-Loss

Approximation

0

R

2Z LC

0

4.34R

ZAttenuation in dB/m

x=0 x=1m

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External Use 10

Dispersion

RS

Z0

lS1

RL

x=0 x=1m

Low-Loss

Approximation

0

R

2Z LC

0

4.34R

ZAttenuation in dB/m

Notice that attenuation is a function of R,

but at high frequencies, R is a function

of frequency due to the skin effect.

Therefore higher frequencies are

attenuated more than lower frequencies.

This can change the shape of the signal

in the time domain, and this effect is

called dispersion.

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External Use 11

RS

Z0

l

RLVS

When is a cable a transmission line?

at midpoint in transmission line

at midpoint in transmission line

Technically, always! But it will be most important to us when

the propagation delay is greater than the transition time.

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External Use 12

When must a cable be modeled as a transmission line?

The answer depends on the application, but generally the following

guidelines apply.

RS

Z0

l

RLVS

For digital signals: When tr < 2 * tpd

For RF signals: When > l/8

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External Use 13

An Important Point

In most applications, anything that must be modeled as a transmission line

must have a matched termination. This is usually undesirable from a cost

and EMC perspective. Therefore, every effort should usually be taken to

ensure that the signal bandwidth is no higher (or transition times are no

shorter) than necessary.

RS

Z0

l

RLVS

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External Use 14

When cables are electrically short …

They can be modeled using their lumped RLCG

parameters.

Often, one or none of these parameters is significant

relative to the source and load impedances

RS

Z0

l<<λ/8

RLVS

RS

RLVS

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External Use 15


Be careful not to model short cables or connectors with the full L or C

unless you have shown the that other parameter can be neglected.

Z01 Z02

l

Z01

Leq

Z01 Z02<

<< 1

Z01 Z01Z01 Z01

Discontinuities with a characteristic impedance greater than the source

and load impedances can be modeled with a lumped inductance.

The value of this inductance is less than the value of the lumped

parameter L in the RLCG model. 2

01eq

02

RL L 1

Z

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External Use 16


Be careful not to model short cables or connectors with the full L or C

unless you have shown the that other parameter can be neglected.

Discontinuities with a characteristic impedance less than the source and

load impedances can be modeled with a lumped capacitance.

The value of this capacitance is less than the value of the lumped

parameter C in the RLCG model.

Z01 Z02

l

Z01 Ceq

Z01Z02<

<< 1

Z01 Z01Z01 Z01

2

02eq

01

ZC C 1

R

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External Use 17

When cables are not electrically short …

RS

Z0

l

RLVS

To eliminate reflections, transmission lines must have a controlled

impedance and must be matched!

For signals with one source and one load, the match can occur

at the source end: RS = Z0.

For signals with one source and more than one load, the match

must generally occur at the load end: RL = Z0.

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External Use 18

When is a wiring harness a transmission line?

RS

Z0

lS1

RL

Steady state solution is always the wire-pair solution

If we don’t care about how we get to the steady state,

then we don’t need to worry about transmission line

solutions.

In most automotive applications, we don’t care!

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External Use 19


RS

Z0

lS1

RL

If the risetime is much greater than the propagation delay,

transmission line can be modeled as lumped element.

length = 5 meters propagation delay ~ 30 nsec

length = 50 cm propagation delay ~ 3 nsec

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External Use 20


1. Every digital signal transition time should be forced to be >100 ns

(unless this would prevent the circuit from working).

2. Signals that must transition faster than 100 ns, should transition in

the longest permissible time.

3. Traces or cables that carry signals with transition times > 100 ns

should not have matched terminations unless the length of the

signal propagation is > 5 meters.

4. Traces or cables that carry signals with transition times > 10 ns

should not have matched terminations unless the length of the

signal propagation is > 50 cm.

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External Use 21

CMOS Driver Model CMOS Input Model

Control Transition Times

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External Use 22


t

t

f

Control transition times of digital signals!

f

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External Use 23


Digital Signal Currents in CMOS Circuits

t

t

f

f

Control transition times of digital signals!

Can use a series resistor or ferrite when load is capacitive.

Use appropriate logic for fast signals with matched loads.

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External Use 24


Reducing risetime with a

parallel capacitor

Bad idea

Reducing risetime with a

series resistor

Good idea

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External Use 25

Example 1: Microcontroller Output Driver

Vsource = 3.3 V

Imax = 20 mA

Cin = 5 pF

Rseries = 0 W

CLK Freq = 100 kHz

Available Information

Rsource = 165 W

T = 10 s

tr = 1.82 ns

Calculated Parameters

Suppose we connected an output of this microcontroller

directly up to an impedance-matched antenna…

Automotive microcontroller in typical application:

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

10 100 1000

3-M

ET

ER

E-F

IEL

D I

N D

B(u

V/M

)

FREQUENCY IN MHZ

Maximum Radiated Field

FCC Limit

Absolute maximum

possible emissions!

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External Use 26


Vsource = 3.3 V

Imax = 20 mA

Cin = 5 pF

Rseries = 20 kW

CLK Freq = 100 kHz


Rsource = 8165 W

T = 10 s

tr = 220.0 ns




Same output with 20-kW series resistor:

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

10 100 1000

3-M

ET

ER

E-F

IEL

D I

N D

B(u

V/M

)

FREQUENCY IN MHZ

Maximum RadiatedField

FCC Limit

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External Use 27


Vsource = 3.3 V

Imax = 20 mA

Cin = 5 pF

Rseries = 0 kW

CLK Freq = 1 MHz


Rsource = 165 W

T = 1 s

tr = 1.82 ns




Same output with 1 MHz output:

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

10 100 1000

3-M

ET

ER

E-F

IEL

D I

N D

B(u

V/M

)

FREQUENCY IN MHZ


FCC Limit

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External Use 28


Vsource = 3.3 V

Imax = 20 mA

Cin = 5 pF

Rseries = 8 kW

CLK Freq = 1 MHz


Rsource = 8165 W

T = 1 s

tr = 90 ns




Same output with 1 MHz output and 8-kW series resistor:

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

10 100 1000

3-M

ET

ER

E-F

IEL

D I

N D

B(u

V/M

)

FREQUENCY IN MHZ


FCC Limit

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External Use 29

0 10 20 30 40 500

2

4

6

8

10

TimeA

mplit

ude

0 10 20 30 40 500

2

4

6

8

10

Time

Am

plit

ude

0 10 20 30 40 500

2

4

6

8

10

Time

Am

plit

ude

Under-damped

LR 2

C

Critically-damped

LR 2

C

Over-damped

LR 2

C

RLC Circuits

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External Use 30

2nd Rule of Automotive EMC Design


you MUST:

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External Use 31

Where does the return current flow?

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External Use 32

Where does the return current flow?

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External Use 33

Identify Current Paths

Current takes the path of least impedance!

> 100 kHz this is generally the path of least inductance

< 10 kHz this is generally the path(s) of least resistance

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External Use 34


Where does the 56 MHz return current flow?

VOLTAGE

REGULATOR

56 MHZ

OSC.

6 VDC

INPUT

+5 volts

+5 volts

+5 volts

ground ground

ground

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External Use 35


Where does the 6 VDC return current flow?

VOLTAGE

REGULATOR

56 MHZ

OSC.

6 VDC

INPUT

+5 volts

+5 volts

+5 volts

ground ground

ground

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External Use 36


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External Use 37


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External Use 38

Where does the 10 kHz return current flow?


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External Use 39

3rd Rule of Automotive EMC Design


you MUST:

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External Use 40

Common Impedance Coupling

VS1

VS2

RS1

RS2

RL1R

L2R

RET

I

I2

1

IRET V

RL2V

RL1

+ +

--

TM

External Use 41

Conducted Coupling

VS1

VS2

RS1

RS2

RL1R

L2R

RET

I

I2

1

IRET V

RL2V

RL1

+ +

--

Requires 2 conductor connections between the source and victim.

The only mechanism that couples DC level shifts.

Most likely to be dominant at low frequencies, when source and victim

share a current return path.

Most likely to be dominant when sources are low impedance (high

current) circuits.

aka: Common Impedance Coupling

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External Use 42

Conducted Coupling Examples

Lights dim and radio dies when automobile engine is started.

Power bus voltage spikes are heard as audible “clicks” on an

AM radio using the same power source.

An electrostatic discharge transient resets a microprocessor

causing a system to shutdown.

A lightning induced transient destroys the electronic

components in a computer with a wired connection to the

internet.

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External Use 43

Electric Field Coupling


Coupling proportional to dV/dt.

Most likely to be dominant at higher frequencies.

Most likely to be dominant when sources are high impedance (high

voltage) circuits.

aka: Capacitive Coupling

VS1

VS2

RS1

RS2

RL1R

L2 VRL2

VRL1

+ +

--

C11

C12

C22

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External Use 44

Electric Field Coupling Examples

Coupling from circuit board heatsinks to cables or enclosures.

AM radio interference from overhead power lines.

Automotive component noise picked up by the rod antenna in

CISPR 25 “radiated” emissions tests.

Microprocessor resets due to indirect electrostatic discharges.

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External Use 45

Magnetic Field Coupling


Coupling proportional to dI/dt.

Most likely to be dominant at higher frequencies.

Most likely to be dominant when sources are low impedance (high

current) circuits.

aka: Inductive Coupling

VS1

VS2

RS1

RS2

RL1R

L2 VRL2

VRL1

+ +

--

M12

L1

L2

TM

External Use 46

Magnetic Field Coupling Examples

Coupling from power transformers or fluorescent lighting ballasts.

Jitter in CRT displays.

60 Hz “hum” in a handheld AM radio.

Hard-drive corruption due to motor or transformer currents.

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External Use 47

4th Rule of Automotive EMC Design


you MUST:

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External Use 48

When do wiring harnesses look like antennas?

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External Use 49

Identifying Antennas

l/2

l/4 Quarter-Wave Monopole

Half-Wave Dipole

• Size

• Two Halves

Electrically Small Loop

What makes an efficient antenna?

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External Use 50

Good Antenna Parts Poor Antenna Parts

<100 MHz >100 MHz <100 MHz >100 MHz

Cables Heatsinks

Power planes

Tall components

Seams in shielding

enclosures

Microstrip or stripline

traces

Anything that is not

big

Microstrip or stripline

traces

Free-space wavelength at 100 MHz is 3 meters


TM

External Use 51

Common-Mode vs. Differential Mode

r

zfI1026.1E

c6

max

r

zsfI1032.1E

2

d14

max

l

s

r

zfI104

d6

z

s


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External Use 52

Voltage-Driven Mechanism

~

Cable Equivalent

voltage

VCM

sinheat kCM DM

board

CV V

C

max

sin

0.2234 boardDM

heat k board cable

C rV E

C F F

~ VDM Noise voltage

Heatsink

How are common-mode currents induced on cables?

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External Use 53

Signal current loop induces a voltage between two good antenna parts.

- Vcm +

At 10 MHz and higher, milliamps of current flowing in a

ground plane produces millivolts of voltage across the

ground plane. A few millivolts driving a resonant antenna can

result in radiated fields exceeding FCC limits.

Current-Driven Mechanism


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External Use 54

What do you think of this automotive

BCM design?

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External Use 55

Signals coupled to I/O lines can carry HF power off the board.

Direct-Coupling Mechanism


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External Use 56

DM-to-CM conversion due to cable and load imbalance

RS

Z0

S1

RL


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External Use 57

Common-mode and Differential-mode Current

I1 = 3 Amps

I2 = 5 Amps

1

2 (1 )

DM CM

DM CM

I I hI

I I h I

DMDM

DM

VI

ZAND

1 2

1 2

1DM

CM

I h I hI

I I I

General Definition

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External Use 58

Driving a Ribbon Cable

h = 0.5 - no plane

h = small - w/ plane

A perfect differential driver driving two

adjacent wires in a ribbon cable

produces no common-mode current on

the ribbon cable.

A single-ended driver driving two

adjacent wires in a ribbon cable

produces a exactly the same amount

of common-mode current as a

common-mode source with half the

signal voltage

Don’t drive ribbon cable wires

with single-ended sources unless

you know the common-mode

current will not be a problem.

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External Use 59

PCB Driving a Twisted Wire Pair

h = 0.5

A perfect differential driver driving a

perfect twisted-wire pair produces no

common-mode current on the wire

pair.

A single-ended driver driving a twisted-

wire pair produces a exactly the same

amount of common-mode current as a

common-mode source with half the

signal voltage

Don’t drive twisted-wire pairs with

single-ended sources unless you

know the common-mode current

will not be a problem.

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External Use 60



you MUST:

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External Use 61

Clocks

Digital Data

Analog signals

Power supply switching

Arcing

Parasitic oscillations

Narrow band, consistent

Not as narrow as clocks, but clock

frequency is usually identifiable.

Bandwidth determined by signal source,

consistent

Appears broadband, but harmonics of

switching frequency can be identified,

consistent

Broadband, intermittent

Narrowband, possibly intermittent

Identify Sources

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External Use 62

Active Devices (Power Pins)

For some ICs, the high-frequency currents drawn from the power pins

can be much greater than the high-frequency currents in the signals!

Identify Sources

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External Use 63

Noise on the low-speed I/O

For some ICs, significant high-frequency currents appear on low-speed

I/O including outputs that never change state during normal operation!

Identify Sources

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External Use 64


To guarantee that your design will meet its EMC requirements the first time:

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External Use 65

EE371 Design Problem

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External Use 66

EMC Design Guideline Collection

http://www.learnemc.com/tutorials/guidelines.html

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External Use 67


To guarantee that your design will meet its EMC requirements the first time:

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External Use 68

“Whenever I see more than one of these symbols on the schematic, I know there is [EMC] work for us here.”

T. Van Doren

AGND

DGND

Ground vs. Signal Return

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External Use 69

The purpose of a system ground is to provide a

reference voltage and/or a safe path for fault currents.

Signal currents flowing on a “ground” conductor can

prevent a ground conductor from serving its intended

purpose.

Don’t confuse ground conductors with signal return

conductors. Rules for the routing of “ground” may conflict

with the rules for routing signal or power returns.


TM

External Use 70

Circuit boards should have high-frequency ground!

Conductors referenced to different grounds can be good antennas.

Signals referenced to two different grounds will be noisy (i.e.

include the noise voltage between the two grounds).

Layouts with more than one ground are more difficult, require more

space and present more opportunities for critical mistakes.

Excuses for employing more than one ground are generally based

on inaccurate or out-dated information.

Why?


TM

External Use 71

If grounds are divided, it is generally to control the flow of low-

frequency (<100 kHz) currents.

For example,

Isolating battery negative (i.e. chassis ground) from digital ground

Isolating digital ground from analog ground in audio circuits.

This can be necessary at times to prevent common impedance

coupling between circuits with low-frequency high-current signals and

other sensitive electronic circuits.

HOWEVER, it is still necessary to ensure that there is

only 1 high-frequency ground.


TM

External Use 72


D/A

Exercise: Trace the path of the digital and analog return currents.

PWM

Driver

Heatsink

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External Use 73


D/A


PWM

Driver

Heatsink

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External Use 74


D/A


PWM

Driver

Heatsink

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External Use 75

Lateral Isolation

Digital GND

Analog GND

Chassis GND

Rarely appropriate

Often the source of significant problems

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External Use 76

Vertical Isolation

Digital GND

Analog GND

Chassis GND

Only one plane usually needs to be full size.

One or zero vias should connect planes with different labels.

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External Use 77

You don’t need to gap a plane to control the flow of high frequency

(> 1 MHz) currents. If you provide a low-inductance path for these

currents to take, they will confine themselves to this path very well.


TM

External Use 78

Rules for gapping a ground plane:

1. Don’t do it!

2. If you must do it, never ever allow a trace or another

plane to cross over the gap.

3. If you must do it, never ever place a gap between two

connectors.

4. Remember that the conductors on either side of the gap

are at different potentials.

5. See Rule #1!


TM

External Use 79

Provide a good HF chassis ground at connector

Exceptions:

When there is no chassis ground

When there are no connectors with cables

Cables and enclosures are both good antenna parts. If they are not held to the same potential, they are likely to create a radiation problem.

Note: Sometimes low-frequency isolation between chassis and digital ground is necessary control the flow of low-frequency currents. However, even in these situations it is usually important to provide a good high-frequency connection.

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External Use 80

Wiring Harness

Digital Return Plane

Chassis Ground Plane

Capacitors connecting

chassis ground to the

digital return plane

Chassis connection

to chassis ground

Chassis connection

to chassis ground

Question:

When do you provide a chassis ground on the board?

Answer: Whenever you have a metal enclosure

OR a connection to vehicle chassis.

TM

External Use 81

Caps with traces

Better implementation

Chassis Ground Capacitors

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External Use 82

Low-Pass Filters

Filtering

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External Use 83

Design Exercise: Which low-pass filter is most appropriate in each case?

Filtering

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External Use 84

Parasitics

Filtering

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External Use 85

Two capacitors are more than twice as good as one.

Filtering with 2 capacitors

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External Use 86



you MUST recognize that:

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External Use 87

Electric

Field

Shielding (b.)

(a.)

(c.)

V+ - V+-

Shielding

TM

External Use 88

Magnetic

Field

Shielding

(at low frequencies)

Shielding

TM

External Use 89

Magnetic

Field

Shielding

(at high frequencies)

Shielding

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External Use 90

Enclosure

Shielding

Shielding

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External Use 91



you MUST:

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External Use 92

The Concept of Power Bus Decoupling

Printed Circuit Board

Power Supply

V V supply board

L G

L P

V inductance

V inductance

TM

External Use 93

Printed Circuit Board

Power Supply

V V supply board

L G

L P


TM

External Use 94

C b

trace L

trace L

trace L

trace L

C d

C d

L d

L d


TM

External Use 95

Effective Strategies for Choosing Amount of Decoupling Capacitance Required

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External Use 96

Rules for PCB Decoupling?

Use small-valued capacitors for high-frequency decoupling.

Use capacitors with a low ESR!

Avoid capacitors with a low ESR!

Use the largest valued capacitors you can find in a given package size.

Locate capacitors near the power pins of active

devices.

Locate capacitors near the ground pins of active

devices.

Location of decoupling capacitors is not relevant.

Run traces from device to capacitor, then to power planes.

Never put traces on decoupling capacitors.

Use 0.01 F for local decoupling!

Use 0.001 F for local decoupling!

Local decoupling capacitors should have a range of values from 100 pF to 1 F!

TM

External Use 97

How much capacitance do you need?

source L f

Z f C

Z C f

2 0

max 0 min

max 0

) 2 (

1

2

1

2

1

p

p

p

=

=

=

) (

) ( ) ( max f I

f V f Z

MAX DEVICE

MAX NOISE =

Impedance approach

max Z

0f 1f

trace fL p 2 C f 1 2 1

p

C b

trace L trace L

trace L trace L

TM

External Use 98


C C

R

D LV

+

-

Recognizing that CMOS loads are capacitances, we are simply

using decoupling capacitors to charge load capacitances.

Total decoupling capacitance is set to a value that is

equal to the total device capacitance times the power

bus voltage divided by the maximum power bus noise.

Capacitance Ratio approach

TM

External Use 99


Let’s do it the way that worked for

somebody at sometime in the past.

“… include one 0.01 uF local decoupling capacitor for each VCC pin of every active component on the board plus 1 bulk decoupling capacitor with a value equal to 5 times the sum of the local decoupling capacitance.” .

Guidelines approach

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External Use 100

Effective Strategies for Locating Printed Circuit Board Decoupling Capacitors

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External Use 101

Boards with Closely Spaced Power Planes

Power Distribution Model ~ (5 - 500 MHz)

C b

C d

Board with power and ground planes

C d

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External Use 102

Boards with Closely Spaced Power Planes

C = 1 F BULK C = 10nF D

L = 5 nH BULK L = 2nH D C = 3.4 nF B

0.1

1.

10.

100.

1 MHz 100 MHz 0.1 MHz 1 GHz 10 MHz

Bare Board

Board with decoupling

TM

External Use 103

For Boards with “Closely-Spaced” Planes

The location of the decoupling capacitors is not critical.

The value of the local decoupling capacitors is not

critical, but it must be greater than the interplane

capacitance.

The inductance of the connection is the most important

parameter of a local decoupling capacitor.

None of the local decoupling capacitors are effective

above a couple hundred megahertz.

None of the local decoupling capacitors are supplying

significant charge in the first few nanoseconds of a

transition.

TM

External Use 104

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External Use 105

Inductance of connections to planes

On boards with closely spaced power and ground planes:

Generally speaking, 100 decoupling capacitors connected

through 1 nH of inductance will be as effective as 500

decoupling capacitors connected through 5 nH of inductance.

5 nH 0.5 nH

TM

External Use 106

Power Bus Decoupling Strategy

With closely spaced (<.25 mm) planes

size bulk decoupling to meet board requirements

size high-frequency decoupling to meet board requirements

mount local decoupling in most convenient locations

don’t put traces on capacitor pads

too much capacitance is ok

too much inductance is not ok

References:

T. H. Hubing, J. L. Drewniak, T. P. Van Doren, and D. Hockanson, “Power Bus Decoupling on Multilayer Printed Circuit

Boards,” IEEE Transactions on Electromagnetic Compatibility, vol. EMC-37, no. 2, May 1995, pp. 155-166.

T. Zeeff and T. Hubing, “Reducing power bus impedance at resonance with lossy components,” IEEE Transactions on

Advanced Packaging, vol. 25, no. 2, May 2002, pp. 307-310.

M. Xu, T. Hubing, J. Chen, T. Van Doren, J. Drewniak and R. DuBroff, “Power bus decoupling with embedded capacitance in

printed circuit board design,” IEEE Transactions on Electromagnetic Compatibility, vol. 45, no. 1, Feb. 2003, pp. 22-30.

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External Use 107

Boards with Power Planes Spaced >0.5 mm

C b

C d C d

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External Use 108

Boards with Power Planes Spaced >0.5 mm

On boards with a spacing between power and ground planes of ~30 mils (0.75 mm) or

more, the inductance of the planes can no longer be neglected. In particular, the mutual

inductance between the vias of the active device and the vias of the decoupling

capacitor is important. The mutual inductance will tend to cause the majority of the

current to be drawn from the nearest decoupling capacitor and not from the planes.

ACTIVE DEVICE

LOOP A LOOP A and LOOP B

DECOUPLING CAPACITOR

SIGNAL PLANE

POWER PLANE

GROUND PLANE SIGNAL PLANE

PORT 1 PORT 2

L TRACE

L TRACE

C BOARD

VIA L

VIA L

M

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External Use 109

Where do I mount the capacitor?

VCC

GND Here?

Here?

POWER

GND

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External Use 110

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External Use 111

For Boards with “Widely-Spaced” Planes

Local decoupling capacitors should be located as close

to the active device as possible (near pin attached to

most distant plane).

The value of the local decoupling capacitors should be

10,000 pF or greater.

The inductance of the connection is the most important

parameter of a local decoupling capacitor.

Local decoupling capacitors can be effective up to 1

GHz or higher if they are connected properly.

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External Use 112

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External Use 113


With widely spaced (>.5 mm) planes


size local decoupling to meet device requirements

mount local decoupling near pin connected to furthest plane

don’t put traces on capacitor pads

too much capacitance is ok

too much inductance is not ok

References:

J. Chen, M. Xu, T. Hubing, J. Drewniak, T. Van Doren, and R. DuBroff, “Experimental evaluation of power bus decoupling on

a 4-layer printed circuit board,” Proc. of the 2000 IEEE International Symposium on Electromagnetic Compatibility,

Washington D.C., August 2000, pp. 335-338.

T. H. Hubing, T. P. Van Doren, F. Sha, J. L. Drewniak, and M. Wilhelm, “An Experimental Investigation of 4-Layer Printed

Circuit Board Decoupling,” Proceedings of the 1995 IEEE International Symposium on Electromagnetic Compatibility,

Atlanta, GA, August 1995, pp. 308-312.

J. Fan, J. Drewniak, J. Knighten, N. Smith, A. Orlandi, T. Van Doren, T. Hubing and R. DuBroff, “Quantifying SMT

Decoupling Capacitor Placement in DC Power-Bus Design for Multilayer PCBs,” IEEE Transactions on Electromagnetic

Compatibility, vol. EMC-43, no. 4, Nov. 2001, pp. 588-599.

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External Use 114


With no power plane

layout low-inductance power distribution


size local decoupling to meet device requirements

two caps can be much better than one

avoid resonances by minimizing L

References:

T. Hubing, “Printed Circuit Board Power Bus Decoupling,” LG Journal of Production Engineering, vol. 3, no. 12, December

2000, pp. 17-20. (Korean language publication) .

T. Zeeff, T. Hubing, T. Van Doren and D. Pommerenke, “Analysis of simple two-capacitor low-pass filters,” IEEE

Transactions on Electromagnetic Compatibility, vol. 45, no. 4, Nov. 2003, pp. 595-601.

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External Use 115


Low-impedance planes or traces?

choice based on bandwidth and board complexity

planes are not always the best choice

it is possible to achieve good decoupling either way

trace inductance may limit current to active devices

Planes widely spaced or closely spaced?

want local or global decoupling?

want stripline traces?

lower impedances obtainable with closely spaced planes.

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External Use 116

Embedded Capacitance

Input impedance of a populated 2” x 3” board with a

plane separation of about 5 microns

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External Use 117

Decoupling Myth

In order to be effective, capacitors must be located within a

radius of the active device equal to the distance a wave can

travel in the transition time of the circuitry.

On boards with closely spaced planes (where this rule is

normally applied) none of the capacitors on the board can

typically respond within the transition time of the circuitry no

matter where they are located.

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External Use 118

Decoupling Myth

Smaller valued capacitors (i.e. 10 pF) respond faster than higher

valued capacitors.

The ability of a capacitor to supply current quickly is

determined by its mounted inductance. The value of the

capacitance only affects its ability to respond over longer

periods of time. For a given value of inductance, higher valued

capacitors are more effective for decoupling.

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External Use 119



you MUST:

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External Use 120

Protecting components from transients on the harness

Digital GND Chassis GND

OR

Design Exercise: Where should the transient protection be grounded?

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External Use 121

Transient Protection Options

0.5 volts to ~10 volts Lowest Energy High Capacitance (10’s of pF) Usually fail short Voltage limiting device

0.5 volts to 10’s of volts Low Energy Higher Capacitance (10’s of pF) Usually fail short Voltage limiting device

10’s of volts to 1000’s of volts High Energy Low Capacitance (< 1 pF) Fail open Crowbar device

Diodes

Thyristors

Gas Discharge

Tubes

0.5 volts to 10’s of volts Low Energy Higher Capacitance (10’s of pF) Usually fail short Voltage limiting device

Varistors

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External Use 122

Current Limiting Devices

100 mA and up Speed depends on current Low impedance Inexpensive Non-resettable

Microamps and up Speed independent of current Moderate inductance Expensive Resettable

100 mA and up Speed depends on current Low to moderate inductance Inexpensive Limit current without opening Resettable

Fuses

Positive Temperature Coefficient

Devices

Circuit Breakers

Microamps and up Speed independent of current Moderate inductance Expensive Resettable

Ground-Fault Interrupters

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External Use 123

1. Control your transition times

2. Know how your currents return to their source

3. Recognize the 4 (not 2) possible coupling mechanisms

4. Identify your antennas

5. Identify your sources

6. Don’t rely on EMC design guidelines

7. Don’t gap your ground planes

8. There are no shielded enclosures in the automotive world

9. Provide adequate power bus decoupling

10. Provide adequate transient protection

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External Use 124

Example Design

Example: How would you modify this design?

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External Use 125

Example Design

Example: A much better design

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External Use 126

Power Inverter Design Example

80 MHZ

OSC.

Connection to power plane

Connection to ground plane

Gate Array

Phase 1

Phase 2

Phase 3

+ Input

- Input

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External Use 127

For More Information

http://www.cvel.clemson.edu

http://www.LearnEMC.com

TM

© 2014 LearnEMC

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Documents

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