EMC Basics of EMC en

© 2018 ROHM Co.,Ltd.

ROHM Co. Ltd.

EMC Basics

ver.A2

Reprinting forbidden

P. 1© 2018 ROHM Co.,Ltd.

Reprinting forbiddenAgenda

The Basics of EMC

1. What is EMC?

2. Spectrum Basics

3. Differential Mode Noise & Common Mode Noise

4. Cross Talk

5. Noises from Switch Mode Power Supplies


Reprinting forbidden1. What is EMC?

What is EMC?

• Without causing electromagnetic interference to other devices, the ability to maintain the inherent performance even subjected to electromagnetic interference from other devices.

• Because of the need to sustain both of the capabilities, the term “electromagnetic compatibility” is used.

• EMC includes the following EMI and EMS.

EMI (Electromagnetic Interference； Emission)

• Due to operation of a electronics, giving an electromagnetic interference to other devices or a thing that is such electromagnetic wave.

• Circuit designing not to generate EMI is required.

EMS (Electromagnetic Susceptibility； Immunity)

• The immunity that do not cause errors, such as a malfunction, when subjected to an EMI.

• Robust circuit designing is required for EMI.

Conducted Emission

Radiated Emission

• Noise that is propagated through wires and PCB wiring.

• Type of noise that is emitted (radiated) through the air.


Reprinting forbidden2. Spectrum Basics

f

dB

πtw

-20dB/dec

-40dB/dec

πts

ts ts

T

A

A tw

2

1 1

Upper right:Fourier transformation spectrum of theoretical pules wave form.

Lower right: Fourier transformation spectrum when ts is slower.

•The spectrum lowers when ts is slower.•Verify how the spectrum changes with changing parameters(e.g. frequency) by calculation.

Pulse Waveform

f

dB

πtw

-20dB/dec

-40dB/dec

πts

1 1

When ts is slower.



Ampli.：10V

f ：400kHz

Duty：50%

tr /tf：10ns

Default

Ampli.：10V

f ：2MHz

Duty：50%

tr /tf：10ns

Blue Line:Default

2. Spectrum Basics

Volt

ag

e [

V]

Am

plitu

de [V

]

Am

plitu

de [d

Bu

V]

Waveform

Waveform

n-th harmonic vs Amplitude


Frequency vs Amplitude [dBµV]

Frequency [kHz]

Frequency [kHz]

Frequency [Multiple]


Time [s]

Time [s]

Volt

ag

e [

V]

Am

plitu

de [V

]


Am

plitu

de [d

Bu

V]



Ampli.：10V

f ：400kHz

Duty：50%

tr /tf：100ns

Blue Line:Default

Ampli.：10V

f ：400kHz

Duty：20%

tr /tf：10ns

Blue Line:Default

Waveform

Waveform




Frequency [kHz]

Frequency [kHz]



Time [s]

Time [s]

Volt

ag

e [

V]

Am

plitu

de [V

]


Volt

ag

e [

V]

Am

plitu

de [V

]

Am

plitu

de [d

Bu

V]

Am

plitu

de [d

Bu

V]



Higher frequency ⇒ Spectrum increase for all.Lower tr and tf ⇒ Decrease -40dB/dec from low frequency.Change in duty cycle ⇒ Even number-th harmonic occurs but not affects

to the peak of spectrum. Fundamental wave lowers.

Only lower tr ⇒ tr component decreases at low frequency.

* Lower frequency and slower tr and tf make spectrum lower.

Ampli.：10Vf ：400kHzDuty：50%tr ：100nstf ： 10ns

Blue Line:Default

Waveform n-th harmonic vs Amplitude Frequency vs Amplitude [dBµV]

Frequency [kHz]Frequency [Multiple]Time [s]

Volt

ag

e [

V]

Am

plitu

de [V

]

Am

plitu

de [d

Bu

V]


Reprinting forbidden3. Differential Mode Noise & Common Mode Noise

Case

PCB

Circuit

PowerSupply

Differential(Normal) Mode Noise Common Mode Noise

Reference GND

•A mode in which a noise current flows on the same path as the power supply current.•Noise voltage occurs across power supply lines.

•Noise voltage does not occur across powersupply lines.•Noise voltage occurs across power supply lineand reference GND.•Noise currents flow in the same direction on thepower supply positive and negative sides.

Case

PCB

Circuit

PowerSupply

Reference GND

Stray Capacitance

VnVn

NoiseSource

（Signal Source）

Reference GND



TX RXLoop Area: S

rId

Radiation due to Differential Mode Noise

Ed ∝ Id x f 2 x S

r

TX RX

Ic

Radiation due to Common Mode Noise

Cable Length: L

r

Ec ∝ Ic x f x L

r

*Equations excerpted from： Detailed Explanation--Electromagnetic Compatibility Engineering, Author: Henry W. Ott, Publisher: John Wiley & Sons




For a 100 MHz differential mode noise current of 1 µA flowing in a loop of area 20 cm2.The value of the electric field intensity at a distance of 1 m (90°) is:

Ed = 1.316 x 10-14 x Id x f 2 x S

r

= 1.316 x 10-14 x 1µA x (100MHz )2 x (0.2 x 0.01)

1

= 0.26µV/m

Differential Mode Noise

For a 100 MHz common mode noise current of 1 µA flowing in a 20 m cableThe value of the electric field intensity at a distance of 1 m (90°) is:

Ec = 1.257 x 10-6 x Ic x f x L

r

= 1.257 x 10-6 x 1µA x 100MHz x 0.2

1

= 25.1µV/m

Common Mode Noise

* Example using an approximate expression.

TX RX20cm

1cm

1m

・For the same noise current values, the radiation due to common mode noise is far greater (in this example, roughly 100 times greater).

・Using a twisted-wire cable, Area S becomes smaller and a differential mode noise decreases.・Area S doen not affect to common mode noise.



Reprinting forbidden4. Crosstalk

Crosstalk increases with high frequency, short distance between patterns, and high impedance coupling.

Capacitive Coupling(Electrostatic Coupling)

Inductive Coupling (Electromagnetic Coupling)

VsVn

Pattern 1

R

CC

R

Pattern 2

Vs Vn

Illustration Equivalent circuit

• Illustration of capacitive coupling between patterns due to stray capacitance.

• Noise occurring on the side of pattern 1 causes a voltage Vn relative to GND due to capacitive coupling:

Vn = jω R C Vs

• Illustration of inductive coupling between patterns due to mutual inductance.•Noise occurring on the side of pattern 1 causes a voltage Vn relative to GND due to inductive coupling:

Vn = jω M Is

Noise source (Signal source)

VsVn

Pattern 1

R

Pattern 2

M

IsM

Is

Vs Vn

Illustration Equivalent circuit

Noise source (Signal source)

R

*Electromagnetic Compatibility Engineering, Author: Henry W. Ott, Publisher: John Wiley & Sons


Reprinting forbidden5. Noise Occurring in Switching Power Supplies

ON

OFF

SW1 ON

OFF

ON

SW2 ON

• The thick lines in the diagram on the left indicate theloop in which there are sharp changes in current due toswitch ON/OFF operation.

• Because high-frequency currents flow in this loop duringswitching, ringing occurs in the loop.

dI

dtV = L ×

Example) When a 1 A current changes in 10 ns in wiringhaving a 10 nH inductance component, a voltage of 1 Vappears.



•Parasitic components on the mounting board cannot all be eliminated through power supply ICs.• Innovations are necessary in the PCB wiring layout and through decoupling capacitors.

V = L ×dt

dI

LVIN

Driver circuit

CIN Cdecap

VOUTL5 L1

L2

L3

L4

Parasitic C

COUT

Parasitic C

Wiring LParasitic L

Wiring L

Intense ringing at 100 to 300 MHz occurs at rises/falls.

Switching node waveform

GND

C1

C2

・ Wiring inductance L is roughly 1 nH per mm.

・ Rise/fall time for a switching MOSFET is several ns.

I = C ×dt

dV

Fundamental wave component

Switching noise components

5. Noise Occurring in Switching Power Supplies



BATTERY

LISN

LISN

Common Mode

Common Mode

Differential

Differential mode ⇒ Common mode

・ Switching noise components that remain even after loop optimization are conducted to the power supply side as common mode noise.

・ Measures must be taken to confine noise by inserting high-impedance components such as inductors into transmission lines.

・ Attention must be paid to crosstalk as well.

5. Noise Occurring in Switching Power Supplies

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