This document provides an introduction to concepts and techniques for electromagnetic interference (EMI) in switched mode power supplies (SMPS). It discusses EMI and electromagnetic compatibility (EMC) standards, types of noise signals, common noise countermeasures like shields and filters, magnetic field and flux basics, core materials and their properties, and common mode chokes. The focus is on helping designers understand and mitigate EMI in their power supply designs.

1. 2011
WURTH ELECTRONICS MIDCOM
SMPS EMI SEMINAR
2011
Introduction to
Concepts and
Techniques
© 2011 – Wurth Midcom 1 1

2. Our Products
© 2011 – Wurth Midcom 2

3. http://www.we-online.com/
http://www.we-online.com/web/en/passive_bauelemente_-
_standard/toolbox_pbs/Toolbox.php
Wurth Electronics Midcom Inc. Headquarters
Phone: (605) 886-4385
Fax: (605) 886-4486
E-Mail: midcom@we-online.com
121 Airport Drive
Watertown, SD 57201
United States

4. An Excellent Resource for EMC
TRILOGY of Magnetics
1. Electromagnetics Fundamentals
2. Passive Components and their characterisitics
3. Principles of Filter
4. Over 300 Detailed Applications
© 2011 – Wurth Midcom 4

5. Basics
EMI = Electromagnetic Interference
How much a device„s own noise affects other components
EMC = Electromagnetic Compatibiltiy
How well a device can handle noise from other components
NOT THIS: E= 2 OR THIS: HipHop Band
© 2011 – Wurth Midcom 5

6. EMI / EMC (CISPR vs. FCC)
CISPR* (22)
Non-regulatory agency, but CISPR has been adopted as part of the EMC tests and limits
0.15MHz to 30MHz - conducted
30MHz to 1000MHz - radiated
>1000MHz - accordance to FCC
FCC** (15)
Regulatory agency that sets the USA EMC tests and limits
0.15MHz to 30MHz - conducted
30MHz to 1000MHz - radiated
* International Special Community on Radio Interference, Pub.22
** Federal Communications Comission, Part 15
© 2011 – Wurth Midcom 6

7. CISPR vs. FCC
960-1000
Class A
0.5 1.6 5 30 88 216
0.15 - 0.5 0.5 - 30 30 - 88 88 - 216 216 - 960 >1000
0.45 – 1.6 1.6 - 30 30 - 88 88 - 216 216 - 1000 >1000
Class B
0.15 - 0.5 0.5 - 5 5 - 30 30 - 88 88 - 216 216 - 960 >1000
0.455 – 1.6 1.6 - 30 30 - 88 88 - 216 216 - 1000 >1000
f [MHz]
0.1 1 10 100 1000
differential mode noise common mode noise
• Class A
 commercial, industrial, business environment equipment
• Class B
 residential environment equipment
f [MHz] CISPR 22 - conducted
f [MHz] FCC 15 - conducted
f [MHz] CISPR 22 - radiated
f [MHz] FCC 15 - radiated
© 2011 – Wurth Midcom 7

8. Basics
Why 30Mhz the cutoff between the conducted and radiated emissions?
30 MHz is roughly equivalent to a wavelenght of ~32 feet (10 meters)
900 MHz is roughly equivalent to a wavelenght of 1 foot (~.33 meters)
In practical terms, the wavelength of a signal and the length of its an antenna need
to be equal to radiate the signal at full power
Most common house cables, wires or power lines are less than 10 meters long!
Short little cables are unlikely
to radiate noise below 30MHz
© 2011 – Wurth Midcom 8

9. Types of Noise Signals
Differential–Mode signal Common–Mode signal
• Noise flows into one line and • Noise flows along both lines
exits through another in the same direction
•Independent from GND • returns by some parasitics path
through system GND
switching switching
supply supply
connection chassis &
connection chassis &
Earth GND
Earth GND
© 2011 – Wurth Midcom 9

10. CISPR vs. FCC
960-1000
Class A
0.5 1.6 5 30 88 216
0.15 - 0.5 0.5 - 30 30 - 88 88 - 216 216 - 960 >1000
0.45 – 1.6 1.6 - 30 30 - 88 88 - 216 216 - 1000 >1000
Class B
0.15 - 0.5 0.5 - 5 5 - 30 30 - 88 88 - 216 216 - 960 >1000
0.455 – 1.6 1.6 - 30 30 - 88 88 - 216 216 - 1000 >1000
f [MHz]
0.1 1 10 100 1000
differential mode noise common mode noise
• Class A
 commercial, industrial, business environment equipment
• Class B
 residential environment equipment
f [MHz] CISPR 22 - conducted
f [MHz] FCC 15 - conducted
f [MHz] CISPR 22 - radiated
f [MHz] FCC 15 - radiated
© 2011 – Wurth Midcom 10

11. Common types of noise countermeasures
Shields radiated noise
© 2011 – Wurth Midcom 11

12. Common types of noise countermeasures
Filtering for conducted and radiated noise
Low Pass Filter
High Pass Filter
Band Pass
Filter
Band Reject
Filter
© 2011 – Wurth Midcom 12

13. The Magnetic Field (H)
Field model
Magnetic field H
N
S
O
O
R
U
T
T
H
H
Current I
© 2011 – Wurth Midcom 1313

14. The Magnetic Flux (B)
Raverage
Raverage
Current I
H1
H2

I
H1  H 2  H  B1 ? B2
2    Raverage

© 2011 – Wurth Midcom 1414

15. The Magnetic Field
The H field corresponds to what is called the magnetic field strength. It is
measured in amps / meter (A/m).
In free space or in air the B field represents magnetic flux density which is
given in units of Tesla byB = μₒ H where μₒ is the absolute
magnetic permeability of free space
More magnetic flux can be produced by the same H value in certain (magnetic)
materials, notably iron, and this is accounted by introducing another factor, the
relative permeability μr, giving B = μₒμr H for magnetic
materials.
© 2011 – Wurth Midcom 1515

16. What is Permeability? µ
Relative Permeability
Describes the capacity of concentration of the 1 B
r 
magnetic flux in the material 0 H
Is a factor of energy needed to magnetize
Typical permeability µr :
• Iron power / Superflux : 50 ~ 150
• Nickel Zinc : 40 ~ 1500
• Manganese Zinc : 300 ~ 20000
© 2011 – Wurth Midcom 1616

17. The Magnetic Field (H)
The magnetic field strength
depends on:
I
Straight wire
H • dimensions
2   R • Number of turns
R
• current
N I
Toroidal H but
R 2   R
NOT ON THE MATERIAL
THROUGH WHICH IT
FLOWS
N I
Rod choke
H
l
l
© 2011 – Wurth Midcom 1717

18. The Magnetic Field (B)
Air Rod core ferrite Ring core ferrite
(Ceramic)
N S N S
N S
O O O O
O O
R U R U
R U
T T T T
T T
H H H H
H H
Induction in air: Induction in Ferrite:
B  0  r  H B  0   r  H
B  0  H
Non-linear function, because the relative
Linear function because µr = 1 permeability depends on:
(a constant)
Material Temperature
Frequency Current
Pressure
© 2011 – Wurth Midcom 1818

19. Reluctance ( A measure of stored magnetic energy)

20. Characteristics Magnetic Parameters H and B (Linear & Hysterises Models)

21. Area of Operation for a Flyback Transformer

22. Area of Operation for a Filter Inductor

23. The Ideal Transformer

24. The air gap and its purpose

25. The air gap and its purpose

26. Saturation Current (power inductor)
© 2011 – Wurth Midcom 26

27. Permeability and Core Material Properties
Permeability depends on temperature
Curie
Temperature
µr
1000
µr = 1
?
770
+15 %
670
-20 %
540
500
-50 -40°C 23°C 50 85°C 150 250 T / °C
© 2011 – Wurth Midcom 2727

28. Permeability – complex Permeability
 j j j
      j
Impedance of winding with Impedance of winding
| || |
  
 | | || ||Core material
core material without corematerial
Z  j L   j j  RjX jX 
| L |  ||  ||   j jX
Z  j L 0 j    R   
Z  j | || | ||
R
0 0
R
L 0
L
© 2011 – Wurth Midcom 28

29. |
    j
||
Permeability – complex permeability

Z  jL0   j  R  jX
| ||

10000
µr=350
1000
µ`
X L  jL0  |
100
RReihe1 L0  ||

Inductance reactance µ`` Frequency dependent
(energy storage)
core losses
(hysteresis & eddy current losses)
10
1 f/MHz
1 10 100 1000 10000
© 2011 – Wurth Midcom 2929

30. Core Material Properties and Applications ( Inductors for Storage)
100%
90%
80%
70%
Impedance
60%
XL(Fe) XL(MnZn) XL(NiZn)
50%
40%
30%
„0“-200kHz „0“-10MHz „0“-40MHz
20%
10%
0% f/MHz
0,01 0,1 1 10 100 1000
© 2011 – Wurth Midcom 3030

31. Core Material Properties and Applications ( Inductors for Filtering)
100%
90%
80%
70%
Impedance
60%
R (Fe) R (MnZn) R (NiZn)
50%
40%
30%
200kHz- 3-60MHz 20-
20%
4MHz 2000MHz
10%
0% f/MHz
0,01 0,1 1 10 100 1000
© 2011 – Wurth Midcom 3131

32. Common Mode Filter
Reduction of noise
• From device to environment
• From environment to device
Conclusion:
• “Almost” no influencing of the signal  Differential mode
• High attenuation of noise  Common mode
© 2011 – Wurth Midcom 3232

33. Common Mode Filter – Signal theories
Filtering
e.g.: USB
VCC
Common mode
D+
D-
GND
Source Signal path Load
© 2011 – Wurth Midcom 3333

34. Common Mode Filter Attenuation feautures
When will be the signal attenuated?
• the Differential mode-Impedance will also attenuate the signal
10000
1000
100
10
1
f/MHz
1 10 100 1000
• The CommonMode-Impedance attenuates just the noise
© 2011 – Wurth Midcom 3434

35. Common mode choke - construction
bifilar sectional
© 2011 – Wurth Midcom 3535

36. Common mode choke - construction
Bifilar Sectional
• Less differential impedance • Low capacitive coupling
• High capacitive coupling • High leakage inductance
• Less leakage inductance • High differential impedance
• Data lines • Power supply input /output filter
 USB, Fire-wire, CAN, etc.  CMC for mains power
• Power supply • High voltage application
• Measuring lines • Measuring lines
• Sensor lines • Switching power supply decoupling
© 2011 – Wurth Midcom 3636

37. Common mode choke - construction
WE-SL2 744227 WE-SL2 744227S
bifilar winding sectional winding
10000
1000
100
10
1 f/MHz
1 10 100 1000
© 2011 – Wurth Midcom 3737

38. Common mode choke - construction
WE-split ferrite – Is it a CMC?
• Yes, CMC with one winding
e.g. 74271712 comparable with bifilar winding CMC
• both will absorb Common Mode interferences
© 2011 – Wurth Midcom 3838

39. Common mode choke: ferrite core
Increase the number of turns means:
2000
1800
1600
1400
1200
1000
800
600
400
200
0
f/MHz
1 10 100 1000
© 2011 – Wurth Midcom 3939

40. 40
Why Filter? – example: Fly back-Converter
Which filter we need?
L1
N
P
E
Parasitic capacities
e.g.: collector to cooling element

41. Transformers

42. Transformers and EMI
• Center leg gap only
– Windings shield
• No gaps in outer legs
– Nothing to shield
No Gaps here
Gap here
No external gaps

43. Inductors and EMI
 Drum core style
 Very large gap
 Much radiation
Not a good solution!

44. Transformers for EMI – Gap issues
• Gap must be perpendicular to flux lines
• Uneven gaps are inefficient. => Why?
– Core saturates at minimum gap
– Requires a larger gap
• Also larger gap – More potential EMI

45. Transformers and EMI – Internal shields
• Shield both conducted and radiated noise
• Copper foil or wound magnet wire?
• Copper foil shields – Expensive, => Why?
– Must build shield
– Must be covered with tape
– Winding machine stopped to apply
• All shields take away from winding area
Internal
shield

46. Transformers and EMI
Y-Cap termination
 Noise couples through the transformer via
CParasitic
• Noise seeks path to primary circuit
• Without path, noise may become conducted
emissions
• Y-Cap across transformer reduces noise
• Tune the capacitor for optimum loss vs. noise
reduction
• Capacitor usually in the 470pF to 4.7nF range
• Place as close to transformer as possible

47. Transformers for EMI – Power Supply
Current CompensatedSnubber Y-Cap
Choke WE-FC Transformer
Output filter
WE-TI
Switch IC

48. Transformers for EMC – Schematic
Current Compensated Snubber Transformer Y-Cap Output filter
Choke WE-FC WE-TI
Switch IC

49. Transformers for EMC – Example 1
• With adjusted Snubber
• Without common mode choke
• Without adjusted Y-Cap
QPeak
Avg.
Peak
Avg.
EMC- Test Failed

50. Transformers for EMC – Example 2
• With adjusted Snubber
• With common mode choke
• Without adjusted Y-Cap
QPeak
Avg.
Peak
Avg.
EMC- Test Failed

51. Transformers for EMC – Example 3
• With adjusted Snubber
• With common mode choke
• Without adjusted Y-Cap
QPeak
Avg.
Peak
Avg.
EMC- Passed

52. Transformers for EMC – Example 4
• Without adjusted Snubber
• With common mode choke
• Without adjusted Y-Cap
QPeak
Avg.
Peak
Avg.
EMC- Passed

53. Transformer for EMC – Conclusion for this power supply
• Necessary to pass EMI:
– Current compensated Choke
(CMC)
– Y-Caps
• Not necessary to pass EMI
– Optimized Snubber

54. Simple EMI detector