EMI in power supplies
Alfred Hesener
Fairchild Semiconductor Europe
www.fairchildsemi.com
Agenda
•
•
•
•
Introduction
Different types of EMI and their characteristics
Regulations and standards for EMI
Measurement and sources of EMI
• Conducted EMI
• Radiated EMI
• EMI as integral part of the design flow
• Conclusion
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Introduction
EMI more and more complex
• Increasing power density, faster switching, higher currents are
causingg more EMI-related issues
• Conducted / radiated EMI
• Further changes complicating things
• New semiconductor switches are faster
• New topologies (e.g. Quasi-resonant)
• How to achieve
hi
a “robust”
“ b ” design?
d i ?
• Embed EMI into the design flow from the beginning
• What is the goal?
• Emit low EMI levels to meet regulations (don’t disturb other
applications nearby) Æ EMI compliance
• Work properly (be self-compliant) Æ Robustness
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Agenda
•
•
•
•
Introduction
Different types of EMI and their characteristics
Regulations and standards for EMI
Measurement and sources of EMI
• Conducted EMI
• Radiated EMI
• EMI as integral part of the design flow
• Conclusion
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Different types of EMI and their characteristics
“Reduce emission
of source”
Emitter
Galvanic
Coupling
Capacitive
Coupling
Inductive
Coupling
Wave
Coupling
“Reduce
transmission
in the system”
“Reduce sensitivity
of receiver”
Receiver
Galvanic coupling of
signals in the circuit
Electric field
Magnetic field
Radiated wave traveling in the
system
Typically <30MHz
Medium-high
frequencies
Typically > 30MHz
High frequencies
Any noisy
An
nois signal in the
system
Large dV/dt
Large dI/dt
Fast switching
s itching
(RC) filtering
Metal shield
Magnetic shield
Electromagnetic shield
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Agenda
•
•
•
•
Introduction
Different types of EMI and their characteristics
Regulations and standards for EMI
Measurement and sources of EMI
• Conducted EMI
• Radiated EMI
• EMI as integral part of the design flow
• Conclusion
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Regulations and standard for EMI
EN550xx and EN61000 most important
•
•
Two main considerations:
• Limit the amount of emission which a given application generates
• Define minimum immunity levels a given application must tolerate
EN550xx – the “EMI” norm (class A = “consumer”, class B = “industrial”)
•
•
•
•
•
•
•
•
CISPR11, EN55011 for industrial, medical, scientific applications
CISPR13, EN55013 for consumer applications
CISPR14 EN55014 for
CISPR14,
f home
h
appliances,
li
power tools,
t l involving
i
l i motion
ti control
t l
CISPR15, EN55015 for lighting equipment
CISPR22, EN55022 for computing applications
CISPR16, EN55016 defines the measurement method
Many applications being tested against a “mix” of different norms (e.g. EN55022 for
frequencies >150kHz, EN55015 for frequencies <150kHz)
EN61000 – the “PFC” norm (equipment classes see next page)
•
•
•
•
•
•
•
Noise current up to the 40th harmonic of the line frequency ( <= 2.0kHz
2 0kHz (e
(e.g.
g EU) / 2.4kHz
2 4kHz (e.g.
(e g US))
EN61000-3-2 for applications < 16A
EN61000-3-12 for applications with 16A…75A
EN61000-4-7 defines the measurement and evaluation method
EN61000 4 16 for common
EN61000-4-16
common-mode
mode disturbances up to 150kHz
Many further standards exist, dealing with more specialized applications
EN61000 specifies maximum harmonic currents, not a power factor
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Regulations and standard for EMI
Equipment classes for EN61000
Class
Equipment
Power
Comment
A
3phase equipment, household appliances,
tools dimmers for incandescent lamps,
tools,
lamps audio
equipment, everything not B, C or D
> 75W
Limit values are defined
as absolute values
B
Portable tools,
tools Arc welding equipment
> 75W
Limit values are defined
as absolute values
> 25W
Limit values defined as
relative values to first
harmonic
C
Lighting
C
Lighting
< 25W
Limit values defined only
for 3rd and 5th harmonic,
relative to first harmonic
D
Personal Computer, Monitor, Television
75W 600W
Limit values relative per
input power
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The power factor
Simulation results
•
•
•
Simulation shows input and bus cap voltage, and current spikes in the input
High dI/dt illustrates significant harmonic content
Simulation below is 100W class A SMPS,
SMPS would require a (active) PFC
• EN61000 considers harmonics to 2kHz/2.4kHz – this would be a pretty large filter
if realized with passive components
• Attenuation of this filters’ components
p
for higher
g
frequencies
q
(conducted
(
EMI))
would be low, due to potentially high parasitic capacitance, and it may not help
with CM noise at all
Limit values for
EN61000 class D
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Agenda
•
•
•
•
Introduction
Different types of EMI and their characteristics
Regulations and standards for EMI
Measurement and sources of EMI
• Conducted EMI
• Radiated EMI
• EMI as integral part of the design flow
• Conclusion
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Measurement and sources of EMI
Conducted EMI test setup
Line Impedance Stabilizer Network (“LISN”):
- Defined impedance for noise voltage measurement
- Blocking the noise coming from the grid
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Measurement and sources of EMI
Conducted EMI limits
Vertical: Amplitude in dbuV
Horizontal: Frequency in MHz
Solid blue line: EN55011/22 limits for average
Solid red line: EN55022 limits for quasipeak
Red spectrum line: quasipeak measurement values
Black spectrum line: average measurement values
Frequency
Limit (dbuV) Limit (V)
Comment
9kHz ... 50kHz
110
316mV EN55011 Quasipeak
50kHz ... 150kHz
90 ... 80
32mV ... 10mV EN55011 Quasipeak
EN55022 B,
B Quasi-peak;
Quasi peak; linearly falling
66 ... 56
2mV ... 0.63mV
with log (frequency)
150kHz ... 500kHz
EN55022 B, Average; linearly falling with
56 ... 46 0.63mV ... 0.2mV
log (frequency)
56
630uV EN55022 B, Quasi-peak
0.5MHz ... 5MHz
46
200uV EN55022 B, Average
60
1mV EN55022 B, Quasi-peak
5MHz ... 30MHz
50
316uV EN55022 B, Average
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Frequency
range
Bandwidth
(-6dB)
9kHz ...
150kHz
200 Hz
150kHz ...
30MHz
9 kHz
30MHz ...
1GHz
120kHz
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Conducted EMI
Differential and common mode noise
•
•
•
In most cases, two different noise voltages will appear at nodes L and N
• Separate into differential (“DM”) and common mode (“CM”) noise
• Different filtering required for both noise types!
Differential mode noise appears out of phase at the nodes
• Noise current flows in a loop between L and N ((“1”)
1 )
Common mode noise appears in phase at both nodes
• Noise current flows via ground and back through the lines (“2”)
L
DM noise current
1
N
2
Ground
CM noise current
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Parasitic
Coupling
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Measurement and sources of EMI
Conducted EMI as result of switching
•
The main switching action will cause a current flow into / out of the bulk cap, at
the main switching frequency
• This current flow causes a noise voltage to appear at the input
• Typical values are ESRmax = 1.9Ω, ESLtyp = 20nH
• Impedance
p
minimum is ESR,, will increase at high
g frequencies
q
Æ EMI is primarily a result from parasitic elements in the circuit
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Conducted EMI
Different filter types
Filter type
Balanced
Unbalanced
Pi filter
18 db / oct
60 db / dec
T filter
18 db / oct
60 db / dec
L filter
filt
12 db / oct
40 db / dec
(Calculation of component values is explained later)
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Conducted EMI
Common mode vs differential mode
• For common mode noise, the line to line capacitors do not help
• Only
y the inductors contribute (but
(
typically
yp
y they
y are too small))
• Introduce a common mode choke
• Designed for (large) leakage inductance to provide DM filter function
Choke (with leakage
inductance)
Line to
line cap
Example of a 200W power supply input stage with a two-stage CM choke
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Conducted EMI
Calculation of the filter components
Design impedance
Input data
Line frequency
Minimal RMS voltage
Maximum RMS load current
Lowest switching frequency
fLine
Vmin
Imax
fswmin
Attenuation
Determine required attenuation
level pper frequency
q
y from
simulation or measurement
Calculate the component values
Filter topology
Determine suitable filter topology
and cutoff frequency so attenuation
ggoals are met with a margin
g of 6...10dB
(but fcut > 10* fLine)
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Conducted EMI
Simulation and results
• Simulation for compliance: Noise generation and filter attenuation
are mostlyy determined byy pparasitic elements in the circuit
• Noise generation: Leakage inductance, ESR, ESL, capacitive
coupling (to ground)
• Attenuation: Core frequency response, capacitive coupling
• Most simulators allow to set parasitics for all passive
components
• Using a behavioural model for the noise (current) source is a
good approximation
• Simulation for function and robustness: Very complex – better to
design accordingly, test a prototype, implement fixes in final circuit
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Conducted EMI
Example values for parasitics
Inductor
Parallel capacitance
e.g.
g 50pF
p for 1mH
Capacitor
Series resistance
e.g.
g 1.9 Ohm for 100uF
Series inductance
e.g. 20nH for 100uF
Transformer
Leakage inductance
e.g. 10uH for 200uH (prim)
Parasitic capacitance
e g 50pF for EF25
e.g.
CM choke
Leakage inductance
e.g. 300uH for 10mH
P
Parasitic
i i capacitance
i
e.g. 100pF for 10mH
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Conducted EMI
Simulation circuit example
Bus cap
Load
±0.05A
100kHz
LISN
Input voltage
230V / 50Hz
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CM filter
DM filter
(T type)
Page 20
Parasitic
coupling
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Conducted EMI
Simulation results
without filter
EN55022 limits
(quasi-peak)
with filter
EN55022 limits
(quasi-peak)
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Agenda
•
•
•
•
Introduction
Different types of EMI and their characteristics
Regulations and standards for EMI
Measurement and sources of EMI
• Conducted EMI
• Radiated EMI
• EMI as integral part of the design flow
• Conclusion
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Radiated EMI
What generates it?
•
•
Magnetic EMI is
caused by changing currents:
Vnoise =
RM
RS + RM
*M*
dI
Current
(di/dt)
RS
Vnoise
+ Vmeas
dt
RM
•
Coupling factor M depends on:
• Distance,
Di t
area andd orientation
i t ti off the
th
disturbing magnetic loops
• Magnetic absorption between the loops
• Current risetime
• Impedance of the receiver
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Radiated EMI
Main contributions to radiated EMI
•
•
•
Avoid high dI/dt – move to softer (slower switching) or zero-current
switching
• Analyze the current flows at normal behavior of the circuit,
circuit and check
which elements will only see current flow in one part of the cycle –
these elements are very likely to be in a current loop with high dI/dt
Reduce the coupling factor M between the magnetic loops
• Orientation
Oi
i off the
h current loops
l
should
h ld be
b orthogonal,
h
l not parallel.
ll l
• The current loop areas should be made as small as possible
• Increase the distance between the emitting current loop and the loop
picking
p
g up
p the noise (energy
(
gy transfer proportional
p p
to power
p
of 3))
• Magnetic shielding
Make the signal processing nodes in the system as low-impedance as
possible
• Current-based
C
b d signal
i l transfer
f
• Add additional resistors to Ground at sensitive
• Differential signaling
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“Reduce emission
of source”
“Reduce
transmission
in the system
system”
“Reduce sensitivity
of receiver”
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Radiated EMI
How to measure radiated EMI
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Radiated EMI
How to identify “hot spots”
• Use a two-channel scope
• Connect a (HV) probe to the main switching signal
• Connect the H-field probe to a probe amplifier (if necessary) and to
the second channel (proper termination required)
• Use the main switching signal as a trigger signal
• “Wander” around the PCB to identify areas of large emission, then
zoom in
• Take (static) pictures of the critical field signals to determine
q
y and quality
q
y factor ((this can be used to identifyy the
frequency
elements of the resonant tanks)
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Transformers radiating magnetic fields
High leakage inductance == leakage field
P core hhas the
Pot
h smallest
ll field
fi ld (not
(
surprising)
ii )
Toroid
T
id with
ith exposedd
core emits more than it
should
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ER core – better
b
than
h E core (tighter
( i h winding)
i di )
Better
B
tt to
t move air
i gap to
t
center leg (may increase
AC losses)
Page 27
E core – stronger field due to
leakage inductance
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Radiated EMI - various issues
(incomplete list…..)
•
•
•
•
•
•
•
•
•
•
Leakage inductance fields
External field of air gaps
Diode reverse recovery
M i l losing
Materials
l i their
h i damping
d
i
Caps becoming inductive
Inductors becoming capacitive
((secondaryy side)) chokes picking
p
g upp
magnetic noise
Ringing between parallel caps
Ringing between parallel rectifiers
Transformer shield ringing
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Example 1: 70W QR flyback supply
18MHz peak from transformer
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Example 1: 70W QR flyback supply
Two different diodes in the snubber
Yellow: Drain voltage of main MOSFET
Blue: Magnetic field at snubber
• Fast snubber diode gives faster rise / fall times and lower losses
• Slow diode with much larger Qrr shows significant magnetic EMI
• Impact can not be seen in the node voltage – need to investigate
magnetic field to find out
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Example 2: 300W CCM PFC board
Strong EMI event at turn-off
Yellow: Drain voltage of MOSFET
Blue: Magnetic field
At MOSFET
•
•
•
At Diode
Inside inductor
Medium EMI spike at the MOSFET,
MOSFET high frequency ((~40MHz)
40MHz) indicates ringing between
Coss (780pF) and the parasitic inductance of the PCB and package (20nH), well damped,
after which the inductor ringing takes over
Smaller EMI spike at the (SiC) diode shows ringing at similar frequency, indicating that
this is imposed
p
byy the power
p
MOSFET ((in this case,, the equivalent
q
charge
g of the diode is
100x smaller so the contribution is too)
Long ringing tail of the inductor shows the energy flowing between the inductor and its
parasitic capacitance
g p) and the tail lasts for 800ns ((high
g Q)
• Field is strongg ((distributed air gap)
• Ringing frequency is ~9MHz, parasitic cap ~20pF (estimated) Æ effective inductance
is reduced by 40x at this frequency! (core material)
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Example 3: 400W interleaved PFC
PCB layout
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Example 3: 400W interleaved PFC
Main difference between two boards
“Prototype”
Prototype
“Production”
Production
Æ 10dB difference in magnetic field peak intensity
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Example 4: 200W LLC power supply
Turn-off of main LLC stage
• Small EMI fields around the
converter, most at leakage
inductor (gapped core) which
itself has small leakage
• Well-damped
Well damped transformer
resonance at 22.7MHz
• 70pF and 0.7uH leakage
• Not visible in the node voltage
100 /di
100ns/div
Red: Magnetic field at leakage ind.
Pink: Phase node voltage
g
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Agenda
•
•
•
•
Introduction
Different types of EMI and their characteristics
Regulations and standards for EMI
Measurement and sources of EMI
• Conducted EMI
• Radiated EMI
• EMI as integral part of the design flow
• Conclusion
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EMI as part of the design flow
Design steps
Write the
specification
• EN550xx
• EN61000
• Time for EMI
testing
• Space for
EMI filter
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Select the
topology
Calculate the
components
Topologies with low EMI
PFC
Page 36
• Consider impedance
of EMI filter
• Make circuit nodes
low impedance
(esp. control loop)
• Avoid high di/dt and
dv/dt
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EMI as part of the design flow
Design steps
Simulate
the design
g
• Simulate with a LISN
model (but without
filter) to predict noise
• Use behavioural model
for the load to save
simulation time
• Chose filter topology
for needed attenuation
levels, simulate again
• Put realistic values for
parasitic elements
• Use an impedance analyzer
t measure typical
to
t i l components and put these in
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Build a
pprototype
yp
• Try to be close to final
arrangement of compo
components, so the coupling
and radiated EMI can
be tested
• Minimize high-current
loop area
• Minimize node area with
high dv/dt
• Leave some space at the
input to put a EMI filter
Page 37
Test the
pprototype
yp
• After checking the function,
perform pre
pre-compliance
compliance
EMI testing to see the
“real” conducted noise
• Check CM noise on a
grounded metal plate
(worst case)
• Perform first radiated EMI
tests to identify critical
spots in the circuit
• Compare with simulation
results and calibrate the
models
d l
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EMI as part of the design flow
Design steps
Add the
EMI filter
• Build the EMI filter into
the pprototype
yp and perform
p
full functional test again
• Check if EMI filter impedance and possible resonances create any issues
• Perform pre-compliance
testing again to see if the
measured attenuation
matches calculation
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Design the
final version
Test the
final version
• Final implementation will
• After full functional testing,
g the noise “signature”
g
change
p
perform
ppre-compliance
p
of conducted DM and CM
testing especially and high
as well as radiated EMI
and low line conditions
• (Alternate source) compoover full load range
nents used in production
• Try out different passive
may have different paracomponents (including
sitics, so the EMI behaviour
different vendors)
may change – need to add
• Build several prototypes
appropriate margins
and check if the noise results
are repeatable
Page 38
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Agenda
•
•
•
•
Introduction
Different types of EMI and their characteristics
Regulations and standards for EMI
Measurement and sources of EMI
• Conducted EMI
• Radiated EMI
• EMI as integral part of the design flow
• Conclusion
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Conclusion
• There is no silver bullet!
• Switching currents and voltages will generate EMI
• Assess implications early in the design cycle, and prepare
g cycle
y the pproblem is detected, the more
• The later in the design
expensive it is to fix
• Use topologies and control ICs that create less noise to begin with
• LLC,
C QR
Q fl
flyback,
b k PSR
S
References:
[1]
[2]
[3]
[4]
[5]
Didier Bozec, David Cullen, Les McCormack, John Dawson, Bryan Flynn: An investigation into the EMC emissions from switched
mode power supplies and similar switched electronic load controllers operating at various loading conditions (IEEE Symposium on
Electromagnetic Compatibility, Santa Clara CA, August 2004)
Bruce Carsten: Application note for H-field probe (http://bcarsten.com)
Jonathan Harper: Electromagnetic compatibility design for power supplies (Fairchild Semiconductor power seminar series 2004/2005)
Richard Lee Ozenbaugh: EMI filter design (CRC, Nov 2000)
Christophe Basso: Switch-Mode Power Supplies SPICE Simulations and Practical Designs“, McGraw-Hill, 2008
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