Understanding and Optimizing Electromagnetic Compatibility in...
Transcript of Understanding and Optimizing Electromagnetic Compatibility in...
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Understanding and Optimizing Electromagnetic Compatibility in
Switchmode Power Supplies
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EMI = Electro Magnetic InterferenceEMC = Electro Magnetic Compatibility (No EMI)
Three Components of EMI:• Noise generator (source)• Noise transmission (coupling)• Noise receptor (victim)
Definitions
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• Electromagnetic Fields Caused by Changing Currents and Voltages
• Maxwell’s Equations:
• Slower Rise/Fall Times (with tradeoffs)
• Low Energy Phenomenon – At 1 Mhz, 20 nW can fail FCC limits
dtdv
Ci,dtdi
Me ==
Understanding EMI
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Radiated (>30 MHz)
Magnetic Electric
System Usage: Class A - Industrial and CommercialClass B - Includes Residential
EMI
Categorizing EMI
Conducted (<30 MHz)
Differential Common Mode Mode
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USA - FCC: CFR Title 47, Part 15 (etc.)Europe - IEC: EN50081 (and others)French - CISPR: Publication 22
Frequency Ranges (FCC) -
Conducted EMI: 450 kHz to 30 MHzRadiated EMI: 30 MHz to 1 GHz
Note: Measurement dependencies on operating conditions
EMI Specifications
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Conducted Noise Limits (FCC vs CISPR)
70
65
60
55
50
45
40
Frequency - MHz
Vol
tage
- dB
uV
0.1 0.3 0.50.2
FCC
FCC
CISPR
CLASS B
CLASS A
1 3 5 102 30 50 10020
CISPR
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Radiated Noise Limits (FCC vs CISPR)
50
45
40
35
30
25
2010 30 50 100 500 1000
Frequency - MHz
Fiel
d S
treng
th -
dBuV
/m
30020020
FCC
CISPR
CISPRCLASS B
CLASS A
Measuring Distance = 10 mFCC
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Measuring Conducted Noise
10 µF10 nF
to330 nF*50 Ω
50 µHPower Source
Ground
Power Supply Input
To Spectrum Analyzer
Line Impedance Stabilization Network (LISN)
* Capacitor value determined by lowest specified frequency
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Elements of Conducted Noise
Two LISN Circuits
SpectrumAnalyzer 50 Ω
SMPSDM
OUTPUTS
CM
CM
CM
NEUTRAL
GROUND
LINE
CM and DM add vectoriallyEMI (line) = CM + DMEMI (neutral) = CM - DM
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Switching action causes current pulses at input and output.
Sources of Differential Mode Noise
INPUTNOISE
OUTPUTNOISE
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Sources of Common Mode Noise
InputNoise
OutputNoise
Capacitive Currents Direct to Chassis
Capacitive Currents Primary to Secondary
Chassis Ground Chassis Ground Chassis Ground
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DMInputFilter
SMPS VOUT+
-
Positive Input
Negative Input
Line
Return
Ground
POWERSOURCE
Combating Differential Mode Noise
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Basic Ideal Filter
VIN VOUT
20 µH
3200 µF+
0
20 dB
40 dB
60 dB
80 dB
100 dB
120 dB100Hz 1 kHz 10 kHz 100 kHz 1 MHz 10 MHz 100 MHz 1 GHz
Frequency - Hz
Atte
nuat
ion
- dB
(a)
20 µH3200 µF
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Actual Filter with Parasitics
3200 µF
VIN VOUT
20 µH
500 pF
+
ESR 0.02 Ω
ESL 16 nH
0
20 dB
40 dB
60 dB
80 dB
100 dB
120 dB100Hz 1 kHz 10 kHz 100 kHz 1 MHz 10 MHz 100 MHz 1 GHz
Frequency - Hz
Atte
nuat
ion
- dB
(a)
(b)
500 pF20 µH
0.02 Ω16 µH
20 µH3200 µF 0.02 Ω
3200 µF
16 n
H50
0 pF
(b)
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Four Paralleled Capacitors Reduces ESL
VIN
800 µF
20 µH
500 pF
+
0.08 Ω
16 nH
800 µF
VOUT
+
0.08 Ω
16 nH
800 µF+
0.08 Ω
16 nH
800 µF+
0.08 Ω
16 nH
0
20 dB
40 dB
60 dB
80 dB
100 dB
120 dB100Hz 1 kHz 10 kHz 100 kHz 1 MHz 10 MHz 100 MHz 1 GHz
Frequency - Hz
Atte
nuat
ion
- dB
(a)
(b)
500 pF20 µH
0.02 Ω4 nH
20 µH3200 µF 0.02 Ω
3200 µF
(c)
(c)
4 nH
500
pF
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Improved Inductor Reduces Shunt CVIN
800 µF
20 µH
50 pF
+
0.08 Ω
16 nH
800 µF
VOUT
+
0.08 Ω
16 nH
800 µF+
0.08 Ω
16 nH
800 µF+
0.08 Ω
16 nH
0
20 dB
40 dB
60 dB
80 dB
100 dB
120 dB100Hz 1 kHz 10 kHz 100 kHz 1 MHz 10 MHz 100 MHz 1 GHz
Frequency - Hz
Atte
nuat
ion
- dB
(a)
(b)
(d)
50 pF20 µH
0.02 Ω4 nH
20 µH3200 µF 0.02 Ω
3200 µF
(c)
4 nH
50 p
F
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Adding a Second Stage Inductor
VIN
800 µF
20 µH
50 pF
+
0.08 Ω
16 nH
800 µF
VOUT
+
0.08 Ω
16 nH
800 µF+
0.08 Ω
16 nH
800 µF+
0.08 Ω
16 nH
5 pF
1 µH
0
20 dB
40 dB
60 dB
80 dB
100 dB
120 dB100Hz 1 kHz 10 kHz 100 kHz 1 MHz 10 MHz 100 MHz 1 GHz
Frequency - Hz
Atte
nuat
ion
- dB
(a)
(b)
(d)
(e)
8 nH50 pF
5 pF1 µH
50 pF20 µH
0.02 Ω4 nH
0.08 Ω1 µH
20 µH3200 µF 0.02 Ω
3200 µF
(c)
(c)
(e)
8 nH
5 pF
(b)
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Reducing Inductor Parasitic Shunt Capacitance
Core
S FWindings
Bobbin
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Natural Resonant Frequency =
Three Potential Problems:
1. Step application of input voltage could ring to 2VP.
2. High frequency noise at input could be amplified by “Q” of filter.
3. Filter output impedance rises at fr with potential oscillation with ZIN of converter.
LC21
π
Filter Resonance
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Filter Damping
Undamped Filter
L
C
LINE
SOURCE
RETURN
DampingComponents
POWERSUPPLY
CL2RD ≈
CCD ≈
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Combating Common Mode Noise
12 pF
+200 VDC
20 ns+48 mA
? 8 mA
100 ns
I
V400 V
5 µs
IRMS = 9.6 mA
(Typical TO-220 insulator capacity is 12 pF, fs = 200 kHz)
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3rd harmonic equivalent noise voltage circuit
• VN = 68 mV• FCC Limit (class A) = 1.0 mV• Required noise filter attenuation:
37 db at 600 kHz
Common Mode Noise Analysis
LISNs
PWR
SMPS
50 Ω50 Ω
12 pF
400 V200 kHz
ICM
RET
VN
VN
12 pF
25 Ω60 VRMS600 kHz
VN
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Allowable parasitic capacitance is unrealistic.
Achieving 37 dB Attenuation With Series Inductor
12 pF
25 Ω
0.95 mVVN
(Xc = 22.1 kΩ)
419 mH
Co < 0.17 pF!
(XL = 1.58 MΩ)(N @ 2 X 250T)
60 VRMS600 kHz
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Required capacitor will not meet safety specifications.
Achieving 37 dB Attenuation With Shunt Capacitor
12 pF(XC = 22.1 kΩ)
25 Ω
0.95 mVVN
60 VRMS600 kHz
760 nF(XC = 0.35 Ω)
ESL < 93 nH
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Optimum Solution Uses Both L and C
12 pF
25 Ω
0.95 mVVN1.07 mH
Co < 66 pF
(XL = 4.03 kΩ)(N @ 2X13T)
60 VRMS600 kHz
4.7 nF(XC = 56 Ω)
153 mV
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Complete Input Filter for Both DM and CM Conducted Noise
Cc2 Cd3Cc1
Cd2
Cc2
AC LineD
isch
arge
LN
CN
RNCc1
Lc1
Inrush Limiter
Cd1 DC Bus
Ld1
Final Cleanup Common Mode Differential Mode
DifferentialMode
Lc1
Notch filter for fS
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Minimizing CM Noise Injection
Case at HF AC
Insulators
PCB
Bracket ContactingPCB Ground
Chassis GroundedHeat Sink
Heat Sink Tied to PCB Ground (return)
PCB MountedHeat Sink
Shielding Heat Sink Bracket
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• Also called a Faraday shield• Connect to V+ if turn-off is fastest, to return with
faster turn-on
Electrostatic Shielding
ALTERNATESHIELD
CONNECTION
CORE
V
UnshieldedTransformer Correct Incorrect!
Use of Primary Shield
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• Noise is easily transformed back and forth between conducted and radiated form
• Conductors become antennas and antennas become receivers
• Testing more difficult– Frequency > 30 MHz
– Test environment and fixturing is critical
Radiated EMI
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Radiated Noise Measurements
SMPS "Load"Power Source
Antenna
SpectrumAnalyzer
Test fixture onrotating turntable
Antenna on variableheight vertical support
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Electric Field:
– Minimize high dv/dt on large surfaces
Magnetic Field:– Minimize high di/dt in conductive loops
Near Field:– Electric and magnetic fields act independently
Far Field:– Electric and magnetic fields merge
At 1 MHz,
=dtdv
CE
=dtdi
MH
πλ<2
πλ>2
meters300=λ
Characteristics of Radiated Noise
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Potential Electric Field EMI Sources
Shielding possible
LARGE AREA,HIGH dV/dtANTENNAS
E EE
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Magnetic Fields from Transformers and Inductors
Stray Transformer andInductor Fields
H
II H
High dI/dtSwitching
Loops
Shielding difficult
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Potential Magnetic Field EMI Sources
Tran
sfor
mer
Transformer
Wide, Closely Spaced CopperStraps
Worst case! Much Better!
Large Loop AreaWith High dI/dt
Rectifier Rectifier
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Leakage fields radiate
with intensity of
Opposing fields tend to cancel
with intensity of3d
14d
1
Magnetic Fieldsfrom Leakage Inductance
Core
SE
C
PR
I
SE
C
PR
ICore
SE
C P
SE
CPP P
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• Continuous copper strap around both windings and core.
• Converts stray magnetic fields to eddy current.
• Eddy current creates a canceling magnetic field.
Minimizing Stray Magnetic Fieldsin Transformers
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Poor Construction Techniques
Minimizing Stray Magnetic Fieldsin Inductors
Core
Coil
Cor
e
Winding
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• Presented a general overview• Defined various categories of noise• Described measurement techniques• Discussed ways to minimize noise
generation• Did not cover shielding or techniques
to minimize susceptibility• Should mention frequency modulation• Valuable additional references listed
Summary