# Simulation of EMI in Hybrid Cabling

```Simulation of EMI in Hybrid
Cabling for Combining
Power and Control Signaling
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Outline
 Motivation
 Hybrid cable design
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Cable parameters, impedance
EMI and screening
 Connector modeling
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 Transient co-simulation (ESD)
 Validation study
 Conclusions
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SMPS/cable/sensor system
Over-braid
V+
V-
IGBT
block
GN
Screen
RS485
RS485
Comm. Physical
Layer
Programmable Switched
Mode Power Supply
Comm. Physical
Layer
Hybrid Cable
Intelligent Sensor
Hybrid connector
Need for EMI analysis
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SMPS Emissions
o PWM IGBT switching may be relatively low frequency
o Fast edge rise time will generate harmonics that may cause EMI
o RS 485 control signal may be corrupted by power switching noise
Example
τr=1ns, τ=10us
F1= 31.8 KHz, F2= 318 MHz
τr
2Aτ/T
A
A/2
τ
t
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F1=1/πτ
F2=1/πτr
f
Simulation methods
Static
field
solvers
DC Parameters
3D full
wave field
solver
Impedance
2D (+TL)
cable
solver
3D full
wave field
solver
Circuit
solver
Equivalent Circuit
S Parameters
System Analysis
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Cable Parameters (Capacitance)
Pin
Potentials
Curved
Tetrahedral
Mesh
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Electrostatic Field
Solution
Cable Parameters (Inductance)
Current
Paths
Curved
Tetrahedral
Mesh
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Magnetostatic Field
Solution
Cable Parameters (Impedance)
Impedances from L/C values (ignoring losses)
Zoo = √ (L11 – L12 / C11 + 2C12)
Zoe = √ (L11 + L12 / C11)
Zc = Zoe/2
Zd = 2 Zoo
Zc = 44.5 Ohm
Zd = 124.4 Ohms
• Electro-static and Magneto-static solvers directly calculate line parameters.
• Differential and common mode impedances can be calculated.
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RS485 Impedance (Full-Wave FEM)
Multi-pin
differential
port
Curved
Tetrahedral
Mesh
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Port Mode
Field
Solution
Port
Impedance
Impedance Optimization
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RS485 insulator radii varied to optimize differential impedance.
Initial design 127.6 Ohms
Impedance “tuned” to 120 Ohms.
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S Parameters and Cable Loss
20m length
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S parameters can be calculated to understand cable losses.
Reference plane can be shifted to account for a longer length of cable.
Avoids having to mesh a long cable.
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Eye Diagram 10 Mbps
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Eye diagram for 10 Mbps; 100 ns pulse with 10 ns rise/fall time.
RS485 protocol recommends maximum of approx 12 m for 10 Mbps.
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Eye Diagram 100m cable
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Eye diagram for 10 Mbps; 100 ns pulse with 10ns rise/fall time.
100m long cable suffers from significant pulse distortion.
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Cable Studio 2D (+TL) Model
Aluminum
Foil
RS485 Twisted Pair
(3 in. twist rate)
36 AWG conductors
FEP insulation
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Copper Braid
Power Wires
14 AWG
PVC insulation
Cable cross-section input using library parts and user-defined groups
In-built calculators for foil and braid screens
2D Method of Moments solver used to calculate field in cross section
Cable parameters automatically extracted from field solution
TL modeling used to simulate a length of cable
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EMI Crosstalk Analysis
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Transient task in Design Studio used for cable/circuit simulation.
20m length of cable modeled to analyze EMI crosstalk.
SMPS transient voltage sources (650 V peak, 100&micro;S period, 0.1&micro;S edges).
Coupling to RS485 line.
Initial design without RS485 shielding.
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EMI Crosstalk Analysis
DM and CM
Currents coupled
into RS485
Switching voltage at SMPS
•1 Amp common mode noise induced
• Will be filtered at receiver
• Induced differential mode must be kept
below a few tens of mV
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Impact of Twist Rate
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Initial study with RS485 shield removed.
Induced differential current monitored at RS485 encoder.
Twist rate of RS485 varied to see impact on EMI rejection.
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Effect of Imbalance
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Imbalance introduced to investigate impact on EMI crosstalk.
Circuit parameters asymmetric for RS485 line.
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Impact of Imbalance
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Differential mode current significantly increases with imbalance.
2 mA peak differential mode current. (0.1 mA for balanced case)
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Shield Transfer Impedance
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ZT is the “intrinsic parameter” of a cable shield, characterizes its shielding
effectiveness (Schelkunoff, 1930s)
ZT = (1/IO)*(dV/dx) where IO is current flowing on one side of the shield and
dV/dx is the voltage per unit length on the other side
Kley (1991) proposed a model for a braided shield, including various
contributions…
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Diffusion of E and H fields through the shield material
Penetration of fields through the small apertures in the braid
Induction phenomena due to overlapping of strand wires (“porpoising” effect)
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Kley’s Model for Over-Braid ZT
Braid
porpoising
aperture
diffusion
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Kley’s Model for RS485 Foil ZT
Foil
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Cable Studio Screen Definitions
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EMI Crosstalk Analysis (Screened)
DM and CM
Currents coupled
into RS485
Switching voltage at SMPS
• 20 mA common mode noise induced
• Almost 1 A in unshielded case
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Common Mode Current Waveform
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Common current pulse on edge of IGBT voltage.
Ringing due to reflections and ring frequency related to length of cable.
In air, pulse takes 0.13&micro;S to travel 40m round trip.
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Improving the Shield
• In some cases, multiple layers of shielding are required
• Foil layers, foil with braid, multiple braids, etc…
• Braids are good barriers at low frequencies, poor
barriers at high frequencies, whereas foils are the
opposite (usually very thin)
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Model for Combined ZT
• Vance’s model (1972) for equivalent transfer impedance of two
shields:
Outer shield
ZT1 = transfer impedance of outer shield
ZT2 = transfer impedance of inner shield
ZS1 = internal impedance of outer shield
ZS2 = internal impedance of inner shield
L12 = inductance of the shield to shield line
where b1 is the outside radius of the inner shield
and a2 is the inside radius of the outer shield
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Inside cable
Predicted Foil/Braid Screen ZT
Braid/Foil
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Imported Transfer Impedance
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Comparison of Results
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Combined foil/braid screen significantly reduces CM current.
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Connector S parameters
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Multi-pin waveguide port. Single-ended.
Distributed computing used to solve for various port excitations.
Transmission losses relatively small due to short length
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Cable
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Connector
Cable and connector blocks cascaded for full system response.
Transient task used to simulate crosstalk.
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Common mode current
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Cascaded system increases peak CM current to 30 mA.
Without connector 20 mA.
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Transient Co-Simulation
Transient Co-Simulation
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2D (TL) cable
solver
Hybrid field/cable/circuit solution
Full bi-directional coupling between
cable and environment
Ideal for susceptibility and emissions
analysis
Circuit solver
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3D Transient
(TLM) full wave
field solver
Cable Susceptibility (ESD)
ESD pulse
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10m cable routed between SMPS and sensor.
ESD event occurs at SMPS end.
Human body contact model (HBM) used in the simulation.
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ESD Transient Co-Simulation
ESD current
ESD Generator Circuit
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Induced Currents – Over-Braid
Copper Braid
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Induced Currents – Power Wires
Power Wires
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Induced Currents – RS485 Line
Aluminum Foil
RS485 Twisted Pair
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Time Animation
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Cable Studio Validation Study
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4 different shielded cables
• RG58 (braid)
• RG6 (foil and braid)
• Twisted pair (spirally wrapped foil/drain)
• STP Shielded Twisted pair (braid)
Test the transfer impedance modeling
Excite source wire loop, measure signal received
on signal wires with and without the shield
Compare measured and simulated results
Thanks to Jeffrey Viel at NTS for providing EMC test facilities and my
colleague Patrick DeRoy for his work on the validation study.
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Twisted pair with drain wire
Drain wire in contact
with foil
Spiraling seam applied to foil wrap
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ZT contributions
Adding a spiraling seam to the foil increases the inductive component of the transfer
impedance. The foil material is aluminum-polyester-aluminum. It is not unreasonable
to assume that this spiraling seam would exist.
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Cable Studio setup, TNC connectors
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Cable Studio model uses different routes and cable cross-sections to
model the shielded section between boxes and unshielded sections
inside the boxes.
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Drain wire cable and pigtail connectors
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Cable Studio setup, pigtail connectors
•
Cable Studio model uses different routes and cable cross-sections to
model the shielded section between boxes and unshielded sections
inside the boxes.
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Coupling results, measured vs. simulated
Coupling / dB
Unshielded
results
Spiral/drain pigtail
Spiral/drain TNC
STP TNC
RG58 TNC
RG6 TNC
Frequency / MHz
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Good correlation between simulation (solid) and measurements (dotted).
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Summary
• Hybrid cable design using various simulation workflows
• Impedance calculation and optimization
• EMI crosstalk analyzed for different configurations
• Effect of twist rate, imbalance and screening investigated
• Transient field/cable/circuit co-simulation
• ESD susceptibility example
• Validation results for shielded cables
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