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![Page 1: EE155/255 Green Electronics - Stanford Universityweb.stanford.edu/class/ee152/lecture_slides/Thermal_EMI_110716.pdf · EE155/255 Green Electronics Thermal and EMI ... 7 10/19/16 Electric](https://reader033.fdocuments.us/reader033/viewer/2022051508/5aa45dad7f8b9ab4788bca01/html5/thumbnails/1.jpg)
EE155/255 Green Electronics
Thermal and EMI11/7/16
Prof. William DallyComputer Systems Laboratory
Stanford University
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YAH
No Date Topic HWout HWin Labout Labck Lab HW1 9/26/16 Intro(basicconverters) 1 1 IntrotoST32F3 PeriodicSteadyState2 9/28/16 EmbeddedProg/PowerElect.3 10/3/16 PowerElectronics-1(switches) 2 1 2 1 ACEnergyMeter PowerDevices4 10/5/16 PowerElectronics-2(circuits)5 10/10/16 Photovoltaics 3 2 3 2 PVMPPT PVSPICE6 10/12/16 FeedbackControl7 10/19/16 ElectricMotors 4 3 4 3 MotorcontrolMatlab Feedback8 10/21/16 IsolatedConverters9 10/24/16 SolarDay 5/PP 4 5 4 Motorcontrol-Lab/ IsolatedConverters10 10/26/16 Magnetics11 10/31/16 SoftSwitching 6 5/PP 6 5 PS MagneticsandInverters12 11/2/16 ProjectDiscussions13 11/7/16 Inverters,Grid,PF,andBatteries 6 P 6 Project14 11/9/16 Thermal&EMI15 11/14/16 QuizReview C116 11/16/16 Grounding,andDebuggingQ 11/16/16 Quiz-intheevening
11/21/16 ThanksgivingBreak C211/23/16 ThanksgivingBreak
17 11/28/1618 11/30/16 MartinFornage,enPhase C319 12/5/16 ColinCampbell,Tesla20 12/7/16 Wrapup
12/15/16 Projectpresentations P12/16/16 Projectwebpagedue 39
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PE Tech Times, Nov 9, 2016
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Thermal Design
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Thermal Design• Many components in green-electronic systems dissipate considerable
power– FETs, IGBTs, Diodes – conduction and switching losses– Magnetics – core and copper losses– Resistors, capacitors, etc…– Typically from 2-20% of system power (2W – 5kW)
• This power must be removed without the components overheating• Heat-transfer mechanisms
– Conduction, convection, radiation, and phase change– Thermal solutions dominated by the first two
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Quantities and Units• Power – (W)
– Flows through thermal system (like current)• Thermal Resistance – (K/W)
– Temperature gradient drives power flow through conduction• Thermal Conductivity (W/(m K))
– Determines resistance of a material– Diamond 1000, Copper 400, Aluminum 205, Water 0.58, Plastic 0.2, Air 0.02
• Specific Heat (J/(g K))– Capacity of a fluid to carry heat – through convection– Determines time constant for conduction (like a capacitor)– ~1 for air, ~4 for water, .39 for copper
• Volumetric Heat Capacity (J/(ml K))– ~10-3 for air, ~4 for water, 3.5 for copper (it takes 4,000 times as much air (by volume) than water to carry heat)
• Heat transfer coefficient (W/(m2 K))– Heat transferred to cooling fluid per unit area– 50W/(m2 K) for air at 200 lfm, 5kW/(m2 K) for water at 10lfm.
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Typical System
SMT Transistor10W, 0.5 K/W J-case
PCB w/Thermal Vias (0.5 K/W)
Heat Sink1cm x 4cm x 4cm128 cm2 fin area
4cm2 cross section
Air flow 200 lfm (1 m/s)4 x 10-4 (m3/s)
Thermal adhesive0.1 K/(W cm2)
0.4 K/W under FET
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Questions• What is the temperature rise of the FET above the ambient air
temperature?
• How hot is the exhaust air temperature?
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Determine Thermal Resistance of Heat Sink
HeatSinkFinArea 0.0128m^2Coeff 50W/m^2KProd 0.64W/KRes 1.56K/W
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Sum Resistance Junction to Ambient
FET 0.5K/WPCB 0.5K/WAdhesive 0.4K/WHeatSink 1.56K/WTotal 2.96K/WPower 10.00WTempRise 29.6K
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Determine Temp Rise of Air
AirTempRisePower 10WAirFlow 400ml/sE/Vol 2.50E-02J/mlCapacityofAir 1.00E-03J/mlKTempRise 25K
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So…• If w.c. inlet air is 60C, how hot does the FET get?
• How hot is the outlet air?
• Only the front of the heat sink sees 60C air. Is our approximation in calculating its thermal resistance correct?
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In Practice• This is done by finite-element modeling (ANSYS, etc…)
– Answer is as good as the model.• Backed up by experiment
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Details are Critical• Interfaces matter
– Even a tiny air-gap is fatal• Upstream devices get cool air
– Downstream devices get air heated by upstream devices• Air follows path of least resistance
– Need plenums to guide it to where its needed
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Littlebox Thermal Design
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Fan Selection - Minimum Airflowto Mantain Outlet Temperature
PF38281BX-000U-S99
12V 6W 38mm x38mmx25mm
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Full Box Simulations
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Windtunnel test config pictures
Thermocouple readout
Anemometer
V_dspower supply and multimeter
Wind tunnel testing area
Fan controls
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ThinPak
Vias A-Pli Gap Filler
Shim + solder
UltrastickGrease
Heat sink
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Thermal Stack - Need 6K/W or lessResistance/Source Estimate
~0.5 (Application note)0.98 Max (Datasheet)
0.8
1-3 (Apllication note)1.2 (1mm board, application note)Our board will be 0.8 mm thick
1
Using L/KA calculation, <0.05 for all shims less than 1mm thick. But what about solder interface?
0.1
0.03°C-in2/W @ 20 psi (datasheet) 0.1
~2 with forced convection (datasheet)Interstitial, non-uniform heat application, unknown airflow
2-5?
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Results
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OOPS
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Thermal Design Summary• Remove heat from power components by conduction and convection• Sum thermal resistances to find total resistance K/W
– Determines temperature rise of component– With multiple components, solve equivalent circuit– Add heat capacities to determine transient response
• Determine heat sink resistance from surface area– Heat transfer coefficient depends on airflow
• Determine cooling fluid temperature rise from heat capacity• Validate with simulation and experiment• Details matter – interfaces, fluid routing, etc…
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EMI
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EMI – Electromagnetic Interference• Noised caused by high currents switching• Affects your circuits
– Large voltages induced across inductance– Currents and voltages coupled into unrelated wires
• Radiates B and E fields– Affects other circuits– Violates regulations
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Cause – Fast Current Transients
LP
C2M2
C1
M1 C3
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Cause – Fast Current Transients
LP
C2M2
C1
M1 C3
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Cause – Fast Current Transients
LP
C2M2
C1
M1 C3
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Fast Current Transients• Littlebox example:• C2, C3 ~ 200pF• DV = 400V, Dt = 5ns• Q = 80nC• Ipeak = 32A + Inorm
• di/dt = 13GA/s
LP
C2M2
C1
M1 C3
i
t
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13GA/s• Induces 13V across every 1nH of inductance• 50V difference from one end of a ground plane to another• Induces a fraction of this in nearby wires (flux coupling)• Everything is an inductor
– All capacitors have inductance– Every foil trace on a PCB has inductance
• Everything is a transformer– Flux coupled with inductor seeing current transient
• There is no such thing as a uniform “ground” plane• Faster devices don’t help
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To Minimize EMI• Keep the “critical loop” very small
– Place Components to minimize trace runs
– Supply current from multiple nearby low-inductance caps
– Minimum loop area– Minimum inductance
• Use one or more “ground” planes– Loop is between signal and plane
• Isolate noisy area with ferrite beads on supply
• Don’t put ferrite beads on ground– Causes huge voltages between
different grounds
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Symptoms – 1 Measurement Noise• Analog measurements become very noisy
– 30mV signal mixed with 10V of noise
• To fix:– Take differential measurements
• And pick reference carefully– Use a good instrumentation amp
• need to reject high-frequency common-mode noise– Sample after the ringing has settled– Use a snubber so the ringing settles quickly
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Symptoms – 2 Actuator Noise• Gate driver switches spuriously
– 5V gate drive signal corrupted by 50V ground noise
• To Fix– Use isolated gate drivers
• High-side drivers – even for low-side FETs• High transient immunity
– Use Kelvin source connection• Decouple gate driver from di/dt noise across source inductance
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Symptoms 3 – Components Fail• Voltage stress on nearby components
– microcontroller, A/D converters, amplifiers, level shifters, gate drivers– Input signals exceed legal range– High currents coupled into output signals
• To Fix – Protect and Isolate– Diode/capacitor (not just diode) protection on all inputs and outputs– Series resistance on outputs or buffer with “robust” driver– Power supplies filtered at point of use– Optical communication where needed
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Protection Circuit
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Symptoms 4 – You violate FCC Regulations• Radiated and/or conducted emissions exceed
specifications
• To Fix– Use snubbers to minimize noise– Use compact layout to minimize loop area– Use ground planes to minimize loop area– Package noisy circuits in a Faraday cage
• Make sure to “gasket” edges – slots radiate– EMI filters on all inputs and outputs
• Ferrite beads to attenuate high frequencies• Bulk inductors for lower frequencies• Common and differential mode filtering• Avoid corrupting signals after the filter
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An EMI Filter
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Ferrite Bead EMI Filter
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3.3uH Inductor Filter
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EMI Summary• Power switching generates EMI
– 10GA/s or larger switching transients– Induces 10s of V of noise across local parasitics
• 50V from one end of a ground plane to another observed
• Take differential measurements at quiet times– Directly across component of interest– With a good instrumentation amplifier
• Use isolated gate drivers and Kelvin source connections• Protect inputs and outputs • Filter inputs and outputs – with ferrite beads to block high frequencies and large
inductors for lower freqeuncies
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YAH
No Date Topic HWout HWin Labout Labck Lab HW1 9/26/16 Intro(basicconverters) 1 1 IntrotoST32F3 PeriodicSteadyState2 9/28/16 EmbeddedProg/PowerElect.3 10/3/16 PowerElectronics-1(switches) 2 1 2 1 ACEnergyMeter PowerDevices4 10/5/16 PowerElectronics-2(circuits)5 10/10/16 Photovoltaics 3 2 3 2 PVMPPT PVSPICE6 10/12/16 FeedbackControl7 10/19/16 ElectricMotors 4 3 4 3 MotorcontrolMatlab Feedback8 10/21/16 IsolatedConverters9 10/24/16 SolarDay 5/PP 4 5 4 Motorcontrol-Lab/ IsolatedConverters10 10/26/16 Magnetics11 10/31/16 SoftSwitching 6 5/PP 6 5 PS MagneticsandInverters12 11/2/16 ProjectDiscussions13 11/7/16 Inverters,Grid,PF,andBatteries 6 P 6 Project14 11/9/16 Thermal&EMI15 11/14/16 QuizReview C116 11/16/16 Grounding,andDebuggingQ 11/16/16 Quiz-intheevening
11/21/16 ThanksgivingBreak C211/23/16 ThanksgivingBreak
17 11/28/1618 11/30/16 MartinFornage,enPhase C319 12/5/16 ColinCampbell,Tesla20 12/7/16 Wrapup
12/15/16 Projectpresentations P12/16/16 Projectwebpagedue 39