EE155/255 Green Electronics

Thermal and EMI11/7/16

Prof. William DallyComputer Systems Laboratory

Stanford University

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

PE Tech Times, Nov 9, 2016

Thermal Design

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

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.

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

Questions• What is the temperature rise of the FET above the ambient air

temperature?

• How hot is the exhaust air temperature?

Determine Thermal Resistance of Heat Sink

HeatSinkFinArea 0.0128m^2Coeff 50W/m^2KProd 0.64W/KRes 1.56K/W

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

Determine Temp Rise of Air

AirTempRisePower 10WAirFlow 400ml/sE/Vol 2.50E-02J/mlCapacityofAir 1.00E-03J/mlKTempRise 25K

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?

In Practice• This is done by finite-element modeling (ANSYS, etc…)

– Answer is as good as the model.• Backed up by experiment

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

Littlebox Thermal Design

Fan Selection - Minimum Airflowto Mantain Outlet Temperature

PF38281BX-000U-S99

12V 6W 38mm x38mmx25mm

Full Box Simulations

Windtunnel test config pictures

Thermocouple readout

Anemometer

V_dspower supply and multimeter

Wind tunnel testing area

Fan controls

ThinPak

Vias A-Pli Gap Filler

Shim + solder

UltrastickGrease

Heat sink

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?

Results

OOPS

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…

EMI

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

Cause – Fast Current Transients

LP

C2M2

C1

M1 C3

Cause – Fast Current Transients

LP

C2M2

C1

M1 C3

Cause – Fast Current Transients

LP

C2M2

C1

M1 C3

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

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

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

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

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

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

Protection Circuit

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

An EMI Filter

Ferrite Bead EMI Filter

3.3uH Inductor Filter

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

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