Heat Generation in Electronics Thermal Management of Electronics Reference: San José State University Mechanical Engineering Department.

Heat Generation in Heat Generation in ElectronicsElectronics

Thermal Management of ElectronicsThermal Management of ElectronicsReference: Reference:

San José State UniversitySan José State UniversityMechanical Engineering DepartmentMechanical Engineering Department

Heat in ElectronicsHeat in Electronics

Heat is an unavoidable by-product of Heat is an unavoidable by-product of operating electronicsoperating electronics

Effects of increased temperature in Effects of increased temperature in electronicselectronics Decreased reliability Decreased reliability Parametric changes may occur in an Parametric changes may occur in an

electronic device’s componentselectronic device’s components

Power DissipationPower Dissipation

Current flowing through active and passive Current flowing through active and passive components results in power dissipation components results in power dissipation and increased temperaturesand increased temperatures

The amount of power dissipated by a The amount of power dissipated by a device is a function of:device is a function of: The type of deviceThe type of device The geometryThe geometry The path from the device to the heat sinkThe path from the device to the heat sink

Components Where Power Components Where Power Dissipation OccursDissipation Occurs

Passive DevicesPassive Devices ResistorsResistors CapacitorsCapacitors InductorsInductors TransformersTransformers

Active DevicesActive Devices TransistorsTransistors Integrated Integrated

CircuitsCircuits

InterconnectionsInterconnections

General TheoryGeneral Theory

Power dissipated will be a function of the Power dissipated will be a function of the type of current that it receivestype of current that it receives

For DC:For DC:

VIP

device the across drop VoltageV

Amps in CurrentI

ondJoules/sec or Watts in PowerP

General TheoryGeneral Theory

For AC:For AC:

1

2

)()(1 t

tM

dttitvT

P

current the for conduction of limit Uppertcurrent the for conduction of limit Lowert

element the through voltage of value ousInstantanev(t)element the through current of value ousInstantanei(t)

Period Waveform TLoss Power MeanP

1

M

2

ResistorsResistors

SymbolSymbol

Power DissipatedPower Dissipated

LawsJouleRIPRIIP

VIPOhm's LawRIV

' )(

2

Temperature Coefficient of Temperature Coefficient of Resistance (TCR)Resistance (TCR)

TCR characterizes the TCR characterizes the amount of drift that amount of drift that takes place in takes place in resistance values over resistance values over temperature changetemperature change

TCR usually has such a TCR usually has such a small effect that (even small effect that (even over large temperature over large temperature gradients) that it can be gradients) that it can be ignored for resistorsignored for resistors

CapacitorsCapacitorsSymbolSymbol

The ideal capacitor would not dissipate The ideal capacitor would not dissipate any power under a DC current any power under a DC current

A real capacitor can be modeled with the A real capacitor can be modeled with the equivalent series circuit below:equivalent series circuit below:

CapacitorsCapacitors

There will be power There will be power dissipated due to the dissipated due to the equivalent series equivalent series resistance (ESR)resistance (ESR)

Power dissipation due Power dissipation due to equivalent series to equivalent series inductance is inductance is negligible compared negligible compared to ESRto ESR

Inductors and TransformersInductors and Transformers

Inductor symbolInductor symbol

Transistor symbolTransistor symbol

Two types of resistance associated with Two types of resistance associated with these devicesthese devices WindingWinding CoreCore

Resistance for Inductors and Resistance for Inductors and TransformersTransformers

Winding Resistance – Resistance that Winding Resistance – Resistance that occurs due to the winding on the componentoccurs due to the winding on the component

Core Resistance – Losses that occur due to Core Resistance – Losses that occur due to use of a ferromagnetic coreuse of a ferromagnetic core Hysteresis Loss – Power dissipation due to the Hysteresis Loss – Power dissipation due to the

reversal of the magnetic domains in the corereversal of the magnetic domains in the core Eddy Current Loss – Heat generated from the Eddy Current Loss – Heat generated from the

conductive current flowing in the metallic core conductive current flowing in the metallic core induced by changing fluxinduced by changing flux

Active DevicesActive DevicesPower dissipation for all standard-product Power dissipation for all standard-product active integrated circuits can be obtained active integrated circuits can be obtained from:from: Device data sheets Device data sheets Calculated from laboratory measurementsCalculated from laboratory measurements

Bipolar devices – power dissipation is Bipolar devices – power dissipation is constant with frequencyconstant with frequency

CMOS devices – power dissipation is a 1CMOS devices – power dissipation is a 1stst order function of frequency and 2order function of frequency and 2ndnd order order function of device geometryfunction of device geometry

Power Dissipation in a CMOS GatePower Dissipation in a CMOS Gate

Power consumption is composed of three Power consumption is composed of three components:components:

Switching powerSwitching powerResults from charging and discharging of the Results from charging and discharging of the capacitance of transistor gates and interconnect capacitance of transistor gates and interconnect lines during the changing of logic stateslines during the changing of logic states

Comprises 70-90% of the power dissipatedComprises 70-90% of the power dissipated

Power Dissipation in a CMOS GatePower Dissipation in a CMOS Gate

Dynamic short-circuit powerDynamic short-circuit powerOccurs when pull-up or pull-down transistors are Occurs when pull-up or pull-down transistors are briefly on during a change of state in the output briefly on during a change of state in the output nodenode

Comprises 10-30% of dissipated powerComprises 10-30% of dissipated power

DC LeakageDC LeakageComprises 1% of dissipated powerComprises 1% of dissipated power

InterconnectionsInterconnections

Interconnections are the connections Interconnections are the connections between componentsbetween components

Power dissipated can be found with Power dissipated can be found with Joule’s Law where resistance of the Joule’s Law where resistance of the interconnection is given by:interconnection is given by:

A

LR

material onarea sectionalCrossA

cm in material of LengthL

cmohm iny Resistivit

Ohms in ResistanceR

Wire BondsWire Bonds

Low power devices (i.e. logic and small analog Low power devices (i.e. logic and small analog devices) usually have bonds fabricated from gold or devices) usually have bonds fabricated from gold or aluminum with a diameter of .001 inchaluminum with a diameter of .001 inch Negligible power is dissipated by a single bond but when Negligible power is dissipated by a single bond but when

many bonds exist these elements should not be ignoredmany bonds exist these elements should not be ignored

High power devices usually have aluminum bond High power devices usually have aluminum bond with diameters ranging from .005 to .025 incheswith diameters ranging from .005 to .025 inches Large amounts of power are dissipated from these bondsLarge amounts of power are dissipated from these bonds

Wire BondsWire Bonds

Ribbon BondsRibbon Bonds

Package PinsPackage Pins

Package pins are the physical connector Package pins are the physical connector on an integrated circuit package that on an integrated circuit package that carries signals into and out of an carries signals into and out of an integrated circuit integrated circuit Pins are made from low-resistance metal Pins are made from low-resistance metal and may be enclosed in glass or ceramic and may be enclosed in glass or ceramic beadbeadPower dissipate can still be calculate with Power dissipate can still be calculate with the relationship outlined for other the relationship outlined for other interconnectionsinterconnections

Package PinsPackage Pins

SubstratesSubstrates

Many different metallizations can be used Many different metallizations can be used for interconnections on substratesfor interconnections on substrates

Each metallization will have its own Each metallization will have its own resistance that will dissipate powerresistance that will dissipate power

Sheet resistivity is used in calculation due Sheet resistivity is used in calculation due to the fact that conductors are much wider to the fact that conductors are much wider than they are thickthan they are thick

SubstratesSubstrates

The resistance of a The resistance of a substrate can be substrate can be found with the sheet found with the sheet resistivityresistivity

Resistivity of the Resistivity of the conductors will vary conductors will vary with temperature with temperature (TCR may be (TCR may be important in some important in some substrate substrate calculations)calculations)

tB

s

W

LR s

film of thicknesst

hohms/lengttivity in Bulk resisρ

re ohms/squastivity inSheet resi

B

s

Various Substrate ConstructionsVarious Substrate Constructions

Substrate Metallization PropertiesSubstrate Metallization Properties

High-Frequency LossHigh-Frequency Loss

DC is evenly distributed DC is evenly distributed throughout a cross throughout a cross section of wiresection of wire

When frequency When frequency increases charge carrier increases charge carrier move to the edges move to the edges because it is easier to because it is easier to move in a conductor in move in a conductor in the edgethe edge

Resistance increases due Resistance increases due to the distribution of to the distribution of charge carrierscharge carriers