11

Final Year ProjectFinal Year Project

Development of method for improving the Development of method for improving the light-load efficiency of VRM’slight-load efficiency of VRM’s

Project Project Supervisor :Supervisor :Dr. Maeve DuffyDr. Maeve DuffyMarch 2009March 2009

22

VRMVRM

Voltage Regulator ModuleVoltage Regulator Module Switching Mode Power Supply for MicroprocessorSwitching Mode Power Supply for Microprocessor The most suitable topography for a VRM The most suitable topography for a VRM

is a Multiphase Synchronous Buck Converteris a Multiphase Synchronous Buck Converter

33

Microprocessor Light Microprocessor Light LoadLoad

Power States Power States Optimize the runtime power consumptionOptimize the runtime power consumption

Sleep States Sleep States Idle Power ManagementsIdle Power Managements

44

Trends in VRM Design Trends in VRM Design is Microprocessor is Microprocessor LoadsLoads Moore’s Law will apply into the future - IntelMoore’s Law will apply into the future - Intel Lower core voltages and higher currents Lower core voltages and higher currents Analysis of recent VRM guideline from IntelAnalysis of recent VRM guideline from Intel

Design Design Guideline Guideline

VRM 9.1 VRM 9.1 VRM 10.0 VRM 10.0 VRM 11.0 VRM 11.0

Release Release DateDate

20022002 20052005 20082008

IccIcc 75A75A 85A85A 130A130A

Icc(max)Icc(max) 81A81A 100A100A 150A150A

Icc(step)Icc(step) 54A54A 70A70A 100A100A

dIcc/dtdIcc/dt 450A/uS450A/uS 560A/uS560A/uS 1200A/uS1200A/uS

VIDVID 1.1-1.85V 1.1-1.85V 0.8375 - 1.6V 0.8375 - 1.6V 0.5 - 1.6V 0.5 - 1.6V

55

Synchronous Buck Synchronous Buck Converter EfficiencyConverter Efficiency

Sources of Losses in the power stageSources of Losses in the power stage Upper and Lower MOSFET sUpper and Lower MOSFET s Output InductorOutput Inductor Output CapacitorOutput Capacitor

Cout RLoad

L11 2

66

Analysis of MOSFET Analysis of MOSFET LossesLosses

Conduction for Losses for Upper Conduction for Losses for Upper and Lower MOSFET sand Lower MOSFET s

Switching Losses for Upper MOSFETSwitching Losses for Upper MOSFET Lower MOSFET Body Diode conduction lossesLower MOSFET Body Diode conduction losses Reverse Recovery Losses Reverse Recovery Losses Gate Driver LossesGate Driver Losses

77

Capacitor and Inductor Capacitor and Inductor Losses Losses

Inductor LossesInductor Losses DCR and ACRDCR and ACR Hysteresis LossesHysteresis Losses Eddy Current LossesEddy Current Losses

Capacitor LossesCapacitor Losses ESR (Equivalent Series Resistance)ESR (Equivalent Series Resistance)

88

SPICE Modelling of SPICE Modelling of Power LossesPower LossesObjective is to use SPICE to measure MOSFET Objective is to use SPICE to measure MOSFET

switching losses from the instantaneous switching losses from the instantaneous products of current and voltageproducts of current and voltage

T

avg dttitvT

P0

)()(1

99

Shoot Through during Shoot Through during Upper MOSFET Switch OnUpper MOSFET Switch On

Time

493.0us 493.1us 493.2us 493.3us 493.4us 493.5us 493.6us 493.7us 493.8us 493.9us 494.0usI(U11:D) I(U13:D)

-20A

-10A

0A

10A

20A

30A

40A

50A

Time

493.0us 493.1us 493.2us 493.3us 493.4us 493.5us 493.6us 493.7us 493.8us 493.9us 494.0usI(U11:D) I(U13:D)

-20A

-10A

0A

10A

20A

30A

40A

50A

Drain Source CurrentDrain Source Current

Upper ONUpper ON Upper OFFUpper OFF

Upper ONUpper ON Upper OFFUpper OFF

1010

Eliminating Shoot Eliminating Shoot ThroughThrough

Introduction of dead eliminated Introduction of dead eliminated shoot though on lower MOSFET shoot though on lower MOSFET switch on but not on upper MOSFET switch on but not on upper MOSFET switch onswitch on

A possible cause of shoot through A possible cause of shoot through on upper MOSFET switch on is Dv/dt on upper MOSFET switch on is Dv/dt induced turn on of lower MOSFET.induced turn on of lower MOSFET.

Alan Elbanhawy - Fairchild Alan Elbanhawy - Fairchild SemiconductorSemiconductor

1111

Dv/dt Induced Turn On Dv/dt Induced Turn On of Lower MOSFETof Lower MOSFET

dt

tdvCti

)()(

1212

MathCAD Efficiency MathCAD Efficiency CalculatorCalculator

MathCAD displays equation and expression in MathCAD displays equation and expression in graphically form, and is aware of SI units.graphically form, and is aware of SI units.Easier to track calculations in MathCADEasier to track calculations in MathCAD

Used a MathCAD worksheet to calculate the Used a MathCAD worksheet to calculate the efficiency of a single phase of a synchronous buck efficiency of a single phase of a synchronous buck converterconverter

The original MathCAD worksheet was a The original MathCAD worksheet was a supplement to a paper called supplement to a paper called “What MOSFET Can “What MOSFET Can Do to Boost the Performance of VRM Design”Do to Boost the Performance of VRM Design”

Alan Miftakhusdinov - Texas InstrumentsAlan Miftakhusdinov - Texas Instruments

1313

Generating Rds(0n) Generating Rds(0n) from SPICE MOSFET from SPICE MOSFET ModelModel

2 4 6 8 10 120

10

20

30

40

Upper MOSFETLower MOSFET

Rds Upper and Lower Mosfet

Gate source Voltage

Dra

in s

ou

rce r

esis

tan

ce

Rds1

Rds2

Vgsx

Rds (0N) for upper and lower MOSFET s

1414

Distributed InductorsDistributed Inductors

Paralleling the output inductorParalleling the output inductor Investigating if losses are reduced Investigating if losses are reduced

if inductor are switched out if inductor are switched out during periods of light loadsduring periods of light loads

The effective inductance of the The effective inductance of the output inductor changes when output inductor changes when inductors are switched out.inductors are switched out.

1515

Effects of changing Effects of changing inductance of inductance of

Output Ripple Current Output Ripple Current Output Capacitor ESR LossesOutput Capacitor ESR Losses

IL

Increasing L

Reducing L

Io

1616

ConclusionConclusion

Switching Inductors

0.00

1.00

2.00

3.00

4.00

5.00

6.00

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Current per phase Amps

Lo

sses

per

Ph

ase

Wat

ts 1 Inductor

2 Inductors

Using Distributed Inductor and switching out Using Distributed Inductor and switching out inductor during periods of light load does inductor during periods of light load does improve VRM efficiencyimprove VRM efficiency

1717

QuestionsQuestions