The Making of The Perfect MOSFET

Alan ElbanhawyFairchild Semiconductor

PESC 2006 June 21, 2006

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Outline

• MOSFET Components• Silicon Losses• Packaging Related losses• Gate Driver Losses• Integration and Design Optimization• Conclusion

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MOSFET Components

Silicon

Package

Gate Driver

• Parasitic Inductances• Parasitic Resistances• Skin Effect• Thermal Resistance• Footprint• Price

• Conduction Losses• Dynamic Losses

• Distributed Parameters Effects, Rg

• Shoot Through• Reverse Recovery Losses• Price

• Rise and Fall Times• Sink and Source Currents • Source Resistance• Parasitic Inductances•Thermal Resistance• Footprint• Price

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:= RonOpt VinputRg FOMKDrive fsw

Iload Vout Vdrive

Optimum on Resistance RDS(ON) as a function of load current and input voltage for the top MOSFET

Power dissipation as a function of the on resistance RDS(ON) and the load current

Conduction Losses

Optimum Range for RDS(ON)

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Distributed and segmented parameters model

C

L

1 2 GateDriver

Synchronous Rectifier

Control MOSFET

R g 10

S 9

R g 9

0

S 1 0

0

C

G a t e D riv e r

1 2

S 1S 2

R g 2

L1 2

R g 1 0

H S

MOSFET Outlines

Gate lead

R g C g d

C g s

Q

C g d

C g s

RRRRRRRRRR d riv e r

C g dC g dC g dC g dC g dC g dC g dC g dC g d

C g sC g sC g sC g sC g sC g sC g sC g sC g s

D

2 14 36 58 710 9

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Effect of distributed Rg-Cgs and die current Density

Gate voltage and drain currents at different Segments

Vgth

Uneven Power dissipation across the die during turn on!

The first and last segment currents for different combinations of Rg and Cgs

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Clearly tri = Rg*Cgs*Constant. This equation shows that the current rise time is directly proportional to Rg*Cgs which dictates that both parameters must be minimized for better performance

Effect of Rg on Current Rise Time

Current Rise time, tri

tri

LambertW e

Vp Gm Cgs Rg tr Gm Vgth tr Gm IloadVp Gm Cgs Rg

Vp Gm Cgs Rg tr Gm Iload :=

Gm

LambertW e

Vp Cgs Rg Vgth trVp Cgs Rg

Vp Cgs Rg Vp Gm/( )

Current Fall time, tif

:= tif

ln

Vgth

VpRg Cgs

ln

Gm Vgth Gm Iload

Vp GmRg Cgs

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Effect of Rg on MOSFET Vds Rise and Fall Times

VDS Rise time

VDS Fall time

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Lab Verification, tri, trv

Rg = 0 Ohm, Current Rise time

Rg=4.7 Ohm, Current rise time

Vds Rise time

Vds Rise time

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Shoot Through in Synchronous Buck Converter

1 2

M1

L11 2

1 2

M2

0

C

• Conduction Losses• Dynamic Losses

• Distributed Parameters Effects, Rg

• Shoot Through• Reverse Recovery Losses• Price

1 2

M1

L11 2

1 2

M2

0

C

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Segm

ent Instantaneous P

ower

Gate Threshold Voltage

Gate-S

ource V

oltage

Segments Currents

Segments gate-source voltage

Distributed Parameters Model Solution, Voltages and Currents

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Distributed Rg Influence on Shoot Through Current

Drain Current

Gate-SourceVoltage

Gate thresholdVoltage

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Gate threshold voltage Vgth

Drain current

Gate to source voltage

Distributed CGD Influence on Shoot Through Current

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Distributed CGS Influence on Shoot Through Current

Gate thresholdVoltage

Gate-SourceVoltage

Drain Current

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Lumped parameters Model with Parasitics

C

L

1 2 GateDriver

Synchronous Rectifier

Control MOSFET

Full parasitic model

Added source and gate inductances

only

Simple model

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Loop Inductance Effects on Shoot Through

Loop Inductance Influence on Shoot Through Current

-2.00E+01

0.00E+00

2.00E+01

4.00E+01

6.00E+01

8.00E+01

1.00E+02

1.20E+02

1.40E+02

1.60E+02

9.89E-06 9.90E-06 9.91E-06 9.92E-06 9.93E-06 9.94E-06 9.95E-06 9.96E-06 9.97E-06 9.98E-06

Time

Dra

in C

urr

ent

- A

mp

0.2nH

0.4nH

1.0nH

2.0nH

4.0nH

6.0nH

8.0nH

10nH

No Inductor

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Reverse Recovery Current

-4.00E+01

-3.50E+01

-3.00E+01

-2.50E+01

-2.00E+01

-1.50E+01

-1.00E+01

-5.00E+00

0.00E+00

5.00E+00

9.90E-06 9.91E-06 9.91E-06 9.92E-06 9.92E-06 9.93E-06 9.93E-06 9.94E-06

Time - Seconds

Cu

rre

nt

- A

mp

Loop Inductance 0nH – 10nH

Loop Inductance Effects on Reverse Recovery

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Parasitics, Current Parasitics, Current rise and fall timesrise and fall times

Parasitics, Current Parasitics, Current sharingsharing

Fall time as a function of the Source inductance Ls and Load Current IL Vgth=1.5, gm=30

Source Inductance Effects

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Low Drain Current

High Drain Current

Source Inductance Effects

trg

LambertW ( )gm Kd Ls Vgth e

Kd gm

2Ls gm Vgth IL

Kd gm2Ls

gm Kd Ls

:=

Kd gm2Ls Kd gm

2Ls gm Vgth IL gm Kd

( )

:= tfg 2 Ls a IL a Ls Vgth

ln

a Vgth2

IL 2 Vgth a IL

a Vgth2

PdHS :=

IL2 ( )RdsonHS( )1 ( )TmaxHS Tamb Rpackage 0.5 Vcc IL fs ( )tr1 tf1

Rise time Fall time

Power Dissipation

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Parasitic Resistance and Skin Effect

Ts Ts t

Ipk i(t)

:= Id Ipk

n 1

2 Ipk ( )sin n ( )cos n tn

The Fourier series for a square wave

for n=20 and n=200

Frequency Spectrum

-1

-0.5

0

0.5

1

1.5

2

2.5

3

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49

Harmonic

Am

plit

ud

e

Square wave synthesis

Square wave Frequency Spectrum

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Package Effects, Parasitics and Skin Effects

Parasitic resistance as a function of frequency

BGA

49568.17374922.8 59179.030982.095504.0FF

DS eeR

09916.8085494.9275894.94F

DS eR

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Skin Effect and Power Loss for HS MOSFET

Conduction loss (z-axes) as a function of the fundamental switching frequency f and the silicon on-resistance for an

The percentage error (z-axes) as a function of the switching frequency f and the silicon on-resistance for a

BGA 5 x 5.5 mm Package

SO8 Package

:= Pc Il2 ( )Rdson DC

:= Pcf Ipk2 2

( )Rdson DC

n 1

2 Ipk2

( )sin n 2( )Rdson f

n2 2

:= ErrorPercent100 ( )Ipk

2 ( )Rdson DC Pcf

Ipk2 ( )Rdson DC

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Package Thermals

• JC = 0.46 C/W• Heat sinking from the top of the package• Very low footprint and profile

Power Ball Grid Array BGA package is an example of the modern packages to address all the requirements of high frequency and high power densities modern DC-DC converters

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Gate Driver Influence on Losses

Uneven current

distribution

:= VgOpt1

2

Id2

bt Vo

VinId

2bb

1

Vo

Vin

( )Cint Cinb fs

Rg = 6

Rg = 4

Rg = 2

Rg = 6

Rg = 4

Rg = 2

Fall Time

Rise Time

Optimum gate Drive voltage

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Parasitic Drain Inductance EffectsParasitic Drain Inductance Effects

• Ls is PCB trace and package inductance, Cp is MOSFET Coss and stray capacitanceLs is PCB trace and package inductance, Cp is MOSFET Coss and stray capacitance

• For tr & tf >> tFor tr & tf >> tresres : :

• Overstress voltage = Overstress voltage =

• Power Dissipation = Power Dissipation =

For 20A, 10nH, 1nF and 1MHz:For 20A, 10nH, 1nF and 1MHz:

Overstress voltage = 60Volt Overstress voltage = 60Volt

Power Dissipation =4W = 4Power Dissipation =4W = 4

VdrainVdrain IindIind

Vin

L1 2

C

0

I

1

2L ID

2fs

IDL

C

Current as a function of L and time

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MOSFET Driver Comparison

70

72

74

76

78

80

82

84

86

88

90

0 20 40 60 80 100 120 140

Load Current

Eff

icie

nc

y %

Driver #1

Driver #2

Driver #3

Why match MOSFETs and gate drivers?

• Use one VRM board with three different drivers• All MOSFETs, Inductors and Filter capacitors are identical in all three cases• The very same PCB• The same test setup and test condition on an ATE

How will the efficiency curves be different?

Gate Driver Influence on Losses

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Current Density (A/m^2)Top MOSFET On

Current Density (A/m^2)Bottom MOSFETs On

Integration and Design Optimization

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Conclusion

The pursuit of the perfect MOSFET must be launched simultaneously on three fronts, silicon, package and gate driver.

The optimization of the MOSFET parameters requires an extremely careful consideration of the individual parameter and its effect on all the loss mechanisms in a given power MOSFET and all of these in conjunction with each other and the effect of a given combination of these parameters on the overall performance of the device in the intended application which in our case is a synchronous buck converter

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Thanks for your attentionThanks for your attentionQuestions?Questions?