© international rectifier 2002 1 directfet mosfets double current density in high current dc-dc...

© International Rectifier 2002 www.irf.com

1

DirectFET MOSFETs Double Current Density

In High Current DC-DC Converters

With Double Sided Cooling


2

Increasing Current Requirement In DC-DC Converters

DC-DC converters for next generation Intel and AMD microprocessors Trending towards >100A or 25A per phase Core processor voltages are stabilizing around 1V

Current and Clock Speed Trends of CPUs

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Processor Frequency

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3

The Challenge: How To Remove The Heat

Current requirements PCB size Less and less area to dissipate the heat 30% of RDS(on) of best SO-8 is due to package

Power Dissipated in a VRM Module

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4

Thermal Limitations Of Single-Sided Cooled SO-8

Top-side cooling is inefficient PCB is the main path of heat dissipation New SO-8 derivatives improve Rth(j-b), to make it easier to dissipate heat into PCB and lower junction temperature

– Fairchild BottomlessSO8™, Siliconix PowerPak™, leadless SO-8 PCB thermal conduction is at its limit now

Standard and derivative SO-8 can only be effectively cooled on 1 side

Rth(j-c): 18°C/W

Rth(j-pcb): 20°C/Wmax

+ Rth(pcb-a)>40 °C/W

SO-8Rth(j-a)top >55°C/W at 400 LFM

Actual measurement

Rth(j-a)bottom >60°C/W


5

What Is DirectFET Power Packaging?

Large area source and gate contacts are directly soldered onto the PCB board Copper drain clip facilitates the drain connection and also improves transfer of heat from

the top of the device. Proprietary passivation system for electrical isolation, soldering mask and die protection

copper track on board

copper ‘drain’ clip

passivated diedie attach material

gate connection

source connection

passivation

source pads

gate pad

solderable top metal

passivation

source pads

gate pad

solderable top metal


6

SO-81.7 mm

Cu-strap SO-8

DirectFET 0.7 mm

Best Package Resistance In Any SMT Package

Reducing Die-Free Package Resistance by over 90% over SO-8

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7

Enabling Efficient Dual Sided Cooling Increases the power dissipated out of the package by removing the

heat very efficiently through the top of the package.

Rth(j-c): 18°C/W

Rth(j-b): 20°C/Wmax

SO-8

Gap filling thermal pad to metallic chassisGap pad cannot justify cost with SO-8

+ Rth(pcb-a) >40 °C/W

DirectFETRth(j-a)top >55°C/W

at 400 LFM

Rth(j-pcb): 1°C/W

Rth(j-c): 3°C/W

Rth(c-a)gap pad <9°C/W

+

Rth(j-a)top<12°C/W

Rth(j-a)bottom >60°C/W Rth(j-a)bottom >41°C/W

+ Rth(pcb-a)>40 °C/W


8

Top side cooling benefits

Increases the power dissipated out of the top of the package by 50% compared to standard SO-8

– And by 100% when compared to Bottomless SO-8

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DirectFET SO-8 LeadlessSO-8

PowerPak HitachiLFPak

BottomlessSO-8

Pd to chassis [W]

Pd to board [W]

105°C PCB Temperature and 70 °C AmbientMinimum footprint. Gap filler.


9

Specifications And Availability

20VN for servers and very high end desk-tops. 30VN devices for notebooks and designers who require a higher

safety margin. Sample available immediately, volume production 3rd quarter 2002 Extensive reliability testing completed.

Part # BVDSS

ID @ 25oC

10VGS

RDS(on) typ

@10VGS QGD typ. Package Typical Application

IRF6601 20V 28A 2.7mOhm 20 nC DirectFETTM SYNC FET Servers

IRF6602 20V 14 A 10.0 mOhm 5 nC DirectFETTM CTRL FET Servers

IRF6603 30V 28 A 2.8 mOhm 13 nC DirectFETTM SYNC FET Notebooks

IRF6604 30V 15 A 8 mOhm @ 7VGS

3.7 nC DirectFETTM CTRL FET Notebooks


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50oC Cooler In Still Air! Measured results at max standard SO-8 current in still air


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35A/Phase Using 2 DirectFET MOSFETs! 3 applications for DirectFET MOSFETs:

– 35A/phase with a single pair of DirectFET MOSFETs using top side cooling– 33% more current in still air– 50°C cooler and up to 3% higher efficiency at same current level

DirectFET 30VN Vs 30VN SO-8 using same silicon technology12VIN, 1.3VOUT, buck Converter @300kHz, 105°C max PCB temperature

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Load Current (A)

Effi

cien

cy (%

)

IRF6604/ IRF6603 (200LFM andHeatsink)

IRF6604/ IRF6603 (still air, noheatsink)

IRF7811W/ IRF7822 (still air, noheatsink)

17A more!

125oC Tj

33% more105oC Tj

3%

75oC Tj


12

100A at 85% Efficiency

Efficiency For 4 Phase converter with DirectFEToperating at 500 kHz, input is 12 volts, output is 1.7 volts

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Amps

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cien

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85% at 100 Ampsin less than 4 sq inches


13

Double Current Density for 30A/phase!

50% smaller solution footprint 100% improvement in Current Density! In real circuit, can cut part count by up to 60% compared to best SO-8 (2

DirectFETs replace 5 SO-8) And lower system cost

Circuit 30A/phase Area [in2] Amps/in2

2-Phase Converter

Single Side Cooled SO-8*

5.5 10.8

Dual Side Cooled DirectFET

2.8 21.7

4-Phase Converter

Single Side Cooled SO-8*

10.5 11.4

Dual Side Cooled DirectFET

4.9 24.6

* including standard SO-8, Bottomless SO-8 or Leadless SO-8


14

Lower Total System Cost

Cost of heat removal using SO-8– Additional 1 oz. Cu for the whole PCB area

• @ >$0.25/sq. in (>$3.50/system)

– Additional fan ($2 to $3/system)– Using heat pipe and additional fan ($4 to $10/system)

DirectFET MOSFETs– Can use chassis for heat removal with no additional fan– Can use minimum PCB footprint and fewer devices

DirectFET MOSFETs cut the cost of heat removal by at least 50%– Reduce cost of heat removal– Cut the size of PCB


15

4-phase Converter Using DirectFET MOSFETs

Single DirectFET pair/phase for current up to > 25A/phase in a 3.8in x

1.25in footprint for a total of 100A. Existing best 60A. Low profile allows the DirectFET to be mounted on the back of the board

with heatsink and still stay within VRM outline specifications.

Heatsink added to remove heat from top of DirectFETTM

away from board


16

solder mask definedpads

drain rail

gate railSource rail

Additional drain contact and/or further mechanical stability

Ease of Layout

Easier board layout Easier to parallel


17

DirectFET can replace SO-8with minimal rework

Gate = Pin 4Source = Pin 1 - 3

Drain = Pin 5 - 8

DirectFET outlineVs

SO-8 Outline

A board can be reworked with a few

simple changes


18

Alternatives to dissipate heat through the top of the package

Forced air cooling + finned sink

Gap filling thermal pad to metallic chassis

Forced air cooling


19

Bend Strength Measurement

DirectFET far outlasted the ceramic capacitors on the board in this measurement Reliability testing demonstrate reliability similar to SO-8


20

DirectFET Package in Summary

Reduce package resistance Increase efficiencyWith MOSFET in sub-10mOhm RDS(ON),

the package becomes an important contributor to the resistance.

Improve thermal characteristicsEnables dual sided cooling in SMTRemove the heat from the device faster

and away from PCB

Reduce stray inductance To enable faster switching

frequencies and better response to current transients

Double current densityCut PCB by 50%

Cut part count by up to 60%Lower total system cost

Compatible with high volume manufacturing processes

Easy to design, easy to parallelDirectFET power

packagingEnvironmentally friendly Lead and bromide free

Low profile 0.7mm high

© international rectifier 2002 1 directfet mosfets double current density in high current dc-dc...

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