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Optimized Load Sharing Control by means of Optimized Load Sharing Control by means of Thermal Reliability ManagementThermal Reliability Management

Carsten Nesgaard* Michael A. E. Andersen

Technical University of Denmark

in collaboration with

*Currently with: International Rectifier HI-Rel Analog Devices

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• Load Sharing

• Power System Evaluation

• Current Sharing

• Thermal Load Sharing

• Reliability

• Conclusion

OutlineOutline

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Load sharing is utilized when applications call for:

• Modular structure – increase maintainability

• Simple power system realization

• Short time to market

• Increased reliability – redundancy and fault tolerance

• High-current low-voltage applications

• Distributed networks

Load SharingLoad Sharing

4

0

10

20

30

40

50

60

0 1 2 3 4 5 6 7 8 9 10 11 12

Number of units in N+1 system

Po

wer

'ove

rsh

oo

t' re

du

ctio

n i

n %

Power System EvaluationPower System Evaluation

Number of parallel-connected units to use:

0.751)-(xindex Price -x indexcircuitry LS index Complexity

100

(x)unit pr. P

1)(xunit pr. P - (x)unit pr. P

Max

MaxMax

• Power ’overshoot’

• Circuit complexity

• Component count

•Overall reliability

Increasing N:

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Power System EvaluationPower System Evaluation

Power system under consideration:

Converter 1 (T 1)

Converter 2 (T 2)

Converter 3 (T 3)

I1

I2

I3

IOUTI in

• N+1 redundant system (N = 2)• Output voltage = 5 V

• Maximum output current = 30 ARMS

• Single MOSFET buck topology• Three different ON-resistances

P RDS(ON) P Radiation + P Convection

R jc R cs

T c T SurfaceT j

T Ambient

Power losses + Power dissipation

Thermal evaluation

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Power System EvaluationPower System Evaluation

System equations and constraints:

R d s (O N ) ()

0 .0 2 5

T e m p e ra tu re1 2 51 0 07 55 02 5 1 5 0-2 5

0 .0 5 0

0 .0 7 5

0 .1 0 0

0 .1 2 5

0 .1 5 0

P C o n ve ctio n (W )

5

1 0

1 5

2 0

2 5

T S u rfa ce (oC )

1 4 01 2 01 0 08 06 0

T Am b ie n t = 4 0oC

A H e a ts in k = 2 0 c m . x 2 0 cm .

P R a d ia tio n (W )

0 .2

0 .4

0 .6

0 .8

1 .0

T S u rfa ce (oC )

1 4 01 2 01 0 08 06 0

T Am b ie n t = 4 0oC

A H e a ts in k = 2 0 c m . x 2 0 c m .

DS(ON)2RMSR RI P

DS(ON)

4

5AmbientSurface

Convection h

T - TA1,34 P

4Ambient

4Surface

8Radiation T - TA015,7 P

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Powercomponents

PWM control

Load sharecontrol

Currentmeas. OutputInput

R MEAS

+ 9V

- 9V

LS controller

R 3

R 1 R 2

R 4

OP-amp

High side sensing

DC/DC converter

Loadcontrol

DC/DC converter

Loadcontrol

DC/DC converter

Loadcontrol

Load

Load

sha

ring

bus

Current SharingCurrent Sharing

Power loss calculations limited to MOSFET conduction losses

Additional losses to include:

• Current sensing resistor losses

• Switching losses

• Diode losses

• Other circuitry losses

Ref [9] in the paper provides calculations for the abovementioned losses.

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Theoretical advantages of the current sharing technique include:

• Equalization of current stress

Among the disadvantages of the technique are:

• Non-equalized thermal stress• Non-optimized overall system reliability• High side sensing in non-isolated systems• Added control circuitry• Increased component count

Transition to thermal load sharing is straight forward, since the same load share controller can be utilized.

Current SharingCurrent Sharing

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Load

sha

ring

bus

DC/DC converter

DC/DC converter

DC/DC converter

Load

Loadcontrol

Temp

Loadcontrol

Temp

Loadcontrol

Temp

Powercomponents

PWM control

Load sharecontrol

Currentmeas. OutputInput

2,7V - 20V

R 1

R 2

T Sense

Part of

Thermal Load SharingThermal Load Sharing

Temperature sensing device is mounted on the MOSFET casing.

Continuous Unequal reliability currentoptimization distribution

Allows for:

Power system realization by means of converters with different power ratings

Different operating environments within the power system

Equal ”operating” temperature

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Another advantage of the thermal load sharing is the dynamic power throughput capability:

Load sharing is now based on both current and thermal information.

Thermal Load SharingThermal Load Sharing

Powercomponents

PWM control

Load sharecontrol

Currentmeas. OutputInput

Current Limit (I LIM )

IMAX

IOUT

TemperatureT MAX

LS controller

V TEMP

R 1

R 2

C 1ISENSE

0

V Temp.

0+

ILimit

0

R 2

R 1+R 2I'SENSE

t

t

t

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Temperature distribution for reliability evaluation:

TAmbient = 40C

TS-avg, current = 104.4C

TS-avg, thermal = 95.7C

Transformer

Heatsink

Transistor

ICIC

Misc. components

Temperature

Distance

T SurfaceT Transformer

T Ambient

T IC

PCB

T End of PCB

Resulting unavailabilities:

Current Sharing

Thermal Load Sharing

Complex calculations

2.60% 0.0260 .97400 - 1 Prob - 1 P System

1.26% 0.0126 .98740 - 1 Prob - 1 P System

ReliabilityReliability

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• Three parallel-connected buck converters controlled by a dedicated load share IC formed the basis for the theoretical assessment.

• The point of origin was a power system controlled by a current sharing scheme.

• Concept of thermal load sharing: Presented and analytically proven.

• After transition to thermal load sharing the power system improved significantly reliability-wise.

• The gain in reliability is solely due to a much lower operating temperature.

• Efficiency improved due to redistribution of losses.

ConclusionConclusion