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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
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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