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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
2
• Load Sharing
• Power System Evaluation
• Current Sharing
• Thermal Load Sharing
• Reliability
• Conclusion
OutlineOutline
3
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:
5
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
6
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
7
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.
8
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
9
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
10
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
11
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
12
• 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