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Optimized Load Sharing Control by means of
Thermal Reliability Management
Carsten Nesgaard*
Michael A. E. Andersen
Technical University of Denmark
in collaboration with
*Currently with: International Rectifier HI-Rel Analog Devices
1
Outline
•
Load Sharing
•
Power System Evaluation
•
Current Sharing
•
Thermal Load Sharing
•
Reliability
•
Conclusion
2
Load Sharing
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
3
Power System Evaluation
Number of parallel-connected units to use:
60
PMax pr. unit (x) - PMax pr. unit (x  1) 
Power 'overshoot' reduction in %
PMax pr. unit (x)
 100
50
Increasing N:
40
• Power ’overshoot’
30
• Circuit complexity
20
• Component count
•Overall reliability
10
Complexity
index  LS circuitry index  x - Price index (x -1)  0.75
0
0
1
2
3
4
5
6
7
8
9
10
11
12
Number of units in N+1 system
4
Power System Evaluation
Power system under consideration:
Iin
Converter 1
(T1)
Converter 2
(T2)
Converter 3
(T3)
I1
I2
IOUT
I3
• N+1 redundant system (N = 2)
• Output voltage = 5 V
• Maximum output current = 30 ARMS
• Single MOSFET buck topology
• Three different ON-resistances
Power losses + Power dissipation
Rjc
Tj
Thermal evaluation
Rcs
TSurface
Tc
PRDS(ON)
PRadiation + P Convection
TAmbient
5
Power System Evaluation
System equations and constraints:
PR DS(ON)  I 2RMS  R DS(ON)
Rds(ON) ()
0.150
0.125

4
4
PRadiation  5,7 10 8  A  TSurface
- TAmbient
PConvection  1,34  A 
4

0.100
0.075
0.050
TSurface - TAmbient 5
h
0.025
-25
PRadiation(W)
25
50
75
100
125
150
Temperature
PConvection (W)
1.0
0.8
TAmbient
= 40oC
25
AHeatsink
= 20 cm. x 20 cm.
20
TAmbient
= 40oC
AHeatsink
= 20 cm. x 20 cm.
0.6
15
0.4
10
0.2
5
60
80
100
120
140
TSurface (oC)
60
80
100
120
140
TSurface (oC)
6
Current Sharing
DC/DC converter
Power loss calculations limited to
MOSFET conduction losses
Load
control
DC/DC converter
Load
control
Load sharing bus
Load
Additional losses to include:
• Current sensing resistor losses
DC/DC converter
• Switching losses
Load
control
• Diode losses
Input
Power
components
Current
meas.
• Other circuitry losses
Output
High side sensing
RMEAS
PWM control
Load share
control
OP-amp
R1
R2
R3
R4
LS controller
Ref [9] in the paper provides calculations
for the abovementioned losses.
- 9V
+ 9V
7
Current Sharing
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.
8
Thermal Load Sharing
DC/DC converter
Temp
Temperature sensing device is
mounted on the MOSFET casing.
Load
control
DC/DC converter
Load
control
Load
Load sharing bus
Temp
Continuous
reliability
optimization
DC/DC converter
Temp
Unequal
current
distribution
Load
control
Allows for:
Input
2,7V - 20V
TSense
Power
components
Part of
PWM control
R1
R2
Load share
control
Current
meas.
Output
Power system realization
Different operating
by means of converters
environments within
with different power
the power system
ratings
Equal ”operating” temperature
9
Thermal Load Sharing
Another advantage of the thermal load sharing is the dynamic power
throughput capability:
VTEMP
C1
ISENSE
Power
components
Input
R1
Current
meas.
Output
Current Limit (I LIM)
R2
R
2
VTemp . R1+R
2
I' SENSE
PWM control
t
0
+
0
t
Load share
control
ILimit
0
IOUT
IMAX
t
LS controller
Temperature
TMAX
Load sharing is now based on both current and thermal information.
10
Reliability
Temperature distribution for reliability evaluation:
Heatsink
Misc. components
Transformer
Transistor
IC
TAmbient
IC
PCB
Temperature
TTransformer
TSurface
TEnd of PCB
TAmbient
= 40C
TS-avg, current = 104.4C
TS-avg, thermal = 95.7C
TIC
Distance
Complex
calculations
Resulting unavailabilities:
Current Sharing
P  1 - ProbSystem  1 - 0.9740  0.0260  2.60%
Thermal Load Sharing
P  1 - ProbSystem  1 - 0.9874  0.0126  1.26%
11
Conclusion
•
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.
12