1. Efficiency Improvement in Redundant Power Systems by Means of Thermal Load Sharing
Carsten Nesgaard Michael A. E. Andersen
Technical University of Denmark
in collaboration with

2. Outline Load Sharing The Power System Experimental Verification
Efficiency
Reliability
Causes of power imbalance
Conclusion

3. 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

4. Load Sharing

5. The Power System Buck topology – simplicity of implementation
125 W converters – 5 V output at 25 A
5% output ripple voltage
4 IC’s – lowers overall system reliability
2 freewheeling diodes and 1 MOSFET
L = 48 H, COut = 200 F, RSense = 10 m

6. Experimental Verification
Duty cycle differences due to component tolerances, off-set voltages and temperature difference.
The output voltage that results is a combination of each converter’s output voltage.

7. Experimental Verification
Current distribution among the two converters as a function of total output current.
Current sharing: Thermal load sharing:

8. Experimental Verification
Power component loss distributions

9. Experimental Verification
MOSFET conduction and switching losses.
Both type of losses increase nonlineary with current and temperature.
Temperature dependance of MOSFET switching losses are described in [3]

10. Experimental Verification
Loss distribution as a function of combined losses.
Diode loss redistribution
MOSFET loss redistribution

11. Efficiency Initial ‘semi-droop’ method Current sharing
Thermal load sharing
The thermal load sharing efficiency
‘Semi-droop’ at low current levels
Current sharing technique at heavier loads but at a higher level.
Lowest temperature

12. Reliability Temperature distribution for reliability assessment
= Accumulated failure
rate per unit
R = Survivability
Q = Unavailability

13. Reliability Annual system downtime – current sharing: 10 min. 14 sec.
– thermal load sharing: min. 11 sec.
Change in unavailability (downtime):
Inserting values – an overall reduction of almost 40% can be calculated.
Achieved by simply choosing a different load sharing technique.

14. Causes of power imbalance
Possible causes of the power imbalance in the two-converter system:
Lower thermal contact between MOSFET and heat-sink
RDS(ON) incremental deviation among the two converters
Unequal switching losses among the two MOSFET’s
Diode parasitic deviations – causing imbalanced diode losses

15. Conclusion
The concept of thermal load sharing has been presented and analytically proven to enhance system reliability and efficiency.
Two parallel-connected buck converters controlled by a dedicated load share IC formed the basis for the experimental verification.
Theoretical evaluations of the experimental measurements provided the explanation for the efficiency gain.
Redistribution of the MOSFET transistor losses proved to be the major contributor to the increased efficiency.
Unequal thermal contact, differences in RDS(ON) and diode parasitic deviations are some of the possible causes.