Efficiency Improvement in Redundant Power Systems by Means of Thermal Load Sharing

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: 6 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.