1 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

2 Load Sharing Power System Evaluation Current Sharing Thermal Load Sharing Reliability ConclusionOutline

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 Sharing

4 Power System Evaluation Number of parallel-connected units to use: Power ’overshoot’ Circuit complexity Component count Overall reliability Increasing N:

5 Power System Evaluation Power system under consideration: N+1 redundant system (N = 2) Output voltage = 5 V Maximum output current = 30 A RMS Single MOSFET buck topology Three different ON-resistances Power losses + Power dissipation Thermal evaluation

6 Power System Evaluation System equations and constraints:

7 Current 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 Sharing

9 Thermal Load Sharing Temperature sensing device is mounted on the MOSFET casing. Continuous Unequal reliability current optimization 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 Sharing

11 Temperature distribution for reliability evaluation: T Ambient = 40  C T S-avg, current =  C T S-avg, thermal = 95.7  C Resulting unavailabilities: Current Sharing Thermal Load Sharing Complex calculationsReliability

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.Conclusion