Optimized Load Sharing Control by means of Thermal

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: Increasing N: • Power ’overshoot’ • Circuit complexity • Component count • Overall reliability 4

Power System Evaluation Power system under consideration: • 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 Thermal evaluation 5

Power System Evaluation System equations and constraints: 6

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. 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 Temperature sensing device is mounted on the MOSFET casing. Continuous reliability optimization Unequal current distribution Allows for: 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 thermal load sharing is the dynamic power throughput capability: Load sharing is now based on both current and thermal information. 10

Reliability Temperature distribution for reliability evaluation: TAmbient = 40 C TS-avg, current = 104. 4 C TS-avg, thermal = 95. 7 C Complex calculations Resulting unavailabilities: Current Sharing Thermal Load Sharing 11

Conclusion • Three parallel-connected buck converters controlled by a dedicated load share IC formed the basis for theoretical assessment. • The point of origin was a power system controlled by a current sharing scheme. • Concept of thermal load sharing: Presented 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