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The cryogenic behaviour of five 200 V rated HEXFET R ⃝ MOSFETs between the temperatures of 20 K to 100 K are presented, including the on-state resistances, breakdown voltages and electro-thermal behaviour. The results indicate negative temperature dependent behaviour for the onstate resistances below 70 K and non-ohmic behaviour below 40 K. The physical reasons behind these phenomenon were suggested to be the combined behaviour of temperature dependent electron mobility, carrier freeze-out and electric field assisted thermal ionisation. The classically assumed linear temperature relationship for the breakdown voltages was found to be invalid below 100 K and a better model was suggested. A simple analysis into the utilisation of these power MOSFETs that considers the electro-thermal behaviour was discussed and parallel MOSFETs configuration was suggested to optimised the temperature dependent power loss at cryogenic temperatures.

This paper proposes a new design approach to the conventional two stages solution to convert the 48-60V low voltage bus to the 1-1.8V point of load (PoL) voltage for microprocessors in large telecoms server farms. The core idea is to stack or cascade the buck voltage regulator module (VRM) input voltage in series and connect their output in parallel. This single stage multi-phase cascaded buck (MCB) VRM has a large degree of design freedom depending on the load requirements. A 150W 6-phase MCB (two 3 phase modules in parallel) achieved a maximum efficiency of 91.3% in simulation. This paper also presents the results of a demonstrator of the same circuit which achieved a measured peak efficiency of 90.1%.

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