A high precision, wide output range, programmable and efficient power supply

The implementation of a wide output range, programmable and efficient DC-to-DC power converter with 16-bit linearity is investigated. A medium power unit (/spl sim/200 W) and a high power one (2 kW) were built and fully characterized. These power converters are intended to be the power supply of the coils of a multipolar magnet for the generation of the magnetic field required for a magnetic resonance imaging system. A mixed switch-mode and linear technique, i.e. the combination of a switching preregulator and a linear final regulator, makes it possible to achieve high accuracy and good efficiency. The linear section is a high performance programmable current generator based on a MOS output stage in the feedback loop of an operational amplifier. The output of the switch-mode section (a conventional buck cell for the medium power version or a quadratic cell for the high power one) is regulated by an outer feedback loop that stabilizes the voltage drop across the MOS at the minimum value required for linear operation, thus allowing fair efficiency. Design criteria and experimental results are reported.