"A Switch Mode Power Supply"

A companion paper (New and Wittey) describes a new Power Equipment Rack concept. The present paper describes the design of a SMPS for the new rack. The SMPS operates on 240V 50Hz s.p. A.C., producing a nominal 28A D.C. at a preset level between 52 and 58V D.C. The active element is a power transistor half-bridge switching the transformer primary winding at 20kHz between the centre tap of series connected capacitors across a D.C. link, using PWM for load-dependent regulation. The D.C. link is provided by a f.w. diode bridge across the line, feeding a small reservoir capacitor, topped up by a shunt-transistor (boost) converter giving line-dependent pre-regulation. The rectified transformer output (40kHz) after filtering for ripple and RFI may be used to provide an output of either polarity. A low power auxiliary inverter provides isolated supplies for the transistor base drives. The module is totally enclosed for better mechanical protection and freedom from RFI, and has a single full length `thermal wall' heat sink carrying insulated devices, which forms the mechanical `back bone' of the module. 95% of the drilling and tapping operations are carried out in this heat sink using a steel jig. Almost all interconnections are made using PCB tracks, leaving only a few wire links between PCB and devices. Overall efficiency is better than 90% at f.l.c. and the psophometric ripple is about 1mV. Line power factor is better than 0.97 and fold back current limit below 40V cuts the current back to 2.