A constant power rectifier for telecommunications using a novel variable turns ratio transformer

The power requirements of modern telecommunications equipment are explored and the need for good half load efficiency and constant power output over a wide DC voltage range is established. This paper then examines the various methods of high frequency AC voltage transformation which can be used within the isolated DC-DC converter stage of a typical power factor corrected rectifier design. For each method considered, the conduction losses in the AC source resistance (switching transistors) are calculated. When the requirement for these conduction losses to be minimised over the voltage transformation range of n:1 through n:1.45 is included, both pulse width modulation (PWM) and series resonant techniques are seen to be far from optimum. Switching losses are ignored in this analysis. The novel aspect of this paper is the presentation of a DC-DC converter stage which makes electronic approximation of a variable transformer having no moving parts and not involving saturating core techniques. Finally it is shown how this invention has allowed the commercial realisation of a 48V, 2.9kW rectifier having an output range of 40V to 70V, with a constant power output over the range 48V to 70V and an efficiency of 91.5% at 54V and half load.