Linearization of the Transfer Characteristic of a Phase- Controlled Converter Under Discontinuous Conduction

An open-loop control circuit is described which, in conjunction with a cosine wave crossing technique, provides a linear transfer characteristics of a phase-controlled converter under both continuous and discontinuous conduction, regardless of ωL/R values of the load. The nonlinear transfer characteristic between the signal and the output voltage at discontinuous conduction has been counteracted by an inversely nonlinear control function for different ωL/R values. The control functions have been approximated by piecewise linear segments, synthesized by analog circuits, and identified by digital addresses. A complete control circuit has been designed and tested with a single-phase center-tap converter. The experimental results agree well with the theory. The principle can be extended to polyphase and multiquadrant converters.