Supplementary charge circuit for dc-to-ac converter

Claim 1

A DC-to-AC converter for converting the potential from an external, variable-voltage DC source into an AC potential, the amplitude of said AC potential varying proportionally to the amplitude of said DC source and including circuitry to maintain satisfactory commutation when the potential of said DC source falls below the level at which satisfactory commutation is normally obtained, which comprises: at least two main semiconductor devices for alternately passing oppositely directed direct currents from an external, variablevoltage DC source to a load; circuitry for supplying potential from an external, fixed, supplemental DC voltage source, said variable voltage source and said fixed voltage source having one, similarly poled terminal connected in common; at least one commutation capacitor for alternately conditioning said at least two main semiconductor devices to effect switching of said oppositely directed direct current therebetween; at least two auxiliary semiconductor devices for applying the charge developed across said at least one commutation capacitor to said at least two main semiconductor devices, each of said at least two auxiliary semiconductor devices being similarly poled to, and connected across, a corresponding one of said at least two main semiconductor devices and across said variablevoltage DC source, said at least one commutation capacitor being connected between similar electrodes of at least one pair of main and auxiliary semiconductor devices; a commutation inductor divided into at least four sections, one section being serially inserted with said at least two main and said at least two auxiliary semiconductor devices, said commutation inductor forming a resonant circuit with said commutation capacitor; at least two commutation diodes connected directly across, and oppositely poled to, said at least two main semiconductor devices to aid the switching thereof; and at least one supplementary semiconductor device interconnecting the other, noncommon connected terminal of said fixed supplemental DC voltage source to the juncture of said at least one commutation capacitor and one of said at least two auxiliary semiconductor devices to supply a supplementary charging current for said at least one commutation capacitor only when the voltage of said variable-voltage DC source is lowered, to lower the amplitude of said AC potential, below the voltage at which satisfactory commutation would otherwise be obtained.