Drive circuit for electroluminescent lamps

Claim 1

Apparatus for providing electrical power to a capacitive load comprising: a source of direct current; a first solid state switch device, said first switch device having first and second terminals and conductive and non-conductive states, a current flow path being established between said terminals when said switch device is in the conductive state, said first switch device being responsive to gating control signals for switching between said states; a second solid state switch device, said second switch device having first and second terminals and conductive and non-conductive states, a current flow path being established between said terminals when said switch device is in the conductive state, said second switch device being responsive to gating control signals for switching between said states; first connecting means for connecting a capacitive load between said first terminals of said first and second switch devices; second connecting means for connecting said second terminals of said first and second switch devices to a first polarity terminal of said direct current source; a third solid state switch device, said third switch device having first and second terminals and conductive and nonconductive states, a current flow path being established between said terminals when said switch device is in the conductive state, said third switch device being responsive to gating control signals for switching between said states; fourth solid state switch device, said fourth switch device having first and second terminals and conductive and nonconductive states, a current flow path being established between said terminals when said switch device is in the conductive state, said fourth switch device being responsive to gating control signals for switching between said states; third connecting means for connecting said second terminal of said third switch device to said first terminal of said first switch device; fourth connecting means for connecting said second terminal of said fourth switch device to said first terminal of said second switch device; fifth connecting means for connecting said first terminals of said third and fourth switch devices to a second polarity terminal of said direct current source, said fifth connecting means including an inductance; means for sensing a parameter of said load and generating timing control signals commensurate therewith; means for generating periodic gating control signals at first and second frequencies for said switch devices, said first frequency being greater than said second frequency, said first frequency gating control signals being applied to said first and second switch devices and said second frequency gating control signals being applied to said third and fourth switch devices whereby the conductive state of said switch devices will be controlled to cause current to alternately flow through said load in opposite directions, said load current also flowing through said fifth connecting means inductance, said gating control signal generating means including pulse width modulator means for varying the duration of at least said first frequency gating control signals; and means for delivering said timing control signals to said pulse width modulator means of said gating control signal generating means.