In an electronic ballast, a half-bridge inverter is powered from a DC voltage and provides an AC output voltage having a waveform with trapezoidally shaped half-cycles. The DC voltage is obtained by way of a pre-converter with a control input operative to permit control of the magnitude of the DC voltage....http://www.google.com.au/patents/US6002210?utm_source=gb-gplus-sharePatent US6002210 - Electronic ballast with controlled-magnitude output voltage
