| 摘要 |
A DC-DC converter 12 is responsive to detected lamp current and prior to lamp striking provides an increased output voltage applied to a voltage controlled oscillator 14 so that an inverter 16 controlled by oscillator 14 initially operates at high frequency to provide a high voltage for striking the lamp. After striking, the inverter 16 operates in a lower frequency range and lamp current is regulated by comparing sensed lamp current with an adjustable reference set by a dimming control resistor VR1, the comparison controlling pulse width modulation of a transistor Q1 in converter 12 to vary the voltage applied to VCO 14 and thus the operating frequency of inverter 16. VCO14 provides narrow negative pulses to switch off FET's Q2 and Q5 alternately in inverter 16. Q2 and Q5 are switched on and off while their drain voltage is at ground potential to reduce switching losses. To limit peak lamp current immediately after lamp striking, a control circuit (Q13), (Fig. 2), responsive to excess current temporarily turns off the converter 12 and causes the inverter switches Q2, Q5 to operate in a linear, rather than saturated mode whereby the voltage applied to the lamp is reduced. This control circuit also provides short circuit protection. <IMAGE>
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