Patent ID: 6121731
Filing Date: 2000-09-19
Classification: H05B,Y10S

Abstract:
A lighting system for a discharge lamp, providing for overcurrent protection of an inverter switch or switches, the lighting system comprising:(a) an inverter circuit for providing a variable frequency output voltage;(b) a load circuit connected to the inverter circuit and including a resonant circuit having a capacitor with which a discharge lamp is to be connected in parallel, in order to cause an inversely frequency dependent voltage to be applied between a pair of electrodes of the lamp according to a predefined resonance characteristic, the resonant circuit having a resonance frequency (f.sub.0) that is less than a discharge start frequency (f.sub.2) at which the lamp is to start glowing;(c) inverter control means connected to the inverter circuit for lighting up the lamp by changing the frequency of the output voltage of the inverter circuit from a first frequency (f.sub.1) which is higher than the discharge start frequency (f.sub.2) to a second frequency (f.sub.3) which is less than the resonance frequency (f.sub.0) of the resonant circuit, and for holding the lamp glowing by maintaining the output voltage of the inverter circuit at the second frequency;(d) phase advance detector means for ascertaining whether or not a current flowing through the load circuit is in phase advance with respect to the output voltage of the inverter circuit; and(e) overriding frequency control means connected between the phase advance detector means and the inverter control means for causing the inverter control means to make the frequency of the output voltage of the inverter circuit higher than the resonance frequency (f.sub.0) of the resonant circuit when the current flowing through the load circuit is ascertained to be in phase advance with respect to the output voltage of the inverter circuit;(f) whereby, when found to be in phase advance with respect to the inverter output voltage, the load current is automatically delayed in phase in order to protect a switch or switches included in the inverter circuit from destruction due to overcurrent.