Patent ID: 7221107

Claim:
A low frequency square wave electronic ballast for high intensity discharge lamps, comprising: an input filter connected to a sinusoidal line voltage source as an external power supply, a bridge rectifier connected to the input filter, a filtered voltage divider connected to the output of the bridge rectifier providing a DC voltage signal proportional to the average of the rectified sinusoidal line voltage, an isolated dimming control unit including a low power external switch and an optocoupler providing isolated two-state control signal, a power factor preregulator connected to the output of said bridge rectifier including a control unit and a boost converter providing sinusoidal input current in phase with the line voltage, and further providing regulated DC output voltage, a constant power DC current source including a buck converter integrated with a nonlinear function generator, a voltage comparator, and a dimming circuit implementing the ideal full power or dimmed ballast curves for high intensity discharge lamps in their typical lamp voltage range, an output unit including a full-bridge inverter providing low frequency square wave current avoiding cataphoretic phenomenon of high intensity discharge lamps, a dual frequency driver unit of the full-bridge inverter, a high frequency to low frequency transition unit, a short circuit protection unit, and further including a high frequency ignition circuit providing appropriate ignition signal for high intensity discharge lamps, a logic supply voltage unit connected to the output of said bridge rectifier providing a first isolated and stabilized logic supply for the isolated dimming control unit, a second isolated and stabilized logic supply for the power factor preregulator, a third isolated and stabilized logic supply for the constant power DC current source, and a fourth isolated and stabilized logic supply for the output unit; wherein the output of the said power factor preregulator is connected to the input of the said constant power current source, and the output of the said constant power current source is connected to the input of the said output unit, and the output of the said output unit is connected to a high intensity discharge lamp implementing a low frequency square wave electronic ballast providing an acoustic resonance free operation of the lamp; and further wherein the output of the said filtered voltage divider is connected to the control unit of said power factor preregulator, and the control unit of said power factor preregulator is connected to the control unit of said constant power DC current source providing automatic dimming of the high intensity discharge lamp if the line voltage is out of its predetermined range; and still further wherein the optocoupler of the said isolated dimming control unit is connected to the control unit of said power factor preregulator and the dimming circuit of the constant power DC current source providing an isoiated external dimming control of the high intensity discharge lamp.