Inductorless ballast

Details Inductorless ballast Inductorless ballast

1999-06-10

2001-04-10

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Periodic switch in the supply circuit

Impedance or current regulator in the supply circuit

C315S291000

Reexamination Certificate

active

06215253

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an inductorless ballast. More specifically, the present invention relates to an inductorless ballast for a fluorescent light.
2. Description of the Prior Art
Existing solutions for driving a fluorescent lamp at high-frequency include some form of current limiting, or “ballast,” between the high-frequency voltage source (usually a pair of transistors arranged in a half-bridge circuit) and the lamp. The ballast may be a resistor (when driving the lamp at D.C.) or an inductor or capacitor (when driving the lamp at high-frequency). These passive components provide a frequency dependent impedance for current limiting and provide smooth, quasi-sinusoidal lamp voltage and current waveforms. The frequency of operation of the ballast is much higher than the ionization time constant of the lamp, therefore eliminating any re-strike during each cycle of lamp voltage.
Conventional ballasts for fluorescent lamps, which typically incorporate an inductor for driving the lamp at high-frequency, are bulky, heavy and unreliable. It would be desirable to provide an inductorless ballast for high frequency fluorescent lamp operation which does not have the above-mentioned disadvantages.
SUMMARY OF THE INVENTION
The present invention provides an inductorless ballast for driving a load, such as a fluorescent lamp, with an AC step-type voltage waveform generated by a full-bridge circuit formed of switching transistors such as power MOSFETs, for example. The circuit utilizes the physical properties of the fluorescent lamp, such as the ionization time constant and the resistance of the lamp, for control.
The circuit drives the lamp with high frequency AC step-type voltage and current waveforms or other optimized lamp waveforms (e.g., elliptical rather than step-type). The circuit of the present invention utilizes an efficient control means for controlling lamp power (i.e., lamp brightness) by independently varying “on” and “off” time of the voltage applied to the lamp (and thereby controlling the operating frequency and the duty-cycle), and also by varying the amplitude of the applied voltage waveform. Rms, average or peak lamp current are sensed and controlled for controlling lamp power.
The AC step-type waveform generated by the circuit of the present invention provides minimum lamp current crest factor (~1) and maximum power factor (phase shift between lamp voltage and current ~0), therefore maximizing lamp life. The present inventive circuit also utilizes the ionization constant of the lamp to achieve high-frequency switching (lamp acts as a pure resistor) and to prevent lamp power and current from running away exponentially. Also, the circuit achieves a reduction in electromagnetic interference (EM
1
), ringing or harmonies produced during switching by utilizing the ionization time constant of the lamp along with proper snubbing techniques.
The circuit provides the potential for direct AC line operation without rectification and power factor correction. The circuit also utilizes the resistance of the lamp to achieve zero-voltage switching by commutating the voltage across each switch through the lamp to zero during a pre-determined deadtime before turning the appropriate switch on.
Advantageously, the circuit of the present invention could fit into the base of the lamp socket in the case of compact fluorescent lamps where the ballast and lamp are combined into one package, or, into one of the sockets holding the ends of the lamp in a fixture where the ballast is separate from the lamp. The circuit results in a substantial reduction in size, weight, and increases reliability and manufacturability.
In addition, the present invention eliminates the need for an inductor, PCB or other external components necessary with existing solutions.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.


REFERENCES:
patent: 4633161 (1986-12-01), Callahan et al.
patent: 5365148 (1994-11-01), Mallon et al.
patent: 5550440 (1996-08-01), Allison et al.

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