This invention relates to electronic ballasts for fluorescent lamps and, in particular, to electronic ballasts which stop operating in response to a fault condition such as a defective lamp or a missing lamp.
A gas discharge lamp, such as a fluorescent lamp, is a non-linear load to a power line, i.e. the current through the lamp is not directly proportional to the voltage across the lamp. Current through the lamp is zero until a minimum voltage is reached, then the lamp begins to conduct. Once the lamp conducts, the current will increase rapidly unless there is a ballast in series with the lamp to limit current.
A resistor can be used as a ballast but a resistor consumes power, thereby decreasing efficiency, measured in lumens per watt. A "magnetic" ballast is an inductor in series with the lamp and is more efficient than a resistor but is physically large and heavy. A large inductor is required because impedance is a function of frequency and power lines operate at low frequency (50-60 hz.)
An electronic ballast typically includes a rectifier for changing the alternating current (AC) from a power line to direct current (DC) and an inverter for changing the direct current to alternating current at high frequency, typically 25-60 khz. Since the frequency of the inverter is much higher than 50-60 hz., the inductors for an electronic ballast are much smaller than the inductor in a magnetic ballast.
Converting from alternating current to direct current is usually done with a full wave or bridge rectifier. A filter capacitor on the output of the rectifier stores energy for powering the inverter. The voltage on the capacitor is not constant but has a 120 hz "ripple" that is more or less pronounced depending on the size of the capacitor and the amount of current drawn from the capacitor.
Some ballasts include a boost circuit between the rectifier and the inverter. As used herein, a "boost" circuit is a circuit which increases the DC voltage, e.g. from approximately 180 volts (assuming a 120 volt input) to 300 volts or more for operating a lamp, and/or which provides power factor correction. "Power factor" is a figure of merit indicating whether or not a load in an AC circuit is equivalent to a pure resistance, i.e. indicating whether or not the voltage and current are sinusoidal and in phase. It is preferred that the load be the equivalent of a pure resistance (a power factor equal to one).
If a lamp is not connected to an electronic ballast while power is applied to the ballast, the voltages and currents within the ballast can become extremely high, destroying the ballast. In addition, if a lamp is disconnected from a ballast, the person disconnecting the lamp is exposed to the high voltages of the ballast, e.g. by touching the terminals at one end of the lamp while the other end of the lamp is connected to the ballast. Many ballasts are designed to generate extra high voltages initially, to assure an instantaneous or a rapid start of a lamp, then to reduce the voltage when the lamp is conducting. When a lamp is removed, the circuitry within such ballasts reverts to a start-up mode and produces an extra high output voltage at the very time a person may be touching the terminals of the lamp.
Some electronic ballasts include a transformer in the output stage to isolate the lamp circuit from electrical ground. An isolation transformer makes an electronic ballast heavy and expensive. This invention relates to electronic ballasts having what is known as a "direct coupled output", i.e. a path exists between the high voltage supply within the inverter and the output terminals of the inverter during each cycle of the AC output.
U.S. Pat. No. 5,004,955 (Nilssen) discloses an electronic ballast having a direct coupled output. The ballast includes a half-bridge, triggered, series resonant, parallel loaded inverter in which lamp current is sensed by a transformer winding in series with the lamps. A lack of current is interpreted as a fault and the inverter is disabled. The inverter is triggered on or off using RC timing circuits and attempts a re-strike every couple of seconds.
A boost circuit or the inverter, or both, can be self-oscillating, triggered, or driven. A driven circuit requires a source of pulses for operation and the pulses are provided by a timer circuit or a more complicated integrated circuit designed for ballasts or electronic power supplies. A triggered circuit typically incorporates a small pulse generator for starting the circuit into oscillation. A capacitor charging up to the firing voltage of a diac or other semiconductor switch is typically used in such circuits, e.g. the Nilssen patent. The pulse generator may or may not be disabled when the ballast is operating normally. A self-oscillating circuit is constructed in such a way that the applied voltage causes the circuit to begin oscillation and typically includes a resistor having a high resistance to provide a temporary bias for initiating oscillation.
U.S. Pat. No. 5,111,114 (Wang) discloses an electronic ballast having a direct coupled output. The ballast includes a half-bridge, triggered, series resonant, parallel loaded inverter in which lamp current is sensed as the voltage drop across an inductor in series with the lamps. Excess voltage is interpreted as a fault and the inverter is disabled. The inverter is triggered into oscillation using an RC/diac timing circuit in which the capacitor is discharged through a parallel transistor if a fault is sensed. An RC circuit controlling the parallel transistor has a longer time constant than the time constant of the RC/diac circuit. The Wang patent also discloses a large capacitor in series with the lamps, "large" being defined as at least ten times the capacitance of the resonant capacitor.
There are many other circuits described in the prior art for automatically shutting off a ballast in the event of a fault. There remains a need for an efficient ballast having automatic shut off capability, i.e. a ballast which dissipates little power in normal operation and in a shut down mode.
In view of the foregoing, it is therefore an object of the invention to provide an electronic ballast including a capacitive sensor for detecting a missing or defective lamp.
A further object of the invention is to provide an electronic ballast which automatically reduces the output voltage in the event of a fault without dissipating a large amount of power.
Another object of the invention is to provide an electronic ballast including automatic shut-off circuitry which dissipates very little power either during a fault condition or during normal operation of the ballast.