|Saturday, 09 December 2006|
Arc discharge lamps, such as fluorescent tube lamps, are powered by ballasts which limit and otherwise control current to the lamps. Fluorescent lamps must use high voltage at an initial drive stage so as to emit thermions required for a discharge operation. A typical fluorescent lamp is constructed from a glass tube which contains two electrodes at opposite ends, a coating of powdered phosphor covering the interior of the tube, and small amounts of mercury. The major components of a fluorescent lamp are the bulb, electrodes, fill gas, phosphor coating and a base used to support the external conductors of the electrodes. Phosphor coats the inside of the glass tube, and each end of the glass tube includes an electrode. In operation, a ballast provides current to the electrodes. When energized, the electrodes produce a large potential between which free electrons initiate an arc. The arc generates some visible radiation which in turn excites the phosphor coating causing it to emit light. Fluorescent lamps are becoming increasingly popular for use in homes or offices because of their high operating efficiency as compared to incandescent lamps. Fluorescent lamps emit light at several times the efficiency of a typical incandescent lamp, and do not generate as much heat as a typical incandescent bulb, thereby conserving radiant energy and eliminating excess heat output. A fluorescent lamp has negative resistance characteristics in that the voltage applied to the fluorescent lamp following the discharge operation is reduced in inverse proportion to an increase in the amount of current flowing through the fluorescent lamp. Therefore, a ballast used for such a fluorescent lamp must serve to supply high voltage required for a turning on of the fluorescent lamp while controlling the amount of current flowing through the fluorescent lamp, following the turning on, thereby maintaining a desired brightness. Ballasts for fluorescent lamps are generally classified into a self-excited type for determining a lamp drive oscillation frequency depending on a value determined by passive elements such as a coil, capacitor, and a separate-excited type for performing frequency modulation in a digital manner to improve the precision of frequency modulation based on an analog manner. The separate-excited ballasts are adapted to control the brightness of lamps in a frequency modulation manner. Ballasts of the separate-excited ballasts have recently been used more than those of the self-excited ballasts in that the ballasts of the self-excited type have many limitations as well as such a large amount of high-frequency components as to jam the surrounding electronic equipment.
A high-intensity discharge (HID) lamp is a light source which allows brighter lighting with higher light output and is operated with a device called ballast in order to achieve more steady lighting. HID lamps are identified by the gas within the lamp, e.g. metal halide (MH), high-pressure sodium (HPS) or mercury vapor (MV). As HID lamps have become more popular, electronic ballasts for HID lamps have been developed. An electronic ballast supplies the power required for starting and then operating a high-intensity discharge (HID) lamp, such as a metal halide lamp. The employment of electronic ballasts for the operation of gas discharge lamps leads to significant energy savings due to reduced ballast losses and improved lamp efficiency. A metal halide lamp is a high-pressure gas discharge lamp in which metal halides are enclosed in a quartz envelope. Because this lamp has a compact geometry and a high efficacy of nearly white light, it is now widely used to illuminate sports stadiums and roadways. The metal halide lamp is characterized by its high luminous efficiency, high color temperature and long life compared with the halogen lamp. The arc tube of the metal halide lamp contains metal halides which are mixtures of some metals such as sodium and scandium with halogen such as iodine, high pressure xenon serving as a starting gas, and mercury. To initiate its operation, a metal halide lamp demands a high ignition voltage. But once an arc discharge is ignited, the lamp is thereafter maintained in operation by a voltage no higher than the voltage of the AC power source to which the ballast is connected. In HID lamps an arc is established between electrodes which cause a metallic vapor (xenon, sodium and mercury) to produce radiant energy in the form of visible light, generally without phosphors. The vapor is highly pressurized. In the operation of HID lamps, the electrodes carry a high-voltage, high-frequency pulse to strike an arc and vaporize the vapor. The ballast must provide sufficient power to the lamp, from the AC source, to provide sufficient open circuit voltage (OVC) to permit polarity reversal without the arc being extinguished (quenched). High intensity discharge (HID) lamps such as mercury, metal halide, and high pressure sodium lamps are usually operated at high wattage and require a different ballast circuit than lamps such as fluorescent lamps which operate at relatively low wattage. The efficiency of an electronic ballast in supplying power to an HID lamp largely depends on its power factor rating. Power factor is defined as the real input power level divided by the apparent input power level. The apparent power level is determined by the RMS (root mean square) voltage value multiplied by the RMS current value. Power factor is a function of the degree to which the load current and voltage are in time phase with each other. The greater the degree to which the load current leads or lags the voltage, the lower is the power factor rating and the less efficient the electronic ballast. Electronic ballasts for powering high-intensity discharge (HID) lamps usually have a timed ignition period of about 20 to 30 minutes, during which time high voltage pulses are provided in order to ignite the lamp.
Ballast circuits are commonly used for operating a lamp to prevent the sudden, large increases in voltage supplied to the lamp that could result in malfunction or damage to the lamp. Electronic ballasts also control operation of a lamp using a preheating mode and an operating mode. An electronic ballast for a gas discharge, fluorescent or high-intensity discharge lamp provides the current, voltage, and wave-form conditions needed to operate the lamp. 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. The inverter includes a direct coupled output that is a pair of switching transistors connected in series between a high voltage rail and a low voltage rail or common rail. The transistors conduct alternately, producing a square wave at their junction that is converted into a sine wave by a series resonant circuit coupled to the junction. A load is coupled in parallel with the series resonant capacitor. 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. Generally, an electronic ballast for a discharge lamp comprises a half-bridge inverter, a current transformer, and a load circuit including a discharge lamp. The current transformer includes a detecting winding and a feedback winding. The feedback winding generates a driving signal of switching elements of the half-bridge inverter. Some ballasts include a boost circuit between the rectifier and the filter capacitor for increasing the voltage to the lamp. Many electronic ballast use what is known as a "flyback" boost circuit in which the energy stored in an inductor is supplied to the filter capacitor as small pulses of current at high voltage, utilizing the .delta.i/.delta.t characteristic of an inductor to produce a high voltage. An electronic ballast typically includes an integrated circuit in the front end of the ballast to operate the boost circuit and provide 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. An electronic ballast is known that uses passive power factor correction means to reduce ballast input current total harmonic distortion. Electronic ballasts for controlling fluorescent or high-intensity discharge (HID) lamps usually require electronics necessary for preheating the lamp filaments, striking the lamp, driving the lamp to a given power, detecting lamp fault conditions, and safely deactivating the circuit.
Electronic ballasts initiate a glow discharge within a gas discharge lamp and thereafter maintain a stable supply of power to the lamp to sustain the discharge. Basically the electronic ballast is a combination of circuits that converts alternating current (AC) into direct current (DC) and then from DC back to AC. In a typical ballast circuit, the same inductor is used to produce the electrical excitation necessary to ignite as well as to operate the lamp. An electronic ballast with a switching half-bridge provides an oscillator that is used to derive the switching signals for the half-bridge switches to appropriately direct current to various components at particular times to establish a power flow to the lamp load. An electronic ballast using an oscillator involves connecting a voltage controlled oscillator (VCO) with a feedback signal from the switching half-bridge to modify the VCO frequency in accordance with desired operational parameters. For a ballast to operate over a wide range of line voltages, a pre-converter stage may be employed that boosts the incoming voltage to a value higher than the peak value of the highest AC voltage that the unit will use. This pre-converter also known as a boost converter typically uses an industry standard integrated circuit to perform this power conversion in a way such that the AC load current follows the incoming AC line voltage. Electronic ballasts with higher power factor correction (PFC) are being imposed worldwide in order to maximum the efficiency of existing power generation capacity. The input of an electronic ballast usually constitutes a high frequency filter connected to the voltage supply network, which filter is connected with a rectifier circuit. Electronic ballasts for gas discharge lamps are often classified into preheat type and instant start type according to how the lamps are ignited. An instant-start ballast is a ballast that is designed to instantly start a gas discharge lamp such as a fluorescent lamp by applying a high voltage across the lamp without preheating the cathode. An instant start electronic ballast does not employ any filament preheats mechanism to assist in thermionic emission from the lamp electrodes, but relies upon sudden application of a high voltage between the lamp electrodes to ignite the gas discharge within the lamp. In preheat ballasts, the lamp filaments are preheated at a relatively high level for a limited period of time before a moderately high voltage is applied across the lamp in order to ignite the lamp.