Electronic ballast having improved power factor and total harmonic distortion

Electronic ballast having improved power factor and total harmonic distortion

Electronic ballast for fluorescent lamps typically can be analyzed as comprising a "front end" and a "back end". The front end typically includes a rectifier for changing alternating current (AC) mains line voltage to a direct current (DC) busvoltage and a filter circuit for filtering the DC bus voltage. Electronic ballasts also often use a boost circuit for boosting the magnitude of the DC bus voltage. The filter circuit typically comprises a capacitive low-pass filter.

The ballast back end typically includes a switching inverter for converting the DC bus voltage to a high-frequency AC voltage, and a resonant tank circuit having a relatively high output impedance for coupling the high-frequency AC voltage to thelamp electrodes. The ballast back end also typically includes a feedback circuit that monitors the lamp current and generates control signals to control the switching of the inverter so as to maintain a desired lamp current magnitude.

In order to maintain stable lamp operation, typical prior art electronic ballasts filter the DC bus voltage to minimize the amount of bus voltage ripple. This is usually accomplished by providing a bus capacitor having a relatively largecapacitance and hence, a relatively large energy storage capacity. By providing a relatively large bus capacitor, the amount of decay from the rectified peak voltage is minimized from one half-cycle to the next half-cycle. Minimizing the amount ofripple on the DC bus also tends to minimize the current crest factor (CCF) of the lamp current. The CCF is defined as the ratio of the magnitude of the peak lamp current to the magnitude of the root-mean-square (RMS) value of the lamp current.

However, using a relatively large bus capacitor to minimize ripple on the DC bus voltage comes with its disadvantages. The bus capacitor must be fairly large, and is therefore more expensive and consumes more area on a printed circuit board, orthe like, and volume within the ballast. Also, because the bus capacitor is discharging whenever the bus voltage level is above the absolute value of the AC mains voltage, the bus capacitor is recharging during a relatively short duration of each linehalf-cycle centered about the peak of the line voltage. This results in the typical prior art ballast having a tendency to draw a relatively large amount of current during the time that the bus capacitor is charging, as shown in FIG. 1. Consequently,this results in a distorted input current waveform giving rise to unwanted harmonics and undesirable total harmonic distortion (THD)) and power factor for the ballast.

In accordance with a first feature of the invention, a novel electronic ballast for driving a gas discharge lamp includes a rectifier to convert an AC mains input voltage to a rectified pulsating voltage, a valley-fill circuit including an energystorage element to which energy is delivered through a switched impedance to fill the valleys between successive rectified voltage peaks to produce a substantially DC bus voltage, a DC-to-AC voltage inverter having series-connected switching devices toconvert the substantially DC bus voltage to a high-frequency AC voltage signal for driving the gas discharge lamp, a resonant tank for coupling the high-frequency AC voltage signal to the gas discharge lamp, and a control circuit for controlling theswitching action of the switching devices to deliver a desired current to the gas discharge lamp.

In a preferred embodiment of the ballast, the energy storage element of the valley-fill circuit includes a bus capacitor that stores energy during a first predetermined charging portion of each half-cycle of the AC mains voltage, and deliversenergy to the gas discharge lamp during a second predetermined discharge portion of each half-cycle of the AC mains voltage. The switched impedance of the valley-fill circuit includes a resistor in series with a controllably conductive device (a fieldeffect transistor), through which the bus capacitor is charged.

In an alternative embodiment, the energy storage device of the valley-fill circuit includes a bus capacitor, and the switched impedance includes a buck inductor in series with a field effect transistor, connected together in a buck converterconfiguration. The bus capacitor stores energy during a first predetermined charging portion of each half-cycle of the AC mains voltage, and delivers energy to the gas discharge lamp during a second predetermined discharge portion of each half-cycle ofthe AC mains voltage. The buck inductor stores energy in response to conduction of the field effect transistor during the charging period of the bus capacitor, and transfers the stored energy to the bus capacitor in response to non-conduction of thefield effect transistor during the charging period of the bus capacitor.

In accordance with a second feature of the invention, a novel electronic ballast for driving a gas discharge lamp includes a rectifier to convert an AC mains input voltage to a rectified pulsating voltage, a valley-fill circuit to fill thevalleys between successive rectified voltage peaks to produce a substantially DC bus voltage, a DC-to-AC voltage inverter having series-connected switching devices to convert the substantially DC bus voltage to a high-frequency AC voltage signal fordriving the gas discharge lamp, a resonant tank for coupling the high-frequency AC voltage signal to the gas discharge lamp, a control circuit for controlling the switching action of the switching devices to deliver a desired current to the gas dischargelamp, and means for drawing input current near the zero crossing of the AC mains input voltage waveform so that the input current THD is substantially reduced, and the power factor of the ballast is increased.

In a preferred embodiment of the ballast, the means for drawing current near the zero crossing is a cat ear power supply that supplies the power necessary to operate the control circuit. The cat ear power supply draws current from the AC mainsfrom near the zero crossing of the AC mains voltage at either the leading edge of each half-cycle, or the trailing edge of each half-cycle, or both. The cat ear power supply derives its name from the shape of its input current waveform which "fills in"the current waveform drawn by the ballast from the AC mains around the zero crossings. The cat ear power supply may be provided with circuitry that "cuts in" and "cuts out" the power supply in response to fixed input voltage levels. Alternatively, thecat ear power supply may be provided with circuitry to monitor the current drawn by the ballast back end and cause the power supply to draw input current only when the back end is not drawing current.

POST COMMENT

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