subject: Electronic ballast having a flyback cat-ear power supply [print this page] Electronic ballast having a flyback cat-ear power supply
Electronic ballast for fluorescent lamps typically include 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) bus voltage and a filtercircuit for filtering the DC bus voltage. 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 couplingthe high-frequency AC voltage to the lamp electrodes.
The front end of electronic ballasts also often include a boost converter, which is an active circuit for boosting the magnitude of the DC bus voltage above peak of line voltage, and for improving the total harmonic distortion (THD) and powerfactor of the input current to the ballast. However, boost converters typically include integrated circuits (IC) and semiconductor switches, such as field effect transistors (FETs). In order to handle the amount of current required to drive the lamp athigh end (i.e. at or near 100% light intensity), the components of such a boost converter are typically large and costly.
A prior art ballast 100 will be described with reference to the block diagram shown in FIG. 1 and the voltage and current waveforms shown in FIGS. 2a-2d and is explained in greater detail in U.S. Pat. No. 6,674,248, issued on Jan. 6, 2004,entitled "Electronic Ballast", which is herein incorporated by reference in its entirety.
The ballast 100 includes an electromagnetic interference (EMI) filter 115 and a rectifier 120 both capable of being connected to an AC power supply such as a typical 120V, 60 Hz AC main. The EMI filter 115 isolates high-frequency noise generatedby the ballast circuitry from the AC power supply. The rectifier 120 converts the AC input voltage to a rectified pulsating DC voltage 210, which has a maximum value of VPEAK (shown as 230 in FIG. 2a). For example, if the AC input voltage has anRMS (root mean square) value of 277V, the value of VPEAK will be approximately 392V. The rectifier 120 is connected to a valley-fill circuit 130 through a diode 140. A high-frequency filter capacitor 150 is connected across the inputs to thevalley-fill circuit 130. The valley-fill circuit 130 selectively charges and discharges an energy-storage device, such as one or more capacitors, so as to fill the "valleys" between successive rectified voltage peaks to produce a substantially DC busvoltage 220. The DC bus voltage is the greater of either the rectified voltage, or the voltage across the energy storage device in the valley-fill circuit 130.
The outputs of the valley-fill circuit 130 are in turn connected to the inputs to an inverter 160. The inverter 160 converts the rectified DC voltage to a high-frequency AC voltage. The outputs of the inverter 160 are connected to an outputcircuit 170, which typically includes a resonant tank, and may also include a coupling transformer. The output circuit filters the inverter 160 output to supply essentially sinusoidal voltage, as well as provide voltage gain and increased outputimpedance. The output circuit 170 is capable of being connected to drive a load 180 such as a gas discharge lamp; for example, a fluorescent lamp.
According to an embodiment of the present invention, an electronic ballast, which draws an input current from an AC power supply and drives a gas discharge lamp, comprises: (1) a rectifier for rectifying an AC input voltage from the AC powersupply to produce a rectified voltage; (2) a filter circuit for converting the rectified voltage to a filtered bus voltage; (3) an inverter for converting the bus voltage to a high-frequency AC drive voltage to drive the lamp; (4) a control circuitcoupled to the inverter for controlling the operation of the inverter; and (5) a ballast power supply that receives the rectified voltage and generates a substantially DC voltage for powering the control circuit. The ballast power supply selectivelydraws current from the AC power supply when the rectified voltage is lower than a predetermined value so that the input current to the ballast is essentially sinusoidal. The ballast power supply is also coupled to the inverter and is operable to providecurrent to the inverter when the rectified voltage is lower than the predetermined level. The inverter is operable to convert the DC voltage of the ballast power supply to the high-frequency AC drive voltage when the rectified voltage is lower than thepredetermined level and to convert the bus voltage to the high-frequency AC drive voltage when the rectified voltage is more than the predetermined level.
A cat-ear circuit for an electronic ballast having a front end for receiving an AC input voltage and a back end for driving a gas discharge lamp is also described herein. The cat-ear circuit comprises: (1) an input for receipt of a rectifiedvoltage; (2) a semiconductor switch in series electrical connection between the input and a circuit common; (3) an energy storage element in series electrical connection with the semiconductor switch; (4) a control circuit operable to receive therectified voltage and to repeatedly switch the semiconductor switch at a high frequency between a conductive state and a non-conductive state only when the rectified voltage is below a predetermined level; (5) a first output for providing a boostedvoltage to the ballast back end only when the rectified voltage is below the predetermined level; and (6) a second output for providing a substantially low-magnitude DC voltage.
According to another embodiment of the present invention, an electronic ballast for driving a gas discharge lamp comprises: (1) a rectifier for rectifying an AC line voltage to produce a rectified voltage; (2) a boost converter for receiving therectified voltage and producing a substantially DC output voltage having an average magnitude less than a peak value of the AC line voltage, when the rectified voltage is less than a predetermined level; and (3) an inverter for inverting a DC voltage toproduce a high-frequency AC drive voltage to be supplied to the lamp. The inverter is operable to convert the output voltage of the boost converter to the high-frequency AC drive voltage when the rectified voltage is less than the predetermined leveland to convert the rectified voltage to the high-frequency AC drive voltage when the rectified voltage is more than the predetermined level. Alternatively, the inverter may be operable to convert the output voltage of the boost converter to thehigh-frequency AC drive voltage when the output voltage of the boost converter is greater than the rectified voltage and to convert the rectified voltage to the high-frequency AC drive voltage when the rectified voltage is greater than the output voltageof the boost converter.
