Fluorescent lamp electronic ballast

Published: 03rd May 2011
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Fluorescent lamps with electronic ballast is used in the aircraft industry because of the efficiency of fluorescent lighting technology. Conventional aircraft generators produce an AC voltage of nearly constant frequency. The conventionalfluorescent lamp electronic ballast accommodates the nearly constant frequency AC voltage and drives one or more lamps with reasonable performance.

Recently, aircraft manufacturers have begun to employ wild frequency generators. Wild frequency AC generators are not able to control the output frequency of the generated waveform. The output frequency of the generator varies with the enginespeed of the aircraft, typically producing a waveform with frequencies between 300 Hz and 800 Hz. Due to the reduction in generator complexity, wild frequency generators are more reliable, lighter, and result in lower maintenance costs when compared toconventional aircraft generators. However, conventional fluorescent lamp electronic ballasts are not able to accommodate the varying-frequency input.

Conventional ballasts that use a hold-up capacitor or passive power factor correction (PFC) L-C filters on the input are not able to operate with a varying frequency. At higher frequencies, these conventional ballasts draw excessive current,exhibit poor power factor, and exhibit poor total harmonic distortion (THD). PFC boost converters have been used in ballasts, but the boost converter topology creates inrush current at turn on. An existing inverter ballast circuit is shown in U.S. Pat. No. 5,466,992.

A recent development used in certain specific 50/60 Hz applications is the PFC flyback topology, which is intended to operate over a frequency range of 45 Hz to 66 Hz. The topology is centered on the L6561 integrated circuit from STMicroelectronics. Some background information may be found in ST Microelectronics application notes AN1059 "Design Equations Of High-Power-Factor Flyback Converters Based On The L6561," AN1060 "Flyback Converters With The L6561 PFC Controller," andAN1089 "Control Loop Modeling Of L6561-Based TM PFC." However, these recent developments have only been discussed for certain specific 50/60 Hz applications and fail to discuss problems that arise with a wild frequency generator power source.

In carrying out the above object, a fluorescent lamp electronic ballast is provided. The ballast comprises a power factor correction flyback circuit and an inverter ballast circuit. The power factor correction flyback circuit is composed of arectifier connected to a DC to DC flyback converter. The flyback converter includes a flyback transformer connected to a diode/capacitor combination. The flyback converter includes a switch used to switch the flyback transformer during operation toproduce a flyback waveform that is rectified by the diode and results in a DC output at the capacitor. The inverter ballast circuit receives the DC output and inverts the DC output to an AC signal for operating the fluorescent lamp. The flybackconverter may provide input to output isolation. The input voltage may be stepped up or stepped down.

At a more detailed level, the present invention comprehends a PFC flyback topology capable of operation over a wide range of frequencies and having low in-rush current at turn on. The preferred PFC flyback circuit implementation has severalaspects that make wide frequency range operation and low in-rush current at turn on possible. In the most preferred implementation, the PFC flyback circuit provides high power factor and low total harmonic distortion (THD) over a wide range offrequencies ranging from 60 Hz to 800 Hz. One aspect that serves to broaden the range of accommodated AC input frequencies received at the rectifier is that the rectifier has a sufficiently low input capacitance (preferably less than 0.5 microfarads)such that the rectifier output substantially takes the form of a rectified AC wave.

An additional aspect of the preferred PFC flyback circuit is the ability to produce multiple DC output supplies. In a preferred implementation, the inverter ballast includes a self-oscillating resonant circuit including a pair of powertransistors. The flyback converter is further used to create a DC bias for use by the power transistors. This technique reduces dissipated power in the bias supply when compared to traditional biasing techniques. Further, a similar approach may beutilized to supply power to arc detect and dim control circuitry when provided. It is appreciated that the flyback converter has a number of advantages in a lamp electronic ballast including the ability to regulate over a very wide variation in inputvoltage. Thus, for low line or high line conditions, the ballast light output remains unchanged. In addition, the preferred flyback circuit provides short circuit protection from gross output short circuit conditions.

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