Balastro electrónico

BALLAST APPLICATIONS INFORMATION

Electronic Ballasts for Fluorescent Lamps using BULD25 .. 125 Transistors
2nd Edition. Mick Maytum and Koenraad Rutgers

SUMMARY
High frequency electronic ballasts place a wide range of demands on their transistor switches. To establish what these demands are this report looks at the basic requirements of high frequency fluorescent lamp operation and howthese are serviced by a typical ballast design. Following this an analysis of the transistor operation is given, concentrating on power losses, frequency control factors and high stress conditions. In conclusion, the key transistor parameters for ballast use are detailed and performance details of the BULD 25, 50, 85 and 125 range of transistors is given. These transistors, and circuits similar tothose described, are suitable also for electronic transformers, used typically to power low-voltage halogen lamps.

1

BALLAST APPLICATIONS INFORMATION

INTRODUCTION
Standard fluorescent lamps are extensively used in industrial and commercial environments. Compared to incandescent bulbs, fluorescent lamps have lower energy consumption and longer life. But in most domestic applications theseadvantages have been outweighed by the fluorescents' fitting size, the need for extra control gear and the higher initial cost. As a result their use is very selective and they are mainly found in kitchens and garages. Also, for applications like task lighting a concentrated light source is needed rather than the distributed source given by the conventional fluorescent lamp. For these reasonsthere is still an enormous market for incandescent (filament) bulbs. Recognising the need for a small efficient light source the lamp manufacturers have developed a range of compact fluorescent lamps. The small size is achieved by reducing the glass tube diameter and folding the tubing one or more times. As with conventional fluorescent lamps these compact fluorescents still required a ballast and astarter. The starter problem was solved by integrating a glow starter in the tube base. Coupling this compact fluorescent tube with a ballast mounted in an adapter gave a light source which could replace incandescent bulbs. Combining a compact fluorescent with a conventional 50/60 Hz ballast gave a light source that was restricted in its application area. The weight and bulk of the ballast madethe resultant light source non-retrofitable in many incandescent bulb fittings. To overcome this restriction small light weight electronic ballasts were developed to replace the 50/60 Hz ballasts. In some cases these lamps were not compatible with the electronic ballasts. Lamps would often flicker continuously or at random times during running. This problem was caused by the lamps glow starter beingactivated by the hf ac generated by the electronic ballast. So a new range of compacts was introduced where the glow starter was removed. This range of lamps is characterised by four connecting pins (two for each heater) compared to the previous two pin lamps which had an integral starter.

HIGH FREQUENCY LAMP OPERATION
When a fluorescent lamp is running at a frequency of 50 or 60 Hz, therelatively slow rate of operation means that the conducting gas reacts faster than the ac line rate. This results in two phenomena (Figure 1). Firstly when the lamp current falls to zero as the current polarity changes, the lamp stops conducting and has to restrike on the opposite current polarity. Second is that the conducting gas has a negative impedance characteristic. To control the lamp current,the lamp must be connected in series with higher positive impedance (ballast). At ac line frequencies the ballast is usually an iron cored wound component.

300 200 Lamp Voltage - V 100 0 -100 V I

1.0

0.5 Lamp Current - A

0

-0.5 -200 -300 0 10 20 Time - ms 30 40 -1.0

Figure 1. 50 Hz tube voltage and current

2

BALLAST APPLICATIONS INFORMATION

When a fluorescent tube is...