Electronic Ballast Fundamentals

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Electronic Ballast Fundamentals Powered By Docstoc
					Electronic Ballast
   Dr. Bryan M.H. Pong
   Hong Kong University
Fluorescent tubes

  Much more efficient than incandescent
   lamps : 2 to 4 times more efficient
  The lamps last a lot longer – 10k to 20k
   hours versus 0.75k to 1k hours for an
   incandescent lamp
  The lamps are much cooler than
   incandescent lamps
Lamp structure

  The lamp is a sealed tube containing
   mercury vapor and some inert gases
   such as argon at very pressure
  The inside of the tube is coated with
  At the ends of the tube there are
   filaments or electrodes
Lamp structure

How does it work?
    When the tube is off it does not conduct
    It is turned on by a high voltage which excites
     the gas inside the tube
    Once the tube conducts on a lower voltage is
     sufficient to maintain conduction
    The electric current passes through the gas
     and emits UV, the internal phosphor coating
     converters the UV to visible light
    The color of the light cab be varied by different
     combinations of phosphors
How does it work?

Lamp starting

  When the lamps is off the tube is non -
  The tube must be excited or started by a
   high voltage
  After the lamps is started the lamp
   voltage drops
  A current limiting “ballast” is needed in
   between the power source and the lamp
Start up circuit

Conventional starter switch

  A mechanical starter switch together with an inductive
   ballast start up the lamp
  The starter switch is a small discharge bulb containing
   a gas like neon.
  When connect to a voltage source the gas breaks
   down and the switch glows
  The heat generated bends the bimetallic switch and
   closes it
  One the switch is closed the glow is turned off and the
   bimetallic switch cools down and opens
  When the switch opens a series connected ballast
   provides a high voltage and strikes on the tube
Starter switch
High frequency ballast

  Conventional ballast produces
   100/120Hz flicker
  High frequency ballast is more efficient
   than magnetic ballast [3]
  High frequency ballast, like most power
   electronics products, has smaller size
   and weight
  Integrates ballast and starting functions
Rapid start
Basics of Electronic Ballasts




    Half bridge configuration
    The resonant circuit strikes on the lamp
Lamp preheat & ignition

  Before strike on the tube has very high resistance and effectively it is an
   open circuit
  Operation at a high frequency preheats the filaments
  Operation near resonance produces high voltage across C2 and strikes
   on the lamp
  Ref <start1>



   In the steady state the lamp resistance reduces greatly
  current is controlled by L & C1
 Ref <steady>
Typical operating points
Types of Ballast circuits

  Self oscillating circuit
  IC driven circuit
  New Capacitor Couple Converter (CCC)
   circuit developed at the HKU Power
   Electronics Lab
Self Oscillating Ballast

    Simple & low cost
IC driven Ballast

    Easy to design
CCC Ballast


      Low loss – zero voltage switching
      Reliable – automatic power limiting
Look up references on the web

    Encyclopedia
    Books
    Company web sites
    IEEE/ IEE papers
What to do

  Understand the fundamentals
  Design the circuit and fill in the
   component parameters
  Start to build the circuit in September
  Expect a working circuit in December
  Further work on lamp characteristics,
   power factor correction, comparison of
   topologies & dimming
 3.   “HID Electronic Ballast Testing” Public Interest Energy Research
      Program (PIER) Program, California, USA
 4.   T. Ribrarich “A Systems Approach to Ballast IC Design” IR Technical

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