electronic ballast fundamentals dr. bryan m.h. pong hong kong university

Electronic Ballast Fundamentals

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 phosphor

At the ends of the tube there are filaments or electrodes

Lamp structure

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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?

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Lamp starting

When the lamps is off the tube is non -conductive

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

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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

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Basics of Electronic Ballasts

Half bridge configuration The resonant circuit strikes on the lamp

C2

C1

L

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>

Run

In the steady state the lamp resistance reduces greatly current is controlled by L & C1Ref <steady>

i(R3)

V(C2)

Typical operating points

Types of Ballast circuits

Self oscillating circuitIC driven circuitNew 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

Load101

Look up references on the web

Encyclopediahttp://www.britannica.com/

Bookshttp://www.knovel.com/knovel2/default.jsp

Company web siteshttp://www.irf.com/technical-info/

IEEE/ IEE papershttp://ieeexplore.ieee.org/Xplore/DynWel.jsp

What to do

Understand the fundamentalsDesign the circuit and fill in the

component parametersStart to build the circuit in SeptemberExpect a working circuit in DecemberFurther work on lamp characteristics,

power factor correction, comparison of topologies & dimming

References

1. http://home.howstuffworks.com/

2. http://www.repairfaq.org/sam/flamp.htm#int0

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 notes.