Electronic Ballast is a device which controls the starting voltage and the operating currents of lighting devices built on the principle of electrical gas discharge. It refers to that part of the circuit which limits the flow of current through the lighting device and may vary from being a single resistor to a bigger, complex device. In some fluorescent lighting systems like dimmers, it is also responsible for the controlled flow of electrical energy to heat the lamp electrodes.
Ballast Basics:
For a lighting device based on electric gas discharge to work, the ionization of gas in the tube is necessary. This phenomenon takes place at a relatively high potential difference and/or temperature than the normal operating conditions of the lamp. After the arc is set up, the conditions can be brought down to normal. To achieve this, three types of methods are generally employed: pre-heat, instant start and rapid start. In pre-heat, the electrodes of the lamp are heated to a high temperature before the voltage is impressed upon them through a starter. Instant start ballasts were developed to start lamps without delay or flashing and use an initial high voltage in place of raised temperatures. Rapid start ballasts make a tradeoff between pre-heat and instant start and use a separate set of windings to initially heat the electrodes for a lesser duration and then, using a relatively lower voltage to start the lamp. Another type, programmed start ballasts is a variant of rapid-start. Any of these starting principles may be used in the ballasts. Initially, when the gas is unionized, it offers a high resistance path to current. But after the ionization takes place and the arc is set up, the resistance drops to a very low value, almost acting like a short circuit. If all this current is allowed to pass through the lamp, the lamp would either burn out or cause the power supply to fail. Thus the ballast needs to perform the current limiting.
Types of Ballast:
There are mainly three types of ballasts: magnetic, electronic and hybrid. Magnetic and hybrid ballasts employ a copper coil wound on a magnetic core as their essential components, while electronic ballasts use solid state electronic circuitry to provide the proper operating electrical conditions to the connected lamps. A brief comparison is tabulated below:
Working of Electronic Ballasts:
The primitive electronic ballast employed a general principle of rectifying the input power and smoothening the waveform by passing it through a simple filter like an electrolytic capacitor. The rectifier converts AC into DC waveform. Improved Electronic ballasts are now generally based on SMPS topology as shown in figure above. The first step is to rectify the input power and then the signal is chopped to increase frequency. This type of ballasts operates between 20 to 60 kHz. Other ballasts like magnetic ballasts generally operate at the line frequency which is about 50-60 Hz. They suffer from problems like flicker and a humming sound which is sometimes a nuisance to ambience. A sample circuit construction of electronic ballast for a CFL Demo board is shown below:
Similar circuit design ideas can be implemented using the application notes provided by chip manufacturers in datasheets. The rationale behind increasing the frequency in electronic ballasts is that the lamp efficiency increases rapidly as the frequency changes from 1 kHz up to 20 kHz and after that improves gradually up to 60 kHz. As the operating frequency of the lamp is increased, the amount of current required to produce the same amount of light is reduced as compared to line frequency, thus increasing the efficiency of the lamp. The increase in efficiency with frequency can be plotted as under:
Advantages:
But some well publicized failures of early ballasts and disadvantages should not be cloud the judgment of buyers. The technology has come a long way from a failure rate of about 20-30% a few years ago to less than 1% now. Ballast reliability ages like wine, the more time it spends in service, the lesser are its chances of failing. The first six months are like incubation period for the electronic ballast, if it survives them, life expectancy increases to 10 or 12 years. The output of the lamps degrades more slowly when used with electronic ballasts as compared to magnetic ballasts. A plot of performance comparing the electronic and magnetic ballast is shown below:
