electronic ballast

The lighting system provides many opportunities for cost-effective energy saving without any sacrifice.

The lighting retrofit is now part of the Energy Conservation program over the world and reduction of energy consumption by implementing energy conservation schemes is needed. Incandescent lamps convert just five per cent of energy into light and the remainder into heat where as fluorescent lamps turn 25 per cent of energy into light.

Fluorescent lighting is now widely used in industrial and commercial applications mainly due to its high efficiency and longer life span compared to incandescent lamps. Conventional fluorescent lighting systems operate from the mains using a simple series choke as the ballast whose function is to create sufficient voltage with the glow starter to start the lamp and also to limit the lamp current after it has started.

Such a system is simple and low-cost, however the drawbacks are a low input power factor, audible humming noise, extra loss in the choke, delayed start, stroboscropic and flicker effects, and no dimming control.

What is electronic ballast

As the rapid development of power electronics technology, inverters are now used for energising the lamp. These inverter systems are now referred as the electronic ballast and can eliminate the conventional magnetic ballast's disadvantages.

Figure 1 shows a functional block diagram of the electronic ballast. The mains is rectified to a DC voltage which is then inverted into a high frequency AC voltage to drive the fluorescent lamp. The power factor correction rectifier is to improve the input power factor to near unity.

The harmonic current can therefore be reduced. Many low-cost electronic ballasts do not use power factor correction, hence the electromagnetic interference is likely to cause disturbance on other equipment, and do not meet international standards.

Electronic ballast operates at a high frequency of 20-60kHz whereas the conventional magnetic ballast operates at 50/60Hz. The high frequency operation of the fluorescent lamp improves the efficiency by approximately 10 per cent because of the increase in phosphor excitation. Flicker is also eliminated. Instant start is possible even at low supply voltage. Lamp life time is hence extended because coating of the filament can last longer.

Electronic ballasts also consume less power than conventional ballasts because the electronics are more efficient than the bulky magnetic chokes. The combined benefit in energy saving is around 25 per cent with the same light output level.

Table 1 shows comparisons of performance of the conventional ballast with the electronic ballast. It can be seen that the drawback of electronic ballast is the price. An electronic ballast is typically several times more expensive than its magnetic counterpart.

The operating benefit of the electronic ballast is very attractive. The efficacy is the term used for describing how effectively a system converts electrical energy into visible light. The electronic ballast has an efficacy 25 per cent higher than the conventional magnetic type. The ballasts are known to generate harmonic current into the distribution system.

This power pollution must be reduced and many international bodies have strict standards on the lighting equipment harmonics such as IEC 1000- 3-2.

Both conventional and electronic types have a low total harmonic distortion (THD). The electronic ballast has a power factor higher than 0.9 which is above what the energy code stated.

The bill on air-conditioning is reduced too because of the lesser heat generated.

Dimmable Electronic Ballasts

Incandescent lamps with dimmers are widely used for lighting control. These dimmers are phase-control circuits with triac to vary the conducting period of the lamp current in order to control the light output. The luminous output of fluorescent lamps cannot be controlled by the same method.

The operation frequency of the electronic ballast is tens of kHz. By controlling the frequency, the power applying to the lamp is varied and hence the luminous output can be adjusted. Dimmable electronic ballast uses a low DC voltage or a rheostat to adjust the light output. Typical dimmability range down to 10 per cent is now available commercially.

Table 2 shows the performance between the dimmable ballasts for the incandescent lamp and fluorescent lamp. An incandescent lamp has a very good power quality when no dimming is applied because the lamp is only a pure resistor. However, when dimming is applied, the power quality is very poor which is the inherent problem of the phase-control of the dimmer.

The fluorescent lamp utilities electronic ballast which has good performance over a wide range of light output.

Tomorrow electronic ballasts

The light sensitive electronic ballast system is now being used for many commercial buildings with the intelligence to sense outdoor light. The system uses a light sensing device to feedback light intensity signal in ambience to a micro-controller to adjust the operation frequency of the inverter in the ballast. The light output can be adjusted automatically according to the sunlight.

Lighting Control Systems incorporating infrared motion detection and remote control are now being installed in many buildings. The utilisation of energy is therefore more effective.

One of the problems of the dimming control of the electronic ballast is the use of a rheostat to adjust the frequency of the electronic ballast. The electronic ballast is located next to the fluorescent tube. This therefore restricts the accessibility of the control button.

Power line communication techniques are now used to overcome the problem. A higher frequency modulating signal is superposed on the mains voltage which is demodulated in the electronic ballast. The modulating signal is a control signal generated in the mains switch. The signal can be transferred to the ballast using the existing power distribution line to control the functionality such as dimming of the electronic ballast,

Moreover, the phase control technique is now developed to adjust the power applied to the electronic ballast. The phase control technique combines the control method for the incandescent lamp and the high frequency inverter techniques. This method is convenient to the users because the wiring is the same as incandescent lamp dimmer. Therefore it simplifies the installation cost. Moreover, very low output intensity is also under development. A very wide dimming range provides various applications to users and it now has significant demand. Less than 0.5 per cent output is now possible in the laboratory but it still takes some time to realise into a product.

Currently, one electronic ballast can supply to one or two fluorescent lamps. Multiple operation of fluorescent lamps is under development. In the very near future, one electronic ballast controlling a number of lamps without depreciation of the output quality will be realised into a product. The actual component cost is indeed very little different.

Today, electronic ballasts of one-for-two and one-for-three are available from the market. One-for-four and one-for-many types have been realised in the laboratory. This technique is using artificial intelligence and model reference adaptive control methods to control the lamps so that even luminous output for each lamp can be obtained. The control algorithm is even more complicated when dimmable electronic ballasts are also required.

The research of this technology is to work on the dimming control of each lamp over the full range of the luminous output. This technology can reduce the component count, wiring connection and initial cost. However they may take some time for commercialization.

Emergency lighting is a typical application of electronic ballast. Combined Electronic Ballasts and Emergency Modules for emergency lighting and Exit signs are now the current trend. In the past, the emergency lighting has been powered by a low voltage battery set through a resonant inverter which converts to a high voltage to drive the fluorescent lamp.

This configuration has also been used in torch-style portable fluorescent lamps. The circuit is based on linear operation mode of the transistors hence the main problem of this circuitry is the excess power loss.

Nowadays, using high frequency magnetic and switched-mode power converter techniques, the whole circuit can be integrated to very compact size. The efficiency can be increased by more than 25 per cent and the size can also be reduced by at least 50 per cent.

Future applications will also concentrate on high power density (HDP) electronic ballasts. This is now integrated into the next generation compact designs to suit special slim luminaires.

The integration of the HDP electronic ballast with the fluorescent lamp will soon become available to replace the compact fluorescent lamp. With the advancement of power electronic devices, the future cost of electronic ballast will certainly become lower and the size more compact. This makes the advantages of electronic ballasts become more attractive.