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Need Of a Ballast
 
Electric lamps are used as a source of light when natural light is not available or is found insufficient by us. Sun is the primary source of light to the earth.
  
 
Electric lamps can be broadly classified into two categories as Incandescent lamps and gas discharge lamps. Examples of incandescent lamps are GLS (General Light Service) bulbs and halogen lamps whereas gas discharge lamps can be Fluorescent Tubular Lamps (FTL's), high-pressure Sodium Vapor (HPSV) lamps etc.
  
 
Any electric lighting system consists of a lamp, fixtures and control gear. Lamp gives light output. Fixtures are required for holding the lamp and providing a means of connecting electric supply to the lamp e.g. lamp holders. A control gear initially supplies high voltage to generate an arc and control the lamp current.
  
 
Incandescent lamp is a resistive load and operates directly on supply voltage. Therefore it does not need any control gear.
  
 
All gas discharge lamps are inductive loads. They need high voltage initially to discharge the lamp and to limit the current to a required value during operation. These tasks are completed with the help of the ballast. Inductive loads cause phase difference between current and voltage waveforms needing power factor correction. Power Factor (PF) correction in magnetic ballast is carried out using a capacitor, while in electronic ballast it can be accomplished internally. Ballast is a non-linear load and which causes distortion of current waveform (i.e. harmonics are generated).
  
 
Magnetic ballast TopΛ
 
Magnetic ballast is used with circuit arrangement as shown in the figure below. This is a popular switch-start circuit in which the inductor is referred to as the ballast (choke) and switch works as a starter.
  
  Block diagram of Magnetic Choke  
   
 
Upon, application of the supply voltage, switch contacts are closed and inductor current preheats the lamp cathodes by resistance heating. A few tenths of seconds later, the contacts open and lamp voltage rises sharply, the process repeats until the lamp strikes. The inductor now assumes its second role as ballast in which its own reactance regulates the lamp current.
  
 
Magnetic ballast has low power factor (typical value is 0.5). Therefore, VA drawn from power supply is double of the wattage consumed. When a capacitor is connected to improve the power factor of magnetic ballast, harmonics are magnified. Thus THD of magnetic ballast increases from 15% to 30% with the introduction of a power factor improvement capacitor.
  
 
Electronic Ballast TopΛ
  High-frequency operation  
 
High-frequency (HF) operation of >20KHz, fluorescent lamp efficiency is increased by 10% i.e. conversion rate of UV radiation increases. Thus advantage of high frequency operation is that the lamp requires less input power for the same light output as it operates at high frequency. This leads to an energy saving. Thus a typical fluorescent lamp of standard four feet lamp and 26mm diameter delivers lumen of 2450. This lamp at 50 Hz consumes 36W. The same lumen are available at 32W with frequency >20KHz. Thus lamp lumen per watt efficiency at 50Hz is 68 Lumen per watt and with high frequency is 76.5 Lumen per watt.
  
 
Similarly, the current requirement for same lamp at 50Hz is 430mA and with high frequency it reduces to 320mA for same lumen. This leads to reduced heat and stress of lamp.
  
 
Also losses in the ballast are as low as 3-4W against 5.5-15W in the case of magnetic ballast. In electronic ballast additional energy savings are possible by control features of electronic designs (Dimming).
 
  What are harmonics?  
 
Harmonics are currents or voltages with frequencies that are integer multiples of the fundamental power frequency e.g. if fundamental frequency is 50Hz, then the 2nd harmonic is 100Hz, the 3rd is 150Hz etc.
  
 
Non-linear loads that draw current in abrupt pulses rather than in smooth sinusoidal manner create harmonics. Non-linear loads consist of inductive or capacitive loads. These pulses cause distorted wave shapes, which in turn cause harmonic currents to flow back into other parts of the power system.
  
 
Harmonics are measured in terms a parameter called Total Harmonic Distortion (THD). It takes into account all the harmonics and is expressed as percentage value of fundamental.
  
 
Harmonics are having adverse effects such as overloading of transformers (de-rating) and rotating equipment, tripping of circuit breakers and fuses, neutral overloading etc.
  
   
  Ballasts are non-linear loads. Low power factor and harmonics generation are problems that evidence themselves on the lamp side. One of these is the lamp current crest factor, which is the ratio of the peak lamp current to the rms lamp current. A sine wave has a crest factor of 1.41. The service life of a fluorescent lamp is significantly shortened when the crest factor exceeds the lamp manufacturer's recommendation, usually 1.7 for most lamps.  
  Simple Inverter Ballast  
 
Electronic ballasts were first introduced as simple inverter ballasts. They perform the basic function of starting the lamp and controlling the lamp current. Power factor correction is not incorporated. This is a basic type of electronic ballast, which consists of various electronic circuits for each element shown in the block diagram below. This is still prominent in integrated Compact Fluorescent lamps (CFL's).
 
  Block diagram of Simple Inverter ballast  
   
 
An AC input is fed to AC-DC converter, which converts the AC voltage to DC. DC is Inverted into high frequency AC using an inverter, which is fed to the lamp using lamp circuit.
 
 

Simple Inverter ballast has disadvantages like very high THD (>120%), low power factor (0.5). The current waveform is highly distorted as shown in fig.. This leads to high VA loads besides, adverse effects of harmonics.

  
  Waveforms for various types of ballasts  
   
 
Passive Power Factor Corrected (PPFC) Ballast TopΛ
 

In order to overcome disadvantages of low power factor in the Simple Inverter Ballasts, Passive Power Factor Corrected (PPFC) Ballast was developed. PPFC ballast consists of additional circuit for power factor correction called as Valley Fill Circuit or Power Factor Correction (PFC) Circuit.

  
  PPFC ballast  
 

 
 
As shown in block diagram above, AC to DC Converter converts AC current into DC. It is pulsating DC, having ripple content. The pulsating DC is fed to Valley Fill Circuit/ Power Factor Correction circuit (Please refer fig.). Capacitors C1 & C2 work for power factor correction. They keep current flowing continuously in both half cycles. This results into input line current drawn continuously from the supply instead of spikes as in the case of Simple Inverter ballast. Please refer waveforms of fig. Thus, this circuit brings input line current waveform close to input voltage waveform. i.e. minimum phase shift exists between them. Thus power factor is corrected (to about 0.95).
 
 
Power Factor Correction Circuit in PPFC Ballast
 
   
 
In an attempt to correct the power factor, current waveform is distorted as shown in fig. Hence, the Total Harmonic Distortion (THD) is more (30%) in case of PPFC ballast. Since the current waveform is distorted, crest factor is also increased (>2). This power factor correction can be achieved by combination of L and C (in lieu of valley fill circuit) for passive power factor correction. But this is very bulky to use.
 
 
Input Filter controls the generation of Electromagnetic/Radio frequency Interference within the stipulated limit and also protects the ballast from those emanating from supply system. On the lamp side, protections are provided against faulty lamp conditions (short circuit, open lamp, transient voltages etc).
  
 
The ballast is designed to operate at rated voltage of lamp i.e. 240V. As the input voltage varies, the variations are transferred as they are to the lamp through the ballast i.e. input wattage to lamp is varied (wattage decreases with decreased voltage and vice versa). The light output also varies directly.
  
  Summarizing, following effects can be observed  
 
  • THD is controlled up to 30% and severity of effects is some what reduced. However, Lamp Current Crest Factor is more (>2). High peaks of Lamp current affects the lamp life. (In Inverter ballast the lamp current crest factor is generally <1.7)
  • KVA demand charges for electricity will be more as KVA requirement is increased.
  • Power Factor is high (0.95)
  • Stroboscopic effect is present due to modulation (due to presence of pulsating DC) in high frequency operation leading to eyestrain.
  
 
 Active Power Factor Corrected (APFC) Ballast TopΛ
 
Active Power Factor Corrected (APFC) ballast consists of Active Harmonic filter to improve the power factor and minimize harmonics
 
  APFC ballast  
   
 
Buck/Boost circuit along with HF rectifier and control circuit forms an Active Harmonic filter. Fig. shows circuitry used for the same. Input to this filter is pulsating DC voltage from converter. The filter is designed to work in such a way that DC voltage at its output i.e. DC bus voltage of the DC-AC inverter is always maintained constant at 400 V (DC). This is explained below.
  
 
When the input line voltage is on higher side (above 240 V) the IC, senses it through feedback from input side of the filter. IC sends control signal to the MOSFET switch. The MOSFET remains 'ON' for lesser time; lesser energy is stored in the inductor L and returned to the output. Thus bucking action takes place and DC bus voltage of the DC-AC inverter is maintained constant at 400V (DC). Similarly, boosting action takes place when the input line voltage is on lesser side (below 240 V). Ultra High Frequency (UF) diode filters high frequency content in the DC voltage.
 
  Active Harmonic Filter in APFC Ballast:  
   
 
The control circuit i.e. IC also detects zero crossings of input line voltage and ensures input line current is in phase with the voltage. Thus power factor is corrected (to about 0.995)
  
 
As discussed above, DC bus voltage of the inverter always remains constant. Hence, the output voltage of inverter and wattage input to the lamp remains constant (for input line voltage variation 145V-275V). Therefore, light output also remains constant. Thus constant wattage operation and constant light output is achieved in case of APFC ballasts using active control.
 
 
Thus, power factor near unity (0.995), THD <10%, crest factor of 1.45 (near ideal value i.e. 1.41 for a pure sine wave, Current waveform follows the voltage waveform in phase as per fig) are the advantages gained. There are no modulations in output current waveform of ballast, so the light output is constant and hence stroboscopic effect is altogether eliminated.
 
  Summarizing, an Intelux APFC ballast has following features  
 
  • Constant wattage operation for 145V-275V. Lamp life is protected and enhanced since constant current is delivered to lamps.
  • THD is <10%, hence minimized effects of harmonics.
  • Power Factor is very high.(0.995)
  • Lamp Current Crest Factor is <1.7 therefore enhanced life of lamp. Lamp life protected.
  • No stroboscopic effect therefore reduced eyestrain.
  • Complies with all the safety, performance, EMI/ RFI and Mains Transient standards.
 
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