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Today more and more lighting applications make use of electronics, and are replacing traditional magnetic ballasts with electronic versions primarily for energy efficiency reasons. Low-voltage halogen lamps are driven by electronic transformers; the classic magnetic ballast for fluorescent lighting is being replaced by the electronic ballast and the new high-power, high-efficacy LEDs are driven by switched mode current sources.
Depending on the application, the return on investment when replacing magnetic with electronic ballasts can be as short as one year, though the electronic version is much more expensive. However, an electronic ballast provides higher performance and instant, flicker-free start without degradation of lamp lifetime at a lower weight and volume, which is a great advantage in state-of-the art fixtures and designer lamps.
While in domestic applications the incandescent lamp — mainly in the form of halogen lamps — is still used most often, in professional applications such as office lighting, the fluorescent lamp dominates, which is the topic of our discussion.
Fluorescent lighting and electronic ballast
A fluorescent lamp (FL) consists of a glass tube with filaments at both ends, filled with low pressure noble gas, and an active component, mercury vapor. A current flow through this gas excites the mercury to emit invisible ultraviolet light. The ultraviolet radiation is converted into visible light by means of the phosphor coating of the tube. Prior to a current flow, the gas has to be ionized by applying a high-ignition voltage. After ignition, the ionized state is maintained by the continuous current flow. Ignition of the lamp is much easier and lifetime is considerably increased by preheating the filaments to at least 600° to 700°C.
The gas discharge has negative differential resistance, i.e., current increases while operating voltage drops. Consequently, any gas discharge needs a current limiting element in a series. The classic magnetic ballast together with the starter as shown in Fig. 1 perfectly implements the requirements to operate a fluorescent lamp. Initially, the starter switch S1 is closed and a current flows through L1 and the filaments of the lamp. When the starter opens after a certain time — how this is implemented is beyond the scope of this article — the filaments are at high temperature and the abrupt change in current induces a high voltage in the inductor and across the lamp.
After ignition the impedance of the inductor limits the discharge current. Some disadvantages of this simple ballast are obvious, others not. First, the starter switch may open when the line voltage is close to crossing zero volts. Current flow is small at this time and the same is true for the ignition voltage. The lamp may not strike and one can easily identify a magnetic ballast by recognizing several attempts to start the lamp. A less obvious disadvantage is the poor system efficiency due to two reasons. First for the sake of cost, a high loss in the inductor itself is acceptable. Secondly, the ions in the discharge recombine during zero crossing of the line voltage and have to be re-ionized in the next half-cycle. The latter effect results in a considerable loss of energy.
Figure 1: Magnetic FL ballast with starter
One of the main advantages of the electronic ballast is that the lamp is driven with a much higher frequency of 30 to 60 kHz typically. Due to the higher frequency, the recombination of ions does not happen and the efficacy of the lamp itself increases about 10% compared to operation at 50/60 Hz. Moreover, the electronic ballast itself is designed to achieve efficiency of more than 90% and together with state-of-the-art high-efficiency FL (so called T5 lamps) the energy savings can easily achieve 30% compared to a magnetic ballast at line frequency. Consequently, the European standard EN 50294 lists four efficiency classes of magnetic ballast and according to directive 2000/55/EG, the class D with "very high" losses has been abandoned since 2002 and class C with "moderate losses" since 2005.
Fluorescent lamp technologies are classified according to their diameter in multiples of an 8th of an inch, e.g., T8 means a Tube with 8/8" (~26 mm) diameter. In most domestic applications in Europe the T8 fluorescent lamp is still used most often while in professional applications the T5 lamp is used more often. For the latter the operation with magnetic ballast is no longer specified within the standards.
Other advantages that an electronic ballast may have are perfect preheating of the filaments, making lifetime of the lamp virtually independent from the number of switching cycles, along with flicker-free start and operation, constant light output with variable input voltage and high power factor. Finally, important for emergency lighting, the electronic ballast can be operated with a DC input voltage (from batteries). The topology for the most popular FL ballast in Europe is the voltage-fed series resonant half-bridge shown in Fig. 2.
Figure 2 : Block diagram of a FL ballast used in professional applications
(Click on Image to Enlarge)
The half bridge is driven with variable frequency and a duty cycle close to 50%. At startup, as long as the FL is not ignited, the ballast controller generates frequency well above the resonant frequency of L1/C1. Thus a high current flows through the lamp filaments heating them up to the desired temperature. After a time that is normally determined by external components, the controller starts to lower operating frequency towards the resonance. A high voltage across the lamp is generated as a result and the lamp will ignite. After ignition the impedance of the FL damps the resonant circuit fairly well and the voltage across the lamp drops close to the operating voltage.
In most applications the lamp current is sensed directly or indirectly and the operating frequency is adjusted until the set-point is met. As long as the operating frequency is above the resonant frequency of L1/C1, the MOSFETs are soft switched and switching losses are negligible while at the same time electromagnetic interference (EMI) is reduced.
MOSFETs with fast recovery body diode (FRFET) are perfectly suited for this application ( ). There are 500-V and 600-V Q-FETs available with fast body diode as well as 600-V SuperFETs. Since the gate of the upper MOSFET needs high voltage drive, a high side gate driver is needed. The high-voltage drivers — FAN7380, FAN7383, FAN7384 and FAN7382 — implement all needs with best in class noise immunity. Finally, there are pure ballast controllers like the FAN7544 that implements the control and safety functions as well as controllers with integrated high-voltage gate drive like the FAN7532.
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