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Advanced Power Electronics Deliver Greater Efficiency in Less Space for Fluorescent and HID Lighting

Advanced Power Electronics Deliver Greater Efficiency in Less Space for Fluorescent and HID Lighting

Power electronics and control circuits are fundamentally changing lighting applications, providing higher efficiency, safety and comfort in a smaller space. Low voltage halogen lamps are driven by electronic transformers, while the classic magnetic ballast for fluorescent and HID lighting is replaced by the electronic ballast and the new high power high efficiency LEDs are driven by switched mode current sources.

The most important reason for replacing the iron cored 50/60Hz transformer or ballast is energy efficiency. Since lighting consumes 10-12% of the total power consumed in commercial and residential applications, replacement of traditional magnetic ballasts and incandescent bulbs with efficient electronic lighting provides substantial gains in overall energy savings and enables compliance with regional and global standards. In addition to lower cost and energy efficiency, Fairchild’s electronic lighting solutions provide higher reliability, safety and comfort that consumer and commercial users expect in next generation lighting. Fairchild’s solutions for linear fluorescent (LFL), compact fluorescent (CFL) and HID lighting include:

• Ballast control ICs
• High Voltage Gate Drivers (HVIC)
• Power MOSFETs, including SuperFET™ and QFET MOSFETs
• PFC Controller ICs
• Diodes and Rectifiers
• Power Transistors for CFL

 

Replacing the magnetic ballast in LFL
Eliminate the power losses, size and weight of the inductor used in traditional magnetic ballasts.

Linear fluorescent lighting is widely used in commercial office and industrial applications, and is gaining acceptance in residential applications as well. Under optimum conditions, linear fluorescent tubes provide high efficiency and long life compared to incandescent tubes. The biggest drawback has been the iron cored 50/60Hz magnetic transformer or ballast required to initially ionize and regulate the plasma.

Electronic ballasts are quickly replacing traditional magnetic ballasts and offer many advantages, including higher efficiency, performance and reliability in addition to smaller overall size and better form factor. Further, magnetic ballasts are becoming less economical – both in the short and long term – due to the rising costs of copper and energy. Depending on the application, the return on investment when replacing magnetic with electronic ballasts can be as short as one year. At the same time, an electronic ballast gives higher performance and comfort at lower weight and volume, a great advantage in state-of-the art fixtures and designer lamps. A common FL ballast topology is the voltage fed series resonant half-bridge shown in Fig. 1.

Gain efficiency and ergonomics

One of the key advantages of the electronic ballast is that the lamp is driven with a much higher frequency, typically 30 – 60 kHz. Due to the higher frequency, the recombination of ions is virtually eliminated and the efficiency of the lamp itself increases about 10% compared to magnetic ballasts operating at 50/60Hz. Moreover, the electronic ballast circuitry is designed to be 90% efficient, and compared to magnetic ballasts overall energy savings of at least 30% can be easily achieved using high efficiency FL (i.e. T5 lamps). Electronic ballasts are also more ergonomic since the high operating frequency eliminates audible 50/60Hz hum produced by the magnetostriction common in magnetic ballasts.

Another advantage of the electronic ballast is that start up timing is electronically controlled. In a traditional magnetic ballast, nothing prevents the starter switch from opening during zero crossing of the line voltage and the lamp may not start immediately and flicker as it attempts to restart. The electronic ballast automatically compensates for voltage fluctuations while providing a high power factor, delivering constant light output and flicker-free start up and operation.

The electronic ballast also provides near ‘perfect’ preheating of the filaments, which makes the lifetime of the lamp virtually independent from the number of switching cycles. Finally, the electronic ballast can be operated with DC input voltage such as batteries, which is important in portable and emergency lighting applications.

The half bridge FL ballast in Figure 2 is driven with variable frequency and a duty cycle close to 50%. At start up, as long as the FL is not ignited, the ballast controller generates frequency well above the resonant frequency of L/C. Thus a high current flows through the lamp filaments and heats 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 L/C resonance. As a result, a high voltage across the lamp is generated and the lamp ignites. 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 L/C the MOSFETs are soft switched, reducing EMI and making switching losses negligible.

Fairchild’s MOSFETs with fast recovery body diode (FRFET™) are perfectly suited for this application (also see Ref. 1), including 500V and 600V Q-FET™
available with fast body diode as well as 600V SuperFET™. Since the gate of the upper MOSFET needs high voltage drive, a high side gate driver is needed. The high voltage drivers http://www.fairchildsemi.com/whats_new/hvic/prod_info.html
FAN7380 , FAN7383 , FAN7384 and FAN7382 are ideal for this application and offer best-in-class noise immunity. Fairchild also offers dedicated ballast controllers such as the FAN7544 that implements the control and safety functions, as well as controllers with integrated high voltage gate drive such as the FAN7532

 

Power Factor Correction

Current international standards require power factor correction in lighting equipment if consumed power is above 25W to reduce losses in the power grid. Since most fixtures have a total power below 150W, critical (also known as boundary, or transition) mode PFC is the most economic solution. In this mode the peak current through the inductor is controlled to be proportional to the rectified input voltage. During off-time, the inductor current falls back to zero, and zero crossing of the current (i.e. de-magnetization of the inductor) initiates the next switching cycle. It is easy to see that the average inductor current is proportional to the input voltage, as desired.

There are two common approaches to control the peak inductor current. In the “current mode” as implemented in the FAN7527 PFC Controller the rectified line voltage is sensed to generate the current reference that sets the actual value of the peak current. However, the necessary divider network can cause considerable loss, which is to be avoided. In “voltage” or “constant on-time” mode (see Figure 3) implemented with the FAN7529 PFC Controller, the on time of the switching device is kept constant during one or more line half-cycles. Keeping on-time constant, peak switch current is again proportional to input voltage according to the basic differential equation dI/dt = V/L. Common to both modes is the sensing and regulation of the output voltage.

Various topologies are available to implement low-cost PFC, but they lack the reliability and ease of design of the integrated PFC. For example, a high inductance iron core choke can be used to smooth input current, but this brute-force method is inefficient. More commonly, the power switch and the control IC are omitted and a charge-pump PFC is used. In this topology the half-bridge is used to simultaneously drive the fluorescent lamp and the PFC as shown in Fig. 4. Since the lamp power has to be regulated and there is no additional degree of freedom that could be used to control the PFC, it is very difficult to find proper L and C values that result in good power factor and stable lamp operation over a wide input voltage range. Although it is an inexpensive solution, this is the main reason that this topology is being replaced with integrated PFC.

End of Lamp Life Detection

In a gas discharge there is a region close to the cathode where the discharge voltage drops steeply and no light is emitted – a phenomenon called “cathode fall’. Due to the voltage drop and current flow there is a certain amount of power dissipated in this region. With increasing operating time the filaments of the lamp become less emissive and the cathode fall increases. In turn, the power dissipation close to the cathode increases and temperature rise in this region of the lamp accelerates. If the diameter of the lamp tube is small, it could be heated up to the melting point. Therefore, for thin tubes a feature called EOL (End of Lamp Life) detection becomes indispensable, especially for small T5 (5/8”) tubes. EOL detection is a standard feature in many of Fairchild’s fluorescent lamp controllers.

Compact Fluorescent Lamps (CFLs)

CFLs contain an electronic ballast integrated with the lamp in a disposable assembly, and are typically used to replace traditional incandescent bulbs. Consequently the electronics of a CFL does not need to have the extraordinary lifetime of an FL ballast (up to 50,000 hours). In addition, CFLs are designed for relatively low power for use in compact fixtures and do not require PFC. While having the same basic structure as an FL ballast, the CFL uses a slightly different inverter circuit. Most CFLs use a self oscillating half bridge without a control IC as shown in Fig. 5.

As shown in Figure 6, newer integrated circuits like Fairchild’s FAN7710 or FAN7711 controller and high voltage gate driver help to simplify the design of CFLs while being competitive in cost. This is especially true if additional performance and safety features implemented by the integrated controllers are considered.

High Intensity Discharge (HID) Lamp Driver

HID lamps are typically used in high power industrial and automotive applications, making efficiency and performance especially critical. An HID drive circuit is shown in Figure 7. Typically, a PFC circuit provides 400V to a buck converter followed by a full wave bridge inverter that drives the HID lamp. In both stages, HVICs are used to drive Power MOSFETs such as SuperFET™ and QFET™ MOSFETs. Fairchild’s HVICs improve system reliability by utilizing an innovative common-mode dv/dt noise canceling circuit that provides excellent noise immunity. With 600V capability and fast 50V/ns switching speed, these devices are optimal for driving MOSFETs and IGBTs in HID applications.

Conclusion

Replacing the traditional magnetic ballast in linear and compact fluorescent lighting saves considerable power, size and weight. In addition to helping to meet worldwide energy standards, power electronics and control circuits are revolutionizing fluorescent and HID lighting applications with advanced features such as smart end of lamp life detection, power factor correction and reliable lamp start and operation. Fairchild’s broad power portfolio features a diverse number of high performance lighting components to meet these needs, including the LFL and CFL controllers, HVIC drivers and power MOSFETs such as SuperFET™ and QFET™ MOSFETs. These advanced components enable the next generation of lighting solutions to provide higher efficiency, reliability, safety and comfort for consumer and commercial users.

For more information on Fairchild lighting solutions, visit: http://www.fairchildsemi.com/markets/lighting/

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