Today's lighting applications include residential lighting, office interior lighting, retail and display lighting, outdoor lighting, mood lighting, and emergency lighting. These applications have different lighting needs and are satisfied by specific lighting technologies. The top five global lighting technologies (excluding incandescent) include compact fluorescent, linear fluorescent, halogen, high-intensity discharge and light-emitting diodes (see the table).
Residential and hotel applications typically use compact fluorescent lamps (CFLs) to replace incandescent lamps, reduce energy costs and for easy insertion into existing ceiling or table fixtures. Office interior lighting applications use tubular fluorescent lamps because of their large area of light coverage. Halogen lighting is the preferred technology for display and retail lighting due to the color and spotlight produced by these lamps. High-intensity discharge (HID) lamps deliver a high-brightness output and typically are used for outdoor and street lighting applications.
Finally, light-emitting diodes (LEDs) are used for mood and emergency lighting applications because of their color-mixing capabilities, low-power consumption, low maintenance and ultra-long lifetime. These lighting technologies have different electrical characteristics, and each technology requires a unique control solution.
Compact Fluorescent
A compact fluorescent lamp consists of a small-diameter fluorescent lamp that has been formed into a compact shape, a polycarbonate mid-section that contains the control circuitry for powering the lamp and an Edison screw base for mounting the lamp into a standard incandescent lamp socket.
The fluorescent lamp consists of a glass tube filled with Argon gas and a small amount of mercury. Filaments are located at each end of the tube and usually are coated with Boron to help emit electrons. As electrons flow across the tube from one filament to the other, they collide with mercury atoms. The excited mercury atoms give off UV light, which is then converted into visible light as it passes through the phosphor coating on the inside of the tube. The higher the number of collisions, the higher the light output from the lamp. The spiral shaped CFL has proved to be the most popular because the light produced in the center of the lamp can escape to the outside, giving a higher light output than other shapes.
Fluorescent lamps have three operating points — preheat, ignition and run — that must be fulfilled in a proper sequence. To accomplish this, a resonant mode circuit is used. During preheat and ignition, the circuit has a high-Q and is under-damped. The circuit starts at the preheat frequency for a fixed time to preheat the lamp filaments and then decreases the operating frequency toward resonance to increase the output voltage to ignite the lamp. After the lamp ignites, the circuit becomes a low-Q, over-damped circuit and the frequency is decreased further until the rated lamp power is reached (Fig. 1).
The running working voltage of a fluorescent lamp ranges from about 50 Vac to 150 Vac, depending on the lamp type and power rating; the operating frequency is typically around 50 kHz. The control circuit first rectifies and smoothes the ac mains voltage to produce a dc bus voltage with an amplitude of several hundreds of volts (Fig. 2). The circuit then generates a high-frequency square-wave using a high-voltage control IC and two MOSFETs connected in a totem-pole half-bridge configuration. The high-voltage square-wave drives the resonant output stage across the lamp and sweeps the frequency to fulfill the lamp operating points.
Tubular Fluorescent
Tubular fluorescent lamps have the same electrical characteristics as compact fluorescents, but the control circuit must fulfill additional performance and protection requirements. The block diagram of a tubular fluorescent ballast circuit depicted in Fig. 3 includes an EMI filter to block ballast generated noise, a bridge rectifier, a power factor correction (PFC) circuit for sinusoidal input current, half-bridge MOSFETs with driver and timing for high-frequency operation, and final resonant output stage for fulfilling the lamp operating points. In addition, the control circuit must protect against various lamp and line fault conditions. The ballast must survive many years of transient voltage spikes or brownouts at the mains input, as well as lamp failures and re-lamping at the output.
The primary difference between the tubular fluorescent ballast and the CFL ballast is the addition of the PFC circuit. PFC is mandatory in Europe for power levels above 25 W and the boost-type topology has proven to be the most popular solution. Not only does the PFC provide a sinusoidal input current that is in phase with the input voltage, but it also regulates the dc bus voltage at a constant level.
This approach means the half-bridge and resonant output stage is fed with a constant voltage so that the light output from the lamp stays constant over a wide ac mains input voltage range. This allows for the ballast to work across a universal input voltage range, and it becomes easy to adapt the output stage to different lamp types or lamp configurations while maintaining the same design platform.
Halogen
Halogen lamps use a tungsten filament that is encased inside a small quartz envelope. Similar to an incandescent lamp, the electrical current causes the tungsten filament to heat up to temperatures above 2500°C and get “white hot” and produce visible light. Halogen and incandescent lighting technologies use heat to excite atoms. With a halogen lamp, the envelope is much closer to the filament. Thus, to prevent it from melting, the envelope is made from quartz. The gas inside the envelope consists of a gas from the halogen group.
These gases have an interesting property in that they combine with tungsten vapor. If the temperature is high enough, the halogen gas will combine with tungsten atoms as they evaporate and redeposit them on the filament. This recycling process lets the filament last much longer than incandescent. In addition, because the filament is running hotter, more light per unit energy is achieved. This is why halogen lamps are ideal for intense “spot” lighting applications.
The halogen lamp has different electrical characteristics than fluorescent. There is no high-voltage ignition required to start the lamp, and the working lamp voltage is limited to between 11.2 V and 11.7 V. Anything below this range will give a reduced light output and anything above will cause premature aging. Halogen control circuits are known as “halogen converters” or “electronic transformers.” They step the mains input voltage down to the required working voltage range, provide galvanic isolation and allow for the lamp to be dimmed with a standard phase-cut triac wall dimmer.
The applications for these lamps include long exposed tracks to which the lamps are connected. These tracks are continuously being touched directly by the user, and lamps are being connected and moved depending on the desired lighting effect. To avoid electrical shock to the user, these tracks must be galvanically isolated from the mains. The control circuit also must protect against open-circuit (no lamps connected), overload (too many lamps connected) and short-circuit (tracks shorted together) fault conditions.
To satisfy these requirements, a typical halogen converter circuit includes EMI filtering, rectification, two half-bridge MOSFETs, a transformer to step the voltage down, and a control IC (Fig. 4). This chip drives the half-bridge at high frequency, regulates the lamp output voltage, provides soft-start and dimming, protects against all fault conditions and restarts the lamp when the fault is removed.
High-Intensity Discharge
HID lamps are available in the form of mercury or sodium vapor and typically are used as street lamps and outdoor lighting for stadiums. These lamps are popular because they are efficient and have a high-brightness output. In the case of sodium vapor, they are twice as efficient as normal fluorescent bulbs.
Mercury and sodium vapor lamps produce light using a technique similar to that used in fluorescent lamps, where a low-pressure mercury vapor produces lots of ultraviolet light that excites a phosphor coating on the tube. In the case of mercury vapor lamps, it is a high-pressure gas, the distance between the electrodes is short and the light is produced directly without the need for the phosphor.
HID lamps require a high ignition voltage (3 kV to 4 kV) and even higher (>20 kV) if the lamp is hot. In addition, they have working voltages of 100 V to 200 V. HID lamps also are typically operated at low frequency (100 Hz to 200 Hz) to avoid damage from acoustic resonance. The HID ballast block diagram shown in Fig. 5 includes an EMI filter, a bridge rectifier, a boost converter for PFC, a buck converter to control the lamp current, an ignition circuit, a full-bridge for driving the lamp at low-frequency, and various high-voltage driver ICs and circuitry for controlling the different blocks.
Light-Emitting Diode
The LED is an emerging lighting technology due to recent breakthroughs in high-brightness LEDs. LEDs have always been attractive light sources because of their ultra-long lifetime and low-power consumption. However, they were limited to only a few colors and low brightness levels. High-brightness LEDs now are available in many colors and have enabled this technology to penetrate into certain mainstream general-purpose lighting applications, including emergency lighting, traffic lights, display and mood lighting.
LEDs work on a completely different principle than ordinary lamps. As in silicon and germanium-based semiconductors, the compound semiconductor materials used to build LEDs exhibit the band-gap property whereby individual electrons move in jumps that involve much higher energy levels than those associated with electron flow in conductors. In one particular family of compound semiconductor materials based on the rare metal gallium, the band-gap is so wide that appreciable energy is needed to make electrons jump.
When each electron recombines with an atom, it emits a particle of light called a photon. These jumps take place across the junction between the n- and p-type regions of the semiconductor crystal. Semiconductor materials based on gallium have the useful property of being transparent, allowing the light generated to escape from the junction.
LEDs are much simpler to control than other lamp types but still have their own set of requirements and challenges. They do not need to be ignited or preheated, but the current should be constant and matched in each LED. Furthermore, depending on the application, the electrical connection to the LEDs may or may not need to be galvanically isolated. A typical nonisolated off-line circuit used is a high-voltage buck converter for controlling the current through a string of LEDs connected in series (Fig. 6).
The buck operating frequency is maximized to reduce the size of the inductor. Each LED has a given voltage drop; thus, the number of LEDs that can be connected depends on the available input voltage to the circuit. The source of the MOSFET is at the high-voltage input each time the MOSFET is turned on, requiring a high-voltage gate driver IC.
The control circuitry then senses the LED current through a current-sensing resistor and controls the buck on- and off-time to keep the current constant. Finally, a shutdown input is used to dim the LEDs using a low-frequency PWM on/off dimming control signal.
Ballast Design Software
Available from International Rectifier is a software program to help designers quickly begin new ballast designs. The program, Ballast Designer V4.0, offers a friendly graphic interface that allows users to select from various control ICs, lamp types, and input and output configurations. When all of the parameters are selected, the user can then generate a schematic, bill of materials and inductor specifications so they can get started on their prototyping. An advanced display page also is available for closer examination of the calculations, operating points and time-domain waveforms.
To download this software and for more information on the control ICs mentioned in this article, visit www.irf.com.