Digital Control Turns On the Lights
For the PDF version of this article, click here.
The goal of power management for lighting is to ensure that electric lights are switched off or dimmed at times when they are not needed. These times are when people are not present in a particular area or when sufficient or partial natural light is available. Fluorescent light sources are used predominantly in office environments and, by means of suitable solid-state control gear, can be dimmed to any level from 100% down to less than 5% of maximum light output or can be switched off altogether.
Such control gear consists of an electronic ballast, which in North America is supplied from a 120-Vac or 277-Vac line, and provides the correct starting and running frequencies for the particular type of fluorescent lamp used in a luminaire (lighting fixture). These systems generally operate a lamp at frequencies from 30 kHz to 100 kHz, at which the light-output to power-input ratio is optimized and flicker is eliminated. Electronic ballasts are often capable of driving two or more lamps and can be controlled remotely through a two-wire digital bus or other means. It is important that high-performance electronic ballasts also be designed to have a high power factor and low harmonic distortion of the ac line current. This presents an essentially resistive load to the ac line, which optimizes the utilization of power supplied by the energy provider. Installations where the overall power factor is low and the harmonic currents are high add additional strain on the generating system and may give rise to additional charges.
One widely adopted control protocol is the industry-standard digitally addressable lighting interface (DALI) system now finding its way to the North American market. This system allows up to 64 individually addressed ballasts to be connected in parallel to a single two-wire, low-voltage bus. The DALI input connections to a ballast are polarity independent, making installation straightforward. In a large installation, several separate DALI lines can be connected to groups of 64 ballasts controlled from a central controller capable of providing several separate DALI output buses.
Of course, there are other light sources used in installations, such as incandescent, halogen and high-intensity discharge (HID), which can be controlled equally with the DALI system. However, HID lamps are generally nondimmable or have limited dimming range compared to fluorescent lamps, and they are not capable of hot restrike without special auxiliary equipment.
Besides DALI, some of the other control methods adopted are 0-V to 10-V control, an older but reliable analog system, although it has the disadvantage of requiring a separate pair of polarity-specific control wires to each ballast, making installation much more complicated. The concept of ballasts being controlled via a wireless connection is attractive. However, such a system would be more expensive and perhaps less reliable, because the ballast and light fixtures generally are made from metal and would absorb some of the signal.
Other control methods include ballasts that can be dimmed through signals superimposed on the ac power line and ballasts that can be dimmed from a traditional phase-cutting dimmer. However, this method is less reliable because there are many different types of dimmers on the market, and it is not possible to guarantee reliable dimming with every different type. Also, it is well known that this type of dimming, while being effective for resistive incandescent lamps, is inherently unsuitable for discharge lamps such as fluorescent tubes.
At the moment, the DALI system appears to be the best tradeoff between cost, complexity of installation and reliability. It also has the added advantage of being capable of backward communication, sending status or other information from each ballast in the system back to the central controller.
Benefits of DALI
The objective of lighting power management is to minimize the amount of energy consumed in an installation firstly by harvesting as much natural light as possible and secondly by switching off lights when no personnel are present. A lighting management system would consist of a central controller connected to many dimmable fluorescent ballasts controlled by a DALI bus using occupancy and light level sensors around the building (Fig. 1). It also can monitor the overall power consumption of the building to enable load shedding, the ability to reduce building power demand by reducing lighting levels, and therefore the power required to light the building. Although an individual light fixture consumes a relatively small amount, the cumulative power utilized to light an entire building is a significant proportion of the overall load.
Electrical energy cost in some locations is a function of the amount of energy used or kilowatt-hours plus a peak-demand charge based on maximum power (kilowatt) draw during a period. Load shedding implements a strategy to keep the kilowatt demand on a minimum cost basis by controlling kilowatt consumption rates to predefined levels.
For example, a large office building pays a demand charge for 250-kW max draw at any one time. If the building rate of power draw climbs above 250 kW, the demand charges go to the next level, for example, to 500 kW, which is substantially higher than the 250-kW rate. Now, assume it is a hot summer day and the interior temperatures of the building are rising to uncomfortable levels. If the air conditioning motors are started, the building power consumption will increase into the next and more expensive tier of pricing. With the load-shedding strategy, the DALI controller can subliminally lower light levels and power consumption to the point where the HVAC motors of the air conditioning system can be started without the building entering the next cost level. Once the building temperature has been stabilized, the system can be programmed to raise lighting levels while not exceeding the 250-kW tier or an appropriate margin.
Another issue worth noting is lumen depreciation, which refers to the fact that over its lifetime the light output from a fluorescent tube will gradually drop. Lighting designers generally are forced to overdesign the lighting system, which results in higher lumen levels when the lamps are new to compensate for the drop in light levels over time. In a closed-loop system where light levels are monitored, it is possible to reduce ballast output levels when the lamps are new and increase ballast output levels slowly over time to compensate for this phenomenon, and therefore maintain a constant light level regardless of the level of lumen depreciation in the lamps. This presupposes that a sufficient number of luminaires have been installed such that if they were all set to maximum brightness, the light level would be higher than necessary.
Evidently, lighting control techniques can play a significant role in the power management of large installations. One of the key components required in such a system is the DALI-controlled electronic ballast for fluorescent lamp fixtures. The design of such a ballast requires the integration of several important analog and digital circuit blocks to create high-performance, high-reliability end product.
The Electronic Ballast
The electronic ballast requires EMI filtering and power-factor correction circuitry in order to meet the requirements of FCC standards and to provide the minimum harmonic distortion of ac supply current possible in its power range.
The ballast output circuit must provide stable and flicker-free dimming in a gas-discharge lamp, which by its nature is hard to control since its impedance varies depending on the level of ionization of gases taking place (Fig. 2a).
The lamp is essentially a resistive load; however, the resistance changes considerably depending on the amount of power that drives it. Consequently, the ballast-dimming control circuitry requires a complex control loop capable of compensating for lamp instabilities and providing a constant regulated light output. In addition, the ballast must include a microcontroller with software that reads the digital signals from the DALI bus and is able to interpret them and then provide control signals to the analog sections of the ballast. The microcontroller is also required to monitor the ballast and detect faults such as a failed or removed lamp and send a return signal via the DALI interface to the central control system. In this way, the system can advise maintenance staff exactly which light fixtures are not operating correctly. A working electronic ballast with all the components of Fig. 2a is shown in Fig. 2b.
One way in which an industry-standard microcontroller can be interfaced with an industry-standard fluorescent ballast dimming control IC is shown in Fig. 3. In this example, a PIC16F628 microcontroller from Microchip has been incorporated with an IR21592 dimming control IC from International Rectifier. These two parts have been chosen here because each one has the necessary functionality, as well as the input and output signals required to enable the DALI ballast design to be accomplished without undue complexity. There are many other alternatives that may be used in each case.
For the microcontroller, it is necessary to use a device that has some on-board EEPROM, because the DALI system stores various parameters within each ballast that need to remain in memory when the ballast is switched off. Such parameters include the ballast address, fade times and default light levels, as well as grouping information. It is impossible to conform to the published DALI standard without the ability for ballasts to store these parameters.
The microcontroller also requires one programmable pulse-width modulated (PWM) output, which is connected through a simple RC filter to produce a dc voltage between 0 V and 5 V that is proportional to the PWM duty cycle. This enables the microcontroller to generate a dimming control voltage, which is then fed into the dimming control IC to set the required light output. This approach is simple because it requires only that a value be placed into a register in the software, which will directly control the dimming level with only one resistor and one capacitor needed to couple the two ICs together.
The control IC can be enabled and disabled through its shutdown pin directly from a logic-level signal supplied from the microcontroller. When this signal is set high in software, the ballast will shut off, and when it is set low again, the ballast will go through a programmed preheat, ignition and start sequence that is provided by the IC and configured by means of a small number of external resistors and capacitors connected around it. In this way, when a DALI command is received telling the ballast to switch off, the microcontroller can determine that it is the ballast being addressed and execute the instruction. Similarly, the microcontroller can also restart the ballast when the appropriate DALI command is received.
The IC also has on-board protection features such as an overcurrent shutdown that will shut it down in the event of a failed ignition. If the control IC is shut down by means of its own built-in protection features, this needs to be communicated to the microcontroller. In this case, the FMIN output of the IC, which is high during normal operation, will go low and this signal can be fed back into the microcontroller, to a port that has been set up by the software to be an input.
Other microcontroller inputs are also used to determine if one or more lamps have been removed from the fixture. If this happens, it is necessary for the microcontroller to monitor the event and shut down the ballast controller IC. It is also necessary for it to use the same scheme to sense when new lamps have been placed into the fixture so that it can enable the IC to start up again and light the lamps. In each of the cases described, the microcontroller can send a suitable coded message on the DALI bus back to the central controller.
Interfacing and Programming
The DALI interface (Fig. 4) has to be galvanically isolated from the ballast circuitry as a safety requirement. The DALI bus is low voltage, and therefore presents no danger to maintenance staff. Isolation is achieved by using two appropriately rated optoisolators, one to receive and one to transmit, located between the DALI input and the microcontroller.
In this example, RB0 has been used as an enable that needs to be set high in software to provide power to the input and output drive circuits. Here a power-saving strategy has been used such that RA1 provides a weak pull up for the input signal when a DALI command is received; the microcontroller sets the receive-drive pin at RB1 high to provide a strong pull up. The ballast is designed to need as little power as possible during periods when the output is switched off. This is necessary to minimize power dissipation in the internal low-voltage supply circuits. This is not an issue when the ballast is running, as some of the high-frequency current can be used efficiently to power the system.
The ballast output stage shown in Fig. 2a consists of a switching half bridge driving a resonant output stage in which a series inductor feeds the lamp, which is in parallel with a resonant capacitor. The half-bridge switching MOSFETs are driven from the control IC that adjusts the output frequency, and in so doing controls the lamp power.
The principle of operation of dimming control is that the control IC senses the phase difference between the half-bridge switching voltage and the current in the resonant output circuit (Fig. 5). This is done by sensing the zero crossing of the voltage signal obtained from a current sense resistor in the lower switch of the half bridge. The phase difference is linearly proportional to the lamp arc power, enabling the lamp brightness to be adjusted simply by increasing the frequency to reduce the lamp power until the phase difference detected corresponds to the desired output.
The software to implement the DALI is quite large but simple to understand when broken into elementary functional blocks. Fig. 6 outlines the basic flow of the software in its most simplified form.
Upon entry into the program after setup, the microcontroller is held in a loop. While in this loop, the microcontroller is checking for errors, plus it is polling the communications circuitry for incoming data. If valid data is received, it is filtered to determine if the address matches the defined address for the ballast, if the group matches the defined group memberships, or if the data is a broadcast command for all ballasts to respond to. Also, the type of command is filtered into two basic choices: standard or special.
Once filtered, the program immediately vectors to the appropriate command and executes. All of the commands are divided into four general categories, including arc power control commands, configuration commands, query commands and special commands. Within each category, the commands are divided again according to related functions.
The arc power control commands are the most commonly used in a functioning lighting system. Within any of these commands, appropriate signals are sent to the ballast controller to adjust light level via pulse-width modulation. This includes scene level selections.
Configuration commands are used to set up the ballast. Examples of such settings include setting minimum and maximum lighting thresholds, fade times and rates, groups and scene levels. Generally, data is stored in an EEPROM, where it is maintained regardless of the power conditions.
Query commands are used to get feedback from the ballast. All of the settings can be queried. Even more useful, the status of the ballast is available. Information about fading, the lamp, general faults and power is available through the appropriate query command.
Special commands are immune to addresses, thus all ballasts on the DALI bus respond to a special command. All the functions for finding new ballasts or ballasts that have no addresses are available. Uploading information to the ballast, typically settings, is available in the special command set.
Sophisticated lighting control plays a significant role in the power management of large installations. The DALI system is ideally suited for this task because of its flexibility and simplicity. The successful DALI ballast is comprised of a cost-effective solution, integrating suitable analog and digital circuits, based on high-functionality integrated circuits.