The word is officially out – inefficient light sources are not cool! Pardon my poor pun, but we are rapidly becoming aware of the impact that our energy consumption has on the environment. Lighting applications consume a large portion of our overall energy consumption. According to the US Department of Energy, 12 percent of residential energy and 25 percent of commercial energy is consumed by lighting. So, there is a significant amount of energy savings to be made by using more efficient lighting technology.
The incandescent bulb has hardly changed since it was designed and has an efficacy of approximately 8 lumens per watt. Efficacy is a measurement of efficiency used by lighting gurus that specifies how many lumens of light output are produced for each watt of input power. Approximately 95 percent of the power put into an incandescent bulb creates heat, not light. There are alternatives to the incandescent bulb that easily provide two to10 times higher efficacy values.
Now, I am not trying to pick on the incandescent bulb. If efficacy were the only important quality of a light source, incandescent bulbs would have long since vanished. Other parameters such as lifetime, durability, and quality of light are important, depending upon the type of lighting application. For best efficiency, all lighting technologies can benefit from switch-mode power supply (SMPS) systems. Intelligent control can be applied to any lighting technology to minimize energy loss through active conservation. Therefore, intelligent embedded-control systems that include SMPS-control features are a necessary component for developing energy-efficient lighting applications.
I am going to show you some examples of how embedded control can be applied to different lighting technologies – even the incandescent light bulb, providing control of SMPS circuits and adding intelligence.
DIM THE LIGHTS, PLEASE
Incandescent bulbs have remained desirable for the high color-rendering index (CRI) that can be obtained. The CRI of a light source is a measure of its ability to faithfully reproduce the colors of an object that is illuminated by the source. A monochromatic light source would have a CRI of 0, since only one color can be reproduced. Incandescent bulbs have a CRI very near 100, which is the maximum possible value. We have grown accustomed to the warm, pleasing light that incandescent bulbs provide in our homes. Incandescent technology is also popular in retail lighting applications, where it increases the appeal of products on display.
The addition of an embedded processor to a lighting application does not have to be complex, as shown in the schematic of Figure 1. A lot of energy can be saved by simply dimming an incandescent bulb. The circuit in Figure 1 uses a six-pin PIC10F200 MCU to control a Triac circuit. The Triac controls the light intensity by controlling the amount of conduction time in each half-cycle of the AC-input voltage. In effect, the Triac performs a PWM function on the incoming AC voltage. The light intensity is reduced by waiting for a longer time from the start of the AC cycle to turn on the Triac, as shown in Figure 2.
Two I/O pins are required to control the Triac. The MCU monitors a sample of the AC-line voltage on an input pin to obtain zero-crossing information. It can then use the zero-crossing information to implement a variable Triac firing delay.
Now some of you engineers might say, “I don’t need a MCU to control a Triac. I can do that with a simple RC-delay circuit.” However, the MCU offers some advantages here. A Triac requires a certain amount of gate-bias current to get current to flow. Also, a Triac has a minimum holding-current specification. When the amount of current flowing through the Triac exceeds the holding current, the gate bias can be removed and the Triac will continue to conduct.
What this means is that the Triac can be energized with just a short pulse on the gate when a MCU is used, so the bias circuit will have a very low current when averaged over each AC cycle. Therefore, smaller and less expensive bias-circuit components can be used. The MCU will need a 5V power supply as well, but it turns out that an inexpensive resistor and Zener-diode circuit can be used because the MCU draws less than 500ìA average current. Two 11K, 1/8W resistors are used in this example application to generate the MCU bias supply.
Now that you have a MCU in the circuit, you can add additional functions, including remote control, motion sensing, and timing-related functions. Additionally, the dimming control can be made linear. Since the AC voltage has a sinusoidal profile, you will not get a linear relationship between the Triac firing delay and light intensity. This can be easily fixed using a lookup table to translate the requested lamp intensity into an appropriate firing angle.
If you want to control the circuit using a photocell or IR sensor, it is best to power these devices using an I/O pin on the MCU. In this way, the sensing devices can be enabled only when required, to conserve power drawn from the 5V bias circuit.
DIGITAL LIGHTING
Fluorescent lamps offer a much more efficient source of light over incandescent bulbs. Although the quality of light is not as pleasing (lower CRI), the efficacy of a fluorescent bulb is typically ten times higher than that of an incandescent bulb. Fluorescent bulbs are most widely used in commercial applications where energy costs must be kept low. they are also finding increased use in residential applications as consumers become more interested in energy savings.
Fluorescent ballast designs have traditionally been based on magnetic (inductor) circuits, but they are rapidly moving to electronic designs to increase system efficacy. At a minimum, the fluorescent ballast must regulate the bulb current. The resistance of the bulb varies widely, depending upon the operational state. The fluorescent bulb consists of a glass tube filled with a small amount of mercury vapor and an inert gas. A tungsten filament is located at each end of the bulb.
Before the bulb is lit, the gas will have a very high resistance. To start the bulb, current is passed through the filaments (not through the gas) so that they are heated and begin to emit electrons. Then, a high voltage potential is applied across the two filaments to strike an arc in the gas mixture. Once the arc is struck, the resistance of the gas mixture drops significantly due to an avalanche effect. The ballast must lower the voltage across the filaments to maintain the proper current flow through the mixture.A resonant circuit is commonly used to control the bulb current for switch-mode ballast applications. An inductor and capacitor are placed in series with the bulb, as shown in Figure 3. A second capacitor is placed across the filaments.
A square wave, variable-frequency oscillator (VFO) drives the resonant circuit through a pair of power transistors connected to a DC bus. A dead-time generator provides complementary signals for the power transistors and ensures that shoot-through currents are eliminated.
The current flow through the lamp is regulated by the frequency of the VFO. To start the lamp, a high frequency is applied to the circuit. This causes current to flow through the filaments and the filament capacitor Cf. The high-frequency operation heats the filaments so the lamp can be started.
After heating the filaments, the frequency of the VFO is changed to a lower frequency. The voltage across the filaments rises rapidly and strikes the arc. When the bulb is lit, the frequency of the VFO can be adjusted to obtain different bulb currents and light-output levels.
A rectifier and filter-capacitor circuit are usually the first things that you will find in an electronic ballast circuit to convert the incoming AC voltage into a DC bus for the resonant converter. Unfortunately, this causes the ballast to consume current only at the peaks of the incoming AC voltage. Power factor correction (PFC) is required in an electronic ballast to increase efficiency and eliminate input-current harmonics.
In many ballast designs, separate ICs are used for the PFC function, ballast-control function and external-control functions. However, a digital signal controller (DSC) can be used to implement a complete digital-ballast solution, as shown in Figure 4. The dsPIC33F DSC has been used in this circuit because its 16-bit CPU has the calculation performance required to simultaneously perform PFC, control the resonant mode inverter, and respond to external control signals if required.
There are many ways to describe how a PFC circuit works, but basically the PFC circuit tries to make the input-current waveform follow the same sinusoidal profile as the input voltage. One of the most popular ways to implement PFC is with a voltage-boost circuit. The inductor current, and therefore the input current, can be controlled by the duty cycle that is applied to the inductor switch. Ultimately, the rectified AC voltage is boosted to a higher value, usually around 400V DC. So, the PFC circuit is a boost-voltage regulator. The voltage-regulation function can easily be performed by a digital control loop. There is a special requirement for this voltage regulator. It uses an inner-current control loop that controls the current profile in the inductor of the PFC circuit. The output of the voltage control loop provides a command to the current control loop, to set the amount of input current. Before the current command is provided to the current control loop, it is mixed with a sample of the rectified input voltage. This mixing forces the input current to have the same shape as the input voltage, as shown in Figure 5.
The ballast application uses two PWM channels to implement a fully digital ballast solution. One PWM channel drives a half-bridge circuit connected to the lamp. The other PWM channel controls the PFC-boost circuit. The ADC monitors two voltages and two currents. The DC-bus voltage, AC-input voltage, and input current are monitored for the PFC function. The lamp current is monitored to control lamp brightness and detect bulb failures.
Proportional-integral-derivative (PID) controllers are used in the PFC algorithm to regulate the bus voltage and input current. The behavior of each PID controller (and the PFC algorithm) can be modified by changing software coefficients. The DSC device has the CPU bandwidth to execute these PID controllers. In particular, the inner-current control loop will be executed at the same frequency as the PWM signal that is applied to the boost-circuit switch. A frequency of 100kHz or more is often used in the boost circuit, to keep the inductor size small.
The circuit currents are measured using simple shunt resistors in series with the power switches. This lowers the circuit cost, but requires a little extra work to get the data. The voltage on the shunt resistor only indicates the circuit current at certain times in the PWM cycle. Therefore, the ADC measurements are automatically triggered by the PWM timebase to ensure that the shunt resistors are sampled at the correct time.
WHATEVER COLOR YOU WANT
The world’s longest lasting light bulb is 105 years old and is installed in a firehouse in California. Unfortunately, most modern-day light bulbs don’t last as long as this one. When lifetime, efficacy, and durability are important, power LEDs have the most to offer.
Present power LED technology provides efficacy values that rival fluorescent technology. In addition, the power-LED industry expects to double present efficacy values in the next few years. The lifetime of LEDs can exceed 50,000 hours, which is a huge benefit in commercial applications where the cost of changing the light bulb is not free.
Although LEDs have tremendous efficiency and lifetime advantages, the CRI of white LEDs can be very low. Many white LEDs have a bluish color. This is because the white light is produced by covering a blue or infrared emitter with yellow phosphorus to shift the light to the desired wavelength. The final color spectrum produced by the LED is limited by the initial wavelength of the emitter and the ability of the phosphorous to spread the light energy across the visible spectrum.
One emerging application for power LEDs is backlighting for LCD video displays. Many current LCD-panel designs use fluorescent technology to provide the backlighting. LED technology improves the quality of the LCD image by using separate red, green and blue (RGB) emitters. The use of separate emitters allows much greater control of the produced color spectrum over white LED technology. Within a range defined by the three component colors, any color can be generated.
RGB LEDs allow the LCD panel to produce a broader range of colors than a typical fluorescent design. Additionally, the LEDs can be modulated on and off using the video scan information. Unlike other lighting technologies, LEDs have instant on and off times. The scan modulation allows the LCD panel to produce a sharper image.
Figure 6 shows a RGB LED-control circuit that could be used for a LCD panel application or even a general-illumination application. The LEDs must be driven with a source of constant current. When multiple LEDs are used, they are normally connected in series so that each LED receives the same amount of current. The choice of current-drive level will be a tradeoff between the amount of light produced, efficacy and possibly thermal limitations.
Three MCP1630 switching devices are used to implement the constant-current drivers. These devices are MCU peripherals that contain the analog components necessary for a SMPS control loop. A clock signal is provided by the MCU, to set the switching frequency and limit the maximum duty cycle. A buck or boost topology could be implemented in this circuit, depending upon the available input voltage and the forward voltage across the string of LEDs.
The PIC18F1330 MCU was selected because it has three 14-bit PWM channels. These PWM channels are used to modulate the outputs of the three constant-current drivers and set the light intensity of each color. The high PWM resolution is required, to allow accurate color control over a wide range of brightness levels.
The wavelength and light intensity of LEDs can change with variations in manufacturing process, age, and drive-current level. In most backlighting applications, active color control is needed to ensure a consistent color, and brightness is produced. A RGB sensor is used to detect each component of the light output. The MCU or DSC calibrates the output of the sensor, determines the amount of color error, and calculates three PID routines that set the R, G and B component levels.
Quite a few topics have been discussed in this article. Hopefully, it has given you many ideas for your next project. Efficient lighting applications require power-circuit control and intelligence. You can integrate both of these functions with a MCU or DSC, decreasing circuit complexity, and increasing flexibility.
Click here for Illustrations:
Figure 1, Figure 2, Figure 3, Figure 4, Figure 5, Figure 6
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