Control Theory

The operation of a fluorescent tube requires several components around the tube, as shown in Figure 2-3. The gas mixture enclosed in the tube is ionized by means of a high voltage pulse applied between the two electrodes.

To make this startup easy, the electrodes are actually made of filaments that are heated during the tube ionization startup (i.e. increasing the electron emission), their disconnection being automatic when the tube goes into the steady state mode. At this time, the tube impedance decreases toward its minimum value (depending upon the tube internal characteristics), the current in the circuit being limited by the inductance L in series with the power line. The starting element, commonly named “starter”, is an essential part to ignite the fluorescent tube. It is made of a bimetallic contact, enclosed in a glass envelope filled with a neon based gas mixture, and is normally in the OPEN state. When the line voltage is applied to the circuit, the fluorescent tube exhibits a high impedance, allowing the voltage across the “starter” to be high enough to ionize the neon mixture. The bimetallic contact gets hot, turning ON the contacts which, in turn, will immediately de-ionize the “starter”. Therefore, the current can flow in the circuit, heating up the two filaments. When the bimetallic contact cools down, the electrical circuit is rapidly opened, giving a current variation in the inductance L which, in turn, generates an over-voltage according to Lenz’s law.

Since there is no synchronization with the line frequency (the switch operates on a random basis), the circuit opens at a current level anywhere between maximum and zero.

If the voltage pulse is too low, the tube does not turn on, and the startup sequence is automatically repeated until the fluorescent tube ionizes. At that time, the tube impedance falls to its minimum value, yielding a low voltage drop across its end electrodes and, hence, across the switch. Since the starter can no longer be ionized, the electrical network of the filaments remains open until the next turning on of the circuit.

We must point out that the fluorescent tube turns off when the current is zero; this is the source of the 50 Hz flickering in a standard circuit. This is an important problem, which can lead to visual problems due to the stroboscopic effect on any rotating machines or computer terminals.

To take care of this phenomena, the fluorescent tubes, at least those used in industrial plants, are always set on a dual basis in a single light spreader, and are fed from two different phases (real or virtual via a capacitor) in order to eliminate the flickering.

On the other hand, the magnetic ballast provides a very low cost solution for driving a low pressure fluorescent tube. To overcome the flickering phenomenon and the poor startup behavior, the engineers have endeavored to design electronic circuits to control the lamp operation at a much higher frequency. The efficiency (Pin/Lux) of the fluorescent lamp increases significantly, as soon as the current through the lamp runs above a few kilo Hertz.

The electronic circuits that can be used to build a fluorescent lamp controller can be divided into two main groups:

•Single switch topology, with unipolar AC current, (unless the circuit operates in the parallel resonant mode)

•Dual switch circuit, with a bipolar AC output current

Manufacturers of fluorescent lamps usually recommend operating the tubes with a bipolar AC current. This avoids constantly biasing the electrodes as an anode-cathode pair, which, in turn, decreases the expected lifetime of the lamp. In fact, when a unipolar AC current flows into the tube, the electrodes behave like a diode and the material of the cathode side is absorbed by the electron flow, yielding a rapid wear out of the filaments. As a consequence, all of the line operated electronic lamp ballasts are designed with either a dual switch circuit (the only one used in Europe), or a single switch, parallel resonant configuration (mainly used in countries with 110 V lines), providing an AC current to the tubes. A few low

Dimmable Light Ballast with Power Factor Correction, Rev. 1

14	Freescale Semiconductor

M68HC08 specifications

Freescale Semiconductor, known for its innovative solutions in the field of embedded systems, developed the M68HC08 microcontroller family, which includes the MC68HC908QT2. This 8-bit microcontroller is engineered to meet the demands of diverse applications, including automotive, industrial, and consumer electronics.

The MC68HC908QT2 is designed around Freescale’s M68HC08 core, which is renowned for its efficient and reliable performance. This microcontroller integrates a powerful instruction set, enabling developers to create high-performance applications with relatively low power consumption. The device operates at a clock frequency of up to 3 MHz, which is adequate for various control tasks.

One of the key features of the MC68HC908QT2 is its memory architecture. It includes a 2 KB Flash memory for program storage, representing a significant advantage for developers requiring non-volatile memory. Additionally, it encompasses 128 bytes of EEPROM memory, allowing for data retention even after power loss. The microcontroller also has 256 bytes of RAM for efficient data manipulation during operation.

In terms of input/output capabilities, the MC68HC908QT2 supports a variety of interfacing options. The microcontroller features up to 20 general-purpose I/O pins for flexibility in connecting with peripheral devices. Additionally, it provides multiple analog-to-digital converters (ADC) and timers that facilitate efficient analog signal processing and precise control through timing functions.

The architecture of the MC68HC908QT2 also incorporates sophisticated on-chip peripherals, enhancing its functionality. These peripherals include PWM (Pulse Width Modulation) outputs, which are essential for applications requiring motor control and other precise duty cycle processes. The integrated watchdog timer ensures reliable operation by resetting the system in the event of an application failure.

Moreover, the MC68HC908QT2 is equipped with an efficient power management system, enabling operation in a low-power mode, ideal for battery-powered applications. This microcontroller is packaged in a compact 28-pin dual in-line package (DIP), making it suitable for space-constrained designs.

In summary, the Freescale Semiconductor MC68HC908QT2 microcontroller is distinguished by its robust performance, extensive memory options, and versatile I/O capabilities. Its advanced features, including built-in timers, ADC, and a power management system, make it an exceptional choice for developers seeking to implement reliable and efficient embedded solutions. With its comprehensive architecture, the MC68HC908QT2 remains a popular choice in the landscape of 8-bit microcontrollers.