Apparatus for driving cold cathode fluorescent lamps

A starting voltage adjustment circuit (70), which can vary starting voltages to CCFLs (40) according to variations in the temperature of the immediate environment, is employed in an apparatus for driving CCFLs. The starting voltage adjustment circuit includes a zener diode (710), a thermal resistor (720), and a voltage dividing resistor (730) connected in series between a buck converter, a resonant boost converter, and ground. The starting voltage adjustment circuit also includes a control chip (740) which includes pins, of which one is connected between the thermal resistor and the voltage dividing resistor and outputs a constant voltage, and another is connected between the voltage dividing resistor and ground. The thermal resistor has a voltage drop thereacross varying with the temperature. The starting voltage adjustment circuit adjusts an input voltage to the resonant boost converter, thereby adjusting the starting voltage to the CCFLs according to variations in temperature.

FIELD OF THE INVENTION

The present invention relates to an apparatus for driving lamps, and particularly to an apparatus for driving Cold Cathode Fluorescent Lamps (CCFLs).

DESCRIPTION OF RELATED ART

Fluorescent lamps are used in a number of applications where light is required but the power required to generate light is limited. One particular type of fluorescent lamp is the Cold Cathode Fluorescent Lamp (CCFL) which provides illumination in a variety of electronic devices, such as flat panel displays, computers, personal digital assistants, scanners, facsimile machines, copiers, and the like.

CCFL tubes typically contain a gas, such as Argon, Xenon, or the like, along with a small amount of Mercury. CCFLs require a high starting voltage, generally from 700–1,700 volts, for a short time at an initial ignition stage to ionize the gas contained within the CCFL tubes and ignite the CCFLs. After the gas in the CCFLs is ionized and the lamps are ignited, less voltage is required to maintain ionization.

The starting voltages of CCFLs vary with the temperature of the environment within which they operate: the higher the temperature, the lower the starting voltage. For example, when the temperature of the immediate environment is about 0 degrees Celsius, the starting voltage needed for CCFL's is approximately 1700 volts, which is significantly higher than the 1400 volts starting voltage required when the temperature is about 25 degrees Celsius. However, to avoid CCFL ignition failure from too little voltage applied in low temperature environments, conventional CCFL driving circuits provide a fixed high starting voltage (e.g., 1700 volts) to ignite the CCFL, regardless of any variation in the temperature, be it relatively high (e.g. 25° C.) or relatively low (e.g., 0° C.).

However, high starting voltages can seriously shorten the life span of CCFLs.

Therefore, what is needed is an apparatus for driving CCFLs which can provide variable voltages to ignite the CCFLs as conditions dictate in a variable temperature working environment.

SUMMARY OF INVENTION

An apparatus for driving Cold Cathode Fluorescent Lamps (CCFL) includes: a buck converter connected to a direct-current power supply; a resonant boost converter connected to the buck converter; one or more CCFLs connected to the resonant boost converter; and a starting voltage adjustment circuit connected between the buck converter and the resonant boost converter, for adjusting the starting voltage applied to the CCFLs according to the temperature of the environment within which they are operating. A feedback loop and a PWM (pulse-width modulation) control circuit are sequentially connected in series between the CCFLs and the buck converter. In addition, the PWM control circuit is also connected with the starting voltage adjustment circuit. The starting voltage adjustment circuit and the feedback loop send voltage signals to the PWM control circuit, and the PWM control circuit accordingly generates a series of PWM waves to control the power-transfer rate of the buck converter.

The starting voltage adjustment circuit comprises a control chip, and a voltage stabilizing circuit, a thermal circuit, and a voltage dividing circuit that are sequentially connected in series between the buck converter, the resonant boost converter, and ground. The voltage stabilizing circuit has one terminal connected between the buck converter and the resonant converter, and another terminal connected with the thermal circuit. The control chip includes a plurality of pins, of which a first pin is connected between the voltage stabilizing circuit and the thermal circuit, a second pin is connected between the thermal circuit and the voltage dividing circuit, a third pin is connected between the voltage dividing circuit and ground, and a fourth pin is connected to the PWM control circuit.

The second pin outputs a constant voltage U0. The thermal circuit senses the temperature of the immediate environment and adjusts a voltage drop U1thereacross according to the reading. In addition, the voltage stabilizing circuit has a constant voltage drop Uz thereacross. Therefore, an input voltage to the resonant boost converter is equal to the sum of the constant voltage U0, the voltage drop U1, and the constant voltage drop Uz. This input varies inversely with the temperature of the immediate environment, whereby the starting voltage of the CCFLs varies inversely with such temperature.

Other advantages and novel features will be drawn from the following detailed description with reference to the attached drawings, in which:

DETAILED DESCRIPTION

FIG. 1is a block diagram of an apparatus for driving Cold Cathode Fluorescent Lamps (CCFLs) (hereinafter, “the apparatus”) according to a preferred embodiment of the present invention. The apparatus includes a buck converter20, and a resonant boost converter30connected to the buck converter20. The buck converter20receives power from a DC power supply10, and transfers the power to one or more CCFLs40via the resonant boost converter30. A feedback loop50and a PWM (pulse-width modulation) control circuit60are positioned sequentially between the CCFLs40and the buck converter20. The PWM control circuit60includes a modulation signal generator and a comparator610. InFIG. 1, the modulation signal generator is detailed as a triangle waveform generator620. However, the modulation signal generator can be provided in any other suitable form, such as a saw-tooth waveform generator, or even a trapezoidal waveform generator. The comparator610includes a plurality of inputs and an output. The inputs of the comparator610are respectively connected to the triangle waveform generator620, the feedback loop50and a starting voltage adjustment circuit70(described below), and the output of the comparator610is connected to the buck converter20. The comparator610receives voltage signals from the feedback loop50or the starting voltage adjustment circuit70, compares the voltage signals with modulation signals generated by the triangle waveform generator620, and outputs a series of PWM waves to modulate the power-transfer rate of the buck converter20accordingly. The starting voltage adjustment circuit70has an input connected between the buck converter20and the resonant boost converter30. In the preferred embodiment, the starting voltage adjustment circuit70senses variations in the temperature of the immediate environment, and adjusts input voltages to the resonant boost converter30. In addition, the starting voltage adjustment circuit70outputs voltage signals according to the variations in temperature of the immediate environment to the comparator60, whereby the comparator60outputs PWM waves to the buck converter20to modulate its power-transfer rate.

The starting voltage adjustment circuit70adjusts an input voltage to the resonant boost converter30whereby the input voltage is inversely proportional to the variation in the temperature of the immediate environment. For example, when the temperature is 0 degrees Celsius, the ignition voltage from the resonant boost converter30as adjusted by the starting voltage adjustment circuit70may be 1700 volts; alternatively, when the temperature is 25 degrees Celsius, the ignition voltage may be 1400 volts.

FIG. 2is similar toFIG. 1, but showing details of the starting voltage adjustment circuit70in accordance with an exemplary embodiment of the present invention. The starting voltage adjustment circuit70includes a voltage stabilizing circuit710, a thermal circuit720, a voltage dividing circuit730, and a control chip740having four pins (symbolically expressed as pin A, pin G, pin K, and pin O). In the illustrated embodiment, the voltage stabilizing circuit710is a zener diode710having a cathode and an anode, the thermal circuit720is a thermal resistor720, and the voltage dividing circuit730is a voltage dividing resistor730. The cathode of the zener diode710is connected between the buck converter20and the resonant boost converter30, forming a common node D thereof. The anode of the zener diode710is respectively connected to one terminal of the thermal resistor720and the pin A of the control chip740, forming a common node B thereof. The other terminal of the thermal resistor720is connected to one terminal of the voltage dividing resistor730and the pin G of the control chip740, forming a common node C thereof. The other terminal of the voltage dividing resistor730and the pin K of the control chip740are grounded. The pin O of the control chip740is connected to an input of the comparator610.

The zener diode710has a constant voltage drop Uz thereacross. In the preferred embodiment, the constant voltage drop Uz is preferably a little greater than an output voltage at the buck converter20after the CCFLs40have been ignited. The voltage dividing resistor730has a constant intrinsic resistance R2. Conversely, the thermal resistor720has a variable intrinsic resistance R1that varies inversely with a change in temperature of the immediate environment. For example, when the temperature is 0 degrees Celsius, the resistance R1of the thermal resistor720may be 6 ohms; and when the temperature is 25 degrees Celsius, the resistance R1of the thermal resistor720may be 4 ohms.

The common node C is supplied with a constant voltage U0from the pin G of the control chip740. Taken together, the constant voltage U0of the common node C, the constant resistance R2of the voltage dividing resistor730, and the variable resistance R1of the thermal resistor720can be used in a formula to calculate a voltage U1at the common node B, whereby U1=(R1+R2)/R2*U0. As described above, R1varies with the temperature of the immediate environment. Therefore, correspondingly, the voltage U1varies with the temperature as well. For example, if setting R2equal to 2 ohms and U0equal to 2 volts and a value for R1of 6 ohms when the temperature of the immediate environment is 0 degrees Celsius, then the value of U1is: (6+2)/2*2=8 volts. Further, when the temperature of the immediate environment is 25 degrees Celsius, then the resistance of R1decreases, for example to 4 ohms, and then correspondingly the voltage U1is: (4+2)/2*2=6 volts. The voltage U1is supplied to the control chip740through the pin A, and accordingly the control chip740outputs voltage signals to the comparator610through the pin O thereof.

By function of the starting voltage adjustment circuit70(i.e., the zener diode710, the thermal resistor720, the voltage dividing resistor730, the control chip740, and combinations therebetween), a voltage U equal to (Uz+U1) is obtained at the common node D and is input to the resonant boost converter30. U1(i.e., (R1+R2)/R2*U0) varies inversely with the temperature of the immediate environment, therefore the starting voltage to the CCFLs varies inversely with the temperature as well. Accordingly, unnecessarily high ignition voltages are avoided, thereby extending the working lifetime of the CCFLs.

According to the preferred embodiment, the PWM control circuit60controls the power-transfer rate of the buck converter20pursuant to voltage signals from the control chip740or the feedback loop50. At an ignition stage of the CCFLs40, the voltage signals from the control chip740are received and compared with the modulation signals from the modulation signal generator at the comparator610, and subsequently a series of PWM waves are produced in accordance with a comparison result to control the power-transfer rate of the buck converter20. After the CCFLs40have been ignited, the voltage signals from the feedback loop50are received and another series of PWM waves are produced at the comparator610, to control the power-transfer rate of the buck converter20.

InFIG. 2, the CCFLs40are shown as including a CCFL141, and a CCFLn42(remark: n is a natural number equal to or greater than 2). Other (n-2) CCFLs arranged between the CCFL141and the CCFLn42are not shown, but all n CCFLs40are arranged in parallel to each other. However, it is to be noted that in some applications, the CCFLs40may in fact include only the CCFL141.