Working Principle of CCFL and Analysis of Electronic Driving Circuit

Cold cathode fluorescent lamps are called Cold Cathode Fluorescent Lamps in English, or CCFL for short. The diameter of this type of lamp is 1.8mm to 3.0mm, and three-primary color phosphors are generally used. Usually, metals such as Ni, Ta, and Zr are used as cold cathodes. Under high starting voltage, glow discharge is formed to make the lamp work. Cold cathode fluorescent lamps have the characteristics of small size, high brightness, and long life, but they need to be preheated before working. This type of light source has been widely used in the backlight source of liquid crystal displays and LCD TVs. With the increasing application of backlighting in office laptops and household appliances such as televisions, digital cameras, and camcorders, high-brightness cold cathode ultra-thin diameter fluorescent lamps have come into being.

Although LEDs have long dominated the backlighting of small-sized color screens in portable electronic products such as phones, mobile phones, PDAs, and digital cameras, CCFLs are still the dominant force in the backlighting of large-sized LCDs such as notebook computers, LCD color TVs, and LCD monitors. Although CCFLs will face competition from LEDs in large-sized LCD backlighting applications, the recent high prices of LEDs and the uneven brightness of dozens or even hundreds of LEDs used as backlights have become a technical barrier to entry. Therefore, CCFL's dominance will not be replaced by LEDs in the next few years.

1. CCFL structure and main features

1. Structure and working principle of CCFL

CCFL is similar to neon lamp, with cold cathode and very thin tube diameter (only about 2~8mm), and the light-emitting principle is basically the same as that of fluorescent lamp. When the output end of the lighting circuit (inverter) is connected to the two electrodes of qualified cold cathode fluorescent lamp, and the rated voltage and current are input to the input end of the lighting circuit, that is, hundreds of volts of voltage and 50KHZ frequency and rated milliampere of high-frequency current are applied to the two electrodes of cold cathode fluorescent lamp. At this time, heat is generated on the two electrodes to emit a large number of electrons. Under the action of the electric field formed between the two electrodes, the electrons collide with the gas molecules to ionize the molecules and produce more ions and electrons. Such rapid growth makes the gas completely ionized quickly. At the same time, the heat of the electrode and the ionized ions and electrons are heated in the high-frequency electric field to quickly vaporize mercury, and the concentration of mercury vapor reaches saturation. The collision probability of electrons, ions and mercury vapor is getting bigger and bigger, and the intensity of ultraviolet rays generated gradually increases and reaches saturation. When ultraviolet rays irradiate the fluorescent layer, the fluorescent layer emits visible light of a certain color temperature, and the cold cathode fluorescent lamp begins to work normally. Although the luminescence process is relatively complicated, it only takes a few milliseconds to complete.

The structure of CCFL is shown in the figure above. The CCFL glass tube is filled with mercury and inert gas (neon-argon mixed gas), and the inner wall is coated with a layer of three-primary color (RGB) phosphor. The CCFL cathode is in the shape of a hollow cylinder, made of metal materials such as nickel and molybdenum, and the inner wall of the tube is coated with luminescent materials. CCFL can be made into different shapes as needed, as shown in the figure below. When a high voltage is applied to both ends of the CCFL, the gas in the tube is ionized to emit ultraviolet light with a wavelength of 253.7nm, which is absorbed by the phosphor on the inner wall of the tube and converted into visible light {wavelength of 400~7OOnm) and then radiated.

2. Main characteristics of CCFL

Since CCFL tubes are very thin and short (10 to 10 cm), the operating current is very small, usually 3~8 mA. However, the operating voltage of CCFL is relatively high, reaching 250~600V (effective value), which is several times or even nearly 10 times the operating voltage of fluorescent lamps. The triggering voltage of CCFL is at least 1.5 times its operating voltage, usually 600~1500V (effective value).

The advantages of CCFL are stable electrical and optical properties, long life (up to 15~20kh), vibration resistance, impact resistance (greater than 100g), and strong resistance to flashing (up to 100,000 times). However, due to the large size of LCD panels, more CCFL tubes are required, and the number of inverters driving the tubes is also large, accounting for a large share of the cost of the whole machine. Since CCFL contains mercury, from an environmental point of view, it does not comply with the EU RoHS Directive on the restriction of hazardous substances and the Weee Directive on waste electrical and electronic equipment, while LED does not have this problem.

2. CCFL electronic drive circuit and its controller IC

The CCFL electronic driver is also called CCFL electronic transformer or electronic ballast. Whether it is powered by AC mains (such as 220V/50Hz) or low-voltage DC power supply (ranging from 3~3OV), the core of the CCFL drive circuit is a DC/AC high-frequency inverter {output frequency is 20~1OOkHz). In order to meet the CCFL startup and operating voltage requirements, the CCFL inverter output is connected to a high-frequency step-up transformer, and the step-up ratio is often 60~100. When CCFL is used as an LCD backlight source, its drive circuit must have analog, especially digital control, dimming functions. CCFL drive circuits all use controller ICs. According to the CCFL controller, CCFL drive circuits are divided into three types: half-bridge, full-bridge and push-pull, and the latter two are the main ones.

1. Half-bridge drive circuit Currently, there are only a few CCFL half-bridge controller ICs such as MAX8729, UCC3976 and UBA2070. The half-bridge topology requires two power switches (mostly MOSFETs), and the circuit efficiency is relatively high. Philips Semiconductor's UBA2070 is suitable for AC power supply up to 277V. The CCFL half-bridge drive circuit is shown in the figure above. In the figure, Q1 and Q2 are half-bridge switches; RT and CT are used to set the switching frequency: T1 is a step-up transformer, and the T1 step-up ratio (i.e., the turns ratio between the secondary and primary windings) is N; C1 is a DC blocking capacitor, and C2 is a T1 secondary parallel capacitor. The resonant tank equivalent circuit of the circuit shown in the above figure is shown in the figure below. Among them, artificial is the leakage inductance of the secondary winding of T1, C1 is the series capacitor reflected to the secondary, RL is the equivalent resistance of the CCFL lamp, and the square wave AC signal source represents the half-bridge power level.

According to the equivalent circuit shown in the figure above, we can understand the generation of CCFL triggering start voltage. Before CCFL is triggered, its resistance is infinite, the resonant tank circuit is a parallel topology, and the resonant frequency FP is determined by L, C1 and C2.

The inverter acts like a voltage source. When in resonance, it generates a high voltage of about 1000V across the lamp tube, ionizing the lamp tube and turning it on. Once the lamp is lit, the CCFL resistance drops sharply, and the inverter is close to operating in series resonant mode, like a current source. The circuit series resonant frequency fs is determined by L and C1.

2. Full-bridge drive circuit

Most CCFL full-bridge driver ICs have one channel, and representative products include Maxim's MAX8709 (QFN28 package), MAX8722A (QSOP24 package), MAX8751 (QPN32 package), MAX8759 (QFN32 package) and Fairchild Semiconductor's FAN7310 (SSOP20 package). These control ICs have analog and digital PWM (DPWM) dimming and load open and short circuit protection functions.

The CCFL full-bridge driver circuit requires four power switches, and the output power is twice that of the half-bridge. The figure above shows a CCFL full-bridge driver circuit using the MAX8722A. The working principle of this circuit is almost exactly the same as the half-bridge circuit. The turns ratio of transformer T1 is 1:93, the CCFL lamp current is 6mA, the trigger voltage is 1600V, and the operating voltage is 650V. The circuit DC input voltage Vin is 8~24V, and the DPWM frequency fdpwm is 209Hz (set by the resistor R6 on the pin FREQ). When a dimming control voltage of 0~2V is added to the IC pin 5 (CNTL), the lamp brightness can be adjusted, and the dimming range is 10%~1OO%. The MAX8722A can also use the DPWM method to achieve digital dimming. The MAX8722A provides T1 secondary overcurrent protection, secondary voltage limiting, lamp open circuit protection, and primary overcurrent protection. R1 is used to sense the lamp current, the capacitor divider C3/C4 is used to detect the secondary voltage of T1, and R3 is used to sense the secondary current of T1.

In fact, the circuit shown in the figure above can drive two CCFL lamps through two transformers with parallel primary terminals. In some applications, two CCFL lamps can also be connected in parallel on the secondary side of the transformer.

3. Push-pull drive circuit The CCFL push-pull drive circuit requires two power switches. The circuit with the center tap on the primary side of the transformer is shown in the figure above. Among them, CO1 and CO2 form a capacitor voltage divider to sense the secondary overvoltage of T1, and R5 is used to sense the lamp current. DS3984 has 4 channels, and each channel can drive 4 CCFL lamps in parallel through the primary side of the transformer.

The figure above shows a push-pull resonant CCFL drive circuit using MAX1610. Among them, T1 (primary), C1, Q1 and Q2 form an oscillation circuit, and the oscillation frequency is determined by the resonant cavity capacitor C1, the inductance of T1 primary winding (LP=44μH) and the inductance reflected from T1 secondary to primary. The turns ratio of T1 primary to secondary winding is 1:67, and the T1 secondary circuit is a dynamic power supply structure (CCFL has no ground terminal). MAXl610 uses a 16-pin SO package, and the internal 5-bit up/down counter is used to complete the dimming of CCFL. R1 is used to detect the oscillator current, and D1, R3 and R4 form a lamp open circuit detection circuit.