[Peter Green] High-brightness LEDs have gradually become an emerging alternative to LCD panel backlighting, but CCFL is still the mainstream and has a cost advantage. Regardless of which light source is used, the uniformity of the backlight is a primary consideration. In CCFL lamp applications, electronic ballasts must be used to equalize the arc current of each lamp. The half-bridge system driven and controlled by the ASIC has significant advantages. In LED backlight applications, the hysteretic buck control chip can provide accurate current control. These chips can directly sense the LED load current and switch the high-voltage MOSFET switch through the internal floating high-voltage terminal drive circuit. . The market is flooded with a wide range of static and interactive visual communication products, including televisions, computer displays and data displays, which are widely used in many different end applications, such as the Global Positioning System (GPS) in the car. Many of these types of applications integrate different sizes of liquid crystal displays (LCDs), many of which use touch screens. In the past few years, liquid crystal displays have quickly replaced traditional bulky cathode ray tube (CRT) displays and televisions. The main driving force behind this trend is cost and volume savings, as well as the significant environmental benefits it brings. For example, a liquid crystal display uses only about one-third of the power of a CRT display of the same size. Other advantages include higher brightness, no flicker, perfect flat display, and sharper images without the need for focusing. It is also longer, about 50,000 hours, while the CRT is only 20,000 hours. Among various display products, the projector's display requires a high-intensity high-intensity light source and a high-intensity discharge (HID) short-arc lamp, so it should be considered. On the other hand, small LCDs used in mobile phones and other handheld devices, such as MP3 players, use multiple light-emitting diodes (LEDs) or single white LEDs to provide backlighting, which is sufficient for small screens. It can also be driven by a low voltage power supply that controls the brightness via a simple voltage regulator chip. The basic principle of the LCD panel can be regarded as an electronically controlled optical valve, so a backlight is required to produce a visible image. The LCD screen is usually backlit using a Cold Cathode Fluorescent Lamp (CCFL). Still the most cost-effective way, and this advantage is expected to continue for some time. CCFL backlights are still cost effective The provision of panel backlights by high-brightness LEDs has gradually become an emerging alternative to liquid crystal displays, but on most liquid crystal displays and televisions, the backlights are still CCFL long airtight small calibers arranged in several horizontally equidistant positions. Provided by the lamp. Depending on the size of the screen, approximately 8 to 32, or even more, CCFL lamps of the same length can be used, as well as lamps of various lengths to meet different screen width requirements. In use, a high voltage is applied to the electrodes at both ends of the CCFL lamp to ionize the internal gas to generate ultraviolet light and excite the phosphor coated inside the lamp to emit white light. These lamps must operate at the same brightness. Although a light guide plate is arranged between the CCFL lamp and the liquid crystal panel to assist in evenly distributing the backlight, the different luminous intensity between the individual lamps cannot be completely ignored, so the electronic ballast (Ballast) for driving the CCFL lamp must be Contains circuitry that equalizes the arc current of each lamp. Regardless of the particular source used, the even distribution of backlighting is a major consideration for this type of application, and the notion of the same applies to systems that use LEDs. CCFL ballasts can be implemented in several different topologies, including Royer self-oscillating, full-bridge, push-pull and half-bridge. In most applications, a fixed stabilization frequency is required to avoid interference problems when the stabilization frequency is close to the image display scan frequency, so the Royer structure is hardly used in such applications. The full-bridge topology works well in applications with lower DC voltages, such as notebook computers, and in a wide range of 7 to 21 volts. Push-pull architectures are sometimes used, but because they require a large transformer with a center tapped at the primary end, they are less efficient than full-bridge or half-bridge systems. Half-bridge CCFL ballast for higher DC voltages The half-bridge architecture is the best choice because it requires only two metal oxide semiconductor field effect transistor (MOSFET) switches, and the transformer design is relatively simple. The half-bridge configuration is also suitable for higher DC supply voltages, such as hundreds of volts at the front end of the input power factor correction stage circuit, which is also more cost effective for cost-conscious applications such as LCD TVs and displays. system. The half-bridge CCFL ballast consists of two MOSFETs driven by a low voltage/high voltage driver chip. It can accept logic level signals provided by a microcontroller or other low voltage control circuit as a general purpose driver circuit. It can also be integrated into an Application Specific Integrated Circuit (ASIC) product that contains all of the control and protection circuitry required by the system. In backlight applications, the brightness of the CCFL lamp must be able to be controlled to a low level without instability or flicker, which can be achieved by Burst Mode brightness control. In this mode, the high-frequency current output to the lamp is intermittently interrupted at a much lower frequency, but is fast enough to avoid the effects on the human eye. In this way, the average current of the lamp can be adjusted by changing the length of the on and off times from 0 to 100% while maintaining a fixed wave width modulation frequency. At the beginning of each intermittent action, the lamp must be subjected to a slow start action to minimize pressure and provide the longest lamp life. The way to achieve is not to change the stability frequency, but to increase the on-time of the switch above the half-bridge, from the low to the effective cycle rate of 50%, while reducing the on-time of the lower switch to maintain a fixed dead time, let the gentle The switch can be maintained during the slow start time. This means that control of the output current can also be used to control the maximum lamp current to provide short circuit and overload protection. Therefore, the advantages of a half-bridge system driven by an ASIC, controlling all stability functions, and providing low voltage and high voltage output are quite obvious. LED backlight control is better The image quality and visual performance of the liquid crystal display are affected by the backlight characteristics. The backlight system using a series of LEDs as a light source has higher controllability than CCFL and better ability to achieve optimization. The LED array consists of three main color combinations of red, green and blue to provide an overall white light output, which allows the color temperature of the backlight to be adjusted by adjusting the intensity of each color to meet the needs of different display environments, such as on television. Applications require a warmer yellowish backlight, while computer screens prefer pure white light. Backlighting systems using LED arrays also need to be carefully designed to provide an even distribution of color and intensity. Since the color of the LED changes with current, it is necessary to provide a precisely controlled steady current, which is also necessary to optimize LED life. The condition, the light source output can be changed by using the same intermittent mode brightness control method as in CCFL applications, and it is also preferable to provide a slow start in each pulse width modulation (PWM) cycle for extended use in such applications. life. Must provide a steady current for the series LED But even LEDs of the same color and type will have significant differences in forward voltage drop, and will also be affected by temperature and different LED colors, so it is obviously necessary to provide stable control for each series of LEDs that make up individual colors. Constant current supply, which includes switched converters, because linear regulators can introduce unacceptable power losses at high-brightness LEDs operating at 350 mA or higher. In such cases, the choice of topological structure is limited to buck or flyback topology. The buck is simpler and is also sufficient for situations where isolation is not required and the bus voltage supplied to the converter is higher than the overall series series drop of the series LED, which is usually the most common case. Hysteretic Buck Control Chip for More Accurate Current Control At present, there are various special control chips on the market, which can be used for applications with different operation modes. The simpler method is to sense the current of the inductor when the buck regulator MOSFET switch is turned on. The maximum voltage is regulated. This method requires only a low voltage side switch, but it is limited. First, its effective period rate can't exceed 50%, which means that the input voltage must be more than twice the LED output voltage, so that the control loop can work stably. Moreover, since this method does not directly control the LED current, there is no It can avoid the partial change of LED current on the line and load. Although these changes are not large, it may still be affected by the change of LED current caused by temperature, which is what you hope to avoid. A hysteretic buck control chip that directly senses the LED load current and switches the high-voltage-side MOSFET through the internal floating high-voltage terminal drive circuit can provide more accurate current control. This arrangement allows the LED current and average current to be precisely controlled, rather than the maximum current, thus creating a stable closed loop control circuit and compensating for any line or load variations. In this case, although the frequency can be freely varied, it may be high enough to cause eye-detectable interference, but the effective period rate can be changed from 0% to nearly 100%, so the input voltage must be slightly higher than the output voltage. Just fine. (The author is a senior lighting system engineer at International Rectifier Corporation)