The typical LCD backlight can be one or more cold cathode fluorescent lamps (CCFLs) or an array of light-emitting diodes (LEDs). An example of each is shown in Figure 1. The quality of the backlight image depends heavily on the backlight driver. In this article, we will discuss the considerations one can make for CCFLs and LEDs, as well as how to power both kinds of backlight.
GENERAL CCFL CONSIDERATIONS CCFL backlights are the most common backlight technology and are used in displays ranging from 5.7 to 23 in. or more diagonal. They can have from one to 24 or more lamps mounted along the edge of the LCD or spaced uniformly over the entire back of the display.
Typically, brightness is controlled by modulating the CCFL current or lamp duty cycle. The basic driver is a dc-to-ac inverter powered by 5 to 48 V dc.
GENERAL LED CONSIDERATIONS LEDs are already used in a wide range of smaller displays. For larger displays, because of their higher power consumption and, in some cases, their mercury content, CCFL backlights are beginning to be replaced by LED backlighting. LEDs may be arranged along the edges of the LCD or as a matrix over the back of the LCD assembly. The LED devices may be arranged in series or parallel. Either configuration will provide uniform LCD lighting. The LED strings can be arranged in parallel using a series resistor in each string to provide string-to-string current balancing as well as lighting redundancy.
While CCFL backlights typically provide white light, LED backlights may supply either white light or a mixture of red, green, and blue. LEDs emit light when biased in the forward direction. For quality performance, a constant current driver is required to compensate for LED voltage drops and changes with temperature. This ensures stable light output.
Unlike a CCFL, LED backlights don’t require high ac voltages; therefore, they don’t require an inverter. The basic LED driver is powered by 5 to 48 V dc and employs dc-dc boost to provide voltage to a constant-current driver that drives the LED string.
CCFL DRIVER CIRCUITS Inverter product circuits can be divided into two groups: those with lower output power, which use power transistors as the primary circuit switching device, and those with higher output power, which use FETs.
The transformer steps up the input voltage. Design considerations include power, copper losses, and core material.
Figure 2 shows one type of CCFL driver in detail. Working backward from secondary to primary, a ballast (or secondary) capacitor, C2, reduces the voltage to the CCFL at the time the CCFL starts and the output current begins to increase. The relationships among the starting voltage (VS), the voltage drop across the CCFL (VR), and the voltage drop across the secondary capacitor (VC), are defined by: VS2 = VR2 + VC2. The value for the secondary capacitor value depends on the output current and output frequency. Increasing the capacitance increases the output current and reduces the frequency.
On the primary side, capacitor C1 fine-tunes the inverter output-current level and output operating frequency once the secondary load is defined, the secondary capacitor is chosen, and the number of transformer primary and secondary turns is decided upon. C1 “rolls off” the output current and frequency, which were determined by the values of the components on the secondary side.
Base-current-limiting resistor R1 establishes enough transistor base current to guarantee transistor saturation. Meanwhile, the choke circuit reduces ripple in the input current as the transistors switch the primary windings. The choke also increases the current rise time at inverter turn-on. The goal is to reduce peak inrush current and choke audible noise from the choke.
Trading off inductance, physical size, saturation current, IR losses, and power losses make selecting an inductor somewhat challenging. Be aware that long input-current rise times can reduce the effectiveness of pulse-width modulation (PWM) dimming at low duty cycle. Also, careless choke selection can produce saddles or worse in the rising input current, and that will adversely affect low duty-cycle PWM dimming as well as CCFL startup.
All inverters with high power outputs or those that incorporate on-board dimming should use input bypass capacitors to reduce input voltage ripple. Without them, each time the inverter power devices switch, the resulting current increase will cause an input voltage decrease.
Another consideration is that CCFLs operate at high voltages. This complicates safety agency approvals, (i.e. CE, UL and CSA), especially when the product is powered by an external power supply. In addition product enclosure costs typically go up due the requirement of fire rated plastic material due to the CCFL operating voltage. LED backlights can operate well below maximum SELV voltage requirements, avoiding all of the above considerations.
DVanditmars -April 25, 2007 (Article Rating: )
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