Liquid crystal display (LCD) is widely used in various display applications such as cellular phones, PC monitors, televisions (TVs), multimedia products, among others. An LCD backlight module usually includes backlight sources, a light-diffusion plate, a reflector, a brightness-enhancement film (BEF) and a light-guide plate (LGP). Conventionally, cold cathode fluorescent lamps (CCFLs) are required to provide sufficient backlighting for LCD panels [1-5]. Growing concerns about environmental issues will inhibit the use of CCFLs that contain poisonous mercury. Owing to improvement in long operative life, wider operation temperature range, and the simplicity of driver circuit work with low and safe voltages, light emitting diode (LED) has gradually substituted the CCFL as backlight [6-9]. This chapter will introduce some LED backlight driving techniques for LCD panels. Some dimming control methods will also be introduced and compared for regulating the LED current and brightness of the LED backlight system. The principal goal of this chapter is to ensure that readers become familiar with LED backlight driving techniques for LCD panels. We begin this chapter with a look at three LCD backlight structures: edge-light type, bottom-light type and hollow type. Figure 1(a) shows the bottom-light structure. Because of its high-luminance feature, the bottom-light structure is commonly used for PC monitors and TVs. With regard to the luminance uniformity on an LCD panel, a light-mixed zone is necessary between the diffusion plate and the light sources. This zone causes undesirable thickness for large-sized TV applications. Figure 1(b) shows the hollow type structure in which an LGP is used to reduce the thickness of the light-mixed zone. The good qualities of this type pertain to its compact shape, high luminance and good thermal dissipation. Figure 1(c) shows the edge-light structure commonly used in a small-scale LCD panel. This type is of compact shape and low power consumption, so it is suitable for notebook PCs and personal digital assistant (PDA) products. There are two types of LEDs for backlight sources, the white-light LEDs and RGB LEDs. The white-light LED is composed of a blue LED coated with yellow phosphor. Simple driving feature make it as a popular choice for new generation of LCD backlight sources in portable display products. Its color filter divides the emitted white light into RGB sub-pixels to present color pictures. Thicknesses of RGB sub-pixels must be adjusted according to the corresponding wavelengths to correct the white balance on LCD panel. This results in the difficulty of manufacturing process. The white-color point may vary after a long working time. Thus, the RGB LEDs mixing three-color lights to white light are more suitable for medium-scale, or even large-size screens [10-16]. White balance of the LCD panel with RGB LED backlight can be easily corrected by regulating the emission luminance of the RGB LEDs individually.
2. RGB LED backlight circuit
Figure 2 shows the block diagram of an LCD TV power supply with RGB LED backlight design. The LCD TV power provides a 12V output for the signal-process board, a 24V output for the backlight driving circuit and an additional 5V standby-power output. As shown in Figure 3, the backlight driving circuit consists of three power converters. Backlight LEDs are connected in series and parallel in the RGB LED backlight modules. LED current/ voltage characteristic variations cause brightness difference. Therefore, dimming control is an important design consideration for LED backlight applications. We studied three dimming methods for current regulation of the parallel connected LED arrays: the transconductance-amplifier (TA) dimming, the current-mirror (CM) dimming and the burst-mode (BM) dimming.
Figure 4(a) shows the TA dimming circuit. The LED current can be expressed as Equation (1).
Figure 4(b) shows the CM dimming circuit. The LED current can be expressed as Equation (2).
where Kn and VTN are the conduction parameter and threshold voltage of the dimming transistors Qr and Qd, respectively. By using the TA dimming and CM dimming circuits, the current regulation of paralleled LED arrays can be achieved. However, the conduction losses of the dimming transistors will be difficult to solve . An adaptive voltage output for the DC-DC converter is usually designed to retain the minimum drain-source voltage on the dimming transistors. As shown in Figure 4(c), the backlight LED current can be also controlled with a BM dimming circuit. Considering the switching loss for the dimming transistors, the burst-mode frequency fb is designed at 400Hz that are unperceivable to the human eye. The duty ratios of the dimming transistors are varied to regulate the LED average current that can be represented as Equation (3).
where Im denotes the peak value of the LED array current. The dimming transistors are operated as low-frequency switches, the thermal problem on the dimming transistors can be improved significantly. The current variations can be minimized by using the TA or BM dimming methods while the CM dimming has the simplest circuit configuration. Anyway, the TA and CM dimming methods are unsuitable to be used in the high-power LED backlight design of LCD panels due to the significant conduction losses of the dimming transistors under dimming operations. The emission luminance of the RGB LEDs is able to be regulated individually for achieving the white balance of the LCD panel. The luminance of the red light is always highest and the luminance of the blue light is lowest among three color lights. In practical applications, the blue light is most sensitive to human eyes such that lower luminance of blue LED is enough to compose white light with the red and green LEDs.
3. Soft-switched LED backlight circuit
Figure 5(a) shows a half-bridge DC-DC Series-Resonant Converter (SRC) topology for driving the RGB LEDs. The soft-switched DC-DC resonant converter includes power switches Q1 and Q2, resonant inductor Lr, resonant capacitor Cr, transformer T1, rectifier diodes Df1 and Df2, filter capacitor Cf, and the LED arrays represented by an equivalent resistance Ro. The characteristic impedance and the resonant frequency are respectively [18-21].
From (4) and (5), Lr and Cr can be expressed as
The turn number of the primary winding is
where VNp is the peak-to-peak amplitude of the transformer primary voltage v Np, fs is the switching frequency, Br is the magnetic flux density, and Ae is the effective core area. To simplify the analysis, the first-order harmonic approximation has been applied, and the circuit elements of the primary side in Figure 5(a) are reflected to the secondary side, as depicted in Figure 5(b). The equivalent impedances Z1 and Z2 are respectively
The voltage divider rule can be used to obtain the output voltage phasor VL as
where the amplitude of VL is assumed equal to Vo. Vin is the phasor of the fundamental component of v a in Figure 5(a).
Substituting (9), (10) and (12) into (11), the voltage gain transfer function can be expressed as
It is clearly seen from (13) that the switching frequency must be varied to regulate the output voltage. The highest switching frequency appears at the highest input voltage and the lightest load. On the other hand, the lowest switching frequency happens at the lowest input voltage and the heaviest load. For the SRC to operate in the zero-voltage-switching (ZVS) region, the lowest switching frequency must be higher than the resonant frequency as expressed in (5). Moreover, due to the switching speed limitations of the power devices, the highest switching frequency is below a specified value. In other words, the variations of the input DC voltage and the load variations must be confined to a small range. Usually a power factor corrector (PFC) is added in front of the DC-DC converter to raise the input power factor and reduce the input current harmonics. A phase-shift pulse width modulation (PSPWM) dimming control can effectively confine the load variation of the DC-DC SRC. Consequently, the output voltage variation of the PFC can be limited to a smaller extent. This results in a better operating condition for the SRC. For the PSPWM dimming strategy, the working durations of the shunt LED arrays are properly phase-shifted to confine the variation of the output current of the SRC. Figure 6 illustrates the circuit arrangement for N shunt single-colored LED arrays with PSPWM dimming method. It is almost the same as the conventional one, except that the dimming signals are applied with a specified phase difference. With the PSPWM dimming, there are always overlaps between the LED driving currents. The maximum duty cycle, or the overlap, is 100 %, corresponding to the highest brightness. To prevent the DC-DC SRC from operating at no load, the minimum duty cycle of the PSPWM dimming signal is 1/N, where N is the number of the shunt LED arrays. Under this circumstance, the overlap is zero, corresponding to the lowest brightness. Compared with the conventional dimming scheme, it is apparently recognized that the load variation of the SRC is less with the proposed PSPWM dimming function. To further investigate the operating principle of the PSPWM dimming, a more general case with N shunt LED arrays is discussed as follows. Figure 7 shows the waveforms of the N driving currents and the output current of the SRC. As stated earlier, the duty cycle range of the dimming signal is from 1/N to 100 %. In terms of the phase angle, if a complete period is 360º, the duty cycle range is from 360º/N to 360º. Assuming that the dimming signal for the LED array 1 starts at 0º, then the dimming signal for the k-th LED array would start at
If the duty cycle of each dimming signal is d, then the average driving current of one LED array is
where Ip is the amplitude of the driving current for each LED array. Therefore the average output current of the SRC is
It can be observed from Figure 7 that if the end of the dimming signal for LED array 1 is at d, where d is between k and k+1 and k 1, then the output current of the SRC in the range of k to k+1 is
This is also the SRCs output current in each duration from j to j+1, where j = 1 to N. Therefore, the average output current of the SRC is now
A favored feature is that the load variation of the SRC is always within one step change of Ip, no matter what the load level is. Therefore, by carefully designing the duty cycle and the amplitude of the driving current for each LED array, the no load operation of the DC-DC SRC may be precluded. Moreover, the output transient of the SRC is improved due to the confined load change. The number of the LED array for one color, and the peak driving current of each LED array are first determined according to the specifications of the LED and the spectrum of the white color. Then a suitable duty cycle is chosen allowing a reasonable span of variation for dimming control.
4. Single-stage LED backlight circuit
Figure 8 shows a single-stage LED backlight driving system. The backlight driving system consists of an AHB DC/DC cell integrated with a charge-pump PFC cell. The power MOSFETs Q1 and Q2, operate with asymmetrical duty ratios, and 1-, which require short and well-defined dead time between the conduction intervals. D1, D2 and Cp1 and Cp2 are the body diodes and the parasitic capacitors of power MOSFETs, respectively. The charge-pump PFC cell is composed of resonant inductor Lr, charge-pump capacitors Cr1 and Cr2, input diodes Di1, and Di2, clamping diodes Dc1, and Dc2. The capacitor Cbus is used as the DC bus capacitor between the charge-pump PFC cell and the post-stage AHB DC/DC cell. The transformer leakage inductor Ll resonates with the parasitic capacitors Cp1 and Cp2 during dead-time intervals to achieve zero-voltage switching for the power MOSFETs. The blocking capacitor Cb is used to assure that the power sent into the transformer primary winding is a pure AC type. A DC voltage is supplied to the LED arrays through the secondary rectifier and filter circuit that are composed of D3, D4, Lo and Co.
The average rectified input current |Iin|,av can be expressed as follows.
where Q is the charge variation of Cr1. From Equation (19), we can see that the average rectified input current is proportional to the rectified input voltage. Thus, high power factor can be achieved. Based on the power balance between the input and output of the AC/DC converter, the following equation has to be satisfied.
where and Po are the overall efficiency and output power of the converter. From Equations (19) and (20), the design equations for the resonant inductor Lr and the charge-pump capacitor Cr1 can be derived as follows [22-25].
The ZVS conditions for power switches depend on the resonant inductance current ILr related with the input voltage. At the zero-crossing of input voltage, the resonant inductance current ILr will be ignorable. Considering the ZVS condition during an entire a line period, the transformer leakage inductance Ll could be determined by using Equation (23).
In practical design, an external inductor Le is usually needed to be added in series connected with Ll for satisfying ZVS condition [26-28]. The input current has a near sinusoidal waveform and in phase with the input voltage. High efficiency and high power factor can be achieved because of single-stage power conversion with soft-switching features.
The advantages of LED backlighting over conventional CCFLs are numerous: fast response, broader color spectrum, longer life span, and no mercury. However, CCFLs still have cost advantages. For a LED backlighting, luminous efficacy and thermal management are the most important issues need to be solved before commercialization. Anyway, rapid advances in material and manufacturing technologies will enable significant developments in high-luminance LEDs for backlighting applications. In this chapter, we introduced some LED backlight driving systems for LCD panels. Dimming control methods are then discussed to regulate the LED current and brightness for the LED backlight system.