1. Field of the Invention
The present disclosure relates to a flyback boost circuit, a LED backlight driving circuit including the flyback boost circuit, and a liquid crystal device including the LED backlight driving circuit.
2. Discussion of the Related Art
With the technology revolution, backlight technology of LCDs has been developed. Typical LCDs adopt cold cathode fluorescent lamps (CCFL) as the backlight sources. However, as the CCFL backlight is characterized by attributes including low color reduction ability, low lighting efficiency, high discharging voltage, bad discharging characteristics in low temperature, and also, the CCFL needs a long time to achieve a stable gray scale, LED backlight source is a newly developed technology. For LCDs, the LED backlight source is arranged opposite to the liquid crystal panel so as to provide the light source to the liquid crystal panel.
The driving circuit of the LED backlight source generally includes a boost circuit for converting a voltage provided by a power module to a needed output voltage to the LED unit. Flyback converter is one widely adopted boost circuit, which is also called as inductive-energy-storage converter. When the main transistor of the flyback converter is turn on, the circuit only stores the energy without transmitting the energy. The circuit transmits the energy only when the main transistor is turn off. The flyback converter is characterized by attributes such as it includes the least amount of component, highest reliability, and lowest cost.
FIG. 1 shows a typical flyback boost circuit including a converter, a MOS transistor (Q), and an output diode (Do). The converter includes a primary coil (P) and a secondary coil (S). The turn ratio of the primary coil (P) to the secondary coil (S) is K. The homonymous end of the primary coil (P) connects to the drain of the MOS transistor (Q), and the other end of the primary coil (P) connects to the input voltage (Vin). The source of the MOS transistor (Q) is grounded. The gate of the MOS transistor (Q) is controlled by pulse signals (DRV). The input voltage (Vin) is grounded via a filter capacitor (C). The homonymous end of the secondary coil (S) connects to a positive end of the output diode (Do), and the other end of the secondary coil (S) is grounded. There is also an output capacitor (Co) connected between the negative end of the output diode (Do) and the ground. In addition, the negative end of the output diode (Do) provides the output voltage (Vout) to the load.
When the MOS transistor (Q) is turned on by the pulse signals (DRV), the input DC voltage (Vin) is applied to the primary coil (P) of the converter. The output diode (Do) is blocked due to it is reversely biased due to the voltage sensed on the secondary coil (S) of the converter. At this moment, the power energy is saved on the primary coil (P) in the form of magnetic energy. When the MOS transistor (Q) is turned off, the polarity of the voltage at two ends of the primary coil (P) is inversed, and the polarity of the voltage of the secondary coil (S) is reversed. As such, the output diode (Do) is turned on, and the energy stored in the converter is released to the load. FIG. 2 is a current waveform of the above-mentioned flyback boost circuit. The pulse signals (DRV) are the control signals to turn on or off the MOS transistor (Q). Ip denotes the current signals of the primary coil (P). Is denotes the current signals of the secondary coil (S). As shown in FIG. 1, the relationship between the input voltage and the output voltage is
      Vout    =                  Vin        *        D        *        K                    1        -        D              ,wherein K denotes the turn ratio of the secondary coil (S) to the primary coil (P) and D denotes the duty cycle ratio of the MOS transistor (Q). D satisfies the equation:
      D    =          Ton              Ton        +        Toff              ,wherein Ton denotes the turn-on period of the MOS transistor (Q), and Toff denotes the turn-off period of the MOS transistor (Q). The parameters K and D have to be considered when designing the boost circuit. After the value of K is determined, the value of D has to be larger for at least 50 percent so as to achieve the boost function. Currently, only single primary coil (P) and single switch component are adopted. When the value of D is larger than 50 percent, the switch component may generate a huge amount of heat during the turn-on process. Thus, generally, the value of D is has to be under 50 percent, which limits the range of the output voltage (Vout).