Abstract:
The present invention relates to a driving circuit for an LCD backlight lamp which can feed back some of a current flowing in a lamp with electrical insulation. This driving circuit for an LCD backlight lamp is able to eliminate stray capacitances which might reside in the secondary side of a boosting transformer, and minimize a leakage current due to the stray capacitances. Therefore, the power feeding time of a battery is extended in a portable device such as laptop computer.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a driving circuit for an LCD backlight lamp, more particularly to a driving circuit minimizing a leakage current due to stray capacitances residing in its lamp driving side.  
           [0003]    2. Description of the Related Art  
           [0004]    Generally, a battery is used as a power source for laptop computers that use an LCD as a main display device. An LCD uses a backlight lamp to supply enough light required for illuminating pixels to display data or information since it can not generate light by itself. In addition, because a high voltage of about 1000-1500V is required to drive a backlight lamp, a low-voltage DC power supplied from a battery should be converted to high-voltage AC power. For satisfying this requirement, a driving circuit for a backlight lamp such as FIG. 1 is used.  
           [0005]    [0005]FIG. 1 depicts a conventional driving circuit for an LCD backlight lamp. The driving circuit of FIG. 1 comprises a DC/DC converter  120  generating a DC voltage of higher level by switching the DC power supplied from a battery  110  according to a PWM (Pulse Width Modulation) control signal from a PWM controller  121 ; an inverter  130  consisting of an AC oscillator  131  which swings sinusoidally with amplitude of the high DC voltage from the DC/DC converter  120  and a transformer T 1  which boosts the AC output of the oscillator  131  to its secondary side; a Ballast capacitor C 2  applying the boosted AC power from the transformer T 1  to a backlight lamp  150  at initial state and absorbing some power to protect the driven lamp at stable state; a current sensor  160  sensing the current flowing in the lamp  150  after rectifying; and a luminosity controller  170  comparing the magnitude sensed by the current sensor  160  with an adjustable reference level which is set from outside, and outputting a control signal to vary duty ratio of the PWM control signal of the PWM controller  121  according to the comparison result.  
           [0006]    The operation of the LCD backlight driving circuit configured as FIG. 1 will be explained in detail.  
           [0007]    The DC/DC converter  120  always provides the inverter  130  with a high DC voltage by switching the DC power supplied from the battery  110  according to PWM control signal, and the inverter  130  converts the high DC voltage from the DC/DC converter  120  to high voltage AC power through the internal AC oscillator  131  and the transformer T 1 . While dissipating the supplied AC power, the lamp  150  emits light. At the moment when the lamp  150  starts to be driven, the Ballast capacitor C 2  enables the high starting voltage (1000-1500V) to be instantly applied to the lamp  150 , and then it absorbs some of the AC power outputted from the inverter  130  to protect the driven lamp  150 , which guarantees stable operation of the lamp  150  after the lamp  150  is driven.  
           [0008]    The current sensor  160  rectifies positive half waves through a diode D 1  because the current driving the lamp  150  is an alternating current, and it flattens the rectified waves through a resister R 7  and a capacitor C 3 . Then, the luminosity controller  170  compares the flattened magnitude outputted from the current sensor  160  with a reference which is adjustable manually, and outputs a difference signal, which is result of the comparison, to change the duty ratio of the PWM—control signal of the PWM controller  121 . Due to this feedback control based on a set reference and the fed back lamp current, it is possible to supply constant electric energy for the lamp  150 , so that the desired brightness is maintained constantly.  
           [0009]    In the conventional lamp driving circuit that operates as described above, the current flowing in the lamp  150  is fed back through the current sensor  160  and the luminosity controller  170  for PWM-control of the DC/DC converter  120  so that constant electric energy might be supplied to the lamp  150  to maintain a desired brightness. However, because the secondary side of the transformer T 1  is connected to the primary side via a ground in order to establish a feedback loop as described above, stray capacitances are formed unwantedly along the high power path of the secondary side and around its windings and the lamp  150 . Because of a leakage current induced by such stray capacitances, the efficiency of power consumption is lowered.  
           [0010]    That is, in the conventional backlight driving circuit, stray capacitances Cx (marked as dot lines) are formed, as depicted in FIG. 2, between the lamp  150  and a lamp protection reflector grounded, and along the high power path of the secondary side of the transformer T 1 . Therefore, a leakage current flows to a ground through the stray capacitances Cx. Because the leakage current due to the stray capacitances Cx is about 10% (in the condition of i=2Πfcv, f=50 kHz, V=700V, and C=about 20 pf) of the lamp driving current, all the energy provided from the secondary side of the transformer T 1  is not used to drive the lamp  150 , thus the efficiency of power consumption is not good.  
         SUMMARY OF THE INVENTION  
         [0011]    It is an object of the present invention to provide an LCD backlight driving circuit being able to minimize a leakage current through stray capacitances by conducting feedback of some load current with electrical insulation between the primary and secondary side of a transformer.  
           [0012]    A driving circuit for LCD backlight according to the present invention, comprises a DC/DC converter changing the level of an input DC power; an inverter converting the level-changed DC power into AC, boosting the converted AC power to higher voltage AC power, which is to be applied to a lamp, according to the ratio of a primary and a secondary winding; a feedback means sensing the AC current flowing in the lamp, feeding back the sensed current with electrical insulation between the primary and the secondary side, and flattening the fed back current; and a level controller comparing the flattened current with a reference signal, and providing the difference signal between the two compared signals to the DC/DC converter which adjusts a target level according to the difference signal. Especially, the feedback means comprises a photo coupler rectifying the AC current flowing in the lamp and feeding back to the primary side, or an auxiliary transformer inducing the AC current of the lamp to its secondary winding with electrical insulation.  
           [0013]    The driving circuit for an LCD backlight lamp according to the present invention, can eliminate stray capacitances which might reside in the lamp driving side, and minimize a leakage current through stray capacitances.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The accompanying drawings, which are included to provide a further understanding of the invention, illustrate the preferred embodiments of the invention, and together with the description, serve to explain the principles of the present invention.  
         [0015]    In the drawings:  
         [0016]    [0016]FIG. 1 depicts a conventional driving circuit for an LCD backlight lamp;  
         [0017]    [0017]FIG. 2 illustrates stray capacitances formed in a lamp driving side of the circuit of FIG. 1;  
         [0018]    [0018]FIG. 3 is a driving circuit for an LCD backlight lamp according to the present invention including a photo coupler as an insulating means; and  
         [0019]    [0019]FIG. 4 is another driving circuit for an LCD backlight lamp according to the present invention including an auxiliary transformer as an insulating means. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]    The accompanying drawings illustrate the preferred embodiments of the present invention, and together with the description, serve to explain the principles of the present invention.  
         [0021]    [0021]FIG. 3 depicts a circuit diagram of a backlight lamp driving circuit according to the present invention. The driving circuit of FIG. 3 comprises a DC/DC converter  320  generating a DC voltage of higher level by switching the DC power supplied from a battery  310  according to an PWM control signal of an internal PWM controller  321 ; an inverter  330  consisting of an AC oscillator  331  which swings sinusoidally with amplitude of the high DC voltage from the DC/DC converter  320  and a transformer T 1  boosting the AC output of the oscillator  331  to its secondary side; a Ballast capacitor C 2  applying the boosted AC power from the transformer T 1  to a backlight lamp  350  at initial driving state, and absorbing some power to protect the driven lamp  350  at stable state; a photo coupler  360  feeding back the AC current flowing in the lamp  350 , with electrical insulation, to the primary side from the secondary of the transformer T 1 ; a DC filter  370  flattening the half-wave current outputted from the photo coupler  360 ; and a luminosity controller  380  comparing the magnitude flattened by the DC filter  370  with a desired set-point which is adjustable manually, and outputting a regulating signal to change the duty ratio of the PWM control signal of the DC/DC converter  320  according to the comparison result. The flattened magnitude and the desired set-point are compared each other at the inverting (−) and non-inverting terminal (+) of a comparator, and the difference between two signals is applied to the PWM controller  321  as the regulating signal.  
         [0022]    The DC/DC converter  320  consists of a power transistor Q 1  whose emitter is connected to the battery  310 ; the PWM controller  321  whose output is applied to the base of the transistor Q 1 ; and an inductor L 1  connected to the collector of the transistor Q 1  to boost switched voltage from the transistor Q 1 .  
         [0023]    The AC oscillator  331  of the inverter  330  consists of resistors R 1  and R 2  connected to the inductor L 1 ; a resistor R 3  connected between the inductor L 1  and the collector of a photo transistor of the photo coupler  360 ; two transistor Q 2  and Q 3  whose bases are connected to the resistor R 2  and R 3  respectively and whose emitters are commonly grounded; a capacitor C 1  connected between the collectors of the transistors Q 2  and Q 3 . In addition, the transformer T 1  of the inverter  330  is connected with the neighboring circuits such that its first winding of the primary side is connected between each collector of the transistors Q 2  and Q 3 , its second winding of the primary side is connected between each base of the transistors Q 2  and Q 3 , the secondary winding is connected between the Ballast capacitor C 2  and the cathode of a photo diode of the photo coupler  360 .  
         [0024]    The input terminal of the lamp  350  is connected to the Ballast capacitor C 2  and its output terminal is connected to the anode of the photo diode of the photo coupler  360 . Another diode D 1  is connected in parallel with the photo diode of the photo coupler  360  such that their connected directions are opposite each other.  
         [0025]    The photo transistor of the photo coupler  360  is connected with a resistor R 4  at its collector and its emitter is grounded.  
         [0026]    The DC filter  370  comprises the resistor R 4  whose the other end is connected to the inverting terminal of the comparator of the luminosity controller  380 ; and a capacitor C 3  connected between the inverting terminal and a ground.  
         [0027]    The luminosity controller  380  comprises the comparator whose non inverting terminal is connected to a ground through serial connected two resistors R 6  and R 7 . The output terminal of the comparator is connected to the PWM controller  321 , and it is also connected with the inverting terminal through a resistor R 5 . And, a reference signal for a desired set-point is applied to the connection point of the two resistors R 6  and R 7 .  
         [0028]    The detail explanation on the operation of the present backlight lamp driving circuit will be followed with reference to FIG. 3.  
         [0029]    If a DC power is supplied from the battery  310 , it is inputted to the first transistor Q 1  of the DC/DC converter  320  and is switched according to the PWM control signal from the PWM controller  321  and is then fed to the inductor L 1 . The inductor L 1  boosts the switched DC voltage and provides it to the inverter  330 . The chopped and boosted DC voltage fed to the inverter  330  is converted to an AC power by the AC oscillator  331  whose transistors Q 2  and Q 3  turn on/off alternatively. The AC voltage inverted by the AC oscillator  331  is transformed to high voltage 1000-1500V in accordance with the ratio of the first winding of the primary side to secondary winding of the transformer T 1 . When the high AC power is supplied from the inverter  330  to the lamp  350 , the lamp  350  emits enough light.  
         [0030]    At the moment when the lamp  150  starts to be driven, the Ballast capacitor C 2  applies the high starting voltage (1000-1500V) to the lamp  150  instantly, and it absorbs some of the AC power outputted from the inverter  130  to protect the driven lamp  150  after initial state diminishes, which guarantees a stable operation of the lamp  150  after the lamp  150  is driven.  
         [0031]    While the lamp  350  is being driven, the AC current flows the lamp  350  and the diode D 1  during negative half wave, and the photo diode of the photo coupler  360  during positive half wave. While a current flows the photo diode, the photo transistor turns on, so that the voltage at the collector of the photo transistor is proportional to the magnitude of the current flowing in the lamp  350 . This signal transmission is conducted by the intensity of radiation with insulation between the two photo elements.  
         [0032]    The rectified half-wave current at the collector of the photo transistor becomes almost flat through the DC filter  370 . The comparator of the luminosity controller  380  receives the flattened DC level from the DC filter  370  at its inverting terminal, and compares the received level with the reference level set based on a desired luminosity. According to the comparison, the difference signal between the two levels is sent to the PWM controller  321  which uses the difference signal for adjusting duty ratio of its own PWM signal to be applied to the base of the transistor Q 1 . Due to this feedback control based on a set reference and the fed back lamp current, it is possible to supply constant electric energy for the lamp  150 , so that the desired backlight brightness is maintained constantly.  
         [0033]    [0033]FIG. 4 depicts another circuit diagram of a backlight lamp driving circuit according to the present invention. The same elements as in FIG. 3 will be assigned to identical numeric codes, and the explanation for them is omitted. However, different elements and their operations will be described.  
         [0034]    In the circuit of FIG. 4, instead of the photo coupler  360 , an auxiliary transformer T 2  feeds back the AC current flowing through the lamp  350  with insulation between the primary and the secondary side of the transformer T 1 , and a diode D 3  connected to the secondary side of the auxiliary transformer T 2  rectifies the AC energy delivered through the transformer T 2  and applies the rectified half-wave current to the DC filter  370 . The resistor R 3  and the diode D 1  of FIG. 3 are removed in the circuit of FIG. 4.  
         [0035]    The newly configured parts of the circuit of FIG. 4 operate as follows. The auxiliary transformer T 2  is electrically insulated between its primary and secondary side and induces a current proportional to the load current flowing through the primary winding at the secondary to feed back some of the current driving the lamp  350 . The induced current is AC and it is rectified into a positive half-wave current through the diode D 3 . The half-wave current is flattened by the DC filter  470  and is then fed to the inverting terminal of the comparator in the luminosity controller  380 . Next operations are all same with the above explanation related with the driving circuit of FIG. 3.  
         [0036]    The backlight lamp driving circuit according to the present invention can insulate electrically between the primary and the secondary side of the boosting transformer in feeding back some of a load current driving the lamp, so that prevent stray capacitance from being formed between a lamp protection reflector and between the secondary side and the ground. Due to the elimination of stray capacitance, there is little power loss caused by the leakage current, which extends a power feeding time of a battery which a portable device such as a laptop computer should be equipped with.