HK1069708B

HK1069708B - Power supply for an lcd panel

Info

Publication number: HK1069708B
Application number: HK05102148.2A
Authority: HK
Inventors: 林永霖; 柳达
Original assignee: 美国凹凸微系有限公司
Priority date: 2003-04-15
Filing date: 2005-03-11
Publication date: 2008-02-29

Description

Power supply for LCD screen

Technical Field

The present invention relates to a power supply for an LCD screen display, and more particularly, to a backlight power supply for supplying power to a plurality of cold cathode fluorescent lamps of an LCD screen television (LCDTV).

Technical Field

Fig. 1 shows a conventional power supply system 10 for an LCD panel. In the conventional system, a 110V/220V ac power source is converted into a high dc voltage by a rectifier circuit or power factor correction circuit 12. The high DC voltage is then stepped down by a direct current/direct current (DC/DC) converter 14 to provide low voltages, such as 5V and 12V, to electronic devices including microcontrollers, memories, TFT drivers, graphics and Cold Cathode Fluorescent Lamps (CCFLs). The inverter 16 then converts the low dc voltage to a high ac voltage to power the CCFLs in the LCD panel 18. Multiple voltage conversions of the dc/dc converter 14 and the dc/ac inverter 16 can affect conversion efficiency and generate heat in the system. For large LCD screens, such as LCD televisions, most of the power is consumed by the CCFL. Therefore, it is important to improve the efficiency of the inverter for CCFL.

Fig. 2 shows a conventional circuit 20 for increasing the efficiency of an inverter system, in which a high dc voltage is fed directly into the dc/ac converter 16. This reduces the intermediate steps of the dc/dc converter, thereby improving the overall efficiency.

As shown in fig. 3, 3A and 4, a high dc voltage to an ac signal requires a low turns ratio transformer to be connected to the CCFL. Fig. 3 shows an inverter circuit built on a half-bridge circuit (two switches) and including an inverter controller 32 driving switches 34 and 36 to supply the necessary voltage to a transformer 38. Both the inverter controller 32 and the half-bridge circuit are well known in the art. Fig. 3A shows a Class D inverter circuit and fig. 4 shows a full bridge (four switch) inverter circuit, all of which are well known in the art. The circuit shown in fig. 4 also shows a feedback line, which is not important in this discussion. Since the size of the transformer is according to the CCFL. However, in the case of a high voltage input, the number of turns of the primary winding is much greater than that of a transformer driven by a low voltage input of 5V to 10V, which increases the complexity and cost of the transformer.

Disclosure of Invention

In one aspect, the present invention provides a power supply system for an LCD panel, comprising:

an inverter controller capable of controlling the plurality of switches to thereby convert a dc signal to a high voltage ac signal;

a plurality of transformers for receiving a high voltage ac signal, each of said transformers producing a high voltage sinusoidal signal, each transformer having a primary winding and at least one secondary winding, each primary winding being connected in series and each passing the same high voltage ac signal; and

an LCD panel includes a plurality of cold cathode fluorescent lamps, each lamp being powered by a corresponding secondary winding of the transformer.

In another aspect, the present invention provides a power supply system for an LCD panel, including:

an inverter controller capable of controlling the plurality of switches so as to convert a direct current signal into an alternating current signal;

a plurality of transformers for receiving said ac signal and each of said transformers producing a sinusoidal signal, each of said transformers having a primary winding and a secondary winding, each of said primary windings being connected in series and each passing the same ac signal; and

an LCD panel includes a plurality of cold cathode fluorescent lamps, wherein a lamp is connected between a positive pole of a first secondary coil and a negative pole of a second secondary coil, and a first feedback circuit is connected to the negative pole of the first secondary coil and a second feedback circuit is connected to the positive pole of the second secondary coil.

In any of the embodiments presented herein, the power supply may be converted from a high dc voltage signal to a high ac voltage for powering the lamp.

It will be appreciated by those skilled in the art that although the following detailed description will proceed with reference being made to preferred embodiments, the present invention is not intended to be limited to these embodiments. Rather, the invention is to be construed broadly and limited in scope only by the appended claims.

Other features and advantages of the present invention will become more fully apparent during the following detailed description, wherein reference is made to the accompanying drawings and accompanying data, wherein like reference numerals designate like elements.

Drawings

FIG. 1 is a block diagram of a conventional power supply system for an LCD panel;

FIG. 2 is a block diagram of another conventional power supply system for an LCD panel;

FIG. 3 is a block diagram of a conventional inverter circuit for an LCD panel;

FIG. 3A is a block diagram of another conventional inverter circuit for an LCD panel;

FIG. 4 is a block diagram of another conventional inverter circuit for an LCD panel;

FIG. 5 illustrates an exemplary embodiment of an inverter circuit for an LCD panel according to the present invention;

FIG. 5A illustrates another exemplary embodiment of an inverter circuit for an LCD panel according to the present invention;

FIG. 6 is a block diagram of the transformer and LCD panel of the inverter circuit of the present invention;

FIG. 7 is another block diagram of the transformer and LCD panel of the inverter circuit of the present invention;

FIG. 8 is a detailed circuit diagram of the LCD power system of the present invention;

FIG. 9 is a block diagram of a transformer and an LCD panel in the inverter circuit of the present invention; and

fig. 10 shows another driving circuit in which each CCFL is driven by two controllers and two inverter circuits.

DETAILED DESCRIPTION OF EMBODIMENT (S) OF INVENTION

Multiple CCFLs are commonly used in LCD screen televisions to provide sufficient brightness to the LCD screen, for example, 4 to 32 CCFLs depending on the size of the LCD screen. In one aspect of the invention, the primary windings of the transformers are connected in series during power conversion.

Fig. 5 shows an exemplary embodiment of an inverter circuit 50 for an LCD panel according to the present invention. In the exemplary embodiment, the primary windings of transformers T1 and T2 are connected in series. Thus, the coil voltage on each primary coil is the input voltage (V) across the entire coil_AB) Half of that. Compared with the primary coil of the transformer in fig. 4, the method can reduce the number of turns of the coil to half of the original number. In the half-bridge circuit shown in fig. 5, the primary windings of the transformers are connected in series, which reduces the voltage across each winding to a quarter of the input voltage. When a half-bridge circuit is used, the voltage across each coil is 1/2N (where N is the number of transformers in series) of the input voltage. Of course, a full bridge circuit may be used instead in the circuit shown in fig. 4, in which case, when the primary windings of the N transformers are connected in series, the voltage across each primary winding is reduced to 1/N of the input voltage. Fig. 5A shows a Class D inverter circuit, which has the same advantages as the circuit described above with reference to fig. 5, since the primary windings of the transformers are also connected in series.

Fig. 6 is a block circuit diagram 52 showing the transformer and LCD panel of the inverter circuit of the present invention. In this figure, the above principle is extended to: between points a and B shown in fig. 5, four primary coils T1, T2, T3, and T4 are connected in series to supply power to four CCFL tubes. Also, the circuit can be extended as shown in FIG. 7, with N transformers powering N CCFL tubes.

Because each primary winding is connected in series, the current flowing through each transformer primary winding is the same during the closing and opening of the switching circuit (e.g., half-bridge, full-bridge, or Class D switching circuits). In fig. 5,6 and 7, the switch circuit is connected between the points a and B. This configuration further improves the current balance of each transformer secondary driving the CCFL.

Fig. 8 is a detailed current block diagram of an exemplary LCD power supply system 100 of the present invention. The power supply includes an inverter controller 52 that drives two switches 54 and 56 in a half-bridge circuit, as described above with respect to fig. 5. The inverter controller 52 includes voltage and current feedback to control the CCFL power conditions connected to the circuit. In accordance with the principles and description above, each CCFL is powered by one primary coil transformer (e.g., T1, T2.. T (n-1), Tn, Tx; where n represents an even number of lamps and x represents an odd number of lamps) connected in series as shown.

The current feedback is generated by a feedback circuit 60 derived from lamps 1 and 2 in the illustrated circuit. The exemplary current feedback circuit 60 includes an optocoupler and a regulator 64. Regulator amplifying current feedback signal C_FBThe optocoupler 62 then transmits a feedback signal to the controller 52. Likewise, voltage feedback information is generated by voltage feedback circuit 70. In the exemplary embodiment, a voltage feedback signal V is generated from voltage feedback information collected from each lamp in the circuit_B。

Other circuits not directly related to the present invention are also included in the detailed circuit shown in fig. 8. For example, a Pulse Width Modulation (PWM) controller 58 generates dc power signals (e.g., 12V and 5V) to power other components (e.g., memory, microcontroller, etc.) associated with the LCD display. Also, as described above, the PFC module 12 may employ any conventional and/or custom circuitry to generate a high dc voltage.

In another aspect, the present invention provides a circuit for driving a long CCFL lamp. The size of the CCFL tubes employed in LCD televisions is typically longer than the CCFL tubes of LCD displays in portable devices. And driving a long CCFL becomes more difficult. For example, as shown in fig. 3, 3A and 4, conventional methods of driving any lamp having a length of about 60 cm or more require applying a high frequency, high voltage (typically around 1000 volts effective) to the CCFL, when the potential on one side of the CCFL is close to chassis ground. These driving methods typically cause one side of the CCFL tube to darken due to the leakage path between the CCFL and the chassis. Long tubes are about 75 to 80cm or more and long tubes usually mean a leakage capacitance, which affects the electron drift between the tube electrodes.

To solve this problem, the present invention proposes a different driving technique. As shown in fig. 9, a long lamp can be driven by two transformers with opposite phase polarities. In fig. 9, the CCFL1 is driven by the positive pole of the T1 secondary coil and the negative pole of the T2 secondary coil (the positive and negative poles are represented by half a cycle of the sinusoidal power signal generated by the transformer). In practice, the CCFL1 is centered at approximately zero potential. For example, when each transformer outputs 500 volts rms, its voltage and mechanical space are reduced considerably in terms of the safety requirements.

On the other hand, the above-described driving technique may also be modified as shown in fig. 10. Fig. 10 shows a driving circuit 200 for driving a CCFL using two controllers 202 and 204 and two inverter circuits 206 and 208. The two inverter circuits are connected to each other by a synchronization signal 210, so that the controller controls the respective inverter circuits to generate approximately sinusoidal signals with a phase difference of approximately 180 degrees as shown. This ensures that the lamp can receive the full voltage from each inverter every half cycle without power supply signal cancellation. Of course, the circuit may also include voltage and/or current feedback to control the supply conditions of the lamp.

The inverter controller of the present invention may be a conventional inverter controller that includes dimming circuitry (e.g., pulse mode, analog and/or phase) for regulating the power delivered to the lamp. Inverter controllers capable of controlling half-bridge, full-bridge, Class D, and/or other inverter circuits are known in the art and are all equivalent to the present invention. For example, U.S. patent nos. 6,259,615 and 5,615,093, both incorporated herein by reference, disclose inverter controllers for full-bridge and half-bridge inverter circuits, respectively. The inverter controller may also be produced by O2Micro international limited, such as No. oz960, OZ961, OZ965, OZ970, OZ971, OZ972, or OZ9 RR.

In addition, it will be apparent to those skilled in the art that the LCD panel as shown includes circuitry that generates a voltage and/or current feedback signal indicative of the voltage and/or current conditions across the lamp load. The inverter controller described herein is also capable of receiving this feedback information to adjust the voltage and/or current supplied to the lamp load. In an exemplary embodiment, the current feedback may be: one of the two lamps in fig. 5 and 5A, or two of the N lamps in fig. 6, 7 and 8. In fig. 9, the current feedback control signal is generated by the secondary winding portion of the transformer which is not connected to the lamp. In this case, each half cycle of the current flowing to the lamp is monitored. Also, the voltage feedback control signal may be generated by methods well known in the art.

In the exemplary embodiment, the transformer is connected to the power supply by control of the inverter controller. The inverter controller converts a high voltage dc signal source into a high voltage ac signal (square wave). Then, the transformer converts the high voltage AC signal into a high voltage sinusoidal power supply to supply power to the lamp tube. Of course, the present invention may also be used with a low voltage dc power source where the transformer will increase the voltage to a suitable level to drive the lamp. Those skilled in the art will recognize many modifications thereto, all of which are deemed to be within the spirit of the invention and are intended to be limited by the claims appended hereto.

Claims

1. An LCD power supply system, characterized by: the power supply system includes:

2. The power supply system according to claim 1, wherein: the switches are arranged in a full-bridge circuit configuration, and the inverter controller is capable of controlling the full-bridge circuit.

3. The power supply system according to claim 1, wherein: the switches are arranged in a half-bridge circuit configuration, and the inverter controller is capable of controlling the half-bridge circuit.

4. The power supply system according to claim 1, wherein: the switches are arranged according to a Class-D type circuit structure, and the inverter controller can control the Class-D type circuit.

5. The power supply system according to claim 1, wherein: the first and second feedback circuits may generate a current feedback signal indicative of the current supplied to at least one of the lamps, and the inverter controller receives the current feedback signal to adjust the current delivered to at least one of the lamps.

6. The power supply system according to claim 1, wherein: the first and second feedback circuits may generate a voltage feedback signal indicative of a voltage supplied to at least one of the lamps, and the inverter controller receives the voltage feedback signal to adjust the voltage delivered to the at least one lamp.