Power Source Device for Driving Liquid Crystal

1. Field of the Invention

The present invention relates to a power source device for a liquid crystal display unit used to display images on an instrument panel of an automobile, a television picture tube, etc., and more particularly to a power source device for a liquid crystal display unit such as a liquid crystal display unit of a dot matrix type, etc. which requires a high driving voltage and which therefore requires a negative power source for the purpose of driving same.

2. Description of the Prior Art

A liquid crystal display unit requires a high driving voltage. An integrated circuit (hereinafter referred to as IC) used in a circuit provided in the signal processing stage prior to the driving voltage stage (the signal processing stage processes the content of the display) is not matched with a power source voltage. In the prior art, when a liquid crystal display unit of this kind is driven, a special IC called a negative power source generating IC 10, as shown in FIG. 4, is used to obtain a negative polarity power source voltage which is applied to a liquid crystal driving IC 11 to effect matching of voltage with that of the liquid crystal display unit 1.

However, in the case where the aforementioned negative power source generating IC 10 is used, and exclusive-use special IC is required, and further circuit parts therefor are also required, thus inevitably increasing the cost. With the recent extensive market need for liquid crystal display units, wherein the number of matrix display units tends to be greatly multistaged, there is a need for higher power negative voltage power sources therefor. The negative voltage capable of being generated by the aforesaid negative voltage generating IC 10 is not enough to cover the aforesaid need, and a few of said ICs 10 need be connected in series, as a consequence of which the device further becomes expensive.

As a specific means for solving the problems noted above with respect to the prior art, the present invention provides a power source device for driving a liquid crystal display unit provided with a negative voltage generating device, characterized in that the negative voltage power source generating device is provided as part of an inverter which serves as a power source device for a light source used for the back illumination of the liquid crystal display unit, whereby a suitable negative voltage, as desired, may be easily obtained, and the manufacturing cost of a liquid crystal display unit including drive circuit parts is not affected by the requirement to provide a sufficient voltage to overcome the above-described prior art problems.

FIG. 1 is an explanatory view showing the schematic connection of a power source device for driving a liquid crystal according to the present invention;

FIG. 2 is a circuit representation specifically showing one embodiment of a power source device for driving a liquid crystal also serving as an inverter according to the present invention;

FIG. 3 is a circuit representation showing an example wherein an intermediate tap is provided in a secondary winding of a step-up transformer according to a further embodiment of the present invention; and

FIG. 4 is a circuit representation showing a prior art power source device.

The present invention will now be described in detail with reference to the drawings. For ease of understanding, parts similar to those of the prior art device of FIG. 4 are indicated by the same reference numerals.

A guest-host type or a dot matrix type liquid crystal display unit 1 is provided with a discharge lamp 2 for back illumination such as a cold cathode fluorescent tube which is superior in color rendering property to an incandescent bulb, and an illuminating device such as a reflecting mirror 3 adjacent thereto. The discharge lamp 2 cannot be normally lighted by a d.c. 12-volt power supply which is a source voltage of an automobile, and therefore there is provided an inverter 4 basically comprising an oscillation circuit and a step-up transformer to generate a sufficient voltage to light the discharge lamp 2.

FIG. 2 specifically shows an internal connection diagram of the inverter 4 according to the present invention, which also includes a negative voltage power source for the liquid crystal display. The inverter 4 comprises an oscillation circuit 41 provided in the conventional inverter, a primary winding 42a of a step-up transformer 42 to which is applied a power source voltage converted from d.c. to a.c. by said oscillation circuit 41, a secondary winding 42b which steps up the voltage of the primary winding 42a to form a voltage high enough to light said discharge lamp 2, a feedback winding 42c for said oscillation circuit 41, and a tertiary winding 42d. To the output of the tertiary winding 42d are connected a diode D1, a capacitor C1 and a resistor R1 which constitute a rectifier circuit so as to obtain a negative d.c. voltage. Further, a Zener diode D2 for regulating and stabilizing the negative d.c. voltage to the intended voltage is connected in parallel with the output of the rectifying circuit. With this arrangement, the intended negative d.c. voltage is obtained, and the thus obtained negative d.c. voltage is supplied to a printed substrate 5 on which liquid crystal driving ICs 11 as previously mentioned in the prior art embodiment are disposed.

In a modified embodiment of the present invention, an intermediate tap L1 is suitably provided at an intermediate position on the secondary winding 42b in place of the tertiary winding 42d of the step-up transformer 42, for example, as shown in FIG. 3. In such a modified embodiment, the rectifying circuit is connected to tap L1 instead of across tertiary winding 42d.

According to the present invention, a negative voltage power source generator is incorporated into a power source device (i.e., an inverter) for a discharge lamp for back illumination of a liquid crystal display unit, whereby a suitable negative d.c. voltage for the liquid crystal display unit may be obtained without use of any special IC or special circuit.

Since the liquid crystal display unit according to the present invention is viewed synthetically (i.e., with artificial back lighting provided by a lamp), a tertiary winding or an intermediate tap or the like is provided on the secondary winding of the inverter (which is a power source device for an indispersibly provided discharge lamp for back illumination), and a rectifier circuit to obtain a negative voltage is coupled to the tertiary winding or to the intermediate tap of the secondary winding to provide a suitable negative voltage without use of a special negative voltage generating IC as in the prior art, and without being restricted at all by a voltage generated by such a prior art IC. This increases the freedom and adaptability of the device in terms of design of circuits as well as enables provision of a liquid crystal driving power source device which is unexpensive and has an excellent performance by virtue of the joint use of parts.