Fast Warm-Up Heater for Liquid Crystal Display

This invention relates to circuitry for increasing thermal operating stability of solid state devices. More specifically, the invention relates to a circuit for heating solid state arrays such as nematic liquid crystal (LCD) displays so that such arrays can perform in an acceptable manner.

Solid state components such as nematic liquid crystal (LCD) displays are often temperature-dependent in their normal operating characteristics. For example, an LCD display relies on the behavior characteristics of twisted nematic crystalline substances when these nematic crystalline substances are exposed to driving voltages. In response to such exposure to driving voltages, the nematic crystals tend to align themselves so as to provide a desired reflectivity of light. When such substances are arrayed in a pattern, different portions of the substances can be provided with different driving voltages, thereby creating a pattern. Because of the different characteristics of these substances at different temperatures, their performance becomes temperature sensitive.

In order to avoid such temperature sensitivity, heating circuits have been provided for such diplays. Prior art liquid crystal displays are provided with a backplane glass which is used as a supporting substrate for the liquid crystal display. In order to allow such displays to operate at low temperatures, a resistive layer of indium tin oxide (ITO) was placed on the back surface of the backplane glass. The backplane glass provided insulation between the ITO heater element and the liquid crystal display. Typically, the backplane glass was approximately 0.75 mm thick. This provided ample electrical insolation between the ITO heater and the LCD but also resulted in an extensive warm-up time. When tested at minus 54.degree. C., the LCD display required approximately 30 seconds warm-up time.

Full operation of a heating element inside an LCD display on the inside surface of the backplane, would cause deterioration of the liquid crystal substance. Therefore prior art heated LCDs had relied on external heaters, and the extended warm-up time.

This extended warm-up time was considered to be inadequate for some applications, such as for certain instrumentation on aircraft. It is therefore desired to provide a quicker warm-up time for the LCD. It is further desired that such a system for providing a quick warm-up time for an LCD not cause aberations in the normal performance of the LCD.

In accordance with the present invention, a solid state device such as a liquid crystal display (LCD) is provided with a heating arrangement in association with a backplane mounting substrate for the LCD. On an inside surface of the backplane, between the backplane and the LCD, an internal heater is provided, using a resistant material. The internal heater is coated with an insulating layer and backplane electrodes for the LCD are juxtaposed to the insulating layer, so that the insulating layer is between the internal heater and the backplane electrodes and nematic liquid crystal substance. On an external side of the backplane, a second heater is juxtaposed. The internal heater is used to provide LCD response, while continuous heating is applied through the external heater in the manner of conventionally heated liquid crystal displays.

In accordance with an alternate embodiment of the invention, the internal heater is not provided with an insulating layer and the internal heater is used in a pulsed-on mode.

Advantages include quick response time, along with the display stability which is obtained from conventional external heating of the LCD.

FIG. 1 shows a cross-sectional view of a preferred embodiment of the invention;

FIG. 2 shows a circuit for driving the embodiment of FIG. 1;

FIG. 3 shows an alternate embodiment of the invention in cross-section; and

FIG. 4 shows a cross-sectional view of an alternate embodiment, in which no insulating layer is used between backplane electrodes and an internal heater.

Referring to FIG. 1, a typical liquid crystal display (LCD) 11 includes a backplane 13 and a cover glass 15. Between the backplane 13 and cover glass 15 are two arrays of electrodes 17, 19 and liquid crystal substance 21.

The liquid crystal substance typically is an organic compound in a transition state between solid and liquid forms. Liquid crystals usually occur as vicious, jelly-like materials that resemble liquids in certain respects (viscosity) and crystals in other properties (light scattering and reflection). The various esters of cholesterol are the best-known examples. They can align with dichroic dye molecules in a thin-layer cell to produce color changes in transmitted light. 0f particular interest with electronic LCD displays are nematic liquid crystals, which react to biasing voltages applied across a layer of the crystals to darken as a result of the voltage disrupting the orderly arrangement of the crystal molecules. The darkening of the liquid crystal is enough to form visible characters in a pattern corresponding to a pattern of the electrodes 17, 19.

The backplane 13 is typically made of an insulator such as glass. In prior art applications (not shown), backplane electrodes are coated or otherwise deposited onto the backplane. The cover glass 15 is also an insulator and the segment electrodes 19 are coated threon. The liquid crystal substance 21 is contained between the backplane and the cover glass by seals 25.

The electrodes 17, 19 are made of conductive or semiconductive materials such as indium tin oxide (ITO). Very little current is caused to flow through the electrodes 17, 19, so that the relative resistivity of thin layers of ITO electrode material do not adversely affect performance of the LCD device 11.

The inventive LCD 11 is provided with a heater element 31 which is used to bring the liquid crystal substance 21 to a preferred operating temperature. The heater element 31 is coated or otherwise placed onto the backplane 13 on the inside surface of the backplane 13. It is also possible to place the heater element 31 on any other inside surface of the LCD 11, much as electrodes 19 are deposited onto the inside surface of the glass cover 15.

The heater element 31 is preferably made of indium oxide, tin oxide or indium tin oxide (ITO), although other resistive materials may be used. In order to allow the backplane electrodes 17 to operate independently of current applied to the heater element 31, and insulating layer 33 is deposited onto the heater element 31, and backplane electrodes 17 are deposited onto the insulating layer 33. The insulating layer may be made of any convenient insulating material such as silicon dioxide, and allows the backplane electrodes to be supported by the backplane 13 through the heater element 31 and the insulating layer 33. The insulating layer 33 further prevents the biasing voltages applied to the heater element 31 from biasing the liquid crystal material 21, thereby preventing the liquid crystal material 21 from reacting to the biasing of the heater element 31. The insulating layer 33 also permits the backplane electrodes 17 to operate in a manner consistent with the backplane electrodes 17 being insulated from conductor materials.

An external heater 41 is placed on the external surface of the backplane 13. The external heater is similar to the external heaters used in prior art heated LCD designs and permits continued operation of the LCD in a manner similar to such prior art heated LCDs.

The operation of the two heater elements 33, 41 is best explained with reference to FIG. 2. FIG. 2 schematically shows the circuitry of the embodiment of FIG. 1 (and applies equally to the embodiment of FIG. 4).

In FIG. 2, the LCD 11 is provided with display signals by an LCD driver 51 which provides biasing voltages to the electrodes 17, 19. When the display 1 is activated a heater power supply 53 connects to the heater elements 33, 41, activating the heater elements 33, 41. Since the internal heater element 33 is only used for a quick initial warm-up, current to the internal heater element is interupted after a short time period by a delay circuit 55. By the time the current is interupted, the external heater element 41 is providing sufficient heat to permit satisfactory performance of the display 11, and current is applied to the external heating element 41 in order to maintain a desired temperature of the LCD display 11. Operation of the external heater element 41 would not provide the desired warm-up characteristics of the display 11 because of the time it takes for heat generated by the external heater element 41 to be conducted through the backplane 13. Since the internal heating element 33 avoids such time delays, with only the insulating layer 33 slowing heat transfer from that heater element 31 to the liquid crystal substance 21, there is no need for a rapid transfer through the backplane 13. Additionaly, since the internal heater element 31 is providing heat to the backlane 13 as well as to the liquid substance 21, the backplane 13 is heated rapidly. Once the backplane 13 is heated, it is only necessary that the backplane 13 transfer enough heat to maintain the liquid crystal substance 21 at a desired temperature.

Referring to FIG. 3, it may be possible to provide a modified display 61, in which an internal heater element 31 is provided and no external heater element is required. In that arrangement, the heater element 31 must be maintained at a relatively low power level after initial warm-up in order to avoid causing the liquid crystal substance 21 to deteriorate.

It was desired that improved performance be obtained from LCD displays in certain aircraft applications. The testing criteria had been established at -54.degree. C. Using conventional backplane heaters and a backplane having a thickness of 0.75 mm, warm-up was approximately 30 seconds before good performance was obtained from the display.

A display was tested, using the configuration of FIG. 1, except the heaters were located on both the inside surface of the cover glass, as well as the inside surface of the backplane. The experiment was designed to test the "on" response time of a twisted-nematic LCD being driven with 12 V DC at 30 HZ.

The LCD's segments were connected together to be driven by the same "on" signal and a common backplane signal. The common signal allowed for observation of possible different LCD response over the entire LCD area. This would show non-uniformity in the heating process. The LCD had its electrodes layered on top of the heater isolated with silicon dioxide. The display featured black characters on a clear background.

The LCD was situated in a thermal chamber with the display facing the viewing window. All connections to the LCD were made through an access hole in the side of the chamber. The internal temperature of the chamber was lowered to -55.degree. C., and the LCD was stabilized at this temperature for half an hour. After stabilization the LCD heater circuit and segment driver signals were switched on to heat and drive the LCD.

When switching the heater and segment driver on, the segments turned black before the timer counted one second 9 out of 12 times. The other three times the segments turned partially black before one second and fully black after 1.25 seconds.

In order to cause the display to become fully operational in a time period of less than 1 second, it was concluded that a display 71 should be constructed as shown in FIG. 4. In that configuration, an internal heater element 81 should be in direct contact with the liquid crystal substance 21 and, as in the configuration of FIG. 1, an external heater element 41 is provided. In that configuration, the internal heater element 81would be pulsed on and the display 71 would be considered inoperative during the time in which the internal heater 81 is pulsed on. As in the configuration of FIG. 1, the external heater 41 would remain on as long as necessary to maintain the display 71 in a proper operating state. It is anticipated that the internal heater element 81 may have to be pulsed on several times during the first minute of operation. If the pulses are sufficiently short, the display 71 would only be momentarily blanked, thus providing a performance which is at least adequate during the initial operation of the display 71. During the time periods in which the internal heater element 81 is pulsed on, the display 71 would effectively be blanked by showing the effects of the biasing inherent from energizing the internal heater element 81. If the time periods of the internal heater element 81 being pulsed on are sufficiently short this pulsing on should not significantly diminish the utility of display 71 during the warm-up period. In order to further enhance the utility of the display 71 when the internal heater element 81 is being used, it is possible to mask the non-segment areas of the display 71 in order that those parts of the display 71 which are directly over the electrodes 17, 19 may be seen. The masked areas 85 are shown in FIG. 4 on an outer surface of the cover glass 15.

An advantage of not using an insulating layer (33, FIG. 1) is that the heater 81 can be deposited between the backplane electrodes 17 in a single deposition step.

It is clear from the above description that there are numerous variations on the inventive concepts. Therefore, the invention should be read only as limited by the claims.