Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2001-05-14
2002-05-21
Vu, David (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169400, C313S495000, C313S496000
Reexamination Certificate
active
06392356
ABSTRACT:
FIELD OF THE INVENTION
The present application teaches an apparatus and method for applying an active matrix to the Vacuum Fluorescent Display (“VFD”) in order to maximize the brightness of the VFD. This invention teaches how to integrate an active matrix to the VFD using either single crystal silicon chips or thin film transistor (“TFT”) techniques for the active matrix, and generally relates to the areas of flat panel displays.
BACKGROUND OF THE INVENTION
The Vacuum Fluorescent Display (“VFD”) is a flat panel display that has been manufactured in Japan and Russia for the last two decades. The VFD has found a marketplace as a messaging display for equipment such as clocks, radios, tape players and CDs in automobiles. It is also found on appliances such as microwave ovens. The VFD is viewed by the industry as a very bright and reliable display for low-resolution alphanumeric and icon displays. It has never found use as a high-resolution graphics display that could be used in the computer monitor, or television markets. The reason this has not occurred is that the high-resolution displays must support animated images, and VFDs presently cannot support such animation.
In order to produce animation the display must be refreshed at some frame rate that is fast enough that the image does not appear to flicker to the human eye. This minimum frame rate is around sixty frames per second. In such displays the frame rate is usually selected at around 75 frames per second so that the frame rate does not coincide with the 60 cycles per second of the alternating current electrical power source.
Cathode ray tubes (“CRTs”) are beam-driven displays. In CRTs the frame is painted pixel-by-pixel by sweeping a beam of electrons in a raster scan from side-to-side and down the frame until the complete image is formed. In such displays the electron beam is only momentarily on each phosphor dot (pixel), once for each frame. The human eye's response is too slow to catch the beam movement and interprets the response as a steady lighted dot, although in reality it is flickering at 60 or 70 times per second. Instead of a flicker the eye sees a low brightness.
Due to the short dwell time of the beam on a particular pixel, the light from the phosphor of the pixel is highly limited. To compensate for the short dwell time the beam power is boosted to extremely high powers and voltages (30,000 volts for the color TV). If the television beam were to remain fixed on the phosphor dot, then that pixel would be extremely bright for a short time and then burn out.
Today all flat displays, including VFDs, are matrix driven devices as opposed to beam driven devices. Matrix driven means that driving the image is obtained by activating columns and rows. The point where a column and row meet defines a pixel. Present matrix driven displays are commonly line-driven as well, as opposed to either raster-scanned (CRTs) or matrix displays that are individually addressable pixel-by-pixel. This means that a total line of the display is enabled together by a single line driver and then image data for the line is fed in parallel to all the columns. The result of this is that the dwell time of the electrons on the phosphor is about a thousand times longer than it is for a raster-scanned display. This means that the electron power can be greatly reduced so that a line scanned VFD need only have 10 to 50 volts to energize the electrons stimulating the phosphors.
The brightness of the line-driven display is impacted not by the number of dots or pixels in a line, but by the total number of lines in the display, as the line may be activated only for the length of time of the given line is active during a particular frame. Hence, increasing the number of lines decreases the amount of time that each line is active, and hence diminishes the brightness of the line. The high brightness of previous VFDs is due to the fact that the matrix in a messaging display has only a few lines (from 1 to 10). The more lines the display has, the dimmer is the image for a particular voltage. This means that a high-resolution display with 500 lines is too dim, and that more brightness has to be attained by turning up the voltage.
However, high voltage displays require that the driver system must be able to handle voltages in the 200 and 300-volt range to obtain the brightness of a line-driven VFD. This causes the driver system to be prohibitively expensive, and therefore not economical. The matrix line scan cannot produce an economically viable high-resolution display.
One solution to this problem is to turn the phosphor pixels on for the total length of the frame. This can be accomplished using a transistor circuit to drive each individual pixel. This was implemented by Peter Brody at Westinghouse in the early 1970s and is called the active matrix (“AM”). Today liquid crystal displays employ the active matrix and are called active matrix liquid crystal diodes (“AMLCDs”). The active matrix is typically made from amorphous silicon, or poly-silicon.
In 1981, the concept of an active matrix vacuum fluorescent display (“AMVFD”) was published by Sahiro Uemura and Kentaro Kiyozumi, engineers working for Ise Electronics, Japan, in the
Transactions on Electron Devices
, Vol. Ed-28, No. 6, June 1981. In that paper they discussed a pixel memory system consisting of two p-channel transistors and a capacitor in a monolithic integrated circuit silicon chip. This enabled the display to operate at 100 percent duty factor with a 60-Hz refresh rate. The results were, “in the enhancement of phosphor brightness up to 4000 to 5000 fL at Vp=30 V.” Having a display with such brightness potential allows it to be used as a projection system, or the filament temperature may be significantly reduced for a substantial power saving, or high filtration can be added to make a daylight-readable display.
Nine years later in the papers for the 1st International CdSe (Cadmium Selenide) Workshop, 1990, a paper presented by Shimojo, Okada and Kamogawa of Ise Electronics, Japan discussed an AMVFD that utilized an active matrix using thin film transistors (“TFTs”) fabricated with cadmium selenide for the thin film semiconductor. In Japan, cadmium selenide is considered to be very poisonous and therefore, Ise dropped the use of cadmium selenide shortly thereafter in favor of single crystal silicon chips.
The semiconductor circuits used in AMVFDs are fabricated utilizing CMOS (Complementary Metal Oxide Semiconductor) technology. The CMOS circuit is constructed with an insulating layer of glass deposited over the circuitry and interconnects, with an aluminum anode pad deposited over the glass and connected to the drain of a power FET (Field Effect Transistor) under it. Phosphor of the proper formulation is then deposited on the aluminum pads. These chips were then mounted on the base glass of the vacuum envelope with filament wires strung over the phosphors. The image is viewed through the filaments, but they are so thin that they are not seen at the viewing distance.
Prototype displays were tested and were found to have four times the brightness of commercially available VFDs. The difficulty with the silicon chip system is that each chip has to be carefully aligned with the chip on either side of it and with the chip over and under it. Also, since the chips cannot be abutted up against each other (because chips need area around the circuitry to be “diced” and for power lines) and because each chip is not exactly like the next chip, some room has to be afforded between each chip. This reduces the amount of phosphor surface area and also the number of pixels per linear unit, because the space between chips must also be the same as the space between pixels on the chip otherwise the display will not be uniform, but will have lines crisscrossing it corresponding to the cracks between the chips. Thus, high-density graphics displays are not possible using the silicon chip technique.
Another problem with present AMVFD displays is that they have no gray scale capability beyond a simple binar
Kolakowski Victoria S.
Telegen Corporation
Vu David
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