|Vacuum fluorescent display (VFD)|
|Monday, 23 October 2006|
The basic operation of a vacuum fluorescent display involves the emission of electrons from a low work function thermionic filament cathode, the acceleration of the thermionically emitted electrons by an electric field and stimulation of a phosphor anode by the impacting electrons resulting in cathodoluminescent light generation in the phosphor. The fluorescent display includes anode electrodes, each forming a segment and being coated with a phosphor layer for emitting light by the bombardment of electrons emanated from a cathode filament, and grid electrodes for controlling acceleration of the colliding electrons. When the anodes and the control grids are at a high voltage and the filament is at a lower voltage the electrons can excite the phosphor layer on the anodes to cause light emission from the anodes. Typically, the anodes are selectively activated by a combination of signals from at least one anode driver and a grid driver. The anode and grid drivers, in turn, are controlled by a system processor that, among its many functions, transmits processed serial display data to the anode drivers and grid data to the grid driver. A typical vacuum fluorescent display device comprises a transparent evacuated envelope containing a plurality of anodes arranged in a pattern of desired light emission, each anode being coated with a fluorescent layer for emitting light when excited, a heated filament serving as a source of electrons, and control grids located between the filament and the anodes for determining which anodes can be excited by the electrons. Generally, vacuum fluorescent displays (VFDs) may be classified into various formats depending upon the structure, the display area, the display content, and the manner of driving. Particularly in view of the display area, the vacuum fluorescent displays can be classified into a usual type, a front luminescent type, and a dual layer type. In view of the display content, the vacuum fluorescent displays can be classified into a number display type, a character display type, and a graphic display type. The graphic display type vacuum fluorescent display has phosphors patterned in a dot matrix type corresponding to the picture signal information, and driver chips for selectively driving the grid electrodes.
Vacuum fluorescent displays are generally manufactured in the form of a glass housing having an evacuated central cavity containing phosphored anode segments arranged in a suitable display pattern, and a grid disposed between the anode segments and a filament (cathode). The vacuum fluorescent display is similar to the common cathode ray tube found in televisions and monitors, except that the cathode ray tube uses electromagnetics to direct electrons to the addressed pixel while the vacuum fluorescent display requires that illuminated pixels be individually biased. A switching power supply is used to provide an isolated power for a filament of a vacuum fluorescent display, and provides other power supplies for use in the vacuum fluorescent display. The filament is heated, such as by an AC current, to a temperature at which it will emit electrons. When the anodes and the control grids are at a high voltage and the filament is at a lower voltage the electrons can excite the fluorescent layer on the anodes to cause light emission from the anodes. A suspended acceleration grid biased at a potential higher than a bias of the filament accelerates electrons emitted from the filament toward an anode, also biased higher than the filament bias. On the anode, a phosphor is deposited and emits light in response to the bombardment of electrons emitted from the filament and accelerated by the grid and anode. In the vacuum fluorescent display, the filament cathode to phosphor anode voltage determines whether the phosphor emits light and the intensity of the emitted light. The color of the light emitted from a VF display is determined by the chemical composition of the phosphor. The most commonly used phosphors emit bluish-green light since such phosphors are long lasting, and emit relatively bright light with low potential electron bombardment. A vacuum fluorescent display is capable of producing multi-colored images with a low voltage, and is well adapted to the semiconductor device appliances. Therefore, the vacuum fluorescent displays are attracted for various display purposes.