Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2001-01-02
2001-09-04
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169300, C315S337000, C345S212000, C345S074100, C345S076000
Reexamination Certificate
active
06285135
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to field emission displays (FEDs), and particularly to control circuits and methods for preventing emission to grid in field emission displays.
BACKGROUND OF THE INVENTION
One type of flat panel display is known as a cold cathode field emission display (FED). A cold cathode field emission display uses electron emissions to illuminate a cathodoluminescent screen and generate a visual image. A single pixel
10
of a prior art field emission display is shown in FIG.
1
A. The pixel
10
includes a substrate
11
having a conductive layer
12
, and an array of emitter sites
13
on the conductive layer
12
. Although each pixel
10
typically contains many emitter sites (e.g., 4-20 for a small display and several hundred for a large display), for simplicity only one emitter site
13
is shown in FIG.
1
A. An extraction grid
15
is associated with the emitter sites
13
and functions as a gate electrode. The grid
15
is electrically isolated from the conductive layer
12
by an insulating layer
18
. The grid
15
-conductive layer
12
-substrate
11
subassembly is sometimes referred to as a baseplate.
Cavities
23
are formed in the insulating layer
18
and grid
15
for the emitter sites
13
. The grid
15
and emitter sites
13
are in electrical communication with a power source
20
. The power source
20
is adapted to bias the grid
15
to a positive potential with respect to the emitter sites
13
. When a sufficient voltage differential is established between the emitter sites
13
and the grid
15
, a Fowler-Nordheim electron emission is initiated from the emitter sites
13
. The voltage differential for initiating electron emission is typically on the order of 20 volts or more.
Electrons
17
emitted at the emitter sites
13
collect on a cathodoluminescent display screen
16
. The display screen
16
is separated from the grid
15
by an arrangement of electrically insulating spacers
22
. The display screen
16
includes an external glass face
14
, a transparent electrode
19
and a phosphor coating
21
. Electrons impinging on the phosphor coating
21
cause the release of photons
25
which forms the image. The display screen
16
is the anode in this system, and the emitter sites
13
are the cathode. The display screen
16
is biased by the power source
20
(or by a separate anode power source) to a positive potential with respect to the grid
15
and emitter sites
13
. The potential at the display screen
16
is termed herein as an anodic potential. In some systems the potential at the display screen
16
is on the order of 1000 volts or more.
One problem that occurs during operation of a field emission display is known as “emission to grid”. Emission to grid refers to an undesirable flow of electrons from the emitter sites
13
to the grid
15
, or to other elements of the field emission display, such as the spacers
22
. Emission to grid is particularly a problem during turn on (power on), and turn off (power off), of the field emission display.
Emission to grid during turn on is illustrated in FIG.
1
B. During the turn on process, electrons
26
emitted from the emitter sites
13
can go directly to the grid
15
rather than to the display screen
16
. This situation can lead to overheating of the grid
15
. Emission to grid can also affect the voltage differential between the emitter sites
13
and the grid
15
. In addition, desorped molecules and ions can be ejected from the grid
15
causing excessive wear of the emitter sites
13
. Electron emission to grid
15
can also lead to electrical arcing
30
between the grid
15
and the conductive layer
12
, or between the grid
15
and the emitter sites
13
. In addition, electrons
26
emitted from the emitter sites
13
can strike the spacers
22
causing a charge build up on the spacers
22
.
All of these problems decrease the lifetime, performance and reliability of a field emission display. Electron emission to grid is particularly a problem in consumer electronic products, such as camcorders, televisions and automotive displays, which are typically turned on and off many times throughout the useful lifetime of the product.
One reason for electron emission to grid, is that electron emission may have commenced from the emitter sites
13
before the large anodic voltage potential (V
Anode
) has been established at the display screen
16
. Typically, the display screen
16
is a relatively large, relatively high voltage structure, that requires some period of time to reach full potential across its entire surface. In addition, the display screen
16
operates at a significantly higher voltage than any other component of the field emission display. Some period of time is required to ramp up to this operating voltage. Consequently, the display screen
16
can be at a low enough positive potential to allow electron emission to grid
15
to occur, as illustrated in FIG.
1
B. Although this situation may only occur for a relatively short period of time, it can cause system problems as outlined above.
A related situation can also occur during turn on of the display screen
16
and grid
15
if the emitter sites
13
are not electrically controlled. If the emitter sites
13
are not limited during turn on, an uncontrolled amount of emission can occur causing the same problems as outlined above.
In addition, a similar situation exists during the turn off process for the FED cell
10
(i.e., power off). If power to the large positive potential at the display screen
16
is lost prior to termination of electron emission from the emitter sites
13
, then electron emission to grid, as illustrated in
FIG. 1B
, can occur.
The present invention is directed to an improved field emission display and control circuit constructed to prevent electron emission to grid.
SUMMARY OF THE INVENTION
In accordance with the present invention, an improved field emission display configured to prevent emission to grid, is provided. Also provided is an improved method for controlling field emission displays to prevent emission to grid.
The field emission display includes emitter sites for emitting electrons, a grid (cathode) for controlling electron emission from the emitter sites, and a display screen (anode) for receiving electrons from the emitter sites to form a visual image. The field emission display also includes a control circuit for preventing electron emission to grid during operation of the field emission display.
The control circuit includes two separate electrical paths for biasing the grid: a high impedance grid bias path and a low impedance grid bias path. The high impedance grid bias path has an impedance selected to not allow electron emission from the emitter sites, which prevents emission to grid. The low impedance grid bias path has an impedance selected to allow electron emission from the emitter sites to occur. The high impedance grid bias path includes an impedance control circuit for controlling an impedance in the path. The low impedance grid bias path includes a sensing-switching circuit for sensing an anode voltage at the display screen, and switching between the separate electrical paths upon detection of a threshold anode voltage (V
t
).
During turn-on of the FED, the display screen and the high impedance grid bias path are enabled. An anode voltage at the display screen is then sensed, and the low impedance grid bias path is enabled only upon detection of the threshold anode voltage. The control circuit permits the display screen to be enabled either before, or after, enabling of the high impedance grid bias path. In either case, the high impedance grid bias path maintains a grid bias level that will prevent electron emission from the emitter sites, and thus emission to grid, until the threshold anode voltage has been established. In a normal situation the display screen reaches full potential prior to the grid, by a time differential measured in milli-seconds or less.
During turn-off of the FED, the low impedance grid bias path is enabled as
Cathey, Jr. David A.
Zimlich David A.
Gratton Stephen A.
Micro)n Technology, Inc.
Philogene Haissa
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