Electric lamp and discharge devices: systems – Current and/or voltage regulation
Reexamination Certificate
2001-09-28
2002-11-26
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
C315S307000, C315S256000, C315S250000, C315S224000
Reexamination Certificate
active
06486618
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to DC-AC inverters. More specifically, it relates to DC-AC inverters that adapt to different input voltages and different loads.
2. Discussion of the Related Art
Producing a color image using a Liquid Crystal Display (LCD) is well known. Such displays are particularly useful for producing images that are updated by frames, such as in LCD desktop and laptop computer. Typically, each image frame is composed of color sub-frames, usually red, green and blue sub-frames.
LCD systems employ a light crystal light panel that is comprised of a large number of individual liquid crystal pixel elements. Those pixel elements are beneficially organized in a matrix comprised of pixel rows and pixel columns. To produce a desired image, the individual pixel elements are modulated in accordance with image information. Typically, the image information is applied to the individual pixel elements by rows, with each pixel row being addressed in each frame period.
Pixel element matrix arrays are preferably “active” in that each pixel element is connected to an active switching element of a matrix of switching elements. One particularly useful active matrix liquid crystal display is produced on a silicon substrate. Thin film transistors (TFTs) are usually used as the active switching elements. Such LCD displays can support a high pixel density because the TFTs and their interconnections can be integrated on the silicon substrate.
FIG. 1
schematically illustrates a single pixel element
10
of a typical LCD. The pixel element
10
is comprised of a twisted nematic liquid crystal layer
12
that is disposed between a transparent common electrode
14
and a transparent pixel electrode
16
. Additionally, image signals are applied to the pixel electrode
16
via a control terminal
24
.
Still referring to
FIG. 1
, the liquid crystal layer
12
rotates the polarization of light
30
that passes through it, with the rotation being dependent on the voltage across the liquid crystal layer
12
(the image signal potential). The light
30
is derived from incident non-polarized light
32
from an external light source (which is not shown in FIG.
1
). The non-polarized light is polarized by a first polarizer
34
to form the polarized light
30
. The light
30
passes through the transparent pixel electrode
16
, through the liquid crystal layer
12
, and through the transparent common electrode
14
. Then, the light
30
is directed onto a second polarizer
36
. During the pass through the liquid crystal layer
12
, the polarization of the light
30
is rotated in accord with the magnitude of the voltage across the liquid crystal layer
12
(the image signal potential). Only the portion of the light
30
that is parallel with the polarization direction of the second polarizer
36
passes through that polarizer. Since the passed portion depends on the amount of polarization rotation, which in turn depends on the voltage across the liquid crystal layer
12
, the voltage on the control terminal
24
controls the intensity of the light that leaves the pixel element.
FIG. 2
schematically illustrates a liquid crystal display comprised of a pixel element matrix. As shown, a plurality of pixel elements
10
, each having an associated switching thin film transistor, are arranged in a matrix of rows (horizontal) and columns (vertical). For simplicity, only a small portion of a pixel element matrix array is shown. In practice there are numerous rows, say 1290, and numerous columns, say 1024. Still referring to
FIG. 2
, the pixel elements of a row are selected by applying a gate (switch) control signal on a gate line, specifically the gate lines
40
a,
40
b,
and
40
c.
Image signals are then applied to column lines
46
a,
46
b,
and
46
c.
The various image signal voltages are then applied to associated control terminals
24
of the pixel elements
10
. When the gate (switch) control signal is removed, the image signal voltages are then stored on capacitances associated with the TFT.
The foregoing processes are generally well known and are typically performed using digital shift registers, microcontrollers, and voltage sources. Beneficially semiconductor processing technology is used extensively.
The principles of the present invention relate to producing the non-polarized light
32
illustrated in FIG.
1
. That non-polarized light
32
is typically produced by a cold cathode fluorescent lamp. This is at least partially because fluorescent lamps are efficient sources of broad-area white light. In battery powered applications, such as portable computers, the efficiency of the fluorescent lamp light source directly impacts battery life, size, and weight.
Fluorescent lamps are typically powered by an inverter. The inverter, in turn, can be powered by a battery or by another power source such as an LCD power supply. In any event, the inverter converts a relatively low DC voltage (say 3-24 volts DC) into a high AC voltage required to drive the fluorescent lamp. Typically over 500 volts are required to operate a cold cathode fluorescent lamp, while a “kick-off” voltage of around 1500 Volts is required to start conduction. Thus, such inverters are DC-to-AC inverters.
FIG. 3
depicts a conventional DC-to-AC inverter
50
in operation. That inverter receives DC power on a line
52
. The operating DC-to-AC inverter includes a filter capacitor
54
, totem pole arranged FET switches
56
and
58
, diodes
57
and
59
, an inductor
60
, one or more fluorescent lamps (modeled by resistors)
62
, each associated with a transformer
64
, and a storage capacitor
66
. The FET switches
56
and
58
are controlled by a controller
68
. In operation, the FET switches
56
and
58
are alternately turned on and off with about equal times (50 % duty cycle) by the controller
68
. When the FET
56
is conducting, the FET
58
is OFF. Then, the input on line
52
is switched across the inductor
60
and transformer(s)
64
and the storage capacitance
66
. When FET
56
is OFF, the FET
58
is conducting. Additionally, under proper bias conditions, the diodes
57
and
59
conduct. Then, the storage capacitor
66
discharges through the inductor
60
and the transformer(s)
64
to ground.
Essentially, the DC-to-AC inverter
50
forms a simplified circuit shown in FIG.
4
. The input voltage supply
80
is formed by the controller
68
selectively switching the FET switches
56
and
58
such that the power input on line
52
is applied to the inductor
60
, and then selectively switching that inductor to ground.
FIG. 4
also shows an equivalent inductor
84
, which is formed by the inductance of the inductor
60
and of the transformer(s)
64
. That equivalent inductor
84
beneficially resonates with an equivalent resonant capacitor
80
, which is the reflected secondary-side capacitance of the lamp-shield capacitance and the inter-winding parasitic capacitance of the transformer.
FIG. 4
also shows an equivalent resistor
90
, which represents the transformed resistance of the fluorescent lamp(s)
62
.
While DC-to-AC inverters as shown in
FIGS. 3 and 4
are generally successful, in some applications they may not be optimal. For example, it is difficult to implement highly efficient DC-to-AC inverters over a wide range of input voltages. That is, the voltage on line
52
becomes critical in the overall design of the DC-to-AC inverters, and thus to the LCD display. In practice DC-to-AC inverters must be tailored to a particular LCD display's backlight inverter input voltage.
Even if a DC-to-AC inverter's input voltage range is acceptable, a DC-to-AC inverter usually only works well when designed for a particular load. That is, the equivalent lamp resistance
90
(see
FIG. 4
) and capacitance
80
must be taken into consideration when designing a particular DC-to-AC inverter. Thus, DC-to-AC inverters are usually designed to operate only with a narrow range of fluorescent lamps. Changes in lamp styles, sizes, or manufacturers can create p
Alemu Ephrem
Wong Don
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