Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Silicon controlled rectifier ignition
Reexamination Certificate
2001-12-21
2003-05-20
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Silicon controlled rectifier ignition
C310S316010
Reexamination Certificate
active
06566821
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal backlight device, and relates more particularly to the drive device for a cold cathode fluorescent lamp using a piezoelectric transformer and used for the backlight device in liquid crystal displays such as used in personal computers, flat panel monitors, and flat panel televisions.
2. Description of Related Art
Piezoelectric transformers achieve extremely high voltage gain when the load is unlimited, and the gain ratio decreases as the load decreases. Other advantages of piezoelectric transformers are that they are smaller than electromagnet transformers, noncombustible, and do not emit noise due to electromagnetic induction. Piezoelectric transformers are used as the power supply for cold cathode fluorescent lamps due to these features.
FIG. 26
shows the configuration of a Rosen-type piezoelectric transformer, a typical piezoelectric transformer according to the prior art. As shown in
FIG. 26
, this piezoelectric transformer has a low impedance part
510
, high impedance part
512
, input electrodes
514
D and
514
U, output electrode
516
, and piezoelectric bodies
518
and
520
. Reference numeral
522
indicates the polarization direction of the piezoelectric body
518
in the low impedance part
510
, reference numeral
524
indicates the polarization direction in piezoelectric body
520
, and reference numeral
610
indicates the piezoelectric transformer.
When piezoelectric transformer
610
is used for voltage gain, the low impedance part
510
is the input side. As indicated by polarization direction
522
the low impedance part
510
is polarized in the thickness direction, and input electrodes
514
U and
514
D are disposed on the primary front and surfaces in the thickness direction. The high impedance part
512
is the output part when the piezoelectric transformer is used for voltage gain. As indicated by polarization direction
524
the high impedance part
512
is polarized lengthwise and has output electrode
516
on the lengthwise end of the transformer.
A specific ac voltage applied between input electrodes
514
U and
514
D excites a lengthwise expansion and contraction vibration, which piezoelectric effect of the piezoelectric transformer
610
converts to a voltage between input electrode
514
U and output electrode
516
. Voltage gain or drop results from impedance conversion by the low impedance part
510
and high impedance part
512
.
A cold cathode fluorescent lamp with a cold cathode configuration not having a heater for the discharge electrode is generally used for the backlight of a LCD. The striking voltage for starting the lamp and the operating voltage for maintaining lamp output are both extremely high in a cold cathode fluorescent lamp due to the cold cathode design. An operating voltage of 800 Vrms and striking voltage of 1300 Vrms are generally required for a cold cathode fluorescent lamp used in a 14-inch class LCD. As LCD size increases and the cold cathode fluorescent lamp becomes longer, the striking voltage and operating voltage are expected to rise.
FIG. 27
is a block diagram of a self-excited oscillating drive circuit for a prior art piezoelectric transformer. Variable oscillator
616
generates the ac drive signal for driving piezoelectric transformer
610
. The variable oscillator
616
generally outputs a pulse wave from which the high frequency component is removed by wave shaping circuit
612
for conversion to a near-sine wave ac signal. Drive circuit
614
amplifies output from wave shaping circuit
612
to a level sufficient to drive the piezoelectric transformer
610
. The amplified voltage is input to the primary electrode of piezoelectric transformer
610
. The voltage input to the primary electrode is stepped up by the piezoelectric effect of the piezoelectric transformer
610
, and removed from the secondary electrode.
The high voltage output from the secondary side is applied to over-voltage protection circuit
630
and the serial circuit formed by cold cathode fluorescent lamp
626
and feedback resistance
624
. The over-voltage protection circuit
630
consists of voltage-dividing resistances
628
a
and
628
b,
and comparator
620
for comparing the voltages detected at the node between voltage-dividing resistances
628
a
and
628
b
with a set voltage. The over-voltage protection circuit
630
controls the oscillation control circuit
618
to prevent the high voltage potential output from the secondary electrode of the piezoelectric transformer from becoming greater than the set voltage. The over-voltage protection circuit
630
does not operate when the cold cathode fluorescent lamp
626
is on.
In the over-voltage protection circuit
630
, the voltage occurring at both ends of the feedback resistance
624
is applied to the comparator
620
as a result of the current flowing to the series circuit of cold cathode fluorescent lamp
626
and feedback resistance
624
. The comparator
620
compares the set voltage with the feedback voltage, and applies a signal to the oscillation control circuit
618
so that a substantially constant current flows to the cold cathode fluorescent lamp
626
. Oscillation control circuit
618
output applied to the variable oscillator
616
causes the variable oscillator
616
to oscillate at a frequency matching the comparator output. The comparator
620
does not operate until the cold cathode fluorescent lamp
626
is on.
Cold cathode fluorescent lamp output is thus stable. This self-exciting drive method enables the drive frequency to automatically track the resonance frequency even when the resonance frequency varies because of the temperature.
This piezoelectric inverter configuration makes it possible to maintain a constant current flow to the cold cathode tube.
As shown in
FIG. 23
, a method of driving the cold cathode fluorescent lamp by parallel driving two piezoelectric transformers, and a drive method wherein the two output electrodes of the piezoelectric transformers are connected to two input terminals of the cold cathode fluorescent lamp, have been proposed as a way to prevent uneven brightness. The cold cathode fluorescent lamp in these cases is connected as shown in FIG.
25
.
Similarly to the drive circuit shown in
FIG. 27
, these drive circuits also need feedback of current flow to the lamp in order to control the frequency or voltage. It is alternatively possible to detect and feed back the cold cathode fluorescent lamp brightness.
Piezoelectric transformer output current or output voltage is held constant in order to hold the cold cathode fluorescent lamp brightness constant, or current flow to the reflector is detected and fed back for control.
A conventional piezoelectric transformer and drive circuit therefore thus connect a resistance near the cold cathode fluorescent lamp ground and use the voltage of this resistance in order to control the brightness of the cold cathode fluorescent lamp when the cold cathode fluorescent lamp is on. A problem with this method is that uneven brightness occurs as a result of current leaks.
To resolve this problem, Japanese Laid-Open Patent Publication No.11-8087 teaches a means for inputting 180° different phase voltages from opposite ends of the cold cathode fluorescent lamp. This configuration is shown in FIG.
22
. However, when a cold cathode fluorescent lamp is connected as shown in
FIG. 22
, current flows to the reflector from the cold cathode fluorescent lamp
330
on the high potential side, and current flows from the reflector to the cold cathode fluorescent lamp on the low potential side. Piezoelectric transformer output current thus contains both current flowing to the lamp and current flowing to a parasitic capacitance. As a result, the output current detection circuit
344
in the drive circuit of a piezoelectric transformer
340
configured as shown in
FIG. 25
thus detects both the current flowing to the cold cathode fluorescent lamp
346
and the leakage current of the parasitic capacitance
348
con
Moritoki Katsunori
Nakatsuka Hiroshi
Takeda Katsu
Yamaguchi Takeshi
Greenblum & Bernstein P.L.C.
Matsushita Electric - Industrial Co., Ltd.
Tran Chuc
Wong Don
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