Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2001-06-14
2004-11-02
Hjerpe, Richard (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S212000, C315S276000
Reexamination Certificate
active
06812916
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a driving circuit for an LCD backlight lamp, more particularly to a driving circuit minimizing a leakage current due to stray capacitances residing in its lamp driving side.
2. Description of the Related Art
Generally, a battery is used as a power source for laptop computers that use an LCD as a main display device. An LCD uses a backlight lamp to supply an amount of light required for illuminating pixels to display data or information since it cannot generate light by itself. In addition, because a high voltage of about 1000-1500V is required to drive a backlight lamp, a low-voltage DC power supplied from a battery should be converted to high-voltage AC power. For satisfying this requirement, a driving circuit for a backlight lamp such as
FIG. 1
is used.
FIG. 1
depicts a conventional driving circuit for an LCD backlight lamp. The driving circuit of
FIG. 1
comprises a DC/DC converter
120
generating a DC voltage of higher level by switching the DC power supplied from a battery
110
according to a PWM (Pulse Width Modulation) control signal from a PWM controller
121
; an inverter
130
consisting of an AC oscillator
131
which swings sinusoidally with an amplitude of the high DC voltage from the DC/DC converter
120
and a transformer T
1
which boosts the AC output of the oscillator
131
to its secondary side; a Ballast capacitor C
2
applying the boosted AC power from the transformer T
1
to a backlight lamp
150
at initial state and absorbing some power to protect the driven lamp at stable state; a current sensor
160
sensing the current flowing in the lamp
150
after rectifying; and a luminosity controller
170
comparing the magnitude sensed by the current sensor
160
with an adjustable reference level which is set externally, and outputting a control signal to vary a duty ratio of the PWM control signal of the PWM controller
121
according to the comparison result.
The operation of the LCD backlight driving circuit configured as
FIG. 1
will be explained in detail.
The DC/DC converter
120
always provides the inverter
130
with a high DC voltage by switching the DC power supplied from the battery
110
according to PWM control signal, and the inverter
130
converts the high DC voltage from the DC/DC converter
120
to high voltage AC power through the internal AC oscillator
131
and the transformer T
1
. While dissipating the supplied AC power, the lamp
150
emits light. At the moment when the lamp
150
starts to be driven, the Ballast capacitor C
2
enables the high starting voltage (1000-1500V) to be instantly applied to the lamp
150
, and then it absorbs some of the AC power outputted from the inverter
130
to protect the driven lamp
150
, which guarantees stable operation of the lamp
150
after the lamp
150
is driven.
The current sensor
160
rectifies positive half waves through a diode D
1
because the current driving the lamp
150
is an alternating current, and it flattens the rectified waves through a resister R
7
and a capacitor C
3
. Then, the luminosity controller
170
compares the flattened magnitude outputted from the current sensor
160
with a reference which is adjustable manually, and outputs a difference signal, which is the result of the comparison, to change the duty ratio of the PWM-control signal of the PWM controller
121
. Due to this feedback control based on a set reference and the fed back lamp current, it is possible to supply constant electric energy for the lamp
150
, so that the desired brightness is maintained constantly.
In the conventional lamp driving circuit that operates as described above, the current flowing in the lamp
150
is fed back through the current sensor
160
and the luminosity controller
170
for PWM-control of the DC/DC converter
120
so that constant electric energy might be supplied to the lamp
150
to maintain a desired brightness. However, because the secondary side of the transformer T
1
is connected to the primary side via a ground in order to establish a feedback loop as described above, stray capacitances are formed unwantedly along the high power path of the secondary side and around its windings and the lamp
150
. Because of a leakage current induced by such stray capacitances, the efficiency of power consumption is lowered.
That is, in the conventional backlight driving circuit, stray capacitances Cx (marked as dotted lines) are formed, as depicted in
FIG. 2
, between the lamp
150
and a lamp protection reflector grounded, and along the high power path of the secondary side of the transformer T
1
. Therefore, a leakage current flows to a ground through the stray capacitances Cx. Because the leakage current due to the stray capacitances Cx is about 10% (in the condition of i=2Π fcv, f=50 kHz, V=700V, and C=about 20 pf) of the lamp driving current, all the energy provided from the secondary side of the transformer T1 is not used to drive the lamp
150
, thus the efficiency of power consumption is not good.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an LCD backlight driving circuit being able to minimize a leakage current through stray capacitances by conducting feedback of some load current with electrical insulation between the primary and secondary side of a transformer.
A driving circuit for an LCD backlight according to the present invention comprises a DC/DC converter changing the level of an input DC power; an inverter converting the level-changed DC power into AC, boosting the converted AC power to higher voltage AC power, which is to be applied to a lamp, according to the ratio of a primary and a secondary winding; a feedback means sensing the AC current flowing in the lamp, feeding back the sensed current with electrical insulation between the primary and the secondary side, and flattening the fed back current; and a level controller comparing the flattened current with a reference signal, and providing the difference signal between the two compared signals to the DC/DC converter which adjusts a target level according to the difference signal. More particularly, the feedback means comprises a photo coupler rectifying the AC current flowing in the lamp and feeding back to the primary side, or an auxiliary transformer inducing the AC current of the lamp to its secondary winding with electrical insulation.
The driving circuit for an LCD backlight lamp according to the present invention, can eliminate stray capacitances which might reside in the lamp driving side, and minimize a leakage current through stray capacitances.
REFERENCES:
patent: 5936359 (1999-08-01), Gibson
patent: 6097158 (2000-08-01), Manor et al.
patent: 6346800 (2002-02-01), Mano et al.
patent: 6466196 (2002-10-01), Isogawa et al.
patent: 6498437 (2002-12-01), Chang et al.
Birch & Stewart Kolasch & Birch, LLP
Fathaiyan M.
Hjerpe Richard
LandOfFree
Driving circuit for LCD backlight does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Driving circuit for LCD backlight, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Driving circuit for LCD backlight will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3279638