Power supply circuit for a cold-cathode fluorescent lamp

Details Power supply circuit for a cold-cathode fluorescent lamp Power supply circuit for a cold-cathode fluorescent lamp

2002-09-20

2003-10-28

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Periodic switch in the supply circuit

Periodic switch in the primary circuit of the supply...

C315S224000, C315S225000, C315SDIG002, C315SDIG005, C323S355000

Reexamination Certificate

active

06639366

ABSTRACT:

This application incorporates by reference Taiwan application Serial No. 90126086, filed Oct. 22, 2001.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates in general to the power supply circuit converting a DC voltage to a AC voltage, and more particularly to the power supply circuit for a cold-cathode fluorescent lamp.
2. Description of the Related Art
The LCD (Liquid Crystal Display) monitor is popular in these years because of being low in radiation, lightweight and compact. For example, portable electronic devices such as the notebook computers are equipped with LCDs for portable purposes.
The LCD panels can be classified into a reflective type and a transmissive type. The LCD panels of the transmissive type require back lighting. Cold-Cathode Fluorescent Lamp (CCFL) is commonly used as back lighting source, because it needs only simple control circuits and has the high power efficiency and longer life. The CCFL is started up by supplying a high AC voltage thereto. In a notebook computer, the high AC voltage is supplied by a power supply circuit, which converts the DC voltage outputted by the battery into the high AC voltage.
FIG. 1
is a diagram of a conventional power supply circuit
100
for the CCFL. The power supply circuit
100
is the Royer type circuit, which includes switches
104
,
106
, and a transformer
108
. The power supply circuit
100
converts the DC voltage outputted by the DC voltage output circuit
102
into a high AC voltage for driving the CCFL
110
. The transformer
108
is used for stepping up the voltage inputted thereto. The switches
104
and
106
are bipolar junction transistors (BJT). The collectors of the switches
104
and
106
are coupled to the two end nodes of the primary side of the transformer
108
, respectively. The middle node of the primary side of the transformer
108
is coupled to the positive node of the DC voltage output circuit
102
. The emitters of the switches
104
and
106
are coupled to the negative node of the DC voltage output circuit
102
. The two nodes of the feedback circuit
112
of the secondary side of the transformer
108
are coupled to the bases of the switches
104
and
106
, respectively. The bias resistance R
1
is coupled between the positive node of the DC voltage output circuit
102
and the base of the switch
104
. The CCFL
110
and the decoupling capacitor C
1
are connected serially with the secondary side of the transformer
108
.
FIG. 2A
is the equivalent circuit diagram of the power supply circuit
100
while the switch
104
is on and the switch
106
is off.
FIG. 2B
is the equivalent circuit diagram of the power supply circuit
100
while the switch
104
is off and the switch
106
is on. The voltage outputted by the DC voltage output circuit
102
controls the on/off status of the switches
104
and
106
, and the polarity of the primary side of the transformer
108
changes accordingly, as shown in
FIGS. 2A and 2B
. The polarity of voltage of the secondary side of the transformer
108
also changes according to that of the primary side. The transformer
108
steps up the AC voltage at the primary side and outputs the high AC voltage to the CCFL
110
via the decoupling capacitor C
1
at the secondary side, according to the turn ratio of the primary side and the secondary side.
The main disadvantage of the power supply circuit
100
is the low power efficiency, which is about 70%~80%. Thus the usage time of the battery after each charge is reduced. The lifetime of the CCFL is also reduced. The transformer
108
has a complex structure that makes it expensive and difficult to manufacture.
FIG. 3
is a diagram of another power supply circuit
300
for the CCFL. The power supply circuit
300
includes switches
304
and
306
, formed with MOSFETs, the capacitor C
1
and a transformer
308
. The switch
304
is an N-channel MOSFET, and the drain thereof is coupled to one node of the primary side of the transformer
308
, and the other node of the primary side is coupled to the positive node of the DC voltage output circuit
302
. The on/off statuses of the switch
304
and
306
are controlled by the switch control circuit
312
. The negative node of the capacitor C
1
is connected to the drain of the switch
306
, and the positive node thereof is connected to both the drain of the switch
304
and one node of the primary side of the transformer
308
. Two nodes of the diode D
1
are connected to the drain and the source of the switch
304
, respectively. And two nodes of the diode D
2
are connected to the drain and the source of the switch
306
, respectively. The diodes D
1
and D
2
are either the intrinsic diodes of the MOSFETs, or external diodes connected to the MOSFETs.
The operation of the power supply circuit
300
is described in
FIGS. 4A
to
4
C.
FIG. 4A
is the equivalent circuit diagram of the power supply circuit
300
when the switch
304
is on and the switch
306
is off. The DC voltage output circuit
302
supplies a positive voltage to the primary side of the transformer
308
, and the corresponding current flows from the DC voltage output circuit
302
, to the transformer
308
, and then to the switch
304
.
FIG. 4B
is the equivalent circuit diagram of the power supply circuit
300
when the switches
304
and
306
are off. At this time, the voltage of the primary side of the transformer
308
is still positive, but the magnitude of the voltage thereof decreases with time. The current flows from the primary side of the transformer
308
to the capacitor C
1
for energy preserving and charges the capacitor C
1
to make the voltage thereof increases with time.
FIG. 4C
is the equivalent circuit diagram of the power supply circuit
300
when the switch
304
is off and the switch
306
is on. At this time, the capacitor C
1
discharges and the voltage of the primary side of the transformer
308
is negative. By alternating the on and off status of the switches
304
and
306
, the polarity of the voltage of the transformer
308
also alternates, as shown in
FIGS. 4A
to
4
C. At the same time, the primary current I
1
that flows through the primary side of the transformer
308
, and the secondary current I
2
that flows through the secondary side of the transformer
308
each also alternates the flow direction accordingly.
The disadvantage of the power supply circuit
300
is that the control mechanism is complex because three phases are required for the switch control circuit
312
to control the on/off status of the switches
304
and
306
. Besides, the precise timing control of the on/off status of the switches
304
and
306
are required and thus the control mechanism is more complex.
FIG. 5
is another well-known diagram of the power supply circuit
500
. The power supply circuit
500
includes the energy-preserving capacitor C
1
coupled to the primary side of the transformer
512
in parallel, the energy-preserving inductor L
1
coupled to the energy-preserving capacitor C
1
and the primary side of the transformer
512
, and four MOSFETs used as switches
504
,
506
,
508
, and
510
. The switch
504
is electrically connected to the positive node of the DC voltage output circuit
502
, energy-preserving inductor L
1
and the switch
506
. The switch
508
is electrically connected to the positive node of the DC voltage output circuit
502
, the primary side of the transformer
512
, the capacitor C
1
and the switch
510
. The switch
506
is further connected to the switch
510
.
The operation scheme is described in FIGS.
6
A~
6
D.
FIG. 6A
is the equivalent circuit diagram of the power supply circuit
500
while the switch
504
and
510
are on, and the switch
506
and
508
are off. At this time, the DC voltage output circuit
502
charges the energy-preserving capacitor C
1
and the energy-preserving inductor L
1
. The polarity of the primary side of the transformer
512
is positive, and the magnitude of the voltage thereof increases with time. The current flows from the energy-preserving inductor L
1
to the prim

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