JP4058530B2 - Separately excited inverter for backlight of liquid crystal display - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a separately-excited inverter for a backlight of a liquid crystal display device , and more particularly to a separately-excited inverter that is used in a cold-cathode tube and can be easily dimmed, a parallel output inverter used in a multi-lamp cold-cathode tube, The present invention relates to a parallel output inverter whose output can be adjusted.
[0002]
[Prior art]
Generally, a backlight is used as a light source in a liquid crystal display panel (LCD), and a plurality of cold cathode tubes are used when high luminance is required.
[0003]
When a plurality of cold-cathode tubes are lit, a beat is generated due to a difference in the lighting frequency between the cold-cathode tubes, and when the lighting frequencies are close to each other, it is visually recognized as flicker. In order to prevent such flickering, it is necessary to synchronize the lighting frequency of each cold-cathode tube. In a conventional multi-lamp cold-cathode tube inverter, a self-excited inverter as shown in FIG. A method of using a plurality of outputs of the transformer 101 and a method of sharing the oscillation circuit on the primary side of the inverter transformer have been adopted. Further, in the separately excited inverter 102 as shown in FIG. 4, a driving pulse is generated by a driving pulse generation circuit, and a switching element connected to one end of the primary winding is turned ON / OFF.
[0004]
In general, a DC / DC converter is used to prevent the brightness of the cold cathode tube from fluctuating due to fluctuations in the primary power supply. In order to prevent flickering due to interference with the inverter transformer, the frequency of the DC / DC converter is usually 3 to 4 times or more the frequency of about 60 kHz that is the oscillation frequency of the inverter transformer.
[0005]
[Problems to be solved by the invention]
However, the conventional cold cathode tube inverter has the following problems. That is, in the self-excited inverter, the oscillation frequency varies depending on the characteristics of the components, and there is a risk that a beat will occur in the brightness of the illuminated LCD. In addition, in the method in which a plurality of self-excited inverter transformer outputs are used to light a plurality of cold cathode tubes, there is a problem that the number of cold cathode tubes is limited due to the output capacity of the inverter transformer. Further, in the method of sharing the oscillation circuit on the primary side of the inverter transformer, there is a problem that the switching element, for example, a transistor, has a large power capacity, and the switching element becomes large.
[0006]
Further, in the above-described method for synchronizing the lighting frequencies of the cold cathode tubes and preventing flickering, the cold cathode tubes cannot be dimmed independently. Further, in the separately excited inverter using the drive pulse, the switching element is connected to only one of the primary windings of the inverter transformer, so the winding ratio between the primary side and the secondary side of the inverter transformer is not increased. Insufficient secondary output voltage is obtained, and the inverter transformer is expensive. In the separately excited inverter 102 shown in FIG. 4, when the cold-cathode tube is lit, the resonance frequency of the inverter transformer and the drive pulse frequency are shifted due to component variations, and the output, for example, the tube current fluctuates. There was a problem that power conversion efficiency fell.
[0007]
Further, since the oscillation frequency of the DC / DC converter used in the inverter is increased to 200 kHz to 300 kHz, the switching loss in the DC / DC converter is large. Further, when the oscillation frequency of the DC / DC converter is lowered, a beat is generated due to interference with the oscillation frequency of the inverter transformer, and there is a problem that the cold cathode tube is flickering.
[0008]
The present invention solves at least one of the problems existing in the prior art, can reduce the cost, and can perform dimming of a cold cathode tube as a load with a simple configuration. Excited inverter, multi-lamp cold-cathode tube inverter capable of dimming each cold-cathode tube independently while performing flicker prevention by synchronizing the lighting frequencies of a plurality of cold-cathode tubes, and a DC / DC converter The purpose is to provide a highly efficient inverter with a reduced frequency.
[0009]
[Means for Solving the Problems]
Other examples type inverter for a back light of a liquid crystal display device according to the present invention, Lee down converter transformer and, complementarily ON and two switching elements of push-pull drive the primary winding of the transformer, the two switching elements A separately-excited inverter having a clock signal generating circuit for supplying a clock signal for -OFF control , wherein the separately-excited inverter includes a plurality of the inverter transformer and the two switching elements; A plurality of PWM signal generation circuits that drive a plurality of the two switching elements by sharing a clock signal and control each of the two switching elements are provided, and each PWM signal generation circuit independently performs PWM modulation. It is characterized by this.
[0011]
The separately-excited inverter includes one PWM signal generating circuit and two AND gates, and the two switching elements of the separately-excited inverter are connected to a clock signal of the clock signal generating circuit and a PWM signal of the PWM signal generating circuit. Are connected so as to be controlled by the AND output.
[0012]
Each of the separately excited inverters includes one PWM signal generating circuit and two AND gates, and each switching element of each separately excited inverter is connected to the clock signal of the clock signal generating circuit and the PWM signal generating circuit. It is connected so as to be controlled by an AND output with the PWM signal.
[0013]
Further, as the input power supplied to the primary winding of the inverter transformer, the output voltage of the DC / DC converter is used, and the DC / DC converter and the oscillation frequency of the clock signal generation circuit are synchronized. is there.
[0014]
Further, a delay circuit is provided for delaying the output of the DC / DC converter until the output oscillation of the clock signal generation circuit is stabilized after the power is turned on.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Embodiment 1
FIG. 1 shows an inverter circuit for a cold cathode tube according to an embodiment of the present invention. An embodiment of the present invention will be described below with reference to the drawings. Reference numerals 14, 15, 24, and 25 denote inverter transformers, and reference numerals 11, 12, 21, and 22 denote switching elements made of FETs that push-pull drive the inverter transformer. The single clock signal i generated by the clock signal generation circuit 4 capable of adjusting the frequency push-pull-drives the switching elements 11, 12, 21, and 22, and one of them is supplied to the AND gate 9 through the inverter gate 8. The other is sent directly to the AND gate 10. The same applies to the inverter gate 18 and the AND gates 19 and 20. When the clock signal generation circuit 4 has an antiphase output, the inverter gates 8 and 18 can be omitted. PWM signals for PWM control of the switching elements 11, 12, 21, 22 and dimming control of the cold cathode tubes 16, 17, 26, 27 are generated by the PWM signal generation circuits 5, 6, and AND gates 9, 10, 19 and 20. The primary power source 1 is connected to the middle point of each inverter transformer via the coils 13 and 23, the DC / DC converter 3, and the power switch 2. Although power supply lines to other electronic circuits are omitted, a power supply voltage is applied in conjunction with the power switch 2. A clock signal generation circuit 4 and a delay circuit 7 are connected to the DC / DC converter 3.
[0016]
In the configuration as described above, the AND gate 9 receives the clock signal e (el) having a phase opposite to that of the single clock signal i and the PWM signal g generated by the PWM signal generation circuit 5. The AND gate 10 receives the clock signal f (f1) in phase with the single clock signal i and the PWM signal g generated by the PWM signal generation circuit 5.
[0017]
Therefore, while the PWM signal g is High, the switching elements 11 and 12 connected to both ends of the primary winding of the inverter transformer 14 are push-pull driven by the signals a and b output from the AND gates 9 and 10, respectively. As shown in FIG. 2, the drive signal Z1 of the cold cathode tube 16 is PWM dimmed. The cold cathode tube 17 is also PWM dimmed by the inverter transformer 15 sharing the primary winding.
[0018]
The AND gate 19 receives a clock signal e (e2) having a phase opposite to that of the single clock signal i and a PWM signal h generated by the PWM signal generating circuit 6. The AND gate 20 receives the clock signal f (f2) having the same phase as the single clock signal i and the PWM signal h generated by the PWM signal generation circuit 6.
[0019]
Therefore, while the PWM signal h is High, the switching elements 21 and 22 connected to both ends of the primary winding of the inverter transformer 24 are push-pull driven by the signals c and d output from the AND gates 19 and 20, respectively. As shown in FIG. 2, the drive signal Z3 of the cold cathode tube 26 is PWM dimmed. The cold cathode tube 27 is similarly PWM dimmed.
[0020]
Here, PWM modulation is performed independently of the PWM signal generation circuits 5 and 6, but the pulse repetition frequency of both is synchronized with one as the master and the other as the slave, or the clock signal of the clock signal generation circuit 4 is counted. It is desirable to synchronize with the common clock signal. As a result, flicker due to interference between the PWM signals g and h can be prevented.
[0021]
As described above, as shown in FIG. 2, the drive signal Z1 of the cold cathode tube 16 and the drive signal Z3 of the cold cathode tube 26 operate in synchronization with a single clock signal, and thus operate in synchronization with each other. Moreover, since the cold cathode tubes 16 and 26 are dimmed by separate PWM signals, each is dimmed independently. The frequency of the single clock signal is adjusted to a predetermined frequency by monitoring the tube current with the tube ammeters 28 and 38 and using a variable resistor or a variable capacitor connected to the clock signal generating circuit 4. As a result, it is possible to prevent fluctuations in output and a decrease in power conversion efficiency due to a difference between the frequency of the single clock signal and the resonance frequency of the inverter transformer.
[0022]
As described above, according to the present invention, since the primary winding of the inverter transformer has a push-pull configuration, the winding ratio of the primary winding and the secondary winding is compared with that which does not have a push-pull configuration. Can be set small. Further, as the dimming means, the AND output of the clock signal e (or clock signal f) and the PWM signal is used, so that the cold cathode tube can be easily dimmed. In addition, each of the cold-cathode tubes can be synchronized to operate independently of each other while preventing flickering, and each of them can be dimmed independently, and the difference between the resonance frequency of the inverter transformer and the frequency of the single clock signal due to component variations. Can prevent fluctuations in output and power conversion efficiency.
[0023]
Embodiment 2
The DC / DC converter 3 in FIG. 1 supplies a constant DC voltage to the inverter transformers 14, 15, 24, 25 even when the power supply voltage fluctuates, and the brightness of the cold cathode tubes 16, 17, 26, 27 is stable. It is provided. That is, the input end of the DC / DC converter 3 is connected to the power source 1 via the power switch 2, and the output end is connected to each middle point of the primary winding of each inverter transformer via the coils 13 and 23. ing. A clock signal for controlling the oscillation of the DC / DC converter 3 is supplied by a connection line from the clock signal generation circuit 4. For oscillation of the DC / DC converter 3, by using the same clock signal as the single clock signal generated by the clock signal generation circuit 4 that drives the switching elements 11, 12, 21, and 22, the inverter transformer and the DC / DC The converter 3 performs a synchronous operation, no beat is generated, and no flickering of the cold cathode tube occurs. Here, the clock signal for synchronizing the DC / DC converter 3 and the inverter circuit is supplied from the clock signal generation circuit 4 of the inverter circuit to the DC / DC converter, but is supplied from the DC / DC converter to the inverter circuit. May be.
[0024]
As described above, according to the present invention, since the DC / DC converter 3 is oscillated at the same low frequency as that of the inverter transformer, no beat is generated, and flickering of the cold cathode tubes 16, 17, 26, 27 is prevented. Can be eliminated.
[0025]
Embodiment 3
The delay circuit 7 in FIG. 1 can be configured by an integration circuit using resistors and capacitors, or a preset counter that counts a fixed number of clock signals, and is connected to an ON / OFF control terminal of the DC / DC converter 3. The delay circuit 7 delays and outputs the output of the DC / DC converter 3 for a predetermined time after the power switch 2 is turned on. Here, the clock signal generation circuit 4, the inverter gates 8 and 18, the AND gates 9, 10, 19, 20 and the PWM signal generation circuits 5 and 6 are linked to the power switch 2 without going through the DC / DC converter 3. Power is being supplied. The power switch 2 is turned on, the clock signal generating circuit 4 oscillates stably, and the output of the AND gates 9, 10, 19, and 20 is delayed after the power is turned on until the output becomes stable, After the stable oscillation, the output of the DC / DC converter is gradually raised in a ramp shape.
[0026]
As described above, all of the AND gates 9, 10, 19, and 20 are output High when the output is indefinite immediately after the power is turned on, and the switching elements 11, 12, 21, and 22 connected to the primary winding of the inverter transformer. Even when is continuously turned on, an inrush current can be prevented by preventing an overcurrent from flowing and raising the output of the switching element in a ramp shape.
[0027]
【The invention's effect】
Backlight separately excited inverter of the liquid crystal display device according to claim 1 of the present invention, complementary and Lee down converter transformer, and two switching elements of push-pull drive the primary winding of the transformer, the two switching elements A separately-excited inverter having a clock signal generation circuit for supplying a clock signal for ON / OFF control in an integrated manner , wherein the separately-excited inverter includes a plurality of the inverter transformer and the two switching elements, and the clock signal A plurality of PWM signal generation circuits that drive the plurality of the two switching elements by sharing the clock signal of the generation circuit and control the two switching elements respectively, and each of the PWM signal generation circuits are independently PWM because and performs modulation, inverter transformer oscillation frequency Without being bound to the resonance frequency can be set freely. Furthermore, the operations of the plurality of cold-cathode tubes can be synchronized to prevent flickering, and the light can be adjusted independently of each other, and output fluctuations due to component variations can be prevented.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing a circuit configuration according to an embodiment of the present invention.
FIG. 2 is a diagram showing signal waveforms at various parts in FIG. 1;
FIG. 3 is a circuit diagram showing a circuit configuration of a conventional self-excited inverter for a cold cathode tube.
FIG. 4 is a circuit diagram showing a circuit configuration of a conventional separately-excited inverter.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Power supply 2 Power switch 3 DC / DC converter 4 Clock signal generation circuit 5, 6 PWM signal generation circuit 7 Delay circuit 8, 18 Inverter gate 9, 10, 19, 20 AND gate 11, 12, 21, 22 Switching element 13, 23 Inductor 14, 15, 24, 25 Inverter transformer 16, 17, 26, 27 Cold cathode tube 28, 38 Tube ammeter

Claims (2)

An inverter transformer, two switching elements that push-pull drive the primary winding of the transformer, and a clock signal generation circuit that supplies a clock signal for complementary ON-OFF control of the two switching elements in the separately excited inverter, said other-commutated inverter, and the inverter transformer, the a plurality of the two switching elements, sharing a clock signal before Symbol clock signal generating circuit to drive a plurality of said two switching elements, A separately-excited inverter for a backlight of a liquid crystal display device, comprising a plurality of PWM signal generation circuits each controlling the two switching elements, wherein each PWM signal generation circuit independently performs PWM modulation . In the separately excited inverter according to claim 1, comprising a plurality of a drive circuit for driving one of said PWM signal generating circuit and two switching elements each, the drive circuit is the PWM signal and the clock signal of the clock signal generating circuit A separately-excited inverter for a backlight of a liquid crystal display device , wherein the switching element is connected to drive a logical product output with a PWM signal of a generation circuit.

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