JP2006164785A - Power supply for backlight - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To allow an inverter part to be always operated in an optimum state, whereby efficiency of the inverter part is improved; and to increase the output voltage of a D.C. power source part to enhance its efficiency, whereby efficiency of a whole backlight lighting circuit is significantly improved. <P>SOLUTION: This power supply for a backlight is provided with the separately-excited inverter part 10 for lighting a plurality of cold-cathode tubes used for a backlight unit, and the D.C. power source part 12 for driving it. The separately-excited inverter part operates at a constant duty not less than 45% and less than 50% in a normal operation state; and stabilization of a tube current flowing to the cold-cathode tube is executed by the control of the output voltage of the D.C. power source part. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a power supply device for driving a backlight unit using a cold cathode tube. More specifically, the present invention relates to a separately excited inverter unit for lighting a cold cathode tube and a direct current power source for driving the separately excited inverter unit. The present invention relates to a backlight power supply apparatus in which the unit is integrated and the tube current is stabilized by controlling the output voltage of the DC power supply unit.

  A plurality of cold-cathode tubes are used for backlights of liquid crystal televisions and liquid crystal display devices. For example, in a 32-inch liquid crystal television, 16 cold cathode fluorescent lamps are arranged on the back surface of the liquid crystal panel at appropriate intervals to maintain the brightness of the entire screen. Such a backlight power supply device generally has a configuration in which an inverter unit for lighting a plurality of cold cathode tubes and a DC power source unit for driving the inverter unit are combined. As an inverter part, there are a thing by a separate excitation type switch circuit and a thing by a self-excitation type switch circuit, but in any case, in the prior art, these inverter part and DC power supply part are completely separated, It is comprised so that it may operate | move independently (for example, refer patent document 1).

  As is well known, the brightness of the panel screen varies depending on the current (tube current) flowing through a cold cathode tube incorporated as a backlight. Therefore, in order to keep the brightness of the panel screen constant, it is necessary to stabilize the tube current. In the conventional backlight power supply device as described above, the DC power supply unit always supplies a DC output of a substantially constant voltage (rated output voltage) to the inverter unit, detects the tube current in the inverter unit, and switches the inverter unit Stabilization is performed by controlling the duty of the circuit (in other words, controlling the on-time of the switch element).

  As a matter of course, the inverter unit is designed to satisfy the input voltage range standard (rated voltage ± tolerance voltage). Usually, since the control is started with a certain margin (with a lower voltage) with respect to the lower limit value of the input voltage range, the switching duty at the rated input voltage in the normal operation state is considerably small (30 to 40). %degree). However, the optimum state of the inverter is obtained when the switching duty is operated at the maximum value (ideally 50%). Therefore, as the duty decreases, the conversion efficiency of the transformer deteriorates and the switching loss increases. For this reason, in the power supply device having the conventional configuration, the efficiency of the inverter unit is low in the operation of the normal rated input voltage.

  Further, in the conventional separately-excited inverter unit, in order to reduce the required number of switch circuits and reduce the cost, a plurality of primary windings of the inverter transformer are usually connected in parallel. However, in the case of parallel connection, due to variations in transformer inductance, variations in tube current occur, resulting in uneven brightness of the panel screen. Although it is only necessary to keep the variation in inductance very small, it is difficult to design and manufacture such a transformer. Furthermore, in such a parallel connection, the switching current increases by the amount of parallel connection, and the loss of the switch element increases accordingly, which also causes the deterioration of efficiency.

Generally, a power supply device has a characteristic that efficiency increases as an output voltage increases. However, in the prior art, the output voltage of the DC power supply unit (input voltage of the inverter unit) is generally 12 to 24V, so that in any case of 24V, a satisfactory efficiency can never be obtained up to 12V. .
JP 2002-134293 A

  The problem to be solved by the present invention is to enable the inverter part to always operate in an optimum state, thereby improving the efficiency of the inverter part. Another problem to be solved by the present invention is to increase the output voltage of the DC power supply unit so as to increase the efficiency, which together improve the efficiency of the entire backlight lighting circuit.

  The present invention includes a separately-excited inverter unit for lighting a plurality of cold-cathode tubes used in a backlight unit, and a direct-current power source unit that drives the separately-excited inverter unit. Operates at a constant duty of 45% or more and less than 50%, and stabilization of the tube current flowing in the cold cathode tube is performed by controlling the output voltage of the DC power supply unit. It is a power supply device. The separately excited inverter unit is preferably set to perform a switching operation with an optimum duty of about 47 to 49%.

  The DC power supply section incorporates an output voltage detection error amplifier that regulates the maximum value of the output voltage, and during the transition period immediately after the power is turned on (the period until all the cold cathode tubes are lit), the switching frequency is switched by the separately excited inverter section. After that, all the cold-cathode tubes are kept lit by controlling to a low normal frequency.

  The switching circuit of the separately excited inverter unit may be a half-bridge circuit, a full-bridge circuit, or an active clamp circuit, and a plurality of primary windings of the inverter transformer may be connected in series or in series and parallel.

  The backlight power supply device according to the present invention is configured such that the separately excited inverter unit is always operated at the optimum duty, and the tube current flowing in the cold cathode tube is stabilized by controlling the output voltage of the DC power supply unit. As a result, the conversion efficiency of the inverter transformer is improved, and at the same time, the switching loss is reduced, whereby the inverter efficiency can be greatly improved.

  Further, in the backlight power supply device of the present invention, when a plurality of primary windings of the inverter transformer are connected in series, the variation in tube current can be reduced, and the voltage and current waveform applied to the cold cathode tube can be made more sinusoidal. The stress applied to the cold cathode tube is also reduced. The output voltage of the DC power supply unit can be set as high as several tens to several hundreds V instead of the low voltage (12 to 24V) of the normal inverter specification. Therefore, the efficiency of the DC power supply unit is greatly improved. The overall efficiency of the backlight power supply device is remarkably improved by the synergistic effect of the improvement of the inverter efficiency and the improvement of the efficiency of the DC power supply unit.

  A schematic configuration of a power supply device for a backlight according to the present invention is shown in FIG. This power supply device for backlight includes a separately excited inverter unit 10 for lighting a plurality of cold cathode tubes used in a backlight unit, and a DC power supply unit 12 for driving the separately excited inverter unit 10. The DC power supply unit 12 is configured to convert a commercial AC input into a DC output and control its output voltage. The separately excited inverter unit 10 is configured to operate at a constant optimum duty of 47 to 49% in a normal operation state, and supplies a sine wave tube current having a predetermined frequency to each cold cathode tube.

  In the present invention, the current (tube current) flowing through the cold cathode tube is detected by the tube current detection circuit 14, and the tube current detection signal is returned to the control unit of the DC power supply unit instead of the control unit of the inverter unit. The present invention is characterized by such a tube current feedback system. Specifically, the tube current detection signal is fed back to the tube current feedback error amplifier 16 of the DC power supply unit 12, thereby controlling the output voltage of the DC power supply unit and stabilizing the tube current flowing in the cold cathode tube. Plan.

  For comparison, the prior art is illustrated in FIG. The prior art also includes a separately-excited inverter unit 20 that turns on a plurality of cold-cathode tubes used in a backlight unit, and a DC power source unit 22 that drives the separately-excited inverter unit 20. The signal controls the ON width of the switch element (pulse width control) in the inverter section to stabilize the tube current. In other words, the DC power supply unit and the inverter unit are completely separated, tube current control is completed in the inverter unit, and the DC power supply unit only outputs a substantially constant output voltage. Yes.

  FIG. 3 is a block diagram showing an embodiment of the backlight power supply device according to the present invention. As described above, the backlight power supply apparatus includes the separately excited inverter unit 10 that lights a plurality of cold cathode tubes used in the backlight unit, and the DC power source unit 12 that drives the separately excited inverter unit 10. It has.

  The DC power supply unit 12 is configured to convert a commercial AC input into a DC output and control its output voltage. The commercial AC input is input to the power factor correction (PFC) circuit 32 via the noise filter (NF) 30, and the power factor of the pulsating flow obtained by full-wave rectifying the AC input and suppressing the harmonics is improved. It always outputs as almost constant and stable direct current. This direct current output is switched by a switch circuit 34 having a switch element, converted into a voltage via a transformer 36, rectified by a rectifier 38, and smoothed by a smoother 40 to obtain a desired direct current output. The operation of the switch circuit 34 is controlled by the control unit / driver unit 42, thereby controlling the DC output voltage.

  The separately excited inverter unit 10 is driven by a DC output of the DC power source unit 12 and supplies a current for lighting to the cold cathode tube. The direct current output of the direct current power supply unit 12 is input to the switch circuit 46 through the input unit 44, converted into alternating current of a desired frequency by switching operation of the switch element, and cold cathode tube through the inverter transformer 48, the resonant capacitor 50, and the like. To supply current. The operation of the switch circuit 46 is controlled by the control unit 52. This switch circuit can be applied to a full bridge circuit, an active clamp circuit, etc. in addition to a half bridge circuit.

  Here, in the present invention, a tube current detection circuit 14 is incorporated in the separately excited inverter unit 10 to detect a current (tube current) flowing in the cold cathode tube as a tube current detection signal, and the tube current detection signal is The feedback is made to the tube current feedback error amplifier 16 incorporated in the DC power supply unit 12, and the error signal is supplied to the control unit 42 of the DC power supply unit 12 so as to control the output voltage of the DC power supply unit 12. Constitute. Therefore, the separately excited inverter unit 10 is not controlled at all with respect to the tube current in the normal operation state. In other words, the separately-excited inverter unit 10 is configured to operate at an optimal duty of 47 to 49% in a normal operation state (an ON time of each switch element is constant), and a sinusoidal tube current having a predetermined frequency is generated. Supply to each cold cathode tube. The maximum value of the duty is ideally 50%, but in order to prevent excessive short-circuit currents from flowing at the same time when the switch elements that should be alternately turned on are turned on, the time when both are turned off (dead time) is set. Therefore, the optimum duty is set to 47 to 49%. By operating at such a high duty, it is possible to improve the efficiency of the inverter transformer and reduce the switching loss.

  Since the tube current by the inverter in the non-control state is a value proportional to the input voltage, if the input voltage fluctuates within a certain range, the tube current changes accordingly. The present invention utilizes this phenomenon, and the DC voltage supplied to the separately excited inverter unit 10 is variably controlled by the DC power source unit 12 so as to stabilize the tube current flowing in the cold cathode tube. The screen brightness is kept constant.

  By the way, the cold cathode tube has a property that current hardly flows when it is cold. In the present invention, since the tube current is stabilized by the DC power supply unit 12 instead of the separately excited inverter unit as described above, the DC power supply unit has a DC voltage much higher than that during normal operation immediately after the power is turned on. If it is left as it is, an excessive voltage may be generated. Therefore, an output voltage detection error amplifier 54 that regulates the maximum value of the output voltage is incorporated in the DC power supply unit 12, and the control unit 42 is controlled by its output. In other words, the output voltage is detected by the output voltage detection error amplifier 54, and a control function is provided so that the output voltage does not become higher than a specified value. The overvoltage protection detection circuit 56 stops the switching operation by the control unit 42 when the output voltage becomes excessive due to some trouble.

  On the other hand, it is expected that the voltage necessary for lighting is not applied to the cold cathode tube when the power is turned on by suppressing the output of the DC power supply unit 12 to the maximum specified value or less. Therefore, only in the transition period immediately after the power is turned on (a period until all the cold-cathode tubes are lit), the switching frequency is increased in the separately excited inverter unit 10 so that all the cold-cathode tubes are lit. Is controlled to return to a low normal operating frequency. Increasing the frequency produces a high voltage, and returning to the normal low frequency settles to a normal low voltage.

  Further, in this embodiment, the separately-excited inverter unit 10 is provided with functions of open voltage detection and tube open detection to control the control unit 52 on the inverter side.

  Next, an output voltage detection control function and a tube current feedback control function in the DC power supply unit 12 will be described. Since the output voltage detection control function and the tube current feedback control function are related, the tube current feedback error amplifier 16 and the output voltage detection error amplifier 54 are connected in series, and the control unit 42 of the switch circuit 34 is controlled by the output. To do. A specific circuit example is shown in FIG.

  Since the tube current is small when the power is turned on, the output voltage of the DC power supply unit becomes the value set in the output voltage detection error amplifier 54. Next, when the cold cathode tube is turned on and a tube current flows and a tube current detection signal exceeding the reference voltage V2 of the tube current feedback error amplifier 16 arrives, the output of the tube current feedback error amplifier 16 increases. When this output rises above the output voltage detection terminal voltage V1 of the output voltage detection error amplifier 54, as a result, the power supply output voltage falls and stabilizes until the tube current reaches a specified value.

  FIG. 5 shows the operation of the separately excited inverter immediately after the power is turned on. Immediately after the power is turned on, the tube current hardly flows, so the output voltage of the DC power supply section tends to increase, but it has a control function that does not exceed the specified value as described above. Cannot light or difficult to light. Therefore, in the separately excited inverter unit 10, immediately after the power is turned on, the switching frequency is increased to, for example, about 80 kHz. As a result, a high voltage (1.5 to 1.7 times the normal time) is generated, and all the cold-cathode tubes are reliably turned on. Thereafter, the switching frequency is returned to a normal frequency (for example, about 50 kHz) so that a normal voltage is applied to the cold cathode tube.

  In this embodiment, as shown in FIG. 6, a plurality of primary windings of the inverter transformer are connected in series. Here, an example of a half-bridge type switch circuit 46 that alternately turns on and off two switch elements 60 is shown. The configuration in which a plurality of inverter transformers 48 are combined with one switch circuit 46 can reduce the number of necessary switch circuits, and can realize simplification of the circuit and cost reduction. Thus, when the primary side windings of the inverter transformer 48 are connected in series, the influence of the individual inductances on the tube current can be offset, and the energy stored on the primary side when the switch circuit is turned on is the same. Therefore, the variation in tube current can inevitably be reduced.

  When a plurality of primary windings of the inverter transformer are connected in series, the current that flows can be the current of one transformer, but the number of voltages required. For example, when five transformers designed at 10V are connected in series, the output of the DC power supply unit needs 50V. However, even if a large number of transformers are driven by a single switch circuit, only one switching current is required, so that there is an advantage that the power loss of the switch elements is much smaller than that in parallel connection. In addition, since the output voltage of the DC power supply unit is not set to a low voltage (12 to 24 V) of normal inverter specifications but is set to be high from several tens of volts to several hundreds V, the efficiency of the DC power supply unit is also improved.

  In the above example, a plurality of primary windings are connected in series. However, a plurality of blocks connected in series can be connected in parallel (that is, in series-parallel connection). However, in this case, since the switching current increases, the loss increases.

The schematic block diagram of the power supply device for backlights which concerns on this invention. The schematic block diagram which shows a prior art for a comparison. The block diagram which shows one Example of the power supply device for backlights which concerns on this invention. The circuit diagram which shows an example of the error amplification system in the direct-current power supply part. Operation | movement explanatory drawing immediately after power activation in a separately excited inverter part. Explanatory drawing of the connection state of the primary side coil | winding of an inverter transformer.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Separately excited inverter part 12 DC power supply part 14 Tube current detection circuit 16 Tube current feedback error amplifier

Claims (3)

  It has a separately excited inverter unit that turns on a plurality of cold cathode tubes used in the backlight unit, and a DC power source unit that drives the separately excited inverter unit. In a normal operation state, the separately excited inverter unit is 45% or more. The backlight power supply apparatus is characterized in that it operates with a constant duty of less than 50% and the stabilization of the tube current flowing in the cold cathode tube is performed by controlling the output voltage of the DC power supply section.   The DC power supply unit incorporates an output voltage detection error amplifier that regulates the maximum value of the output voltage, and during the transition period immediately after the power is turned on, the switching frequency is increased in the separately excited inverter unit to light all the cold cathode tubes, 2. The backlight power supply device according to claim 1, wherein the power supply is controlled to a low normal constant frequency. The switching circuit of the separately excited inverter unit is any one of a half bridge circuit, a full bridge circuit, and an active clamp circuit, and a plurality of primary windings of the inverter transformer are connected in series or connected in series and parallel. Power supply for backlight.

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