JP2007265897A - Inverter circuit for discharge tube - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inverter circuit for a discharge tube capable of lighting different kinds of discharge tubes even if the same booster transformer is used, and preventing a lifetime of the discharge tube from being shortened. <P>SOLUTION: In the inverter circuit for the discharge tube to carry out lighting by applying an alternate current of a high voltage to the discharge tube which is the lighting target, the circuit is provided with a booster transformer in which the discharge tube is connected to a secondary side in series, a capacitor having a prescribed electrostatic capacity is connected to the discharge tube in parallel, and a resonance circuit of a resonance frequency determined by the electrostatic capacity including leakage inductance, the capacitor, and the discharge tube is formed on the secondary side, a transformer driving circuit to convert a direct current into an alternate current and to bi-directionally excite the primary side of the booster transformer, and a driving frequency forming circuit to control a frequency to drive the transformer driving circuit so that a phase of a tubular electric current flowing in the discharge tube and a tubular voltage applied to the discharge tube is adjusted. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a low-voltage drive inverter circuit having versatility with respect to a discharge tube for lighting a discharge tube using a power-coupled transformer.

In recent years, LCD (liquid crystal) displays have been used as represented by notebook computers. A backlight is used for the LCD display, and a cold cathode tube, which is a kind of discharge tube, is used for the light source of the backlight. It is necessary to apply a high-voltage AC voltage to light the cold cathode tube, and it is necessary to convert a low-voltage DC voltage to a high-voltage AC voltage. The demand for inverter circuits that obtain AC power has increased.
In addition, the brightness adjustment method of the cold cathode tube includes a current dimming method that adjusts the current flowing through the cold cathode tube, that is, the tube current value, and a burst control that intermittently lights the cold cathode tube and adjusts its lighting rate. An optical system or the like is used. The burst dimming method is generally used because it has a wider dimming range than the current dimming method, although there is a concern that the lifetime of the cold cathode tube may be reduced. (For example, see Patent Document 1)

In addition, when a high voltage AC voltage is applied to a cold cathode tube by the burst dimming method, in order to light a plurality of cold cathode tubes with a single step-up transformer, each cold cathode tube is connected to the output of the step-up transformer. In this case, the amount of current flowing through all the CCFLs should be adjusted due to the negative impedance characteristic of the CCFLs that the impedance decreases as the amount of current increases. However, adjustment was performed by connecting a current suppressing element in series to each cold cathode tube.
JP 2003-168585 A

  However, the output voltage of the step-up transformer is divided by the stray capacitance of the current suppressing element, the winding of the step-up transformer, the board pattern, the connector, and the cable, and the stray capacitance of the cold cathode tube. The type, size, manufacturer, and peripheral equipment such as the LCD housing vary greatly. As a result, the voltage applied to the cold cathode tube differs, so different types of cold cathode tubes can be driven with the same step-up transformer. Is difficult. For this reason, the step-up transformer used in the inverter circuit for the discharge tube cannot be properly lit unless it is custom designed for each cold-cathode tube to be lit. Therefore, the step-up transformer must be custom designed according to the type of cold-cathode tube. There is a problem that the cost increases.

  In addition, the cold cathode tube has a negative impedance characteristic in which the resistance value decreases as the tube current value increases, and when a plurality of cold cathode tubes are connected in parallel and lit, only one of the cold cathode tubes is provided. There is a problem that single lighting in which current flows occurs. In addition, the cold cathode tube has a characteristic that the luminance does not change so much even if a certain current value or more is passed, and even when one-side lighting does not occur, there is a difference in the amount of current flowing through each cold cathode tube. In order to make the cold cathode tube have a desired luminance, an extra tube current is passed, and there is a problem that the efficiency is lowered and the life of the cold cathode tube is shortened.

  An object of the present invention is to provide an inverter circuit for a discharge tube that can light up different types of discharge tubes even when the same step-up transformer is used, and can prevent a decrease in the life of the discharge tube.

  A discharge tube inverter circuit according to the present invention is a discharge tube inverter circuit that is lit by applying a high-voltage alternating current to one discharge tube to be lit or a plurality of discharge tubes connected in series. A plurality of discharge tubes are connected in series on the secondary side, a capacitor having a predetermined capacitance is connected in parallel with the discharge tube, and a leakage inductance on the secondary side, including the capacitor and the discharge tube A step-up transformer in which a resonance circuit having a resonance frequency determined by a capacity is formed; a transformer driving circuit that converts direct current into alternating current and separately excites the primary side of the step-up transformer; a tube current that flows through the discharge tube; and the discharge tube And a drive frequency generation circuit for controlling the frequency for driving the transformer drive circuit so that the phase with the tube voltage applied to is matched.

  The effect of the discharge tube inverter circuit according to the present invention is that the secondary side leakage inductance and the capacitor constituting the resonance circuit are connected to the secondary side of the step-up transformer to which the discharge tube is connected, and the phase between the tube current and the tube voltage is The primary side of the step-up transformer is separately excited and the step-up transformer operates as a power-coupled type so that different types of discharge tubes can be lit using a general-purpose step-up transformer operating in the power-coupled type. Therefore, it is not necessary to design a step-up transformer for each type of discharge tube.

Embodiment 1 FIG.
1 is a circuit diagram of an inverter circuit for a cold cathode tube according to Embodiment 1 of the present invention.
A cold cathode tube 8A, 8B will be described below as an example of the discharge tube according to the first embodiment of the present invention. As shown in FIG. 1, an inverter circuit for a cold cathode tube according to Embodiment 1 of the present invention includes a step-up transformer 6 in which two cold cathode tubes 8A and 8B to be lit are connected to a secondary coil 6b, Connected in series with the cathode tubes 8A and 8B, connected in parallel with the tube current detection resistor 9 and the cold cathode tubes 8A and 8B for detecting the tube current of the cold cathode tubes 8A and 8B, the voltage of the cold cathode tubes 8A and 8B is A high voltage resistance resistor 10A and a tube voltage detection resistor 10B for detecting voltage, a DC voltage variable circuit 13 that intermittently cuts off and smoothes the DC power supplied from the outside based on a burst signal, and changes the voltage to a desired DC voltage, DC A transformer drive circuit 5 that generates alternating current by intermittently exciting the step-up transformer 6 and a burst signal that is transmitted for a predetermined time so that the tube current and the luminance signal are input from outside and the tube current matches the luminance signal. output That the dimming circuit 11 is inputted and the tube current and the tube voltage, a driving frequency generating circuit 12 for outputting a timing signal phase between the tube current and tube voltage to control the transformer drive circuit 5 so as to approach zero.

The tube current detection resistor 9 is connected to the secondary coil 6b of the step-up transformer 6 in series with the cold cathode tubes 8A and 8B, and converts the tube current into a voltage corresponding to the magnitude thereof.
The dimming circuit 11 compares the luminance signal input from the outside with the tube current, and adjusts the time of the burst signal so that the tube current matches the luminance signal.

The DC voltage variable circuit 13 includes a switching element 1 that intermittently cuts off current from a DC power supply, a choke coil 3 that smoothes intermittently flowing current, a capacitor 4, and a diode 2.
The switching element 1 intermittently cuts off the current from the DC power supply by the burst signal output from the dimmer circuit 11 and supplies it to the choke coil 3 and the capacitor 4.
The choke coil 3 smoothes the current that is intermittently interrupted by the switching element 1 and prevents the return voltage generated from the step-up transformer 6 from flowing into the external power source.
The diode 2 is a flywheel diode.
The capacitor 4 smoothes the DC that is intermittently supplied by the switching element 1 together with the choke coil 3.

The transformer drive circuit 5 is composed of four switching elements 14 connected in a full bridge type by converting DC supplied from the DC voltage variable circuit 13 to AC to excite the step-up transformer 6. The transformer drive circuit 5 is a separately excited drive circuit that can drive the step-up transformer 6 even at a low voltage. The direct current smoothed by the choke coil 3 and the capacitor 4 is input to the transformer drive circuit 5, converted into alternating current, and then input to the step-up transformer 6.
In the step-up transformer 6, when the primary coil 6a is excited in a bipolar manner by the transformer drive circuit 5, a high-voltage alternating current is output to the secondary coil 6b, and the cold cathode tubes 8A and 8B connected to the secondary coil 6b are turned on. To do.
The voltage dividing high withstand voltage resistor 10A and the tube voltage detection resistor 10B divide the tube voltages of the cold cathode tubes 8A and 8B, and input the voltages at both ends of the tube voltage detection resistor 10B to the drive frequency generation circuit 12 as tube voltages.

The drive frequency generation circuit 12 receives the tube current detected by the tube current detection resistor 9 and the tube voltage detected by the tube voltage detection resistor 10B, compares the phase difference between the tube voltage and the tube current, and the phase difference is zero. Then, a transformer driving frequency signal is output to the transformer driving circuit 5.
The transformer drive circuit 5 forms a resonance circuit on the secondary side of the step-up transformer 6 by exciting the step-up transformer 6 in accordance with the frequency signal for driving the transformer from the drive frequency generation circuit 12, and power-couples the step-up transformer 6 Act as a type.

  On the secondary side of the step-up transformer 6, a capacitor 7 having a capacitance of about 50 pF to 100 pF is connected in parallel with the cold cathode tubes 8A and 8B. Since the stray capacitance difference between the cold cathode tubes 8A and 8B is usually about 5 pF to 10 pF, the capacitance of the capacitor 7 is an order of magnitude larger.

Further, the step-up transformer 6 is of a winding type and has a very large number of secondary turns compared to a general power supply transformer, and its structure has a lower coupling rate than a general transformer. Impedance is high. However, the step-up transformer 6 is driven by driving the step-up transformer 6 at the resonance frequency of the resonance circuit determined by the capacitance of the capacitor 7 connected in parallel to the cold cathode tubes 8A and 8B and the leakage inductance on the secondary side of the step-up transformer 6. 6, the output impedance of the step-up transformer 6 is efficiently transmitted without being reflected to the secondary side circuit. On the other hand, since the primary side impedance seen from the secondary side is high, regeneration to the primary side of the secondary side power is suppressed.
Such a step-up transformer 6 has characteristics different from those of a general transformer in which the primary side is bi-directionally excited, and is in a state where the primary side and the secondary side are power-coupled like a transformer used in a flyback converter or the like. It becomes.

  In order to resonate the secondary side of the step-up transformer 6, the phase difference between the tube voltage and the tube current may be made to coincide with each other. If the primary side of the step-up transformer 6 is excited at the output frequency of the PLL, resonance can be easily generated on the secondary side, and the cold cathode tubes 8A and 8B to be connected can be made flexibly even when they are different types. Can respond.

  By using the power-coupled step-up transformer 6 in this way, the difference in stray capacitance that varies depending on the types, sizes, manufacturers, and peripheral devices of the cold cathode tubes 8A and 8B is alleviated, and various types of cold cathode tubes 8A and 8B. It is possible to provide an inverter circuit for a discharge tube corresponding to the above.

  When the secondary side is made to resonate with a power-coupled transformer, a filter is composed of the leakage inductance of the transformer, the capacitor, the stray capacitance generated in the discharge tube, the secondary winding of the transformer, the connector, the wiring, etc. Are cut off. Even if the primary side of the transformer is driven by a rectangular wave, the filter blocks high-frequency components, the output of the step-up transformer becomes a sine wave, and the transformer drive circuit is not limited to a conventional sine wave output, but a rectangular wave output. Can be used without problems. In this case, the cut off high frequency component does not cause power loss, is effectively used by the power coupling type transformer, and does not reduce efficiency. Even when the step-up transformer is driven separately, a sinusoidal output waveform suitable for discharge tube driving can be obtained.

In addition, electronic devices have been driven at a low voltage, and most of the supplied power is 5 V or less. When a step-up transformer is driven by a self-excited type typified by a Royer type, the self-excitation is stabilized. Requires a DC power supply of about 12V, and it is necessary to provide a dedicated booster circuit. However, the transformer drive circuit 5 excites the booster transformer 6 in a bipolar manner so that a cold cathode can be used even when a DC voltage of 5V is used. The tubes 8A and 8B can be turned on, and a dedicated booster circuit is not required.
In this way, the primary side of the step-up transformer 6 is driven by a separate excitation method, and the power-coupled step-up transformer is used, so that the discharge tube can be stably lit even at a low voltage comparable to semiconductor logic drive. It becomes possible.

Here, the tube current flowing through the cold cathode tubes 8A and 8B is observed as shown in FIG. 2A shows a tube current observed when the cold cathode tube inverter circuit according to Embodiment 1 is lit, and FIG. 2B shows a cold current as shown in Patent Document 1 for reference. This is the tube current observed when the cathode tube inverter circuit is lit.
As shown in FIG. 2B, when the output of the step-up transformer controlled by the burst signal is input to the cold cathode tubes 8A and 8B, a steep tube current change occurs when power is re-supplied by the burst signal, This is a factor that reduces the lifetime of the cold cathode fluorescent lamps 8A and 8B.
On the other hand, in the cold-cathode tube inverter circuit according to the first embodiment, by performing smoothing using the capacitor 4 in addition to the diode 2 and the choke coil 3, a steep tube current change is suppressed, and the cold-cathode tubes 8A and 8B are suppressed. It is possible to remove the cause of the life reduction.

When the inverter circuit for a cold cathode tube is driven by a DC power source having a DC voltage of 5V, in order to obtain the same brightness of the cold cathode tubes 8A and 8B as when driven by a DC power source having a DC voltage of 12V, a DC voltage of 12V is used. The input current becomes larger than that when driving the conventional inverter circuit for a cold cathode tube using a bipolar transistor as a switching element, and the loss due to the switching element is large and not efficient.
On the other hand, in the cold cathode tube inverter circuit according to the first embodiment of the present invention, the primary side and the secondary side of the step-up transformer 6 are power-coupled and driven by the separate excitation type. Since the switching element 14 constituting the drive circuit 5 is a unipolar transistor, the voltage drop due to the switching elements 1 and 14 can be minimized even when a low voltage is input, and the cold cathode tubes 8A and 8B are stably stabilized. Can be turned on.

Further, since the step-up transformer 6 operates in a power-coupled manner, that is, an operation in which the input power and the output power are substantially equal, the output voltage of the step-up transformer 6 is not determined by the input voltage and the primary / secondary turns ratio. The tube current is small and a voltage larger than the value determined by the turn ratio can be taken out.
Further, by connecting a plurality of cold cathode tubes 8A and 8B in series on the secondary side of the step-up transformer 6, the tube currents for all the cold cathode tubes 8A and 8B can be made equal, and light having an equivalent luminance can be obtained. Can be issued.

Further, the capacitor 4 stabilizes the primary voltage of the step-up transformer 6, suppresses the spike voltage applied to the switching element 14 constituting the transformer driving circuit 5, and protects the switching element 14 constituting the transformer driving circuit 5. There is also work.
If the capacity of the capacitor 4 is too large, the input voltage value to the transformer drive circuit 5 decreases at the start and at low luminance, which causes a problem in stability.

On the contrary, when the capacitor 4 has a large capacity, a chopper circuit is formed together with the switching element 1, the diode 2, and the choke coil 3, and a voltage higher than the power supply voltage at which the step-up transformer 6 operates stably is supplied. Since the voltage is stepped down by the chopper circuit so that the tube current is optimized, the operating power supply voltage range can be widened.
However, in this case, since the low luminance is not stable when a low voltage is input, the dimming range is narrower than that in the first embodiment.

Although the full-bridge type has been described as an example of the transformer drive circuit 5, a half-bridge type or a push-pull type can be similarly applied to the present invention.
In the first embodiment, a cold cathode tube has been described as an example of a discharge tube. However, the discharge tube inverter circuit of the present invention can be applied to a discharge tube, such as a fluorescent lamp, a mercury lamp, or a sodium lamp. it can.
Further, instead of the discharge tube, an electroluminescence light (EL) can be driven by the discharge tube inverter circuit of the present invention.

Embodiment 2. FIG.
FIG. 3 is a circuit diagram of an inverter circuit for a cold cathode tube according to Embodiment 2 of the present invention.
The cold-cathode tube inverter circuit according to the second embodiment of the present invention has two cold-cathode tube tubes connected in series to the cold-cathode tube inverter circuit according to the first embodiment and the step-up transformer 6 as shown in FIG. Since 8C and 8D are different in that they are connected in series, and the others are the same, the same parts are denoted by the same reference numerals, and description thereof is omitted.

In this way, by connecting the two cold cathode tubes 8C and 8D connected in series to the secondary side to the step-up transformer 6, the tube currents flowing through the cold cathode tubes 8C and 8D become equal, Two cold-cathode tubes 8C and 8D can be lit without a suppressing element.
In the second embodiment, the example in which two cold cathode tubes 8C and 8D are connected in series has been described. However, three or more cold cathode tubes connected in series are connected to the secondary side of the step-up transformer 6. The same effect can be obtained even if connected in series.

In the discharge tube inverter circuit according to the present invention, it is possible to reduce the discharge tube life reduction factor as compared to the conventional power supply voltage range, so that it can be applied to the discharge tube drive used in the FA industry and the like. it can.
In addition, since driving at a low voltage is possible, it is not necessary to provide a dedicated power source, which contributes to downsizing of the entire device.

It is a circuit diagram of the inverter circuit for cold cathode tubes concerning Embodiment 1 of this invention. This is a tube current observed when the cold cathode tube inverter circuit according to the first embodiment and Patent Document 1 is lit. It is a circuit diagram of the inverter circuit for cold cathode tubes concerning Embodiment 2 of this invention.

Explanation of symbols

  1 switching element, 2 diode, 3 choke coil, 4 capacitor, 5 transformer drive circuit, 6 step-up transformer, 6a primary coil, 6b secondary coil, 7 capacitor, 8A, 8B, 8C, 8D cold cathode tube, 9 tube current detection Resistor, 10A high voltage resistance for voltage division, 10B tube voltage detection resistor, 11 dimmer circuit, 12 drive frequency generation circuit, 13 DC voltage variable circuit, 14 switching element.

Claims (3)

In an inverter circuit for a discharge tube that is lit by applying a high-voltage alternating current to one discharge tube to be lit or a plurality of discharge tubes connected in series,
The discharge tube or the plurality of series-connected discharge tubes are connected in series on the secondary side, a capacitor having a predetermined capacitance is connected in parallel with the discharge tube, a leakage inductance on the secondary side, and the capacitor And a step-up transformer in which a resonance circuit having a resonance frequency determined by the capacitance including the discharge tube is formed,
A transformer driving circuit for converting direct current to alternating current and separately exciting the primary side of the step-up transformer;
A drive frequency generation circuit for controlling the frequency for driving the transformer drive circuit so that the phase of the tube current flowing through the discharge tube and the tube voltage applied to the discharge tube are aligned;
An inverter circuit for a discharge tube, comprising:   2. The inverter circuit for a discharge tube according to claim 1, wherein the transformer driving circuit excites the primary side of the step-up transformer in a bipolar manner. A dimming circuit that emits a burst signal for controlling a DC voltage input to the transformer driving circuit so that a tube current flowing in the discharge tube matches an externally input luminance signal;
A DC voltage variable circuit that varies a DC voltage input from the outside in accordance with the burst signal;
With
3. The discharge tube inverter circuit according to claim 1, wherein the DC voltage variable circuit includes a choke coil and a capacitor for smoothing a DC interrupted according to the burst signal.

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