JPH0311595A - Cold cathode tube lighting circuit - Google Patents

Abstract

PURPOSE:To obtain a stable constant cold cathode tube brightness even through the input voltage is changed, by providing a pulse width modulating circuit to modulate the pulse width of the output of a switch element at a duty ratio depending on a DC voltage, and to maintain the mean voltage output to a self-exciting type resonant oscillation circuit constant through the switch element. CONSTITUTION:The pulse output E of the collector of a transistor 25 is divided by resisters 39 and 40, smoothed by a capacitor 38, and made into a positive input F to a comparator 37. This positive input F is made equal in the mean voltage to the negative input G to a capacitor 37, which is the voltage limited by a Zener diode 32 and divided by resisters 33 and 34. In such a way, the voltage VO made by averaging the pulse presented in the collector of the transistor 25 during a cold cathode tube 28 is lighted is dependent only on the resistance values of the resisters 33, 34, 39, and 40, and the property of the Zener diode 32, and receives no influence from input voltage. As a result, an average pulse voltage is applied to a self-exciting type resonant oscillation circuit 50 as the input. Consequently, the brightness of the cold cathode tube 28 can be obtained in a stable condition to the variation of the input voltage.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a dimmable cold-cathode tube lighting circuit that can stably maintain lighting of a cold-cathode tube over a wide range of input voltage changes. .

[Conventional technology]

Figure 2 shows, for example, the self-excited resonance described in the document "Transistor Technology" (CQ Publishing, March 1987, pages 406 and 407, "9.3 Self-excited resonant DC-DC converter"). 1 is a circuit diagram of a DC-DC converter using a type oscillation circuit.

In the figure, 1 is an input terminal, 2 and 3 are bias resistors, 4 is a coil for current averaging, 5 and 6 are NPN transistors, 7 is a capacitor, 8 is a transformer, 9 and 10 are rectifier diodes, 11 is a smooth choke coil,
12 is a smoothing capacitor, and 13 is an output terminal.

The converter described above causes a resonant current to flow between the primary winding of the capacitor 7 and the transformer 8 by alternately conducting the transistors 5 and 6, and gives a sine wave output to the secondary side of the lance 8, which is smoothed. A choke coil 11 and a smoothing capacitor 12 perform rectification and smoothing to obtain a predetermined DC voltage at an output terminal 13.

Further, FIG. 3 is a circuit diagram of a conventional cold cathode tube lighting circuit to which the converter of FIG. 2 is applied, and FIG. 4 is an operating waveform diagram thereof.

In the figure, 50 is a self-excited resonant oscillation circuit, and 51 is a dimming circuit. The dimming circuit 51 includes an input terminal 21, a square wave generator 22, resistors 23 and 24, a PNP type transistor 25, and a diode 26. On the other hand, 27
2 is a high-voltage, small-capacity capacitor, and 28 is a cold cathode tube. still,
Regarding the self-excited resonant oscillation circuit 50, the same parts as in the configuration of FIG. 2 are given the same reference numerals. However, in this example, the number of turns of the secondary winding of the transformer 8 is increased so that a voltage of about 500 Vrms can be obtained on the secondary side with no load.

In the above configuration, when a DC voltage is applied to the input terminal 21, the square wave generator 22 outputs a square wave (A in FIG. 4) having a constant pulse width and a constant period to the base of the transistor 25. Therefore, from the collector of the transistor 25, a square wave with a voltage substantially equal to the DC voltage is output at a constant cycle (B in FIG. 4), and the self-excited resonant oscillator circuit 50 receives this output signal and performs self-excited oscillation. Then, turn on the cold cathode tube 28 (C in Fig. 4). Incidentally, the on/off frequency of the output pulse of the dimming circuit 51 is selected to be about 200 Hz or higher, at which it is difficult to visually recognize flickering.

[Invention or problem to be solved]

However, the cold cathode tube lighting circuit shown in FIG. 3 has the following problems.

First, since the collector output voltage of the transistor 25 input to the self-excited resonant oscillation circuit 50 changes due to fluctuations in the input voltage of the input terminal 21, there is a problem that the brightness of the cold cathode tube 28 is not stable.

Second, the cold cathode tube 28 is often used in the backlight 1 for liquid crystal display of word processors and the like, which have different battery voltages depending on the model (for example, +5V to +24V), but according to the above conventional configuration, The specifications of the dimming circuit 51 have to be changed for each model, which is problematic in that it lacks versatility and is inconvenient in terms of design, manufacturing, and manufacturing management.

Thirdly, in the self-excited resonant oscillation circuit 50, the input frequency is kept as low as possible within the range of audible frequencies or higher (usually 2
5kHz to 100kHz) Is it desirable to reduce the high frequency loss of the capacitor 7?If the frequency is kept constant,
As the input voltage becomes higher, the number of windings of the transformer 8 must be increased, which poses a problem of hindering the miniaturization of the circuit.

In addition, the number of windings of the transformer N, (turns) and the input voltage
The relationship between (■) and V is the saturation voltage of the switching transistor. , (ON) (v), the maximum magnetic flux density of the core material is ΔB (crow), and the effective cross-sectional area of the core is A. (Cm2)
, when the self-excited oscillation frequency is f (Hz), it is expressed by the following formula.

Therefore, the present invention has been made to solve the above-mentioned problems of the prior art, and its purpose is to stably light a cold cathode tube at a constant brightness even if the input voltage changes over a wide range. The object of the present invention is to provide a cold cathode tube lighting circuit that can be used.

[Means to solve the problem]

The cold cathode tube lighting circuit of the present invention includes a square wave generator that outputs a square wave at a predetermined period by applying a DC voltage, and a square wave generator that turns on and off based on the output of this square wave generator, and A resonator having a switch element that outputs a voltage based on the voltage, and a self-excited resonant oscillator circuit that receives a voltage based on the DC voltage during the ON period of the switch element and causes a resonant current of a predetermined frequency to flow through the cold cathode tube. In the cathode tube lighting circuit, the output of the switching element is pulse width modulated with a duty ratio based on the DC voltage, and the average voltage output to the self-excited resonant oscillation circuit through the switching element is maintained constant. It is characterized by being equipped with a circuit.

[For production]

In the present invention, the output of a switching element is pulse width modulated using a duty ratio based on a DC voltage, and the average voltage outputted to a self-excited resonant oscillation circuit through this switching element is kept constant. Therefore, even if the value of the DC voltage changes, the average voltage output to the self-excited resonant oscillator circuit will not change.
Cold cathode tubes light up at a constant brightness.

〔Example〕

The present invention will be explained below based on illustrated embodiments.

FIG. 1 is a circuit diagram showing an embodiment of the cold cathode tube lighting circuit according to the present invention, FIG. 5 is an operational waveform diagram of this embodiment, and FIG. 6 shows the change in waveform E in FIG. 5 due to input voltage. FIG.

In the figure, 21 is an input terminal, 22 is a square wave generator, 2
3. and 24 are resistors, 25 are PNP type transistors, 26 are diodes, 31 are resistors, 32 are Zener diodes, 33 to 36 are resistors, 37 are comparators, 3
8 is a capacitor, 39 and 40 are resistors, and the above constitutes a dimming circuit 52. 50 is a self-excited resonant oscillation circuit having the same configuration as the conventional example shown in FIG. 3, and the same parts are given the same reference numerals. Also, 27 is a capacitor, 28
is a cold cathode tube.

In the dimming circuit 52, the input terminal 21 is connected to the square wave generator 22 and the emitter of the transistor 25, and is further connected to the positive input (
”-).

The output end of the square wave generator 22 is connected via a resistor 31 to a Zener diode 32 that limits the peak value to a certain value or less, and further connected to a resistor 33.3 that divides the peak value into a predetermined value.
4 and the negative input (-) of the comparator 37.

Further, the positive input (10) of the comparator 37 is connected to the collector of the transistor 25 via a resistor 40, and
It is connected to ground through a capacitor 38 and three resistors, respectively.

The output terminal of the comparator 37 is connected to the base of a transistor 25 through a resistor 24, and a resistor 23 is connected between the base and emitter of the transistor 25. Also,
The collector of the transistor 25 is connected to the cathode side of a diode 26 whose anode side is grounded, and further connected to the negative input (-) of a comparator 37 through a resistor 35. The collector of the transistor 25 is connected to the midpoint of the primary main winding of the transformer 8 via the coil 4 as an input terminal of the self-excited resonant oscillation circuit 50, and is further connected via the resistor 2 to the NPN type transistor. 6 and is connected to the base of an NPN transistor 5 via a resistor 3.

Further, the hot side (the side marked with ".") of the primary main winding of the transformer 8 is connected to the collector of the transistor 5, and the cold side is connected to the collector of the transistor 6. Further, resistors 3 and 2 for starting oscillation are connected to the bases of transistors 5 and 6, respectively. Further, the collectors of the transistors 5 and 6 are connected to each other via a capacitor 7, and the emitters of each are grounded.

On the other hand, the hot side of the primary auxiliary winding of the transformer 8 is connected to the base of the transistor 5, and the cold side is connected to the base of the transistor 6.
A capacitor 27 and a cold cathode tube 28 are connected in series to the next winding.

The operation of the cold cathode tube lighting circuit having the above configuration will be explained. First, when an input voltage is applied to the input terminal 21, the square wave generator 22 immediately operates and outputs a square wave (D in FIG. 5) having a maximum voltage equal to the input voltage. This square wave voltage is limited to below a certain amplitude by the Zener diode 32, and is divided by the resistors 33 and 34 and input to the negative input (=) of the comparator 37.

The direct current voltage of the input terminal 21 is applied to the positive input (+) of the comparator 37 through the resistor 36 .

Here, the resistor 36 is a resistor for improving the brightness change of the cold cathode tube 28 due to input fluctuation, and its resistance value is selected to be about 100 times larger than that of the resistor 39, so that when the input voltage is turned on, the The voltage applied to the input (10) is sufficiently smaller than the voltage applied to the negative input (-).

Therefore, when the input voltage is turned on, the output voltage of the comparator 37 becomes a low level, and the base current flows through the 1 transistor 25, making it active, and rapidly turning the 0 1 transistor 25 on. A voltage approximately equal to the DC voltage of the input terminal 21 appears at the collectors of the transistors 1 to 25, and this voltage is positively fed back to the negative input (-) of the comparator 37 through the resistor 35.

Also, the collector voltage of the transistor 25 is the resistor 39.4
0, smoothed by a capacitor 38, and input to the positive input (+) of a comparator 37. Therefore, the positive input (+)@ position gradually increases (F in FIG. 5).

- After a certain period of time has passed, the positive input (+) potential of the comparator 37 becomes greater than the negative input (-) potential, and the comparator 37
is inverted and its output becomes high level. As a result, transistor 25 is suddenly turned off. In this state, the collector voltage of the transistor 25 is almost zero, so the charge stored in the capacitor 38 starts discharging through the resistors 39 and 40, and the potential of the positive input (10) gradually decreases (see Fig. 5). F). Then, the positive input of the comparator 37 (
When the potential (+) falls below the potential of the negative input (-), the output of the comparator 37 becomes low level again, and transistors 1 to 25 are turned on.

In this way, a modulated pulse output (E in FIG. 5) appears at the collector of transistor 25 during the period when the output of dimming square wave generator 22 appears.

Here, the average voltage V 1 of the pulse output E during the lighting period in FIG. 5 is determined as follows.

First, the pulse output E of the collector of the transistor 25 is divided by the resistor 39.40, smoothed by the capacitor 38, and becomes the positive input F of the comparator 37, and this positive input F is limited by the Zener diode 32. It is noted that the average voltage is equal to the negative input G of the comparator 37, which is obtained by dividing the voltage by the resistors 33 and 34. However, resistance 3
The resistance value of 5.36 is chosen to be sufficiently large compared to other resistances, so it can be ignored in calculations. Resistance 33.3
4.The resistance values of 39 and 40 are R33°R34' respectively.
Assuming that R39 and R4o are R39 and the voltage of the Zener diode 32 is 7, the relationship between the positive input and the negative input of the above-mentioned inline comparator 37 is expressed by the following equation.

vox <R39/ (R39+R40))-V7×(
R34/(R33+R34)) Therefore, ■ - ((R39+R40)134/(R39+R34)R3
9) Becomes VZ.

The above equation is the voltage (■) that is the average of the pulses appearing at the collector of the transistor 25 during the lighting period of the cold cathode tube 28. indicates that it depends only on the resistance values of the resistors 33, 34, 39, and 40 and the characteristics of the Zener diode 32, and is not affected by the input voltage. For example, as shown in FIG. 6, when the voltage increases from IOV to 15V, the duty ratio of waveform E is decreased to control the averaged voltage to be constant.

In this case, the amplitude of the oscillation waveform of the self-excited resonant oscillation circuit 50 gradually increases because the current is limited by the coil 4 during the ON period of the transistor 25 (■, period a in FIG. 5). Next, when the transistor 25 is turned off, energy is no longer supplied from the input terminal, and either the current due to the energy stored in the coil 4 or the ground,
The current oscillation is continued in the path of the diode 26, coil 4, and transformer 8 (period b in FIG. 5). During this period b, the energy of the coil 4 decreases, the electromotive force also gradually decreases, and the amplitude of the oscillation waveform also gradually decreases. Next, when the transistor 25 is turned on again, energy is supplied to the coil 4, and the amplitude of the oscillation waveform increases (period C in FIG. 5). In this way, the oscillation output amplitude is amplitude modulated by the switching frequency of the I transistor 25, but since the switching frequency is as high as 20 kHz or more, flickering of the cold cathode tube 28 is not recognized.

As explained above, in this embodiment, since a pulse voltage that is constant on average regardless of the input voltage is applied as an input to the self-excited resonance type oscillation circuit 50, the brightness of the cold cathode tube 28 is controlled. It can be obtained stably against input voltage changes. Therefore, there is no need to change the specification of the cold cathode tube lighting circuit even if the device to be incorporated is different. Furthermore, even if the input voltage increases, it is possible to operate by lowering the average voltage to 5 to 6 V, which allows the oscillation transformer to be made smaller.
Pulse voltage can be used without converting it to direct current using a smoothing choke coil or an electrolytic capacitor, making it possible to miniaturize the circuit.

〔Effect of the invention〕

As explained above, according to the present invention, it is possible to input a pulse voltage that keeps the effective voltage constant during the lighting period to the self-excited resonant oscillator circuit, so that even if the input voltage changes, the voltage remains stable and constant. cold cathode tube brightness can be obtained.

Furthermore, even if the battery voltage differs depending on the model, such as in the case of the backlight 1 for liquid crystal display, there is no need to change the specifications, which is advantageous in terms of manufacturing control and the like.

Furthermore, even when the input voltage is high, it can be operated at a low average voltage, so the oscillation transformer can be made smaller, and the pulse voltage can be used without converting it to DC using a smoothing choke coil or an electrolytic capacitor. This has the effect that the installation space can be saved significantly and it is suitable for miniaturizing the circuit.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an embodiment of the cold cathode tube lighting circuit according to the present invention, Fig. 2 is a circuit diagram of a DC-DC converter using a self-excited resonant oscillator circuit, and Fig. 3 is the converter shown in Fig. 2. 4 is an operating waveform diagram of FIG. 3, FIG. 5 is an operating waveform diagram of this embodiment, and FIG. 6 is an explanation of waveform E in FIG. 5. It is a diagram. 22...Square wave generator, 25...Transistor, 3
7... Comparator, 28... Cold cathode tube, 50...
・Self-excited oscillation type resonant circuit, 52...Dimmer circuit.

Claims (1)

[Claims] A square wave generator that outputs a square wave at a predetermined period by applying a DC voltage, and a voltage that is turned on and off based on the output of the square wave generator and that is based on the DC voltage. and a self-excited resonant oscillator circuit that receives a voltage based on the DC voltage during the ON period of the switch element and causes a resonant current of a predetermined frequency to flow through the cold cathode tube. , comprising a pulse width modulation circuit that pulse width modulates the output of the switching element with a duty ratio based on the DC voltage and maintains constant the average voltage output to the self-excited resonant oscillation circuit through the switching element. A cold cathode tube lighting circuit characterized by: