JPH088150B2 - Discharge tube drive circuit - Google Patents

Description

The present invention relates to a discharge tube drive circuit for driving a fluorescent discharge tube or the like, and particularly to a PWM (Pulse Width Modulatio).
n) The present invention relates to a discharge tube drive circuit that employs control.

[Outline of Invention]

The present invention is a discharge tube drive circuit for driving a fluorescent discharge tube or the like, which is activated by applying a PWM-controlled drive pulse to the discharge tube, thereby stabilizing the drive during steady operation. Further, the brightness of the discharge tube can be adjusted by providing a circuit for detecting the current of the discharge tube.

[Conventional technology]

Conventionally, a liquid crystal display device in an image device has a light transmission type or a light reflection type display function, so that the entire screen becomes dark. Therefore, a small fluorescent lamp for high-frequency lighting was provided at the rear of the LCD to compensate for the darkness of the screen.

A discharge tube driving circuit used as a driving device for a high-frequency lighting fluorescent lamp or the like comprises a fluorescent discharge tube, a driving transformer for applying a high frequency high voltage to the fluorescent discharge tube, and a circuit for applying a high frequency to the driving transformer.

Generally, in a discharge tube such as a fluorescent tube, when the ambient temperature of the discharge tube is lowered, the cathode part of the discharge tube becomes inactive, and it becomes difficult for electrons to be emitted from the cathode part. Therefore, in order to make sure that the discharge tube drive circuit is started at the time of start-up, by increasing the primary and secondary turns ratio of the drive transformer and applying a high voltage between the cathode and the anode of the discharge tube. , The electron of the cathode part was emitted.

Further, when the discharge tube drive circuit of the conventional example is used as a drive device for a backlight of a liquid crystal display device, the brightness of the screen may change due to fluctuations in the power supply voltage.

[Problems to be solved by the invention]

As described above, in the discharge tube drive circuit of the conventional example, since the winding ratio of the primary coil and the secondary coil of the drive transformer is increased and the high voltage of the discharge tube is applied, the power efficiency is poor. There is a point.

Further, the discharge tube drive circuit has a problem that the brightness of the screen of the liquid crystal display device changes due to the fluctuation of the power supply voltage.

[Means for solving problems]

A discharge tube drive circuit according to the present invention corresponds to a pulse width modulation pulse generation circuit that supplies a drive pulse to a discharge tube, a current detection circuit that detects a tube current of the discharge tube, and a change in ambient temperature of the discharge tube. A pulse width adjusting circuit for outputting an electric potential that changes with the pulse width adjusting pulse generating circuit, and the output obtained by comparing the detection output of the current detecting circuit with the output of the pulse width adjusting circuit by the pulse width modulation pulse generating circuit. The pulse width of the discharge tube is controlled to drive the discharge tube by pulse width modulation.

[Work]

The discharge drive circuit according to the present invention is activated by controlling the pulse width of the drive pulse with an output obtained by comparing the detection output of the current detection circuit and the output of the pulse width adjustment circuit by the pulse width modulation pulse generation circuit. It facilitates and stabilizes driving during steady operation. Further, the brightness of the discharge tube can be adjusted by detecting the current of the discharge tube and returning it.

〔Example〕

An embodiment of the discharge tube drive circuit according to the present invention will be described in detail below with reference to the drawings.

The embodiment shown in FIG. 1 applies the present invention to a backlight lighting drive circuit of a liquid crystal display device.

In this embodiment, the fluorescent discharge tube for backlight 1 is used.
The anode 2 is connected to the current detection circuit 7 via the secondary coil 5 of the drive transformer 46. Cathode 3 of the discharge tube 1
Has one end grounded and the other end connected to the power supply terminal 20 through the heater circuit 13. The primary coil 6 of the drive transformer 4 has one end connected to the power supply terminal 20 and the other end grounded via a switching circuit 21. The switching circuit 21 is driven by a pulse width modulation pulse output from a pulse width modulation pulse generation circuit 27 whose pulse width is feedback-controlled by the detection force of the current detection circuit 7. The pulse width modulation pulse generation circuit 27 includes an input terminal 34 to which a signal having a frequency that is an integral multiple of a video frequency is input, the switching circuit 21 via a resistor 26, and the current detection circuit via a voltage dividing circuit 35. 7 and
The pulse width adjusting circuit 39 is connected.

The pulse width modulation pulse generation circuit 27 forms an externally synchronizable oscillation circuit 28 that outputs a triangular wave, and compares the triangular wave with a DC level to form a width modulation (PWM) having a pulse width according to the DC level. An output amplifier that supplies the comparator 29 and the PWM pulse to the switching circuit 21 in the subsequent stage.
30, a maximum value circuit 31 that forms a DC level signal to be given to the comparator 29, an operational amplifier 32 that feeds back and amplifies the output from the current detection circuit 7, and a reference voltage to a voltage dividing circuit 35, and the oscillation circuit It is composed of a reference voltage circuit 33 connected to 28.

In the pulse width adjusting circuit 39, the movable portion has the maximum value circuit.
Variable resistor connected to 31 with one end connected to power terminal 20
42 and a posistor 40 having one end connected to the variable resistor 42 in parallel with the resistor 41 and the other end grounded.

The current detection circuit 7 has a resistor 8 having one end grounded and the other end connected to the secondary coil 5 of the drive transformer 4.
Diode 9 whose cathode side is connected to this resistor 8
A capacitor 10 connected to the anode side of the diode 9 and one end of which is grounded; a variable resistor 11 connected to the anode side of the diode 9;
11 and a resistor 12 having one end connected and the other end connected to the pulse width modulation pulse generation circuit 27.

The switching circuit 21 has a transistor 22 whose base side is connected to the resistor 8 and whose emitter side is connected to the power supply terminal 20, and one end which is connected to the collector side of the transistor 22 and whose other end is connected to the base side of the transistor 22. Resistor 24 and one end is grounded and the other end is a transistor
It is composed of a resistor 25 connected to the base side of 23 and a transistor 23 whose collector side is connected to the primary coil 6 of the drive transformer 4 and whose emitter side is grounded.

The heater circuit 13 has a transistor 14 whose emitter side is connected to the power supply terminal 20 and whose collector side is connected to the cathode part of the discharge tube 1, and whose collector side is connected to the base side of the transistor 14 and whose one end is A resistor 18 connected to the power supply terminal 20 and the other end thereof connected to the base side of the transistor 14, and a capacitor 19 having one end connected to the power supply terminal 20 and the other end connected to the base side of the transistor 15, It is composed of a resistor 16 having one end connected to the emitter side of the transistor 15 and the other end grounded, and a resistor 17 having one end grounded to the base side of the transistor 15 and the other end grounded.

Next, the operation of the above embodiment will be described with reference to the waveform charts shown in FIG. 2 and FIG.

In the pulse width modulation pulse generating circuit 27, the oscillating circuit 28 synchronizes with the signal S _A having a frequency twice as high as the video frequency input from the input terminal 34, for example, the signal S _B having a triangular wave as shown in FIG. Is supplied to one end of the comparator 29. The comparator 29 compares the triangular wave signal S _B with the potential V _A supplied from the maximum value circuit 31, and outputs a signal S _C having a pulse width of a period T _A higher than the potential V _A level. This signal S _C is supplied to the output amplifier 30 and signal-amplified,
It is supplied to the switching circuit 21 via the resistor 26. The maximum value circuit 31 performs level comparison between the potential feedback-amplified by the operational amplifier 32 and the potential output from the pulse width adjusting circuit 39, and supplies the maximum value level to the comparator 29. The reference voltage circuit 33 supplies the potential at which the oscillation circuit 28 operates to the voltage dividing circuit 35 as a reference potential.

The switching circuit 21 operates the transistor 23 by the input signal S _C to cause a high-frequency primary current to flow in the primary coil 6 of the drive transformer 4. Above heater circuit 13
Supplies the heater current to one end of the cathode portion 3 of the discharge tube 1 by operating the transistor 14 for a predetermined period with the time constant of the capacitor 19 and the resistor 17.

The current detection circuit 7 detects the current of the discharge tube 1 with a resistor 8 and rectifies it with a diode 9 and a capacitor 10, and through a variable resistor 11 capable of adjusting the current, a pulse width modulation pulse via a voltage dividing circuit 35. It is supplied to the generation circuit 27.

In the pulse width adjusting circuit 39, the posistor 40 is
The fluorescent discharge tube 1 is arranged such that the resistance value decreases as the ambient temperature decreases, and the resistance value increases as the ambient temperature increases. Therefore, the pulse width adjusting circuit 39 pulses a low potential when the ambient temperature of the fluorescent discharge tube 1 is lowered and a high potential when the ambient temperature is increased via the variable resistor 42. It is supplied to the width modulation pulse generation circuit 27.

In the above embodiment, when the discharge tube drive circuit is started when the power is turned on, the heater current from the heater circuit 13 preheats the cathode portion 3 of the discharge tube 1, so that the cathode portion 3 emits electrons. Activated to facilitate. Further, since the temperature inside and outside the discharge tube 1 is low in the pulse width adjusting circuit 39, the ambient temperature of the posistor 40 is also low, and the low potential V _A due to the temperature characteristic deterioration is supplied to the maximum value circuit 31 of the pulse width modulation pulse generating circuit 27. Supply. Pulse width modulation pulse generation circuit
Reference numeral 27 denotes a triangular wave signal signal S _B synchronized with the frequency signal S _A that is twice the horizontal image frequency from the oscillation circuit 28 as shown in FIG.
Is supplied to one end of the comparator 29, and the potential V _A from the maximum value circuit 31 is supplied to the other end of the comparator 29.

The comparator 29 outputs a signal S _C having a wide pulse width in the period T _A higher than the potential V _A level and supplies it to the switching circuit 21. The switching circuit 21 voltage-amplifies the signal S _C to generate the primary coil 6 of the drive transformer 4.
Drive pulse S _C is supplied to. The drive transformer 4 is supplied with the drive pulse S _C having a wide pulse width to the primary coil 6, so that the secondary coil 5 is induced with a high voltage to apply the high voltage between the electrodes of the discharge tube 1. Since the cathode part 3 of the discharge tube 1 is preheated by the heater circuit 13 and is in an active state, it emits electrons to the anode part 2.

Therefore, in the above-described embodiment, a high voltage is induced in the secondary coil 5 of the drive transformer 4 without increasing the winding ratio of the primary coil and the secondary coil of the drive transformer as in the conventional example. The adjustment circuit 39 and the heater circuit 13 make it easier for the discharge tube drive circuit of the above-described embodiment to start when the power is turned on.

In the steady operation of the discharge tube drive circuit, a tube current flows when a high voltage is applied to the discharge tube 1, and this current causes a potential difference in the detection resistor 8 of the current detection circuit 7. The current due to the potential difference of the detection resistor 8 is rectified by the diode 9 and the capacitor 10 and is supplied to one end of the comparator 32 of the pulse width modulation pulse generation circuit 27 via the voltage dividing circuit 35 via the variable resistor 11. Further, the reference potential from the reference voltage circuit 33 of the pulse width modulation pulse generation circuit 27 is supplied to the comparator via the voltage dividing circuit 35.
It is supplied to the other end of 32. The comparator 32 compares the supplied potential from the current detection circuit 7 with the potential from the pulse width modulation pulse generation circuit 27, and negatively amplifies the difference to amplify the potential close to the reference potential to the maximum value. Supply to one end of the circuit 31. Further, from the pulse width adjusting circuit 39, the temperature inside the tube rises due to electron emission from the discharge tube 1 and the ambient temperature around the posistor 40 rises, so a potential higher than the potential at the time of starting the discharge tube drive circuit is a pulse width modulation pulse generating circuit. It is supplied to the other end of the maximum value circuit 31 of 27. The maximum value circuit 31 selects one of the supplied potentials which is higher and supplies the selected potential to the comparator 29. In the pulse width modulation pulse generating circuit 27, a comparator with the triangular wave in signal S _C to be output from the oscillation circuit 28 as shown in FIG. 3
At 29, the high potential V _B is applied, and a signal S _D having a narrow pulse width of the period T _B higher than the potential V _B level is output and supplied to the switching amplification circuit 21. The switching circuit 21 voltage-amplifies the signal S _D and outputs the signal to the drive transformer 4 1
A drive pulse S _D having a narrow pulse width is supplied to the next coil 6.
The drive transformer 4 induces a voltage having a shorter pulse width than the high voltage generated at the time of start-up in the secondary coil 5, and gives the discharge tube 1 a voltage having a shorter application period. The applied voltage reduces the tube current, and the detection resistor 8 of the current detection circuit 7
A potential difference lower than the potential difference at the time of startup occurs. Therefore, immediately after start-up, the current detection circuit 7 gives a high potential to the pulse width modulation pulse generation circuit 27, but with the passage of time, gives a lower potential to the pulse width modulated pulse generation circuit 27 than immediately after start-up. . Further, since the electric potential that changes in accordance with the temperature characteristic from the pulse width adjusting circuit 39 is also given to the pulse width modulating pulse generating circuit 27, the tube current of the discharge tube 1 approaches a constant current with the passage of time. It will be.

Therefore, the above-mentioned discharge tube driving circuit is provided with a heater circuit 13 for preheating the cathode portion 3 of the discharge tube 1 and a pulse width adjusting circuit.
Discharge tube 1 and the potential that changes according to the temperature characteristics change of 39
By feeding back the current of (1), electrons are emitted from the discharge tube 1, stable driving is obtained, and power efficiency is improved.

Further, the discharge tube current detection circuit 27 can detect the current of the discharge tube 1 and adjust the potential supplied to the comparator 32 by the variable resistor 24. Therefore, as shown in FIG. An arbitrary potential V _C is applied to the triangular wave S _B from 28. Therefore, the pulse width modulation pulse generating circuit 27 outputs the signal S _C having the pulse width T _C which can be arbitrarily adjusted from the comparator 29 by performing PWM control with the given potential V _C , and thus the variable The current of the discharge tube 1 can be changed by adjusting the resistance 11. Further, by changing the current of the discharge tube 1, the brightness of the discharge tube can be adjusted.

〔The invention's effect〕

When the discharge tube driving circuit according to the present invention is powered on, the cathode width of the discharge tube is preheated by the current from the heater circuit, and the pulse width is changed by the potential that changes in response to the change in the temperature characteristic of the pulse width adjusting circuit. The modulated pulse generating circuit applies a drive pulse having a pulse width according to the above potential to the discharge tube to cause electronic discharge of the discharge tube, thereby facilitating activation of the discharge drive circuit.

During steady operation of the discharge tube drive circuit, the voltage is fed back to the discharge tube detection circuit, and a potential that changes in accordance with the temperature characteristics of the pulse width adjustment circuit is applied to the pulse width modulation pulse generation circuit, thereby providing the discharge tube with a potential. , Electronic discharge is performed, and the discharge tube drive circuit is stably driven. In addition, the discharge tube current detection circuit detects the discharge tube current, and the pulse width of the drive pulse can be arbitrarily adjusted by the variable resistor, so that the discharge tube current can be adjusted and the brightness of the discharge tube can be adjusted. it can.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a discharge tube drive circuit according to the present invention. 2 and 3 are waveform diagrams for explaining the operation of the above embodiment. 7: Discharge tube current detection circuit 27: Pulse width modulation pulse generation circuit

Claims (1)

[Claims] 1. A pulse width modulation pulse generation circuit for supplying a drive pulse to a discharge tube, a current detection circuit for detecting a tube current of the discharge tube, and a potential which changes in response to a change in ambient temperature of the discharge tube. A pulse width adjusting circuit for outputting, and controlling the pulse width of the drive pulse with an output obtained by comparing the detection output of the current detecting circuit with the output of the pulse width adjusting circuit by the pulse width modulation pulse generating circuit. Then, the discharge tube drive circuit is characterized in that the discharge tube is driven by pulse width modulation.