JP2010200481A - Power supply circuit device and backlight device using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To generate a duty signal, not synchronized with a vertical synchronization frequency of a liquid-crystal display device, in order to prevent beat noise or flickering or the like occurring on a screen of the liquid-crystal display device. <P>SOLUTION: A vibration sensor 22 is installed in a power supply circuit device so as to detect a repetition frequency of a vibration state and a stop state of a piezoelectric transformer 12 due to time-division drive. A value of a resistor 20 for frequency adjustment connected to a triangle-wave forming circuit 18 is controlled on the basis of the frequency. By this, it is possible to exert control such that the repetition frequency of the piezoelectric transformer 12 does not become a value within a frequency range in which it is likely that the repetition frequency is synchronized with a vertical synchronization frequency of a liquid-crystal display device using a load 13. Accordingly, it is possible to prevent the occurrence of trouble even if a secular change, ambient temperature change or the like occurs. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a power supply circuit device used as a power source for driving a backlight device mounted on a liquid crystal display device, and a backlight device using the same.

  In recent years, demand for flat panel displays is rapidly expanding due to the advantages of thinness and light weight. Among them, a liquid crystal display device has features such as a small size, a thin shape, and low power consumption, and is applied to a display screen of a mobile phone or a display of a notebook personal computer. Recently, it has been actively applied to large screen televisions.

  In the liquid crystal display device, since the liquid crystal itself does not emit light, a backlight device is installed on the back surface of the liquid crystal panel, and image display is performed by controlling transmitted light by the light emission by the liquid crystal panel. In the backlight device, a discharge tube such as a cold cathode tube, a hot cathode tube, or an external electrode tube is used as a light source. Of these, the cold cathode tube can realize a discharge tube having a small diameter and an elongated shape, so that the backlight device is thin. Therefore, it is used as a light source in many backlight devices because of its long product life.

  In order to drive a discharge tube typified by a cold cathode tube, a high voltage alternating current is generally required. Therefore, it is necessary for the power supply circuit of the backlight device to convert a low-voltage direct current supplied from the outside into an alternating current for driving the discharge tube and a high-voltage current. For this purpose, a boost type inverter circuit having a step-up transformer is used. ing. In recent years, the use of piezoelectric transformers is increasing as the step-up transformer because of its advantages such as high safety, small size, thinness, and high conversion efficiency.

  Unlike a wound transformer, a piezoelectric transformer is an element that realizes high-efficiency voltage conversion by once converting electrical energy into mechanical energy and then converting it back into electrical energy. Piezoelectric transformers are generally formed of a piezoelectric ceramic, and a type of rectangular plate shape called a Rosen type is mainly used. A Rosen-type piezoelectric transformer is a piezoelectric ceramic rectangular plate polarized in the thickness direction as the primary side and in the length direction as the secondary side. A large voltage is obtained on the secondary side due to the vibration of the rectangular plate. Can do. Here, the step-up ratio of the piezoelectric transformer depends on its driving frequency. Therefore, if you want to increase the output voltage of the piezoelectric transformer, you can increase the boost ratio by bringing its drive frequency close to the resonance frequency of the piezoelectric transformer, and if you want to decrease it, decrease the boost ratio by moving away from the resonance frequency. Is possible. In this way, the output voltage of the piezoelectric transformer can be controlled to a desired value by changing the drive frequency. The boost type inverter circuit is used for controlling the driving frequency.

  Patent Document 1 describes a conventional example relating to a power supply circuit that lights a discharge tube using a piezoelectric transformer. The power supply circuit described in Patent Document 1 is intended for application to a drive power source of a backlight device of a liquid crystal display device using a cold cathode tube. Among these, the third embodiment of Patent Document 1 is used. Shows an example in which the average output power that the piezoelectric transformer inputs to the load is controlled by time-division driving. FIG. 5 shows a block diagram of a power supply circuit in the third embodiment of Patent Document 1. In FIG.

In FIG. 5, an alternating current for driving is input from the driving circuit 51 to the piezoelectric transformer 52 in order to light a load 53 which is a discharge tube such as a cold cathode tube. The input alternating current is boosted by the piezoelectric transformer 52, and then input to the load 53 to turn on the discharge tube and consumed. The load current that flows through the load 53 by the input of the alternating current is input to the load current comparison circuit 54. The load current comparison circuit 54 performs current-voltage conversion on the input load current, and the converted voltage is compared with a reference voltage V _ref corresponding to a predetermined load current value given in advance. The load current comparison circuit 54 is connected to a frequency sweep oscillator 55, and the frequency sweep oscillator 55 adjusts the frequency of the drive signal when the load 53 is lit according to the load current comparison result in the load current comparison circuit 54. .

The frequency sweep oscillator 55 is an element for determining the frequency of the driving AC voltage that is the output from the driving circuit 51, and in order to sweep the frequency of the AC current from the piezoelectric transformer 52 when the load 53 is lit. It has a role of changing the driving frequency. This driving frequency is generally a high frequency of several tens of kHz to several hundreds of kHz. As described above, the frequency of the alternating current after the lighting of the load 53 is determined by the comparison result with the reference voltage V _ref in the load current comparison circuit 54.

The time-division drive control circuit 56 is an element provided for adjusting the luminance of the load 53 that is a discharge tube. By using the piezoelectric transformer 52 in time-division drive, the lighting period when the load 53 is driven can be reduced. Set the time ratio of the light-off period. The luminance of the load 53 is adjusted by adjusting the time ratio of turning on and off the discharge tube. The time-division drive control circuit 56 controls the time ratio between the driving and non-driving periods of the driving circuit 51 based on the luminance adjustment signal input to the duty setting terminal V _duty . The signal for adjusting the luminance (hereinafter referred to as duty signal) is generally a rectangular wave of about several hundred Hz.

  The output from the time-division drive control circuit 56 is also input to the frequency sweep oscillator 55. This is the initial value of the frequency of the driving AC voltage input to the drive circuit 51 after the extinguishing period. It is for returning. The drive frequency output from the frequency sweep oscillator 55 to the drive circuit 51 is reset by this input, and at the start of the lighting period, the sweep is always started from the initial frequency set for lighting the discharge tube. .

  Note that the method of adjusting the brightness of the discharge tube by time-division driving is particularly low brightness compared to the method of adjusting the brightness by adjusting the amount of alternating current input while the discharge tube is lit. It has a feature that the range in which the luminance can be adjusted is wide in the region.

  By the way, as described above, the time division frequency set when the discharge tube, which is the light source of the backlight device, is time division driven is generally about several hundred Hz. However, when this time division frequency is a value in the vicinity of a frequency that is an integral multiple of the vertical synchronization frequency of the liquid crystal display device (hereinafter referred to as the case of synchronization), when the time division driving of the discharge tube is performed, the liquid crystal There has been a problem that defects such as beat noise (noise generated as a vertical stripe pattern on the screen) and flickering may occur on the screen of the display device. Here, when the time-sharing frequency of the discharge tube and the vertical synchronization frequency of the liquid crystal display device are synchronized, details of the cause of the above-described malfunction on the screen of the liquid crystal display device are not necessarily clear, but the characteristics of the human eye It is considered that both the visual reason involved and the electrical reason related to the interference of the driving signal of the liquid crystal display device are involved.

  First, the visual reason can be considered as follows. During the response time required for rewriting one liquid crystal screen, light from the discharge tube is blocked by the liquid crystal, resulting in a blackout state. Therefore, when the time-sharing frequency of the discharge tube and the vertical synchronization frequency of the liquid crystal display device are synchronized, the blackout period of the liquid crystal screen and the extinguishing period of the discharge tube are continuous or sandwiched with a very short light emission period. become. For this reason, when both frequencies are synchronized, the state in which the screen of the liquid crystal display device is effectively turned off for a long time is repeated at regular intervals, which is perceived as flickering of the screen by human eyes. Presumed.

  Further, beat noise is considered to be generated by superimposing a discharge tube driving signal in a backlight device as a noise on a signal for drawing on a liquid crystal screen. When the two frequencies are synchronized, the position of the light and shade stripe pattern due to the noise is fixed on the screen, so that it is observed as beat noise. Moreover, the superimposition of noise on the drawing signal of the liquid crystal screen may be related to the occurrence of screen flicker.

In order to solve the problem of occurrence of beat noise and flickering on the screen of the liquid crystal display device, in the example of Patent Document 1, the frequency for driving the discharge tube in a time-sharing manner is set to the vertical synchronization frequency of the liquid crystal display device. It is set so as to avoid a region of 10 Hz up and down from the frequency that is an integral multiple. That is, the luminance adjustment signal input to the duty setting terminal V _duty of the time division drive control circuit 56 in FIG. 5 is set in advance to a value that avoids the frequency region, and is generated on the screen of the liquid crystal display device. We try to suppress the occurrence of defects.

JP-A-8-107678

When the piezoelectric transformer for driving the discharge tube is time-division driven to adjust the luminance of the liquid crystal display device, the duty signal input to the duty setting terminal V _duty of the time-division drive control circuit constituting the piezoelectric transformer drive circuit Is a rectangular wave of about several hundred Hz as described above. For this duty signal, an external signal generated by a motherboard or the like that controls the power supply for driving the liquid crystal display device, or a triangular wave created by a triangular wave generation circuit provided inside the power supply circuit device for lighting the discharge tube is used. An originally generated signal is used, and a PWM (Pulse Width Modulation) signal is generally used.

  Here, when the triangular wave is created inside the power supply circuit device, the frequency is determined by the value of the capacitance and the resistance of the frequency adjusting capacitor connected to the triangular wave generating circuit. However, it is generally known that the value of the capacitance of a capacitor fluctuates depending on the secular change due to long-term use and the temperature (ambient temperature) of the operating environment at the time of operation. The frequency of the signal also varies. This frequency is a frequency for driving the discharge tube in a time-sharing manner. Therefore, even if the frequency of the duty signal is set to a value that is not synchronized with the vertical synchronization frequency of the liquid crystal display device in advance as in the example of Patent Document 1, the time division frequency of the discharge tube varies due to a change in the ambient temperature or the like. As a result, synchronization with the vertical synchronization frequency may occur. In this case, as described above, beat noise or flicker may occur on the screen of the liquid crystal display device.

  In addition, even when an external PWM signal is used as the duty signal input to the time division drive control circuit, there is no particular consideration of the problem of synchronization with the vertical synchronization frequency of the liquid crystal display device in the external signal. In addition, when a signal whose frequency varies depending on the use environment or the like is used, the same problem as described above may occur. For the above reasons, in the power supply circuit device, a reliable internal signal is used as the duty signal input to the time-division drive control circuit, and control is performed so that the internal signal never synchronizes with the vertical synchronization frequency of the liquid crystal display device. There was a need for a means to do this.

  The present invention has a function of generating a duty signal that is not synchronized with the vertical synchronization frequency of a liquid crystal display device, and a power supply circuit device having a configuration for inputting such a signal to a time-division drive control circuit, and The liquid crystal display device used is proposed.

  In the power supply circuit device, when an internal signal is used as the duty signal input to the time division drive control circuit, the triangular wave that is the source of the duty signal is created by the triangular wave generation circuit inside the power supply circuit device. . The elements that determine the frequency are a frequency adjusting capacitor and resistor connected to the triangular wave generating circuit. Here, let us consider a case where the capacitance of the capacitor changes and the time division frequency fluctuates to a frequency range that may be synchronized with the vertical synchronization frequency of the liquid crystal display device. Even in such a case, if the function of detecting the time division frequency is provided in the power supply circuit device, and the value of the frequency adjustment resistor can be changed based on the detection result, this synchronization can be achieved. The problem can be solved. As a result of investigations, the inventors have paid attention to the fact that the piezoelectric transformer in the power supply circuit device is accompanied by vibrations when driven, and detects the vibration of the piezoelectric transformer and adjusts the frequency based on the frequency of the fundamental vibration. It has been found that it is appropriate to have a configuration that controls the value of the resistor for use.

  In order to control the value of the frequency adjusting resistor connected to the triangular wave generating circuit, this resistor needs to be a variable resistor. For this purpose, it is preferable to use a resistance circuit using a semiconductor element such as a field effect transistor. Here, a method of controlling the capacitance of the frequency adjusting capacitor instead of the resistance value is also conceivable. However, in general, in order to change the capacitance of a capacitor, a relatively large structural component such as a varicap is required, and it is difficult to achieve a circuit configuration using only small-scale elements such as semiconductors. Since the power supply circuit device of the present invention is supposed to be used in applications that require miniaturization such as for discharge tube lighting, it is difficult to control the time division frequency by controlling the capacitance of the capacitor. .

  The piezoelectric transformer in the power supply circuit device vibrates at a high frequency of several tens kHz to several hundreds kHz in order to input a high voltage alternating current to the discharge tube and light it. The vibration operation of the piezoelectric transformer repeats a vibration state and a stop state at a frequency of several hundred Hz when the discharge tube is driven in a time-sharing manner. The frequency of the fundamental vibration (repetition frequency) resulting from repetition of the vibration state and the stop state is the frequency of the time-division drive of the discharge tube. Therefore, the repetition frequency is detected, and the value of the frequency adjusting resistor is adjusted so that the value does not fall within a frequency range in which synchronization with the vertical synchronization frequency of the liquid crystal display device is possible. In this case, the frequency synchronization does not occur. Accordingly, it is possible to prevent occurrence of problems such as beat noise and flickering on the screen of the liquid crystal display device.

  A vibration sensor is used as means for detecting the repetition frequency of the vibration state and the stop state of the piezoelectric transformer, and the use of a surface mount type charge output type piezoelectric vibration sensor is particularly suitable. The charge output type piezoelectric vibration sensor is generally characterized by being small and inexpensive and having little secular change. However, the present invention is not limited to the surface mount type charge output type piezoelectric vibration sensor as long as it can be installed in the power supply circuit device and can repeatedly detect the vibration of the piezoelectric transformer without any problem. Here, the installation location of the vibration sensor in the power supply circuit device is the surface of the circuit board provided in the power supply circuit device, the surface of the mounting component installed on the surface of the circuit board, or the inside of the mounting component. Either is preferable. In any case, it is necessary to install it in the vicinity of the piezoelectric transformer that is the source of vibration.

  However, it is not preferable to directly fix the vibration sensor to the piezoelectric transformer itself because it may affect the operation of the piezoelectric transformer. In addition, since a high voltage is generated in the piezoelectric transformer, the piezoelectric transformer may be housed and arranged in a case made of an insulator in order to ensure insulation from surrounding substrate circuits. It is preferable to install the piezoelectric transformer together with the piezoelectric transformer or to arrange the insulator on the surface of the case.

  Furthermore, the detection range of the repetition frequency by the vibration sensor used is preferably 50 Hz to 2 kHz. This is because the vertical synchronization frequency value of the liquid crystal display device is generally about 50 Hz to 240 Hz even when the same speed drive, double speed drive, or quadruple speed drive is performed on a dedicated screen or television of a mobile phone, personal computer or the like. It depends. Accordingly, the time division frequency for adjusting the luminance of the discharge tube used in the backlight device of the liquid crystal display device is a value several times that value. A vibration sensor with a detection range of 50 Hz to 2 kHz is sufficient for detecting the time-division frequency, and can detect harmonic components obtained by multiplying the time-division frequency without problems as described later. .

  Further, as described above, the use of a cold cathode tube is particularly suitable as a discharge tube constituting the backlight device of the liquid crystal display device. However, in the present invention, the power supply circuit device having a vibration sensor and driving a load that is a discharge tube in a time-sharing manner is a case where not only a cold cathode tube but also various discharge tubes such as a hot cathode tube and an external electrode tube are driven. Can be used well.

  Furthermore, the duty signal by the internal signal obtained by detecting the repetition frequency of the piezoelectric transformer by the vibration sensor and adjusting the value of the frequency adjusting resistor based on the result is sufficiently reliable. Therefore, even when an external signal can be used as the duty signal, it is possible to prevent the occurrence of problems on the screen of the liquid crystal display device by using this internal signal without using the external signal.

  That is, the present invention is a power supply circuit device for lighting a discharge tube, wherein the discharge tube is time-division driven by the power supply circuit device, and the power supply circuit device inputs an alternating current to the discharge tube and is time-division driven. A piezoelectric transformer for driving the piezoelectric transformer, and a drive circuit unit for driving the piezoelectric transformer.The power supply circuit device includes a vibration sensor, and a time division frequency in the time division driving is determined by a vibration signal obtained by the vibration sensor. A power supply circuit device having a function of adjusting to a predetermined value.

  Further, the present invention is the power supply circuit device, wherein the vibration sensor detects time-division driving of the piezoelectric transformer and outputs the vibration signal.

  Further, according to the present invention, the vibration sensor is provided on at least one of a surface of a circuit board included in the power supply circuit device, a surface of a mounting component installed on the surface of the circuit board, and an inside of the mounting component. It is the power supply circuit device characterized by being installed.

  Further, the present invention is characterized in that the power supply circuit device includes a case, the case is installed on a surface of the circuit board, and the piezoelectric transformer and the vibration sensor are installed together in the case. Is a power supply circuit device.

  Further, according to the present invention, the power supply circuit device includes a case, the case is installed on a surface of the circuit board, the piezoelectric transformer is installed in the case, and the vibration sensor is installed on the surface of the case. It is the power supply circuit device characterized by being installed in.

  Furthermore, the present invention is the power supply circuit device, wherein the vibration sensor is a surface mount type charge output type piezoelectric vibration sensor.

  Furthermore, the present invention is a power supply circuit device characterized in that a frequency detection range by the vibration sensor is 50 Hz to 2 kHz.

  Furthermore, the present invention is the power supply circuit device, wherein the discharge tube is a cold cathode tube.

  Furthermore, the present invention provides the power supply circuit device, wherein the power supply circuit device includes a triangular wave generation circuit and a comparator, and a frequency of the triangular wave generated by the triangular wave generation circuit is a time division frequency in the time division drive. The triangular wave generating circuit is connected to a resistor circuit for adjusting the value of the generated frequency, and the resistance value in the resistor circuit is set to a predetermined value by the vibration signal obtained by the vibration sensor. A power supply circuit device having a function of adjusting.

  Furthermore, the present invention is a backlight device comprising the power supply circuit device.

  According to the present invention, when a piezoelectric transformer is mounted on a power supply circuit device by installing a vibration sensor on the surface of the circuit board of the power supply circuit device for lighting the discharge tube, or inside or on the surface of the component mounted on the circuit board. The repetition frequency of the vibration state and the stop state by the division drive is detected. Then, based on the detected frequency, the value of the frequency adjusting resistor connected to the triangular wave generating circuit included in the power supply circuit device of the present invention is controlled.

  By the above method, even if the frequency for time division driving of the discharge tube in the power supply circuit device can fluctuate from a preset frequency due to the influence of secular change or ambient temperature change, the frequency adjustment This frequency can be adjusted by controlling the value of the resistance. As a result, the frequency of the triangular wave generated by the triangular wave generating circuit, which is the repetition frequency of the piezoelectric transformer, does not become a value within a frequency range that may be synchronized with the vertical synchronization frequency of the liquid crystal display device in which the discharge tube is used. It becomes possible to control to. Therefore, the frequency for the time-division driving of the discharge tube in the power supply circuit device of the present invention is kept within a certain range even when there is a change in the ambient temperature. Occurrence of problems such as beat noise and flickering on the screen can be prevented.

1 is a block diagram of a power supply circuit device according to an embodiment of the present invention. The example of the circuit diagram of the triangular wave generation circuit in embodiment of this invention of FIG. The figure which shows each relationship of the triangular wave _Vtri in the time-division drive in embodiment of this invention of FIG. 1, the luminance adjustment signal _Vadj , the PWM signal _Vpwm , and the envelope of a load current. The block diagram containing the backlight apparatus based on Embodiment of this invention using the power supply circuit apparatus of FIG. The block diagram which concerns on the conventional power circuit device.

  Hereinafter, embodiments of the present invention will be described in detail with reference to FIGS. 1 to 4.

FIG. 1 is a block diagram of a power supply circuit device according to an embodiment of the present invention. The configuration of each element of the drive circuit 11, the piezoelectric transformer 12, the load 13, the load current comparison circuit 14, the frequency sweep oscillator 15, and the time-division drive control circuit 16 in the power supply circuit device of the present invention is shown in FIG. It is the same as that of the block diagram concerning the power supply circuit device. However, the present invention is characterized by a duty signal for brightness adjustment input to the duty setting terminal V _duty provided in the time division drive control circuit 16, and this signal is a frequency that is an integral multiple of the vertical synchronization frequency of the liquid crystal display device. PWM signal V _pwm of the PWM signal generated based on the internal signal, controlled so as not to have a value in the vicinity of.

This PWM signal V _pwm is generated as follows. First, the triangular wave V _tri generated by the triangular wave generating circuit 18 and the luminance adjustment signal V _adj which is a direct current signal input from the outside by the setting of the user of the liquid crystal display device are input to the comparator 17. The comparator 17 compares the two, and the result is output as the PWM signal V _pwm . As a method of combining both signals by the comparator 17, for example, the following method is suitable. First, during a period when the signal level of the luminance adjustment signal V _adj is higher than that of the triangular wave V _tri , the signal level of the PWM signal V _pwm generated by the comparator 17 is set to a high level. Conversely, during the period when the signal level of the DC luminance adjustment signal V _adj is lower than that of the triangular wave V _tri, the signal level of the PWM signal V _pwm is set to a low level. In this method, the PWM signal V _pwm output from the comparator 17 is a rectangular wave, and the frequency thereof is the same as the input triangular wave V _tri regardless of the value of the luminance adjustment signal V _adj .

An example of a circuit diagram of the triangular wave generating circuit is shown in FIG. The circuit diagram of FIG. 2 is an example of a triangular wave generating circuit used in the power supply circuit device of the present invention, and a triangular wave generating circuit having a circuit configuration other than that of FIG. 2 may be used. In FIG. 2, a DC voltage from DC power supplies 25 and 26 is input to the comparator 23. When the output from the comparator 23 is High, it is output from the triangular wave generation circuit as a triangular wave V _tri through the transistor 24 and charges are accumulated in the capacitor 19. For this reason, the output signal from the triangular wave generation circuit input to the comparator has a waveform in which the voltage gradually increases. As the charge accumulation in the capacitor 19 proceeds, the output voltage gradually increases.

  However, since this output is also connected to the positive input terminal of the comparator 23, when the output voltage reaches a certain upper limit voltage value, the output of the comparator 23 is inverted and becomes Low, and is stored in the capacitor 19. Charge is released through transistor 24. Therefore, the output signal input to the comparator has a waveform in which the voltage gradually decreases. When the discharge of the charge accumulated in the capacitor 19 proceeds and the output voltage of the triangular wave generation circuit reaches a certain lower limit voltage value, the output of the comparator 23 is inverted and becomes High again, and the voltage of the output signal gradually increases. At the same time, the accumulation of electric charges in the capacitor 19 is started again.

As described above, the triangular wave generating circuit generates a triangular wave by repeating accumulation and discharge of electric charges in the connected capacitor 19. The frequency at that time is determined by the constants of the capacitor 19 and the resistor 20 connected in parallel thereto. The resistor 20 is constituted by a resistor circuit using a field effect transistor and the resistance value is changed to change the frequency of the triangular wave V _tri output from the triangular wave generating circuit, that is, the time for driving the discharge tube in a time-sharing manner. It is possible to adjust the division frequency.

FIG. 3 shows the relationship between signals input to and output from the comparator 17. FIG. 3 shows the relationship between the triangular wave V _tri , the luminance adjustment signal V _adj , the PWM signal V _pwm , and the load current envelope graph in the time-division drive in the embodiment of the present invention shown in FIG. FIG. In the following, reference numerals in the names of the devices correspond to those in FIG. Here, the load current is a current that flows through the load 13 which is a discharge tube and lights the discharge tube. In each graph of FIG. 3, the horizontal axis of each signal is time and is synchronized with each other. The vertical axis represents voltage or current.

In FIG. 3, during a period when the signal level of the DC luminance adjustment signal V _adj input from the outside is higher than the triangular wave V _tri generated by the triangular wave generation circuit 18 and input to the comparator 17, the comparator 17 The level of the generated PWM signal V _pwm becomes a high level, and a load current that is a high-frequency driving current flows through the load 13. Therefore, during this period, the load 13 which is a discharge tube is in a lighting state. On the other hand, the signal level of the PWM signal V _pwm output from the comparator 17 is low during the period when the signal level of the luminance adjustment signal V _adj is lower than the triangular wave V _tri , and no current flows through the load 13 during this period. Accordingly, the load 13 is turned off.

Here, when the signal level of the brightness adjustment signal V _adj is changed, the time ratio between the high level and the low level of the PWM signal V _pwm output from the comparator 17 changes accordingly, and the load 13 The time ratio between the lighting period and the extinguishing period will change. Since the ratio between the lighting period and the extinguishing period of the discharge tube is the luminance of the backlight device used in the liquid crystal display device, by using the power supply circuit device according to the embodiment of the present invention, the screen of the liquid crystal display device is displayed. The brightness can be adjusted.

Note that the comparison method of the two signals in the comparator 17 of FIG. 1 may be other than the above method, but in the power supply circuit device of the present invention, the output PWM signal V _pwm is a rectangular wave, and the frequency thereof is It is necessary to be the same as the triangular wave V _tri . By _adopting a configuration in which such a PWM signal V _pwm is input to the time-division drive control circuit 16 constituting the power supply circuit device of the present invention, the signal level of the PWM signal V _pwm according to the signal level of the luminance adjustment signal V _adj. The time ratio between the high level and the low level can be adjusted, whereby the load current of the power supply circuit device can be adjusted, that is, the luminance of the backlight device in the liquid crystal display device can be adjusted.

Here, the frequency of the triangular wave V _tri output from the triangular wave generating circuit 18, that is, the time division frequency, is determined by the capacitance value of the capacitor 19 externally attached to the triangular wave generating circuit 18 and the resistance value of the resistor 20. In the initial state, these values are set so that the time division frequency is not a value in the vicinity of a frequency that is an integral multiple of the vertical synchronization frequency of the liquid crystal display device. However, it is known that the value of the capacitor 19 externally attached to the triangular wave generation circuit 18 varies depending on the secular change and the temperature (ambient temperature) of the operating environment during operation as described above. Therefore, it is necessary to change the resistance value of the externally attached resistor 20 so that the time division frequency is not synchronized with the vertical synchronization frequency of the liquid crystal display device due to this variation.

  As such a resistance whose resistance value is variable, it is preferable to use a resistance circuit using a field effect transistor or the like. For example, when an N-channel enhancement type field effect transistor is used as the resistance circuit, the resistance value between the drain and the source can be increased by applying a bias voltage as a control signal from the vibration frequency detection circuit 21 described later between the gate and the source. It is possible to configure a changing resistance circuit.

As means for adjusting the resistance value of the resistor 20 in FIG. 1, a vibration frequency detection circuit 21 and a vibration sensor 22 are provided in the power supply circuit device of the present invention. The vibration sensor 22 is an element that detects vibration of the power supply circuit device by the piezoelectric transformer 12 that drives the load 13 in a time-sharing manner. The piezoelectric transformer 12 vibrates at a high frequency of several tens kHz to several hundreds kHz in order to light up the load 13 which is a discharge tube. The vibration is transmitted to the time division drive control circuit 16 for adjusting the luminance of the load 13. The vibration state and the stop state are repeated at the frequency of the input PWM signal V _pwm . The vibration sensor 22 has a function of detecting a time division frequency that is a repetition frequency of the vibration state and the stop state. However, in order to make the frequency value detected by the vibration sensor 22 more accurate, it is necessary to simultaneously detect harmonic components that are multiplied by the time division frequency. Therefore, the vibration sensor 22 needs to be able to detect the time division frequency of the load 13 that is a discharge tube and a harmonic component that is multiplied by the time division frequency.

The vibration signal of the piezoelectric transformer 12 detected by the vibration sensor 22 is input to the vibration frequency detection circuit 21 where the repetition frequency of the vibration of the piezoelectric transformer 12, that is, the time division frequency is detected. If the detected frequency is an integer multiple of the vertical synchronization frequency of the liquid crystal display device and is a value near the frequency synchronized with the vertical synchronization frequency, the resistance value of the resistor 20 that is a variable resistor is adjusted. The frequency of the triangular wave V _tri output from the triangular wave generation circuit 18 is changed. Since this frequency is the frequency of the PWM signal V _pwm input to the time division drive control circuit 16 and the time division frequency of the vibration of the piezoelectric transformer 12, the power supply circuit device is synchronized with the vertical synchronization frequency of the liquid crystal display device. Therefore, occurrence of beat noise and flicker in the liquid crystal display device is prevented. In order to avoid synchronization with the vertical synchronization frequency of the liquid crystal display device, it is sufficient that the time division frequency of the piezoelectric transformer 12 is 10 Hz or more away from the value that is an integral multiple of the vertical synchronization frequency.

  Here, the vibration frequency detection circuit 21 is a circuit having a charge amplifier, an F / V converter (frequency-voltage converter), and the like. A vibration signal of the piezoelectric transformer 12 input from the vibration sensor 22 is first input to the charge amplifier and converted into a voltage signal having a pulsed signal waveform corresponding to the measured frequency. Next, this voltage signal is input to the F / V converter and converted into a DC voltage having a voltage value corresponding to the height of the frequency. A DC voltage value corresponding to this frequency is applied to a resistor 20 externally attached to the triangular wave generation circuit 18. When the resistor 20 is a resistor circuit having a field effect transistor, the DC voltage is input between the gate and the source of the field effect transistor, and the resistance value between the drain and the source of the field effect transistor according to the voltage value. To change.

At the initial time, the bias voltage input to the gate of the field effect transistor constituting the resistance circuit is set to a predetermined value. This predetermined value is a value at which the PWM signal V _pwm finally output from the comparator becomes a specific time division frequency different from an integer multiple of the vertical synchronization frequency of the liquid crystal display device. With these series of circuit configurations, when the time division frequency in driving the piezoelectric transformer 12 fluctuates due to a change in ambient temperature, the time division frequency is first detected by the vibration sensor 22. Next, the value of the bias voltage is changed based on the value, and the resistance value of the resistor 20 which is a resistor circuit can be adjusted. This makes it possible to correct the value so as to cancel the fluctuation of the time division frequency in the power supply circuit device, and the time division frequency at the time of driving is always a value different from an integer multiple of the vertical synchronization frequency of the liquid crystal display device. Can continue to hold.

Since the triangular wave V _tri and the PWM signal V _pwm are signals having a vibration period of a time division frequency, these are used instead of the vibration signal from the vibration sensor 22 in order to adjust the resistance value of the resistor 20 in FIG. If the above signal is used, the vibration sensor 22 and the like can be omitted from the power supply circuit device. However, unlike the detection signal from the vibration sensor 22, the triangular wave V _tri and the PWM signal V _pwm do not include a harmonic component of the time-division frequency, so that there is another problem for accurate detection. As a method of detecting the frequency of vibration, a method of detecting not a fundamental frequency but a multiplied harmonic thereof is known. Since harmonics have a higher frequency, the waveform is steeper than the fundamental frequency. Therefore, by using the harmonics, a more accurate frequency can be detected.

As described above, in the power supply circuit device of the present invention, it is possible to obtain a more accurate time division frequency by using the vibration sensor 22 to detect a harmonic component that is multiplied by the time division frequency. It is a feature of the present invention that this method makes it possible to control the time division frequency more precisely than when using the triangular wave V _tri or the PWM signal V _pwm . Therefore, providing the vibration sensor 22 in the power supply circuit device is an essential condition in the power supply circuit device of the present invention.

  Here, since the vibration sensor 22 in FIG. 1 needs to be installed in the vicinity of the piezoelectric transformer 12, on the surface of the circuit board on which the piezoelectric transformer 12 is installed, the mounted components installed on the surface of the circuit board. It is preferable that it is installed on at least one of the surface and the inside of the mounted component. In addition, since the secondary side of the piezoelectric transformer 12 has a high voltage, the piezoelectric transformer 12 may be housed in an insulating case to ensure insulation, in which case the vibration sensor 22 is It is preferable to be installed inside or on the surface of the case. As the vibration sensor 22 used in the power supply circuit device, a surface mount type charge output type piezoelectric vibration sensor is suitable. This is because the charge output type piezoelectric vibration sensor is generally small and inexpensive, and has a feature that the secular change is small. However, other types of vibration sensors may be used as long as they can be installed in the power supply circuit device and can detect repeated vibrations of the piezoelectric transformer without any problem.

  Furthermore, it is sufficient if the frequency detection range by the vibration sensor 22 is 50 Hz to 2 kHz. As described above, the vertical synchronization frequency of the liquid crystal display device actually used is generally about 50 Hz to 240 Hz, and the time division frequency when the piezoelectric transformer 12 is driven is an integral multiple thereof, and the vibration sensor 22 further includes this time division. It is necessary to detect harmonic components of the frequency. However, even in such a case, since the frequency of the detected harmonic component is about 1 kHz at the maximum, even if a margin is taken into consideration, the detectable range of the vibration sensor 22 only needs to be about 2 kHz. When the vibration sensor 22 detects the drive frequency of the piezoelectric transformer, which is generally several tens of kHz, the vibration frequency detection circuit 21 uses this fact that the frequency is much higher than the frequency of the harmonic component. The influence can be removed.

  As the load 13 used in the power supply circuit device of the present invention, any discharge tube in which the piezoelectric transformer 12 is used for driving the load 13 can be suitably used. A cold cathode tube is particularly preferred when used as a light device. As described above, a cold cathode tube can be realized as a discharge tube having a small diameter and an elongated shape, and has a longer product life than a hot cathode tube, etc., and thus constitutes a backlight device for a liquid crystal display device that requires a thin shape. It is particularly good for. It is also suitable for controlling the lighting with a piezoelectric transformer.

  FIG. 4 is a block diagram including a backlight device using the power supply circuit device according to the embodiment of the present invention, and constitutes a liquid crystal display device as a whole. Here, the case where two cold cathode tubes are used as the discharge tube of the backlight device is described. Two cold-cathode tubes 42 and 43 are installed in parallel with the back panel 44 on the back side of the screen of the liquid crystal panel 41. When these cold-cathode tubes 42 and 43 are lit, the liquid crystal panel 41 is turned on. An image is displayed on the front. The cold cathode tubes 42 and 43 are loads of the power supply circuit device 46 and are time-division driven by the power supply circuit device 46. A luminance adjustment signal 47 is input to the power supply circuit device 46 from the outside. The cold cathode tubes 42 and 43 and the power supply circuit device 46 are components of the backlight device in the present invention.

  On the other hand, the liquid crystal panel 41 is driven by a driving device 45 independent of the power supply circuit device 46, and both the driving device 45 and the power supply circuit device 46 are supplied with power from a common power supply 48. There is a possibility that the drive signals of the cold cathode tubes 42 and 43 are superimposed as noise on the drawing signal of the liquid crystal panel 41 via 48. However, in the case of using the backlight device driven by the power supply circuit device of the present invention, it is possible to prevent the occurrence of problems such as beat noise and flickering on the screen of the liquid crystal display device even if such noise is superimposed. It becomes possible.

  As described above, in the power supply circuit device according to the present invention, the vibration sensor is installed in the device, and the repetition frequency of the vibration state and the stop state by the time-division driving of the piezoelectric transformer included in the device is detected. Based on this frequency, the value of the frequency adjusting resistor connected to the triangular wave generating circuit in the apparatus is controlled. As a result, the drive frequency in the power supply circuit device of the present invention is maintained within a certain range even if there is a secular change or a change in ambient temperature. It is possible to prevent the occurrence of problems such as beat noise and flicker, and to provide a backlight device that prevents these problems from occurring. Further, the above description is for explaining the embodiment of the present invention, and does not limit the invention described in the scope of claims. Moreover, each part structure of this invention is not restricted to the said embodiment, A various deformation | transformation is possible within the technical scope as described in a claim.

  The power supply circuit device according to the present invention is suitably used as a driving power source for a backlight device mounted on a lighting device, particularly a liquid crystal display device.

11, 51 Drive circuit 12, 52 Piezoelectric transformer 13, 53 Load 14, 54 Load current comparison circuit 15, 55 Frequency sweep oscillator 16, 56 Time division drive control circuit 17 Comparator 18 Triangle wave generation circuit 19 Capacitor 20 Resistance 21 Vibration frequency detection Circuit 22 Vibration sensor 23 Comparator 24 Transistors 25 and 26 DC power supply 41 Liquid crystal panel 42 and 43 Cold cathode tube 44 Back panel 45 Drive device 46 Power supply circuit device 47 Luminance adjustment signal 48 Common power supply

Claims (10)

A power supply circuit device for lighting a discharge tube, wherein the discharge tube is time-division driven by the power supply circuit device, and the power supply circuit device inputs an alternating current to the discharge tube to be time-division driven, and A drive circuit unit for driving the piezoelectric transformer,
The power supply circuit device includes a vibration sensor,
A power supply circuit device having a function of adjusting a time division frequency in the time division drive to a predetermined value by a vibration signal obtained by the vibration sensor.   The power supply circuit device according to claim 1, wherein the vibration sensor detects time-division driving of the piezoelectric transformer and outputs the vibration signal.   The vibration sensor is installed on at least one of a surface of a circuit board provided in the power supply circuit device, a surface of a mounting component installed on the surface of the circuit board, and an inside of the mounting component. The power supply circuit device according to claim 1, wherein the power supply circuit device is a power supply circuit device. The power supply circuit device includes a case, and the case is installed on a surface of the circuit board;
The power supply circuit device according to claim 1, wherein the piezoelectric transformer and the vibration sensor are both installed in the case. The power supply circuit device includes a case, the case is installed on a surface of the circuit board, and the piezoelectric transformer is installed in the case,
The power supply circuit device according to claim 1, wherein the vibration sensor is installed on a surface of the case.   6. The power supply circuit device according to claim 1, wherein the vibration sensor is a surface mount type charge output type piezoelectric vibration sensor.   The power supply circuit device according to claim 6, wherein a frequency detection range of the vibration sensor is 50 Hz to 2 kHz.   8. The power supply circuit device according to claim 1, wherein the discharge tube is a cold cathode tube. The power supply circuit device has a triangular wave generation circuit and a comparator, and the frequency of the triangular wave created by the triangular wave generation circuit is a time division frequency in the time division drive,
A resistor circuit for adjusting the value of the generated frequency is connected to the triangular wave generating circuit,
The power supply circuit device according to any one of claims 1 to 8, wherein the power supply circuit device has a function of adjusting a resistance value in the resistance circuit to a predetermined value based on a vibration signal obtained by the vibration sensor.   A backlight device comprising the power supply circuit device according to claim 1.

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