JP5380450B2 - Piezoelectric transformer drive circuit and drive method - Google Patents

Description

  The present invention relates to a driving circuit for a piezoelectric transformer used for cold cathode tubes such as various backlights and a driving method thereof.

  In recent years, a liquid crystal display monitor is attached as a liquid crystal display device such as a notebook personal computer. In such a liquid crystal display monitor, a piezoelectric transformer has been used as a step-up transformer for driving a cold cathode tube such as a backlight. A piezoelectric transformer is a voltage that applies an input alternating voltage to the primary electrode of a piezoelectric element to generate mechanical vibration using the piezoelectric effect, and takes out a voltage amplified from the secondary electrode at a step-up ratio determined by the shape of the piezoelectric transformer. It is a conversion element.

  Since this piezoelectric transformer is not a method of transforming with magnetic energy using windings, no leakage magnetic flux is generated. For this reason, there is an advantage that no noise is generated outside the inverter. In addition, since the piezoelectric transformer selects and outputs only the resonance frequency determined by the external dimensions, there is an advantage that the output waveform is close to a sine wave and the generation of high frequency noise is small. Further, since the piezoelectric transformer is an inorganic material obtained by sintering a ceramic material, it has an advantage that there is no danger of smoke or ignition.

  The piezoelectric transformer has resonance characteristics, and the output obtained from the secondary side electrode differs depending on the frequency of the AC voltage input to the primary side electrode. Therefore, in order to keep the brightness of a liquid crystal display or the like constant, it is possible to adjust the voltage output from the secondary electrode to a desired level by controlling the frequency of the AC voltage input to the piezoelectric transformer. is there.

  A conventional piezoelectric transformer drive circuit is composed of an analog circuit, and an analog oscillator is used to control the frequency of the voltage. On the other hand, the piezoelectric transformer has a phenomenon that the frequency characteristic changes due to a change in frequency characteristic due to an ambient temperature or a temperature change of the piezoelectric transformer itself, or due to variations in dimensions and piezoelectric characteristics of the piezoelectric transformer. In such a case, it is necessary to adjust the frequency of the voltage supplied to the piezoelectric transformer in accordance with the changed frequency characteristics. However, with the conventional analog oscillator, it is difficult to control the frequency of the output voltage at an appropriate timing. Therefore, it is desirable to use a digital oscillator that can freely control the frequency of the voltage output from the oscillator and change the frequency of the output voltage to an arbitrary form instead of the analog oscillator.

  However, when digitizing a conventional piezoelectric transformer drive circuit configured in an analog manner, a CPU that outputs a clock signal of 500 MHz to 1 GHz in order to obtain the frequency accuracy necessary for controlling the piezoelectric transformer. Was necessary. However, such a high clock frequency is not a practical clock frequency for a piezoelectric transformer for driving a cold cathode tube in consideration of radiation interference, supply power, and the like.

  Therefore, in Patent Document 1, the frequency necessary for controlling the piezoelectric transformer is improved by dividing the drive pulse of the low-frequency clock signal and determining the ratio for the frequency division to improve the resolution of the average frequency. A method for obtaining accuracy has been proposed.

JP 2000-133485 A

  However, in the method of Patent Document 1, there is a limit to the ratio of dividing the clock signal, and the voltage corresponding to the frequency below the resolution cannot be controlled. There is also a problem that a complicated configuration is required as a frequency dividing circuit. In order to solve the above problems, an object of the present invention is to provide a piezoelectric transformer drive circuit and drive capable of obtaining an arbitrary output voltage by a simpler means than using a digital oscillator to divide a clock signal. Is to provide a method.

In order to achieve the above object, the present invention provides a piezoelectric transformer having a predetermined resonance frequency characteristic, a digital oscillator that outputs a voltage of a predetermined frequency to the piezoelectric transformer, and an output voltage V _{1 of the} piezoelectric transformer. In the piezoelectric transformer drive circuit, comprising: an output detection unit for detecting 'and a control unit for controlling the frequency F ₁ ′ output from the digital oscillator based on a detection signal from the output detection unit. Is based on the detection signal from the output detection unit, the output voltage setting unit for setting the output voltage V ₁ ′ of the piezoelectric transformer, and the digital oscillator outputs the frequency F ₁ ′ corresponding to the output voltage V ₁ ′ If not, the corresponding frequency detector to determine the corresponding frequencies F ₁ of the two possible outputs which sandwich _'frequencies F _1, F _2, piezoelectric preparative said two frequencies F _1, F ₂ A frequency output unit alternating output Nsu, and alternating ratio setting unit that sets a ratio for the piezoelectric transformer output voltage alternating generation corresponding to the two frequencies F _1, F _2, said two frequencies F _1, F ₂ And an alternating interval control unit for outputting a control signal to the frequency output unit for outputting the control signal based on the alternating rate set by the alternating rate setting unit.

In another aspect of the present invention, a piezoelectric transformer having a predetermined resonance frequency characteristic, a digital oscillator that outputs a voltage having a predetermined frequency to the piezoelectric transformer, and an output voltage V ₁ ′ of the piezoelectric transformer are detected. In the piezoelectric transformer drive circuit comprising: an output detection unit; and a control unit that controls the frequency F ₁ ′ output from the digital oscillator based on a detection signal from the output detection unit, the control unit includes the output Based on a detection signal from the detection unit, an output voltage setting unit for setting an output voltage V ₁ ′ of the piezoelectric transformer, and a frequency F ₁ ′ corresponding to the output voltage V ₁ ′ set by the output voltage setting unit is set to the digital with an oscillator to detect whether it can output, in the case where the digital oscillator _'frequencies F ₁ corresponding _to' the output voltage V ₁ is not be output across the corresponding frequencies F _{1 '} It outputs the corresponding frequency detecting unit for determining an output two possible frequencies F _1, F _2, the frequencies F _{1 'when} the output voltages V ₁ can _output' frequencies F ₁ corresponding to _'the piezoelectric transformer, the output When the frequency F ₁ ′ corresponding to the voltage V ₁ ′ cannot be output, a frequency output unit that alternately outputs _two frequencies F ₁ and F ₂ sandwiching the frequency F ₁ ′ to the piezoelectric transformer, and the corresponding frequency detection unit and alternating ratio setting unit that sets a ratio for alternating generate the output voltage of the piezoelectric transformer corresponding to the two frequencies F _1, F ₂ was detected, two frequency F _1, F ₂ in the frequency output unit, the alternating ratio And an alternating interval control unit for outputting a control signal for outputting based on the alternating ratio set by the setting unit to the frequency output unit.

In the present invention, the alternating ratio set by the alternating ratio setting unit is set to the output voltages V ₁ and V ₂ corresponding to the frequencies F ₁ and F ₂ determined by the corresponding frequency detecting unit and the set output voltage V ₁ ′. From the above, ΔV ₂ −V ₁ ′: ΔV ₁ ′ −V ₁ can be determined.

  In the present invention, the alternating ratio setting unit may set a predetermined set value as the alternating ratio.

  In the present invention having such a configuration, the voltage corresponding to the resonance frequency of the piezoelectric transformer can be controlled without using a high-performance CPU by setting the voltage for driving the piezoelectric transformer as a time-varying voltage. become.

Here, the time-varying voltage to be generated is generated by alternately generating the frequencies F ₁ and F ₂ corresponding to the voltages V ₁ and V ₂ before and after the output voltage V ₁ ′ of the piezoelectric transformer, whereby an approximate voltage is applied to the piezoelectric transformer. generate. Since the piezoelectric transformer circuit has many capacitor components, the voltage changing reaction is slow. Therefore, the voltage during integration is output. By alternating the frequencies, it is possible to generate a voltage that is almost the same as the state in which the frequency for controlling the output voltage of the piezoelectric transformer is included.

  When generating a voltage corresponding to a frequency below the resolution of the control circuit using the piezoelectric transformer drive circuit as described above, by alternately generating the frequencies corresponding to the voltages before and after the voltage, A method of generating an approximate voltage in the piezoelectric transformer is also an aspect of the present invention.

According to the present invention as described above, when the piezoelectric transformer is driven by a digital circuit, the frequency F ₁ and the frequency F ₂ before and after the resonance frequency F ₁ ′ corresponding to the output voltage V ₁ ′ of the piezoelectric transformer are alternately generated. be to, from the piezoelectric transformer, the frequency F _1, F ₂ output voltages V ₁ corresponding _to, V ₂ is alternating output, voltage approximately equal result both output voltage is averaged output voltages V _{1 '} Can be generated. This eliminates the need to always generate the same frequency as the resonance frequency of the piezoelectric transformer. Therefore, even if the CPU is relatively easy to handle without requiring a high-performance CPU that is impractical in consideration of radiation interference, supply power, etc., the piezoelectricity due to frequency deviation, load, operating temperature, etc. due to variations in the piezoelectric transformer. A desired output voltage can be generated in response to changes in the transformer.

The block diagram which shows the structure of one Embodiment of this invention The block diagram which shows the internal structure of the control part 4 in embodiment of FIG. The block diagram which shows the internal structure of the digital oscillator 5 in embodiment of FIG. Flowchart showing the operation of the embodiment of FIG. The figure which showed the relationship between the output voltage of a piezoelectric transformer and resonance frequency in embodiment of FIG.

[This embodiment]
[1. Constitution]
The configuration of the piezoelectric transformer drive circuit of the present embodiment is shown in the block diagram of FIG. 2 is a block diagram showing the inside of the control unit 4 in FIG. 1, and FIG. 3 is a block diagram showing the inside of the digital oscillator 5. As shown in FIG.

  The circuit of this embodiment is a digital circuit that drives the piezoelectric transformer 1, and in this embodiment, it is a circuit including a cold cathode tube. The piezoelectric transformer 1 is a transformer element for taking out the amplified voltage, and the voltage output from the piezoelectric transformer 1 is applied to the cold cathode tube 2. The output detector 3 detects the current flowing through the cold cathode tube 2 and converts it into a voltage signal. The control unit 4 determines a set value n for the digital oscillator 5 based on the voltage signal detected by the output detection unit 3 and outputs the signal. In the digital oscillator 5, a time-varying voltage having a predetermined frequency is generated based on the signal of the set value n sent from the control unit 4, and the time-varying voltage is input to the piezoelectric transformer 1, whereby the frequency-voltage of the piezoelectric transformer 1 A desired voltage corresponding to the input frequency can be output according to the characteristics.

As shown in FIG. 2, the control unit 4 includes an output voltage setting unit 41 that sets the output voltage V ₁ ′ of the piezoelectric transformer 1. The output voltage setting unit 41 sets the output voltage V ₁ ′ of the piezoelectric transformer 1 based on the voltage signal detected by the output detection unit 3. Further, not only the information from the output voltage of the piezoelectric transformer 1 but also the output voltage V ₁ ′ can be set based on a command from the operation unit 6 that outputs a signal for setting an output voltage such as a brightness adjustment signal. .

The controller 4 includes a corresponding frequency detector 42 that detects whether the digital oscillator 5 can output the frequency F ₁ ′ corresponding to the set output voltage V ₁ ′. When the digital oscillator 5 cannot output the frequency F ₁ ′ corresponding to the output voltage V ₁ ′ of the piezoelectric transformer 1, the corresponding frequency detection unit 42 can output the frequency F ₁ across the corresponding frequency F ₁ ′. _1, to determine the _{F 2.}

When the digital oscillator 5 can output the frequency F ₁ ′ corresponding to the output voltage V ₁ ′ of the piezoelectric transformer 1, the control unit 4 sets the setting value n for outputting the frequency F ₁ ′ to the digital oscillator. A frequency output unit 45 for outputting is provided. Further, the frequency output unit 45 outputs a set value n for outputting a time-varying voltage to the digital oscillator 5 based on the alternating ratio set by the alternating ratio setting unit 43.

The control unit 4 includes an alternating ratio setting unit 43 that determines a ratio for alternating the frequencies F ₁ and F ₂ , and an alternating interval at which the alternating ratio setting unit 43 outputs a signal for switching the frequencies F ₁ and F ₂ according to the set ratio. And a control unit 44.

  As shown in FIG. 3, the digital oscillator 5 includes a clock generation circuit 51 that is a circuit that generates a reference frequency fs, and a counter 52 that constantly calculates a reference clock es of the frequency fs generated by the clock generation circuit. Further, the comparator 53 that outputs the divided pulse f1 based on the clock reference frequency fs and the set value n from the control unit 4, and the flip-flop circuit that is a retiming circuit for shaping the edge of the signal output from the comparator 53 54.

[2. Effect]
[2.1. Outline]
FIG. 5 is a diagram showing the relationship between the output voltage of the piezoelectric transformer 1 and the resonance frequency. As shown in FIG. 5, the output voltage of the piezoelectric transformer 1 is determined with respect to the frequency of the input voltage. However, since the resonance frequency of the piezoelectric transformer 1 changes due to environmental changes such as temperature and load, when the piezoelectric transformer 1 is driven at a constant frequency, a relative relationship between the resonance frequency and the frequency of the input voltage is obtained. Will change. That is, when the frequency of the input voltage is far away from the resonance frequency of the piezoelectric transformer 1, the voltage boost ratio by the piezoelectric transformer 1 is significantly reduced. Therefore, a sufficient current cannot be passed through the cold cathode tube 2 as a load, and the luminance of the cold cathode tube 2 cannot be maintained.

  Therefore, in order to cope with the change in the characteristics of the piezoelectric transformer 1, in the circuit of FIG. 1, the current flowing through the cold cathode tube 2 is detected by the output detector 3. When the characteristics of the resonance frequency of the piezoelectric transformer 1 change due to environmental changes such as temperature and load, the current flowing through the cold cathode tube 2 changes. The output detection unit 3 detects the change and sends a signal to the control unit 4.

  The control unit 4 outputs a signal to the digital oscillator 5 so that the current flowing through the cold cathode tube 2 is constant. The digital oscillator 5 drives the piezoelectric transformer 1 with a frequency based on the signal. By this control, the cold cathode fluorescent lamp 2 is lit with a substantially constant luminance.

However, when the piezoelectric transformer 1 is driven by a digital circuit, the frequency of the input voltage corresponding to the output voltage may not be generated due to the resolution of the CPU. In that case, a voltage substantially equal to the output voltage V ₁ ′ is generated by alternately generating the frequency F ₁ and the frequency F ₂ before and after the resonance frequency F ₁ ′ corresponding to the output voltage V ₁ ′ of the piezoelectric transformer 1. To do.

[2.2. Details]
Next, operation | movement of the control part 4 of this embodiment shown in FIGS. 1-3 is demonstrated concretely according to the flowchart of FIG. This process is a process of outputting a desired output voltage from the piezoelectric transformer 1 based on the voltage signal detected by the output detection unit 3 and the instruction from the operation unit 6.

First, as Step 1, the output voltage V ₁ ′ of the piezoelectric transformer 1 is set by the output voltage setting unit 41 based on the signal detected by the output detection unit 3 or by an instruction from the operation unit 6. The instruction from the operation unit 6 at this time is only the value of the output voltage V ₁ ′ of the piezoelectric transformer 1, and there is no need for the value of the alternating ratio of the alternating voltage.

In step 2, the frequency F ₁ ′ corresponding to the output voltage V ₁ ′ set in step 1 is detected. At this time, when a CPU having a high resolution is used, if the frequency F ₁ ′ is between frequencies that can be output, the frequency F ₁ ′ cannot be output because of the resolution. This determination is performed in step 3. If the digital oscillator 5 can generate the frequency F ₁ ′, the process proceeds to step 4; otherwise, the process proceeds to step 7.

Step 4 is a step when the digital oscillator 5 can output the frequency F ₁ ′ corresponding to the set output voltage V ₁ ′. In this step, the corresponding frequency detector 42 sets a set value n for the digital oscillator 5 to output the frequency F ₁ ′. Then, the frequency output unit 45 outputs the set value n to the digital oscillator 5.

In step 5, a voltage of frequency F ₁ ′ is generated by the digital oscillator 5 based on the set value n set in step 4. In this step, the reference frequency fs is generated inside the digital oscillator 5 by the clock generation circuit. The reference clock es of this reference frequency fs is always counted by the counter 52. The count value and the set value n set by the control unit 4 are input to the comparator 53. Whenever the count value matches the set value n, the comparator 53 operates to reset the counter 52 and output the divided pulse e1. In this case, the frequency f1 of the divided pulse e1 is f1 = fs / n. The digital oscillator 5 outputs the frequency f1 of the divided pulse e1 to the piezoelectric transformer 1 as the frequency F ₁ ′ corresponding to the voltage V ₁ ′.

Step 6 outputs an output voltage V ₁ ′ based on the voltage F ₁ ′ output from the digital oscillator 5.

On the other hand, if the frequency F ₁ ′ corresponding to the set output voltage V ₁ ′ cannot be output in step 3 because the resolution of the digital oscillator 5 is large, the process proceeds to step 7. In step 7, the corresponding frequency detection unit 42 determines _two frequencies F ₁ and F ₂ that can sandwich and output F ₁ ′. Then, in step 8, sets the setting value n1, n2 for digital oscillator 5 to output the frequencies _{F 1} and the frequency _{F 2.}

In step 9, the alternating ratio setting unit 43, the voltage _{V 1} and the voltage _{V 2} corresponding to the frequencies _{F 1} and the frequency _{F 2} is calculated, 'from a, [Delta] V ₂ -V _1' output voltages _{V 1,} [Delta] V ₁ ' to calculate the -V _1. In step 10, the alternating interval control unit 44 outputs the set values n 1 and n 2 to the digital oscillator 5 at an alternating ratio corresponding to ΔV ₂ : V ₁ ′: ΔV ₁ ′ −V ₁ .

In step 11, the digital oscillator 5 alternately generates frequencies F ₁ and F ₂ based on the set values n 1 and n 2 set in step 8 at the alternating ratio set in step 10. Thereafter, in step 12, an output voltage V ₁ ′ based on the time-varying voltage of the frequency F ₁ and the frequency F ₂ output from the digital oscillator 5 is output.

By the above operation, even when the frequency F ₁ ′ corresponding to the output voltage V ₁ ′ of the piezoelectric transformer 1 cannot be output, the two frequencies F ₁ and F ₂ sandwiching the corresponding frequency F ₁ ′ are alternately generated. Approximately, the output voltage of the piezoelectric transformer 1 can be generated. In particular, since the circuit of the piezoelectric transformer 1 generally has a large number of capacitor components, since the response of the voltage change is slow, the output voltage V ₁ between the voltage V ₁ and the voltage V ₂ is generated by alternately generating the frequency F ₁ and the frequency F _{2. 1} 'can be generated approximately.

  This eliminates the need to directly generate a frequency corresponding to the output voltage, so that a voltage corresponding to the resonance frequency of the piezoelectric transformer can be obtained without using a high-performance CPU that outputs a clock signal of 500 MHz to 1 GHz. It becomes possible to control.

[3. Other Embodiments]
In another embodiment of the present invention, a setting value preset in the alternating ratio setting unit may be set as the alternating ratio. By alternately generating the frequency F ₁ and the frequency F ₂ at a ratio of 1: 1, the resolution of the digital oscillator can be halved without calculating the alternating ratio. As a result, a frequency close to the frequency F ₁ ′ corresponding to the output voltage V ₁ ′ can be output by a simpler method.

DESCRIPTION OF SYMBOLS 1 ... Piezoelectric transformer 2 ... Cold cathode tube 3 ... Output detection part 4 ... Control part 5 ... Digital oscillator 6 ... Operation part 41 ... Output voltage setting part 42 ... Corresponding frequency detection part 43 ... Alternating ratio setting part 44 ... Alternating space control part 45 ... frequency output unit 51 ... clock generation circuit 52 ... counter 53 ... comparator 54 ... flip-flop circuit

Claims (5)

A piezoelectric transformer having a predetermined resonance frequency characteristic;
A digital oscillator that outputs a voltage of a predetermined frequency to the piezoelectric transformer,
An output detector for detecting the output voltage V ₁ ′ of the piezoelectric transformer;
A piezoelectric transformer drive circuit comprising: a control unit that controls a frequency F ₁ ′ output from the digital oscillator based on a detection signal from the output detection unit;
The controller is
An output voltage setting unit for setting an output voltage V ₁ ′ of the piezoelectric transformer based on a detection signal from the output detection unit;
When the digital oscillator cannot output the frequency F ₁ ′ corresponding to the output voltage V ₁ ′, a corresponding frequency detector that determines _two frequencies F ₁ and F ₂ that can be output across the corresponding frequency F ₁ ′ When,
A frequency output unit that alternately outputs the _two frequencies F ₁ and F ₂ to a piezoelectric transformer;
An alternating ratio setting unit for setting a ratio for alternately generating output voltages of piezoelectric transformers corresponding to the _two frequencies F ₁ and F ₂ ;
An alternating interval control unit that outputs a control signal for outputting the two frequencies F ₁ and F ₂ to the frequency output unit based on the alternating ratio set by the alternating ratio setting unit. A drive circuit for a piezoelectric transformer characterized by the above. A piezoelectric transformer having a predetermined resonance frequency characteristic;
A digital oscillator that outputs a voltage of a predetermined frequency to the piezoelectric transformer,
An output detector for detecting the output voltage V ₁ ′ of the piezoelectric transformer;
A piezoelectric transformer drive circuit comprising: a control unit that controls a frequency F ₁ ′ output from the digital oscillator based on a detection signal from the output detection unit;
The controller is
An output voltage setting unit for setting an output voltage V ₁ ′ of the piezoelectric transformer based on a detection signal from the output detection unit;
Said digital together, the _'frequencies F ₁ corresponding _to' the output voltages V ₁ wherein the digital oscillator _'frequencies F ₁ corresponding _to' the output voltages V ₁ to the output voltage setting unit is configured to detect whether it is possible to output When the oscillator cannot output, a corresponding frequency detector that determines _two frequencies F ₁ and F ₂ that can be output with the corresponding frequency F ₁ ′ sandwiched therebetween,
_'Outputs to the piezoelectric transformer, the output voltages V _1' Part frequencies F ₁ when the _'frequencies F ₁ corresponding _to' the output voltages V ₁ can output the frequencies F ₁ if it can not output frequencies F _{1 'corresponding} to A frequency output unit that alternately outputs _two frequencies F ₁ and F ₂ sandwiching 'to a piezoelectric transformer;
An alternating ratio setting unit for setting a ratio for alternately generating output voltages of piezoelectric transformers corresponding to the _two frequencies F ₁ and F ₂ detected by the corresponding frequency detecting unit;
An alternating interval control unit that outputs a control signal for outputting the two frequencies F ₁ and F ₂ in the frequency output unit based on the alternating ratio set by the alternating ratio setting unit to the frequency output unit; A drive circuit for a piezoelectric transformer, comprising: The alternating ratio set by the alternating ratio setting unit is expressed as ΔV based on the output voltages V ₁ and V ₂ corresponding to the frequencies F ₁ and F ₂ determined by the corresponding frequency detecting unit and the set output voltage V ₁ ′. _3. The piezoelectric transformer drive circuit according to claim 1, wherein the drive circuit is determined by a ratio of ₂₋ V ₁ ′: ΔV ₁ ′ −V ₁ .   The piezoelectric transformer drive circuit according to claim 1, wherein the alternating ratio setting unit sets a predetermined set value as an alternating ratio. A piezoelectric transformer having a predetermined resonance frequency characteristic; a digital oscillator that outputs a voltage having a predetermined frequency F ₁ ′ to the piezoelectric transformer; an output detection unit that detects an output voltage V ₁ ′ of the piezoelectric transformer; In a driving method of a piezoelectric transformer comprising: a control unit that controls a frequency F ₁ ′ output from the digital oscillator based on a detection signal from the output detection unit;
The step of setting the output voltage V ₁ ′ of the piezoelectric transformer based on the detection signal from the output detection unit and the case where the digital oscillator cannot output the frequency F ₁ ′ corresponding to the output voltage V ₁ ′ Determining _two frequencies F ₁ and F ₂ that can be output across the frequency F ₁ ′;
Setting a ratio for alternately generating output voltages of the piezoelectric transformer corresponding to the detected two frequencies F ₁ and F ₂ ;
And a step of outputting the two frequencies F ₁ and F ₂ to the piezoelectric transformer based on the set alternating ratio.

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