Piezoelectric transformer driver and power supply using the same

Abstract

To provide a drive technique of a piezoelectric transformer that has improved utility.SOLUTION: A drive device for a piezoelectric transformer includes: a piezoelectric transformer drive unit 110 for applying a drive voltage to a piezoelectric transformer PZT1 to be driven; a coupling circuit pad for taking out an output voltage from the piezoelectric transformer PZT1 through capacitive coupling; a multivibrator 130 for inputting and oscillating the taken-out output voltage feedback signal to control a frequency of the drive voltage applied to the piezoelectric transformer PZT1 by the piezoelectric transformer drive unit 110; and a λ / 2 frequency oscillation prevention unit 140 for preventing an oscillation frequency of the multivibrator 130 from deviating from a prescribed drive frequency.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a piezoelectric transformer driver and a power supply device that uses the same to generate an output voltage to be applied to a load. [Background technology]

[0002] Conventionally, there are known prior art technologies for controlling the drive voltage of piezoelectric transformers and power supply devices using piezoelectric transformers (see, for example, Patent Document 1). This prior art detects the output voltage value of the piezoelectric transformer and controls the output voltage by switching between an operation that variably controls the duty ratio of the drive voltage so that the output voltage value matches a set reference voltage value, and an operation that variably outputs a dropper control voltage. This makes it possible to perform output control corresponding to a wide voltage variable range and a variable power output width, and also improves overall efficiency by efficiently using frequency and duty ratio. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 4729468 Summary of the Invention [Problem to be solved by the invention]

[0004] The above prior art is extremely useful in that it can drive the piezoelectric transformer using an efficient duty ratio near the resonant frequency by automatically switching between two voltage controls using the output detection voltage of the piezoelectric transformer.

[0005] Furthermore, the demand for greater usability in driving and controlling piezoelectric transformers cannot be ignored. For example, when driving a piezoelectric transformer in a desired resonance mode (λ mode: resonating at the full wavelength λ), immediately after startup, an oscillation circuit with a frequency set near the λ mode resonance frequency is started, and the piezoelectric transformer is driven and controlled at that frequency.

[0006] On the other hand, the piezoelectric transformer itself resonates not only in the λ mode but also in the λ / 2 mode (half wavelength λ / 2) and 3λ / 2 mode (λ·3 / 2 wavelength), and since each mode has a frequency at which the step-up ratio is maximized, even if the resonant mode deviates from the desired one, it is possible for the resonance to continue in that mode. This phenomenon is likely to occur in drive control systems (so-called single-loop) in which the piezoelectric transformer's output voltage is input to the oscillator circuit as a feedback input voltage. One possible solution is to insert a band-pass filter into the single-loop to reduce the loop gain for frequencies other than the λ mode. However, this filter would be a high-order band-pass filter, which would pose obstacles to precision adjustment and eliminating deviations in the piezoelectric transformer itself, making it difficult to apply to actual products.

[0007] Therefore, the present invention provides a piezoelectric transformer driving technique that is more useful. [Means for solving the problem]

[0008] In order to solve the above problems, the present invention employs the following solution. Note that the following parenthetical expressions are merely examples, and the present invention is not limited thereto.

[0009] The present invention provides a driving device for a piezoelectric transformer. The present invention also provides a power supply device that uses the driving device for a piezoelectric transformer to generate an output voltage in the piezoelectric transformer to be applied to a load. The driving device for a piezoelectric transformer includes a driving circuit, an oscillation circuit, and a coupling circuit. Of these, the driving circuit applies a driving voltage to the piezoelectric transformer to be driven. The coupling circuit extracts the output voltage output from the piezoelectric transformer by capacitive coupling. Although the oscillation circuit has a basic circuit configuration that is self-oscillating, when the driving circuit controls the driving frequency of the driving voltage applied to the piezoelectric transformer, it oscillates using the output voltage extracted by the coupling circuit as a feedback input voltage.

[0010] In the driving device of the present invention, the frequency of the driving voltage is controlled by using the output voltage extracted by the coupling circuit as a feedback input voltage to the oscillation circuit as described above, and the following open loop is established for frequency control: the piezoelectric transformer outputs an output voltage at a frequency based on (dependent on) the frequency of the driving voltage applied by the driving circuit, the oscillation circuit oscillates based on (dependent on) the frequency of the feedback input voltage when the output voltage of the piezoelectric transformer is extracted by the coupling circuit, and the frequency of the driving voltage applied by the driving circuit is controlled based on (dependent on) the oscillation frequency of the oscillation circuit.

[0011] When driving and controlling a piezoelectric transformer using such a single-loop, it is possible to continue oscillation in the resonant mode at the frequency where the piezoelectric transformer's step-up ratio is maximized, as described above. However, it is difficult to stably maintain the desired resonant mode using only the single-loop configuration. For this reason, the present invention includes a deviation oscillation prevention circuit. The deviation oscillation prevention circuit prevents the frequency of the drive voltage applied to the piezoelectric transformer from deviating from the specified drive frequency. This maintains the frequency of the drive voltage within the single-loop close to the specified drive frequency, so by appropriately setting the "specified drive frequency," it is possible to continue oscillating the piezoelectric transformer in the desired resonant mode.

[0012] When the desired resonance mode is λ mode, if the resonance of the piezoelectric transformer changes to a resonance mode other than λ mode, the oscillation frequency of the oscillator circuit within the single loop will deviate from the specified drive frequency. In such a case, the deviation oscillation prevention circuit changes (forcibly changes) the frequency of the drive voltage controlled by the oscillator circuit to a frequency that returns to λ mode. This enables drive control that allows the piezoelectric transformer to continue oscillating stably in λ mode.

[0013] Preferably, the deviation oscillation prevention circuit can include a specific frequency detector and a frequency converter. The specific frequency detector detects whether the oscillation frequency of the oscillator circuit is in a specific frequency range different from a predetermined drive frequency. The frequency converter forcibly converts the frequency of the drive voltage controlled by the oscillator circuit to the predetermined drive frequency based on the detection result of the specific frequency detector. This effectively realizes a transition from frequency deviation detection to forcible frequency conversion (regeneration), ensuring reliable operation. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a piezoelectric transformer driving technique that is more useful. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a block diagram showing the overall configuration of a power supply device 100 using a piezoelectric transformer driver. [Figure 2] FIG. 1 is a conceptual diagram of a single loop formed in a driving device for a piezoelectric transformer PZT1. [Figure 3] FIG. 10 is a diagram showing an example of the circuit configuration of a driving device in which no measures are taken to deal with frequency deviation within a single loop. [Figure 4] 10 is a timing chart showing changes in various values ​​when a circuit without countermeasures is operating. [Figure 5] FIG. 10 is a diagram showing an example of the circuit configuration of a driving device in which a measure is taken against deviation of frequencies within a single loop. [Figure 6] 10 is a diagram showing an example of the relationship between the peak voltage of the inter-terminal voltage Vc1 caused by driving the piezoelectric transformer PZT1 and the oscillation frequency. FIG. [Figure 7] 10 is a timing chart showing changes in various values ​​when the countermeasure circuit operates. DETAILED DESCRIPTION OF THE INVENTION

[0016] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. Note that the following preferred embodiments are preferred examples, and the present invention is not limited to these examples.

[0017] [Overall structure] 1 is a block diagram showing the overall configuration of a power supply device 100 that uses a piezoelectric transformer driver. The power supply device 100 of this embodiment also includes an embodiment of the driver.

[0018] The power supply device 100 includes, for example, a Rosen-type three-terminal piezoelectric transformer PZT1, and can apply an output voltage Vo generated by this piezoelectric transformer PZT1 to a desired load L. To this end, the power supply device 100 includes a piezoelectric transformer driver 110 on the primary side of the piezoelectric transformer PZT1, and a rectifying and smoothing unit 120 on the secondary side.

[0019] The piezoelectric transformer driving unit 110 is connected to the primary electrodes T1 and T2 of the piezoelectric transformer PZT1, and the rectifying and smoothing unit 120 is connected to the secondary electrode T3 of the piezoelectric transformer PZT1. The piezoelectric transformer driving unit 110 switches the input voltage Vin using a switching element (e.g., MOSFET) and applies a driving voltage to the piezoelectric transformer PZT1 via the transformer. This drives the piezoelectric transformer PZT1, generating an output voltage that is a boost of the input voltage Vin.

[0020] The rectifying smoothing unit 120 rectifies the output voltage of the piezoelectric transformer PZT1 and supplies a DC power supply of an output voltage Vo (output current Io) to the load L. The rectifying smoothing unit 120 also has an output detection unit 122, which detects the output voltage of the piezoelectric transformer PZT1 and feeds back the divided voltage value Vt to the drive device side.

[0021] In addition, a detection electrode T4 is formed near the output electrode T3 on the secondary side of the piezoelectric transformer PZT1, and the output voltage of the piezoelectric transformer PZT1 is extracted through the detection electrode T4 (coupling circuit pad) due to the capacitive coupling between this detection electrode T4 and the piezoelectric transformer PZT1, and is fed back and input to the driver side of the piezoelectric transformer PZT1 as an output voltage feedback signal. It should be noted that it is also possible to use an output voltage feedback signal by directly attaching a capacitor to the output side of the output electrode T3, without using the detection electrode T4.

[0022] The driving device for the piezoelectric transformer PZT1 includes the piezoelectric transformer driving unit 110 and a multivibrator 130. When the multivibrator 130 starts up (when the input voltage Vin is turned on), it oscillates at a self-oscillation frequency, but once the piezoelectric transformer PZT1 starts outputting, it oscillates at a frequency determined by a single-loop using the output voltage feedback signal of the piezoelectric transformer PZT1, and controls the driving frequency of the piezoelectric transformer driving unit 110. As shown by the "multivibrator / amplifier" block in Figure 1, when oscillating at a frequency determined by a single-loop, the multivibrator 130 operates as an amplifier.

[0023] In this embodiment, the piezoelectric transformer driver 110 is controlled to target a drive frequency at which the piezoelectric transformer PZT1 continues to resonate in λ mode. To this end, the driver of the piezoelectric transformer PZT1 includes a λ / 2 frequency oscillation prevention unit 140. The λ / 2 frequency oscillation prevention unit 140 is disposed between the multivibrator 130 and the piezoelectric transformer driver 110, and includes a λ / 2 frequency detector 142 and a frequency converter 144. The λ / 2 frequency detector 142 detects when the oscillation frequency of the multivibrator 130 changes to a frequency range that deviates from the target drive frequency (e.g., a λ / 2 mode frequency), and activates the frequency converter 144. The frequency converter 144 forcibly converts the oscillation frequency of the multivibrator 130 back to the target drive frequency. Details of the λ / 2 frequency oscillation prevention unit 140 will be described later with reference to another drawing.

[0024] The driving device for the piezoelectric transformer PZT1 also includes a driving voltage control unit 150. The driving voltage control unit 150 has an error amplifier 152 and a duty ratio control unit 154. The error amplifier 152 compares the divided voltage value Vt of the output voltage with a set reference voltage value Vs, and applies a control voltage Vb obtained by amplifying the error to the duty ratio control unit 154 as a duty ratio control signal. The duty ratio control unit 154 receives the duty ratio control signal (control voltage Vb) and variably controls the duty ratio of the driving voltage that the piezoelectric transformer driving unit 110 applies to the piezoelectric transformer PZT1.

[0025] [One-circle loop] FIG. 2 is a conceptual diagram of a single loop formed in the driving device for the piezoelectric transformer PZT1. For example, if the amplifier 130, which also serves as a multivibrator, and the duty ratio control section 154 are defined as the "oscillating circuit," the piezoelectric transformer driving section 110 as "driving circuit 1," and the piezoelectric transformer PZT1 and the coupling circuit pad as "driving circuit 2," then these are connected in a single loop. This single loop is a positive feedback loop in which the output of the amplifier 130 is duty-ratio-controlled by the duty ratio control section 154 and input to the piezoelectric transformer driving section 110, whose output is input to the piezoelectric transformer PZT1, whose output is extracted by the coupling circuit pad and input to the amplifier 130.

[0026] When the entire drive device begins operating in the single-loop configuration shown in Figure 2, the multivibrator / amplifier 130 shifts from its own oscillation frequency to a frequency determined within the single-loop configuration. However, this single-loop configuration does not include any measures to monitor the oscillation frequency and forcibly convert or correct the frequency if it deviates from the desired frequency. Therefore, if the entire circuit continues to oscillate at the frequency determined by the single-loop configuration, and some factor (e.g., external disturbance) causes the resonance of the piezoelectric transformer PZT1 to deviate from λ mode and shift to another mode (e.g., λ / 2 mode), the multivibrator / amplifier 130 will also deviate from the original target oscillation frequency, and the entire single-loop configuration will continue to oscillate at a frequency that deviates from the original target. This point will be explained below.

[0027] [Example of circuit configuration without countermeasures] 3 is a diagram showing an example of the circuit configuration of a drive device in which no measures are taken to prevent frequency deviation within a single loop. Note that here, the configuration equivalent to the multivibrator 130 and drive voltage control unit 150 in FIG. 1 is represented by extremely simplified symbols (differential amplifiers and peripheral elements, etc.). Also, for the sake of convenience, the range of the multivibrator 130 is shown separately from the dual-purpose amplifier AMP1.

[0028] When the driver operates at the frequency determined by the single loop, the multivibrator 130 of such an uncountered circuit shifts from its own oscillation frequency to the frequency determined by the single loop. Therefore, the capacitor C1 of the multivibrator 130 repeatedly charges and discharges at the frequency of the single loop, and this frequency appears as a triangular wave in the inter-terminal voltage Vc1.

[0029] The drive voltage control unit 150, represented by the error amplifier COMP1, compares the duty ratio control signal (error voltage output signal Vb) with the terminal voltage Vc1 of the capacitor C1, and when the terminal voltage Vc1 exceeds the output signal Vb, the output voltage goes low, causing the input voltage of the piezoelectric transformer drive unit 110 (inverting switching circuit) to go low. The output of the error amplifier COMP1 is, for example, an open collector. As the duty ratio control signal (error voltage output signal Vb) rises, the time during which the output voltage of the error amplifier COMP1 remains high increases, and therefore the output voltage of the piezoelectric transformer PZT1 is varied by duty ratio control.

[0030] [Example of operation of an unmeasured circuit] Figure 4 is a timing chart showing the changes in various values ​​when a circuit without countermeasures is operating. Of these, (A) shows the feedback input voltage (pad signal) from the coupling circuit pad, (B) shows the output ON / OFF state of amplifier AMP1, (C) shows the inter-terminal voltage Vc1 of capacitor C1 and the control voltage Vb, and (D) shows the high / low output Vosc level of error amplifier COMP1. For ease of illustration, the horizontal axis in Figure 4 shows time in a greatly abbreviated manner (the number of waveform repetitions is omitted). The following explanation follows the time series on the horizontal axis.

[0031] [Time t0-t1] For example, consider a case where the power supply to the drive device is turned on at time t0, and time t1 is the state immediately after the power supply is turned on. In Figure 4 (A): Immediately after power-on, the pad signal is not yet accurately observed. In FIG. 4(B): Immediately after the power is turned on, the amplifier AMP1 is turned off. In FIG. 4(C): The terminal voltage Vc1 is at a valley (discharge state). In FIG. 4(D): The output Vosc of the error amplifier COMP1 is at a low level.

[0032] [Time t1-t8] For example, from time t1 to time t8, the entire driving device oscillates at the frequency of the multivibrator 130 itself. The oscillation frequency at this time is set as a target frequency at which the piezoelectric transformer PZT1 resonates in the λ mode. In Figure 4 (A): Because this is the pre-oscillation stage of the first loop, the pad signal has not yet been accurately observed. In FIG. 4(B): The output of the amplifier AMP1 repeatedly turns ON and OFF at the oscillation frequency of the multivibrator 130 itself (times t1, t3, t4, t6, t7). In FIG. 4(C): As a result, the capacitor C1 repeatedly charges and discharges (times t1, t3, t4, t6, t7), and the inter-terminal voltage Vc1 rises and falls in a triangular wave shape. In Figure 4 (D): The output Vosc of the error amplifier COMP1 goes high when the terminal voltage Vc1 starts to rise from the valley voltage of the triangular wave (times t1, t4, t7), and goes low when the terminal voltage Vc1 exceeds the control voltage Vb (times t2, t5, t8). A drive voltage is applied to the piezoelectric transformer PZT1 with a duty ratio that is the same as while the amplifier AMP1 is ON and the output Vosc is low (times t2-t3, t5-t6).

[0033] [Time t8-t14] Next, for example, from time t8 to time t14, the entire drive device oscillates at a frequency determined by the pad signal (single loop). The piezoelectric transformer PZT1 resonates in the initially targeted λ mode. In FIG. 4(A): As the piezoelectric transformer PZT1 is driven, a pad signal is fed back and input. In FIG. 4(B): The output of the amplifier AMP1 repeatedly turns ON and OFF at an oscillation frequency determined by the pad signal (single loop) (times t9, t10, t12, t13). In Figure 4 (C): Capacitor C1 repeatedly charges and discharges at the same frequency (times t9, t10, t12, and t13), causing the inter-terminal voltage Vc1 to rise and fall in a triangular waveform. The triangular waveform has a different profile from the one up to time t8. FIG. 4(D): Similarly, the output Vosc repeats high and low states, and the λ mode oscillation continues.

[0034] Up to this point, the timing chart shows the case where the entire drive unit oscillates at a frequency determined by the pad signal (single loop) and the piezoelectric transformer PZT1 continues to resonate in λ mode. However, even if the mode shifts to a mode other than λ mode for some reason, as described above, the following operation will occur.

[0035] [From time t14 onwards] Even after time t14, the entire driving device oscillates at a frequency determined by the pad signal, but consider a case where the piezoelectric transformer PZT1 shifts to a mode other than the λ mode, for example, a λ / 2 mode. In Figure 4 (A): The feedback input pad signal has a frequency in the λ / 2 mode. FIG. 4B: As a result, the output of the amplifier AMP1 begins to repeatedly turn on and off at the oscillation frequency of the λ / 2 mode (times t15 and t16). In Figure 4 (C): Capacitor C1 also repeats charging and discharging at the λ / 2 mode oscillation frequency (times t15 and t16), and the triangular waveform of terminal voltage Vc1 changes to the profile for λ / 2 mode oscillation. The peak voltage value at this time is higher than the peak voltage value for λ mode oscillation because the wavelength is longer than before. FIG. 4(D): Then, the output Vosc of the error amplifier COMP1 repeats high and low at the oscillation frequency of the λ / 2 mode, and the piezoelectric transformer PZT1 is driven and controlled in the λ / 2 mode.

[0036] [Summary of circuits that have not been addressed] In this way, in the operation of an uncountered circuit, even if the oscillation frequency changes to a frequency range different from the target frequency (λ mode), oscillation continues without any problems due to the single-loop. Therefore, the piezoelectric transformer PZT1 continues to be driven in a resonance mode different from the original target, but until now there has been no method for actively correcting this state.

[0037] [Example of countermeasure circuit configuration] Therefore, the inventors of the present invention propose the following countermeasure circuit suitable for the driving device of the piezoelectric transformer PZT1. 5 is a diagram showing an example of the circuit configuration of a driving device that has measures in place to deal with the frequency deviation within a single loop. The countermeasure circuit has the configuration of a λ / 2 frequency oscillation prevention unit 140, and here, the configuration equivalent to the λ / 2 frequency oscillation prevention unit 140 in FIG. 1 is represented by an extremely simplified symbol (such as a differential amplifier).

[0038] The countermeasure circuit in FIG. 5 is similar to the circuit without countermeasure in FIG. 3 in that it uses a single loop to control the frequency to near the frequency at which the step-up ratio of the piezoelectric transformer PZT1 is maximized, and that it uses control voltage Vb to control the duty ratio of the output voltage of the piezoelectric transformer PZT1.

[0039] However, the countermeasure circuit of FIG. 5 adds an error amplifier COMP2, so that it functions as the λ / 2 frequency oscillation prevention unit 140 shown in FIG. 1. This error amplifier COMP2 monitors the terminal voltage Vc1 (triangular wave) of capacitor C1 and constantly compares the terminal voltage Vc1 with a predetermined determination voltage Vdet. If the oscillation frequency of the entire drive device shifts to a lower frequency than the initial target, the charge / discharge cycle of capacitor C1 becomes longer, and the terminal voltage Vc1 exceeds the determination voltage Vdet, the error amplifier COMP2 functions and drives the inverting input terminal of the error amplifier COMP1 low. As a result, even if the terminal voltage Vc1 exceeds the control voltage Vb and the output of the error amplifier COMP1 is low, the output of the error amplifier COMP1 is forcibly changed to high when the terminal voltage Vc1 exceeds the determination voltage Vdet.

[0040] Here, the value of the terminal voltage Vc1 (triangular wave) monitored by the error amplifier COMP2 is determined by the charge / discharge time constant and frequency of the capacitor C1. If the charge and discharge time constants are each constant, the peak and valley voltages of the terminal voltage Vc1 will be determined depending on the frequency. From this relationship, we can see that if the frequency becomes lower than before, the peak voltage of the terminal voltage Vc1 will increase and conversely, the valley voltage will decrease (see (C) in Figure 4).

[0041] When the above relationship is replaced with the resonance mode of the piezoelectric transformer PZT1, when the entire drive circuit changes from the oscillation operation in the λ mode to the oscillation operation in the λ / 2 mode, the peak voltage (Vhλ) of the terminal voltage Vc1 becomes higher than the peak voltage (Vλ) during the oscillation in the previous λ mode. At this time, if the value of the determination voltage Vdet is set such that Vλ < Vdet < Vhλ, when the entire drive circuit operates at the frequency of the λ / 2 mode, the output of the error amplifier COMP2 monitoring the terminal voltage Vc1 becomes Low. Therefore, even if the terminal voltage Vc1 exceeds the control voltage Vb on the drive voltage control unit 150 side, the output Vosc of the error amplifier COMP1 can be forcibly changed to the High level. As a result, the input of the piezoelectric transformer drive unit 110 is converted to a frequency twice that of the λ / 2 mode frequency, so that oscillation in the λ / 2 mode can be prevented. The basis for the usefulness of monitoring the terminal voltage Vc1 in this way for frequency conversion is shown below.

[0042] 〔Relationship between the peak voltage (Vtop) of the terminal voltage Vc1 and the frequency〕 Fig. 6 is a diagram showing an example of the relationship between the peak voltage of the terminal voltage Vc1 and the oscillation frequency accompanying the drive of the piezoelectric transformer PZT1. Here, when the duty ratio (for example, 50%) is fixed and the charge and discharge of the capacitor C1 are repeated with a variable period, the peak voltage of the terminal voltage Vc1 is plotted on the vertical axis with the frequency on the horizontal axis. The value of the peak voltage Vtop is calculated using a general formula from parameters such as, for example, the period T, the charging time constant τ h , the discharging time constant τ1, and the power supply voltage Vreg of the drive device.

[0043] The inventors of the present invention performed trial calculations for the frequency fλ (kHz) in λ mode and the frequency fhλ (kHz) in λ / 2 mode and found that at a relatively high frequency fλ (kHz), the peak voltage of the terminal voltage Vc1 is Vλ (V), but at a frequency fhλ (kHz) that is about half this, the peak voltage of the terminal voltage Vc1 rises to Vhλ (V), a voltage increase of more than 1.4 times. From this, the inventors of the present invention noticed that by monitoring the terminal voltage Vc1 in the drive device and detecting a significant increase in its value, it can be determined that the entire drive device is oscillating at a lower frequency than before.

[0044] [Example of countermeasure circuit operation] The above operation will be explained using a timing chart. Figure 7 is a timing chart showing the changes in various values ​​when the countermeasure circuit operates. Of these, (A) shows the feedback input voltage (pad signal) from the coupling circuit pad, (B) shows the output ON / OFF state of amplifier AMP1, (C) shows the voltage Vc1 across capacitor C1, the control voltage Vb, and the determination voltage Vdet, (D) shows the high-low output levels of error amplifier COMP2, and (E) shows the high-low output Vosc levels of error amplifier COMP1. Note that in (C), the determination voltage Vdet is greater than the control voltage Vb. For ease of illustration, the horizontal axis shows time in a greatly abbreviated form (the number of waveform repetitions is omitted).

[0045] [Time t0, time t1-t8] Similarly, for example, the power supply to the driving device is turned on at time t0, and immediately thereafter, from time t1 to time t8, the entire device oscillates at the frequency of the multivibrator 130 itself. Therefore, the changes in (A), (B), (C), and (E) in Figure 7 are the same as those in (A), (B), (C), and (D) in Figure 4, respectively. 7(D): During this time, the output of the added error amplifier COMP2 is maintained at a high level because the inter-terminal voltage Vc1 does not exceed the determination voltage Vdet. Therefore, during this period, the λ / 2 frequency oscillation prevention unit 140 does not intervene in the oscillation frequency, and the output Vosc of the error amplifier COMP1 is not forcibly changed.

[0046] [Time t8-t14] Next, for example, from time t8 to time t14, the entire driving device oscillates at a frequency determined by the pad signal (single loop), but the piezoelectric transformer PZT1 resonates in the initially targeted λ mode. Therefore, here too, the changes in (A), (B), (C), and (E) in Figure 7 are the same as those in (A), (B), (C), and (D) in Figure 4, respectively. 7(D): Because the piezoelectric transformer PZT1 is resonating in the λ mode, the inter-terminal voltage Vc1 does not exceed the determination voltage Vdet during this period either. Therefore, the λ / 2 frequency oscillation prevention unit 140 does not intervene in the oscillation frequency during this period either.

[0047] [From time t14 onwards] After time t14, if the piezoelectric transformer PZT1 shifts to a mode other than the λ mode, for example, to the λ / 2 mode, the following occurs. In Figure 7 (A): The feedback input pad signal has a frequency in the λ / 2 mode. In FIG. 7(B): The output of the amplifier AMP1 begins to repeatedly turn on and off at the oscillation frequency of the λ / 2 mode (times t16 and t18). In Figure 7 (C), capacitor C1 also repeats charging and discharging at the λ / 2 mode oscillation frequency (times t16 and t18), and the triangular waveform of terminal voltage Vc1 changes to the profile of λ / 2 mode oscillation. The peak voltage value at this time is higher than the peak voltage value during λ mode oscillation (for example, about 1.4 times). In FIG. 7(D): When the inter-terminal voltage Vc1 exceeds the determination voltage Vdet, the output of the error amplifier COMP2 becomes Low (time t15-t17). FIG. 7(E): As a result, even if the inter-terminal voltage Vc1 exceeds the control voltage Vb, the output Vosc of the error amplifier COMP1 is forced to change from low to high level (time t15-t16).

[0048] Although not shown, after time t18, when the inter-terminal voltage Vc1 exceeds the determination voltage Vdet, the error amplifier COMP2 goes low and converts the frequency of the output Vosc of the error amplifier COMP1. As a result, the drive frequency of the piezoelectric transformer driver 110 is forcibly changed, and the piezoelectric transformer PZT1 can be restored to the initially targeted λ-mode resonance.

[0049] In the power supply device 100 of Figure 1, the λ / 2 frequency oscillation prevention unit 140 monitors the inter-terminal voltage Vc1 of the multivibrator 130 and performs detection based on a comparison with the determination voltage Vdet. Therefore, when the oscillation frequency of the multivibrator 130 reaches the λ / 2 mode frequency, the λ / 2 frequency detector 142 detects this, and the frequency converter 144 forcibly converts the output of the multivibrator 130 to the λ mode frequency. This restores the drive frequency of the piezoelectric transformer drive unit 110, allowing the piezoelectric transformer PZT1 to return to λ mode resonance and subsequently maintain the λ mode stably.

[0050] The power supply device 100 and the piezoelectric transformer driving device of the above-described embodiment provide the following advantages. (1) The drive unit operates autonomously and can stably maintain the resonant frequency of the piezoelectric transformer PZT1 at the set target frequency. This makes it possible to provide a robust drive control system that has been difficult to achieve until now. (2) A control system can be constructed using only general-purpose circuit elements (such as error amplifiers) without using particularly high-precision circuit elements (such as bandpass filters) or digital elements, thereby reducing the number of parts, manufacturing steps, and costs. (3) The set value (determination voltage Vdet, etc.) applied to the λ / 2 frequency oscillation prevention unit 140 is uniquely determined from the specifications of the element (capacitor C1) used, so that individual circuit design and adjustment are clear and easy. (4) The output voltage of the power supply device 100 can be stabilized, making it possible to provide a highly reliable product.

[0051] The present invention is not limited to the above-described embodiment, and can be practiced in various modified forms. The circuit configurations shown in FIGS. 1 and 5 are both simple, and when implementing the drive device and power supply device, other necessary peripheral elements, switching elements, IC elements, etc. can be applied as appropriate.

[0052] In one embodiment, the λ mode oscillation frequency is set as the target and then shifted to λ / 2 mode oscillation. However, the method of detecting deviation from the target by monitoring the inter-terminal voltage Vc1 can also be applied to cases where the target frequency is set to a mode other than λ mode. For example, if the λ 3 / 2 mode is set as the target frequency, deviation of the single-loop oscillation frequency to the lower frequency side can be detected, and the frequency can be converted in the same way as in the first embodiment.

[0053] The piezoelectric transformer is not limited to a three-terminal type, but may be a four-terminal type. Furthermore, the specifications and product dimensions of the piezoelectric transformer are arbitrary, and an appropriate one can be adopted depending on the intended use.

[0054] In addition, the various values ​​and waveforms in the timing chart shown in FIG. 7 are merely examples, and it goes without saying that the actual observed values ​​will differ depending on the specific circuit configuration of each individual. [Explanation of symbols]

[0055] 100 Power supply device using piezoelectric transformer driver 110 Piezoelectric transformer drive unit 120 Rectification and smoothing section 122 Output detection unit 130 Multivibrator and amplifier 140 λ / 2 frequency oscillation prevention section 142 λ / 2 frequency detector 144 Frequency Converter PZT1 piezoelectric transformer pad coupling circuit

Claims

1. a drive circuit that applies a drive voltage to the piezoelectric transformer to be driven; a coupling circuit that extracts an output voltage output from the piezoelectric transformer by capacitive coupling in response to application of a drive voltage; an oscillation circuit that oscillates using the output voltage extracted by the coupling circuit as a feedback input voltage and controls the frequency of the drive voltage that the drive circuit applies to the piezoelectric transformer; a deviation oscillation prevention circuit that prevents the frequency of the drive voltage applied to the piezoelectric transformer from deviating from a predetermined drive frequency by changing the frequency of the drive voltage controlled by the oscillation circuit to a frequency that returns the resonance mode of the piezoelectric transformer to the target resonance mode when the resonance mode of the piezoelectric transformer changes to a resonance mode different from a target resonance mode due to the application of a drive voltage; A piezoelectric transformer driving device comprising:

2. 2. The piezoelectric transformer driving device according to claim 1, The differential oscillation prevention circuit includes: A piezoelectric transformer driving device characterized in that, when the application of a driving voltage causes the resonance of the piezoelectric transformer to change to a resonance mode different from the λ mode, causing the oscillation frequency of the oscillation circuit to deviate from the specified driving frequency, the frequency of the driving voltage controlled by the oscillation circuit is changed to a frequency that returns the resonance of the piezoelectric transformer to the λ mode.

3. 3. The piezoelectric transformer driving device according to claim 1, The differential oscillation prevention circuit includes: a specific frequency detector that detects whether the oscillation frequency of the oscillation circuit is in a specific frequency range different from the predetermined drive frequency; a frequency converter that forcibly converts the frequency of the drive voltage controlled by the oscillation circuit to the predetermined drive frequency based on the fact that the oscillation frequency of the oscillation circuit is detected by the specific frequency detector to be within the specific frequency range; A piezoelectric transformer driving device comprising:

4. 4. A power supply device using the piezoelectric transformer driving device according to claim 1, in which the piezoelectric transformer generates an output voltage to be applied to a load.

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