Piezoelectric transformer driver and power supply using the same

Abstract

To provide a driving technique for a piezoelectric transformer with further improved serviceability.SOLUTION: A driving device for a piezoelectric transformer comprises: a switching circuit 110 for applying a driving voltage to a to-be-driven piezoelectric transformer PZT1; a coupling circuit pad for taking out an output voltage from the piezoelectric transformer PZT1 by capacitive coupling; and a multi-vibrator 130 to which a taken-out output voltage feedback signal is input to oscillate to allow the switching circuit 110 to control a driving frequency of the driving voltage applied to the piezoelectric transformer PZT1. An oscillation circuit has a phase leading circuit 132 for varying a leading phase during the oscillation.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, there is a need for greater utility in the drive control of piezoelectric transformers. For example, from the perspective of driving piezoelectric transformers more efficiently, it is necessary to pinpoint the actual drive frequency to the frequency at which the piezoelectric transformer's step-up ratio is maximized. However, piezoelectric transformers have a high Q value, which is a characteristic of their resonance, so extremely high precision is required for controlling the drive frequency. Furthermore, when the output voltage of a piezoelectric transformer is used as a power source for a load, the frequency at which the step-up ratio is maximized and the step-up ratio itself vary depending on the load. Therefore, it is not possible to control the drive frequency by setting a uniform value for the frequency at which the step-up ratio is maximized.

[0006] Therefore, the present invention provides a piezoelectric transformer driving technique that is more useful. [Means for solving the problem]

[0007] In order to solve the above problems, the present invention employs the following means for solving the problems. Note that the following parenthetical expressions are merely examples, and the present invention is not limited to these.

[0008] The present invention provides a piezoelectric transformer driver. The present invention also provides a power supply device that uses the piezoelectric transformer driver to generate an output voltage in the piezoelectric transformer to be applied to a load. The piezoelectric transformer driver includes a drive circuit, an oscillation circuit, and a coupling circuit. The drive circuit applies a predetermined drive frequency to the piezoelectric transformer to be driven. Note that "predetermined" here does not have a uniform meaning, but also includes frequencies determined by external voltage signals. The coupling circuit extracts the output voltage from the piezoelectric transformer through capacitive coupling. While the oscillator circuit has a basic circuit configuration that is self-oscillating, when the drive circuit controls the drive frequency of the drive voltage applied to the piezoelectric transformer, it oscillates using the output voltage extracted by the coupling circuit as a feedback input voltage.

[0009] In the driving device of the present invention, the drive frequency of the drive circuit 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 drive frequency of the drive 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 drive frequency of the drive circuit is controlled based on (dependent on) the oscillation frequency of the oscillation circuit.

[0010] Within this single loop, the piezoelectric transformer driver of the present invention has an oscillator circuit that includes a phase lead circuit. The phase lead circuit varies the phase lead (amount of phase lead) of the oscillator circuit itself when it oscillates. Here, "phase lead" refers to the phase of the output voltage leading relative to the phase of the input voltage, and can be thought of as a phase angle (°). Therefore, the phase lead circuit varies the angle by which the phase of the output voltage of the oscillator circuit leads, which is the "phase lead."

[0011] When the lead phase of the oscillator circuit is changed within a single loop, the effect is reflected in the lead phase of the piezoelectric transformer. Within a single loop, the lead phase of the oscillator circuit and the lead phase of the piezoelectric transformer are in a trade-off relationship; the smaller the lead phase of the oscillator circuit, the larger the lead phase of the piezoelectric transformer.

[0012] On the other hand, it is known that the phase lead from the input voltage to the output voltage of a piezoelectric transformer has a fixed relationship with the frequency at which the step-up ratio is maximized. In other words, the value of the phase lead of a piezoelectric transformer is approximately 90° at a frequency slightly higher than the frequency at which the step-up ratio is maximized. It is known that this relationship hardly fluctuates even with changes in the load when a piezoelectric transformer is used as a power supply, or changes in the frequency at which the step-up ratio is maximized.

[0013] The inventors of the present invention realized that by utilizing the above characteristics and varying the lead phase in the oscillator circuit, it would be possible to control the drive frequency of the piezoelectric transformer to be close to the frequency at which the step-up ratio is maximized. In other words, when the phase lead circuit varies the lead phase of the oscillator circuit, it changes the lead phase of the piezoelectric transformer in the opposite direction, thereby eliminating the deviation of the drive frequency from the frequency at which the maximum step-up ratio is obtained.

[0014] The above single loop as a whole forms a single oscillation circuit. At this time, the phase change of the entire single loop is 0°, but within the single loop, the lead phase in the drive circuit is -180° (phase inversion), and the lead phase in the coupling circuit is 90°. Therefore, the oscillation circuit of the entire single loop continues to oscillate at a frequency where the sum of the lead phase (φpzt) of the remaining piezoelectric transformer and the lead phase (φc) of the oscillation circuit is 90°.

[0015] By utilizing this characteristic, the phase lead circuit can bring the lead phase of the piezoelectric transformer closer to 90° by bringing the lead phase of the oscillator circuit within the single loop closer to 0°. As mentioned above, the lead phase of the frequency at which the piezoelectric transformer's maximum step-up ratio is obtained is approximately 90°, so bringing the lead phase closer to 90° maximizes the step-up ratio of the piezoelectric transformer (driving the piezoelectric transformer at the frequency of the maximum step-up ratio), resulting in improved drive efficiency.

[0016] The piezoelectric transformer driver of the present invention may further include an error amplifier that receives a feedback input of a detected value of the output voltage of the piezoelectric transformer applied to the load, and applies a phase control voltage to the phase lead circuit based on a comparison with a predetermined reference voltage.

[0017] As mentioned above, piezoelectric transformers have a resonant characteristic in which the frequency at which the maximum step-up ratio is achieved and the step-up ratio itself fluctuate depending on the load to which the output voltage is applied. Therefore, when the detected output voltage is fed back and compared with the reference voltage, if the error becomes large, it means that the piezoelectric transformer is not being driven at its maximum step-up ratio. In this case, the phase lead circuit controls the oscillator circuit to decrease the lead phase as the phase control voltage increases. This controls the drive frequency of the piezoelectric transformer to be close to the frequency at which the maximum step-up ratio is achieved, improving drive efficiency. [Effects of the Invention]

[0018] According to the present invention, it is possible to provide a piezoelectric transformer driving technique that is more useful. [Brief explanation of the drawings]

[0019] [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 and a leading phase configured in a driving device for a piezoelectric transformer PZT1. [Figure 3] FIG. 10 is a diagram showing the relationship between the leading phase φpzt of the piezoelectric transformer PZT1 and the step-up ratio. [Figure 4] 1 is a diagram showing a part of a configuration corresponding to a drive device for a piezoelectric transformer PZT1 in a power supply device 100. FIG. [Figure 5] 10 is a diagram showing how the lead phase φpzt of the piezoelectric transformer PZT1 is controlled as the lead phase φc is varied. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0020] 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.

[0021] [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.

[0022] 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 switching circuit 110 on the primary side of the piezoelectric transformer PZT1, and a rectifying and smoothing unit 120 on the secondary side.

[0023] The switching circuit 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 switching circuit 110 switches the input voltage Vin using a switching element (e.g., a MOSFET) and applies a drive 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.

[0024] The rectifying smoothing unit 120 rectifies the output voltage of the piezoelectric transformer PZT1 to produce an output voltage Vo. 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 Vdet to the drive device side.

[0025] 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.

[0026] The driving device for the piezoelectric transformer PZT1 includes a multivibrator 130 in addition to the above-mentioned switching circuit 110. The multivibrator 130 oscillates at a self-oscillation frequency when started (when the input voltage Vin is turned on), 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 switching circuit 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.

[0027] Here, the driver for the piezoelectric transformer PZT1 includes a phase lead circuit 132 and an error amplifier 134. Of these, the phase lead circuit 132 compares a triangular wave voltage generated in association with oscillation of the single loop with the output voltage Vb of the error amplifier 134, and varies the lead phase of the oscillation generated in the single loop. The triangular wave voltage is the voltage between the terminals of a capacitance element (not shown) built into the multivibrator 130 to GND.

[0028] The error amplifier 134 compares the divided value Vdet of the output voltage with a reference voltage Vref, and applies the output voltage Vb obtained by amplifying the error as a phase control signal to the phase lead circuit 132. That is, the phase lead circuit 132 decreases the amount of phase lead when the output voltage Vb of the error amplifier 134 increases, and increases the amount of phase lead when the output voltage Vb decreases. The phase control of the phase lead circuit 132 will be described in more detail below.

[0029] [One-circle loop] FIG. 2 is a conceptual diagram of a single loop and a leading phase configured in the driving device of the piezoelectric transformer PZT1. 2(A): For example, if the amplifier 130, which also serves as a multivibrator, and the phase lead circuit 132 are defined as the "oscillating circuit," the switching circuit 110 as the "drive circuit 1," and the piezoelectric transformer PZT1 and the coupling circuit pad as the "drive circuit 2," then these are connected in a single loop. Such a single loop is a positive feedback loop in which the output of the amplifier 130 is input to the switching circuit 110 via the phase lead circuit 132, the output of which is input to the piezoelectric transformer PZT1, and the output of which is extracted by the coupling circuit pad and input to the amplifier 130.

[0030] [Lead phase within a single loop] In Figure 2 (B): If the vertical axis represents the lead phase (°) and the horizontal axis represents the section (dimensionless) where each circuit forms a single loop, the section p0-p1 on the horizontal axis corresponds to the piezoelectric transformer PZT1, the section p1-p2 corresponds to the coupling circuit pad formed by the capacitive coupling of the output electrode T3 and detection electrode T4 of the piezoelectric transformer PZT1, the section p2-p3 corresponds to the amplifier 130, the section p3-p4 corresponds to the phase lead circuit 132, the section p4-p5 corresponds to the connecting line from the phase lead circuit 132 to the switching circuit 110, the section p5-p6 corresponds to the switching circuit 110, and the section p6-p0 corresponds to the connecting line from the switching circuit 110 to the piezoelectric transformer PZT1.

[0031] For example, let us consider the phase of the output voltage of the piezoelectric transformer PZT1 as the starting point (0°), and the phase change from this point as a leading phase. Below, the leading phase within a single loop will be explained by dividing it into sections. Section p1-p2: The coupling circuit pad is a capacitive coupling between the output electrode T3 and detection electrode T4 of the piezoelectric transformer PZT1, so the phase lead between the input and output is 90°. Note that the horizontal axis is dimensionless, so there is no difference due to the slope of the phase lead. Section p2-p3: Since the amplifier 130 uses the non-inverting input terminal, the phase does not change here and the leading phase is 0°. Section p3-p4: As described above, the phase lead circuit 132 varies the lead phase φc (°) by comparing the triangular wave voltage with the output voltage Vb of the error amplifier 134. For convenience, it is assumed here that the phase lead circuit 132 has a lead phase φc. Section p4-p5: There is no change in phase on the connecting line, and the leading phase is 0°. Section p5-p6: Since the switching circuit 110 has inverted polarity, the phase here is delayed by 180°. Therefore, the leading phase is −180°. Section p6-p0: There is no change in phase on the connecting line, and the leading phase is 0°. Section p0-p1: The piezoelectric transformer PZT1 has an output voltage phase that leads relative to the input voltage, but this lead phase φpzt (°) returns to the starting point (0°) through a single loop. Therefore, within the single loop, the relationship φpzt+φc=90° uniquely holds between this and the lead phase φc (°) in the phase lead circuit 132.

[0032] In other words, the single-loop oscillation circuit continues to oscillate at a frequency where the lead phase φpzt of the piezoelectric transformer PZT1 satisfies the above relationship (φpzt + φc = 90°). Therefore, it can be seen that by variably controlling the lead phase φc with the phase lead circuit 132, the lead phase φpzt of the piezoelectric transformer PZT1 can be controlled in conjunction with it. In this case, there is a trade-off between φpzt and φc within 90°.

[0033] [Relationship between lead phase φpzt and boost ratio] FIG. 3 is a diagram showing the relationship between the leading phase φpzt of the piezoelectric transformer PZT1 and the step-up ratio. 3(A): As mentioned above, the phase lead φpzt of the piezoelectric transformer PZT1 is the amount of phase lead of the output voltage vo relative to the phase of the input voltage vi, and this is shown in the figure. Also, the frequency and step-up ratio that result in the maximum step-up ratio of the piezoelectric transformer PZT1 here depend on the resistance value of the load R. In Figure 3 (B): The relationship between the step-up ratio (times) of the piezoelectric transformer PZT1 and the drive frequency (kHz) is shown by a solid line for each resistance value, and the relationship between the lead phase φpzt and the drive frequency (kHz) is shown by a dashed line for each resistance value of the load R. Here, resistance values ​​A (MΩ), B (MΩ), and C (MΩ) are given as examples (note that A>B>C).

[0034] The following characteristics are evident from FIG. [Frequency at which the maximum boost ratio is achieved] For resistance values ​​A (MΩ), B (MΩ), and C (MΩ), there is a frequency at which the step-up ratio of the piezoelectric transformer PZT1 is maximum (hereinafter referred to as the "maximum step-up ratio frequency"), and this maximum step-up ratio frequency is close for all resistance values ​​A (MΩ), B (MΩ), and C (MΩ). However, it can be seen that the maximum step-up ratio depends on the resistance value of the load R, being largest for resistance value A (MΩ) and smallest for resistance value C (MΩ).

[0035] [Leading phase with maximum boost ratio frequency] The leading phase φpzt is approximately 90° at a frequency slightly higher than the maximum step-up ratio frequency. This relationship remains constant for all resistance values ​​A (MΩ), B (MΩ), and C (MΩ), and is hardly affected by changes in the maximum step-up ratio frequency itself.

[0036] [Maximum frequency control of the step-up ratio of piezoelectric transformers] FIG. 4 is a diagram partially illustrating the configuration of the power supply device 100 that corresponds to the drive device for the piezoelectric transformer PZT1. This diagram particularly illustrates the configuration of the portion related to phase control within the single loop. FIG. 5 is a diagram illustrating how the lead phase φpzt of the piezoelectric transformer PZT1 is controlled in accordance with the variation of the lead phase φc within the single loop. The following describes the maximum frequency step-up ratio control of the piezoelectric transformer PZT1 by the phase lead circuit 132.

[0037] [Before first loop oscillation] As shown in FIG. 4, for example, immediately after power-on of the input voltage Vin, if the output voltage feedback signal from the coupling circuit pad is equal to or lower than a predetermined threshold, the amplifier 130 oscillates at the oscillation frequency of the multivibrator it also serves.

[0038] [Single loop oscillation] After this, when the piezoelectric transformer PZT1 starts to be driven and the output voltage feedback signal from the coupling circuit pad exceeds the threshold, if the value of the lead phase φc of the phase lead circuit 132 (oscillator circuit) is 44°, then from the above relationship, the value of the lead phase φpzt from the input voltage to the output voltage of the piezoelectric transformer PZT1 becomes 46°, as shown by the two-dot chain line in Figure 5. At this time, the amplifier 130 outputs at a frequency where the phase of the entire loop is 0°.

[0039] 5, the phase lead circuit 132 changes the value of its own (oscillator circuit) lead phase φc from, for example, 44° to 0° (by turning off the phase lead circuit 132 itself), thereby changing the lead phase φpzt from the input voltage to the output voltage of the piezoelectric transformer PZT1 from, for example, 46° to 90°. This controls the drive frequency of the drive voltage applied to the piezoelectric transformer PZT1 to near the maximum frequency for the step-up ratio, thereby generating the output voltage Vout at the maximum step-up ratio. In this way, the step-up ratio of the piezoelectric transformer PZT1 can be controlled by the phase lead circuit 132 varying the value of the lead phase φc.

[0040] In the power supply device 100 shown in Figure 1, when the output voltage Vb of the error amplifier 134 increases, the lead phase φc of the phase lead circuit 132 decreases, and the lead phase φpzt of the piezoelectric transformer PZT1 increases and approaches 90°, causing the drive frequency to approach the maximum step-up ratio frequency and increase the step-up ratio of the piezoelectric transformer PZT1. On the other hand, when the output voltage Vb of the error amplifier 134 decreases, the lead phase φc of the phase lead circuit 132 increases, and conversely, the lead phase φpzt of the piezoelectric transformer PZT1 decreases, causing the drive frequency to move away from the maximum step-up ratio frequency and decrease the step-up ratio of the piezoelectric transformer PZT1. Therefore, taking these control characteristics into consideration, it is preferable to appropriately set the reference voltage Vref of the error amplifier 134 and to set the range of error that may occur between the reference voltage Vref and the divided voltage value Vdet.

[0041] The power supply device 100 and the piezoelectric transformer driving device of the above-described embodiment provide the following advantages. (1) By incorporating a phase lead circuit 132 into the multivibrator 130 (which also functions as an amplifier) ​​and varying the amount of phase lead φc, it is possible to control the step-up ratio of the piezoelectric transformer PZT1 to be driven. (2) Since there is no need to uniformly set the maximum step-up ratio frequency and control the drive frequency with high precision, drive control of the piezoelectric transformer PZT1 with a high Q value can be easily achieved. (3) Even if the resistance value of the load used changes, the piezoelectric transformer PZT1 can always be driven at the maximum step-up frequency by phase control within the single loop, which is extremely convenient.

[0042] (4) If the phase lead circuit 132 is configured to actually use a circuit that brings the phase lead φc closer to 0° as the oscillation proceeds, it can automatically drive the piezoelectric transformer PZT1 at the maximum step-up ratio once a single loop is formed. This allows the piezoelectric transformer PZT1 to be driven efficiently, further increasing its usefulness. (5) Furthermore, since the drive frequency can be precisely controlled to the maximum step-up ratio frequency of the piezoelectric transformer PZT1, it can be easily applied to other piezoelectric transformers with high Q-value resonance characteristics, further increasing its usefulness.

[0043] The present invention is not limited to the above-described embodiment, and can be practiced in various modified forms. The circuit configuration shown in FIG. 1 is 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.

[0044] 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.

[0045] In addition, the example of the phase change shown in FIG. 2 and the example of the frequency characteristics shown in FIG. 3 are merely examples, and are not limited to the examples. [Explanation of symbols]

[0046] 100 Power supply device using piezoelectric transformer driver 110 Switching Circuit 120 Rectification and smoothing section 122 Output detection unit 130 Multivibrator and amplifier 132 Phase Lead Circuit 134 Error amplifier PZT1 piezoelectric transformer pad coupling circuit

Claims

1. a drive circuit that applies a drive voltage at a predetermined drive frequency 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 that controls the drive frequency of the drive voltage applied to the piezoelectric transformer by the drive circuit; The piezoelectric transformer outputs an output voltage at a frequency based on the drive frequency of the drive circuit, the oscillation circuit oscillates based on the frequency of a feedback input voltage when the output voltage of the piezoelectric transformer is extracted by the coupling circuit, and the drive frequency of the drive circuit is controlled based on the oscillation frequency of the oscillation circuit, in a single loop: The oscillator circuit comprises: A piezoelectric transformer driver comprising a phase lead circuit that varies the lead phase during oscillation to have a trade-off relationship with the lead phase of the piezoelectric transformer within a range of 90°.

2. 2. The piezoelectric transformer driving device according to claim 1, The oscillator circuit comprises: A piezoelectric transformer driving device characterized in that the phase lead circuit varies the lead phase during oscillation, thereby applying a drive signal to the driving circuit at a drive frequency at which the piezoelectric transformer obtains a maximum step-up ratio.

3. 3. The piezoelectric transformer driving device according to claim 1, When the phase change of the entire one-loop is 0°, the lead phase of the driving circuit in the one-loop is −180°, and the lead phase of the coupling circuit is 90°, The phase lead circuit A piezoelectric transformer driving device characterized in that the lead phase of the oscillator circuit in the one-loop is brought close to 0°, thereby bringing the lead phase of the piezoelectric transformer close to 90°.

4. 4. The piezoelectric transformer driving device according to claim 1, an error amplifier that receives a feedback input of a detected value of the output voltage of the piezoelectric transformer applied to a load, and applies a phase control voltage to the phase lead circuit based on a comparison with a predetermined reference voltage; The phase lead circuit A piezoelectric transformer driving device, characterized in that the leading phase in the oscillation circuit is decreased as the phase control voltage increases.

5. 5. 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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