JP2012038788A - Cooling use piezoelectric actuator drive circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To configure a cooling use piezoelectric actuator drive circuit which is automatically actuated when cooling is necessary, without having to add new electronic components or circuits.SOLUTION: The piezoelectric actuator drive circuit includes a positive feedback circuit which amplifies a detection signal generated according to the voltage applied to a piezoelectric actuator vibrating a cooling fan, for positive feedback to the piezoelectric actuator. The impedance of the piezoelectric actuator has a negative temperature characteristic, so that while the ambient temperature of the piezoelectric actuator does not exceed a prescribed temperature, the impedance of the piezoelectric actuator is high, with a loop gain less than 1 (less than 0 dB). Therefore, only when a temperature is reached which requires cooling, the piezoelectric actuator is driven to vibrate by self-excitation.

Description

  The present invention relates to a drive circuit for a piezoelectric actuator that vibrates a cooling fan.

  2. Description of the Related Art In recent electronic devices such as personal computers, heat-generating components such as processors that consume a large amount of power are mounted with an increase in arithmetic processing capability. For example, Patent Document 1 is disclosed as a small cooling device that cools such heat-generating components.

  FIG. 1 is a perspective view showing a configuration of a piezoelectric fan (cooling piezoelectric actuator) disclosed in Patent Document 1. As shown in FIG. This piezoelectric fan is a piezoelectric fan in which a blower plate 202 is joined to the tip of a piezoelectric element 201 and the piezoelectric element 201 is fixed to a support 203. When the piezoelectric element 201 is energized and driven, the blower plate 202 moves up and down. It vibrates and sends air to the object to be cooled.

JP 2006-191123 A

  In general, in a device that sends air to a cooling object and cools it, assuming that the temperature difference between the air to be blown and the cooling object and the surface thermal conductivity of the cooling object are constant, the amount of cooling heat depends on the amount of air blown. Is determined. The purpose of cooling is to prevent the temperature of the object to be cooled from exceeding the specified value so that it does not operate abnormally. It is desirable from the viewpoint of conversion.

  However, this requires a temperature sensor that detects the temperature of the object to be cooled and a control circuit that controls the cooling device in accordance with the detection result. In the first place, the piezoelectric actuator for cooling is used as a small and highly efficient cooling device, but the provision of the temperature sensor and the control circuit is a factor that hinders downsizing.

  Therefore, an object of the present invention is to provide a cooling piezoelectric actuator drive circuit that automatically operates when cooling is required without adding new electronic components and circuits.

In order to solve the above problems, a piezoelectric actuator drive circuit according to the present invention includes a positive feedback circuit that amplifies a detection signal generated in response to voltage application to a piezoelectric actuator that vibrates a cooling fin and positively feeds back to the piezoelectric actuator. In the cooling piezoelectric actuator drive circuit,
The positive feedback circuit includes a current detection resistor connected in series to the piezoelectric actuator, and an amplification circuit that amplifies a voltage drop of the current detection resistor and applies a drive voltage to the piezoelectric actuator.
The piezoelectric actuator has a negative impedance temperature characteristic, and the positive feedback circuit is configured such that a loop gain of the positive feedback circuit is 1 (0 dB) or more when the piezoelectric actuator is driven at a temperature. The characteristics are set.

  With this configuration, the piezoelectric actuator is not driven when the ambient temperature of the piezoelectric actuator is lower than a predetermined temperature, and the piezoelectric actuator is automatically driven when the ambient temperature becomes high and cooling is required. This can be realized without using a circuit.

  In addition, adjustment means for adjusting the loop gain of the positive feedback circuit is provided as necessary. Thereby, it can set so that a piezoelectric actuator may be driven at the desired temperature suitable for the electronic device of an assembly destination.

  According to the present invention, it is possible to configure a cooling device including a piezoelectric actuator that automatically operates when cooling is required without adding new electronic components or circuits.

FIG. 1 is a perspective view showing a configuration of a piezoelectric fan (cooling piezoelectric actuator) disclosed in Patent Document 1. As shown in FIG. FIG. 2A is a perspective view of the piezoelectric actuator 1 constituting the piezoelectric fan. FIG. 2B is a perspective view of a cooling unit including the piezoelectric actuator 1. FIG. 3 is a circuit diagram of the cooling piezoelectric actuator drive circuit according to the first embodiment. 4 is a waveform diagram of the applied voltage Va to the first terminal, the applied voltage Vb to the second terminal, and the applied voltage Vab between both terminals of the piezoelectric actuator 1 shown in FIG. It is. FIG. 5 is a diagram showing a change in the frequency characteristic of the impedance with respect to the temperature change of the piezoelectric actuator 1. FIG. 6 is a diagram showing the characteristics of the loop gain when the impedance of the piezoelectric actuator 1 is changed in the drive circuit shown in FIG. FIG. 7 is a diagram showing the characteristics of the loop gain when the resistance value of the current detection resistor R11 flowing through the piezoelectric actuator 1 shown in FIG. 3 is changed in the piezoelectric actuator drive circuit according to the second embodiment. .

<< First Embodiment >>
FIG. 2A is a perspective view of the piezoelectric actuator 1 constituting the piezoelectric fan. FIG. 2B is a perspective view of a cooling unit including the piezoelectric actuator 1.

  As shown in FIG. 2A, the piezoelectric actuator 1 includes a vibration plate 2 made of a thin metal plate such as a stainless steel plate, and a flat substrate portion 2a is provided on one end side in the length direction of the vibration plate 2. The bimorph piezoelectric actuator is configured by attaching the piezoelectric elements 3 and 3 to the front and back surfaces of the substrate portion 2a. The diaphragm 2 is bent at a 90 ° bent portion 2b. A plurality (seven in this case) of fins 2d are divided and formed on the other end side in the length direction of the diaphragm 2.

  The fins 2 d extend in a direction orthogonal to the main surface direction of the piezoelectric element 3. An extension 2c to which the piezoelectric element 3 is not attached is formed on the terminal side of the substrate 2a of the diaphragm 2, that is, the end opposite to the bent portion 2b. The extension portion 2c is held by a support member 5 fixed to a fixing portion (not shown). The two piezoelectric elements 3 and 3 and the diaphragm 2 are electrically connected to the piezoelectric actuator drive circuit 6.

  As shown in FIG. 2B, the cooling unit includes a piezoelectric actuator 1 and a heat sink 10. The heat sink 10 includes a plurality of (eight in this case) radiating fins 11 arranged in parallel at intervals. The heat sink 10 is attached in a state where it is thermally coupled to the upper surface of a heating element (CPU or the like) mounted on, for example, a circuit board.

  Each fin 2 d of the piezoelectric actuator 1 is inserted between each heat radiation fin 11 in a non-contact manner in a direction perpendicular to the bottom surface of the heat sink 10. The piezoelectric actuator composed of the substrate portion 2 a of the diaphragm 2 and the piezoelectric element 3 is arranged in parallel along the upper end of the heat sink 10.

  When the diaphragm 2 vibrates by the piezoelectric actuator, the fin 2d vibrates in a direction parallel to the side surface of the radiation fin 11, and the warm air in the vicinity of the radiation fin 11 is scraped off by the fin 2d, so that the heat sink 10 is efficiently cooled. The

FIG. 3 is a circuit diagram of the cooling piezoelectric actuator drive circuit 6 according to the first embodiment.
The power supply circuit PS generates a stable reference potential VM (for example, DC6V) by, for example, equally dividing the power supply voltage DC12V by the resistors R31 and R32 and inputting the voltage obtained by DC6V to the voltage follower circuit by the operational amplifier OP5.

  The filter circuit A21 includes an operational amplifier OP1, amplifies the signal output from the feedback circuit A24, and supplies the amplified signal to the non-inverting amplifier circuit A23. The non-inverting amplifier circuit A23 amplifies the output voltage of the filter circuit A21 with a predetermined gain, and applies it to the first terminal of the piezoelectric actuator 1 via the resistor R11. The inverting amplifier circuit A22 inverts and amplifies the output voltage of the non-inverting amplifier circuit A23 with a gain of 1, and applies it to the second terminal of the piezoelectric actuator 1. The non-inverting amplifier circuit A23 and the inverting amplifier circuit A22 constitute a balanced drive circuit A25.

  The feedback circuit A24 takes out a current (detection signal) flowing through the piezoelectric actuator 1 in response to voltage application to the piezoelectric actuator 1 from both ends of the resistor R11, differentially amplifies it, and applies it to the non-inverting input terminal of the filter circuit A21.

  A voltage proportional to the current flowing through the piezoelectric actuator 1 appears at both ends of the resistor R11. The feedback circuit A24 amplifies the voltage across the resistor R11 and outputs an unbalanced signal. At this time, the output voltage of the feedback circuit A24 is determined so that a positive feedback circuit having a loop gain of more than 1 at the time of operation is configured along the path A24 → A21 → A25. That is, the voltage applied to the piezoelectric actuator 1 increases as the current flowing through the piezoelectric actuator 1 increases.

  A band rejection filter BEF is connected in the negative feedback loop NFL of the filter circuit A21. This band rejection filter BEF has a large insertion loss of the signal component of the fundamental resonance frequency of the piezoelectric device and a small insertion loss of the signal component of the higher order resonance frequency. That is, it acts to lower the loop gain at the higher order resonance frequency.

  Since the amplitudes of the output voltages of the non-inverting amplifier circuit A23 and the inverting amplifier circuit A22 are both equal to the power supply voltage and in the opposite phase, the piezoelectric actuator 1 is driven with a voltage twice the power supply voltage.

  The band pass filter BPF of the filter circuit A21 is connected in the positive feedback loop PFL. This band pass filter BPF passes the fundamental frequency and suppresses harmonic components. That is, it acts as a harmonic suppression filter that suppresses the signal of the higher-order resonance frequency of the piezoelectric device. Therefore, positive feedback is not applied to the harmonic frequency component, the loop gain in the harmonic frequency band is 1 or less, and the harmonic does not oscillate. That is, it oscillates at the fundamental frequency of the piezoelectric device to which the piezoelectric actuator 1 is attached.

The non-inverting amplifier circuit A23 includes an operational amplifier OP3 and resistors R7 and R8, and performs non-inverting amplification with a predetermined gain.
The inverting amplifier circuit A22 includes an operational amplifier OP4 and resistors R9 and R10, and performs inverting amplification with a gain of 1. That is, the inverting amplifier circuit A22 inverts and amplifies the output signal of the amplifier circuit A23 with an equal amplitude.

The feedback circuit A24 includes an operational amplifier OP2 and resistors R3, R4, R5, R6, and R11, and differentially amplifies the voltage across the resistor R11.
The filter circuit A21 also has a function of adjusting the phase amount of the positive feedback loop for self-excited oscillation. That is, the characteristics of the band pass filter BPF are determined so that the phase amount of the positive feedback loop is 0 °.

  4 is a waveform diagram of the applied voltage Va to the first terminal, the applied voltage Vb to the second terminal, and the applied voltage Vab between both terminals of the piezoelectric actuator 1 shown in FIG. It is. Since the amplification circuits A22 and A23 operate with a positive power supply of + 12V and a negative power supply of 0V, a voltage in the range of 0V to + 12V is applied to the first terminal of the piezoelectric actuator 1, and the second terminal of the piezoelectric actuator 1 is applied. A voltage in the range of + 12V to 0V is applied. Therefore, the applied voltage Vab across the piezoelectric actuator 1 is (Va−Vb). That is, 24 Vp-p is applied as a peak-to-peak voltage.

FIG. 5 is a diagram showing a change in the frequency characteristic of the impedance with respect to the temperature change of the piezoelectric actuator 1. In FIG. 5, the correspondence relationship between the characteristic curves Ta to Te and the temperature is as follows. Ta: 0 ° C., Tb: 25 ° C., Tc: 35 ° C., Td: 50 ° C., Te: 70 ° C.
Thus, the higher the temperature, the lower the impedance of the piezoelectric actuator. That is, the temperature characteristic of the impedance of the piezoelectric actuator is a negative characteristic.

  In FIG. 5, the frequency fr is the resonance frequency of the piezoelectric actuator, and the frequency fa is the anti-resonance frequency of the piezoelectric actuator. The oscillation frequency of the piezoelectric actuator drive circuit 6 shown in FIG. 3 is the resonance frequency fr or a slightly higher frequency. In this example, the piezoelectric actuator 1 is driven to oscillate at about 41 to 42 Hz. The impedance at this oscillation frequency is about 7 kΩ to about 8 kΩ at 70 ° C. and about 11 kΩ to about 12 kΩ at 0 ° C.

FIG. 6 is a diagram showing the characteristics of the loop gain when the impedance of the piezoelectric actuator 1 is changed in the drive circuit shown in FIG. In FIG. 6, the correspondence relationship between the characteristic curves Ra to Re and the impedance of the piezoelectric actuator 1 is as follows. Ra: 8 kΩ, Rb: 9 kΩ, Rc: 10 kΩ, Rd: 11 kΩ, Re: 12 kΩ
Thus, the loop gain increases as the impedance of the piezoelectric actuator 1 decreases. In this example, when the oscillation frequency is about 42 Hz, the impedance of the piezoelectric actuator 1 is less than about 11 kΩ and the loop gain is 0 dB or more, and when it is about 11 kΩ or more, the loop gain is less than 0 dB. Since one of the oscillation conditions is that the loop gain is 1 or more (0 dB or more), oscillation does not occur (the oscillation stops) when the impedance of the piezoelectric actuator 1 is about 11 kΩ or more.

  When the piezoelectric actuator 1 having the characteristics shown in FIG. 5 is connected to a drive circuit having the characteristics shown in FIG. 6 and driven to oscillate at about 42 Hz, when the ambient temperature of the piezoelectric actuator 1 is around 0 ° C., the piezoelectric actuator Since the impedance of 1 is about 12 kΩ and the loop gain of the drive circuit is 0 dB or less, the piezoelectric actuator 1 is not driven to oscillate. When the ambient temperature rises and becomes about 25 ° C. or more, the impedance of the piezoelectric actuator 1 becomes 11 kΩ or less, the loop gain of the drive circuit becomes 0 dB or more, and the piezoelectric actuator 1 is driven to oscillate.

  Due to the above-described operation, the operation of the piezoelectric actuator 1 is stopped when the ambient temperature of the piezoelectric actuator 1 is low, and the piezoelectric actuator 1 is operated when the ambient temperature becomes high and cooling is required. This can be done automatically without providing a temperature control circuit.

<< Second Embodiment >>
The piezoelectric actuator drive circuit according to the second embodiment includes loop gain adjusting means. In the piezoelectric actuator drive circuit according to the second embodiment, the resistor R11 of the piezoelectric actuator drive circuit 6 shown in FIG. 3 is configured by a variable resistor.

FIG. 7 is a diagram showing the characteristics of the loop gain when the resistance value of the current detection resistor R11 flowing through the piezoelectric actuator 1 shown in FIG. 3 is changed in the piezoelectric actuator drive circuit according to the second embodiment. . Here, the characteristics are obtained when the ambient temperature of the piezoelectric actuator 1 is about 25 ° C. (the impedance of the piezoelectric actuator 1 is about 11 kΩ). In FIG. 7, the correspondence relationship between the characteristic curves R11a to R11e and the resistance value of the resistor R11 is as follows. R11a: 70Ω, R11b: 60Ω, R11c: 50Ω, R11d: 40Ω, R11e: 30Ω
Thus, the loop gain increases as the resistance value of the current detection resistor R11 increases. In this example, when the oscillation frequency is about 42 Hz, if the resistance value of the resistor R11 is 50Ω or more, the loop gain is 0 dB or more, and if it is about 50Ω or less, the loop gain is less than 0 dB. Therefore, when the resistance value of the resistor R11 is about 50Ω or less, it does not oscillate (oscillation stops). Therefore, if the resistance value of the resistor R11 is set to about 50Ω, the loop gain becomes 0 dB or more at about 25 ° C. or more, and the piezoelectric actuator 1 is driven to oscillate. If the resistance value of the resistor R11 is made smaller than 50Ω, the resistor R11 is still stopped at 25 ° C., and is driven to oscillate at a higher temperature.

  In this way, by setting the resistance R11 for detecting the current flowing through the piezoelectric actuator 1, the driving start temperature of the piezoelectric actuator can be set to a predetermined temperature.

  As a means for adjusting the loop gain of the piezoelectric actuator drive circuit, in addition to setting the current detection resistor (R11), a circuit constant related to the gain of the feedback loop may be variable. For example, the gain of the feedback circuit A24 may be made variable by making the resistance values of the resistors R5 and R6 in FIG. 3 variable. Further, the gain of the non-inverting amplifier circuit A23 may be made variable by making the resistance values of the resistors R7 and R8 in FIG. 3 variable.

A21 ... Filter circuit A22 ... Inverting amplifier circuit A23 ... Non-inverting amplifier circuit A24 ... Feedback circuit A25 ... Balance drive circuit BEF ... Band-pass filter BPF ... Band-pass filter NFL ... Negative feedback loop OP1-OP5 ... Operational amplifier PFL ... Positive feedback loop PS Power supply circuit R11 ... Current detection resistor 1 ... Piezo actuator 2 ... Diaphragm 2a ... Substrate 2c ... Extension 2d ... Fin 3 ... Piezo element 5 ... Support member 6 ... Piezo actuator drive circuit 10 ... Heat sink 11 ... Radiation fin

Claims (2)

In a driving circuit for a cooling piezoelectric actuator, comprising a positive feedback circuit that amplifies a detection signal generated in response to voltage application to a piezoelectric actuator that vibrates a cooling fin and positively feeds back to the piezoelectric actuator.
The positive feedback circuit includes a current detection resistor connected in series to the piezoelectric actuator, and an amplification circuit that amplifies a voltage drop of the current detection resistor and applies a drive voltage to the piezoelectric actuator.
The piezoelectric actuator has a negative temperature characteristic of impedance, and the positive feedback circuit has a characteristic such that a loop gain of the positive feedback circuit becomes 1 or more when the temperature is a temperature for driving the piezoelectric actuator. The set piezoelectric actuator drive circuit for cooling.   The cooling piezoelectric actuator drive circuit according to claim 1, further comprising an adjusting unit that adjusts a loop gain of the positive feedback circuit.

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