Coil drive device

Abstract

The coil temperature is controlled using a simple configuration. [Solution] The coil drive device 10a includes an amplification unit 30 that amplifies and outputs a signal Ain, and a control signal output unit 40 that outputs an amplification control signal to control the input signal to the amplification unit 30 based on the voltage difference between the voltage of the signal Va at terminal a of a speaker 50, which includes a coil driven by the output signal from the amplification unit 30, and the voltage of the signal Vb that appears across the resistor Rm connected to terminal b.

Description

Technical Field

[0001] The present disclosure relates to a coil driving device.

Background Art

[0002] As is well known, a speaker generates sound by flowing an audio signal through a voice coil and vibrating a cone paper to change air pressure. There is a type of speaker called an SR (Sound Reinforcement) speaker used in concerts, live performances on the road, speeches, etc. In this type of speaker, since enhancing sound is one of the purposes, it is often driven by a relatively large current. Therefore, it is necessary to prevent failures caused by large current driving. In order to prevent failures of speakers, techniques have been proposed for detecting the current and voltage flowing through the voice coil of a speaker and operating the protection circuit of the speaker according to the detected current and voltage (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in the technique described in Patent Document 1 above, the current and voltage flowing through the voice coil are detected, and the temperature of the speaker is estimated based on the detected current and voltage. For this temperature estimation, it is necessary to store various parameters in the storage unit in advance.

Means for Solving the Problems

[0005] To solve the above problems, a coil drive device according to one aspect of the present disclosure includes an amplification unit that amplifies and outputs an input signal, and a control signal output unit that outputs an amplification control signal for controlling the amplification of the amplification unit based on the voltage difference between a first voltage appearing at one end of a coil driven by the output signal from the amplification unit and a second voltage appearing across both ends of a detection resistor connected to the other end of the coil. [Brief explanation of the drawing]

[0006] [Figure 1] This is a block diagram illustrating the configuration of an acoustic system including a coil drive device according to the first embodiment. [Figure 2] This is a diagram showing the comparison section in a coil drive device. [Figure 3] This diagram illustrates the frequency-impedance characteristics of a speaker. [Figure 4] This is a diagram illustrating the operation of the heat detection unit in an acoustic system. [Figure 5] This is a diagram illustrating the operation of the heat detection unit in an acoustic system. [Figure 6] This is a diagram illustrating the operation of the heat detection unit in an acoustic system. [Figure 7] This figure illustrates the characteristic change in resistance of copper with respect to temperature. [Figure 8] This is a diagram illustrating the operation of the control signal output unit. [Figure 9] This is a diagram illustrating the operation of a limiting circuit in an acoustic system. [Figure 10] This is a block diagram illustrating the configuration of an acoustic system including a coil drive device according to the second embodiment. [Figure 11] This is a block diagram illustrating the configuration of an acoustic system including a coil drive device according to the third embodiment. [Figure 12] This diagram illustrates the operation when the amplification section is a constant voltage amplifier. [Figure 13] This is a block diagram illustrating the configuration of an acoustic system including a coil drive device according to the fourth embodiment. [Modes for carrying out the invention]

[0007] Hereinafter, coil drive devices according to embodiments of this disclosure will be described with reference to the drawings. The embodiments described below are preferred examples and are subject to various technically preferred limitations, but the scope of the present invention is not limited to these forms unless otherwise stated in the following description.

[0008] Figure 1 is a block diagram showing the configuration of an acoustic system 1a including a coil drive device 10a according to the first embodiment. The sound system 1a includes a coil drive device 10a and a speaker 50. The coil drive device 10a is a device that amplifies the signal Ain in a single-ended manner and emits sound from the speaker 50, which includes a voice coil to be driven.

[0009] Signal Ain is an acoustic signal output from a microphone, electric instrument, or playback device. The frequency range of signal Ain is 20-20kHz, which is within the human audible range. Speaker 50 converts the amplified audio signal into sound, which is a physical vibration, and outputs it. In the diagram, for the sake of explanation, the coil drive unit 10a and speaker 50 are shown separately, but the coil drive unit 10a and speaker 50 may also be housed in a single enclosure.

[0010] The coil drive device 10a includes a limiting circuit 20, an amplification unit 30, and a control signal output unit 40. The limiting circuit 20 is a circuit that limits the amplitude of the signal Ain input via the resistor R1 to a range corresponding to the voltages of signals Vlw and Vup. More specifically, the limiting circuit 20 includes diodes D1 and D2 that clamp (clip) the amplitude of the signal Ain, and a preamplifier 22 that preamps the signal appearing at the connection point n1 between the anode of diode D1 and the cathode of diode D2. Note that voltage means the potential difference between two points. Unless otherwise specified, it refers to the potential difference between the target point and the ground potential Gnd which is the reference of zero voltage.

[0011] A signal Vup is supplied to the cathode of the diode D1, and a signal Vlw is supplied to the anode of the diode D2. The signals Vup and Vlw are output from the control signal output unit 40.

[0012] The preamplifier 12 includes an operational amplifier Op, resistor elements R2 and R3, and pre-amplifies the signal appearing at the connection point n1 and outputs it as the signal Bin.

[0013] The amplification unit 30 is a main amplifier that amplifies the signal Bin with a constant current. Note that the amplification unit 30 is not limited to constant current amplification and may be constant voltage amplification as described later. The output terminal of the amplification unit 30 is connected to one of the two terminals of the speaker 50, terminal a. Also, of the two terminals of the speaker 50, the other terminal b, together with terminal a, is connected to the control signal output unit 40. Let the signal at terminal a be Va and the signal at terminal b be Vb.

[0014] The speaker 50 generates sound by vibrating the cone paper with a voice coil. As is well known, in the speaker 50, the electrical equivalent circuit between terminals a and b includes a series connection of the resistance component and the reactance component of the coil.

[0015] The control signal output unit 40 has a resistor element Rm, a filter unit 42, a rectifying and smoothing unit 43, and a comparison unit 44. The resistive element Rm is an example of a "detection resistor." In this embodiment, one terminal of the resistive element Rm is connected to the other terminal b of the speaker 50, and the other terminal c of the resistive element Rm is at ground potential Gnd. The resistance value of the resistive element Rm is lower than the resistance component of the speaker 50 and is 1Ω or less. The change in the resistance value of the resistive element Rm with respect to temperature is about several tens of ppm, which is negligibly small compared to the change in the resistance component of the speaker 50 with respect to temperature.

[0016] The filter section 42 includes bandpass filters 421 and 422. Bandpass filter 421 passes a specific frequency band component of the signal Va and outputs it as signal V1. Bandpass filter 422 passes the same frequency band component of the signal Vb as bandpass filter 421 and outputs it as signal V2. The passband characteristics of bandpass filters 421 and 422 are almost the same, and in this embodiment, they are set to include 500 Hz.

[0017] The rectifier and smoothing section 43 includes rectifier circuits 431 and 432, and low-pass filters 435 and 436. Rectifier circuit 431 full-wave rectifies the AC signal V1 and outputs it. Rectifier circuit 432 full-wave rectifies the AC signal V2 and outputs it. The low-pass filter 435 passes the low-frequency components of the signal V1, which has been full-wave rectified by the rectifier circuit 431, through, i.e., smooths (filters) them, and outputs them as a DC signal Vsp. The low-pass filter 436 smooths the signal V2, which has been full-wave rectified by the rectifier circuit 432, and outputs it as a DC signal Vm.

[0018] The bandpass filters 421 and 422, and the lowpass filters 435 and 436 are well-known analog filters composed of operational amplifiers, capacitive elements, resistive elements, etc., and the gain of these filters will be discussed later.

[0019] Figure 2 is a circuit diagram showing an example of the comparison unit 44. The comparison unit 44 is a circuit that compares the relative voltages of signal Vsp and signal Vm. In this example, the operational amplifier A11, resistors R11 to R14, and voltage source Eref1 subtract the voltage of signal Vsp from the voltage of signal Vm, and output a signal Vup which is the result of this subtraction multiplied by a coefficient Kp. In addition, the operational amplifier A21, resistors R21 to R24, and voltage source Eref2 subtract the voltage of signal Vm from the voltage of signal Vsp, and output a signal Vlw which is the result of this subtraction multiplied by a coefficient Km. In practice, if the coefficients Kp and Km are sufficiently large, and the voltage of signal Vsp is lower than the voltage of signal Vm, the voltage of signal Vup will swing to the higher voltage of the power supply voltage of op-amp A11, and the voltage of signal Vlw will swing to the lower voltage of the same power supply voltage. Conversely, in the above case, if the voltage of signal Vsp is higher than the voltage of signal Vm, the voltage of signal Vup will swing to the lower voltage of the power supply voltage of op-amp A21, and the voltage of signal Vlw will swing to the higher voltage of the same power supply voltage.

[0020] Figure 3 shows an example of the impedance (ohms) characteristics with respect to frequency in speaker 50. The impedance frequency characteristics of speaker 50 can be mainly divided into the following three regions. More specifically, the impedance frequency characteristics can be divided into a low-frequency region (A) where mechanical impedance is dominant, a mid-frequency region (B) where electrical resistance is dominant, and a high-frequency region (C) where the inductance of the coil is dominant. Of these, if a large amount of signal in the region where electrical resistance is dominant (B) is supplied to speaker 50, speaker 50 will be prone to overheating due to the resistive component. In other words, by extracting and monitoring the component (B) in the region where electrical resistance contributes significantly to the signal supplied to speaker 50, it is possible to accurately estimate the heat generated in the speaker 50's coil.

[0021] Therefore, in this embodiment, the 500Hz component of the signal Va, which is approximately in the center of region (B), that is, the component that contributes most to the heat generation in the speaker 50's coil, is extracted by the bandpass filter 421 and output as signal V1. Similarly, the 500Hz component of the signal Vb is extracted by the bandpass filter 422 and output as signal V2.

[0022] Signals V1 and V2 are signals that drive the speaker 50, and are both AC signals. Therefore, if signal V1 has a waveform like the one shown in Figure 4, for example, it is full-wave rectified by the rectifier circuit 431 as shown in Figure 5, smoothed by the low-pass filter 435 as shown in Figure 6, and output as signal Vsp. Similarly, signal V2 is full-wave rectified by the rectifier circuit 432, smoothed by the low-pass filter 436, and output as signal Vm.

[0023] The coil of speaker 50 is generally constructed by winding a wire, such as copper, around a bobbin (cylinder).

[0024] Figure 7 shows the characteristics of resistance change with temperature when copper is used as the conductor. More specifically, the figure shows the ratio of resistance to temperature when the resistance at the reference temperature of 20°C is normalized to "1". The temperature coefficient of resistance of copper at 20°C is said to be "0.00393". Therefore, for example, at 150°C, which is only 130°C higher than at 20°C, the resistance of copper becomes 1.51 (= 1 + 0.00393 × 130) times that of copper at 20°C. In this embodiment, we assume that copper is used as the material for the wire in the coil, but the temperature coefficient of resistance differs depending on the material of the wire.

[0025] To estimate the temperature of a coil, one could calculate it by working backward from the rate of change in the resistance of the wires that make up the coil. However, in this embodiment, instead of estimating the coil temperature, it is detected that the temperature has exceeded a threshold temperature due to heat generation.

[0026] The voltage of signal Vsp reflects the voltage across terminals a and b at speaker 50. Therefore, when signal Bin is amplified with a constant current, the resistance of the coil increases as its temperature rises, and thus the voltage of signal Vsp also increases. On the other hand, the same amount of current flows through the resistor Rm as the current flowing through the speaker 50, but the resistance value of the resistor Rm hardly changes due to heat generation. Therefore, the voltage across the resistor Rm, that is, the voltage across terminal b and terminal c which is at ground potential Gnd, remains almost constant even when the signal Bin is amplified with a constant current. In practice, since signal Bin is AC, signal Va is converted to a DC signal Vsp by the rectifier and smoothing unit 43 after filtering signal V1 by the bandpass filter 421. Similarly, signal Vb is converted to a DC signal Vm by the rectifier and smoothing unit 43 after filtering signal V2 by the bandpass filter 422.

[0027] Figure 8 is a diagram illustrating the operation of the control signal output unit 40. In the figure, the left half shows the signal voltages Va, Vb, Vm, and Vsp when the coil temperature is 20°C, and the right half shows the voltages when the coil temperature is 150°C. This example shows the case where the threshold temperature Tth is set between 20°C and 150°C. As described above, as the temperature of the coil in speaker 50 increases, the voltage of the signal Va at terminal a increases, but the signal Vb, which is the voltage across the resistor Rm, hardly changes.

[0028] Therefore, when the temperature of the coil in speaker 50 increases, the voltage of signal Vsp increases, reflecting the signal Va, but the voltage of signal Vm remains almost constant, reflecting the signal Vb. When the coil temperature is low, the voltage of signal Vsp is lower than the voltage of signal Vm. However, as the coil temperature increases, it increases, reflecting the signal Va, and eventually reverses with signal Vm, meaning that the voltage of signal Vsp becomes higher than the voltage of signal Vm. In other words, when the coil changes across the threshold temperature Tth, the voltages of signal Vsp and signal Vm reverse.

[0029] The voltage of signal Vsp is determined by the gain of bandpass filter 421 and the gain of lowpass filter 435. Similarly, the voltage of signal Vm is determined by the gain of bandpass filter 422 and the gain of lowpass filter 436. Therefore, the gains of bandpass filters 421, 422, and lowpass filters 435 and 436 are set so that when the coil temperature rises to the threshold temperature Tth, the voltage of signal Vsp reverses with the voltage of signal Vm.

[0030] When the coil temperature rises and reaches the threshold temperature Tth, the voltage of signal Vsp becomes higher than the voltage of signal Vm. Therefore, the comparison unit 44 lowers the voltage of signal Vup from a high level of the power supply voltage and raises the voltage of signal Vm from a low level of the power supply voltage. For this reason, the limiting circuit 20 limits the amplitude of signal Ain to a range corresponding to the voltages of signals Vlw and Vup.

[0031] Specifically, in Figure 9, when signal Ain is shown as a dashed line, signal Bin is shown as a solid line. Diode D1 clips the upper limit (positive side) of the amplitude of signal Ain to a voltage obtained by adding the forward voltage Vf of diode D1 to the voltage of signal Vup. Also, the lower limit (negative side) of the amplitude of signal Ain is clipped to a voltage obtained by subtracting the forward voltage Vf of diode D2 from the voltage of signal Vlw. The signal Bin, whose amplitude is limited compared to the signal Ain, is amplified by the amplification unit 30 and supplied to the speaker 50, thereby reducing the volume of sound emitted by the speaker 50. As a result, in the first embodiment, heat generation of the speaker 50 can be suppressed. Furthermore, if the coil temperature falls below the threshold temperature Tth after exceeding the threshold temperature, the amplitude limiting of the signal Ain by the limiting circuit 20 is released.

[0032] In the first embodiment, the coil drive unit 10a and the speaker 50 were housed in a single enclosure, but the coil drive unit 10a and the speaker 50 may be separate components. In such a configuration, the speaker 50 is detachably connected to the coil drive unit 10a. Generally, the impedance of the speaker 50 differs depending on the model. Next, a second embodiment will be described in which it is possible to manage the speaker at a constant threshold temperature Tth even if the impedances differ. For the sake of explanation, we will assume here that the impedance of the speaker 50 connected to the coil drive unit 10a is 2Ω, 4Ω, 6Ω, or 8Ω.

[0033] Figure 10 is a block diagram showing the configuration of an acoustic system 1a including a coil drive device 10b according to the second embodiment. The difference between the coil drive device 10b and the coil drive device 10a according to the first embodiment is that the bandpass filter 421 is provided with switch Sw1, the bandpass filter 422 is provided with switch Sw2, the lowpass filter 435 is provided with switch Sw3, and the lowpass filter 436 is provided with switch Sw4.

[0034] Switch Sw1 selects one of the elements K12, K14, K16, or K18, and switches the gain of the bandpass filter 421 to the value determined by the selected element. Similarly, switch Sw2 selects one of elements K22, K24, K26, or K28 and switches the gain of bandpass filter 422 to the value determined by the selected element. Switch Sw3 selects one of elements K32, K34, K36, or K38 and switches the gain of lowpass filter 435 to the value determined by the selected element. Switch Sw4 selects one of elements K42, K44, K46, or K48 and switches the gain of lowpass filter 436 to the value determined by the selected element.

[0035] When the impedance of speaker 50 is 2Ω, elements K12, K22, K32, and K42 are selected in order by switches Sw1 to Sw4. When elements K12, K22, K32, and K42 are selected, the gains of the bandpass filters 421 and 422 and the lowpass filters 435 and 436 are set so that at threshold temperature Tth, the voltage of signal Vsp and the voltage of signal Vm are reversed. Furthermore, if the impedance of speaker 50 is 4Ω, elements K14, K24, K34, and K44 are selected in order by switches Sw1 to Sw4. When elements K14, K24, K34, and K44 are selected, the gains of the bandpass filters 421 and 422 and the lowpass filters 435 and 436 are set so that the voltage of signal Vsp and the voltage of signal Vm are reversed at the same threshold temperature Tth as when the impedance is 2Ω.

[0036] When the impedance of speaker 50 is 6Ω, elements K16, K26, K36, and K46 are selected in order by switches Sw1 to Sw4. When elements K16, K26, K36, and K46 are selected, the gains of the bandpass filters 421 and 422 and the lowpass filters 435 and 436 are set so that the voltage of signal Vsp and the voltage of signal Vm are reversed at the same threshold temperature Tth as when the impedance is 2Ω and 4Ω. When the impedance of speaker 50 is 8Ω, elements K18, K28, K38, and K48 are selected in order by switches Sw1 to Sw4. When elements K18, K28, K38, and K48 are selected, the gains of the bandpass filters 421 and 422 and the lowpass filters 435 and 436 are set so that the voltage of signal Vsp and the voltage of signal Vm are reversed at the same threshold temperature Tth as when the impedance is 2Ω, 4Ω, and 8Ω.

[0037] In the second embodiment, even if the impedance of the speaker 50 is changed, it can be managed at the same threshold temperature Tth. In this example, we assumed that speaker 50 has an impedance of 2Ω, 4Ω, 6Ω, or 8Ω, but the impedance can be any other value, and is not limited to these four types.

[0038] In this configuration, which is switchable in accordance with the impedance of the speaker 50, for example, when a new speaker 50 to be driven is connected or when the speaker 50 is replaced, a control circuit (not shown) may cause the amplification unit 30 to output a small DC current signal to the speaker 50 that does not generate heat. The control circuit can then determine the impedance of the speaker 50 by dividing the voltage of the signal Vb when the small DC current signal is output by the current output to the speaker 50. Furthermore, the control circuit may be configured to cause switches Sw1 to Sw4 to select an element corresponding to the calculated impedance.

[0039] In the first and second embodiments, the speaker 50 was driven in a single-ended configuration, but it may also be driven in a BTL (Balanced Transformer Less) configuration. Therefore, a third embodiment in which the speaker 50 is driven in a BTL configuration will be described next.

[0040] Figure 11 is a block diagram showing the configuration of an acoustic system 1a including a coil drive device 10c according to the third embodiment. In the third embodiment, the positive phase signal of signal Bin is input to terminal a of the speaker 50, and the negative phase signal of signal Bin is input to terminal b.

[0041] Therefore, the coil drive device 10c includes an inverter 32 that inverts the signal Bin, and an amplifier 34 that amplifies the inverted signal. In the third embodiment, the other terminal c of the resistive element Rm is connected to the output terminal of the amplification unit 34. In the third embodiment, the signal at terminal c is Vc.

[0042] In the third embodiment, since the other end of the resistive element Rm is not grounded, it is necessary to subtract the voltage of signal Vc from the voltage of signal Vb in order to find the voltage across the resistive element Rm. For this reason, subtractor 412 subtracts the voltage of signal Vc from the voltage of signal Vb and outputs it as signal Vbc. Subtractor 411 subtracts the voltage of signal Vb from the voltage of signal Va and outputs it as signal Vab. In the third embodiment, the bandpass filter 421 extracts the 500Hz component from the signal Vab and outputs it as signal V1. The bandpass filter 422 extracts the same 500Hz component from the signal Vbc and outputs it as signal V2.

[0043] The rectifier and smoothing unit 43 rectifies and smooths signal V1 and outputs it as signal Vsp, and rectifies and smooths signal V2 and outputs it as signal Vm, and the comparison unit 44 outputs signals Vup and Vlw based on signals Vsp and Vm, which is the same as in the first embodiment.

[0044] In the third embodiment, similar to the first embodiment, when the temperature of the coil in speaker 50 rises to the threshold temperature Tth, the voltage of signal Vsp becomes higher than the voltage of signal Vm, so the amplitude of signal Ain is limited to a range corresponding to the voltages of signals Vlw and Vup. Therefore, in the third embodiment as well, the amplitude of the signal Ain is limited, which helps to suppress heat generation in the speaker 50.

[0045] In the third embodiment, the voltage was subtracted first and then filtered by a bandpass filter, but it is also possible to filter by a bandpass filter first and then subtract the voltage.

[0046] The first to third embodiments described above (hereinafter referred to as "embodiments, etc.") can be modified or applied in various ways as follows. Specific examples of modifications that can be applied to the embodiments are given below. Two or more embodiments arbitrarily selected from the following examples may be combined to the extent that they do not contradict each other.

[0047] In the embodiments described, the amplification of the amplification unit 30 is suppressed by limiting the amplitude of the signal Ain that is to be amplified, thereby preventing heat generation in the speaker 50. However, the configuration is not limited to this. For example, heat generation in the speaker 50 can also be suppressed by reducing the amplification factor of the preamplifier 12 or the amplification unit 30. In other words, "controlling the amplification of the amplification unit" includes not only controlling the amplification result of the amplification unit 30 itself, but also controlling it through other elements such as the limiting circuit 20 or the output of the preamplifier 12.

[0048] In the embodiments described, the amplification unit 30 was configured to amplify the signal Bin with a constant current, but a configuration that amplifies with a constant voltage is also possible. The constant voltage amplification will be described using the configuration applied to the first embodiment.

[0049] Figure 12 is a diagram illustrating the operation of the control signal output unit 40 in a coil drive device according to the first modified example, in which the amplification unit 30 amplifies the signal Bin by a constant voltage. In this example, since the signal Bin is amplified by a constant voltage, the output terminal of the amplifier 30, and the voltage at one terminal a of the speaker 50, i.e., the voltage of the signal Va, remain constant even if the coil temperature rises or the resistance component increases. Since the voltage of the signal Va remains constant with respect to temperature changes, the voltage of the signal Vsp also remains constant.

[0050] However, in constant voltage amplification, as the temperature of the coil rises and its resistance increases, the current flowing through the coil decreases. Therefore, the voltage of signal Vb, that is, the voltage across the terminals of the resistive element Rm, decreases with increasing temperature. Since the voltage of signal Vb decreases with increasing temperature, the voltage of signal Vm also decreases with increasing coil temperature. Therefore, in the third embodiment, when the coil temperature is below the threshold temperature Tth, the voltage of signal Vm is set higher than the voltage of signal Vsp. In this setting, when the coil temperature rises and exceeds the threshold temperature Tth, the voltages of signal Vm and signal Vsp reverse, and the voltage of signal Vm becomes lower than the voltage of signal Vsp.

[0051] Therefore, even in constant voltage amplification, if the temperature of the coil in speaker 50 rises to the threshold temperature Tth, the voltage of signal Vsp will become higher than the voltage of signal Vm, thus limiting the amplitude of signal Ain to a range corresponding to the voltages of signals Vlw and Vup. Therefore, even in constant voltage amplification, the amplitude of the signal Ain is limited, which helps to suppress heat generation in the speaker 50.

[0052] In the embodiments described above, the configuration was such that the amplitude of the signal Bin is limited if the temperature of the coil constituting the speaker 50 is above the threshold temperature Tth, that is, the configuration indirectly determines whether or not the temperature of the coil is above the threshold temperature Tth. However, a configuration that estimates the temperature of the coil may also be used. Specifically, in the case of constant current amplification, as shown in Figure 8, the difference between the voltage of signal Va and the voltage of signal Vb increases as the temperature of the coil rises. For example, a conversion table linking this difference to temperature can be prepared in advance, and a control circuit (not shown) can read out the temperature corresponding to the calculated difference by referring to the conversion table and output it.

[0053] In the case of constant voltage amplification, as shown in Figure 12, the difference between the voltage of signal Va and signal Vb increases as the temperature of the coil rises. Similarly, the temperature corresponding to the calculated difference can be read out by referring to a conversion table and output. In a configuration for estimating the coil temperature, the amplitude of the limiting signal Ain, which is limited by the limiting circuit 20, may be narrowed in steps or continuously as the estimated temperature increases. Furthermore, in Figures 8 and 12, V20 represents the difference between the voltage of signal Va and the voltage of signal Vb when the coil temperature is 20°C. V150 represents the difference between the voltage of signal Va and the voltage of signal Vb when the coil temperature is 150°C.

[0054] In the embodiments described, the drive target of the coil drive device 10a, etc., was a speaker 50 including a voice coil, but it is not limited to this. For example, it can also be applied to a coil that generates a magnetic field in an electric motor as the drive target.

[0055] Figure 13 shows the configuration of the motor system 1b driven by the coil drive device 10d according to the second modified example. The electric motor 52 is a three-phase AC motor having U-phase coils, V-phase coils, and W-phase coils. One end of each of these coils is commonly connected to terminal a. Since the current flowing through each phase coil is considered to be equal, in the second modification, one end of the resistive element Rm is connected to the other end of the W-phase coil, which represents the coils of each phase. More specifically, one end of the resistive element Rm is connected to terminal b, which is the other end of the W-phase coil, and the other end of the resistive element Rm is terminal c.

[0056] The current flowing through the U-phase coil is controlled by switching the DC power supply Ve using transistors Pu and Nu. Similarly, the current flowing through the V-phase coil is controlled by switching the DC power supply Ve using transistors Pv and Nv, and the current flowing through the W-phase coil is controlled by switching the DC power supply Ve using transistors Pw and Nw.

[0057] In the motor system 1b, signal Ain specifies the output of motor 52 as a voltage. In the second modified example, the limiting circuit 20 includes an operational amplifier A31 and a variable resistor 24. The variable resistor 24 changes the resistance value that is negatively fed back to the operational amplifier A31 according to the control signal output from the decoder 45. The PWM control circuit 36 ​​outputs switching control signals for transistors Pu and Nu, Pv and Nv, and Pw and Nw, respectively, according to the voltage of the signal output from the limiting circuit 20.

[0058] The subtractor 411 is the same as in the third embodiment in that it subtracts the voltage of signal Vb at terminal b from the voltage of signal Va at terminal a and outputs it as signal Vab. Similarly, the subtractor 412 is the same as in the third embodiment in that it subtracts the voltage of signal Vc at terminal c from the voltage of signal Vb and outputs it as signal Vbc.

[0059] Unlike the speaker 50, the coils of each phase in the electric motor 52 operate mainly only in region (A) where mechanical impedance is dominant. Therefore, in the second modified example, the filter unit 42 extracts the bandwidth of region (A) from the frequency of signal Vab and outputs it as signal V1, and similarly extracts the bandwidth of region (A) from the frequency of signal V2 and outputs it as signal V2. The rectifier and smoothing unit 43, similar to the first and third embodiments, rectifies and smooths the signal V1 that has been filtered by the filter unit 42 and outputs it as the signal Vsp. Furthermore, the rectifier and smoothing unit 43 rectifies and smooths the signal V2 that has been filtered by the filter unit 42 and outputs it as the signal Vm.

[0060] In the second modified example, the comparison unit 44 outputs only the signal Vup, unlike in the embodiment. More specifically, the comparison unit 44 outputs only the signal Vup, which is the voltage obtained by multiplying the voltage obtained by subtracting the voltage of signal Vsp from the voltage of signal Vm by a predetermined gain. The signal Vup voltage decreases as the temperature of the W-phase coil increases, once the W-phase coil reaches a threshold temperature. The decoder 45 outputs a control signal that gradually decreases the resistance value in the variable resistor 24 as the voltage of the signal Vup decreases.

[0061] Therefore, when the W-phase coil is above the threshold temperature, and the temperature of the coil increases, the resistance value negatively fed back in the operational amplifier A31 decreases, causing the voltage output from the operational amplifier A31 to decrease. As a result, the current flowing through the coils of each phase in the motor 52 is suppressed, thus reducing the heat generated in the coils of each phase.

[0062] From the above description, preferred embodiments of this disclosure can be understood, for example, as follows:

[0063] A coil drive device according to one aspect 1 of the present disclosure includes an amplification unit that amplifies and outputs an input signal, and a control signal output unit that outputs an amplification control signal for controlling the amplification of the amplification unit based on the voltage difference between a first voltage appearing at one end of a coil driven by the output signal from the amplification unit and a second voltage appearing across both ends of a detection resistor connected to the other end of the coil.

[0064] In the coil drive device according to Embodiment 1, when the coil heats up, the resistance of the coil changes, which changes the current flowing through the coil and thus changes the first voltage, which is the voltage across the coil. In contrast, if the resistance value of the detection resistor is insensitive to heat generation, the second voltage, which is the voltage across the detection resistor, hardly changes regardless of the heat generation of the coil. Therefore, with the coil drive device according to Embodiment 1, the heating state of the coil can be determined by the difference between the first voltage and the second voltage, without having to store the rated impedance value of the coil as an initial parameter. Furthermore, since an amplification control signal is output according to the heating state, damage to the coil can be prevented.

[0065] Terminal a is an example of "one end of the coil," and terminal b is an example of "the other end of the coil." The voltage of signal Va is an example of a "first voltage," and the voltage of signal Vb or signal Vbc is an example of a "second voltage." Signals Vup and Vlw are examples of "amplification control signals."

[0066] In a coil drive device according to a specific embodiment 2 of embodiment 1, the control signal output unit outputs a signal that limits the amplitude of the input signal as the amplification control signal when the coil reaches a threshold temperature or higher. According to the coil drive device of embodiment 2, when the coil reaches a threshold temperature or higher, the amplitude of the input signal is limited, thereby suppressing heat generation in the coil.

[0067] In a coil drive device according to another specific embodiment 3 of embodiment 1, the first signal indicating the first voltage is a signal that has passed through a specific frequency band, and the second signal indicating the second voltage is a signal that has passed through the specific frequency band. According to the coil drive device of embodiment 3, when alternating current flows through the coil, the signal is processed based on a signal that has passed through a specific frequency band where the contribution of electrical resistance is dominant. Therefore, compared to a configuration without processing, the output accuracy of the amplification control signal can be improved. Signal V1 is an example of a "first signal," and signal V2 is an example of a "second signal."

[0068] In a coil drive device according to a specific embodiment 4 of embodiment 3, the control signal output unit includes a first rectifier circuit for rectifying the first signal, a first low-pass filter for filtering the signal rectified by the first rectifier circuit, a second rectifier circuit for rectifying the second signal, and a second low-pass filter for filtering the signal rectified by the second rectifier circuit. According to the coil drive device of embodiment 4, processing is performed based on the voltage difference using the rectified and smoothed signal, so the output accuracy of the amplified control signal can be further improved compared to a configuration without rectification and smoothing. Rectifier circuit 431 is an example of a "first rectifier circuit," low-pass filter 435 is an example of a "first low-pass filter," rectifier circuit 432 is an example of a "second rectifier circuit," and low-pass filter 436 is an example of a "second low-pass filter." Furthermore, the rectifier circuit can be a full-wave rectifier or a half-wave rectifier.

[0069] In a coil drive device according to a specific embodiment 5 of embodiment 4, the control signal output unit outputs a signal that limits the amplitude of the input signal as the amplification control signal when the temperature of the coil changes and the voltage of the signal filtered by the first low-pass filter reverses in the high-low relationship with the voltage of the signal filtered by the second low-pass filter. According to the coil drive device of embodiment 5, for example, when the temperature of the coil rises and the voltage of the signal filtered by the first low-pass filter reverses in relation to the voltage of the signal filtered by the second low-pass filter, an amplification control signal can be output.

[0070] A coil drive device according to another specific embodiment 6 of embodiment 1 includes a limiting circuit that limits the amplitude of the input signal by the output of the amplification control signal. According to the coil drive device of embodiment 6, the amplitude of the input signal is limited by the output of the amplification control signal, so that amplification of the input signal is suppressed and the coil can be protected.

[0071] In a coil drive device according to another specific embodiment 7 of embodiment 6, the limiting circuit includes a diode that clips the amplitude of the input signal in positive and negative values. According to the coil drive device of embodiment 7, when an amplification control signal is output, the amplitude of the input signal can be limited to positive or negative by a diode. Note that a positive amplitude refers to a position higher than the amplitude center, while a negative amplitude refers to a position lower than the amplitude center.

[0072] A coil drive device according to a specific embodiment 8 of embodiment 2 or 5 includes a changeover switch corresponding to the impedance of the coil. According to the coil drive device of embodiment 8, it is possible to handle cases where there are multiple types of coils driven by the output signal, and the impedance of each coil is different. [Explanation of Symbols]

[0073] 1a...Acoustic system, 1b...Electric motor system, 10a, 10b, 10c, 10d...Coil drive device, 20...Limiting circuit, 30...Amplification section, 40...Control signal output section, 41...Filter section, 43...Rectifier and smoothing section, 44...Comparison section, 411, 412, 413...Bandpass filters, 431, 432...Rectifier circuits, 435, 436...Lowpass filters, D1, D2...Diodes.

Claims

1. An amplification section that amplifies the input signal and outputs it, A control signal output unit outputs an amplification control signal to control the amplification of the amplification unit based on the voltage difference between a first voltage appearing at one end of a coil driven by the output signal from the amplification unit and a second voltage appearing across a detection resistor connected to the other end of the coil. A coil drive device having the following features.

2. The aforementioned control signal output unit is When the coil reaches a threshold temperature or higher, The amplification control signal outputs a signal that limits the amplitude of the input signal. The coil drive device according to claim 1.

3. The first signal representing the first voltage is a signal that has passed through a specific frequency band. The second signal indicating the second voltage is a signal that has passed through the specific frequency band. The coil drive device according to claim 1.

4. The aforementioned control signal output unit is A first rectifier circuit for rectifying the first signal, A first low-pass filter that filters the signal rectified by the first rectifier circuit, A second rectifier circuit for rectifying the second signal, A second low-pass filter that filters the signal rectified by the second rectifier circuit, The coil drive device according to claim 3, including the following:

5. The aforementioned control signal output unit is The temperature of the coil changes, When the voltage of the signal filtered by the first low-pass filter reverses its relative height relationship with the voltage of the signal filtered by the second low-pass filter, the amplification control signal outputs a signal that limits the amplitude of the input signal. The coil drive device according to claim 4.

6. A limiting circuit is provided that limits the amplitude of the input signal based on the output of the amplification control signal. A coil drive device according to claim 1.

7. The limiting circuit is Includes a diode that clips the amplitude of the input signal in both positive and negative directions, The coil drive device according to claim 6.

8. Includes a selector switch corresponding to the impedance of the aforementioned coil, The coil drive device according to claim 2 or 5.

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