Power supply circuits and equipment

Abstract

A power supply circuit (100) has: an input terminal (PVi) to which an input voltage (Vin) is supplied; an inductor (L100); a switching circuit (120) that is electrically connected to the inductor and switches between a storage period in which energy is stored in the inductor and a discharge period in which the energy stored in the inductor is discharged; an output terminal (PVo) that is supplied with current from the inductor during the storage period and the discharge period; and a transistor (FT1) that is electrically connected to the inductor and becomes part of the path of current flowing through the inductor during the storage period and the discharge period. The output terminal outputs a voltage (Vout) based on the storage period, the discharge period, and the input voltage. The transistor suppresses the current flowing through the inductor during a signal processing period in which a signal processing circuit processes a signal, thereby stopping a switching operation in which the storage period and the discharge period are switched.

Description

[Technical Field]

[0001] The present invention relates to a power supply circuit and a device. [Background technology]

[0002] Switching regulators are known as power supply circuits. For example, Patent Document 1 discloses an ultrasonic flowmeter that uses multiple switching regulators as its power supply. The switching regulator outputs a predetermined voltage by switching between a period in which energy is stored in an inductor and a period in which the energy stored in the inductor is released, for example. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-206127 Summary of the Invention [Problem to be solved by the invention]

[0004] However, when a switching regulator (power supply circuit) is placed near a circuit that processes a minute signal, such as an output signal from a sensor, switching noise generated by the switching operation may affect the processing of the minute signal. For example, switching noise may cause errors in the processing results of the minute signal output from the sensor. In consideration of the above circumstances, one aspect of the present invention aims to suppress the generation of switching noise during processing of a predetermined signal, such as a minute signal. [Means for solving the problem]

[0005] According to a preferred aspect of the present invention, there is provided a power supply circuit comprising: an input terminal to which an input voltage is supplied; an inductor including a first terminal and a second terminal; a switching circuit electrically connected to the first terminal and configured to switch between a first period in which energy is stored in the inductor and a second period in which the energy stored in the inductor is released; an output terminal to which a current is supplied from the inductor during at least the second period of the first period and the second period; and a first element electrically connected to the second terminal and configured to be part of a path of a current flowing through the inductor during the first period and the second period, wherein the output terminal outputs a voltage based on the first period, the second period, and the input voltage; and the first element stops the switching operation of switching between the first period and the second period by suppressing the current flowing through the inductor during a predetermined period in which a predetermined circuit is processing a predetermined signal.

[0006] A device according to a preferred aspect of the present invention includes the above-described power supply circuit, a sensor, and the above-described predetermined circuit, wherein the predetermined circuit processes an output signal of the sensor as the predetermined signal. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 2 is an explanatory diagram for explaining an example of a power supply circuit according to the first embodiment. [Figure 2] 2 is an explanatory diagram for explaining an example of a device including the power supply circuit shown in FIG. 1. FIG. [Figure 3] 2 is an explanatory diagram for explaining an example of timing at which the switching operation of the power supply circuit shown in FIG. 1 is stopped. FIG. [Figure 4] FIG. 2 is an explanatory diagram for explaining advantages of the power supply circuit shown in FIG. [Figure 5] FIG. 10 is an explanatory diagram for explaining an example of a power supply circuit according to a second embodiment. [Figure 6] FIG. 10 is an explanatory diagram for explaining an example of a power supply circuit according to a third embodiment. [Figure 7] FIG. 10 is an explanatory diagram for explaining an example of a power supply circuit according to a first modified example. [Figure 8]FIG. 10 is an explanatory diagram for explaining an example of another power supply circuit according to the first modified example. [Figure 9] FIG. 10 is an explanatory diagram for explaining an example of a power supply circuit according to a second modified example. [Figure 10] FIG. 10 is an explanatory diagram for explaining an example of a power supply circuit according to a third modified example. [Figure 11] FIG. 10 is an explanatory diagram for explaining an example of another power supply circuit according to a third modified example. [Figure 12] FIG. 10 is an explanatory diagram for explaining an example of a power supply circuit according to an application example. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In each drawing, the dimensions and scale of each part are appropriately different from those of the actual parts. Furthermore, the embodiments described below are preferred examples of the present invention, and therefore various technically preferable limitations are applied. However, the scope of the present invention is not limited to these embodiments unless otherwise specified in the following description to the effect that the present invention is limited.

[0009] A. Embodiment DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, an example of an outline of a power supply circuit 100 according to an embodiment will be described with reference to FIG.

[0010] A1: First embodiment FIG. 1 is an explanatory diagram for explaining an example of a power supply circuit 100 according to the first embodiment.

[0011] For example, as shown in FIG. 2 (described later), the power supply circuit 100 generates a power supply voltage to be supplied to a circuit that processes minute signals, such as the output signal of a sensor 300. The power supply circuit 100 according to this embodiment is a step-down switching power supply that outputs a voltage Vout that is smaller than an input voltage Vin. For example, the power supply circuit 100 generates the voltage Vout by a switching operation that switches between a storage period in which energy is stored in an inductor L100 and a discharge period in which the energy stored in the inductor L100 is discharged. As a result, the voltage Vout based on the storage period, the discharge period, and the input voltage Vin is output from the output terminal PVo. The storage period is an example of a "first period," and the discharge period is an example of a "second period." Note that this embodiment assumes that the power supply circuit 100 is a synchronous rectification power supply circuit.

[0012] For example, the power supply circuit 100 includes an input terminal PVi to which an input voltage Vin is supplied, an inductor L100, a switching circuit 120, an output terminal PVo to which a voltage Vout is output, and transistors FT1 and FT2. The transistor FT1 is an example of a "first element." Each of the transistors FT1 and FT2 is, for example, an N-channel field-effect transistor. For example, the power supply circuit 100 further includes a control terminal PW to which a control signal Wsig is supplied for controlling the voltage supplied to the gate of the transistor FT2. The power supply circuit 100 further includes capacitors C100 and C102, resistors R100, R102, R104, and R106, and a diode D100.

[0013] For example, switching circuit 120 switches between a storage period in which energy is stored in inductor L100 and a discharge period in which the energy stored in inductor L100 is discharged. For example, switching circuit 120 includes terminals Pv, Ps, Pf, Pg, and Pe, and switching elements SW120 and SW122 such as field-effect transistors. Note that, in FIG. 1, for ease of viewing, only some of the elements included in switching circuit 120 are shown, and other elements are omitted.

[0014] Terminal Pv of switching circuit 120 is electrically connected to input terminal PVi. This supplies input voltage Vin to switching circuit 120. Terminal Ps is also electrically connected to terminal P1 of inductor L100, and terminal Pg is electrically connected to ground terminal GND to which ground voltage is supplied. Terminal P1 of inductor L100 is an example of a "first terminal," and terminal P2 of inductor L100 is an example of a "second terminal."

[0015] Furthermore, for example, the switching element SW120 is connected between terminals Pv and Ps and switches between conduction and non-conduction between terminals Pv and Ps. Furthermore, the switching element SW122 is connected between terminals Ps and Pg and switches between conduction and non-conduction between terminals Ps and Pg. For example, when the switching element SW120 is set to the ON state, conduction occurs between terminals Pv and Ps, and when the switching element SW120 is set to the OFF state, non-conduction occurs between terminals Pv and Ps. Similarly, when the switching element SW122 is set to the ON state, conduction occurs between terminals Ps and Pg, and when the switching element SW122 is set to the OFF state, non-conduction occurs between terminals Ps and Pg. Note that in this embodiment, the switching elements SW120 and SW122 are controlled so that they are not simultaneously in the ON state.

[0016] For example, during the accumulation period, switching element SW120 is set to the ON state, and switching element SW122 is set to the OFF state. Also, during the discharge period, switching element SW120 is set to the OFF state, and switching element SW122 is set to the ON state. That is, during the accumulation period, switching circuit 120 sets the connection between input terminal PVi and terminal P1 of inductor L100 to a conductive state, and during the discharge period, it sets the connection between input terminal PVi and terminal P1 of inductor L100 to a non-conductive state.

[0017] In this embodiment, the switching operation that switches between the accumulation period and the discharge period can be stopped by controlling the state of the transistor FT1, which will be described later, separately from controlling the states of the switching elements SW120 and SW122. For this reason, hereinafter, the period during which the switching element SW120 is in the on state and the switching element SW122 is in the off state may be referred to as the on period, focusing on the state of the switching element SW120. Similarly, the period during which the switching element SW120 is in the off state and the switching element SW122 is in the on state may be referred to as the off period, focusing on the state of the switching element SW120. The switching circuit 120 performs a switching operation that switches between the on period and the off period so that the voltage Vout becomes a predetermined voltage value.

[0018] Terminal Pf of switching circuit 120 is a terminal to which a feedback signal is supplied for adjusting voltage Vout to a predetermined voltage value. In the example shown in Fig. 1, the feedback signal is a signal obtained by resistively dividing voltage Vout using resistors R100 and R102 connected in series between output terminal PVo and ground terminal GND. Voltage Vout is the voltage of capacitor C102 connected between output terminal PVo and ground terminal GND. Resistors R100 and R102 connected in series are connected in parallel to capacitor C102.

[0019] For example, the switching circuit 120 controls a switching operation for switching between an on period and an off period based on a feedback signal supplied to the terminal Pf. Note that well-known control methods such as PWM (Pulse Width Modulation) and PFM (Pulse Frequency Modulation) can be used as a control method for the switching operation for switching between the on period and the off period.

[0020] Furthermore, the terminal Pe of the switching circuit 120 is a terminal to which an enable signal for determining whether or not to operate the switching circuit 120 is supplied. For example, when a low-level enable signal is supplied to the terminal Pe, the operation of the switching circuit 120 stops, and both switching elements SW120 and SW122 are maintained in the off state. In the example shown in FIG. 1, an input voltage Vin is supplied to the terminal Pe as a high-level enable signal for operating the switching circuit 120. In the example shown in FIG. 1, a capacitor C100 for suppressing fluctuations in the input voltage Vin is connected between the input terminal PVi and the ground terminal GND.

[0021] The transistor FT1 is connected, for example, between the terminal P2 of the inductor L100 and the output terminal PVo, and switches between conduction and non-conduction between the terminal P2 of the inductor L100 and the output terminal PVo. For example, when the transistor FT1 is set to the on state, conduction occurs between the terminal P2 of the inductor L100 and the output terminal PVo, and when the transistor FT1 is set to the off state, non-conduction occurs between the terminal P2 of the inductor L100 and the output terminal PVo.

[0022] Therefore, when transistor FT1 is set to the on state, during the on period of switching element SW120, ground terminal GND and terminal P1 of inductor L100 are not electrically connected, and input terminal PVi and output terminal PVo are electrically connected via inductor L100 and transistor FT1. As a result, current is supplied from input terminal PVi to output terminal PVo via switching element SW120, inductor L100, and transistor FT1. Furthermore, as current flows from input terminal PVi to inductor L100, energy is stored in inductor L100. Furthermore, as electrical continuity occurs between input terminal PVi, to which input voltage Vin is supplied, and output terminal PVo, capacitor C102, connected between output terminal PVo and ground terminal GND, is charged.

[0023] Furthermore, when transistor FT1 is set to the ON state, during the OFF period of switching element SW120, the input terminal PVi and terminal P1 of inductor L100 are not electrically connected, and the ground terminal GND and output terminal PVo are electrically connected via inductor L100 and transistor FT1. When switching element SW120 switches from the ON state to the OFF state, inductor L100 acts to continue to pass current to output terminal PVo, thereby releasing the energy stored during the ON period. As a result, current is supplied from ground terminal GND to output terminal PVo via switching element SW122, inductor L100, and transistor FT1. In this way, in this embodiment, current is supplied from inductor L100 to output terminal PVo during both the charge period and the discharge period.

[0024] When transistor FT1 is set to the off state, there is no conduction between terminal P2 of inductor L100 and output terminal PVo, and the current flowing through inductor L100 is suppressed. For example, when transistor FT1 is set to the off state, the flow of current from inductor L100 to output terminal PVo is interrupted, and the supply of current from inductor L100 to output terminal PVo is stopped. In this case, current is supplied to output terminal PVo from capacitor C102 connected between output terminal PVo and ground terminal GND.

[0025] In this embodiment, the power supply circuit 100 also includes a diode D100 whose anode is electrically connected to the terminal P2 of the inductor L100 and whose cathode is electrically connected to the input terminal PVi. When the transistor FT1 switches from the on state to the off state, thereby suppressing the current flowing through the inductor L100, the diode D100 functions as a circuit that returns the energy stored in the inductor L100 as a current to the input terminal PVi. When the release of the energy stored in the inductor L100 ends, the switching operation of switching between the storage period and the release period stops. For example, even when the switching element SW120 is set to the on state, the transistor FT1 is in the off state and a voltage greater than the forward voltage is not applied to the diode D100, so no current flows through the inductor L100. Therefore, the switching operation of switching between the storage period and the release period stops. In this way, when the switching operation stops, diode D100 becomes part of the path of the current flowing through inductor L100 until the release of the energy stored in inductor L100 is completed.

[0026] In a configuration in which the diode D100 is omitted, the energy stored in the inductor L100 is not released as a current when the transistor FT1 switches from an on state to an off state. Therefore, in a configuration in which the diode D100 is omitted, when the transistor FT1 switches from an on state to an off state, a high voltage is generated across the inductor L100, which may damage elements such as the transistor FT1. In this embodiment, even when the transistor FT1 switches from an on state to an off state, the diode D100 allows the energy stored in the inductor L100 to be released as a current to the input terminal PVi. Therefore, in this embodiment, it is possible to prevent elements such as the transistor FT1 from being damaged.

[0027] In this way, transistor FT1 is electrically connected to terminal P2 of inductor L100 and is part of the path of the current flowing through inductor L100 during the accumulation period and the discharge period. Furthermore, transistor FT1 suppresses the current flowing through inductor L100, thereby stopping the switching operation that switches between the accumulation period and the discharge period. Therefore, in this embodiment, by setting transistor FT1 to the off state, the switching operation that switches between the accumulation period and the discharge period can be stopped.

[0028] For example, in this embodiment, the transistor FT1 is set to an off state during a predetermined period when a predetermined circuit located near the power supply circuit 100 processes a predetermined signal, thereby stopping the switching operation that switches between the accumulation period and the discharge period. Here, in this embodiment, it is assumed that the predetermined circuit is a circuit that receives a power supply voltage from the power supply circuit 100. For example, in this embodiment, the transistor FT1 is set to an off state during a predetermined period when a circuit that operates using the voltage Vout output from the output terminal PVo as its power supply voltage processes a predetermined signal, thereby stopping the switching operation that switches between the accumulation period and the discharge period. This makes it possible to suppress the occurrence of switching noise due to the switching operation during the predetermined period. As a result, it is possible to suppress the occurrence of errors due to switching noise in the processing results of the predetermined signal. Note that, although this embodiment assumes that the power supply voltage of the predetermined circuit is supplied from the power supply circuit 100, the power supply voltage of the predetermined circuit does not have to be supplied from the power supply circuit 100. In other words, the predetermined circuit may be a circuit that receives a power supply voltage from a power supply circuit other than the power supply circuit 100. In this case, for example, transistor FT1 is set to an off state during a predetermined period when a predetermined circuit operating on a power supply voltage supplied from a power supply circuit other than power supply circuit 100 is processing a predetermined signal, thereby stopping the switching operation that switches between the accumulation period and the discharge period.

[0029] Here, the on / off switching of the transistor FT1 is controlled by, for example, a control signal Wsig supplied to a control terminal PW.

[0030] For example, the control terminal PW is connected to the gate of transistor FT2. The gate of transistor FT2 may be connected to the ground terminal GND via a pull-down resistor so that the gate is maintained at a low level when the control terminal PW is at a high impedance. The source of transistor FT2 is connected to the ground terminal GND, and the drain of transistor FT2 is connected to the gate of transistor FT1.

[0031] Therefore, when a high-level control signal Wsig is supplied to the control terminal PW, transistor FT2 is turned on, and therefore the gate of transistor FT1 is electrically connected to the ground terminal GND. That is, when a high-level control signal Wsig is supplied to the control terminal PW, transistor FT1 is set to an off state. On the other hand, when a low-level control signal Wsig is supplied to the control terminal PW, transistor FT2 is turned off, and therefore the gate of transistor FT1 is electrically disconnected from the ground terminal GND.

[0032] The gate of transistor FT1 is connected to the drain of transistor FT2 as well as to resistors R104 and R106, which are connected in series between the input terminal PVi and the ground terminal GND. Specifically, the gate of transistor FT1 is connected to the connection node between the resistors R104 and R106. As a result, for example, while the control signal Wsig is at a low level, i.e., while transistor FT2 is in an off state, the gate of transistor FT1 is maintained at a voltage obtained by resistively dividing the input voltage Vin using the resistors R104 and R106. In this embodiment, the power supply circuit 100 is designed so that transistor FT1 is turned on when the voltage obtained by resistively dividing the input voltage Vin using the resistors R104 and R106 is supplied to the gate of transistor FT1.

[0033] For example, if the gate voltage of transistor FT1 is Vg1, the voltage Vg1 during the period when transistor FT2 is in the off state is expressed by equation (1) using the input voltage Vin. Note that R104 in equation (1) represents the resistance value of resistor R104, and R106 in equation (1) represents the resistance value of resistor R106.

[0034] Vg1=Vin×R106 / (R104+R106) …(1)

[0035] The transistor FT1 is turned on when the voltage between the gate and source is equal to or greater than the threshold voltage. Therefore, if the threshold voltage of the transistor FT1 is Vth1, the transistor FT1 is set to the on state when the voltage Vout satisfies the condition expressed by the formula (2), for example.

[0036] Vout <Vg1-Vth1 …(2)

[0037] Furthermore, if the maximum rating of the voltage between the gate and source of the transistor FT1 is Vmax1, the voltage Vout must satisfy the condition expressed by equation (3).

[0038] |Vg1-Vout| <Vmax1 …(3)

[0039] For example, when the transistor FT2 is in an on state, the voltage Vg1 of the gate of the transistor FT1 is 0 V (or approximately 0 V). Therefore, according to the formula (3), the voltage Vout needs to satisfy the condition expressed by the formula (4).

[0040] Vout <Vmax1 …(4)

[0041] Furthermore, for example, when the power supply circuit 100 is started up, the capacitor C102 is not charged, and therefore the voltage Vout is 0 V (or approximately 0 V). Therefore, according to equation (3), the gate voltage Vg1 of the transistor FT1 needs to satisfy the condition expressed by equation (5).

[0042] Vg1 <Vmax1 …(5)

[0043] The power supply circuit 100 is designed to satisfy, for example, the above-mentioned formulas (2) to (5). For example, the power supply circuit 100 shown in FIG. 1 is applied when generating a relatively low voltage Vout that satisfies formula (4). Specifically, for example, the power supply circuit 100 shown in FIG. 1 can be applied when generating a voltage Vout of 5 [V] from an input voltage Vin of 24 [V]. Note that the voltage values of the input voltage Vin and the voltage Vout are not limited to the above-mentioned examples.

[0044] Furthermore, the configuration of power supply circuit 100 is not limited to the example shown in Fig. 1. For example, while Fig. 1 shows power supply circuit 100 in which a field effect transistor (transistor FT1) is used as an element that suppresses the current flowing through inductor L100, this embodiment is not limited to this configuration. Specifically, for example, a bipolar transistor, an analog switch, or the like may be used as an element that suppresses the current flowing through inductor L100.

[0045] Furthermore, although the present embodiment has been described assuming that the power supply circuit 100 is a synchronous rectification power supply circuit, the power supply circuit 100 may be an asynchronous rectification (diode rectification) power supply circuit. When the power supply circuit 100 is an asynchronous rectification power supply circuit, for example, the switching circuit 120 has, instead of the switching element SW122, a diode whose anode is connected to the ground terminal GND and whose cathode is connected to the terminal Ps.

[0046] Next, a device including the power supply circuit 100 will be described with reference to FIG.

[0047] FIG. 2 is an explanatory diagram illustrating an example of a device 10 including the power supply circuit 100 shown in FIG.

[0048] The device 10 is, for example, a measuring device (so-called ultrasonic flowmeter) that measures the flow rate of a fluid FL in a pipe PL. The device 10 has a power supply circuit 100, a signal processing circuit 200 that operates using a voltage Vout supplied from the power supply circuit 100 as a power supply voltage, and a sensor 300 that is disposed on an outer surface OF of the pipe PL.

[0049] The sensor 300 has, for example, two paired ultrasonic probes 320 (320a and 320b). Each ultrasonic probe 320 is disposed on the outer surface OF of the pipe PL. Each ultrasonic probe 320 also has an ultrasonic vibrator 322 that transmits and receives ultrasonic waves and a wedge 324. In FIG. 2, the elements included in each of the ultrasonic probes 320a and 320b (for example, the ultrasonic vibrator 322) are designated by the same alphabet as that designated by the ultrasonic probe 320.

[0050] Each of wedges 324a and 324b includes, for example, a surface SP (SPa or SPb) that is inclined with respect to the outer surface OF of pipe PL. Ultrasonic vibrator 322a is attached to surface SPa of wedge 324a, and ultrasonic vibrator 322b is attached to surface SPb of wedge 324b. Wedge 324 is made of, for example, resin.

[0051] The ultrasonic waves generated by the ultrasonic transducer 322a or 322b propagate through the wedge 324, the pipe PL, and the fluid FL at an angle corresponding to the inclination angle of the surface SP with respect to the outer surface OF of the pipe PL. The ultrasonic transducers 322a and 322b are connected to switches 240a and 240b, respectively, included in the signal processing circuit 200, which will be described later. The switch 240 (240a and 240b) sets one of the ultrasonic transducers 322a and 322b as a transducer that transmits ultrasonic waves, and sets the other of the ultrasonic transducers 322a and 322b as a transducer that receives ultrasonic waves.

[0052] The signal processing circuit 200 is connected to, for example, the output terminal PVo of the power supply circuit 100, and receives the voltage Vout from the output terminal PVo of the power supply circuit 100 as a power supply voltage. The signal processing circuit 200 processes the signal Rsig output from the sensor 300. Note that the signal processing circuit 200 is an example of a "predetermined circuit," and the signal Rsig is an example of an "output signal of the sensor" and an example of a "predetermined signal."

[0053] For example, the signal processing circuit 200 includes a control unit 220, switches 240 (240a and 240b), a transmitting unit 260, a receiving unit 262, a time measuring unit 280, and a flow rate calculating unit 282.

[0054] The control unit 220 controls each element of the signal processing circuit 200. The control unit 220 may be realized by hardware such as a GPU (Graphics Processing Unit), a DSP (Digital Signal Processor), or an FPGA (Field Programmable Gate Array). Alternatively, the control unit 220 may be a processor that controls each element of the signal processing circuit 200. Specifically, the control unit 220 may be configured to include, for example, one or more CPUs (Central Processing Units). In this case, the control unit 220 functions as a functional block that controls each element of the signal processing circuit 200 by, for example, executing a control program read from a storage device (not shown).

[0055] Furthermore, for example, when the control unit 220 is configured to include multiple CPUs, some or all of the functions of the control unit 220 may be realized by these multiple CPUs operating in cooperation with each other in accordance with a program such as a control program. Furthermore, the control unit 220 may be configured to include one or more CPUs and hardware such as a GPU, DSP, or FPGA. In this case, some or all of the functions of the control unit 220 may be realized by hardware such as a DSP.

[0056] In response to an instruction from the control unit 220, the switch 240a connects the ultrasonic probe 320a (more specifically, the ultrasonic transducer 322a) to one of the transmitting unit 260 and the receiving unit 262. Similarly, in response to an instruction from the control unit 220, the switch 240b connects the ultrasonic probe 320b (more specifically, the ultrasonic transducer 322b) to one of the transmitting unit 260 and the receiving unit 262. The switches 240 (240a and 240b) are controlled so that one of the transmitting unit 260 and the receiving unit 262 is connected to the ultrasonic transducer 322a, and the other of the transmitting unit 260 and the receiving unit 262 is connected to the ultrasonic transducer 322b.

[0057] In this embodiment, the switch 240 (240a and 240b) switches the connection destination (ultrasonic transducer 322a or 322b) of each of the transmitting unit 260 and the receiving unit 262 in response to an instruction from the control unit 220. This switches the relationship between transmission and reception of ultrasonic waves by the ultrasonic transducers 322a and 322b.

[0058] The transmitter 260 transmits ultrasonic waves to the fluid FL in the pipe PL by driving the ultrasonic transducer 322a or 322b of the sensor 300 in response to an instruction from the controller 220. Specifically, the transmitter 260 transmits a signal Tsig for driving the ultrasonic transducer 322 to the ultrasonic transducer 322a or 322b via the switch 240a or 240b.

[0059] The receiving unit 262 receives a signal Rsig corresponding to the ultrasonic wave received by the ultrasonic transducer 322a or 322b from the ultrasonic transducer 322a or 322b via the switch 240a or 240b. The signal Rsig corresponding to the ultrasonic wave received by the ultrasonic transducer 322 is a signal obtained by converting the ultrasonic wave that has reached the ultrasonic transducer 322 into an electrical signal by the ultrasonic transducer 322. The receiving unit 262 also generates a pulse signal STsig for measuring the time from transmission to reception of the ultrasonic wave based on the signal Rsig received from the sensor 300 (more specifically, the ultrasonic transducer 322a or 322b). The receiving unit 262 then outputs the pulse signal STsig to the time measurement unit 280. A method for generating the pulse signal STsig will be described later with reference to FIG. 3.

[0060] In the example shown in FIG. 2, the transmitter 260 is connected to the ultrasonic transducer 322a via the switch 240a, and the receiver 262 is connected to the ultrasonic transducer 322b via the switch 240b. In this case, ultrasonic waves generated by the ultrasonic transducer 322a in response to the signal Tsig from the transmitter 260 pass through the wedge 324a and the pipe PL and are incident on the fluid FL in the pipe PL. The ultrasonic waves incident on the fluid FL in the pipe PL are reflected by the inner surface IF of the pipe PL, pass through the pipe PL and the wedge 324b, and propagate to the ultrasonic transducer 322b. The ultrasonic waves propagated to the ultrasonic transducer 322b are converted into a signal Rsig, which is an electrical signal, by the ultrasonic transducer 322b. The signal Rsig is received by the receiver 262 via the switch 240b. The receiver 262 then outputs a pulse signal STsig based on the signal Rsig to the time measurement unit 280.

[0061] The time measurement unit 280 measures the time from transmission to reception of ultrasonic waves (hereinafter also referred to as ultrasonic propagation time) based on the pulse signal STsig, etc. In this embodiment, the control unit 220 switches between the switches 240a and 240b, so that the time measurement unit 280 measures the propagation times in two directions in which the relationship between transmission and reception of ultrasonic waves by the ultrasonic transducers 322a and 322b is reversed. In other words, the time measurement unit 280 measures the propagation time of ultrasonic waves propagating from the ultrasonic transducer 322a to the ultrasonic transducer 322b and the propagation time of ultrasonic waves propagating from the ultrasonic transducer 322b to the ultrasonic transducer 322a.

[0062] The flow rate calculation unit 282 calculates the flow rate of the fluid FL in the pipe PL based on the propagation time of the ultrasonic waves measured by the time measurement unit 280.

[0063] For example, if the propagation time of an ultrasonic wave propagating from ultrasonic transducer 322a to ultrasonic transducer 322b is defined as Tab, then the propagation time Tab is expressed by equation (6). Furthermore, if the propagation time of an ultrasonic wave propagating from ultrasonic transducer 322b to ultrasonic transducer 322a is defined as Tba, then the propagation time Tba is expressed by equation (7). Note that in equations (6) and (7), Cf represents the sound speed of the fluid FL, V represents the velocity (flow velocity) of the fluid FL flowing in the Z direction, θf represents the angle of incidence within the fluid FL, and Lf represents the length of the sound ray SL of the ultrasonic wave propagating within the fluid FL.

[0064] Tab=Lf / (Cf+Vsinθf) …(6) Tba=Lf / (Cf-Vsinθf) …(7)

[0065] Furthermore, by solving equations (6) and (7) for the flow velocity V, the flow velocity V is expressed by equation (8).

[0066] V=(Lf / 2sinθf)×(1 / Tab-1 / Tba) …(8)

[0067] If the flow rate of the fluid FL is Q and the cross-sectional area of the pipe PL is A, the flow rate Q is expressed by equation (9). Furthermore, if the inner diameter of the pipe PL is 2r, for example, the cross-sectional area A of the pipe PL is expressed by equation (10). Note that ^ in equation (10) is a power operator.

[0068] Q = V × A …(9) A = π × (r^2) … (10)

[0069] The angle of incidence θf in the fluid FL is calculated based on, for example, Snell's law. For example, if the speed of sound in the wedge 324 is Cw, the angle of incidence in the wedge 324a is θw, the speed of sound in the pipe PL is Cp, and the angle of incidence in the pipe PL is θp, the relationship between the speed of sound Cf of the fluid FL and the angle of incidence θf in the fluid FL is expressed by Equation (11).

[0070] Cw / sinθw=Cp / sinθp=Cf / sinθf …(11)

[0071] For example, in the sensor 300, the sound speed Cw of the wedge 324 and the incident angle θw within the wedge 324a are known values. Therefore, if the sound speed Cf of the fluid FL is known, the incident angle θf within the fluid FL can be calculated from equation (11). Therefore, for example, the flow rate calculation unit 282 can calculate the flow rate Q of the fluid FL based on the incident angle θf within the fluid FL, the length Lf of the sound ray SL within the fluid FL, the propagation times Tab and Tbap of the ultrasonic waves measured by the time measurement unit 280, and equations (8) and (9).

[0072] Next, the timing at which the switching operation of the power supply circuit 100 is stopped will be described with reference to FIG.

[0073] Fig. 3 is an explanatory diagram for explaining an example of the timing to stop the switching operation of the power supply circuit 100 shown in Fig. 1. Fig. 3 shows a signal Tsig transmitted from the transmitter 260, a control signal Wsig for switching the transistor FT1 on and off, a signal Rsig received by the receiver 262, and a pulse signal STsig received by the time measurement unit 280. The horizontal axis in Fig. 3 represents time. First, an example of a method for generating the pulse signal STsig will be described.

[0074] The pulse signal STsig is generated by the receiving unit 262 based on the signal Rsig, as described in Fig. 2. The voltage Vc shown in Fig. 3 is, for example, the voltage of the signal Rsig output from the sensor 300 when the ultrasonic vibrator 322 is not transmitting ultrasonic waves. The voltage Vthld shown in Fig. 3 is, for example, a predetermined voltage that is compared with the voltage of the signal Rsig by a comparator (not shown) included in the receiving unit 262.

[0075] For example, the receiving unit 262 identifies a specific timing (e.g., time T41) at which the voltage of the signal Rsig relative to the voltage Vc becomes equal to or greater than the voltage Vthld relative to the voltage Vc, based on a comparison result from a comparator (not shown) that compares the voltage of the signal Rsig with the voltage Vthld. Then, the receiving unit 262 raises the pulse signal STsig at the first timing (e.g., time T42) at which the voltage of the signal Rsig becomes equal to or greater than the voltage Vc after the specific timing. Furthermore, after raising the pulse signal STsig, the receiving unit 262 lowers the pulse signal STsig at the first timing at which the voltage of the signal Rsig becomes equal to or less than the voltage Vc. Note that the timing at which the pulse signal STsig falls may be a timing at which a predetermined time has elapsed since the pulse signal STsig was raised.

[0076] The time measurement unit 280 determines the propagation times Tab and Tba, for example, by measuring the time Tst from the time T10 when the signal Tsig for transmitting ultrasound from the ultrasonic transducer 322 is transmitted to the time T42 when the pulse signal STsig first rises.

[0077] Next, the timing of signals Tsig, Rsig, etc. will be described using an example in which the transmitting unit 260 is connected to the ultrasonic transducer 322a via the switch 240a and the receiving unit 262 is connected to the ultrasonic transducer 322b via the switch 240b.

[0078] For example, the transmitting unit 260 transmits a signal Tsig to the ultrasonic transducer 322a from time T10 to time T20. This causes the ultrasonic transducer 322a to transmit ultrasonic waves. Then, for example, the ultrasonic waves transmitted from the ultrasonic transducer 322a are propagated to the ultrasonic transducer 322b, and the receiving unit 262 receives a signal Rsig corresponding to the ultrasonic waves received by the ultrasonic transducer 322b.

[0079] In the example shown in FIG. 3, the ultrasonic wave transmitted from the ultrasonic transducer 322a by the signal Tsig is converted into an electrical signal by the ultrasonic transducer 322b, and the resulting signal Rsig is received by the receiving unit 262 from time T40 to time T50. In this case, for example, the period from time T40 to time T50 corresponds to a signal processing period TPs during which the signal Rsig is processed by the signal processing circuit 200 (more specifically, the receiving unit 262). For example, at time T41, the voltage of the signal Rsig relative to the voltage Vc becomes equal to or higher than the voltage Vthld relative to the voltage Vc. Then, at time T42 when the voltage of the signal Rsig becomes equal to or higher than the voltage Vc, the receiving unit 262 raises the pulse signal STsig. Note that the signal processing period TPs is an example of a "predetermined period."

[0080] In this embodiment, at least during the signal processing period TPs, the switching operation of switching between a storage period in which energy is stored in inductor L100 and a release period in which the energy stored in inductor L100 is released is stopped. That is, during the signal processing period TPs, the switching operation of power supply circuit 100 is stopped.

[0081] For example, the control signal Wsig for switching the transistor FT1 on and off transitions from low to high at time T30, which is before time T40, the start time of the signal processing period TPs. The control signal Wsig is then maintained at high during a period TPw, from time T30 to time T60, which is after time T50, the end time of the signal processing period TPs, and transitions from high to low at time T60. This causes the transistor FT1 to be set to an off state at time T30 and maintained in the off state during the period TPw. In other words, in this embodiment, the transistor FT1 is set to an off state during the period TPw, which includes the signal processing period TPs, and is set to an on state during periods other than the period TPw. As a result, the switching operation for switching between the accumulation period and the discharge period is stopped during the period TPw.

[0082] In this manner, in this embodiment, the switching operation that switches between the accumulation period and the discharge period is stopped during the period TPw that includes the signal processing period TPs, so that it is possible to prevent switching noise caused by the switching operation from occurring during the signal processing period TPs. As a result, in this embodiment, it is possible to prevent the switching noise from propagating to the signal processing circuit 200 and the sensor 300 during the signal processing period TPs.

[0083] Here, for example, if switching noise occurs during the signal processing period TPs, the SNR (Signal-to-Noise Ratio) of the signal Rsig processed by the receiving unit 262 will be lower than when switching noise does not occur during the signal processing period TPs. In particular, if the signal Rsig is a small signal such as the output signal of the sensor 300, it is more susceptible to switching noise than when the signal is large, and the SNR will be lower. When the SNR is low, errors are more likely to occur in the results of signal processing than when the SNR is high. For example, if switching noise occurs during the signal processing period TPs, jitter may occur in the pulse signal STsig due to the influence of the switching noise. If jitter occurs in the pulse signal STsig, for example, the measurement results of the ultrasonic propagation times Tab and Tba may vary more, which may reduce the accuracy of calculating the flow rate Q of the fluid FL.

[0084] In this embodiment, as described above, since the occurrence of switching noise during the signal processing period TPs can be suppressed, the occurrence of errors due to switching noise in the processing results of the signal Rsig can be suppressed. As a result, in this embodiment, for example, the device 10 can suppress the occurrence of variations in the measurement results of the ultrasonic propagation times Tab and Tba, and can accurately calculate the flow rate Q of the fluid FL.

[0085] Here, one possible method for suspending the switching operation of the power supply circuit 100 during the signal processing period TPs is to set the enable signal supplied to the terminal Pe of the switching circuit 120 to a low level during the signal processing period TPs. However, when the operation of the switching circuit 120 is suspended using the enable signal supplied to the terminal Pe, it may take longer to switch between suspending and resuming the switching operation than when using the transistor FT1. For example, in an ultrasonic flowmeter, a period of tens to hundreds of microseconds is sufficient as the period TPw. In contrast, when the operation of the switching circuit 120 is suspended using the enable signal supplied to the terminal Pe, the switching operation may be suspended for a period of several milliseconds. In this case, the period during which the switching operation is suspended may be longer than necessary.

[0086] For example, during a period when switching operation is stopped, current is supplied from capacitor C102 to output terminal PVo. Therefore, when the period when switching operation is stopped is long, the capacitance of capacitor C102 needs to be larger than when the period when switching operation is stopped is short. When the capacitance of capacitor C102 is large, capacitor C102 becomes larger than when the capacitance of capacitor C102 is small, and the cost of power supply circuit 100 increases.

[0087] In this embodiment, as described above, the switching operation of the power supply circuit 100 can be stopped during the signal processing period TPs by setting the transistor FT1 to an off state without stopping the operation of the switching circuit 120. In this case, since the operation of the switching circuit 120 is not stopped, the switching operation resumes when the control signal Wsig transitions to a low level. Therefore, in this embodiment, the period during which the switching operation is stopped can be prevented from becoming longer than necessary, thereby preventing the capacitor C102 from becoming larger. Therefore, in this embodiment, the generation of switching noise during the signal processing period TPs can be prevented while preventing an increase in the cost of the power supply circuit 100. Furthermore, in this embodiment, the period during which the switching operation is stopped can be prevented from becoming longer than necessary, thereby improving the usability of the power supply circuit 100.

[0088] The intervals between the transmission times (times T10 and T20) of the signal Tsig and the start and end times (times T40 and T50) of the signal processing period TPs are estimated to some extent based on, for example, parameters of the wedge 324, the pipe PL, and the fluid FL. Then, a period with a certain margin between the estimated start and end times of the signal processing period TPs is set as the period TPw. The actual measured values of the start and end times of the signal processing period TPs may be fed back to set the period TPw.

[0089] Next, with reference to FIG. 4, other advantages of using the power supply circuit 100 will be described.

[0090] FIG. 4 is an explanatory diagram illustrating the advantages of the power supply circuit 100 shown in FIG. 4. "With element for suppressing current flowing through inductor" in FIG. 4 shows an overview of device 10 having power supply circuit 100 according to this embodiment. Also, "Without element for suppressing current flowing through inductor" in FIG. 4 shows an overview of device 10Z having power supply circuit 100Z, which is contrasted with power supply circuit 100. Power supply circuit 100Z is similar to power supply circuit 100 shown in FIG. 1, except that an element (such as transistor FT1) for suppressing current flowing through inductor L100 has been omitted from power supply circuit 100.

[0091] 4, it is assumed that the signal processing circuit 200 is included in a printed circuit board PB. Therefore, for example, the device 10 has a power supply circuit 100, a printed circuit board PB including the signal processing circuit 200, and a sensor 300. First, a device 10Z that is comparable to the device 10 will be described.

[0092] The device 10Z is similar to the device 10 except that it includes a power supply circuit 100Z instead of the power supply circuit 100 and includes a shield member SLD. For example, the device 10Z includes the power supply circuit 100Z, a printed circuit board PB including a signal processing circuit 200, a shield member SLD, and a sensor 300. The power supply circuit 100Z is similar to the power supply circuit 100 except that the transistors FT1 and FT2, the resistors R104 and R106, and the diode D100 are omitted from the power supply circuit 100. Therefore, in the power supply circuit 100Z, switching noise occurs during a signal processing period TPs in which the signal RSig is processed.

[0093] For this reason, in the device 10Z, as shown in “No element suppressing current flowing through the inductor” in FIG. 4 , a shield member SLD that prevents switching noise from propagating from the power supply circuit 100Z to the sensor 300 is disposed between the power supply circuit 100Z and the sensor 300. The shield member SLD may be a shielding sheet or a shielding plate. In the device 10Z, the shield member SLD disposed between the power supply circuit 100Z and the sensor 300 prevents switching noise from being included in the signal RSig output from the sensor 300 (the output signal of the sensor 300). However, in the device 10Z, there is a risk that switching noise may propagate from the power supply circuit 100Z to the sensor 300 via the connection wiring between the sensor 300 and the signal processing circuit 200. The connection wiring between the sensor 300 and the signal processing circuit 200 may be, for example, a wiring for transmitting the signal RSig from the sensor 300 to the signal processing circuit 200.

[0094] Furthermore, since the shield member SLD is disposed between the power supply circuit 100Z and the sensor 300, the device 10Z has a more complex structure than the device 10. As a result, the device 10Z is larger than the device 10. Furthermore, the cost of the device 10Z is higher than that of the device 10.

[0095] In contrast, in this embodiment, since it is possible to suppress the occurrence of switching noise during the signal processing period TPs in which the signal RSig is processed, it is possible to omit the shield member SLD, as shown in "Element for suppressing current flowing through the inductor" in FIG. 4. As a result, in this embodiment, the structure of the device 10 can be simplified. Therefore, in this embodiment, it is possible to reduce the size and cost of the device 10.

[0096] As described above, in this embodiment, the power supply circuit 100 includes an input terminal PVi to which an input voltage Vin is supplied, an inductor L100 including terminals P1 and P2, a switching circuit 120 electrically connected to terminal P1 and switching between a storage period in which energy is stored in inductor L100 and a discharge period in which the energy stored in inductor L100 is discharged, an output terminal PVo to which current is supplied from inductor L100 during the storage period and the discharge period, and a transistor FT1 electrically connected to terminal P2 and forming part of the path of current flowing through inductor L100 during the storage period and the discharge period. The output terminal PVo outputs a voltage Vout based on the storage period, the discharge period, and the input voltage Vin. During a signal processing period TPs in which a signal processing circuit 200, which operates using the voltage Vout output from the output terminal PVo as a power supply voltage, processes a signal Rsig, the transistor FT1 suppresses the current flowing through inductor L100, thereby stopping the switching operation that switches between the storage period and the discharge period. Furthermore, the device 10 of this embodiment includes a power supply circuit 100, a sensor 300, and a signal processing circuit 200 that receives a power supply voltage (voltage Vout) from the output terminal PVo of the power supply circuit 100 and processes the output signal of the sensor 300 as a signal Rsig.

[0097] As described above, in this embodiment, the switching operation of switching between the accumulation period and the discharge period is stopped during the signal processing period TPs in which the signal Rsig is processed by the signal processing circuit 200. As a result, in this embodiment, it is possible to suppress the occurrence of switching noise due to the switching operation during the signal processing period TPs. As a result, in this embodiment, it is possible to suppress the occurrence of switching noise due to the switching operation during the signal processing period TPs without providing a shield member SLD that prevents noise from propagating from the power supply circuit 100 to the sensor 300. As a result, in this embodiment, it is possible to suppress the occurrence of errors due to switching noise in the processing results of the signal Rsig while suppressing an increase in the cost of the device 10.

[0098] Furthermore, in this embodiment, the switching operation can be stopped by suppressing the current flowing through inductor L100 without stopping the operation of switching circuit 120. Therefore, in this embodiment, even if it takes time to switch between stopping and restarting the operation of switching circuit 120, it is possible to prevent the time required for switching between stopping and restarting the switching operation from becoming long. As a result, in this embodiment, the usability of the power supply circuit 100 can be improved.

[0099] In this embodiment, the switching circuit 120 establishes a conductive state between the input terminal PVi and the terminal P1 during the accumulation period, and establishes a non-conductive state between the input terminal PVi and the terminal P1 during the discharge period. The transistor FT1 establishes a conductive state between the terminal P2 and the output terminal PVo during the accumulation period and the discharge period, and establishes a non-conductive state between the terminal P2 and the output terminal PVo during the signal processing period TPs. A current is supplied to the output terminal PVo from the inductor L100 during the accumulation period and the discharge period. The output terminal PVo outputs a voltage Vout based on the accumulation period, the discharge period, and the input voltage Vin.

[0100] Therefore, in this embodiment, the power supply circuit 100 operates as a step-down switching power supply that outputs a voltage Vout that is smaller than the input voltage Vin. In this way, in this embodiment, even when the power supply circuit 100 operates as a step-down switching power supply, the switching operation can be stopped during the signal processing period TPs by suppressing the current flowing through the inductor L100. Therefore, even when the power supply circuit 100 operates as a step-down switching power supply, the above-mentioned effects, such as suppressing the generation of switching noise due to the switching operation during the signal processing period TPs, can be obtained.

[0101] In this embodiment, the power supply circuit 100 further includes a diode D100 electrically connected to terminal P2 of inductor L100 and forming part of the path of current flowing through inductor L100 when switching operation is stopped. For example, the power supply circuit 100 includes a diode D100 whose anode is electrically connected to terminal P2 and whose cathode is electrically connected to input terminal PVi. As a result, in this embodiment, when transistor FT1 switches from the on state to the off state, diode D100 can release the energy stored in inductor L100 as a current to input terminal PVi. Therefore, in this embodiment, it is possible to prevent elements such as transistor FT1 from being destroyed.

[0102] A2: Second embodiment Figure 5 is an explanatory diagram for explaining an example of a power supply circuit 100A according to a second embodiment. Elements similar to those explained in Figures 1 to 4 are given the same reference numerals, and detailed explanations will be omitted. In this embodiment, the power supply circuit 100A is also a step-down switching power supply that outputs a voltage Vout that is smaller than an input voltage Vin.

[0103] The power supply circuit 100A has a switching circuit 120A instead of the switching circuit 120 shown in Fig. 1. Furthermore, the resistors R104 and R106 shown in Fig. 1 are omitted from the power supply circuit 100A, and instead resistors R108 and R110, a capacitor C104, and a Zener diode ZD100 are provided. The rest of the configuration of the power supply circuit 100A is the same as that of the power supply circuit 100 shown in Fig. 1.

[0104] 1, switching circuit 120A switches between a storage period during which energy is stored in inductor L100 and a discharge period during which the energy stored in inductor L100 is discharged. For example, switching circuit 120A has a diode D120 having an anode connected to ground terminal GND and a cathode connected to terminal Ps, instead of switching element SW122 shown in FIG. 1. In other words, power supply circuit 100A including switching circuit 120A is an asynchronous rectification (diode rectification) power supply circuit.

[0105] For example, during the accumulation period, the switching element SW120 is set to the ON state, and a current is supplied from the input terminal PVi to the output terminal PVo via the switching element SW120, inductor L100, and transistor FT1. Also, during the discharge period, the switching element SW120 is set to the OFF state, and a current is supplied from the ground terminal GND to the output terminal PVo via the diode D120, inductor L100, and transistor FT1.

[0106] Resistors R108 and R110 are connected in series between the drain of transistor FT2 and the input terminal PVi. Specifically, one end of resistor R108 is connected to the drain of transistor FT2, the other end of resistor R108 is connected to one end of resistor R110, and the other end of resistor R110 is connected to the input terminal PVi. In addition, the connection node between resistors R108 and R110 is connected to the gate of transistor FT1.

[0107] One end of the capacitor C104 is connected to the gate of the transistor FT1, and the other end of the capacitor C104 is connected to the drain of the transistor FT2. For example, when the transistor FT2 is set to the ON state, the capacitor C104 has the function of quickly reducing the voltage Vg1 at the gate of the transistor FT1, thereby quickly transitioning the transistor FT1 to the OFF state.

[0108] The anode of the Zener diode ZD100 is connected to the source of the transistor FT1, and the cathode of the Zener diode ZD100 is connected to the gate of the transistor FT1. The Zener diode ZD100 is an example of a "voltage control element," the anode of the Zener diode ZD100 is an example of a "fourth terminal," and the cathode of the Zener diode ZD100 is an example of a "third terminal."

[0109] 5, transistor FT1 is turned on when transistor FT2 is turned off. For example, when transistor FT2 is turned off, no current flows through resistors R108 and R110, and therefore the gate voltage Vg1 of transistor FT1 becomes equal to the input voltage Vin. Therefore, the condition for transistor FT1 to be turned on when transistor FT2 is turned off is expressed by equation (12) using the threshold voltage Vth1 of transistor FT1.

[0110] Vout <Vin-Vth1 …(12)

[0111] Generally, the input voltage Vin is significantly greater than the threshold voltage Vth1, and therefore the power supply circuit 100A shown in Fig. 5 is applied to cases where a relatively high voltage Vout is generated.

[0112] Furthermore, when transistor FT2 is set to the ON state, transistor FT1 is set to the OFF state. For example, when transistor FT2 is set to the ON state, the gate voltage Vg1 of transistor FT1 is expressed by equation (13) using the input voltage Vin. Note that R108 in equation (13) represents the resistance value of resistor R108, and R110 in equation (13) represents the resistance value of resistor R110.

[0113] Vg1=Vin×R108 / (R108+R110) …(13)

[0114] The condition for the transistor FT1 to be in the off state is expressed by equation (14) using the threshold voltage Vth1 of the transistor FT1.

[0115] Vg1 <Vout+Vth1 …(14)

[0116] From equation (14), it can be seen that the power supply circuit 100A is more applicable when the voltage Vout is large. For example, the power supply circuit 100A shown in FIG. 2 can be applied when generating a voltage Vout of 18 V from an input voltage Vin of 24 V. Note that the voltage values of the input voltage Vin and the voltage Vout are not limited to the above examples.

[0117] Consider the gate voltage Vg1 of transistor FT1 when Zener diode ZD100 is omitted from power supply circuit 100A. When power supply circuit 100A is started, capacitor C102 is not charged. Therefore, when power supply circuit 100A is started, gate voltage Vg1 of transistor FT1 becomes the gate-to-source voltage. Therefore, when transistor FT2 is set to the off state and gate voltage Vg1 of transistor FT1 becomes equal to input voltage Vin, there is a risk that the gate-to-source voltage of transistor FT1 will exceed the maximum rated voltage Vmax1, as shown in equation (15).

[0118] Vin=Vg1>Vmax1 …(15)

[0119] In this embodiment, a Zener diode ZD100 is connected between the gate and source of transistor FT1. The Zener diode ZD100 operates so that the voltage between the gate and source of transistor FT1 does not exceed a maximum rating Vmax1. The maximum rating Vmax1 is an example of a "predetermined voltage." The voltage between the gate and source of transistor FT1 is limited to, for example, the breakdown voltage of the Zener diode ZD100. Therefore, if the breakdown voltage of the Zener diode ZD100 satisfies equation (16), the voltage between the gate and source of transistor FT1 does not exceed the maximum rating Vmax1. Note that Vzd1 in equation (16) represents the breakdown voltage of the Zener diode ZD100.

[0120] Vzd1 <Vmax1 …(16)

[0121] In this way, in the power supply circuit 100A shown in FIG. 5, the Zener diode ZD100 that satisfies the formula (16) is used, so that the voltage between the gate and source of the transistor FT1 is limited so as not to exceed the maximum rating Vmax1.

[0122] The configuration of the power supply circuit 100A is not limited to the example shown in Fig. 5. For example, the element (voltage control element) that limits the voltage between the gate and source of the transistor FT1 is not limited to the Zener diode ZD100, and may be a diode, a surge absorber, a varistor, or the like, as long as it can limit the voltage. Also, for example, the power supply circuit 100A may have the switching circuit 120 shown in Fig. 1 instead of the switching circuit 120A. Also, the device 10 according to this embodiment is similar to the device 10 shown in Fig. 2 and the like, except that it has the power supply circuit 100A instead of the power supply circuit 100 shown in Fig. 1.

[0123] As described above, this embodiment can also achieve the same effects as the first embodiment. Furthermore, in this embodiment, the power supply circuit 100A includes a Zener diode ZD100 that includes a cathode and an anode and operates so that the voltage of the cathode relative to the voltage of the anode does not exceed the maximum rated voltage Vmax1 between the gate and source of transistor FT1. Transistor FT1 is a field-effect transistor. The cathode of the Zener diode ZD100 is connected to the gate of the field-effect transistor (transistor FT1), and the anode of the Zener diode ZD100 is connected to the source of the field-effect transistor (transistor FT1). In this way, in this embodiment, the Zener diode ZD100 limits the voltage between the gate and source of transistor FT1 so that it does not exceed the maximum rated voltage Vmax1, thereby preventing breakdown of elements such as transistor FT1.

[0124] A3: Third embodiment FIG. 6 is an explanatory diagram illustrating an example of a power supply circuit 100B according to a third embodiment. Elements similar to those described in FIGS. 1 to 5 are assigned the same reference numerals, and detailed description thereof will be omitted. In this embodiment, the power supply circuit 100B is a step-up switching power supply that outputs a voltage Vout higher than an input voltage Vin. For example, the power supply circuit 100B shown in FIG. 6 can be applied when generating a voltage Vout of 35 V from an input voltage Vin of 24 V. Note that the voltage values of the input voltage Vin and the voltage Vout are not limited to those described above.

[0125] The power supply circuit 100B has a switching circuit 120B instead of the switching circuit 120 shown in FIG. 1. Furthermore, the resistors R104 and R106 and the transistor FT1 shown in FIG. 1 are omitted from the power supply circuit 100B, and instead resistors R112 and R114, a bipolar transistor BT1, and diodes D102 and D104 are provided. The rest of the configuration of the power supply circuit 100B is the same as that of the power supply circuit 100 shown in FIG. 1. However, in FIG. 6, the polarity of the control signal WNsig for suppressing the current flowing through the inductor L100 is opposite to that of the control signal Wsig described in FIG. 1 and elsewhere. For this reason, the control terminal PW to which the control signal WNsig is supplied has a lowercase alphabet "n" suffixed to its reference symbol.

[0126] The bipolar transistor BT1 is an example of a "first element." For example, the bipolar transistor BT1 is a PNP transistor whose collector is connected to the terminal P2 of the inductor L100 and whose emitter is connected to the input terminal PVi, and which switches between conduction and non-conduction between the terminal P2 of the inductor L100 and the input terminal PVi. Specifically, for example, when the bipolar transistor BT1 is set to an ON state, conduction occurs between the terminal P2 of the inductor L100 and the input terminal PVi, and when the bipolar transistor BT1 is set to an OFF state, non-conduction occurs between the terminal P2 of the inductor L100 and the input terminal PVi. The base of the bipolar transistor BT1 is connected to the connection node between resistors R112 and R114, which will be described later. Switching the bipolar transistor BT1 on and off will be described later.

[0127] The resistors R112 and R114 are connected in series between the drain of the transistor FT2 and the input terminal PVi. Specifically, one end of the resistor R112 is connected to the drain of the transistor FT2, the other end of the resistor R112 is connected to one end of the resistor R114, and the other end of the resistor R114 is connected to the input terminal PVi. As described above, the connection node between the resistors R112 and R114 is connected to the base of the bipolar transistor BT1.

[0128] The anode of diode D102 is connected to terminal P1 of inductor L100, and the cathode of diode D102 is connected to output terminal PVo.

[0129] Similar to the switching circuit 120 shown in FIG. 1, the switching circuit 120B switches between a storage period during which energy is stored in inductor L100 and a discharge period during which the energy stored in inductor L100 is discharged. For example, the switching circuit 120B includes terminals Ps, Pf, Pg, and Pe, and a switching element SW124 such as a field-effect transistor. Note that in FIG. 6 as well, for ease of viewing, only some of the elements included in the switching circuit 120B are shown, while other elements are omitted. Furthermore, the terminals Pf, Pg, and Pe of the switching circuit 120B are similar to the terminals Pf, Pg, and Pe of the switching circuit 120 described in FIG. 1, and therefore will not be described here.

[0130] The switching element SW124 is connected between the terminals Ps and Pg, and switches between conduction and non-conduction between the terminals Ps and Pg. For example, when the switching element SW124 is set to the on state, conduction occurs between the terminals Ps and Pg, and when the switching element SW124 is set to the off state, non-conduction occurs between the terminals Ps and Pg.

[0131] For example, during the accumulation period, switching element SW124 is set to the ON state, and current flows from input terminal PVi to ground terminal GND via bipolar transistor BT1, inductor L100, and switching element SW124. This causes energy to be accumulated in inductor L100. Furthermore, during the discharge period, switching element SW124 is set to the OFF state. This causes the energy accumulated in inductor L100 to be discharged, and current is supplied from input terminal PVi to output terminal PVo via bipolar transistor BT1, inductor L100, and diode D102.

[0132] Here, the bipolar transistor BT1 is switched on and off by switching the transistor FT2 on and off.

[0133] For example, transistor FT2 is set to the off state by supplying a low-level control signal WNsig to the control terminal PWn connected to the gate. When transistor FT2 is set to the off state, no current flows through resistors R112 and R114, and therefore no current is supplied to the base of bipolar transistor BT1. This turns bipolar transistor BT1 off, suppressing the current flow through inductor L100. As a result, the switching operation that switches between the charge accumulation period and the discharge period is stopped. Therefore, in this embodiment, for example, by maintaining the control signal WNsig at a low level during period TPw shown in FIG. 3, the switching operation can be stopped during period TPw. As a result, in this embodiment, it is possible to suppress the occurrence of switching noise during signal processing period TPs, which is included in period TPw.

[0134] In this embodiment, as described above, the resistor R114 is connected between the base and emitter of the bipolar transistor BT1. For example, when the bipolar transistor BT1 is set to the off state, the resistor R114 has the function of quickly reducing the voltage between the emitter and base of the bipolar transistor BT1, thereby quickly transitioning the bipolar transistor BT1 to the off state.

[0135] Furthermore, when a high-level control signal WNsig is supplied to the gate of transistor FT2 via the control terminal PWn, transistor FT2 is turned on, and current is supplied to the base of bipolar transistor BT1. As a result, bipolar transistor BT1 is turned on, and switching operation resumes. In this embodiment, the switching operation can be stopped without stopping the operation of the switching circuit 120B, thereby preventing the time required to stop and resume the switching operation from becoming long. The control signal WNsig is maintained at a high level, for example, during a period other than the period TPw shown in FIG. 3. The gate of transistor FT2 may be connected to the input terminal PVi via a pull-up resistor so that the gate is maintained at a high level when the control terminal PWn is in a high impedance state.

[0136] Furthermore, in this embodiment, the power supply circuit 100B includes a diode D104 whose anode is grounded and whose cathode is electrically connected to terminal P2 of inductor L100. When bipolar transistor BT1 switches from the on state to the off state, thereby suppressing the current flowing through inductor L100, diode D104 acts as a circuit that transfers the energy stored in inductor L100 as a current to output terminal PVo. In other words, when switching operation stops, diode D104 becomes part of the path of the current flowing through inductor L100 until the release of the energy stored in inductor L100 is completed.

[0137] In a configuration without diode D104, the energy stored in inductor L100 is not released as a current when bipolar transistor BT1 switches from an on state to an off state. Therefore, in a configuration without diode D104, when bipolar transistor BT1 switches from an on state to an off state, a high voltage is generated across inductor L100, which may damage switching circuit 120B and other components. In this embodiment, even when bipolar transistor BT1 switches from an on state to an off state, diode D104 allows the energy stored in inductor L100 to be released as a current to output terminal PVo. Therefore, in this embodiment, it is possible to prevent the switching circuit 120B and other components from being damaged.

[0138] The configuration of the power supply circuit 100B is not limited to the example shown in FIG. 6. For example, while FIG. 6 shows the power supply circuit 100B in which a bipolar transistor BT1 is used as an element for suppressing the current flowing through the inductor L100, this embodiment is not limited to this configuration. Specifically, for example, a field-effect transistor, an analog switch, or the like may be used as the element for suppressing the current flowing through the inductor L100. Furthermore, the device 10 according to this embodiment is similar to the device 10 shown in FIG. 2 and the like, except that it has a power supply circuit 100B instead of the power supply circuit 100 shown in FIG. 1.

[0139] As described above, this embodiment can also achieve the same effects as the first and second embodiments. Furthermore, in this embodiment, the switching circuit 120B establishes a conductive state between the ground terminal GND, to which a ground voltage is supplied, and the terminal P1 during the accumulation period, and establishes a non-conductive state between the ground terminal GND and the terminal P1 during the discharge period. The bipolar transistor BT1 establishes a conductive state between the terminal P2 and the input terminal PVi during the accumulation period and the discharge period, and establishes a non-conductive state between the terminal P2 and the input terminal PVi during the signal processing period TPs.

[0140] Therefore, in this embodiment, the power supply circuit 100B operates as a step-up switching power supply that outputs a voltage Vout that is greater than the input voltage Vin. In this way, in this embodiment, even when the power supply circuit 100B operates as a step-up switching power supply, the switching operation can be stopped during the signal processing period TPs by suppressing the current flowing through the inductor L100. As a result, even when the power supply circuit 100B operates as a step-up switching power supply, the same effects as those of the above-described embodiment can be obtained, such as suppressing the generation of switching noise due to the switching operation during the signal processing period TPs.

[0141] In this embodiment, the power supply circuit 100B further includes a diode D104 electrically connected to terminal P2 of inductor L100 and forming part of the path of current flowing through inductor L100 when switching operation is stopped. For example, the power supply circuit 100B includes a diode D104 whose cathode is electrically connected to terminal P2 and whose anode is grounded. As a result, in this embodiment, when the bipolar transistor BT1 switches from the on state to the off state, the diode D104 can release the energy stored in inductor L100 as a current to the output terminal PVo. Therefore, in this embodiment, it is possible to prevent breakdown of the switching circuit 120B and the like.

[0142] B: Modified example The above-described exemplary embodiments may be modified in various ways. Specific examples of modifications that may be applied to the above-described embodiments are given below. Two or more of the following examples may be combined together as long as they are not mutually inconsistent.

[0143] B1: First modified example In the first and second embodiments described above, diode D100 is used as an element that forms part of the path of current flowing through inductor L100 when switching operation stops, but the present invention is not limited to this. For example, as shown in Figure 7 or 8, power supply circuit 100 or 100A may include Zener diode ZD102, instead of diode D100, whose cathode is electrically connected to terminal P2 of inductor L100 and whose anode is electrically connected to ground terminal GND.

[0144] 7 is an explanatory diagram for explaining an example of a power supply circuit 100C according to a first modification. Elements similar to those explained in FIGS. 1 to 6 are given the same reference numerals, and detailed explanations thereof will be omitted.

[0145] The power supply circuit 100C is similar to the power supply circuit 100 shown in Fig. 1, except that it has a Zener diode ZD102 instead of the diode D100 shown in Fig. 1. The Zener diode ZD102 is an example of a "diode."

[0146] The cathode of the Zener diode ZD102 is electrically connected to the terminal P2 of the inductor L100, and the anode of the Zener diode ZD102 is electrically connected to the ground terminal GND. When the transistor FT1 switches from the on state to the off state, suppressing the current flowing through the inductor L100, the Zener diode ZD102 functions as a circuit that returns the energy stored in the inductor L100 as a current to the ground terminal GND. When the release of the energy stored in the inductor L100 ends, the switching operation that switches between the storage period and the release period stops. For example, even when the switching element SW120 is set to the on state, the transistor FT1 is in the off state, and a voltage greater than the breakdown voltage is not applied to the Zener diode ZD102, so no current flows through the inductor L100. Therefore, the switching operation that switches between the storage period and the release period stops. In this way, when the switching operation stops, Zener diode ZD102 becomes part of the path of the current flowing through inductor L100 until the release of the energy stored in inductor L100 is completed. Here, Zener diode ZD102 is preferably a Zener diode whose breakdown voltage is greater than voltage Vin and smaller than the maximum rated voltage between the drain and source of transistor FT1, for example.

[0147] 8 is an explanatory diagram for explaining an example of another power supply circuit 100D according to the first modification. Elements similar to those explained in FIGS. 1 to 7 are given the same reference numerals, and detailed explanations thereof will be omitted.

[0148] Power supply circuit 100D is similar to power supply circuit 100A shown in Fig. 5, except that it has a Zener diode ZD102 instead of diode D100 shown in Fig. 5. In power supply circuit 100D, as in power supply circuit 100C shown in Fig. 7, the cathode of Zener diode ZD102 is electrically connected to terminal P2 of inductor L100, and the anode of Zener diode ZD102 is electrically connected to ground terminal GND. As described in Fig. 7, when switching operation stops, Zener diode ZD102 becomes part of the path of current flowing through inductor L100 until release of energy stored in inductor L100 is completed.

[0149] As described above, in this modification, the power supply circuit 100C or 100D has a Zener diode ZD102 that is electrically connected to the terminal P2 of the inductor L100 and that becomes part of the path of the current that flows through the inductor L100 when the switching operation stops. In this modification, too, the same effects as in the first and second embodiments described above can be obtained.

[0150] B2: Second variant In the third embodiment described above, the diode D104 is used as an element that forms part of the path of the current that flows through the inductor L100 when the switching operation stops, but the present invention is not limited to this. For example, as shown in Fig. 9, the power supply circuit 100B may have a Zener diode ZD104, instead of the diode D104, whose anode is electrically connected to the terminal P2 of the inductor L100 and whose cathode is electrically connected to the input terminal PVi.

[0151] 9 is an explanatory diagram for explaining an example of a power supply circuit 100E according to a second modification. Elements similar to those explained in FIGS. 1 to 8 are given the same reference numerals, and detailed explanations thereof will be omitted.

[0152] The power supply circuit 100E is similar to the power supply circuit 100B shown in Fig. 6 except that it has a Zener diode ZD104 instead of the diode D104 shown in Fig. 6. The Zener diode ZD104 is an example of a "diode".

[0153] The cathode of the Zener diode ZD104 is electrically connected to the input terminal PVi, and the anode of the Zener diode ZD104 is electrically connected to the terminal P2 of the inductor L100. When the bipolar transistor BT1 switches from the on state to the off state, thereby suppressing the current flowing through the inductor L100, the Zener diode ZD104 acts as a circuit that transfers the energy stored in the inductor L100 as a current to the output terminal PVo, etc. In other words, when the switching operation stops, the Zener diode ZD104 becomes part of the path of the current flowing through the inductor L100 until the release of the energy stored in the inductor L100 is completed. The Zener diode ZD104 is preferably, for example, a Zener diode whose breakdown voltage is greater than the input voltage Vin and less than the maximum rated voltage between the collector and emitter of the bipolar transistor BT1.

[0154] As described above, in this modification, the power supply circuit 100E has the Zener diode ZD104 that is electrically connected to the terminal P2 of the inductor L100 and that becomes part of the path of the current that flows through the inductor L100 when the switching operation stops. In this modification, too, the same effects as in the third embodiment described above can be obtained.

[0155] B3: Third variant The power supply circuit 100 and the like may have the main components shown in FIG. 10 or FIG.

[0156] 10 is an explanatory diagram for explaining an example of a power supply circuit 100F according to a third modification. Elements similar to those explained in FIGS. 1 to 9 are given the same reference numerals, and detailed explanations thereof will be omitted.

[0157] The power supply circuit 100F is, for example, a step-down switching power supply that outputs a voltage Vout that is smaller than an input voltage Vin. For example, the power supply circuit 100F includes an input terminal PVi to which the input voltage Vin is supplied, an inductor L100 including terminals P1 and P2, a switching circuit 120C, an output terminal PVo, and a switching element SW106.

[0158] For example, the switching circuit 120C has a terminal Ps electrically connected to a terminal P1 of the inductor L100, and switches between a storage period in which energy is stored in the inductor L100 and a discharge period in which the energy stored in the inductor L100 is discharged. For example, the switching circuit 120C switches between the storage period and the discharge period by switching between conduction and non-conduction between the terminal Pv, to which the input voltage Vin is supplied from the input terminal PVi, and the terminal Ps. Specifically, for example, the switching circuit 120C may include switching elements SW120 and SW122, similar to the switching circuit 120 shown in FIG. 1. Alternatively, the switching circuit 120C may include a switching element SW120 and a diode D120, similar to the switching circuit 120A shown in FIG. 5.

[0159] Terminal P2 of inductor L100 is connected to output terminal PVo via switching element SW106. That is, switching element SW106 is electrically connected to terminal P2 of inductor L100. Furthermore, when switching element SW106 is in the on state, it becomes part of the path of current flowing through inductor L100 during the charge period and discharge period. The on and off of switching element SW106 is controlled by a control signal Wsig supplied to control terminal PW. For example, a field-effect transistor, a bipolar transistor, or an analog switch may be used as switching element SW106.

[0160] A current is supplied to the output terminal PVo from the inductor L100 during the accumulation period and the discharge period, and the output terminal PVo outputs a voltage Vout based on the accumulation period, the discharge period, and the input voltage Vin.

[0161] During the signal processing period TPs, the switching element SW106 stops the switching operation that switches between the storage period and the discharge period by suppressing the current flowing through the inductor L100. As described in FIG. 3, the signal processing period TPs is a period during which the signal processing circuit 200, which operates using the voltage Vout output from the output terminal PVo as its power supply voltage, processes the signal Rsig.

[0162] 11 is an explanatory diagram for explaining an example of another power supply circuit 100G according to Modification 3. Elements similar to those explained in FIGS. 1 to 10 are given the same reference numerals, and detailed explanations thereof will be omitted.

[0163] The power supply circuit 100G is, for example, a step-up switching power supply that outputs a voltage Vout that is greater than an input voltage Vin. For example, the power supply circuit 100G includes an input terminal PVi to which the input voltage Vin is supplied, an inductor L100 including terminals P1 and P2, a switching circuit 120D, an output terminal PVo, and a switching element SW108.

[0164] For example, the switching circuit 120D has a terminal Ps electrically connected to a terminal P1 of the inductor L100, and switches between a storage period in which energy is stored in the inductor L100 and a discharge period in which the energy stored in the inductor L100 is discharged. For example, the switching circuit 120D switches between the storage period and the discharge period by switching between conduction and non-conduction between the terminal Ps and a ground terminal GND to which a ground voltage is supplied. Specifically, for example, the switching circuit 120D may include a switching element SW124, similar to the switching circuit 120B shown in FIG. 6.

[0165] Terminal P2 of inductor L100 is connected to input terminal PVi via switching element SW108. That is, switching element SW108 is electrically connected to terminal P2 of inductor L100. Furthermore, when switching element SW108 is in the on state, it becomes part of the path of current flowing through inductor L100 during the charge period and discharge period. The on and off of switching element SW108 is controlled by a control signal Wsig supplied to control terminal PW. For example, a field-effect transistor, a bipolar transistor, or an analog switch may be used as switching element SW108.

[0166] The output terminal PVo is electrically connected to the terminal P1 of the inductor L100, and outputs a voltage Vout based on the accumulation period, the discharge period, and the input voltage Vin. Note that the output terminal PVo is supplied with a current from the inductor L100, for example, during the discharge period of the accumulation period and the discharge period.

[0167] During the signal processing period TPs, the switching element SW108 suppresses the current flowing through the inductor L100, thereby stopping the switching operation that switches between the storage period and the release period.

[0168] As described above, in this modification, the same effects as those of the above-described embodiment and modification can be obtained.

[0169] B4: Fourth variant In the above-described embodiment and modified example, the device 10 is an ultrasonic flowmeter, but the present invention is not limited to this. For example, the device 10 may be an analyzer such as a gas analyzer. The sensor 300 is also not limited to one that uses ultrasonic waves. For example, the sensor 300 may be one that uses infrared light, etc. The sensor 300 may also be one that detects the concentration of a gas such as oxygen. In the above-described embodiment and modified example, the signal processing circuit 200 is a "predetermined circuit," but the "predetermined circuit" is not limited to the signal processing circuit 200. For example, the signal processing circuit 200 may be a circuit that receives a power supply voltage from a power supply circuit separate from the power supply circuit 100. As described above, the modified example can also achieve the same effects as the above-described embodiment and modified example.

[0170] C: Application examples If the time it takes for switching circuit 120 to resume operation after stopping is within an allowable time, the switching operation may be stopped by stopping the operation of switching circuit 120. Similarly, the switching operation may be stopped by stopping the operation of switching circuit 120A, or the switching operation may be stopped by stopping the operation of switching circuit 120B.

[0171] 12 is an explanatory diagram for explaining an example of a power supply circuit 100H according to an application example. Elements similar to those explained in FIGS. 1 to 11 are given the same reference numerals, and detailed explanations thereof will be omitted.

[0172] The power supply circuit 100H is similar to the power supply circuit 100 except that the transistors FT1 and FT2, the resistors R104 and R106, and the diode D100 shown in FIG. 1 are omitted from the power supply circuit 100, and the connection destination of the terminal Pe of the switching circuit 120 is different. For example, the terminal Pe of the switching circuit 120 is connected to a control terminal PWn to which the control signal WNsig is supplied. The polarity of the control terminal PWn is opposite to the polarity of the control signal Wsig shown in FIG. 3.

[0173] For example, the control signal WNsig is maintained at a low level during the period TPw shown in FIG. 3 and at a high level during periods other than the period TPw. During the period TPw when the control signal WNsig is maintained at a low level, the operation of the switching circuit 120A is stopped, and the switching operation of switching between the accumulation period and the discharge period is stopped. As a result, even in this application example, it is possible to suppress the occurrence of switching noise during the signal processing period TPs included in the period TPw.

[0174] Note that power supply circuit 100H may have switching circuit 120A shown in Fig. 5 instead of switching circuit 120. Alternatively, power supply circuit 100H may be a step-up power supply circuit. For example, power supply circuit 100H may be configured such that bipolar transistor BT1, transistor FT2, resistors R112 and R114, and diode D104 shown in Fig. 6 are omitted from power supply circuit 100B, and terminal P2 of inductor L100 is connected to input terminal PVi.

[0175] As described above, in this application example, the power supply circuit 100H includes an input terminal PVi to which an input voltage Vin is supplied, an inductor L100 including terminals P1 and P2, a switching circuit 120 electrically connected to terminal P1 and switching between a storage period in which energy is stored in the inductor L100 and a discharge period in which the energy stored in the inductor L100 is discharged, and an output terminal PVo to which current is supplied from the inductor L100 during at least the discharge period after the storage period and the discharge period. The output terminal PVo outputs a voltage Vout based on the storage period, the discharge period, and the input voltage Vin. The switching circuit 120 stops its switching operation during a signal processing period TPs in which the signal processing circuit 200, which operates using the voltage Vout output from the output terminal PVo as a power supply voltage, processes the signal RSig. That is, the power supply circuit 100H is controlled to stop its switching operation during the signal processing period TPs. As a result, in this application example as well, as in the above-described embodiment and modification, it is possible to suppress switching noise caused by switching operation during the signal processing period TPs. [Explanation of symbols]

[0176] 10, 10Z... Equipment, 100, 100A, 100B, 100C, 100D, 100E, 100F, 100G, 100H, 100Z... Power supply circuit, 120, 120A, 120B, 120C, 120D... Switching circuit, 200... Signal processing circuit, 220... Control unit, 240a, 240b... Switch, 260... Transmitter, 262... Receiver, 280... Time measurement unit, 282... Flow rate calculation unit, 300... Sensor, 320a, 320b... Ultrasonic probe, 322a, 322b... Ultrasonic vibrator, 324a, 324b... Wedge, BT1... Bipolar transistor Starter, C100, C102, C104...capacitors, D100, D102, D104, D120...diodes, FL...fluid, FT1, FT2...transistor, L100...inductor, PB...printed circuit board, PL...piping, PVi...input terminal, PVo...output terminal, R100, R102, R104, R106, R108, R110, R112, R114...resistors, SLD...shield material, SW120, SW122, SW124...switching elements, TPs...signal processing period, ZD100, ZD102, ZD104...zener diodes.

Claims

1. an input terminal to which an input voltage is supplied; an inductor including a first terminal and a second terminal; a switching circuit electrically connected to the first terminal, for switching between a first period in which energy is stored in the inductor and a second period in which the energy stored in the inductor is released; an output terminal to which a current is supplied from the inductor during at least the second period of the first period and the second period; a first element electrically connected to the second terminal and forming a part of a path of a current flowing through the inductor during the first period and the second period; Equipped with The output terminal is outputting a voltage based on the first period, the second period, and the input voltage; The first element is a current flowing through the inductor is suppressed during a predetermined period in which a predetermined circuit processes a predetermined signal, thereby stopping a switching operation in which the first period and the second period are switched over; power circuit.

2. The switching circuit In the first period, a conductive state is established between the input terminal and the first terminal; In the second period, a non-conductive state is established between the input terminal and the first terminal; The first element is During the first period and the second period, a conductive state is established between the second terminal and the output terminal; During the predetermined period, a non-conductive state is established between the second terminal and the output terminal.

2. The power supply circuit according to claim 1.

3. The inverter further includes a diode electrically connected to the second terminal and serving as a part of a path of a current flowing through the inductor when the switching operation is stopped.

3. The power supply circuit according to claim 2.

4. a voltage control element including a third terminal and a fourth terminal, the voltage control element operating so that the voltage of the third terminal relative to the voltage of the fourth terminal does not exceed a predetermined voltage; the first element is a field effect transistor, the third terminal is connected to the gate of the field effect transistor; the fourth terminal is connected to the source of the field effect transistor; 3. The power supply circuit according to claim 2.

5. The switching circuit During the first period, a conductive state is established between a ground terminal to which a ground voltage is supplied and the first terminal; In the second period, a non-conductive state is established between the ground terminal and the first terminal, The first element is During the first period and the second period, a conductive state is established between the second terminal and the input terminal; During the predetermined period, a non-conductive state is established between the second terminal and the input terminal.

2. The power supply circuit according to claim 1.

6. The inverter further includes a diode electrically connected to the second terminal and serving as a part of a path of a current flowing through the inductor when the switching operation is stopped.

6. The power supply circuit according to claim 5.

7. a power supply circuit according to any one of claims 1 to 6; A sensor, the predetermined circuit; Equipped with The predetermined circuit is processing an output signal of the sensor as the predetermined signal; device.

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