Voltage amplifier circuit, switching power supply device, and initial setting method for switching power supply device

Abstract

To provide a voltage amplification circuit in which the input-output characteristic exhibits a linear relation passing through the origin and in which fine adjustment of gain can be easily performed, as well as a switching power supply device using the same and an initial setting method thereof.SOLUTION: A voltage amplification circuit 54(1) includes an operational amplifier 56(1), a fifth resistor 58(1) connected between a point P and a non-inverting input terminal, and a sixth resistor 60(1) connected between the non-inverting input terminal and a ground. The voltage amplification circuit includes a seventh resistor 62(1) connected between the inverting input terminal and an output terminal, an eighth resistor 64(1) having one end connected to the inverting input terminal, and a variable resistance circuit 66(1) connected between the eighth resistor 64(1) and a ground. The variable resistance circuit 66(1) performs arithmetic processing on the basis of an externally input digital signal to change a setting of an effective resistance value Rk of itself. The input-output characteristic of the voltage amplification circuit 54(1) exhibits a linear relation passing through the origin, and the slope (gain G) of the line can be adjusted by changing the setting of the effective resistance value Rk from the outside.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a voltage amplifier circuit, a switching power supply device using the same, and an initial setting method for the switching power supply device. [Background technology]

[0002] Conventionally, in switching power supplies, in order to detect the output voltage Vo or the output current Io, a voltage corresponding to the output voltage Vo or a signal obtained by converting the output current Io into a voltage is amplified using a voltage amplifier circuit. For example, FIG. 1 of Patent Document 1 describes a configuration in which a shunt resistor is inserted in the output line of a DC-DC converter and the output current Io is detected by amplifying the voltage across the shunt resistor using a differential amplifier or the like. Also, FIG. 14A of Patent Document 1 describes a technology in which two series resistors are connected to the output terminals of a differential amplifier and the connection of the high-side resistor is switched using a switch to change the output voltage of the differential amplifier (effectively, a technology to change the gain of the differential amplifier).

[0003] Another common technique is to apply a bias voltage Vb to a specific point in a differential amplifier circuit to shift the input / output characteristics, as shown in Figure 5. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2020-141538 Summary of the Invention [Problem to be solved by the invention]

[0005] When a voltage amplifier circuit (or a switching power supply device equipped with the same) is mass-produced, there is a problem that variations in the characteristics of internal components can cause individual differences in the gain G of the voltage amplifier circuit. Therefore, in order to detect the output voltage Vo and output current Io with a certain level of accuracy or higher, it is necessary to fine-tune the gain G after assembling the voltage amplifier circuit to correct for individual differences.

[0006] However, the differential amplifier shown in FIG. 1 of Patent Document 1 does not have a function to adjust the gain G. The circuit shown in FIG. 14A of Patent Document 1 switches the gain G of the differential amplifier between two stages, and does not allow fine adjustment of the gain G.

[0007] On the other hand, the technology shown in Figure 5 allows fine adjustment of the input / output characteristics of the differential amplifier circuit by adjusting the bias Vb. However, it is difficult to use this technology as a voltage amplifier circuit for a switching power supply.

[0008] When detecting the output voltage Vo and output current Io in a switching power supply, it is convenient if the input / output characteristics of the voltage amplifier circuit, i.e., the relationship between the input voltage (voltage to be amplified Vzi) and the output voltage (amplified voltage Vzo), is a straight line passing through the origin (Vzo = G Vz1).

[0009] In particular, when it is desired to accurately detect the output current Io from zero amperes to large values, it is preferable that the input-output characteristics be such that the amplified voltage Vzo is exactly zero volts when the output current Io is zero amperes, and that the amplified voltage Vzo is exactly a predetermined finite value when the output current Io is large. However, because the technology shown in Figure 5 shifts (translates) the line of the input-output characteristics, adjusting and matching the amplified voltage Vzo when the output current Io is large may result in a deviation from the amplified voltage Vzo when the output current Io is zero amperes.

[0010] The same issue applies when you want to accurately detect the output current Vo from zero volts to large values: if you adjust the amplified voltage Vzo when the output voltage Vo is large, there is a possibility that the amplified voltage Vzo when the output voltage Vo is zero volts will be off. Therefore, the technology shown in Figure 5 is difficult to use as a voltage amplifier circuit for a switching power supply.

[0011] The present invention has been made in view of the above-mentioned background art, and has as its object to provide a voltage amplifier circuit in which the input / output characteristics are in a linear relationship passing through the origin and in which fine adjustment of the gain is easy, a switching power supply unit using the same, and an initial setting method for the same. [Means for solving the problem]

[0012] The present invention is a voltage amplifier circuit that sets the voltage at point P in a circuit network having a ground as a voltage to be amplified Vzi (Vzi≧0), amplifies the input voltage to be amplified Vzi, and outputs an amplified voltage Vzo (≧0), an operational amplifier having an inverting input terminal, a non-inverting input terminal, and an output terminal, and outputting the amplified voltage Vzo from the output terminal; a fifth resistor connected between the point P and the non-inverting input terminal; a sixth resistor connected between the non-inverting input terminal and the ground; a seventh resistor connected between the inverting input terminal and the output terminal; an eighth resistor having one end connected to the inverting input terminal; and a variable resistor circuit connected between the eighth resistor and the ground, The variable resistance circuit has a function of changing the setting of its own effective resistance value Rk by performing arithmetic processing based on an externally input digital signal, and the relationship between the voltage to be amplified Vzi and the amplified voltage Vzo is Vzo=G·Vzi ( G is a positive coefficient), and the value of the coefficient G can be adjusted by externally changing the setting of the effective resistance value Rk.

[0013] The variable resistance circuit includes a low-side resistor connected between the other end of the eighth resistor and ground, an auxiliary circuit connected in parallel to the low-side resistor and consisting of a series circuit of an auxiliary resistor and a switching element, a switching control unit that changes the setting of the duty that has been calculated based on an externally input digital signal, and a smoothing capacitor that is connected in parallel to the low-side resistor and generates a DC voltage across both ends that changes in accordance with the value of the duty, and the value of the effective resistance value Rk can be changed by changing the setting of the duty.

[0014] The present invention also provides a voltage amplifier circuit that sets a voltage at a point P in a circuit network having a ground as a voltage to be amplified Vzi (Vzi≧0), amplifies the input voltage to be amplified Vzi, and outputs an amplified voltage Vzo (≧0), The inverter has an inverting input terminal, a non-inverting input terminal, and an output terminal, and the inverting input terminal is connected to the output terminal, and the output terminal is connected to the The aforementioned an operational amplifier that outputs an amplified voltage Vzo; a ninth resistor connected between the point P and the non-inverting input terminal; input a tenth resistor connected to the terminal; and a variable resistance circuit connected between the other end of the tenth resistor and the ground, the variable resistance circuit has a function of performing arithmetic processing based on an externally input digital signal to change the setting of its own effective resistance value Rk, This is a voltage amplification circuit in which the relationship between the voltage to be amplified Vzi and the amplified voltage Vzo satisfies Vzo=G·Vzi (G is a positive coefficient), and the value of the coefficient G can be adjusted by externally changing the setting of the effective resistance value Rk.

[0015] The variable resistance circuit includes a low-side resistor inserted between the tenth resistor and ground, an auxiliary circuit connected in parallel to the low-side resistor and consisting of a series circuit of an auxiliary resistor and a switching element, a switching control unit that changes the setting of the duty after performing arithmetic processing based on an externally input digital signal, and a smoothing capacitor connected in parallel to the low-side resistor and generating a DC voltage across both ends that changes in accordance with the value of the duty, and the value of the effective resistance value Rk can be changed by changing the setting of the duty.

[0016] Furthermore, the present invention provides a power conversion circuit that converts an input voltage into a DC output voltage Vo and outputs the converted voltage, and a control circuit that controls the operation of the power conversion circuit, wherein the voltage amplification circuit is connected so that the output voltage Vo of the power conversion circuit becomes the voltage to be amplified Vzi, and the voltage amplification circuit outputs The aforementioned In this switching power supply device, the amplified voltage Vzo is used as a detection signal for the output voltage Vo to control the power conversion circuit.

[0017] The present invention also provides an initial setting method for the switching power supply device, comprising: The aforementioned When the voltage to be amplified Vzi is input The aforementioned Measure the value of the amplified voltage Vzo, The aforementioned This is an initial setting method for a switching power supply device in which the setting of the coefficient G is adjusted externally so that the value of the amplified voltage Vzo falls within a specified range, and the program of the voltage amplifier circuit is rewritten so that the state after adjustment becomes the default setting.

[0018] Furthermore, the present invention provides a power conversion circuit that converts an input voltage into a DC output voltage Vo and outputs the converted voltage, a current detection resistor through which a voltage drop occurs when an output current Io that is output from the power conversion circuit to a load flows, and a control circuit that controls the operation of the power conversion circuit, wherein the voltage amplification circuit is connected so that the voltage drop that occurs in the current detection resistor becomes the voltage to be amplified Vzi, and the voltage amplification circuit outputs The aforementioned In this switching power supply device, the amplified voltage Vzo is used as a detection signal for the output current Io to control the power conversion circuit.

[0019] The present invention also provides an initial setting method for the switching power supply device, comprising: The aforementioned When the voltage to be amplified Vzi is input The aforementioned Measure the value of the amplified voltage Vzo, The aforementioned This is an initial setting method for a switching power supply device in which the setting of the coefficient G is adjusted externally so that the value of the amplified voltage Vzo falls within a specified range, and the program of the voltage amplifier circuit is rewritten so that the state after adjustment becomes the default setting. [Effects of the Invention]

[0020] The voltage amplifier circuit of the present invention has input / output characteristics that are linearly related to the origin, and allows for easy fine adjustment of the gain. Furthermore, the gain adjustment circuit block (variable resistor circuit) performs arithmetic processing based on a digital signal to adjust the gain, resulting in a highly intelligent voltage amplifier circuit. Furthermore, the gain adjustment circuit block (variable resistor circuit) is configured to be connected to ground, minimizing the effects of noise entering through the signal line for digital signal input.

[0021] By using the voltage amplifier circuit with the excellent performance described above, the switching power supply device of the present invention can detect the output voltage or output current with high accuracy and can be used for various controls. Furthermore, by executing the initial setting method for the switching power supply device of the present invention, individual differences in gain G caused by variations in the characteristics of the internal components of the voltage amplifier circuit can be corrected by rewriting the program, making it easy to achieve the desired performance. [Brief explanation of the drawings]

[0022] [Figure 1] 1 is a circuit diagram showing a first embodiment of a switching power supply device according to the present invention; [Figure 2] 2A is a circuit diagram showing the voltage amplifier circuit (first embodiment of the voltage amplifier circuit of the present invention) in FIG. 1, and FIG. 2B is a graph showing the input / output characteristics of this voltage amplifier circuit. [Figure 3] 3 is a graph showing a change in effective resistance value Rk when duty D is changed in the voltage amplifier circuit of FIG. 2. [Figure 4] 2A is a circuit diagram showing a modified example of the voltage amplifier circuit in FIG. 1 (a second embodiment of the voltage amplifier circuit of the present invention), and FIG. 2B is a graph showing the input / output characteristics of this voltage amplifier circuit. [Figure 5] 1A is a circuit diagram showing a conventional differential amplifier circuit, and FIG. 1B is a graph showing the input / output characteristics of this differential amplifier circuit. DETAILED DESCRIPTION OF THE INVENTION

[0023] <Switching power supply device 52 and voltage amplifier circuits 54(1), 54(2) of the first embodiment> First, a first embodiment of a switching power supply device and a voltage amplifier circuit according to the present invention will be described with reference to FIGS.

[0024] 1, a switching power supply device 52 of this embodiment includes a power conversion circuit 14 that converts an input voltage supplied from an input power source 12 into a DC output voltage Vo and outputs the converted voltage, and a control circuit 18 that controls the operation of the power conversion circuit 14. The switching power supply device 10 further includes a current detection resistor 20 through which a voltage drop occurs when an output current Io that is output from the power conversion circuit 14 to a load 16 flows, and voltage amplifier circuits 54(1) and 54(2) of the first embodiment.

[0025] The voltage amplifier circuit 54(1) uses the high-voltage side of the two output terminals of the power conversion circuit 14 as point P1 and the low-voltage side as ground, and the output voltage Vo generated at point P1 becomes the voltage to be amplified Vzi(1), which is amplified to generate an amplified voltage Vzo(1) and output to the control circuit 18. The amplified voltage Vzo(1) is then used to control the power conversion circuit 14 as a detection signal for the output voltage Vo.

[0026] The voltage amplifier circuit 54(2) uses the high-voltage side of the current detection resistor 20 as point P2 and the low-voltage side as ground, and the voltage drop across the current detection resistor 20 that occurs at point P2 becomes the voltage to be amplified Vzi(2), which is amplified to generate an amplified voltage Vzo(2) and output to the control circuit 18. The amplified voltage Vzo(2) is then used to control the power conversion circuit 14 as a detection signal for the output current Io.

[0027] The internal configurations of the voltage amplifier circuits 54(1) and 54(2) are similar, so the configuration of the voltage amplifier circuit 54(1) will be mainly described here.

[0028] The voltage amplifier circuit 54(1) is a type of so-called inverting amplifier circuit, and as shown in FIG. 2(a), the voltage at point P1 with respect to ground is the voltage to be amplified Vzi (Vzi≧0), and the input voltage to be amplified Vzi is amplified and output as an amplified voltage Vzo (≧0).

[0029] The voltage amplifier circuit 54(1) includes an operational amplifier 56(1) having an inverting input terminal, a non-inverting input terminal, and an output terminal, and outputting an amplified voltage Vzo(1) from the output terminal. A fifth resistor 58(1) [resistance value R5] is connected between point P1 and the non-inverting input terminal, and a sixth resistor 60(1) [resistance value R6] is connected between the non-inverting input terminal and ground. A seventh resistor 62(1) [resistance value R7] is connected between the inverting input terminal and the output terminal. One end of an eighth resistor 64(1) [resistance value R8] is connected to the inverting input terminal, and a variable resistor circuit 66(1) is connected between the other end of the eighth resistor 64(1) and ground.

[0030] The variable resistance circuit 66(1) is a circuit whose own resistance value (effective resistance value Rk) changes. The effective resistance value Rk is set as a default based on the duty D described below, and the setting of the effective resistance value Rk can be changed by performing arithmetic processing based on an externally input digital signal DS.

[0031] The internal configuration of the variable resistance circuit 66(1) will be described. The variable resistance circuit 66(1) includes a low-side resistor 68(1) [resistance value Rb] connected between the other end of the eighth resistor 64(1) and ground, and an auxiliary circuit 74(1) consisting of a series circuit of an auxiliary resistor 70(1) [resistance value Rc] and a switching element 72(1) is connected to both ends of the low-side resistor 68(1).

[0032] A switching control unit 76(1) that turns the switching element 72(1) on and off is connected to the drive terminal of the switching element 72(1). The switching control unit 76(1) turns the switching element 72(1) on and off with a default set duty D, but also has a function of changing the setting of duty D by performing arithmetic processing based on an externally input digital signal DS. A smoothing capacitor 78(1) that smoothes the voltage across the low-side resistor 68(1) to convert it into a DC voltage is connected in parallel to both ends of the low-side resistor 68(1).

[0033] Next, the operation of the voltage amplifier circuit 54(1) will be described. The input / output characteristics of the voltage amplifier circuit 54(1) are expressed by a straight line passing through the origin, as shown in equation (1). The gain G in equation (1) is determined based on equations (2) and (3). Vzo(1)=G·Vzi(1) (1) G=[1+R7÷(R8+Rk)]·R6÷(R5+R6) (2) Rk=Rb / / (Rc÷D) (3) As shown in equation (2), the gain G can be adjusted by changing the effective resistance Rk. Also, as shown in equation (3), the effective resistance Rk is a combined value of the resistances of the low-side resistor 68(1) [resistance Rb] and the auxiliary resistor 70(1) [resistance Rc], and can be adjusted by changing the duty D.

[0034] The graph in Figure 3 is a graph of equation (3). For example, when D = 100%, the switching element 72(1) is fixed to ON, so Rk = Rb / / Rc. When D = 0%, the switching element 72(1) is fixed to OFF, so Rk = Rb. Therefore, by changing the duty D within the range of 0 to 100%, the effective resistance value Rk can be changed within the range of Rb to (Rb / / Rc).

[0035] From the above, the input / output characteristics of the voltage amplifier circuit 54(1) are represented by a straight line passing through the origin as shown in FIG. 2(b), and the slope of the line (gain G) can be adjusted by changing the duty D.

[0036] When switching power supply devices 52 are mass-produced, individual differences occur in gain G due to variations in the characteristics of the internal components of voltage amplifier circuit 54(1). Therefore, it is advisable to implement the following initial setting method (one embodiment of the initial setting method for a switching power supply device of the present invention) in the manufacturing process of switching power supply device 52.

[0037] First, a known voltage to be amplified Vzi(1) is input to the voltage amplifier circuit 54(1), and the value of the amplified voltage Vzo(1) is measured. If the value of the amplified voltage Vzo(1) is within a specified range, the initial setup process is complete. However, if it is not within the specified range, a digital signal DS is sent to the switching control unit 76(1), and the duty D setting is adjusted so that the value of the amplified voltage Vzo(1) is within the specified range. Then, the program of the switching control unit 76(1) is rewritten so that the adjusted duty D becomes the default setting. By performing this initial setup process for each voltage amplifier circuit 54(1), individual differences in gain G can be reliably corrected.

[0038] So far, the configuration, operation, and initial setting method of the voltage amplifier circuit 54(1) have been explained, but the same applies to the voltage amplifier circuit 54(2).

[0039] As described above, the voltage amplifier circuit 54(1) has input / output characteristics that are linear and pass through the origin, and the gain can be easily fine-tuned. Furthermore, the gain-adjusting variable resistor circuit 66(1) adjusts the gain by performing arithmetic processing based on a digital signal, resulting in a highly intelligent voltage amplifier circuit 54(1). Furthermore, the variable resistor circuit 66(1) is configured to be connected to ground, which minimizes the effects of noise that enters the voltage amplifier circuit 54(1) through the signal line for inputting the digital signal DS. The same is true for the voltage amplifier circuit 54(2). By using the excellent performance voltage amplifier circuits 54(1) and 54(2), the switching power supply 52 can detect the output voltage Vo and output current Io with high accuracy and can be used for various controls. Furthermore, by executing the above initial setting method, individual differences in gain G caused by variations in the characteristics of the internal components of the voltage amplifier circuits 54(1) and 54(2) can be corrected by rewriting the program, making it easy to achieve the desired performance. Furthermore, the resistance variable circuit 66(1) is characterized by a configuration in which the on / off duty D of the switching element 70(1) is changed as a method for adjusting the effective resistance value Rk. This configuration requires that the part (calculation unit) that performs digital arithmetic processing in the switching control unit 76(1) be provided within a digital processor, but has the advantage that the effective resistance value Rk can be changed with high resolution even when an inexpensive general-purpose digital processor is used. The same is true for the resistance variable circuit 66(2).

[0040] <Other embodiments, modifications, etc.> The voltage amplifier circuit of the present invention is not limited to the above embodiment.

[0041] The voltage amplifier circuit 54(1) can be replaced with a modified voltage amplifier circuit 80(1) shown in Fig. 4(a). The voltage amplifier circuit 80(1) is a type of non-inverting amplifier circuit, similar to the voltage amplifier circuit 54(1), and as shown in Fig. 6(a), the voltage at point P1 with respect to ground is the voltage to be amplified Vzi (Vzi ≧ 0), and the input voltage to be amplified Vzi is amplified and output as an amplified voltage Vzo (≧ 0).

[0042] The voltage amplifier circuit 80(1) includes an operational amplifier 82(1) having an inverting input terminal, a non-inverting input terminal, and an output terminal, and outputs an amplified voltage Vzo(1) from the output terminal. A ninth resistor 84(1) [resistance value R9] is connected between point P1 and the non-inverting input terminal, and the input terminal and output terminal are connected. In addition, a tenth resistor 86(1) [resistance value R10] is connected to the non-inverting input terminal, and a variable resistance circuit 66(1) is connected between the other end of the tenth resistor 86(1) and ground.

[0043] The variable resistance circuit 66(1) is similar to the variable resistance circuit 66(1) in FIG. 2(a), and the effective resistance value Rk is set as a default based on the duty D. The switching control unit 76(1) performs arithmetic processing based on the digital signal DS, thereby enabling the setting of the effective resistance value Rk to be changed.

[0044] To explain the operation of the voltage amplifier circuit 80(1), the input / output characteristics of the voltage amplifier circuit 80(1) are expressed by a straight line passing through the origin, as shown in equation (4). The gain G in equation (4) is determined based on equations (5) and (6). Vzo(1)=G·Vzi(1) (4) G = (R10 + Rk) ÷ (R9 + R10 + Rk) (5) Rk=Rb / / (Rc÷D) (6) As shown in equation (5), the gain G can be adjusted by changing the effective resistance value Rk, and as shown in equation (6), the value of the effective resistance value Rk can be adjusted by changing the duty D.

[0045] The graph in FIG. 3, which was explained above, is a graph of equation (3), and equation (6) also has a similar graph. For example, when D=100%, switching element 72(1) is fixed to ON, so Rk=Rb / / Rc. Also, when D=0%, switching element 72(1) is fixed to OFF, so Rk=Rb. Therefore, by changing the duty D within the range of 0 to 100%, the effective resistance value Rk can be changed within the range of Rb to (Rb / / Rc).

[0046] From the above, the input / output characteristics of the voltage amplifier circuit 80(1) are represented by a straight line passing through the origin as shown in Figure 4(b), and the gain G, which is the slope of the line, can be adjusted by changing the duty cycle D. Therefore, even if the voltage amplifier circuit 54(1) is replaced with the voltage amplifier circuit 80(1), the same effects can be obtained.

[0047] Furthermore, it goes without saying that the voltage amplifier circuit of the present invention is highly versatile and can be used for purposes other than switching power supplies.

[0048] The switching power supply of the present invention is not limited to the above embodiment. For example, in the case of switching power supply 52, two sets of voltage amplifier circuits with the same configuration are installed to detect output voltage Vo and output current Io, but two other sets may be selected from the above-mentioned voltage amplifier circuits 54(1) and 80(1) and installed. Furthermore, if only one of output voltage Vo or output current Io is to be detected, only one set of voltage amplifier circuits 54(1) and 80(1) may be selected and installed.

[0049] The initial setting method for the switching power supply of the present invention is not limited to the above embodiment. For example, in the above description, it was stated that "duty D is set as a default in switching control unit 76(1)." However, it is not necessary to set it as a default. For example, when performing the initial setting work, the initial duty D may be set by a digital signal from an external device, and the adjusted duty D may be written into the program of switching control unit 76(1). [Explanation of symbols]

[0050] 52 Switching power supply 14 Power Conversion Circuit 18 Control circuit 20 Current detection resistor 54(1), 54(2), 80(1) Voltage amplifier circuit 56(1),82(1) Op-amp 58(1) Fifth resistance 60(1) Sixth resistance 62(1) Seventh Resistance 64(1) Eighth Resistance 66(1) Variable Resistor Circuit 68(1) Low-side resistor 70(1) Auxiliary resistor 72(1) Switching elements 74(1) Auxiliary circuit 76(1) Switching control section 78(1) Smoothing capacitor 84(1) Ninth Resistance 86(1) Tenth Resistance D Duty G Gain Io output current Rk Effective resistance Vo output voltage Vzi(1), Vz1(2) Voltage to be amplified Vzo(1), Vzo(2) Amplified voltage

Claims

1. A voltage amplifier circuit that sets a voltage at a point P in a circuit network having a ground as a voltage to be amplified Vzi (Vzi≧0), amplifies the input voltage to be amplified Vzi, and outputs an amplified voltage Vzo (≧0), an operational amplifier having an inverting input terminal, a non-inverting input terminal, and an output terminal, and outputting the amplified voltage Vzo from the output terminal; a fifth resistor connected between the point P and the non-inverting input terminal; a sixth resistor connected between the non-inverting input terminal and the ground; a seventh resistor connected between the inverting input terminal and the output terminal; an eighth resistor having one end connected to the inverting input terminal; and a variable resistor circuit connected between the eighth resistor and the ground, the variable resistance circuit has a function of performing arithmetic processing based on an externally input digital signal to change the setting of its own effective resistance value Rk, the relationship between the voltage to be amplified Vzi and the amplified voltage Vzo satisfies Vzo=G·Vzi (G is a positive coefficient), and the value of the coefficient G can be adjusted by externally changing the setting of the effective resistance value Rk; the variable resistance circuit includes: a low-side resistor connected between the other end of the eighth resistor and ground; an auxiliary circuit connected in parallel to the low-side resistor and consisting of a series circuit of an auxiliary resistor and a switching element; a switching control unit that turns on and off the switching element at a predetermined duty and changes the setting of the duty after performing arithmetic processing based on an externally input digital signal; and a smoothing capacitor connected in parallel to the low-side resistor and generating, across both ends, a DC voltage that changes according to the value of the duty, A voltage amplifier circuit, characterized in that the effective resistance value Rk can be changed by changing the duty setting.

2. A voltage amplifier circuit that sets a voltage at a point P in a circuit network having a ground as a voltage to be amplified Vzi (Vzi≧0), amplifies the input voltage to be amplified Vzi, and outputs an amplified voltage Vzo (≧0), an operational amplifier having an inverting input terminal, a non-inverting input terminal, and an output terminal, the inverting input terminal being connected to the output terminal and outputting the amplified voltage Vzo from the output terminal; a ninth resistor connected between the point P and the non-inverting input terminal; a tenth resistor having one end connected to the non-inverting input terminal; and a variable resistor circuit connected between the other end of the tenth resistor and ground, the variable resistance circuit has a function of performing arithmetic processing based on an externally input digital signal to change the setting of its own effective resistance value Rk, the relationship between the voltage to be amplified Vzi and the amplified voltage Vzo satisfies Vzo=G·Vzi (G is a positive coefficient), and the value of the coefficient G can be adjusted by externally changing the setting of the effective resistance value Rk; the variable resistance circuit includes: a low-side resistor inserted between the tenth resistor and ground; an auxiliary circuit connected in parallel to the low-side resistor and consisting of a series circuit of an auxiliary resistor and a switching element; a switching control unit that turns the switching element on and off at a predetermined duty and changes the setting of the duty after performing arithmetic processing based on an externally input digital signal; and a smoothing capacitor connected in parallel to the low-side resistor and generating a DC voltage across both ends that changes in accordance with the value of the duty, A voltage amplifier circuit, characterized in that the effective resistance value Rk can be changed by changing the duty setting.

3. a power conversion circuit for converting an input voltage into a DC output voltage Vo and outputting the converted voltage; and a control circuit for controlling the operation of the power conversion circuit; the voltage amplifier circuit is connected so that the output voltage Vo of the power conversion circuit becomes the amplified voltage Vzi, and the amplified voltage Vzo output by the voltage amplifier circuit is used to control the power conversion circuit as a detection signal for the output voltage Vo.

4. 4. The method for initializing a switching power supply according to claim 3, a setting of the coefficient G externally adjusted so that the value of the amplified voltage Vzo falls within a specified range; and a program for the voltage amplifier circuit is rewritten so that the state after the adjustment becomes a default setting.

5. a power conversion circuit that converts an input voltage into a DC output voltage Vo and outputs the converted voltage; a current detection resistor through which a voltage drop occurs when an output current Io that is output from the power conversion circuit to a load flows; and a control circuit that controls the operation of the power conversion circuit, the voltage amplifier circuit is connected so that the voltage drop occurring across the current detection resistor becomes the amplified voltage Vzi, and the amplified voltage Vzo output by the voltage amplifier circuit is used to control the power conversion circuit as a detection signal for the output current Io.

6. 6. The method for initializing a switching power supply according to claim 5, a setting of the coefficient G externally adjusted so that the value of the amplified voltage Vzo falls within a specified range; and a program for the voltage amplifier circuit is rewritten so that the state after the adjustment becomes a default setting.

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