JP2020014302A - Switching power supply and usb power supply device - Google Patents

Abstract

To provide a switching power supply and the like having a temperature protection function achieved by a simple circuit.SOLUTION: A switching power supply 10 includes a rectifier circuit 11 and a DC/DC converter 12. The DC/DC converter 12 includes: a switching element 17; a current detection circuit 19; a non-inverting amplifier circuit 20 for amplifying a voltage output from the current detection circuit 19; and a control circuit 18 for controlling switching of the switching element 17 according to a voltage output from the non-inverting amplifier circuit 20. The non-inverting amplifier circuit 20 includes: an operational amplifier Amp; a first resistor element R1 connected between an output terminal of the current detection circuit 19 and a non-inverting input terminal of the non-inverting amplifier circuit 20; and a second resistor element R2 connected between a power supply voltage Vcc and the non-inverting input terminal of the non-inverting amplifier circuit 20. At least one of the first resistor element R1 and the second resistor element R2 is a thermistor.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a switching power supply and a USB power supply device including the switching power supply.

  2. Description of the Related Art Conventionally, various types of switching power supplies having a function of stabilizing an output voltage using a switching element have been proposed (for example, see Patent Document 1).

  Patent Literature 1 proposes a flyback switching power supply including a switching element and a control IC for controlling the switching element.

International Publication No. 2009/069649

  By the way, in a switching power supply, when current supply to a load is continued, there is a possibility that heat is generated in a plurality of places such as a switching element, a flyback transformer, and a rectifier diode on an output side, but a small power supply circuit is manufactured. , It becomes difficult to provide a heat dissipation structure. For example, in a USB power supply device such as a power adapter, it is necessary to provide a switching power supply in a limited space. Therefore, it is required that the switching power supply be provided with a temperature protection function for suppressing the output when the temperature of the switching power supply rises abnormally.

  Here, some control ICs for a switching power supply have a built-in temperature protection function. However, such a control IC has a problem that the threshold value is extremely high, around 150 ° C., and the operation stops when the threshold value is exceeded.

  Therefore, in order to add a temperature protection function to a small switching power supply incorporated in a product that has a limited heat radiation structure, a control IC with some advanced functions or a programmable control IC must be used. Unnecessarily, the cost is increased.

  Then, this invention is made in order to solve the said problem, and an object of this invention is to provide the switching power supply and USB power supply device which have the temperature protection function implement | achieved by the simple circuit.

  In order to achieve the above object, a switching power supply according to one embodiment of the present invention includes a rectifier circuit for rectifying input AC power and a DC for converting a first DC voltage output from the rectifier circuit to a second DC voltage. The DC / DC converter includes a switching element, a current detection circuit that detects a current flowing through the switching element and outputs a voltage corresponding to the detected current, and a voltage output from the current detection circuit. A non-inverting amplifier circuit for amplifying, and a control circuit for controlling switching of the switching element according to a voltage output from the non-inverting amplifier circuit, wherein the non-inverting amplifier circuit has a non-inverting input terminal and an inverting input terminal. A first resistor connected between the output terminal of the current detection circuit and the non-inverting input terminal; and a first resistor connected between the power supply voltage and the non-inverting input terminal. And a second resistor element, at least one of the first resistive element and the second resistive element is a thermistor.

  To achieve the above object, a USB power supply device according to one embodiment of the present invention includes a USB connector, a circuit board supporting the USB connector, and the switching power supply mounted on the circuit board. The provided DC / DC converter supplies the second DC voltage to the power terminal of the USB connector.

  According to the present invention, a switching power supply and a USB power supply device having a temperature protection function realized by a simple circuit are provided.

FIG. 1A is an external view illustrating a structure of a USB power supply device according to an embodiment. FIG. 1B is an external view of the back surface of the circuit board shown in FIG. 1A. FIG. 2 is a circuit diagram of a switching power supply included in the USB power supply device shown in FIG. 1A. FIG. 3 is a circuit diagram showing a detailed configuration of the non-inverting amplifier circuit in FIG. FIG. 4 is a circuit diagram showing a non-inverting amplifier circuit according to a modification of the above embodiment.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Each of the embodiments described below shows one specific example of the present invention. Numerical values, shapes, materials, constituent elements, arrangement positions and connection forms of constituent elements, operation procedures, and the like shown in the following embodiments are merely examples and do not limit the present invention. In addition, among the components in the following embodiments, components not described in the independent claims that indicate the highest concept of the present invention are described as arbitrary components. In addition, each drawing is not necessarily strictly illustrated. In each of the drawings, substantially the same configuration is denoted by the same reference numeral, and redundant description will be omitted or simplified.

  FIG. 1A is an external view showing a structure of a USB power supply device 6 according to the embodiment. The USB power supply device 6 is a power supply device used for a power adapter or an embedded wiring device that supplies power via the USB connector 8. As shown in the figure, the USB power supply device 6 includes a USB connector 8, a circuit board 7 supporting the USB connector 8, and a switching power supply 10 mounted on the circuit board 7.

  The USB connector 8 is, for example, a USB Type-A receptacle (female).

  The circuit board 7 has a USB connector 8 mounted on the front surface (upper surface in the figure) and various electronic components constituting the switching power supply 10 mounted on the back surface.

  The switching power supply 10 receives an AC power (here, commercial AC power) as an input, and supplies a stabilized DC voltage (here, 5 V) to a power supply terminal VBUS of the USB connector 8. Power supply circuit.

  FIG. 1B is an external view of the back surface of the circuit board 7 shown in FIG. 1A. At the center of the circuit board 7, a terminal of a USB connector 8 attached to the opposite side (the surface of the circuit board 7) is penetrated and fixed by soldering. Among the terminals of the USB connector 8, the terminal located at the left end in the figure is the power supply terminal VBUS, and the terminal located at the right end is the ground terminal GND.

  On the upper left of the circuit board 7, a diode 15, which is an example of a semiconductor element that rectifies AC power on the secondary side of a flyback transformer described later and outputs DC power, is mounted.

  The power supply terminal VBUS and the diode 15 of the USB connector 8 are electronic components having a high risk of generating heat. Therefore, in the present embodiment, between the power supply terminal VBUS and the diode 15 in plan view of the circuit board 7, the first resistance element R1 which is a thermistor for detecting the temperature of the power supply terminal VBUS and the diode 15 is mounted. ing. That is, the first resistance element R1 also functions as a sensor that detects the temperature of both the power supply terminal VBUS and the diode 15.

  FIG. 2 is a circuit diagram of the switching power supply 10 included in the USB power supply device 6 shown in FIG. 1A. The switching power supply 10 receives an AC power (here, a commercial AC power) from the AC power supply 5 and supplies a stabilized DC voltage (here, 5 V) to a power supply terminal VBUS of the USB connector 8. This is a flyback type power supply circuit having a function.

  As shown in FIG. 1, the switching power supply 10 includes a rectifier circuit 11 for rectifying input AC power, a first DC voltage output from the rectifier circuit 11 converted into a second DC voltage, and a A DC / DC converter 12 for supplying power to the power supply terminal VBUS.

  The rectifier circuit 11 is a circuit that rectifies AC power (in this case, commercial AC power) from the AC power supply 5 into DC power, and includes, for example, a diode bridge or the like.

  The DC / DC converter 12 includes a snubber circuit 13, a flyback transformer 14, a diode 15, a capacitor 16, a switching element 17, a control circuit 18, a current detection circuit 19, and a non-inverting amplifier circuit 20.

  The snubber circuit 13 is a protection circuit connected to both ends of the primary winding of the flyback transformer 14 and absorbing a high voltage generated in the primary winding when the switching element 17 is turned off.

  The flyback transformer 14 is an insulating transformer having a primary winding and a secondary winding.

  The diode 15 is an example of a semiconductor element that rectifies AC power generated in a secondary winding of the flyback transformer 14. The semiconductor element for such rectification is not limited to a diode, but may be a switching element for synchronous rectification (for example, an FET or the like).

  The capacitor 16 is a smoothing capacitor that smoothes the waveforms of the voltage and current rectified by the diode 15.

  The switching element 17 is connected in series to the primary winding of the flyback transformer 14, and is an element that repeatedly turns on and off under the control of the control circuit 18, and is, for example, a MOS transistor.

  The current detection circuit 19 is a circuit that detects a current flowing through the switching element 17 and outputs a voltage corresponding to the detected current, and is, for example, a resistance element connected in series with the switching element 17.

  The non-inverting amplifier circuit 20 is a circuit that amplifies the voltage output from the current detection circuit 19 in a non-inverting manner and outputs the amplified voltage to the control circuit 18.

  The control circuit 18 is a circuit that controls the switching of the switching element 17 according to the voltage output from the non-inverting amplifier circuit 20, and is, for example, an IC. Specifically, the control circuit 18 changes the ON time in switching of the switching element 17 so that the voltage input from the current detection circuit 19 via the non-inverting amplifier circuit 20 does not exceed a predetermined limit value. It performs PWM (Pulse Width Modulation) control.

  FIG. 3 is a circuit diagram showing a detailed configuration of the non-inverting amplifier circuit 20 in FIG. The non-inverting amplifying circuit 20 amplifies the voltage Vi output from the current detecting circuit 19 (that is, the input voltage to the non-inverting amplifying circuit 20) at an amplification factor depending on a temperature detected by an internal thermistor. This is an amplifier circuit that outputs the voltage Vo obtained as described above to the control circuit 18. As shown in the figure, the non-inverting amplifier circuit 20 includes a low-pass filter (LPF) 21, an operational amplifier Amp, a first resistor R1, a second resistor R2, a third resistor R3, and a fourth resistor. R4.

  The low-pass filter 21 is a filter that generates a flat voltage waveform by smoothing the input voltage Vi, and is, for example, a CR low-pass filter including a resistance element and a capacitor.

  The operational amplifier Amp is an operational amplifier (Operational Amplifier) having a non-inverting input terminal and an inverting input terminal, operates by receiving power supply from the power supply voltage Vcc, and outputs the voltage Vo to the control circuit 18. Power supply voltage Vcc is generated in DC / DC converter 12 using a voltage regulator (not shown) or the like.

  The first resistance element R1 is an example of a first resistance element connected between the output terminal of the current detection circuit 19 and the non-inverting input terminal of the operational amplifier Amp. In the present embodiment, the first resistance element R1 is connected between the output terminal of the low-pass filter 21 provided at the input stage of the non-inverting amplifier circuit 20 and the non-inverting input terminal of the operational amplifier Amp. It is a positive temperature coefficient thermistor whose resistance value increases when it rises.

  The second resistance element R2 is an example of a second resistance element connected between the power supply voltage Vcc and the non-inverting input terminal of the operational amplifier Amp, and is a fixed resistance element in the present embodiment.

  The resistance values of the first resistance element R1 and the second resistance element R2 are set to values satisfying the following conditions. That is, when the temperature detected by the first resistance element R1 that is a positive temperature coefficient thermistor is normal (that is, the temperature range determined to be normal), the resistance value of the second resistance element R2 >> the resistance value of the first resistance element R1 The resistance value holds. Then, when the temperature detected by the first resistance element R1 is abnormally high, the resistance value of the second resistance element R2 抵抗 the resistance value of the first resistance element R1 holds.

  The third resistance element R3 is a fixed resistance element connected between the inverting input terminal of the operational amplifier Amp and a reference voltage (ground).

  The fourth resistance element R4 is a fixed resistance element for feedback connected between the inverting input terminal of the operational amplifier Amp and the output terminal of the operational amplifier Amp.

  Next, an operation of the USB power supply device 6 according to the present embodiment configured as described above will be described.

  The AC power from the AC power supply 5 is rectified by the rectifier circuit 11 into DC power, and the rectified DC power is stabilized by the DC / DC converter 12 to become a DC voltage (here, 5 V), and the USB connector 8 power supply terminal VBUS.

  In the DC / DC converter 12, the DC power applied to the primary winding of the flyback transformer 14 is chopped by the switching element 17 under the control of the control circuit 18 to become AC power. AC power is generated in the secondary winding. The AC power generated in the secondary winding of the flyback transformer 14 is rectified by the diode 15 and further smoothed by the capacitor 16 to become a DC voltage (here, 5 V).

  The control circuit 18 controls the ON time in switching of the switching element 17 so that the voltage input from the current detection circuit 19 via the non-inverting amplifier circuit 20 does not exceed a predetermined limit value. Specifically, when the voltage input from the current detection circuit 19 via the non-inverting amplifier circuit 20 exceeds the limit value, the control circuit 18 controls the switching element 17 so as to limit the current flowing through the switching element 17. The on-time in the switching of. Thus, the output current and the output voltage from the power supply terminal VBUS of the USB connector 8 are limited so that the current detected by the current detection circuit 19 does not exceed the value corresponding to the limit value.

  Here, the operation of non-inverting amplification in the non-inverting amplifier circuit 20 is as follows.

  Assuming that the voltage at the non-inverting input terminal of the operational amplifier Amp is Vm, the voltage Vo output from the operational amplifier Amp is expressed by the following equation 1 using the third resistance element R3 and the fourth resistance element R4.

  Vo = [(R3 + R4) / R3] × Vm (Equation 1)

  Here, if the attenuation of the input voltage Vi in the low-pass filter 21 is neglected, the voltage Vm is a voltage obtained by dividing the potential difference between the power supply voltage Vcc and the input voltage Vi by the first resistor R1 and the second resistor R2. Therefore, it is expressed by the following equation 2.

  Vm = (R1 × Vcc + R2 × Vi) / (R1 + R2) (Equation 2)

  Note that R1, R2, R3, and R4 in the above equations 1 and 2 are resistance values of the first resistance element R1, the second resistance element R2, the third resistance element R3, and the fourth resistance element R4, respectively.

  Here, in a normal state, that is, when the temperature detected by the first resistance element R1 which is a positive temperature coefficient thermistor is a normal temperature, R2 >> R1, and therefore Vm ≒ Vi from the above equation (2). . By substituting this into the above equation 1, the value shown in the following equation 3 is obtained as the voltage amplification factor Vo / Vi of the non-inverting amplifier circuit 20.

  Vo / Vi ≒ (R3 + R4) / R3 (Equation 3)

  Therefore, when the temperature is normal, the voltage gain of the non-inverting amplifier circuit 20 is a fixed value that does not depend on the first resistance element R1 that is a positive temperature coefficient thermistor. Therefore, the control circuit 18 performs a normal control for limiting the current flowing through the switching element 17 when the voltage input from the current detection circuit 19 via the non-inverting amplifier circuit 20 exceeds the limit value.

  On the other hand, when abnormal, that is, when the temperature detected by the first resistance element R1 which is a positive temperature coefficient thermistor is abnormally high (for example, heat generation of an electronic component is detected), the resistance of the first resistance element R1 is increased. , R1 ≒ R2, Vm = (Vcc + Vi) / 2 from the above equation (2). By substituting this into the above equation 1, the value shown in the following equation 4 is obtained as the voltage amplification factor Vo / Vi of the non-inverting amplifier circuit 20.

  Vo / Vi ≒ [(R3 + R4) / R3] × (Vcc + Vi) / 2 (Equation 4)

  Here, the power supply voltage Vcc is about 2.5 to 5 V, and the input voltage Vi is about 1 to 2 V. Therefore, the second term ((Vcc + Vi) / 2) on the right side of the above equation 4 is 1.75 to 3.5, and the voltage amplification factor Vo / Vi of the non-inverting amplifier circuit 20 at the time of abnormality is calculated by It is 1.75 to 3.5 times that of the normal time shown. As a result, the control circuit 18 controls the output voltage of the DC / DC converter 12 (the voltage of the power supply terminal VBUS) to be less than 20 to 50% of the normal state by suppressing the current flowing through the switching element 17. Do. Thereby, a temperature protection function is realized.

  If the temperature detected by the first resistance element R1 which is a positive temperature coefficient thermistor is abnormally high, R1 ≒ ∞ is obtained by further increasing the resistance of the first resistance element R1. ≒ Vcc. By substituting this into the above equation 1, the value shown in the following equation 5 is obtained as the voltage Vo output from the non-inverting amplifier circuit 20.

  Vo ≒ [(R3 + R4) / R3] × Vcc (Equation 5)

  The voltage Vo is a value exceeding the voltage Vth (for example, Vth <(R1 / R2) × Vcc) output from the non-inverting amplifier circuit 20 when the current detection circuit 19 detects the maximum current in the normal state. Therefore, the control circuit 18 to which the voltage Vo is input controls the output of the DC / DC converter 12 (the voltage of the power supply terminal VBUS) to zero by setting the current flowing through the switching element 17 to zero. Do.

  As described above, when the temperature detected by the first resistance element R1 which is a positive temperature coefficient thermistor is abnormally high, the voltage Vo output from the non-inverting amplifier circuit 20 becomes higher than when the temperature is normal. This is equivalent to the fact that the current detected by the current detection circuit 19 is apparently detected to be larger than the actual one for the control circuit 18 that receives the voltage Vo output from the non-inverting amplifier circuit 20, in other words, This corresponds to a decrease in the determined current limit value. As a result, the control circuit 18 limits the output voltage and the output current from the power supply terminal VBUS of the USB connector 8 as compared with the normal state. When an extremely high temperature is detected by the first resistance element R1, the resistance value of the first resistance element R1 increases exponentially, and the output voltage and output current of the DC / DC converter 12 are suppressed to zero. Is done.

  Thus, the switching power supply 10 having the temperature protection function is realized by adding the non-inverting amplifier circuit 20 to the normal switching power supply having no temperature protection function without changing the control IC. You.

  As described above, switching power supply 10 according to the present embodiment includes rectifier circuit 11 for rectifying input AC power, and DC / DC converter for converting the first DC voltage output from rectifier circuit 11 to the second DC voltage. The DC / DC converter 12 includes a switching element 17, a current detection circuit 19 that detects a current flowing through the switching element 17, and outputs a voltage corresponding to the detected current. A non-inverting amplifier circuit that amplifies the output voltage; and a control circuit that controls switching of the switching element according to the voltage output from the non-inverting amplifier circuit. An operational amplifier Amp having an input terminal and an inverting input terminal; and an operational amplifier Amp connected between the output terminal of the current detection circuit 19 and the non-inverting input terminal. A resistance element R1, and a second resistive element R2 connected between the power supply voltage Vcc and the non-inverting input terminal, at least one of the first resistor element R1 and the second resistor element R2 is a thermistor.

  Thus, the voltage output from the current detection circuit 19 is input to the control circuit 18 after the non-inversion amplification using the thermistor is performed in the non-inversion amplification circuit 20. Therefore, the signal based on the temperature detection is superimposed on the signal based on the current detection and input to the control circuit 18, and the output of the temperature is suppressed by the control of the output voltage stabilization of the control circuit 18. Become. That is, the switching power supply 10 having the temperature protection function is realized by adding the non-inverting amplifier circuit 20 to the normal switching power supply having no temperature protection function without changing the control IC. . Therefore, a switching power supply 10 having a temperature protection function realized by a simple circuit is realized.

  The first resistance element R1 is a positive temperature coefficient thermistor, and the second resistance element R2 is a fixed resistance element.

  Thus, by using a positive temperature coefficient thermistor as the first resistance element R1, a temperature protection function can be easily realized.

  The USB power supply device 6 according to the present embodiment includes a USB connector 8, a circuit board 7 supporting the USB connector 8, a switching power supply 10 mounted on the circuit board 7, and a DC power supply included in the switching power supply 10. The / DC converter 12 supplies the second DC voltage to the power supply terminal VBUS of the USB connector 8.

  Thus, the USB power supply device 6 is provided with the switching power supply 10 having a temperature protection function realized by a simple circuit, so that a small-sized and low-cost USB power supply device is realized.

  Further, the DC / DC converter 12 includes a semiconductor element (the diode 15 in the embodiment) for rectifying AC power generated by switching by the switching element 17, and includes a thermistor (the first resistance element R1 in the embodiment). Detects the temperature of at least one of the power supply terminal VBUS and the semiconductor element.

  Thus, the temperature of at least one of the power supply terminal VBUS of the USB connector 8 and the rectifying semiconductor element which is likely to generate heat is detected.

  In the above embodiment, in the non-inverting amplifier circuit 20, the first resistance element R1 is a positive temperature coefficient thermistor, and the second resistance element R2 is a fixed resistance element. However, the present invention is not limited to this. As shown in FIG. 4, the first resistance element R1 may be a fixed resistance element, and the second resistance element R2 may be a negative characteristic thermistor.

  FIG. 4 is a circuit diagram showing a non-inverting amplifier circuit 20a according to a modification of the non-inverting amplifier circuit 20 included in the switching power supply 10 of the above embodiment. The non-inverting amplifier circuit 20a according to this modified example has a fixed resistance element as the first resistance element R1, and has a negative characteristic thermistor as the second resistance element R2. The resistance values of the first resistance element R1 and the second resistance element R2 are set to values satisfying the following conditions. That is, when the temperature detected by the second resistance element R2, which is a negative characteristic thermistor, is normal (that is, the temperature range determined to be normal), the resistance value of the second resistance element R2 >> the resistance value of the first resistance element R1 The resistance value holds. Then, when the temperature detected by the second resistance element R2 is abnormally high, the resistance value of the second resistance element R2 抵抗 the resistance value of the first resistance element R1 holds.

  Even the non-inverting amplifier circuit 20a having such a configuration performs the same operation as the non-inverting amplifier circuit 20 according to the above embodiment. That is, the voltage gain Vo / Vi of the non-inverting amplifier circuit 20a is expressed by the above equation (1). Then, in a normal state, that is, when the temperature detected by the second resistance element R2, which is a negative characteristic thermistor, is a normal temperature, R2 >> R1, so that the voltage amplification factor Vo / The above equation 3 is used as Vi. On the other hand, when the temperature is abnormal, that is, when the temperature detected by the second resistance element R2, which is a negative characteristic thermistor, is abnormally high, R1 ≒ R2 due to the low resistance of the second resistance element R2. The voltage amplification factor Vo / Vi of the circuit 20 is expressed by the above equation (4). This is equivalent to the fact that the current detected by the current detection circuit 19 is apparently detected to be larger than the actual one for the control circuit 18 that receives the output voltage from the non-inverting amplifier circuit 20, in other words, the predetermined value is used. This corresponds to a reduction in the current limit value. As a result, the control circuit 18 limits the output voltage and the output current from the power supply terminal VBUS of the USB connector 8 as compared with the normal state. When an extremely high temperature is detected by the second resistance element R2, the resistance value of the second resistance element R2 decreases exponentially, and the output voltage and the output current are suppressed to zero.

  As described above, according to the switching power supply including the non-inverting amplifier circuit 20a according to the present modification, the same effects as in the above-described embodiment can be obtained. That is, a switching power supply having a temperature protection function is realized by adding a non-inverting amplifier circuit 20a to a normal switching power supply having no temperature protection function without changing the control IC.

  Further, in the non-inverting amplifier circuit 20a according to the present modification, the first resistance element R1 is a fixed resistance element, and the second resistance element R2 is a negative characteristic thermistor.

  Thus, by using a negative temperature coefficient thermistor as the second resistance element R2, the temperature protection function can be easily realized.

  As described above, the switching power supply and the USB power supply device according to the present invention have been described based on the embodiments and the modifications, but the present invention is not limited to these embodiments and the modifications. Unless departing from the gist of the present invention, various modifications conceived by those skilled in the art are applied to this embodiment or modification, or another form constructed by combining some components in the embodiments and modifications. Are also included in the scope of the present invention.

  For example, in the above-described embodiment, the USB power supply device 6 includes a USB Type-A receptacle (female) as the USB connector 8, but is not limited thereto. The USB Type-A or the USB Type-C may be used. It may be a connector.

  In the above embodiment, the USB power supply device 6 includes one USB connector 8, but may include a plurality of USB connectors. In that case, it is preferable that the non-inverting amplifier circuit 20 detect the temperatures of the plurality of USB connectors. For example, by using a negative temperature coefficient thermistor as the second resistance element R2 and using a positive temperature coefficient thermistor as the first resistance element R1, a switching power supply including a plurality of thermistors and having a temperature protection function for a plurality of remote locations is realized. You.

  Further, in the above embodiment, the thermistor included in the non-inverting amplifier circuit 20 detects the temperature of both the power supply terminal VBUS of the USB connector 8 and the diode 15, but may detect the temperature of only one of them. Good.

  In the switching power supply 10 according to the above-described embodiment, the output voltage on the secondary side is not fed back to the primary side, but the output voltage on the secondary side is fed back to the control circuit 18 using a photocoupler or the like, Control for outputting a constant voltage may be performed.

  Further, the switching power supply 10 according to the above-described embodiment is a flyback type (that is, an insulation type) switching power supply, but is not limited to such a type and may be a non-insulation type. Further, the DC / DC converter 12 may be a step-down type, a step-up type, or a step-up / step-down type. The non-inverting amplifier circuit according to the present invention can be applied to all types of switching power supplies by adding a switching power supply including a current detection circuit and a control circuit between the circuits.

  Further, in the switching power supply 10 according to the above-described embodiment, the low-pass filter 21 is provided before the non-inverting amplifier circuit 20; however, the present invention is not limited to this. The low-pass filter may not be provided in the non-inverting amplifier circuit 20, and a low-pass filter may be provided in the input stage of the control circuit 18.

  In the switching power supply 10 according to the above embodiment, the low-pass filter 21 is a CR low-pass filter, but may be an active low-pass filter using an operational amplifier or an LC low-pass filter.

5 AC power supply 6 USB power supply device 7 Circuit board 8 USB connector 10 Switching power supply 11 Rectifier circuit 12 DC / DC converter 15 Diode (an example of a semiconductor element)
Reference Signs List 17 switching element 18 control circuit 19 current detection circuit 20, 20a non-inverting amplifier circuit R1 first resistance element R2 second resistance element Amp operational amplifier

Claims (5)

A rectifier circuit for rectifying the input AC power,
A DC / DC converter for converting a first DC voltage output from the rectifier circuit into a second DC voltage,
The DC / DC converter comprises:
A switching element;
A current detection circuit that detects a current flowing through the switching element and outputs a voltage corresponding to the detected current;
A non-inverting amplifier circuit for amplifying the voltage output from the current detection circuit,
A control circuit that controls switching of the switching element according to a voltage output from the non-inverting amplifier circuit;
The non-inverting amplifier circuit,
An operational amplifier having a non-inverting input terminal and an inverting input terminal;
A first resistance element connected between an output terminal of the current detection circuit and the non-inverting input terminal;
A second resistance element connected between a power supply voltage and the non-inverting input terminal,
A switching power supply, wherein at least one of the first resistance element and the second resistance element is a thermistor. The first resistance element is a positive temperature coefficient thermistor,
The switching power supply according to claim 1, wherein the second resistance element is a fixed resistance element. The first resistance element is a fixed resistance element;
The switching power supply according to claim 1, wherein the second resistance element is a thermistor having a negative characteristic. A USB connector,
A circuit board supporting the USB connector;
The switching power supply according to any one of claims 1 to 3, which is mounted on the circuit board,
The USB power supply device, wherein the DC / DC converter included in the switching power supply supplies the second DC voltage to a power terminal of the USB connector. The DC / DC converter has a semiconductor element that rectifies AC power generated by switching by the switching element,
The USB power supply device according to claim 4, wherein the thermistor detects a temperature of at least one of the power terminal and the semiconductor element.

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