WO2014133188A1

WO2014133188A1 - Switching regulator

Info

Publication number: WO2014133188A1
Application number: PCT/JP2014/055525
Authority: WO
Inventors: Atsushi NISHII; Junji Nishida; Masaya Uchida
Original assignee: Ricoh Company, Ltd.
Priority date: 2013-02-28
Filing date: 2014-02-26
Publication date: 2014-09-04
Also published as: JP2014168342A; JP6154158B2

Abstract

A switching regulator includes a DC power source which generates a variable first voltage, a switching element which operates by a predetermined switching frequency and a predetermined duty ratio, and generates a second voltage from the first voltage, an immittance converter including an input terminal pair and an output terminal pair, so as to generate an output current having a third voltage from the output terminal pair, a resistor connected to the output terminal pair of the immittance converter, so as to generate an output voltage having a fourth voltage lower than the third voltage by the flow of the output current, and a control circuit which changes the first voltage such that the second voltage is equal to the sum of a product of a predetermined current value and the resistance value and the fourth voltage, so that the output current conforms to the predetermined current value.

Description

DESCRIPTION

TITLE OF INVENTION: SWITCHING REGULATOR

PRIORITY CLAIM

The present application is based on and claims priority from Japanese Patent Application No. 2013-039306, filed on February 28, 2013, the disclosure of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

[0001 ]

The present invention relates to a switching regulator which operates as a DC power source having a constant current or a constant voltage.

BACKGROUND ART

[0002]

An impedance and admittance converter or an immittance converter such as a two-terminal-pair circuit is known in which an impedance in one terminal pair is proportional to an admittance of an element or a circuit connected to the other terminal pair (refer to JP H08-308 1 51 A, JP 2004-086833 A, and JP 4379622B).

[0003]

The immittance converter has a property in which an output current is only proportional to an input voltage. When the immittance converter is connected between a load and an AC power source having a constant voltage, a load current is determined only by a power source voltage, and has no relationship to the voltage of the load. Therefore, the voltage is only changed, and the current becomes a constant even if the load is changed. Namely, the constant voltage source is converted into a constant current source by the immittance converter.

[0004] If the immittance converter is connected between the load and the AC power source having a constant current in addition to the above connection, the load voltage becomes a constant value which is determined only by the magnitude of the power source current, and has no relationship to the magnitude of the load current. Namely, the constant current source is converted into a constant voltage source by the immittance converter.

[0005]

FIG. 9 is a circuit view illustrating a configuration of a constant current DC power source including an immittance converter according to the related art. A DC power source E101 , switching transistors SW101 , SW 102, and capacitors C l Ol , C 102 constitute a half bridge type inverter circuit, and operate as a high-frequency voltage source. The capacitors C l O l , C 102, an inductor L101 , a transformer TR101 , and a capacitor C I 03 constitute an immittance converter. Capacitors C 103, C 104, and diodes D 101 , D 102 constitute a rectification circuit, and supply a direct current to a load 100. An LC resonator including the capacitors C l O l , C I 02 and a primary winding of the transformer TR101 and the inductor L I 01 has a resonance frequency which is equal to the operation frequency of the inverter circuit. An LC resonator including the capacitor C I 03 and a secondary winding of the transformer TR101 also has a resonance frequency which is equal to the operation frequency of the inverter circuit. The power source in FIG. 9 is configured to convert the high frequency constant voltage generated by the inverter circuit into a high frequency constant current by the immittance converter, and convert the converted current into a direct current by the rectification circuit, so as to supply a DC constant current.

[0006]^· In the power source in FIG. 9, the immittance converter and the transformer share the same circuit configuration element, so that the entire circuit configuration can be simplified.

[0007]

However, in order to change the input voltage or the output voltage in the power source in FIG. 9, it is necessary to change the winding ratio of the transformer TR101 . Moreover, in the power source in FIG. 9, direct current is converted into alternate current by the inverter circuit, and the alternate current is further converted into direct current by the rectification circuit. For this reason, a decrease in the number of circuit configuration elements is limited.

SUMMARY OF INVENTION

[0008]

It is, therefore, an object of the present invention to provide a switching regulator which operates as a DC power source having a constant current with a simple circuit configuration, so as to easily generate a constant current.

[0009]

One embodiment of the present invention provides a switching regulator, including: a DC power source which generates a variable first voltage: a switching element which operates by a predetermined switching frequency and a predetermined duty ratio, and generates a second voltage from the first voltage; an immittance converter including an input terminal pair and an output terminal pair, the immittance converter having a resonance frequency equal to the switching frequency, an impedance in one terminal pair of the input terminal pair and the output terminal pair being proportional to an admittance of an element or a circuit connected to the other terminal pair., the second voltage being applied to the input terminal pair, so as to generate an output current having a third voltage from the output terminal pair; a resistor including a predetermined resistance value, the resistor being connected to the output terminal pair of the immittance converter, so as to generate an output voltage having a fourth voltage lower than the third voltage by the flow of the output current; and a control circuit which changes the first voltage such that the second voltage is equal to the sum of a product of a predetermined current value and the resistance value and the fourth voltage, so that the output current conforms to the predetermined current value.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]

The accompanying drawings are included to provide further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the specification, serve to explain the principle of the invention.

FIG. 1 is a circuit view illustrating a schematic configuration of a switching regulator according to an embodiment of the present invention.

FIG. 2 is a circuit view illustrating a first modified example of an immittance converter in the switching regulator in FIG. 1.

FIG. 3 is a circuit view illustrating a second modified example of an immittance converter in the switching regulator in FIG. 1 .

FIG. 4 is a circuit view illustrating a third modified example of an immittance converter in the switching regulator in FIG. 1.

FIG. 5 is a circuit view illustrating a configuration of a switching regulator according to a first example of the embodiment of the present invention.

FIG. 6 is a circuit view illustrating a configuration of a switching regulator according to a second example of the embodiment of the present invention. g

FIG. 7 is a circuit view illustrating a configuration of a switching regulator according to a third example of the embodiment of the present invention.

FIG. 8 is a circuit view illustrating a configuration of a switching regulator according to a fourth example of the embodiment of the present invention.

FIG. 9 is a circuit view illustrating a configuration of a constant current DC power source including an immittance converter according to a related art.

DESCRIPTION OF EMBODIMENT

[001 1 ]

Hereinafter, a switching regulator according to the embodiment of the present invention will be described with reference to the drawings.

[0012]

FIG. 1 is a circuit view illustrating a schematic configuration of a switching regulator according to the embodiment of the present invention. The switching regulator in FIG. 1 includes a variable voltage source E l , switching elements SWl , SW2, a control circuit 10, immittance converter Ml , and resistor Ro, and supplies an output voltage Vo to a load 20.

[0013]

The variable voltage source El is a DC voltage source which generates a variable input voltage Vi under the control of the control circuit 10. The switching elements SWl , SW2 operate by a predetermined switching frequency and a predetermined duty ratio, or a predetermined switching frequency and a variable duty ratio under the control of the control circuit 10, and generate a primary current I I having a primary voltage V I for the immittance converter Ml from the input voltage Vi. The immittance converter Ml includes an input terminal pair and an output terminal pair, and a resonance frequency equal to a switching frequency. In the immittance converter Ml , an impedance in one terminal pair of the input terminal pair and the output terminal pair is proportional to an admittance of an element or a circuit connected to the other terminal pair. The primary current II having the primary voltage V I is supplied to the input terminal pair of the immittance converter Ml , and the secondary current 12 having the secondary voltage V2 is generated from the output terminal pair. The output terminal pair of the immittance converter Ml is connected to the load 20 through the resistor Ro. The resistor Ro includes a predetermined resistance value (Ro). The secondary current 12 of the immittance converter Ml flows in the resistor Ro. With the flow of the secondary current 12 in the resistor Ro, the output voltage Vo of the switching regulator, which is lower than the secondary voltage V2 of the immittance converter Ml , is generated, and the output voltage Vo is applied to the load 20. The control circuit 10 changes the input voltage Vi or the duty ratio so as to generate a constant current or a constant voltage.

[0014]

In this case, the operation principle of the immittance converter Ml is described based on JP 4379622B.

[0015]

The immittance converter Ml is a T-type circuit including inductors L I , L2, and a capacitor C I , for example. An angular resonance frequency ω of the immittance converter Ml is expressed by the following formula where L is an inductance of the inductors L I , L2, and C is a capacitance of the capacitor CI .

[0016] ω = (LC) (1)

[0017] The immittance converter Ml is configured such that the angular resonance frequency co is equal to the switching frequency of the switching elements SW1 , SW2. In this case, the primary voltage V I , primary current II , secondary voltage V2, and secondary current 12 of the immittance converter M l are expressed by a matrix of a four-terminal constant in the following formula.

[0018]

[0019]

In this case, Zo = (L / C) is a characteristic impedance of the immittance converter Ml . In formula (2), voltages VI , V2 and currents II , 12 are vectors, respectively.

[0020]

The characteristic of the immittance converter is that A and D of the four-terminal constant are 0. In the following formulas, the secondary current 12 is proportional to the primary voltage V I , and the secondary current V2 is proportional to the primary current I I .

[0021 ]

[0022]

Therefore, when the constant voltage source is connected to the input terminal pair of the immittance converter M l , and the primary voltage VI is constant, the secondary current 12 becomes constant and has no relationship to the primary current II and second voltage V2, so that the immittance converter Ml operates as a constant current source. When the constant current source is connected to the input terminal pair of the immittance converter M l , and the primary current II is constant, the secondary voltage V2 becomes constant and has no relationship to the primary voltage V I and the secondary current 12, so that the immittance converter M l operates as a constant voltage source.

[0023]

The following formula can be derived from formulas (3), (4).

[0024]

[0025]

According to formula (5), the input power is equal to the output power, and the immittance converter M l operates as a power converter.

[0026]

The following formula is satisfied where Z l is the input impedance of the immittance converter Ml in the input terminal pair of the immittance converter M l and Z2 is the load impedance (namely, impedance of load 20 and resistor Ro) of the immittance converter Ml .

[0027]

[0028] More specifically, the input impedance Z l is proportional to the inverse of the load impedance Z2, namely, the load admittance.

[0029]

According to formula (6), the positive and negative of the phase of the impedance are inverted after passing through the immittance converter M l , so that the magnitude of the input power factor of the immittance converter is the same as the load power factor, and the lead lag is inverted. Moreover, the impedance in the input terminal pair becomes 0 if the output terminal pair is released. The impedance in the input terminal pair becomes infinity if the output terminal pair is short-circuited. The linear circuit which satisfies these conditions is the immittance converter.

[0030]

The switching regulator in FIG. 1 includes the immittance converter M l , so that the switching regulator in FIG. 1 operates as a constant current source or a constant voltage source.

[003 1 ]

The immittance converter is not limited to the immittance converter M l of a T-type circuit including the inductors L I , L2 and the capacitor C I as illustrated in FIG. 1 . FIG. 2 is a circuit view illustrating a first modified example of the immittance converter in the switching regulator in FIG. 1 . The switching regulator in FIG. 1 may include an immittance converter M2 of a T-type circuit including capacitors C l l , C 12 and an inductor L I I instead of the immittance converter M l . FIG. 3 is a circuit view illustrating a second modified example of the immittance converter in the switching regulator in FIG. 1 . The switching regulator in FIG. 1 may include an immittance converter M3 of a Π-type circuit having capacitors C21 , C22 and an inductor L21 instead of the immittance converter Ml . FIG. 4 is a circuit view illustrating a third modified example of the immittance converter in the switching regulator in FIG. 1 . The switching regulator in FIG. 1 may include an immittnace converter M4 of a Π-type circuit having a capacitor C31 and inductors L3 1 , L32 instead of the immittance converter M l .

[0032]

Hereinafter, the features of the switching regulator according to the embodiment of the present invention will be described with reference to FIGS. 5-8.

[0033]

FIG. 5 is a circuit view illustrating the configuration of a switching regulator according to a first example of the embodiment of the present invention. The switching regulator in FIG. 5 includes an adder 31 and a multiplier 32 in addition to the configuration of the switching regulator in FIG. 1. The switching regulator in FIG. 5 also includes a control circuit 10A instead of the control circuit 10 in FIG. 1 . The control circuit 10A controls the switching elements SW1 , SW2 so as to operate by a predetermined switching frequency and a predetermined duty ratio. The control circuit 10A changes the input voltage Vi by controlling the variable voltage source E l , so as to generate the output current of the switching regulator equal to a predetermined current value Iset. The switching regulator in FIG. 5 operates as a DC power source having a constant current.

[0034]

In this case, the operation principle of the switching regulator in FIG. 5 will be described.

[0035] The primary voltage VI of the immittance converter Ml is expressed by the following formula with the input voltage Vi and the duty ratio d.

[0036]

Vl=Vi Xd (7)

[0037]

According to formula (7), when the duty ratio d is a fixed value, the primary voltage VI of the immittance converter Ml is changed by changing the input voltage Vi.

[0038]

The secondary voltage V2 of the immittance converter Ml is expressed by the following formula.

[0039]

V2 = Vo + Ro I2 (8)

[0040]

The following formula can be obtained when substituting formula (8) into the formula (5).

[0041]

Vl=(Vo + Ro I2)x(I2÷Il) (9)

[0042]

It is considered that a current hardly flows in the capacitor CI because the primary current II is a direct current. Therefore, the following formula is satisfied between the primary current II and the secondary current 12.

[0043] II =F 12 (10)

[0044]

The following formula can be therefore derived from formulas (9), ( 10).

[0045]

Vl = Vo + Ro I2 (11)

[0046]

When generating the output current of the switching regulator which is equal to the predetermined current value Iset, the output current is equal to the secondary current 12 of the immittance converter Ml , so that the following formula can be obtained by substituting Iset into the 12 of formula ( 1 1).

[0047]

VI = Vo + Ro Iset (12)

[0048]

In formula (12), the output voltage Vo is a value which is actually monitored, and the resistance value Ro is a known value. The output current equal to a predetermined current value Iset can be thus generated by changing the primary voltage V I of the immittnace converter Ml , so as to satisfy the formula (12). More specifically, an output current equal to a predetermined current value Iset can be generated by changing the input voltage Vi.

[0049]

Referring to FIG. 5, the multiplier 32 multiplies the resistance value Ro and the predetermined current value Iset. The multiplied result is added to the output voltage Vo by the adder 31 , and the added result is input to the control _lg circuit 1 0A. The control circuit 10A changes the input voltage Vi of the switching regulator such that the primary voltage V I of the immittance converter Ml is equal to the left-hand side of formula (12) (namely, the sum of the product of the predetermined current value Iset and the resistance value Ro and the output voltage Vo of the switching regulator), so that the output current of the switching regulator conforms to the predetermined current value Iset.

[0050]

As described above, when the primary voltage V I of the immittance converter Ml is a constant voltage, the switching regulator in FIG. 5 operates as a DC power source having a constant current.

[0051 ]

According to the embodiment of the present invention, a switching regulator can be provided which operates as a DC power source having a constant current, has a simple circuit configuration, and easily generates a constant current.

[0052]

FIG. 6 is a circuit view illustrating the configuration of the switching regulator according to a second example of the embodiment of the present invention. The switching regulator in FIG. 6 includes a comparator 41 in addition to the configuration of the switching regulator in FIG. 1. The switching regulator in FIG. 6 also includes a fixed voltage source E2 instead of the variable voltage source E l in FIG. 1 and a control circuit 10B instead of the control circuit 10 in FIG. 1. The comparator 41 determines whether or not the output voltage Vo conforms to the predetermined voltage value Vset. The control circuit 10B controls the switching elements SW1 , SW2, so as to operate by a predetermined switching frequency and a variable duty ratio.

[0053] The output voltage Vo equal to the predetermined voltage value Vset can be generated by changing the primary voltage V I of the immittance converter Ml , so as to satisfy formula ( 1 ). In other words, the output voltage Vo equal to the predetermined voltage value Vset can be generated by changing the duty ratio d of formula (7).

[0054]

The control circuit 10B therefore changes the duty ratio of the switching elements SW1 , SW2 such that the output voltage Vo is equal to the predetermined voltage value Vset. The switching regulator in FIG. 6 operates as a DC power source having a constant voltage. In other words, the switching regulator in FIG. 6 includes a feedback loop of a voltage similar to a conventional DC-DC converter, so as to generate a constant output voltage.

[0055]

In addition, the switching regulator in FIG. 6 uses a fixed voltage source E2 because it is not necessary to change the input voltage Vi as long as a voltage corresponding to the power necessary for the load 20 can be generated.

[0056]

According to the embodiment of the present invention, a switching regulator can be provided which operates as a DC power source having a constant voltage, has a simple circuit configuration, and can easily generate a constant voltage.

[0057]

FIG. 7 is a circuit view illustrating the configuration of the switching regulator according to a third example of the embodiment of the present invention. The switching regulator in FIG. 7 includes an adder 3 1 , multiplier 32, and comparator 51 in addition to the configuration of the switching regulator in FIG. 6. The switching regulator in FIG. 7 also includes a control circuit I OC instead of the control circuit 10B in FIG. 6. The control circuit I OC controls switching elements SW I , SW2 so as to operate by a predetermined switching frequency and a variable duty ratio similar to the control circuit 10B in FIG. 6.

[0058]

In the switching regulator in FIG. 5, an output current of the switching regulator equal to a predetermined current value Iset is generated based on formula ( 1 1 ). On the other hand, in the switching regulator in FIG. 7, the output current is limited to a predetermined upper limit current value Ilimit or below based on formula ( 1 1 ). When the output current of the switching regulator is equal to the upper limit current value Ilimit, the following formulas can be obtained by substituting Ilimit into 12 of formula ( 1 1 ).

[0059]

VI = Vo +.Ro x Ilimit (13)

Ilimit - (VI - Vo) ÷ Ro (14)

[0060]

Therefore, when formula ( 13) or formula ( 14) is satisfied, the primary voltage V I of the immittance converter M l becomes the upper limit voltage value Vlimit corresponding to the upper limit current value Ilimit.

[0061 ]

Vlimit = Vo + Ro Ilimit (15)

Ilimit = (Vlimit - Vo) ÷ Ro (16) [0062]

The primary voltage V I of the immittance converter Ml is set to the upper limit voltage value Vlimit of formula ( 15) or below, so that the output current can be limited to a predetermined upper limit current value Ilimit or below.

[0063]

Referring to FIG. 7, the multiplier 32 multiplies the resistance value Ro and the upper limit current value Ilimit, the multiplied result is added to the output voltage Vo by the adder 3 1 , and the added result is input to the comparator 51 as the upper limit voltage value Vlimit. The comparator 51 determines whether or not the primary voltage V I of the immittance converter Ml reaches the upper limit voltage value Vlimit. The control circuit I OC controls the range which changes the duty ratio of the switching elements SW 1 , SW2 such that the primary voltage V I of the immittance converter M l becomes the upper limit voltage value Vlimit of formula (15) (namely, the sum of the product of the upper limit current value Ilimit and the resistance value Ro and the output voltage Vo of the switching regulator) or below.

[0064]

Accordingly, when the primary voltage V I of the immittance converter Ml reaches the upper limit voltage value Vlimit, the control circuit I OC does not further increase the primary voltage V I even if the output voltage Vo is less than the predetermined voltage Vset. In the normal operation of the switching regulator, the control circuit I OC changes the duty ratio so as to be Vo = Vset. On the other hand, when the consumed power of the load 20 is excessive (when the primary voltage V I of the immittance converter M l reaches the upper limit voltage Vlimit), the control circuit I OC controls the duty ratio so as to be the upper limit or below. [0065]

As described above, the switching regulator in FIG. 7 controls the output current to the upper limit current value Ilimit or below while generating the output voltage Vo equal to a desired voltage value Vset.

[0066]

FIG. 8 is a circuit view illustrating the configuration of the switching regulator according to a fourth example of the embodiment of the present invention. The switching regulator in FIG. 8 includes a comparator 61 in addition to the configuration of the switching regulator in FIG. 6. The switching regulator in FIG. 8 also includes a control circuit 10D instead of the control circuit 10B in FIG. 6. The comparator 61 operates as a detector which detects a reverse current in the input terminal pair of the immittance converter M l . More specifically, the comparator 61 compares the electric potential of the node between the switching elements SWl , SW2 to a ground battery, and determines the generation of a reverse current when it is detected that the electric potential is lower than that of the ground battery. Similar to the control circuit 10B in FIG. 6, the control circuit 10D controls the switching elements SWl , SW2, so as to operate by a predetermined switching frequency and a variable duty ratio. The control circuit 1 0D also disconnects the switching elements SW l , SW2 when the comparator 61 detects a reverse current.

[0067]

The switching regulator in FIG. 8 operates as a DC power source having a constant voltage, and disconnects the switching elements SWl , SW2 when a reverse current is detected. Power loss can thus be prevented by disconnecting the switching elements SW l , SW2.

[0068] According to the switching regulator of the embodiment of the present invention, constant current control can be easily achieved by using a simple circuit configuration which only adds an output voltage to the product of the predetermined current value and the resistance value, so that both of the circuit and control can be simplified. Constant voltage control can thus be easily achieved instead of constant current control.

[0069]

The switching regulator according to the embodiment of the present invention includes the following features. An output current equal to a desired current value is generated by performing the power conversion with the immittance converter and changing the input voltage Vi. With this configuration, a feedback loop in the constant current control becomes unnecessary, the phase design also becomes unnecessary, and thus, stability can be easily achieved. Moreover, conventional constant voltage control can be performed with a similar configuration, so that the circuit design and the circuit can be simplified.

[0070]

The switching regulator according to the embodiment of the present invention includes the following configurations.

[0071 ]

A switching regulator according to a first aspect of the present invention includes: a DC power source which generates a variable first voltage: a switching element which operates by a predetermined switching frequency and a predetermined duty ratio, and generates a second voltage from the first voltage; an immittance converter including an input terminal pair and an output terminal pair, the immittance converter having a resonance frequency equal to the switching frequency, an impedance in one terminal pair of the input terminal pair and the _1Q output terminal pair being proportional to an admittance of an element or a circuit connected to the other terminal pair, the second voltage being applied to the input terminal pair, so as to generate an output current having a third voltage from the output terminal pair; a resistor including a predetermined resistance value, the resistor being connected to the output terminal pair of the immittance converter, so as to generate an output voltage having a fourth voltage lower than the third voltage by the flow of the output current; and a control circuit which changes the first voltage such that the second voltage is equal to the sum of a product of a predetermined current value and the resistance value and the fourth voltage, so that the output current conforms to the predetermined current value.

[0072]

A switching regulator according to a second aspect of the present invention includes: a DC power source which generates a first voltage; a switching element which operates by a predetermined switching frequency and a variable duty ratio, and generates a second voltage from the first voltage; an immittance converter including an input terminal pair and an output terminal pair, the immittance converter having a resonance frequency equal to the switching frequency, an impedance in one terminal pair of the input terminal pair and the output terminal pair being proportional to an admittance of an element or a circuit connected to the other terminal pair, the second voltage being applied to the input terminal pair, so as to generate an output current having a third voltage from the output terminal pair; a resistor having a predetermined resistance value, the resistor being connected to an output terminal pair of the immittance converter, so as to generate an output voltage having a fourth voltage lower than the third voltage by the flow of the output current; and a control circuit which changes the duty ratio such that the fourth voltage is equal to a predetermined voltage value. [0073]

According to a switching regulator of a third aspect of the present invention, in the switching regulator according to the second aspect, the control circuit controls a range which changes the duty ratio such that the second voltage becomes less than or equal to a sum of a product of a predetermined upper limit current value and the resistance value and the fourth voltage.

[0074]

A switching regulator according to a fourth aspect of the present invention, in the switching regulator according to the second aspect or the third aspect includes a detector which detects a reverse current in the input terminal pair of the immittance converter, wherein the control circuit disconnects the switching element when the detector detects the reverse current.

[0075]

Although the embodiment of the present invention has been described above, the present invention is not limited thereto. It should be appreciated that variations may be made in the embodiment described by persons skilled in the art without departing from the scope of the present invention.

Claims

1. A switching regulator, comprising:

a DC power source which generates a variable first voltage;

a switching element which operates by a predetermined switching frequency and a predetermined duty ratio, and generates a second voltage from the first voltage;

an immittance converter including an input terminal pair and an output terminal pair, the immittance converter having a resonance frequency equal to the switching frequency, an impedance in one terminal pair of the input terminal pair and the output terminal pair being proportional to an admittance of an element or a circuit connected to the other terminal pair, the second voltage being applied to the input terminal pair, so as to generate an output current having a third voltage from the output terminal pair;

a resistor including a predetermined resistance value, the resistor being connected to the output terminal pair of the immittance converter, so as to generate an output voltage having a fourth voltage lower than the third voltage by the flow of the output current; and

a control circuit which changes the first voltage such that the second voltage is equal to the sum of a product of a predetermined current value and the resistance value and the fourth voltage, so that the output current conforms to the predetermined current value.

2. A switching regulator, comprising:

a DC power source which generates a first voltage; a switching element which operates by a predetermined switching frequency and a variable duty ratio, and generates a second voltage from the first voltage;

a resistor having a predetermined resistance value, the resistor being connected to an output terminal pair of the immittance converter, so as to generate an output voltage having a fourth voltage lower than the third voltage by the flow of the output current; and

a control circuit which changes the duty ratio such that the fourth voltage is equal to a predetermined voltage value.

3. The switching regulator according to Claim 2, wherein the control circuit controls a range which changes the duty ratio such that the second voltage becomes less than or equal to a sum of a product of a predetermined upper limit current value and the resistance value and the fourth voltage.

4. The switching regulator according to Claim 2 or Claim 3 , further comprising a detector which detects a reverse current in the input terminal pair of the immittance converter, wherein the control circuit disconnects the switching element when the detector detects the reverse current.