JP5488230B2 - DC-DC converter - Google Patents

Description

  The present invention relates to a DC-DC converter that converts a DC input voltage and outputs a stable DC output voltage to a load connected between a pair of output lines. More specifically, the current value of an output current flowing through a load is expressed as DC. The present invention relates to a DC-DC converter that outputs from the DC converter side.

  The DC-DC converter is used as a converter for converting an unstable DC input voltage into a stable predetermined output voltage, for a charger or an AC adapter of a portable electronic device such as a mobile phone or a portable music player.

  FIG. 4 shows an example of an insulated flyback DC-DC converter 100 among conventional DC-DC converters (Patent Document 1). In FIG. 4, 1a is a high-voltage terminal of a DC power source, 1b is A low voltage terminal, 2a is a primary winding of the transformer 2, and 2b is a secondary output winding of the transformer 2.

  The intermittent oscillation element 3 is connected in series with the primary winding 2a of the transformer 2 and the DC power source 1, and normally operates continuously so as to open and close the primary winding 2a and the low-voltage terminal 1b at a constant frequency. While a stop control signal for supplying a constant current to the control terminal 3a is input, the control terminal 3a is stopped in a state where the primary winding 2a and the low voltage terminal 1b are disconnected.

  Reference numeral 39 denotes a photocoupler light-receiving element that is photocoupled with a photocoupler light-emitting element 35 provided on the output side, which will be described later, and is connected between the control terminal 3 a of the intermittent oscillation element 3 and the low-voltage terminal 1 b of the DC power supply 1.

  On the secondary side (output side) of the transformer 2, a rectifying diode 4 and a smoothing capacitor 13 constituting a rectifying / smoothing circuit are provided, and the output of the secondary output winding 2b is rectified and smoothed to obtain a high-voltage side output line. Output between 20a and the low-voltage side output line 20b.

  The output voltage and output current are monitored between the pair of output lines 20a and 20b, and when one of them exceeds a set voltage or set current set to a predetermined value, the photocoupler light emitting element 35 in the figure emits light. An output monitoring circuit including a voltage monitoring circuit and a current monitoring circuit is provided.

  The intermittent oscillation element 3 described above is switched on and off so that the output power and output current generated between the high-voltage side output line 20a and the low-voltage side output line 20b gradually increase during continuous operation. Duty is set. When a rising output voltage exceeds a set voltage with a load connected between the high-voltage side output line 20a and the low-voltage side output line 20b, the voltage dividing resistors 30 and 31 input to the inverting input terminal of the error amplifier 33a. The potential of the intermediate tap 32 also rises, and the potential difference from the first comparison voltage of the voltage monitoring reference power supply 34a is inverted and amplified to a potential that exceeds the light emission threshold value of the photocoupler light emitting element 35.

  Further, even when the output current rises beyond the set current due to the load connected between the high voltage side output line 20a and the low voltage side output line 20b, the current detection resistor 43 connected in series to the low voltage side output line 20b. The voltage drop due to the output current increases, the voltage input to the inverting input terminal of the error amplifier 33b increases, the potential difference with the second comparison voltage of the current monitoring reference power supply 34b is inverted and amplified, and the photocoupler light emitting element 35 The potential exceeds the light emission threshold.

  As a result, as long as either the output voltage or the output current exceeds the set voltage or set current, the photocoupler light emitting element 35 continues to emit light, and a limit signal indicating that these values have been exceeded continues to the photocoupler light receiving element 39. Is output.

  Since the photocoupler light receiving element 39 receives the limit signal from the photocoupler light emitting element 35, a constant current flows from the control terminal 3a of the intermittent oscillation element 3 to the low voltage terminal 1b of the DC power supply 1, and thus the control terminal 3a. In this state, the pause control signal is continuously input. Meanwhile, the opening / closing operation of the intermittent oscillation element 3 is stopped, the current flowing through the primary winding 2a of the transformer 3 is cut off, and the excitation energy accumulated in the secondary output winding 2b of the transformer 3 is gradually consumed by the load. As a result, the output voltage or output current that has exceeded the set voltage or set current decreases and becomes lower than the set voltage or set current.

  As a result, the photocoupler light emitting element 35 stops emitting light, and the photocoupler light receiving element 39 does not receive the limit signal. Therefore, the intermittent oscillation element 3 repeats the continuous opening / closing operation again, and sets the output voltage and output current to the set voltage. Then, constant voltage control and constant current control for setting current are performed.

  There is also known a DC-DC converter that outputs an abnormal operation signal to a load when the output voltage between a pair of output lines exceeds a set voltage for some reason and the operation is stopped (Patent Document 2).

Japanese Patent No. 3391774 JP 2005-323437 A

  A conventional DC-DC converter of this type is used as a charger or an AC adapter by connecting to an electronic device that is a load that conforms to its output rating. In recent years, a general-purpose USB (Universal Serial) is used. Since a configuration in which a charger or an AC adapter is connected via a bus connector is adopted, there is a possibility that a non-conforming DC-DC converter is erroneously connected. If a DC-DC converter with an output rating exceeding the power consumption of the load is erroneously connected, there is no means to stop the operation of the DC-DC converter from the load side or to control its operation. There was a risk of burns and fires due to abnormal heat generation.

  In spite of such a problem being predicted, the conventional DC-DC converter represented by the DC-DC converter described in Patent Document 1 has a wrong connection state when an incompatible load is connected. Even if the DC-DC converter described in Patent Document 2 has no output means on the load side, it only outputs its own abnormal operation to the load side. There was no means for transmitting erroneous connections that could occur to the load side.

  On the other hand, on the load side, a shunt resistor corresponding to the current detection resistor 43 in FIG. 4 is connected in series to the internal wiring connected to the output line of the DC-DC converter, and the output current of the DC-DC converter is determined from the voltage drop. In the case where is different from the rated current, it is conceivable to display an erroneous connection of the DC-DC converter on the display unit and warn the user. However, in general, when a DC-DC converter is connected to a load as a charger, the output current (charging current) is as large as about 1 A, and the voltage drop of the shunt resistor connected to the inside of the load cannot be ignored. A battery or the like is charged with a voltage lower than the output voltage of a DC-DC converter that performs constant voltage output control, which causes a problem of insufficient charging voltage. In particular, a lithium ion battery needs to be charged under strict control of the charging voltage, and even if the output voltage on the DC-DC converter side is controlled at a constant voltage, the charging voltage fluctuates due to a change in the output current. Therefore, as described above, it is practically difficult to detect the connection with the incompatible DC-DC converter by monitoring the output current on the load side.

  Further, each circuit in the load generally operates with the potential of the low-voltage side output line 20b of the DC-DC converter at the ground level as the reference potential, so that the voltage drop caused by the current detection resistor 43 in FIG. 4 representing the output current. Is represented by a negative polarity, and even if output to the load side, it cannot be detected by a normal load side comparison circuit. When the current detection resistor 43 is connected in series to the high-voltage side output line 20a so that the voltage drop due to the current detection resistor 43 is represented by a positive polarity, the voltage drop of the shunt resistor representing the output current is small with respect to the output voltage. However, there has been a problem that the output current cannot be detected with high accuracy due to the influence of the output voltage fluctuation, and as a result, the value of the output current is transmitted from the DC-DC converter side to the load.

  The present invention has been made in consideration of such a conventional problem, and provides a DC-DC converter capable of transmitting the fact that an incompatible DC-DC converter is connected to a load connected to the output side. The purpose is to do.

  It is another object of the present invention to provide a DC-DC converter having a simple configuration that outputs a voltage signal representing an output current to a load with a positive polarity and that can be detected by a comparison circuit on the load side.

In order to achieve the above object, a DC-DC converter according to claim 1 is connected to a switching element connected in series with a DC power supply and a current flowing from the DC power supply intermittently by switching operation of the switching element. Switching between a pair of high-voltage output line and low-voltage output line according to the output voltage between the high-voltage output line and the low-voltage output line A DC-DC converter provided with a constant voltage control circuit that controls opening and closing of an element and performs constant voltage control of an output voltage,
The main feature is that a shunt resistor is connected in series to either the high-voltage side output line or the low-voltage side output line, and a voltage signal representing the potential difference between both ends of the shunt resistor is output to the load.

  The voltage signal output to the load represents the output current flowing through the high-voltage side output line or low-voltage side output line connected in series with the shunt resistor, so the load side does not change the voltage to the internal load. The output current of the DC-DC converter can be detected from the voltage signal.

  Further, in the DC-DC converter according to claim 2, a shunt resistor is wired in series with the low-voltage output line, and one is connected between the high-voltage output line and the low-voltage output line connected to the input side of the shunt resistor. A buffer resistor connected to the high-voltage side output line and a constant voltage circuit element that generates a constant voltage higher than the potential difference between both ends of the shunt resistor are connected in series, and voltage is applied from the series connection point of the constant voltage circuit element and the buffer resistor. A signal is output.

  The potential on the input side of the shunt resistor is expressed as a negative potential proportional to the output current flowing through the low-voltage main power line, with the potential of the low-voltage output line connected to the load as the reference potential. The voltage signal output from the series connection point of the constant voltage circuit element and the buffer resistor via the positive voltage represents the output current flowing through the load with a positive polarity.

  The constant voltage circuit element generates a constant voltage that does not depend on the fluctuation of the output voltage, and the voltage of the buffer resistor changes according to the fluctuation of the output voltage.

  The DC-DC converter according to claim 3 is characterized in that the constant voltage circuit element is a shunt regulator.

  The shunt regulator is connected in parallel with the load, generates a constant voltage that does not depend on fluctuations in the DC output voltage output to the load, and has a constant voltage that is less than a few V DC output voltage and greater than the potential difference across the shunt resistor. Can be easily generated.

  According to a fourth aspect of the present invention, there is provided a DC-DC converter that controls opening and closing of a switching element in accordance with an output current flowing through a low-voltage side output line detected from a potential difference between both ends of a shunt resistor, and controls the output current at a constant current. A control circuit is further provided.

  A shunt resistor used for outputting a positive voltage signal is used as a current detection resistor of the constant current control circuit.

  According to the first aspect of the present invention, the output current of the DC-DC converter connected on the load side can be detected without lowering or changing the charging voltage to the battery of the load.

  On the load side, an erroneous connection with a DC-DC converter having a different rated output can be detected from the output current of the DC-DC converter, and the danger of abnormal heat generation of the load can be prevented by alerting the user.

  According to the second aspect of the present invention, the voltage signal output from the DC-DC converter is positive with respect to the reference potential of the low-voltage side output line and represents the output current flowing through the load. The output current output from the DC-DC converter can be easily detected by a normal comparison circuit on the load side that operates with reference potential.

  Since the shunt resistor is connected in series to the low-voltage output line, the potential that represents the output current that compares the potential of the low-voltage output line with the reference potential does not affect the fluctuation of the output voltage, and the output current is detected with high accuracy. it can.

  According to the invention of claim 3, it is suitable as a constant voltage circuit element for outputting a positive voltage signal from a rated DC-DC converter having an output voltage of several volts and an output current of about 1A.

  According to the invention of claim 4, since the current detection resistor of the constant current control circuit can be used as a shunt resistor, a positive voltage signal representing an output current can be loaded only by connecting the constant voltage circuit element and the buffer resistor. Can be connected to.

1 is a circuit diagram showing an outline of a DC-DC converter 10 according to a first embodiment of the present invention. It is a circuit diagram of the DC-DC converter 50 which concerns on 2nd Embodiment of this invention. It is a circuit diagram of the DC-DC converter 70 which used the bipolar transistor 60 as a constant voltage circuit element. 1 is a circuit diagram showing a conventional DC-DC converter 100. FIG.

  Hereinafter, a DC-DC converter 10 according to an embodiment of the present invention will be described with reference to FIG. The DC-DC converter 10 according to the present embodiment has the same basic configuration as the conventional flyback type DC-DC converter 100 shown in FIG. 4, and the constant voltage circuit element 7 and the buffer are added to the conventional DC-DC converter 100. A resistor 8 is provided. Therefore, circuits and circuit elements common to the conventional DC-DC converter 100 are denoted by the same reference numerals, and detailed description thereof is omitted. The DC-DC converter 10 also includes a voltage monitoring circuit of a constant voltage control circuit and an AC negative feedback element, but these configurations are not shown in FIG.

  In the present embodiment, it is assumed that the DC-DC converter 10 is connected to a commercial AC power supply of 100 V and is built in a charger that charges the mobile phone 40 that is a load with a rated output of 5 V and 1 A. Accordingly, in the figure, reference numeral 1 denotes an unstable DC power source in which the voltage obtained by rectifying a commercial AC voltage of 100 V may fluctuate. The DC input voltage between the high voltage terminal 1a and the low voltage terminal 1b is 141V. 2a is a primary winding of the transformer 2, 2b is a secondary output winding of the transformer 2, and the primary winding 2a is connected in series to the intermittent oscillation element 3 and the DC power source 1.

  The intermittent oscillation element 3 incorporates an oscillator, a control element that controls oscillation and pause of the oscillator, and a switching element that interrupts the current of the primary winding 2a of the transformer 2, and the control element is usually a transmitter. The switching element is controlled to open and close at a constant frequency output from the control terminal 3a, and while the pause control signal for passing a constant current is input to the control terminal 3a, the oscillation of the oscillator is paused and the switching element is controlled to open. Operate.

  Between the control terminal 3a of the intermittent oscillation element 3 and the low voltage terminal 1b of the DC power source 1, a photocoupler light emitting element 35 and a photocoupler light receiving element 39 that are photocoupled provided on the secondary side of the transformer 2 are connected. 2, the output voltage and output current signals output from the secondary side to the load 40 are fed back to the primary side of the insulated transformer 2 to control the intermittent oscillation operation of the intermittent oscillation element 3.

  Both ends of the secondary output winding 2b of the transformer 2 are connected to the high-voltage side output line 20a and the low-voltage side output line 20b connected to the load 40, and the output generated in the secondary output winding 2b is the high-voltage side output line 20a. Is rectified and smoothed by a rectifying / smoothing circuit including a rectifying diode 4 connected in series to the rectifying diode 4 and a smoothing capacitor 13 connected in parallel to the secondary output winding 2b, and the high-voltage side output line 20a and the low-voltage side output Output between lines 20b. Reference numeral 14 denotes a capacitor for obtaining the stability of the output voltage against the load fluctuation of the load 40, but it is not essential.

  The output voltage and the output current are monitored between the pair of output lines 20a and 20b, and the voltage monitor for causing the photocoupler light emitting element 35 in the figure to emit light when either exceeds a predetermined set voltage or set current. Although an output monitoring circuit including a circuit and a current monitoring circuit is provided, the illustration and description of the voltage monitoring circuit are omitted as described above.

  In the current monitoring circuit, the shunt resistor 11 is connected in series to the low-voltage side output line 20b, one end on the output side of the shunt resistor 11 is connected to the inverting input terminal of the error amplifier 33b, and the other end on the input side is connected to the reference power supply 34b for current monitoring. To the non-inverting input terminal. As a result, the output current flowing through the low-voltage side output line 20b is expressed by the potential difference between both ends of the shunt resistor 11, and is compared with the second comparison voltage of the current monitoring reference power supply 34b by the error amplifier 33b. Determine whether it has been exceeded.

  The set current, which is a target value for constant current control of the output current, can be set to an arbitrary value by changing the resistance value of the shunt resistor 11 or the second comparison voltage of the current monitoring reference power supply 34b. On the other hand, since an output current flows through the shunt resistor 11, a resistor having a small resistance value is used as the shunt resistor 11. Here, in order to control the output current to 1 A at a constant current, the set current is 1 A, and the resistance value is 0. A 3Ω shunt resistor 11 and a 0.3V current monitoring reference power supply 34b are used.

  A photocoupler light emitting element 35 is connected to the output side of the error amplifier 33b. The photocoupler light emitting element 35 is connected to the high voltage side output line 20a via an electric resistor 36 and is supplied with drive power. Accordingly, while the output current flowing through the low-voltage side output line 20b exceeds the set current, the photocoupler light emitting element 35 emits light continuously, and a limit signal indicating that the set current has been exceeded by light emission is output to the photocoupler light receiving element 39. .

  The low-voltage side output line 20b on the input side of the shunt resistor 11 is connected to the low-voltage side of the shunt regulator 7 serving as a constant voltage circuit element. It is connected to the line 20a. The shunt regulator 7 is a circuit element that generates a constant potential difference at both ends regardless of the flowing current. The potential difference is larger than the potential difference of the shunt resistor 11 through which the output current flows, and has a constant potential smaller than the output voltage to be controlled with constant voltage. A constant voltage circuit element to be generated is used. Here, the shunt operates with an output voltage of 5 V or less, at least an output current of 4 A or less, and a high-voltage side potential of +1.25 V with respect to the low-voltage side so that the high-voltage side potential is a positive potential. A regulator is used.

  Although the output voltage between the high-voltage side output line 20a and the low-voltage side output line 20b is controlled at a constant voltage by a constant voltage control circuit, there are temporary fluctuations due to load fluctuations and input voltage fluctuations. The flowing current is changed, and the voltage drop is changed with the fluctuation of the output voltage, so that the potential difference of the shunt regulators 7 connected in series is kept at a constant potential.

  A series connection point of the shunt regulator 7 and the buffer resistor 8 is connected to the load 40 via the signal line D, and the potential on the high voltage side of the shunt regulator 7 is output to the load 40 as a voltage signal.

  The operation of the DC-DC converter 10 configured as described above is that the output voltage between the high-voltage side output line 20a and the low-voltage side output line 20b and the output current flowing through these output lines exceed a predetermined set voltage and set current. While not in operation, the intermittent oscillation element 3 oscillates at a constant frequency, and the output power or output current generated in the secondary output winding 2b increases.

  When the output current flowing through the shunt resistor 11 exceeds the set current by the load 40 connected between the high-voltage side output line 20a and the low-voltage side output line 20b, the voltage input to the inverting input terminal of the error amplifier 33b is used for current monitoring. The second comparison voltage of the reference power supply 34b is exceeded, the potential difference from the second comparison voltage is inverted and amplified, and the potential exceeds the light emission threshold value of the photocoupler light emitting element 35.

  As long as the output current exceeds the set current, the photocoupler light emitting element 35 continues to emit light, and a limit signal indicating that these values have been exceeded is continuously output to the photocoupler light receiving element 39. Since the photocoupler light receiving element 39 receives the limit signal from the photocoupler light emitting element 35, a constant current flows from the control terminal 3a of the intermittent oscillation element 3 to the low voltage terminal 1b of the DC power supply 1, and thus the control terminal 3a. In this state, a pause control signal is input. As a result, the intermittent oscillation element 3 stops the oscillation of the oscillator while the photocoupler light receiving element 39 receives the limit signal, and the built-in switching element is opened.

  When the switching element of the intermittent oscillation element 3 is opened, the current flowing in the primary winding 2a of the transformer 3 is cut off, and no output power is generated in the secondary output winding 2b of the transformer 3, so that the set current is exceeded. The output current that has been reduced gradually falls below the set current. As a result, the photocoupler light-emitting element 35 stops emitting light, and the photocoupler light-receiving element 39 does not receive the limit signal. Therefore, the intermittent oscillation element 3 repeats oscillation again, and a stable output corresponding to the power of the load 40 is obtained. As a result, the output current is constant-current controlled to the set current.

  Here, each circuit in the load 40 to which the low-voltage output line 20b of the DC-DC converter 10 is connected generally operates with the potential of the low-voltage output line 20b at the ground level as the reference potential. On the other hand, the potential on the input side of the shunt resistor 11, that is, on the low voltage side of the shunt regulator 7, becomes a negative potential proportional to the current value of the output current. For example, if a set current flows through the low-voltage side output line 20b, a potential of −0.3V is obtained by multiplying the resistance value 0.3Ω of the shunt resistor 11 by 1A of the set current.

  The shunt regulator 7 generates a constant potential difference of +1.25 V on the high voltage side with respect to the low voltage side, and the resistance value of the shunt resistor 11 is 0.3Ω. Therefore, if the output current is 4.16 A or less, The potential on the high voltage side, that is, the potential of the signal line D output to the load 40 as a voltage signal becomes positive with respect to the reference potential, and the potential decreases as the output current flowing in the low voltage side output line 20b increases. Potential. For example, if a set current of 1 A flows through the low-voltage side output line 20 b, it is +0.95 V, and if 1.2 A more than the set current flows, a potential of +0.89 V is loaded from the signal line D as a voltage signal. 40.

  Since the potential of the signal line D is positive on the load 40 side, the positive difference voltage compared with the reference potential can be easily determined as a predetermined potential difference. In the load 40 using the DC-DC converter 10 that outputs the rated output current of 1 A as a charger, if the potential of the signal line D is around +0.95 V with respect to the reference potential, a suitable DC-DC converter 10 is provided. Recognized as connected. On the other hand, when a DC-DC converter with an excessive rated output with respect to the power consumption during charging of the load 40 is erroneously connected, an output current larger than the set current of the DC-DC converter 10 flows to the shunt resistor 11. The potential of the signal line D falls below + 0.95V. Conversely, when a DC-DC converter with a small rated output is erroneously connected, the output voltage increases, the output current flowing through the shunt resistor 11 becomes smaller than the set current, and the potential of the signal line D becomes +1. It rises from + 0.95V with 25V as the upper limit. Accordingly, it is possible to detect that the load 40 is erroneously connected to a DC-DC converter other than the suitable DC-DC converter 10 from the potential of the signal line D. The operator can be informed that the “DC-DC converter” has been connected by mistake.

  In the above embodiment, the shunt regulator 7 is used to make the potential of the signal line D representing the output current positive with respect to the reference potential of the low-voltage side output line 20b. If the current can be discriminated even if it is expressed as a negative potential, as shown in FIG. 2, the signal line D is directly connected to the input side of the shunt resistor 11 and expressed as a negative potential with respect to the reference potential. The DC-DC converter 50 which outputs a voltage signal to the load 40 may be used.

  Further, although the shunt regulator 7 is used as the constant voltage circuit element, any constant voltage circuit element that generates a constant potential at both ends that is larger than the potential difference of the shunt resistor 11 through which the output current flows and smaller than the output voltage to be constant voltage controlled can be used. The shunt regulator 7 may be replaced with a diode whose forward direction is the direction from the high-voltage side output line 20a to the low-voltage side output line 20b. Since the forward voltage drop Vf of the diode is a constant potential of about 0.6 V, if a diode is substituted for the shunt regulator 7 in FIG. When the resistor 8 is connected in series and a set current of 1 A flows, a positive voltage signal of +0.3 V representing the output current can be output from the signal line D.

  Further, a bipolar transistor 60 can be used as the constant voltage circuit element. FIG. 3 is a circuit diagram of a DC-DC converter 70 using an NPN transistor 60 in place of the shunt regulator 7, and bias resistors 61 and 62 of 1 kΩ are connected between the base and the collector and between the base and the emitter, respectively. The resistance value of the buffer resistor 8 is 3.8 kΩ, which is sufficiently high with respect to the internal resistance of the load 40 so that a large current does not flow through the NPN transistor 60.

If an output voltage of 5V is generated between the high-voltage side output line 20a and the low-voltage side output line 20b, and the NPN transistor 60 is in the cut-off state, the base voltage V _BE is _reduced to the operating voltage of 0 by the divided voltage generated in the bias resistor 62 .6V is exceeded, transistor 60 transitions to the active state. At this time, since the base voltage V _EB does not substantially change when the base voltage V _EB is 0.6 V, the current that flows through the bias resistor 62 does not change at 0.6 mA. Since it is negligibly small in comparison, the currents flowing through the bias resistor 61 and the bias resistor 62 are substantially equal, and the collector-base voltage V _CB is also a constant voltage of 0.6V. As a result, the collector-emitter voltage V _CE is stabilized at 1.2 V, and the potential connected to the signal line D rises by +1.2 V with respect to the negative potential on the input side of the shunt resistor 11. The output current can be expressed by a sex potential.

  Although the above embodiment has been described with an insulated flyback type DC-DC converter, other DC-DC converters such as an insulated forward back type or a choke converter may be used.

  Further, although the shunt resistor for detecting the output current is also used as the current detection resistor used for constant current control of the output current, it may be provided separately.

  In the case of using a constant voltage circuit element, an element other than the above, such as a Zener diode having a low impedance, may be used as long as it can obtain a constant voltage addition voltage that does not vary depending on the output voltage.

  INDUSTRIAL APPLICABILITY The present invention is suitable for a charger that charges a device to be charged that it is difficult to detect a charging current output from the charger with an internal circuit.

1 DC power supply 2a Transformer primary winding (inductor)
2b Transformer secondary winding (inductor)
3 Intermittent oscillation device (switching device, constant voltage control circuit)
7 Shunt regulator (constant voltage circuit element)
8 Buffer resistor 10, 50, 70 DC-DC converter 11 Shunt resistor 20a High voltage side output line 20b Low voltage side output line 60 Transistor (constant voltage circuit element)

Claims (4)

A switching element connected in series to the DC power supply;
An inductor for converting the input voltage of the DC power source into a different DC output voltage between a pair of high-voltage side output lines and a pair of low-voltage side output lines connected to the load, with the current flowing from the DC power source intermittently by the switching operation of the switching element; ,
A DC-DC converter including a constant voltage control circuit that controls opening and closing of the switching element according to an output voltage between the high-voltage side output line and the low-voltage side output line, and performs constant voltage control of the output voltage,
Connect a shunt resistor in series to either the high-voltage side output line or the low-voltage side output line,
A DC-DC converter that outputs a voltage signal representing a potential difference between both ends of a shunt resistor to a load. Connect a shunt resistor in series with the low-voltage output line,
Between the high-voltage side output line and the low-voltage side output line connected to the input side of the shunt resistor, a buffer resistor with one connected to the high-voltage side output line, and a constant voltage circuit that generates a constant voltage higher than the potential difference between both ends of the shunt resistor Connect the device in series,
2. The DC-DC converter according to claim 1, wherein a voltage signal is output from a series connection point between the constant voltage circuit element and the buffer resistor. The DC-DC converter according to claim 2, wherein the constant voltage circuit element is a shunt regulator. 3. A constant current control circuit for controlling opening and closing of the switching element in accordance with an output current flowing in the low-voltage output line detected from a potential difference between both ends of the shunt resistor, and for controlling the output current at a constant current. Or the DC-DC converter of Claim 3.

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