JP2011083130A - Switching power supply device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To commonalize a charging circuit for soft start with a charging and discharging circuit for protective operation. <P>SOLUTION: A switching power supply device includes: an IC 101, input voltage detecting resistors R101, R102, a brown-out detecting (input low voltage detecting) circuit 103, a timer capacitor C101, switching elements Q101, Q102, a transformer T101 consisting of a primary winding Np and secondary windings Ns1, Ns2, a resonant capacitor C102, a current detecting resistor R103, rectification diodes D201, D202, a rectification capacitor C201, output voltage detecting resistors R201, R202, a shunt regulator IC201, and a photocoupler PC101. The timer capacitor C101 is connected to an SST terminal of the IC 101 so as to work as the soft start circuit as well as protection in abnormal conditions such as overcurrent, and the brown-out detecting circuit 103 is further connected to execute protecting operation. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a switching power supply, and more particularly to a switching power supply having a soft start operation function and a protection operation function at the time of startup.

  Conventionally, a switching power supply has compared an error amplification circuit that compares a divided voltage of an output voltage with a reference voltage, and compares the output signal of the error amplification circuit with a reference triangular wave signal to obtain the comparison result. A PWM comparator that generates a pulse signal with a corresponding duty cycle, and performs switching control of a transistor that forms a switching element in accordance with an output signal of the PWM comparator so that the output voltage becomes constant at a predetermined value. .

  When starting the switching power supply, it is common to increase the output voltage from 0 V in a light load state. At this time, the inrush current is suppressed to the input side power supply such as a battery. In order to reduce the burden, a soft start circuit that gradually increases the output voltage of the switching power supply is frequently used.

  This soft start circuit is realized by gradually increasing the voltage of the output signal of the error amplification circuit, which is a voltage to be compared with the reference triangular wave signal, in the PWM comparator. Specifically, many soft start circuits realize a soft start function by charging a capacitor with a constant current to generate a gradually increasing voltage and outputting the voltage to the error amplifier circuit.

  Further, as one of the protection circuits for the switching power supply, there is a latch protection circuit which is a timer latch type protection circuit. When an abnormality such as an output short-circuit occurs, the output voltage of the error amplifier circuit increases to the maximum value and exceeds the maximum voltage value of the reference triangular wave signal, that is, the duty cycle in PWM control reaches the maximum value. The state and the duration of the state are detected, and when the state continues for a predetermined period, it is latched and the operation of the switching power supply is stopped.

  In many cases, the delay time until the latch is determined by the charging time required for connecting the constant current source to the capacitor and setting the voltage of the capacitor to a predetermined voltage, as in the soft start.

  However, as shown in FIG. 7, in the conventional switching power supply as described above, the circuit for setting the soft start time (soft start circuit in the drawing) and the circuit for setting the delay time of the timer latch type protection circuit (in the drawing) Each has a latch protection circuit).

  Accordingly, two sets of comparators, current sources, and the like having the same configuration are required, and it is useless when considering the chip area when forming an IC.

  When a time setting capacitor is externally attached to the IC, the IC needs two IC pins for connecting these capacitors. In addition, as electric appliances and the like have been reduced in size, it has been necessary to reduce the area of the power supply by reducing the number of IC pins and external components.

  For this reason, the time setting means included in the soft start circuit and the latch protection circuit are shared, and the purpose is to reduce the number of pins of the IC, reduce the chip area and external parts, and reduce the size of the device. A switching power supply is known (see, for example, Patent Document 1).

JP 2007-159316 A

  However, in the technique described in Patent Document 1, by sharing the time setting means, it is possible to reduce the number of IC pins, reduce the chip area, reduce external components, and reduce the size of the device. However, when sharing the time setting means, during the soft start operation, the protection operation is reset, prohibiting the execution of the protection operation, and after the soft start operation is completed, the reset is released and the protection operation is started. At the same time, since the soft start operation is prohibited until the start-up, it is difficult to protect the abnormal state of the switching power supply by directly controlling the common IC pin voltage after the soft start operation ends. In order to realize this, there is a problem that the circuit configuration becomes complicated, and the expandability of further functions is impaired.

  In addition, when the soft start terminal and the latch control terminal are shared, the latch control unit counts that the terminal voltage has reached a predetermined voltage with a counter, and executes a latch operation when this count value reaches a predetermined value. In general, however, if the terminal voltage (SS terminal voltage in the figure) remains high as shown in the conventional operation waveform of FIG. 8, the counter miscounts due to the influence of noise or the like. There was also a problem.

  Therefore, the present invention has been made in view of the above-described problems, and a soft start charging circuit and a charge / discharge circuit for protection operation are used as a common terminal. Control is performed while maintaining a predetermined voltage, and when the common terminal voltage reaches a predetermined threshold value, the common terminal voltage is stabilized, thereby reducing the number of IC pins, reducing the chip area, and externally attaching. An object of the present invention is to provide a switching power supply capable of reducing the number of parts, downsizing the device, and enabling stable operation.

  The present invention proposes the following matters in order to solve the above problems.

  (1) The present invention provides a switching element that performs switching according to an input control signal, an inductor that accumulates electric power from a DC voltage by the switching operation of the switching element, and discharge of electric power accumulated in the inductor And a switching control circuit that performs switching control so that the output voltage output from the output terminal becomes a predetermined voltage, and converts the DC voltage input to the input terminal into a predetermined constant voltage. A switching power supply that outputs from an output terminal, wherein a capacitor is connected to a single control terminal, and the capacitor is charged with a first set current by a charging circuit at the time of start-up, and the terminal voltage of the capacitor is set By increasing the voltage to the set voltage value over time and performing switching control with the terminal voltage of the capacitor, soft star A soft start circuit for performing the operation, and after the soft start operation is completed, the abnormal state of the switching power supply operation is detected while maintaining the voltage of the set voltage value, and at the same time, the capacitor is charged from the set voltage value. And when the abnormal state is detected continuously for a predetermined time, the capacitor is discharged by a discharge circuit and a protection operation circuit for stopping the switching power supply operation is provided, and the terminal voltage of the capacitor is a threshold voltage. A clamp circuit that stabilizes the terminal voltage of the capacitor at the threshold voltage when the voltage reaches the single control terminal after the soft start operation is completed. A switching power supply characterized by performing a soft-start operation by discharging. It is.

  According to the present invention, a capacitor is connected to a single control terminal, and a soft operation execution circuit that executes a soft start operation at the time of start-up, and an abnormal state in a state in which the voltage of the set voltage value is maintained after the soft start operation ends Is detected together with a protection operation execution circuit that stops the switching power supply operation when it is detected for a predetermined time, and when the capacitor terminal voltage reaches the threshold voltage, the capacitor terminal voltage is stabilized at the threshold voltage. With a clamping scheme, the voltage of a single control terminal after the end of the soft start operation performs the soft start operation by discharging the capacitor from the set voltage value state, thereby reducing the number of IC pins. In addition to reducing the size of the equipment by reducing external parts and preventing erroneous counting due to noise of the common terminal voltage. It is possible.

  (2) The present invention monitors the input DC voltage with respect to the switching power supply of (1), and when the DC voltage falls below a set voltage, the capacitor connected to the single control terminal is There has been proposed a switching power supply including an input low voltage detection circuit that performs a protective operation by discharging.

  According to the present invention, the input DC voltage is monitored, and when the DC voltage falls below the set voltage, a protective operation is performed by discharging the capacitor connected to the single control terminal. Therefore, by connecting the input low voltage detection circuit to a single control terminal, the circuit protection operation can be performed at one terminal even when the input voltage falls below the set voltage as an additional function. Can do.

  (3) The present invention relates to the switching power supply of (1), wherein the charging circuit is connected to a constant current source, a constant voltage element in which an output of the constant current source determines the steady voltage, and the current source and the anode. The switching power supply is characterized in that the cathode is composed of a diode connected to the single control terminal.

  According to the present invention, the charging circuit includes a constant current source, a constant voltage element whose output is determined by a constant current source, a current source and an anode connected, and a cathode connected to a single control terminal. It consists of a diode. Therefore, an accurate charging circuit can be configured with a simple circuit configuration.

  (4) In the switching power supply of (1), the present invention provides a comparator in which the charging circuit compares the single control terminal voltage with a reference power supply that determines the steady voltage, and according to the comparison result, There has been proposed a switching power supply comprising a buffer for supplying a current to a capacitor, and a diode having an output connected to the anode and a cathode connected to the single control terminal.

  According to the present invention, the charging circuit includes a comparator that compares a single control terminal voltage and a reference power source that determines a steady voltage, a buffer that supplies current to the capacitor according to the comparison result, and an output of the buffer is an anode. And a cathode having a cathode connected to a single control terminal. Therefore, an accurate charging circuit can be configured with a simple circuit configuration.

  (5) In the switching power supply of (1), the present invention provides a comparator in which the charging circuit compares the single control terminal voltage with a reference power supply that determines the steady voltage, and the output of the comparator is a constant current source. And a cathode of the diode is connected to the single control terminal, and a current is supplied to the capacitor according to the comparison result. .

  According to the present invention, the charging circuit includes a comparator that compares a single control terminal voltage and a reference power source that determines a steady voltage, and the output of the comparator is connected to the output of the constant current source and the anode of the diode, and the cathode of the diode Is connected to the single control terminal and supplies current to the capacitor according to the comparison result. Therefore, an accurate charging circuit can be configured with a simple circuit configuration.

  According to the present invention, the conventional soft start terminal and the protection circuit terminal can be shared, and the number of IC terminals and the number of components can be reduced. In addition, erroneous counting due to noise of the shared terminal voltage can be prevented by the action of the clamp circuit.

It is a figure which shows the structure of the switching power supply which concerns on 1st Embodiment. It is an operation | movement waveform diagram of the switching power supply which concerns on 1st Embodiment. It is a figure which shows the structure of the soft start circuit of the switching power supply which concerns on 1st Embodiment. It is an operation | movement waveform diagram of the soft start circuit of the switching power supply which concerns on 1st Embodiment. It is a figure which shows the structure of the switching power supply which concerns on a modification. It is a figure which shows the structure of the charging circuit which concerns on a modification. It is a figure which shows the structure of the charging circuit which concerns on a modification. It is the figure which showed the internal structure of the soft start / latch protection circuit which concerns on a prior art example. It is an operation | movement waveform diagram of the soft start circuit which concerns on a prior art example.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
Note that the constituent elements in the present embodiment can be appropriately replaced with existing constituent elements and the like, and various variations including combinations with other existing constituent elements are possible. Therefore, the description of the present embodiment does not limit the contents of the invention described in the claims.

<First Embodiment>
A first embodiment of the present invention will be described with reference to FIGS.

<Configuration of switching power supply>
As shown in FIG. 1, the switching power supply according to the present embodiment mainly includes an IC 101, input voltage detection resistors R101 and R102, a brownout detection (input low voltage detection) circuit 103, a timer capacitor C101, and a switching Transformer T101 composed of elements Q101 and Q102, primary winding Np and secondary windings Ns1 and Ns2, resonant capacitor C102, current detection resistor R103, rectifier diodes D201 and D202, rectifier capacitor C201, output voltage It comprises detection resistors R201 and R202, a shunt regulator IC201, and a photocoupler PC101.

  Further, in the IC 101, an SS charging circuit 101, a discharging circuit 102, an oscillation circuit 104, a driving circuit 105, an OCP comparator 106, and a control circuit 107 are provided.

  The switching power supply according to the present embodiment is a switching power supply that controls output power by frequency modulation. The output power is controlled by frequency modulation using the FB terminal current of the IC 101, and the SST terminal voltage of the IC 101 is also controlled. The control circuit 107 can perform frequency modulation in proportion to the output power and limit the output power.

  In the control by the SST terminal voltage, when the SST terminal voltage is low, the output power is limited by increasing the oscillation frequency. As the SST terminal voltage increases, the oscillation frequency is lowered and the output power restriction is relaxed. Further, when the voltage rises above the set voltage value, control is performed to release the restriction on the output power.

  Here, the timer capacitor C101 is connected to the SST terminal which is a functional terminal according to the present invention, and the SST terminal voltage is set by charging and discharging. The SS charging circuit 101 includes a constant current source that supplies a first set current Iss, a constant voltage element CL101 that determines a steady voltage, and a diode D101, and charges the timer capacitor C101 with a soft start function. Execute. When an abnormal signal such as an overcurrent is detected, the switch connected to the second set current source Ira is turned on, and the steady state set by the voltage of the constant voltage element CL101 and the forward voltage of the diode D101. By charging the timer capacitor C101 with respect to the voltage at the time of operation, the SST terminal voltage is further increased with respect to the voltage at the time of steady operation.

  The discharge circuit 102 is a circuit for discharging the electric charge of the timer capacitor C101. The control circuit 107 includes a discharge circuit using a current from the third set current source Irf1 and a discharge circuit using a current from the fourth set current source Irf2. It is connected to the SST terminal via a controlled switch, and the other end is connected to the timer capacitor C101, and detects an abnormal signal such as an overcurrent in which the SST terminal voltage is higher than the voltage during steady operation. In this state, the respective switches are turned on, and the electric charge is extracted from the timer capacitor C101 to lower the SST terminal voltage.

  The oscillation circuit 104 supplies a desired oscillation waveform to the drive circuit 105 via the control circuit 107 in accordance with a signal from the control circuit 107. For example, at the time of soft start, an oscillation frequency is gradually changed from a high frequency to a low frequency by a signal from the control circuit 107 and supplied, and when a reset signal is input, oscillation is quickly stopped.

  The drive circuit 105 drives the switching elements Q101 and Q102 by a signal supplied from the control circuit 107. The OCP comparator 106 monitors the switched current flowing through the resonant capacitor C102 by the current detection resistor R103, and generates an abnormal signal for the overcurrent of the switching current.

  The control circuit 107 controls the entire IC 101 according to a predetermined program. Specifically, the switching power supply protection operation is executed by detecting overcurrent of the switching current, Vcc voltage, abnormal heat generation of the IC, and the like. The brown-out detection (input low voltage detection) circuit 103 discharges electric charge from the timer capacitor C101 when the input voltage becomes lower than a predetermined voltage value, and lowers the voltage of the SST terminal, whereby the oscillation circuit 104 It has a function of protecting the circuit by increasing the oscillation frequency.

  Therefore, in the switching power supply according to this embodiment, a single SST terminal has both a soft start circuit and protection in the event of an abnormality such as overcurrent, and more easily connects a brownout detection circuit or the like to execute a protection operation. can do.

<Operation sequence of switching power supply>
Next, an operation sequence of the switching power supply according to the present embodiment will be described with reference to FIGS. 1 and 2.

  First, in the start-up (A in FIG. 2), when the DC input voltage VDCin is input and the input monitoring voltage Vsen becomes equal to or higher than the predetermined value Vssr, the current supplied from the SS charging circuit 101 (first set current Iss in FIG. 2) By gradually charging the timer capacitor C101 and raising the voltage of the SST terminal to a steady voltage (first set voltage Vss in FIG. 2), a so-called soft start operation is performed, and the oscillation frequency of the oscillation circuit 104 is increased from a high frequency. Transition to low frequency and gradually increase the output power of the switching power supply.

  At this time, the input monitoring voltage Vsen for monitoring the input voltage becomes equal to or higher than the predetermined value Vssr due to the start-up, but when the input monitoring voltage Vsen becomes lower than the predetermined value Vssr for some reason, the collector is connected to the timer capacitor C101. The transistor is turned on, and a rapid discharge is performed by drawing out the charge of the timer capacitor C101 by the discharge current Irf3 due to the restart signal, and the voltage at the SST terminal is lowered.

  When the input monitoring voltage Vsen returns to the predetermined value Vssr or higher, the timer capacitor C101 is gradually charged again with the current (first set current Iss in FIG. 2) supplied from the SS charging circuit 101, and the SST terminal Is raised to a steady voltage (first set voltage Vss in FIG. 2), so-called soft start operation is performed, and the oscillation frequency of the oscillation circuit 104 is changed from high frequency to low frequency, and the output power of the switching power supply is increased. Increase gradually.

  Next, in the case of a short-time overload state (“B” in FIG. 2), an abnormal signal is output from the OCP comparator 106 to the control circuit 107. At this time, the control circuit 107 closes the switch having one end connected to the SST terminal and the other end connected to the second set current source Ira (second set current source Ira in FIG. 2), and charges the timer capacitor C101. Then, a voltage corresponding to the pulse width of the abnormal signal is generated as the SST terminal voltage.

  When the overload state is resolved by the abnormal signal, the refresh timer in the control circuit 107 is started, and after the refresh timer period elapses, the fourth set current source Irf2 (fourth set current source in FIG. 2) is started. The charge of the timer capacitor C101 is rapidly extracted by Irf2), and the SST terminal voltage is set to a steady voltage (first set voltage Vss in FIG. 2).

  When the overload state continues (after “C” in FIG. 2), an abnormal signal is output from the OCP comparator 106 to the control circuit 107. At this time, the control circuit 107 closes the switch having one end connected to the SST terminal and the other end connected to the second set current source Ira (second set current source Ira in FIG. 2), and charges the timer capacitor C101. Then, a voltage corresponding to the pulse width of the abnormal signal is generated as the SST terminal voltage.

  When the abnormal signal is continued and the SST terminal voltage becomes equal to or higher than the second set voltage Vra, the oscillation is stopped, one end is set to the SST terminal, and the other end is set to the second set current source Ira (the second set current source Ira in FIG. 2). The switch connected to the set current source Ira) is opened, the charge of the timer capacitor C101 is extracted by the third set current source Irf1 (third set current source Irf1 in FIG. 2), and the SST terminal voltage is set to the third level. To the set voltage Vrs (the third set voltage Vrs in FIG. 2).

  Thereafter, when the overload state continues, the above operation is repeated. Therefore, in the switching power supply according to the present embodiment, when the overload state continues, that is, when the terminal voltage of the capacitor reaches a predetermined set voltage that is higher than the steady voltage (first set voltage). In this case, the capacitor is rapidly discharged to stop the oscillation, and when the capacitor terminal voltage reaches a predetermined set voltage higher than 0 V, the capacitor is started to be charged and restarted intermittently. Doing so protects the circuit.

<Schematic configuration of soft start circuit>
The soft start circuit gradually changes the oscillation frequency or on-off ratio at the time of start-up and input / output abnormality to suppress the switching of the primary circuit, the stress applied to the switching element, the overshoot of the output voltage, the circuit It has a function to reduce the oscillation noise of parts.

Here, the characteristic part of this invention is demonstrated using FIG. 3 and FIG.
As shown in FIG. 3, the soft start circuit according to the present invention includes a voltage detection circuit 111, a switch SW100, and a clamp circuit 115. Further, as described above, the timer capacitor C101, the SS charging circuit 101, and the discharging circuit 102 are connected to the common terminal SST.

  The voltage detection circuit 111 detects the voltage of the common terminal SST, and turns on the switch SW100 to connect the common terminal SST to the clamp circuit 115 when the voltage reaches the first set voltage Vss set in advance. . The clamp circuit 115 clamps the common terminal SST voltage to a preset first setting voltage Vss.

  The above operation is shown in FIG. As shown in FIG. 4, the common terminal SST voltage is clamped to a preset first set voltage Vss except when an abnormal state is detected. On the other hand, when the abnormality detection signal is input to the voltage detection circuit 111, the connection with the clamp circuit 115 is released, the timer capacitor C101 is charged, and when the voltage exceeds the second set voltage Vra, the oscillation is stopped. The discharge capacitor 102 discharges the timer capacitor C101 to the third set voltage Vrs. Thereafter, this operation is repeated.

  Therefore, according to this embodiment, the conventional soft start terminal and the protection circuit terminal can be shared, and the number of IC terminals and the number of parts can be reduced. In addition, the clamp circuit can prevent the common terminal voltage from reaching the second set voltage Vra due to noise.

<Modification of switching power supply>
FIG. 5 is a modification to the first embodiment. In the first embodiment, the brownout detection (input low voltage detection) circuit 103 is an external circuit of the IC 101. However, in this modification, the brownout detection (input low voltage detection) circuit is provided inside the IC101. 103 is built-in.

  By doing so, although the chip area of the IC 101 is slightly increased, the entire switching power supply can be reduced in size. Further, since the brownout detection (input low voltage detection) circuit 103 is built in the IC 101, it is not necessary for the user to configure the circuit externally, and it is possible to perform a high function without taking extra time. The switching power supply can be configured.

<Modification of SS charging circuit>
FIG. 6 is a diagram showing a modification of the SS charging circuit 101 shown in the first embodiment. According to this figure, in the SS charging circuit of this example, the first set current (Iss) source 201 is connected to the supply source of the buffer IC 202, and is connected to the first set current (Iss) source 201. Is connected to a comparator IC 203 that compares a predetermined reference voltage Vr3 with the SST terminal voltage, and in response to the comparison result of the comparator IC 203, the current of the first set current Iss is inverted from the buffer IC 202 via the diode D101. And is supplied to the timer capacitor C101.

  That is, the function of this charging circuit is the same as that of the first embodiment, and when the SST terminal voltage is lower than the predetermined reference voltage Vr3, at the time of start-up, etc., the timer capacitor C101 is charged via the diode D101. It has a function of supplying and charging.

  This charging circuit is slightly more complicated in its circuit configuration than the SS charging circuit 101 in the first embodiment, but can set a voltage with higher accuracy than the SS charging circuit 101 in the first embodiment. Has the advantage of being.

  FIG. 7 is a diagram showing another modification of the SS charging circuit 101 shown in the first embodiment. According to this figure, the SS charging circuit of this example includes a first set current (Iss) source 202, an open collector type comparator IC 204 that compares a predetermined reference voltage Vr3 and the SST terminal voltage, and a charging current. It comprises a diode D101 supplied to the timer capacitor C101.

  That is, the function of this charging circuit is the same as that of the first embodiment, and at the time of start-up or the like when the SST terminal voltage is lower than the predetermined reference voltage Vr3, the timer is connected from the constant current source 202 via the diode D101. The capacitor C101 has a function of supplying a charge and charging it.

  This charging circuit is slightly more complicated in its circuit configuration than the SS charging circuit 101 in the first embodiment, but can set a voltage with higher accuracy than the SS charging circuit 101 in the first embodiment. Has the advantage of being.

  The embodiments of the present invention have been described in detail with reference to the drawings. However, the specific configuration is not limited to the embodiments, and includes designs and the like that do not depart from the gist of the present invention.

DESCRIPTION OF SYMBOLS 101 ... SS charge circuit 102 ... Discharge circuit 103 ... Brown-out detection (input low voltage detection) circuit 104 ... Oscillation circuit 105 ... Drive circuit 106 ... OCP comparator 107 ... Control Circuit 110 ... Latch circuit 111 ... Voltage detection circuit 112 ... Counter circuit 115 ... Clamp circuit 201 ... Constant current source 202 ... Constant current source C101 ... Timer capacitor C102 ... Resonant capacitor C201 ... Rectifier capacitor D101 ... Diode D201 ... Rectifier diode D202 ... Rectifier diode IC101 ... Control IC
IC201 ... Shunt regulator IC202 ... Buffer IC203 ... Comparator IC204 ... Comparator Np ... Primary winding Ns1 ... Secondary winding Ns2 ... Secondary winding PC101 ... Photocoupler Q101 ... Switching element Q102 ... Switching element R101 ... Input voltage detection resistor R102 ... Input voltage detection resistor R103 ... Current detection resistor R201 ... Output voltage detection resistor R202 ... Output voltage detection Resistance T101 ... Transformer

Claims (5)

A switching element that performs switching in accordance with an input control signal, an inductor that accumulates power from a DC voltage by a switching operation of the switching element, a rectifier circuit that discharges power accumulated in the inductor, and A switching control circuit that performs switching control so that the output voltage output from the output terminal becomes a predetermined voltage, and converts the DC voltage input to the input terminal into a predetermined constant voltage and outputs it from the output terminal A power supply,
A capacitor is connected to a single control terminal, and the capacitor is charged with a first set current by a charging circuit at the time of start-up, and the terminal voltage of the capacitor is increased to a set voltage value at a set time. A soft start circuit that performs a soft start operation by performing switching control according to a terminal voltage, and at the same time as detecting an abnormal state of the switching power supply operation in a state where the voltage of the set voltage value is maintained after the soft start operation is completed A protective operation circuit that starts charging the capacitor from the set voltage value and continuously detects the abnormal state for a predetermined time, and discharges the capacitor by a discharge circuit and stops the switching power supply operation. As well as
A clamp circuit that stabilizes the capacitor terminal voltage to the threshold voltage when the terminal voltage of the capacitor reaches a threshold voltage, and the voltage of the single control terminal after the soft start operation ends is set to A switching power supply, wherein a soft start operation is performed by discharging the capacitor from a voltage value state.   An input low voltage detection circuit that performs a protective operation by monitoring the input DC voltage and discharging the capacitor connected to the single control terminal when the DC voltage is lower than a set voltage. The switching power supply according to claim 1, further comprising: The charging circuit is
A constant current source;
A constant voltage element in which the output of the constant current source determines the steady voltage;
A diode in which the current source and the anode are connected and a cathode is connected to the single control terminal;
The switching power supply according to claim 1, comprising: The charging circuit is
A comparator that compares the single control terminal voltage with a reference power supply that defines the steady voltage;
A buffer for supplying current to the capacitor according to the comparison result;
2. The switching power supply according to claim 1, wherein an output of the buffer is connected to an anode, and a cathode is connected to the single control terminal. The charging circuit is
A comparator that compares the single control terminal voltage with a reference power supply that defines the steady voltage;
The output of the comparator is connected to the output of a constant current source and the anode of a diode, the cathode of the diode is connected to the single control terminal, and current is supplied to the capacitor according to the comparison result. The switching power supply according to claim 1.

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