JP5566656B2 - Switching power supply - Google Patents

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. 8, in the conventional switching power supply as described above, the circuit for setting the soft start time (soft start circuit in the figure) and the circuit for setting the delay time of the timer latch type protection circuit (in the figure) 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 by reducing the number of IC pins and external components in the power supply portion.

  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 pins of the IC, reduce the chip area, and reduce the number of external parts, thereby reducing 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.

  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. A switching power supply that reduces the number of IC pins, reduces the chip area and external components, reduces the size of the device, and enables further expansion of functions by maintaining the specified voltage and performing control. The purpose is to provide.

  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 a 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, the capacitor is charged with a first set current by a charging circuit at the time of startup, and a terminal voltage of the capacitor is set for a set time To increase the voltage to the first set voltage and perform switching control according to the terminal voltage of the capacitor. A soft start circuit for executing a switching operation, and after the soft start operation is completed, the abnormal state of the switching power supply operation is detected in a state where the voltage of the set voltage value is maintained, and at the same time, the capacitor is charged from the set voltage value When the abnormal state is detected continuously for a predetermined time, the capacitor is discharged by a discharge circuit, and a protection circuit for stopping the switching power supply operation is also provided. The voltage of one control terminal performs a soft start operation by discharging the capacitor from the state of the set voltage value, and the switching control circuit includes a latch circuit, a counter, and a reset circuit, and the set voltage value Detection of abnormal state of switching power supply operation while maintaining the voltage of If the abnormal state is continuously detected, the capacitor is charged with a second set current, and when the single control terminal voltage reaches a second set voltage higher than the set voltage value, When the counter counts and the count value reaches a predetermined value, the switching power supply operation is stopped by the latch circuit, and the abnormal state is released before the count value reaches the predetermined value. When a control terminal voltage reaches the first set voltage, a switching power supply is proposed in which the count value of the counter is reset by the reset circuit.

  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 where 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, and the capacitor is discharged from the state that the voltage of the single control terminal after the soft start operation ends is the set voltage value. Thus, since the soft start operation can be performed, the device can be reduced in size by reducing the number of IC pins and the number of external parts, and further the protection function can be expanded. Further, if the abnormal state is canceled within a predetermined count value by the function of the count reset circuit, the count value is reset, and the single control terminal voltage becomes a constant voltage and can be returned to the normal state.

  (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 unit 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 unit to a single control terminal, as an additional function, even when the input voltage becomes lower than the set voltage, the circuit protection operation is executed with one terminal. be able to.

  (3) The present invention detects the abnormal state of the switching power supply operation for the switching power source of (1), and after the abnormal state is canceled when the abnormal state is canceled within the predetermined time. A switching power supply is proposed in which the capacitor is discharged to the predetermined voltage after a predetermined timer period has elapsed.

  According to the present invention, when the abnormal state of the switching power supply operation is detected and the abnormal state is canceled within a predetermined time, the capacitor is set to the predetermined time after the predetermined timer period elapses after the abnormal state is canceled. By discharging to a value voltage, the steady state can be restored in a short time.

  (4) The present invention detects the abnormal state of the switching power supply operation in the state where the voltage of the set voltage value is maintained for the switching power source of (1) and continuously detects the abnormal state. The capacitor is charged with a set current of 2, and when the single control terminal voltage reaches a second set voltage higher than the set voltage value, the switching power supply operation is stopped, and the third set current When the capacitor is discharged and the single control terminal voltage reaches a third set voltage smaller than the set voltage value, the switching power supply operation is started, and at the same time, the capacitor is charged with the first set current. The power supply to the output is resumed by a soft start operation, the single control terminal voltage is increased, and the single control terminal voltage is set to the set voltage value again. When remote reach high second set voltage, stops the switching power supply operation, has proposed a switching power supply characterized by repeating the sequence of control for discharge of the capacitor by a third set current.

  According to the present invention, 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), the capacitor is quickly turned on. To stop the switching operation, and when the terminal voltage of the capacitor reaches a predetermined set voltage higher than 0 V, the capacitor is started to be charged and restarted intermittently, Protect the circuit.

  (5) The present invention relates to the switching power supply of (1) to (4), wherein the charging circuit includes 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 A switching power supply is proposed in which an anode is connected and a cathode is connected to 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.

  (6) According to the present invention, for the switching power supply of (1) to (4), the charging circuit compares the single control terminal voltage with a reference power supply that determines the steady voltage, and the comparison result Accordingly, there is proposed a switching power supply comprising a buffer for supplying current to the capacitor, and a diode having an output connected to the anode and a cathode connected to the single control terminal. Yes.

  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.

  (7) The present invention relates to the switching power supply of (1) to (4), wherein the charging circuit compares the single control terminal voltage with a reference power supply that determines the steady voltage, and an 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 supplies a current to the capacitor according to the comparison result. is suggesting.

  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, 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 where 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, and the capacitor is discharged from the state that the voltage of the single control terminal after the soft start operation ends is the set voltage value. Thus, since the soft start operation can be performed, there is an effect that the size of the device can be reduced by reducing the number of pins of the IC and reducing the number of external parts.

  In addition, if the abnormal state is canceled within a predetermined count value by the function of the reset circuit, the count value is reset, and the single control terminal voltage becomes a constant voltage, which can return to the normal state. .

It is a figure which shows the structure of the switching power supply which concerns on 1st Embodiment. It is a sequence diagram which shows operation | movement of the switching power supply which concerns on 1st Embodiment. It is a figure which shows the structure of the latch type protection circuit of the switching power supply which concerns on 1st Embodiment. It is a figure which shows the operation | movement waveform of the latch type protection 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.

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, Q102, primary winding Np and secondary windings Ns1, Ns2, resonance capacitor C102, current detection resistor R103, rectifier diodes D201, 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 constant current Iss, a constant voltage element CL101 that determines a steady voltage, and a diode D101, and charges the timer capacitor C101 to execute a soft start function. . 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, each switch is turned on, and the SST terminal voltage is lowered by acting to draw out the charge from the timer capacitor C101.

  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 resonance 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 is also 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 the rapid discharge is performed by extracting the charge of the timer capacitor C101 by the discharge current Irf3 by the restart signal, and the voltage of 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 supplied from the SS charging circuit 101 (first set current Iss in FIG. 2), 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 continues and the SST terminal voltage becomes equal to or higher than the second set voltage Vra, the oscillation is stopped, one end is the SST terminal, and the other is 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 drawn by the third set current source Irf1 (third set current source Irf1 in FIG. 2), and the SST terminal voltage is set to Vrs ( Discharge is performed up to the third set voltage Vrs) in FIG.

  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.

<Protection operation by SST terminal voltage>
The protection operation by the SST terminal voltage will be described using FIG. 3 and FIG.

  FIG. 3 is a block diagram illustrating an example of a latch-type protection circuit. According to this figure, the latch type protection circuit of this example includes a timer capacitor C101 connected to the SST terminal, a voltage detection circuit 111 for detecting the voltage of the timer capacitor C101, and a detection result of the voltage detection circuit 111. The counter circuit 112 includes a counter circuit 112 for counting, a reset circuit 113, and a latch circuit 110.

  The counter circuit 112 outputs a signal to the latch circuit 110 when the count value becomes a predetermined value or more. The latch circuit 110 outputs a latch signal to the driving circuit 105, stops the driving circuit 105, and protects the circuit. If the abnormal state is canceled within the count value of the counter circuit 112 within the predetermined count value, the counter circuit 112 is reset by the reset signal from the reset circuit 113, and the count value is cleared.

  The protection operation by the SST terminal voltage will be described with reference to FIG. 4. As for the SST terminal voltage, the connected timer capacitor C101 is charged from the initial state, and the voltage rises. When the SST terminal voltage reaches the first set voltage Vss, the SST terminal voltage is clamped to the first set voltage Vss by a clamp circuit (not shown). At this time, a reset signal is supplied from the reset circuit 113 to the counter circuit 112, and the count value of the counter circuit 112 is cleared.

  On the other hand, when an abnormality detection signal is input to the voltage detection circuit 111, the connection with a clamp circuit (not shown) is released, the timer capacitor C101 is charged to the second set voltage Vra, and the timer capacitor C101 is discharged by the discharge circuit 102. Is discharged to the third set voltage Vrs. After that, when the timer capacitor C101 is charged and there is no pulse signal for detecting an abnormal state until the first set voltage Vss is reached, the SST terminal voltage is clamped to the first set voltage by a clamp circuit (not shown). Is done. At this time, a reset signal is supplied from the reset circuit 113 to the counter circuit 112, and the count value of the counter circuit 112 is cleared.

  Therefore, when the abnormal state is canceled within the predetermined count value by this circuit, the count value is reset, and the single control terminal voltage becomes a constant voltage, and can return to the normal state.

<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 constant current (Iss) source 201 is connected to the supply source of the buffer IC 202, and is connected to the constant current (Iss) source 201 and a predetermined reference voltage Vr3 Is connected to the comparator IC 203 for comparing the SST terminal voltage and the comparison result of the comparator IC 203 is configured to supply Iss current from the buffer IC 202 to the timer capacitor C101 via the diode D101 in the inverted logic. Yes.

  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 constant current (Iss) source 202, an open collector type comparator IC 204 that compares a predetermined reference voltage Vr3 and an SST terminal voltage, and a charging current to a timer capacitor C101. And a diode D101 to be supplied.

  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 113 ... Reset 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 (7)

A switching element that performs switching in accordance with the 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 the power accumulated in the inductor, and an output A switching power supply that includes a switching control circuit that performs switching control so that an output voltage output from a terminal becomes a predetermined voltage, converts a DC voltage input to the input terminal into a predetermined constant voltage, and outputs the voltage from the output terminal Because
A capacitor is connected to a single control terminal, and the capacitor is charged with a first set current by a charging circuit at start-up, and the terminal voltage of the capacitor is increased to the first set voltage in a set time, A soft start circuit that performs a soft start operation by performing switching control according to the terminal voltage of the capacitor, and after the soft start operation is finished, the terminal voltage of the capacitor is maintained at the first set voltage. At the same time as detecting the abnormal state, charging of the capacitor is started from the first set voltage, and when the abnormal state is continuously detected for a predetermined time, the capacitor is discharged by the discharge circuit and the switching power supply operation is performed. In addition to having a protection circuit to stop ,
And a latch circuit and a counter and a reset circuit before Symbol switching control circuit,
When the terminal voltage of the capacitor is maintained at the voltage of the first set voltage, an abnormal state of the switching power supply operation is detected, and when the abnormal state is continuously detected, the capacitor is generated by a second set current. Charge
When the voltage of the single control terminal reaches a second set voltage that is higher than the first set voltage, the counter counts to stop switching power supply operation, and the capacitor is set by a third set current. When the voltage of the single control terminal reaches a third set voltage smaller than the first set voltage, the capacitor is charged by the charging circuit, and the terminal voltage of the capacitor is set to the first voltage. The soft start operation is executed by raising the voltage to the set voltage of 1 and performing the switching control by the terminal voltage rising of the capacitor,
When the count value of the counter reaches a predetermined value, the latch circuit continues to stop the switching power supply operation,
When the abnormal state is canceled before the count value of the counter reaches a predetermined value, when the voltage of the single control terminal reaches the first set voltage, the counter circuit counts the counter. A switching power supply characterized by resetting a value.   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:   When the abnormal state of the switching power supply operation is detected and the abnormal state is canceled within the predetermined time, the capacitor is set to the predetermined value after a predetermined timer period elapses after the abnormal state is canceled. The switching power supply according to claim 1, wherein the switching power supply is discharged to a voltage of   When the terminal voltage of the capacitor is maintained at the voltage of the first set voltage, an abnormal state of the switching power supply operation is detected, and when the abnormal state is continuously detected, the capacitor is generated by a second set current. When the voltage of the single control terminal reaches the second set voltage higher than the first set voltage, the switching power supply operation is stopped and the capacitor is turned on by the third set current. When discharging, when the voltage of the single control terminal reaches a third setting voltage smaller than the first setting voltage, the switching power supply operation is started, and at the same time, the capacitor is charged with the first setting current. Then, the power supply to the output is resumed by the soft start operation, the single control terminal voltage is increased, and the single control terminal voltage again becomes the first set voltage. 2. The switching power supply according to claim 1, wherein when a higher second set voltage is reached, the switching power supply operation is stopped and a series of controls for discharging the capacitor by a third set current is repeated. . The charging circuit is
A constant current source;
A diode in which the constant current source and the anode are connected, and a cathode is connected to the single control terminal;
A constant voltage element that determines the voltage of the output terminal of the constant current source as the sum of the first set voltage and the forward voltage of the diode;
The switching power supply according to any one of claims 1 to 4, characterized by comprising: The charging circuit is
A comparator that compares the single control terminal voltage with a reference power supply that defines the first set voltage;
A buffer for supplying current to the capacitor according to the comparison result;
5. 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 first set 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 any one of claims 1 to 4.

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