JP2002136114A - Inrush current preventing circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to omit resistors or switches for controlling an inrush current with regard to the inrush current preventing circuit of a switching power supply. SOLUTION: This inrush current preventing circuit of a switching power supply comprises a control circuit CNT1 that performs the on-off control of a switching transistor Q1, connected to the primary winding n1 of a transformer T1; and an overcurrent detecting means, including an overcurrent detecting part OC1 that compares a detected current value obtained by detecting the current flowing in the primary winding n1 by a resistor R1 or a current transformer with a first reference voltage Vr1 by a first operational amplifier OA1, and that performs overcurrent protection by inputting an overcurrent detection signal into the control circuit CNT1, if the detected current value exceeds the reference voltage Vr1. On this circuit, a voltage- adding means is provided that impresses a voltage to the detected current value, that is compared with the reference value Vr1, in such a way as to make it exceed the reference voltage Vr1 by a second operational amplifier OA2 and the like, when power supply is turned on that impresses a DC voltage to the primary winding n1 of the transformer T1, and that gradually reduces this voltage by a capacitor C2 and the like, as time elapses.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rush current prevention circuit for preventing a rush current flowing when a power supply is turned on in a switching power supply device for outputting a predetermined DC voltage by switching control.

[0002]

2. Description of the Related Art FIG. 8 is an explanatory view of a conventional example, in which a main part of a switching power supply device of a forward converter type is shown.
S1 is a switch, R21 to R24 are resistors, CNT4 is a control circuit, D6 and D7 are diodes, L2 is a choke coil, C5 is a capacitor, OA6 is an operational amplifier, Vr6 is a reference voltage, and OC3 is an overcurrent detector.

The switch S1 is controlled by a control unit or a timer (not shown). When an AC power supply (not shown) is turned on, the switch S1 is turned off as shown. Therefore, an AC voltage is applied to the rectifier circuit REC via the resistor R21, and the DC voltage rectified by the rectifier circuit REC is applied to the primary winding of the transformer T6, the switching transistor Q3, and the current detecting resistor R22.
And the control circuit CNT4 controls on / off of the switching transistor Q3,
The current flowing through the primary winding of the transformer T6 is on / off controlled. At this time, the rush current due to the power-on is equal to the resistance R2
Limited by one.

When a timer or the like detects that the time during which the inrush current has dropped below the predetermined value has elapsed, or when it has detected that the current has dropped below the predetermined value, the switch S1 is turned on and the resistor R21 is turned on. To avoid power loss due to the resistor R21 in the steady state.

When a current flowing in the primary winding of the transformer T6 is turned on and off by the switching transistor Q3, a voltage is induced in the secondary winding, and the voltage is generated by the diodes D6 and D7, the choke coil L2, the capacitor C
5 is rectified and smoothed by a rectifying and smoothing circuit including
It is applied to a load not shown. The output voltage applied to the load is detected by the control circuit CNT4, and the ON period of the switching transistor Q3 is controlled so that the output voltage becomes the set voltage, thereby making the output voltage constant.

The overcurrent detecting section OC3 includes an operational amplifier OA
6, a reference voltage Vr6, and resistors R22 to R24.
24 and a reference voltage V by an operational amplifier OA6.
Compare with r6. When the load connected to the rectifier / smoothing circuit on the secondary side of the transformer T6 is short-circuited or in a state close to the short-circuit, a large current also flows on the primary side of the transformer T6. As a result, the control circuit CNT4 shortens or turns off the on-period of the switching transistor Q3 by a signal from the operational amplifier OA6 due to an overcurrent state in which the divided voltage by the resistors R23 and R24 exceeds the reference voltage Vr6. And Thereby, the output voltage on the secondary side is drooped to prevent overcurrent.

[0007]

In the conventional switching power supply device, various types such as a forward converter type and a flyback converter type are known, but as shown in FIG. In order to prevent an inrush current at the time of turning on, a resistor R21 for limiting current and a switch S1 for short-circuiting the resistor R21 in a steady state are required. Was necessary. Since the resistor R21 and the switch S1 require a relatively large current capacity, there is a problem that the size is increased and the cost is increased. An object of the present invention is to prevent a rush current by omitting a resistor or a switch for limiting a current.

[0008]

Referring to FIG. 1, a rush current prevention circuit according to the present invention is a control circuit CNT1 for controlling on / off of a switching transistor Q1 connected to a primary winding n1 of a transformer T1. And the primary winding n1
Is compared with a reference voltage Vr1 and a current detection value obtained by detecting a current flowing through the
And an overcurrent detection means such as an overcurrent detection unit OC1 for inputting an overcurrent detection signal to the control circuit CNT1 when the output voltage exceeds 1 to provide overcurrent protection. When a power supply for applying a DC voltage to the primary winding n1 of the transformer T1 is turned on, a voltage is applied to the detected current value to be compared with Vr1 by an operational amplifier OA2 or the like so as to exceed the reference voltage Vr1. For example, a voltage adding means for gradually lowering the voltage over time is provided.

Further, a current detection value obtained by detecting a current flowing through the primary winding n1 of the transformer T1 by a resistor R1 or the like is compared with a first reference voltage Vr1, and the current detection value is determined by the first reference voltage Vr1.
r1; an ON / OFF control of a first operational amplifier OA1 that outputs an overcurrent detection signal and a switching transistor Q1 connected to a primary winding n1 of a transformer T1; and an overcurrent from the first operational amplifier OA1. A control circuit CNT1 for shortening the ON period of the switching transistor Q1 by a current detection signal, a capacitor C2 for rectifying an induced voltage of the tertiary winding n3 of the transformer T1 by a diode D3 and charging the same via a resistor R8; The terminal voltage of C2 is compared with the second reference voltage Vr2, and when the terminal voltage is equal to or lower than the second reference voltage Vr2, the first
And a second operational amplifier OA2 for adding a voltage to the current detection value so as to exceed the reference voltage Vr1.

A current detection value obtained by detecting a current flowing in the primary winding of the transformer is compared with a first reference voltage, and an overcurrent detection signal is output when the current detection value exceeds the first reference voltage. A control circuit for performing on / off control of a switching transistor connected to a primary winding of a transformer, and for shortening control of an on-period of the switching transistor by an overcurrent detection signal from the first operational amplifier; A terminal voltage of a capacitor for applying a DC output voltage on the secondary side of the transformer or a divided voltage obtained by dividing the terminal voltage is compared with a third reference voltage. And a third operational amplifier for adding a voltage to the current detection value so as to have a value exceeding the reference voltage.

A control circuit for controlling on / off of a switching transistor connected to a primary winding of the transformer;
An overcurrent detecting means for comparing a detected current value obtained by detecting a current flowing through the primary winding with a reference voltage and inputting an overcurrent detection signal to a control circuit when the reference voltage is exceeded to perform overcurrent protection; An inrush prevention circuit in a switching power supply device, wherein a capacitor charged by a DC voltage via a resistor when a power supply for applying a DC voltage to a primary winding of a transformer is turned on is referred to a terminal voltage of the capacitor. An operational amplifier that compares a current detection value as a voltage and inputs an overcurrent detection signal to a control circuit when the current detection value exceeds a reference voltage to perform on-period shortening control of the switching transistor. I have.

[0012]

FIG. 1 is an explanatory view of a first embodiment of the present invention, wherein REC is a rectifier circuit, CNT1 is a control circuit, Q1 is a switching transistor, T1 is a transformer, n1 is a primary winding, n2 is a secondary winding, n3 is a tertiary winding, OC1 is an overcurrent detector, R1 to R8 are resistors, C1,
C2 is a capacitor, L1 is a choke coil, D1 to D3
Represents a diode, OA1 and OA2 represent first and second operational amplifiers, and Vr1 and Vr2 represent first and second reference voltages.

This embodiment shows a switching power supply device to which a forward converter system is applied, in which a rectifier circuit REC is connected to an AC power supply via a power switch (not shown), and a full-wave rectified voltage is converted to a primary voltage of a transformer T1. A control circuit CNT1 is applied to a series circuit of a winding n1, a switching transistor Q1, and a current detecting resistor R1.
To control the on / off of the switching transistor Q1, and to apply the induced voltage of the secondary winding n2 of the transformer T1 to the diodes D1, D2, the choke coil L1, and the capacitor C1.
The rectifying and smoothing circuit includes a rectifying and smoothing circuit, and applies the voltage to a load (not shown). The output voltage applied to the load is detected by the control circuit CNT1 along a path indicated by a dotted line to become a set voltage. In addition, the on period of the switching transistor Q1 is controlled. Therefore, when the power is turned on, the output voltage applied to the load is zero or lower than the set voltage, so that the control circuit CNT1 controls so as to lengthen the ON period of the switching transistor Q1.

The overcurrent detecting section OC1 constituting the overcurrent detecting means includes a current detecting resistor R1 and a voltage dividing resistor R2.
To R4, a first operational amplifier OA1, and a first reference voltage Vr1.
When an overcurrent value set by the reference voltage Vr1 is exceeded, an overcurrent detection signal is input from the first operational amplifier OA1 to the control circuit CNT1. Thereby, the control circuit CNT
1 shortens the ON period of the switching transistor Q1 or makes it zero, thereby preventing overcurrent. The function of preventing this overcurrent is the same as in the conventional example.

A voltage corresponding to the turns ratio is induced in the tertiary winding n3 of the transformer T1 as in the secondary winding n2. This induced voltage is rectified by the diode D3 and charges the capacitor C2 via the resistor R8. Therefore, when the power is turned on, the terminal voltage of the capacitor C2 is zero, and the second operational amplifier OA2 compares the divided voltage by the resistors R6 and R7 with the second reference voltage Vr2. Since the reference voltage Vr2 is higher, the resistors R3 and R3 are connected via the resistor R5.
A voltage corresponding to the reference voltage Vr2 is applied to the connection point of R4. That is, it constitutes voltage adding means. Note that the terminal voltage of the capacitor C2 increases in accordance with a time constant including the resistor 8 from the time when the power supply for applying the DC voltage to the primary winding n1 of the transformer T1 is turned on.

As described above, the overcurrent detection unit OC1 divides the voltage across the current detection resistor R1 by the resistors R2 to R4, and divides the voltage at the connection point between the resistors R2 and R3 and the reference voltage Vr1. Are compared by the operational amplifier OA1. When the divided voltage exceeds the reference voltage Vr1, an overcurrent state is set. As described above, the operational amplifier OA2 when the power is turned on is turned on.
Is applied to the connection point between the resistors R3 and R4 via the resistor R5 to increase the potential at the connection point between the resistors R2 and R3. As a result, the overcurrent detection state is established,
The switching transistor Q1 is controlled by the control circuit CNT1.
To shorten the on-period. By shortening the ON period of the switching transistor Q1, it is possible to prevent an inrush current when the power is turned on.

Further, as time elapses from the turning on of a power switch (not shown) in the preceding stage of the rectifier circuit REC, the terminal voltage of the capacitor C2 rises, and the divided voltage by the resistors R6 and R7 becomes equal to the second reference voltage Vr2. Then, the output voltage of the second operational amplifier OA2 becomes zero. Thus, the overcurrent detection unit OC1 outputs the resistance R1 as in the case of the conventional example.
And the first reference voltage Vr1 is compared with the detected current value to detect whether or not an overcurrent state is present.
That is, the voltage obtained by dividing the terminal voltage of the capacitor C2 in the steady state by the resistors R6 and R7 is used as the second reference voltage Vr.
The respective circuit constants and the like are set to be equal to 2.

FIG. 2 is an explanatory diagram of output voltage / output current characteristics. For example, FIG. 2 is a characteristic curve diagram in which the vertical axis represents the output voltage of the rectifying and smoothing circuit in FIG. 1 and the horizontal axis represents the output current. The ON period of the switching transistor Q1 is controlled by the control circuit CNT1 so that the output voltage becomes constant at V1 until I1. Then, when the output current exceeds I1, an overcurrent detection signal is added from the overcurrent detection unit OC1 to the control circuit CNT1, and the control circuit CNT1 shortens the ON period of the switching transistor Q1. Thereby,
The output voltage decreases along curve C. The area according to the curve C is a constant power area.

When this state is continued, the output current increases according to the dotted curve B, and the output voltage decreases.
Normally, when the point P is reached, the on-period of the switching transistor Q1 is controlled to be reduced or set to zero so that the output voltage drops along the curve A. A current corresponding to the output current on the secondary side flows through the primary winding n1 of the transformer T1, so that the overcurrent state on the secondary side can be detected by the resistor R1 on the primary side.

In the above-mentioned overcurrent detecting section OC1, when the power is turned on, the output voltage of the operational amplifier OA2 is connected to the resistors R3 and R4.
By applying the voltage to the connection point, a state equivalent to an output current of 11 or more is obtained. It is also possible to make it equivalent to the case where the output current exceeds the point P, and it is possible to prevent a rush current when the power is turned on by adding a simple configuration.

FIG. 3 is an explanatory view of a second embodiment of the present invention, showing a switching power supply device to which a flyback converter system is applied, wherein the same reference numerals as those in FIG. 1 denote the same parts, T2 denotes a transformer, D4 Is a diode, and C3 is a capacitor. That is, the switching transistor Q
1 is turned on, and the rectifier circuit R is connected to the primary winding of the transformer T2.
When the switching transistor Q1 is turned off after the DC current flows from the EC, the induced voltage of the secondary winding of the transformer T2 at that time is rectified by the diode D4, and the capacitor C3 is charged. The output voltage is applied to a load (not shown).

The voltage induced in the tertiary winding of the transformer T2 is rectified by the diode D3, and the capacitor C2 is charged via the resistor R8. The operational amplifier OA2 compares the reference voltage Vr2 with a voltage obtained by dividing the terminal voltage of the capacitor C2 by the resistors R6 and R7. Therefore, when the power is turned on, the terminal voltage of the capacitor C2 is zero, so that the output voltage of the operational amplifier OA2 is applied to the connection point of the resistors R3 and R4 of the overcurrent detection unit OC1 via the resistor R5, and the resistance R
2, the potential at the connection point of R3 is raised from the reference voltage Vr1 to cause an overcurrent state, and the output signal of the operational amplifier OA1 causes the control circuit CNT1 to shorten the ON period of the switching transistor Q1. Inrush current when the power is turned on can be prevented.

FIG. 4 is an explanatory view of the third embodiment of the present invention. The same reference numerals as those in FIG. 1 denote the same parts, T3 denotes a transformer, OC2 denotes an overcurrent detection unit, CNT2 denotes a control circuit, C
T is a current transformer, D5 is a diode, OA3, O
A4 is an operational amplifier, Vr3 and Vr4 are reference voltages, R10
R17 indicates a resistance.

This embodiment shows a switching power supply device to which a forward converter system is applied, and an overcurrent detection unit OC2 has a function similar to that of the overcurrent detection unit OC1 in FIG. It has a configuration including a current transformer CT for detecting a current, a diode D5, an operational amplifier 3, and resistors R10 to R12, R16, and R17, and detects a current flowing on the primary side of the transformer T3 by the current transformer CT. Then, using the resistor R16 as a load resistor, the voltage across the resistor R16 is rectified by the diode D5, and the divided voltage at the connection point of the resistors R10 and R11 is input to the operational amplifier OA3 as a current detection value.
Compare with the reference voltage Vr3. When the voltage detection value exceeds the reference voltage Vr3, an overcurrent detection signal is input to the control circuit CNT2. Thereby, the control circuit CNT2
Is to perform overcurrent protection by shortening the ON period of the switching transistor Q1.

The control circuit CNT2 detects the output voltage of the rectifying and smoothing circuit on the secondary side of the transformer T3, and controls the switching transistor Q so that the output voltage becomes the set voltage.
1 to control the ON period of the switching transistor Q1, and when the overcurrent detection signal is input from the overcurrent detection unit OC2, as described above, the control to shorten the ON period of the switching transistor Q1 is performed. Have.

Further, the third operational amplifier OA4 controls the resistance R
14 and R15 and the third reference voltage Vr4, and when the divided voltage is higher than the third reference voltage Vr4, the resistors R11 and R12 of the overcurrent detection unit OC2 are connected via the resistor R13. A voltage is applied to raise the potential of the connection point. Therefore, when the power is turned on, the terminal voltage of the capacitor C1 on the secondary side of the transformer T3 is zero, and the divided voltage is lower than the third reference voltage Vr4, so that the output voltage of the third operational amplifier OA4 becomes lower. Through the resistor R13, the resistor R11,
Applied to the connection point of R12, the potential at the connection point of the resistors R10 and R11 is increased.

That is, a state equivalent to that when an overcurrent is detected by the current transformer CT is obtained, an overcurrent detection signal is input from the operational amplifier OA3 to the control circuit CNT2, and the control circuit CNT2 sets the output voltage of the rectifying and smoothing circuit. Even when it is lower than the value, the ON period of the switching transistor Q1 is shortened. That is, it is possible to prevent a rush current when the power is turned on. Then, when the terminal voltage of the capacitor C3 increases and the divided voltage by the resistors R14 and R15 becomes equal to the reference voltage Vr4, the output voltage of the operational amplifier OA4 becomes zero, and the overcurrent detection unit OC2 shifts to a normal operation. Will be.

FIG. 5 is an explanatory view of a fourth embodiment of the present invention. The same reference numerals as those in FIGS. 3 and 4 denote the same parts, and FIG.
4 indicates a transformer. This embodiment relates to a switching power supply device to which a flyback converter system is applied, and includes a capacitor C3 on the secondary side of a transformer T4.
Is divided by resistors R14 and R15 to obtain a third terminal voltage.
Of the third operational amplifier O so as to be compared with the reference voltage Vr4 of
Input to A4.

The overcurrent detection section OC2 and the control circuit CNT
2 is the same as the configuration in FIG. 4, and the duplicate description is omitted. Also in this embodiment, when the power is turned on,
Since the terminal voltage of the capacitor C3 is zero, the reference voltage V
The divided voltage by the resistors R14 and R15 is lower than that of r4. Therefore, the output voltage of the operational amplifier OA4 is applied to the connection point between the resistors R11 and R12 via the resistor R13, and the resistor R1
The potential at the connection point of 0, R11 is raised from the reference voltage Vr3 to be in an overcurrent state, an overcurrent detection signal is input from the operational amplifier OA3 to the control circuit CNT2, and the control circuit CNT2
Reduces the on-period of the switching transistor Q1 and prevents inrush current when the power is turned on.

FIG. 6 is an explanatory view of a fifth embodiment of the present invention, wherein T5 is a transformer, Q2 is a switching transistor shown as a bipolar transistor, CNT3 is a control circuit, D1 and D2 are diodes, L1 is a choke coil, C1 and C4 are capacitors, R18 and R19 are resistors,
OA5 indicates an operational amplifier.

This embodiment shows an outline of a switching power supply device to which a power factor improving type forward converter system is applied. An AC voltage is full-wave rectified by a rectifier circuit REC. 60
On / off of the switching transistor Q2 is controlled at a frequency of several tens kHz to several hundreds kHz with respect to Hz. As a result, a current corresponding to the voltage of the sine wave of the AC power supply flows, so that the power factor for the AC power supply can be improved. The output voltage of the rectifying / smoothing circuit on the secondary side of the transformer T5 is supplied to the control circuit CNT through a path not shown.
3, the ON period of the switching transistor Q2 is controlled so as to reach the set voltage.

The current flowing through the primary winding of the transformer T5 via the switching transistor Q2 is input to the operational amplifier OA5 so as to compare the current detection value detected by the resistor R18 with the terminal voltage of the capacitor C4 as a reference voltage. The capacitor C4 is charged by turning on the power via the resistor R19. Therefore, when the power is turned on, the terminal voltage of the capacitor C4 is zero and the current detection value by the resistor R18 is larger than the terminal voltage of the capacitor C4, so that the operational amplifier OA5 outputs the control circuit CNT as an overcurrent detection signal.
Enter 3 Therefore, the control circuit CNT3 shortens the ON period of the switching transistor Q2. As a result, an inrush current when the power is turned on can be prevented.

When the terminal voltage of the capacitor C4 rises with the passage of time from the power-on and the current detection value by the resistor R18 indicates a steady state value, the terminal voltage of the capacitor C4 is set to be higher than the current detection value. Set the circuit constants. When an overcurrent flows, the current detection value by the resistor R18 becomes larger than the terminal voltage (reference voltage) of the capacitor C4, and the operational amplifier OA5 sends the control circuit CNT3
, An overcurrent detection signal is input to the switching transistor Q2 to shorten the on-period of the switching transistor Q2, thereby performing overcurrent protection.

FIG. 7 is a diagram for explaining the operation of the fifth embodiment of the present invention. The vertical axis represents voltage, the horizontal axis represents time t, the terminal voltage of the capacitor C4 is Vc, and the voltage across the resistor R18 is V.
i, the terminal voltage Vc of the capacitor C4.
Rises from the power-on (t = 0) according to the time constant of the resistor R19. When the power is turned on, the control circuit CNT3
As a result, on / off control of the switching transistor Q2 is started.

When the power is turned on, the terminal voltage Vc of the capacitor C4 is 0, and increases as time t elapses. Therefore, the terminal voltage Vc is compared with the voltage Vi across the resistor R18. When the voltage Vi corresponding to the current detection value exceeds the terminal voltage Vc of the capacitor C4, the control circuit CNT3 switches according to the output signal of the operational amplifier OA5. The transistor Q2 is turned off. Thus, the current flowing through the primary winding of the transformer T5 via the switching transistor Q2 is limited. That is, an inrush current due to power-on can be prevented.

The terminal voltage Vc of the capacitor C4 rises with the passage of time from the power-on, and the ON period of the switching transistor Q2 is correspondingly extended until the voltage Vi across the resistor R18 exceeds the terminal voltage Vc. You. Then, a DC output voltage from the rectifying and smoothing circuit on the secondary side of the transformer T5 is applied to the load, and a steady current corresponding to the current flows through the primary winding of the transformer T5 via the switching transistor Q2. The voltage Vi across the resistor R18 due to the steady current continues to be equal to or lower than the terminal voltage Vc of the capacitor C4. However, when an overcurrent state occurs due to a load short circuit or the like, the voltage Vi across the resistor R18 exceeds the terminal voltage Vc. Therefore, the overcurrent detection signal from the operational amplifier OA5 causes the control circuit CNT3 to delay the on-period of the switching transistor Q2. Shorten.

The present invention is not limited to the above-described embodiments, but can be variously modified. For example, as a power factor improving type, a switching power supply using a flyback converter system is used. The present invention can be applied to a device, and a current transformer CT as shown in FIGS. 4 and 5 can be used instead of the current detecting resistor R18. It is also possible to adopt a configuration in which the terminal voltage of the capacitor C4 in FIG. 6 is divided by a resistor and input to the operational amplifier OA5. In that case, when the power is off,
It is also possible to adopt a configuration in which the capacitor C4 is discharged via a voltage dividing resistor. Further, as shown in FIG. 7, as a voltage adding means in the first to fourth embodiments, a capacitor is charged by using a voltage obtained by turning on the power supply of the primary side of the transformer, and the terminal voltage of the capacitor is compared with the second voltage. Alternatively, a configuration for comparing with the third reference voltage can be applied.

[0038]

As described above, the present invention has a means for detecting a current flowing in the primary winding of a transformer and comparing it with a reference voltage to perform overcurrent protection. By applying a voltage to the current detection value at the time of power-on by the voltage adding means and equivalently in an overcurrent state, the on-period of the switching transistor Q1 is shortened, thereby reducing the inrush current. By adding a slight configuration, it is possible to omit a resistor and a switch means for preventing inrush current having a large current capacity as in the conventional example. Therefore, there is an advantage that size reduction and cost reduction can be achieved.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a first embodiment of the present invention.

FIG. 2 is an explanatory diagram of output voltage / output current characteristics.

FIG. 3 is an explanatory diagram of a second embodiment of the present invention.

FIG. 4 is an explanatory diagram of a third embodiment of the present invention.

FIG. 5 is an explanatory diagram of a fourth embodiment of the present invention.

FIG. 6 is an explanatory diagram of a fifth embodiment of the present invention.

FIG. 7 is an operation explanatory view of a fifth embodiment of the present invention.

FIG. 8 is an explanatory diagram of a conventional example.

[Explanation of symbols]

 REC rectifier circuit CNT1 control circuit T1 transformer n1 primary winding n2 secondary winding n3 tertiary winding Q1 switching transistor OC1 overcurrent detector OA1 first operational amplifier OA2 second operational amplifier Vr1 first reference voltage Vr2 second Reference voltage D1 to D3 Diode R1 to R8 Resistance C1, C2 Capacitor

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Noriyuki Ito 1-17-3 Sakado, Takatsu-ku, Kawasaki-shi, Kanagawa F-term in Fuji Denso Co., Ltd. 5G004 AA05 AB02 DA02 DC04 EA01 5G065 AA08 BA04 DA06 DA07 EA06 HA04 JA01 KA02 KA05 LA01 LA02 MA09 MA10 NA01 NA02 NA04 NA06 NA09 5H730 AA20 AS01 BB23 BB57 CC01 DD04 EE02 EE08 EE72 FD01 FD41 FD47 FG01 VV03 XC04 XC09 XX02 XX15 XX22 XX35 XX48

Claims (4)

[Claims] A control circuit for controlling on / off of a switching transistor connected to a primary winding of a transformer;
An overcurrent detecting means for comparing a detected current value obtained by detecting a current flowing in the primary winding with a reference voltage, and inputting an overcurrent detection signal to the control circuit when the reference voltage is exceeded to perform overcurrent protection; In the inrush prevention circuit in the switching power supply device having the following, the current detection value to be compared with the reference voltage, when the power supply that applies a DC voltage to the primary winding of the transformer is turned on, exceeds the reference voltage. A voltage adding means for applying a voltage to the power supply and gradually reducing the voltage with the passage of time. 2. A current detection value obtained by detecting a current flowing through a primary winding of the transformer is compared with a first reference voltage, and an overcurrent detection signal is output when the current detection value exceeds the first reference voltage. And a first operational amplifier for outputting a signal, and turning on and off a switching transistor connected to the primary winding of the transformer.
A control circuit for performing off-control and shortening control of the on-period of the switching transistor by an overcurrent detection signal from the first operational amplifier, and rectifying an induced voltage of the tertiary winding of the transformer by a diode, and And a terminal voltage of the capacitor and a second
A second operational amplifier that compares the detected voltage with the current detection value so that the terminal voltage is higher than the first reference voltage when the terminal voltage is equal to or lower than the second reference voltage. The inrush current prevention circuit according to claim 1, further comprising: 3. A current detection value obtained by detecting a current flowing through a primary winding of the transformer is compared with a first reference voltage, and an overcurrent detection signal is output when the current detection value exceeds the first reference voltage. And a first operational amplifier for outputting a signal, and turning on and off a switching transistor connected to the primary winding of the transformer.
A control circuit for performing off-control and shortening the on-period of the switching transistor by an overcurrent detection signal from the first operational amplifier; and a terminal voltage of a capacitor for applying a DC output voltage on the secondary side of the transformer. Further, a voltage divided from the terminal voltage is compared with a third reference voltage, and when the third reference voltage is large, the current detection value is set to a value exceeding the first reference voltage. 2. The rush current prevention circuit according to claim 1, further comprising a third operational amplifier for applying a voltage. 4. A control circuit for controlling on / off of a switching transistor connected to a primary winding of a transformer,
An overcurrent detecting means for comparing a detected current value obtained by detecting a current flowing in the primary winding with a reference voltage, and inputting an overcurrent detection signal to the control circuit when the reference voltage is exceeded to perform overcurrent protection; In a rush prevention circuit in a switching power supply having: a capacitor that is charged via a resistor by the DC voltage when a power supply that applies a DC voltage to the primary winding of the transformer is turned on, and a terminal of the capacitor A voltage is used as the reference voltage to compare the current detection value, and when the current detection value exceeds the reference voltage, an overcurrent detection signal is input to the control circuit to perform on-time reduction control of the switching transistor. An inrush current prevention circuit, comprising: