JPH06165488A - Switching power supply device - Google Patents

Abstract

PURPOSE:To reduce inflow currents produced at a time when load is diminished when a plurality of switching power supply devices are connected in parallel. CONSTITUTION:A current-limiting circuit consisting of a first transistor 21 connected between a positive-side DC load terminal OP and a third resistor 9, a fifth resistor 22 connected between the collector and emitter of the first transistor and a Zener diode 23 bonded between the node of the third resistor 9 and the photodiode 5A of a first photocoupler 5 and the base of the first transistor 21 is mounted. Currents flowing through the main terminals C, A of a shunt type stabilized power circuit 6, through which currents are made to flow mainly when there are inflow currents, are limited so as not to exceed a specified current value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply device widely used as a constant voltage DC power supply for electronic devices.

[0002]

2. Description of the Related Art FIG. 5 is a circuit diagram of a conventional switching power supply device. In FIG. 5, the primary coil and the switching element, for example, the collector and the emitter of the transistor 1 are connected in series to the positive and negative DC power supply terminals IP and IN, and the secondary coil of the transformer 3 has a diode (not shown). A rectifying / smoothing circuit 4 including a full-wave rectifying bridge and a smoothing capacitor is connected. Rectifying / smoothing circuit 4
The positive and negative side DC output terminals p and n are drawn out and connected to the positive and negative side DC load terminals OP and ON. Loads such as electronic devices are connected to the positive and negative DC load terminals O
It is connected to P and ON. A series circuit including resistors 7 and 8 is connected between the positive and negative DC load terminals OP and ON, and a resistor 9 and a photodiode of the photocoupler 5 are provided between the positive and negative DC output terminals p and n. 5A, a series circuit composed of main terminals C and A of the shunt-type stabilized power supply circuit 6 is connected. Phototransistor 5B of photocoupler 5
Is connected to the signal input terminal of the control circuit 2 of the transistor 1. The reference voltage terminal R of the shunt type stabilized power supply circuit 6 is connected to the connection point of the resistors 7 and 8 of the series circuit composed of the resistors 7 and 8.

FIG. 10 is a circuit diagram for explaining the function of the shunt-type stabilized power supply circuit 6. The shunt-type stabilized power supply circuit 6 is, for example, a NEC-made μPC1093 high-precision variable shunt-type stabilized power supply circuit. Terminal C (CATHOD
E), A (ANODE) and reference voltage terminal R (REF
The voltage applied to the reference voltage terminal R is higher than a predetermined reference voltage by a Zener diode (not shown) provided inside the main terminal C,
When the voltage applied to the reference voltage terminal R is equal to or lower than a predetermined reference voltage, the operation between A and the non-conduction side is performed between the main terminals C and A. As shown in FIG. 10, the shunt-type stabilized power supply circuit 6 has its main terminals C and A connected to the positive and negative side DC power supply terminals IP and IN via a resistor R3, and its main terminals C and A, respectively.
Pull A to positive and negative DC load terminals OP and ON
Connect to. A series circuit composed of resistors R1 and R2 is connected between the positive and negative side DC load terminals OP and ON, and the reference voltage terminal R of the shunt-type stabilized power supply circuit 6 is connected to the connection point of the resistors R1 and R2 of this series circuit. Connect.

In the shunt-type stabilized power supply circuit 6, the current flowing in the main terminals C and A changes according to the voltage input to the reference voltage terminal R, and this current causes a voltage drop in the resistor R3, and the reference voltage terminal R It operates so that the voltage of becomes the reference voltage. Therefore, the DC load terminal voltage is now V ₀ and the reference voltage is V
_{If R} , V ₀ ≈ (1 + R1 / R2) · V _R ,
The voltage between the positive and negative DC load terminals OP and ON is constant.

The operation of the switching power supply device shown in FIG. 5 will be described below. The transistor 1 is repeatedly turned on and off by the control circuit 2 at high frequency, for example, at a frequency of 100 KHz. Due to this on / off, the primary coil of the transistor 1 is opened / closed, an AC voltage is induced in the secondary coil of the transistor 1, this AC voltage is converted into a DC voltage by the rectifying / smoothing circuit 4, and its positive and negative sides are converted. It is output from the DC output terminals p and n. This DC voltage is applied to resistors R7 and R8
A voltage corresponding to that is taken out from the connection point of the resistors R7 and R8 of the series circuit consisting of and is input to the reference voltage terminal R of the shunt type stabilized power supply circuit 6. Now, assuming that the voltage is higher than a predetermined reference voltage by this amount, the current flowing through the main terminals C and A increases, and this current causes a signal to be input from the photodiode 5A of the photocoupler 5 to the phototransistor 5B, and the phototransistor 5B. A signal is input to the signal input terminal of the control circuit 2 from 5B. In the control circuit 2, the switching width of the transistor 1 is reduced to reduce the DC voltage. When the voltage falls below a predetermined reference voltage by this amount, the current flowing through the main terminals C and A decreases, and this current increases the switching width of the transistor 1 to increase the DC voltage. In this way, the DC voltage is controlled to be constant. This DC voltage is applied to the positive and negative DC load terminals O
Since it is applied to P and ON, the output characteristics of the positive and negative side DC load terminals OP and ON, that is, the DC load terminal voltage V ₀ and the output current I ₀ characteristic are constant voltage characteristics as shown in A of FIG. Become.

The transformer 3 is usually provided with an auxiliary coil (not shown), and a control load is connected to the auxiliary coil to prevent the demagnetization due to the direct current component of the transformer 3. In the above switching power supply, the DC load terminal voltage is controlled to be constant with respect to the output current, which is highly desirable as a constant voltage power supply. A problem arises when a plurality of power supplies are connected in parallel. FIG. 7 is an output characteristic diagram when the first switching power supply device A and the second switching power supply device B are connected in parallel. Normally, the direct current loads between the switching power supply devices A and B are different due to variations in products. Δ between the terminal voltage V ₀₁ and V ₀₂
There is a voltage difference of V ₀ . Although the value of ΔV ₀ is not large, if the constant voltage characteristic is good, the switching power supply device having a high voltage even with a slight difference has the switching power supply device A in FIG.

To solve this problem, a switching power supply device shown in the circuit diagram of FIG. 6 has been proposed. The switching power supply device shown in FIG. 6 is different from the switching power supply device shown in FIG. 5 in that a resistor 10 is provided between the negative side DC output terminal n of the rectifying / smoothing circuit 4 and the negative side DC load terminal ON. It is in. By providing the resistor 10, the positive side and negative side DC load terminals OP and ON are connected to the reference voltage terminal R of the shunt type stabilized power supply circuit 6, that is, between the reference voltage terminal R and the main terminal C. The sum of the divided voltage and the voltage drop of the resistor 10 (that is, the product of the resistance value of the resistor 10 and the output current value) is applied, and the shunt-type stabilized power supply circuit 6 controls the voltage of this sum to be constant. Therefore, the output characteristic comes to have a regulation of a magnitude corresponding to the voltage drop of the resistor 10 as shown in A of FIG.

FIG. 8 shows a plurality of switching power supply devices whose output characteristics are regulated, two in FIG.
That is, in the output characteristic diagram when the first switching power supply device A and the second switching power supply device B are connected in parallel, the DC load terminals of the switching power supply devices A and B are different due to product variations. There is a voltage difference of ΔV ₀ between the voltages V ₀₁ and V ₀₂ . Since the DC load terminal voltages of the two switching power supply devices A and B connected in parallel are the same V _0S , the output characteristics of the switching power supply device A and the switching power supply device B which intersect this voltage V _0S . The output currents I ₀₁ and I ₀₂ are shared based on the output characteristics.

[0009]

However, the above-mentioned newly proposed switching power supply device also has the following problems. That is, a high load when the output current is large in the switching power supply device A and a switching power supply device B as shown in the output characteristic diagram of FIG. 8 and the output current I ₀₁ respectively output currents in the case of connecting a plurality in parallel Although I ₀₂ is shared, when the load decreases and the output current I ₀₂ of the switching power supply B becomes 0 as shown in the output characteristic diagram of FIG. 9, current is supplied to the load only from the switching power supply A. Become so. When the load is further reduced, a current starts to flow from the switching power supply A to the switching power supply B, which causes a large loss in the switching power supply B and, in some cases, an accident in which the device is damaged.

As a countermeasure for this, a method of connecting a diode to the output side of each switching power supply device in a positive polarity to prevent the inflow of current can be considered, but the forward voltage drop of this diode becomes a loss during normal operation. It reduces the efficiency of the device. An object of the present invention is to reduce the inflow current generated when the load is reduced when a plurality of switching power supply devices are connected in parallel.

[0011]

In order to achieve the above-mentioned object, the present invention provides a transformer having a primary coil connected to a DC power source, and a transformer between the DC power source and the primary coil of the transformer in series. A connected switching element,
A rectifying / smoothing circuit connected to the secondary coil of the transformer, and a first rectifying / smoothing circuit connected between the positive side and negative side DC load terminals drawn from the positive side and negative side DC output terminals, respectively. The phototransistor is connected to the control circuit of the switching element and the third resistor connected between the positive side and negative side DC output terminals of the rectifying / smoothing circuit, and the series circuit including the second resistor and the second resistor. Of a photo-coupler (hereinafter, referred to as a first photo-coupler) and a reference voltage terminal of the first resistance and the second resistance of a series circuit including the first resistance and the second resistance. A fourth circuit connected between a series circuit composed of the main terminal of the shunt-type stabilized power supply circuit connected to the connection point and the negative side DC output terminal of the rectifying / smoothing circuit and the negative side DC load terminal. From resistance In that the switching power supply device,
A current limiting circuit is provided to limit the current flowing through the main terminal of the shunt-type stabilized power supply circuit so as not to exceed a predetermined current value. The current limiting circuit is connected between a first transistor whose collector and emitter are connected between the positive side DC load terminal and the third resistor, and between the collector and the base of the first transistor. The fifth resistor, the third resistor and the connection point between the photodiode of the first photocoupler and the first resistor
And a Zener diode connected between the base and the transistor. Alternatively, in this current limiting circuit, its emitter is connected to the positive side DC load terminal through its sixth resistor and its collector are connected in series to the negative side DC load terminal, and its base is connected to the third resistor. The second transistor connected to the connection point of the photodiode of the first photocoupler, the collector thereof is the reference voltage terminal of the shunt-type stabilized power supply circuit, the emitter thereof is the negative side DC load terminal, and the base thereof is the first. The third transistor is connected to the connection point of the resistor and the second resistor. Further or alternatively, the current limiting circuit includes an eighth and a ninth resistors, each of which is connected in series with the third resistor in parallel, a photodiode of which is parallel to the eighth resistor and a phototransistor of which is parallel to the second resistor. And a second photocoupler connected to each. Further, in this current limiting circuit, the emitter is connected to the positive side DC load terminal, the collector is connected to the negative side DC load terminal through the tenth resistor, and the base is connected to the third resistor and the photodiode of the first photocoupler. A fourth transistor connected to the point, and a fifth transistor whose emitter / collector is connected between the positive side DC load terminal and the first resistor and whose base is connected to the collector of the fourth transistor. To consist of.

[0012]

According to the present invention, a transformer having a primary coil connected to a DC power source, a switching element connected in series between the DC power source and the primary coil of the transformer, and a secondary coil of the transformer are provided. The connected rectifying / smoothing circuit, and the first and second resistors connected between the positive and negative DC load terminals drawn from the positive and negative DC output terminals of the rectifying / smoothing circuit, respectively. , A third resistor connected between the positive side and negative side DC output terminals of the rectifying / smoothing circuit, and a photocoupler having the phototransistor connected to the control circuit of the switching element (hereinafter referred to as And a reference voltage terminal thereof is connected to a connection point of a first resistor and a second resistor of a series circuit including the first resistor and the second resistor. And a fourth resistor connected between the negative side DC output terminal and the negative side DC load terminal of the rectifying / smoothing circuit. The switching power supply device is provided with a current limiting circuit for limiting the current flowing through the main terminal of the shunt-type stabilized power supply circuit so as not to exceed a predetermined current value. When a plurality of switching power supply devices are connected in parallel, the inflow current generated when the load decreases mainly flows into the main terminal of the shunt-type stabilized power supply circuit, so that the current flowing through this main terminal in the current limiting circuit is set to a predetermined value. The inflow current can be reduced by limiting the current so that it does not exceed the current value. The current limiting circuit is connected between a first transistor whose collector and emitter are connected between the positive side DC load terminal and the third resistor, and between the collector and the base of the first transistor. Since it is composed of the fifth resistor, the third resistor, the connection point of the photodiode of the first photocoupler, and the Zener diode connected between the base of the first transistor,
When the current flowing through the main terminal of the shunt-type stabilized power supply circuit increases, this current increases the voltage drop of the third resistor. When the voltage drop of the third resistor exceeds the voltage of the Zener diode, the first transistor operates in the OFF direction, and the current flowing through the main terminal of the shunt-type stabilized power supply circuit causes the voltage drop of the third resistor and The current is limited so as not to exceed a predetermined current value determined by the voltage of the Zener diode. Alternatively, in this current limiting circuit, its emitter is connected to the positive side DC load terminal through its sixth resistor and its collector are connected in series to the negative side DC load terminal, and its base is connected to the third resistor. Second transistors respectively connected to the connection points of the photodiodes of the first photocoupler,
The collector is composed of a reference voltage terminal of the shunt-type stabilized power supply circuit, the emitter is composed of a negative side DC load terminal, and the base is composed of a third transistor connected to the connection point of the first resistor and the second resistor, respectively. Therefore, when the current flowing through the main terminal of the shunt-type stabilized power supply circuit increases,
This current increases the voltage drop across the third resistor. When the voltage drop of the third resistor exceeds the base-emitter voltage of the second transistor, the second transistor operates in the ON direction, which causes the third transistor to operate in the ON direction. When the third transistor operates in the ON direction, the voltage of the reference voltage terminal of the shunt-type stabilized power supply circuit decreases, the main terminals operate on the non-conducting side, and the current flowing through this main terminal is the voltage of the third resistor. The current is limited so as not to exceed a predetermined current value determined by the drop and the base-emitter voltage of the second transistor. Further or alternatively, the current limiting circuit includes an eighth and a ninth resistors, each of which is connected in series with the third resistor in parallel, a photodiode of which is parallel to the eighth resistor and a phototransistor of which is parallel to the second resistor. Since the second photocoupler is connected to each of the second photocouplers, when the current flowing to the main terminal of the shunt-type stabilized power supply circuit increases, the voltage drop of the third resistor increases due to this current. When the voltage division of the voltage drop of the third resistor exceeds the armpit voltage of the photodiode of the second photocoupler, the phototransistor of the second photocoupler operates in the ON direction. When the phototransistor of the second photocoupler operates in the ON direction, the voltage at the reference voltage terminal of the shunt-type stabilized power supply circuit decreases, the main terminals operate on the non-conducting side, and the current flowing through this main terminal is reduced. The current is limited so that it does not exceed a predetermined current value determined by the voltage drop of the third resistor and the armature voltage of the photodiode of the second photocoupler. Further alternatively, in the current limiting circuit, the emitter is connected to the positive side DC load terminal, the collector is connected to the negative side DC load terminal via the tenth resistor, and the base is connected to the third resistor and the photodiode of the first photocoupler. A fifth transistor, whose emitter / collector is connected between the fourth transistor connected to the connection point and the positive side DC load terminal and the first resistor, and whose base is connected to the collector of the fourth transistor
When the current flowing to the main terminal of the shunt-type stabilized power supply circuit increases, the voltage drop of the third resistor increases due to this current. The voltage drop across this third resistor is
When the emitter voltage is exceeded, the fourth transistor operates in the ON direction, which causes the fifth transistor to operate in the OFF direction. When the fifth transistor operates in the OFF direction, the voltage at the reference voltage terminal of the shunt-type stabilized power supply circuit decreases, the main terminals operate on the non-conducting side, and the current flowing through this main terminal causes the third resistance to flow. The current is limited so as not to exceed a predetermined current value determined by the voltage drop and the base-emitter voltage of the fourth transistor.

[0013]

1 is a circuit diagram showing an embodiment of a switching power supply device of the present invention. The switching power supply device of the present invention shown in FIG. 1 is the same as the conventional switching power supply device shown in FIG. 6, and includes a transistor 21 whose collector / emitter is connected between a positive side DC load terminal OP and a resistor 9,
A resistor 22 connected between the collector and the base of the transistor 21, a connection point between the resistor 9 and the photodiode 5A of the photocoupler 5 and a Zener diode 23 connected to the base of the first transistor 21 are provided. Is. The transistor 21, the resistor 22 and the Zener diode 23 form a current limiting circuit, and limit the current flowing through the main terminals C and A of the shunt-type stabilized power supply circuit 6 so as not to exceed a predetermined current value. That is, when the current flowing through the main terminals C and A of the shunt type stabilized power supply circuit 6 increases, the voltage drop of the resistor 9 increases due to this current. When the voltage drop of the resistor 9 exceeds the voltage of the Zener diode 23, the transistor 21 operates in the OFF direction, and the current flowing through the main terminals C and A of the shunt-type stabilized power supply circuit 6 causes the voltage drop of the resistor 9 and the Zener diode 23. Limit the current so that it does not exceed the specified current value determined by the voltage and.

When a plurality of the switching power supply devices are connected in parallel and a load is reduced and an inflow current is generated in a certain switching power supply device among them, the inflow current is mainly the main current of the shunt type stabilized power supply circuit 6. Terminal C,
The current flows through A (the resistors 7 and 8 are resistors for voltage division, and normally, their resistance values are high and the current of this circuit is small). Here, since the current flowing through the main terminals C and A of the shunt-type stabilized power supply circuit 6 is limited so as not to exceed a predetermined current value, the inflow current can be reduced.

FIG. 2 is a circuit diagram of essential parts showing a different embodiment of the switching power supply device of the present invention. The switching power supply device of the present invention shown in FIG. 2 is different from the conventional switching power supply device shown in FIG. 6 in that its emitter is connected to the positive side DC load terminal OP and its collector is connected to the negative side via resistors 33 and 34 connected in series. A transistor 31 whose base is connected to the connection point of the resistor 9 and the phototransistor 5A of the photocoupler 5 to the DC load terminal ON, and its collector is the reference voltage terminal R of the shunt-type stabilized power supply circuit 6.
Is provided with a transistor 32 whose emitter is connected to the negative side DC load terminal ON and whose base is connected to the connection point of the resistors 33 and 34, respectively, and these transistors 31, 32 and the resistors 33, 34 are limited. It constitutes a flow circuit. When the current flowing through the main terminals C and A of the shunt type stabilized power supply circuit 6 increases, the voltage drop of the resistor 9 also increases due to this current. When the voltage drop of the resistor 9 exceeds the base-emitter voltage of the transistor 31, the transistor 31 operates in the ON direction, which causes the transistor 32 to operate.
Operates in the on direction. When the transistor 32 operates in the ON direction, the voltage at the reference voltage terminal R of the shunt-type stabilized power supply circuit 6 decreases, the main terminals C and A operate on the non-conducting side, and the current flowing through the main terminals C and A. Is limited so as not to exceed a predetermined current value determined by the voltage drop of the resistor 9 and the base-emitter voltage of the transistor 31.

FIG. 3 is a circuit diagram of essential parts showing a further different embodiment of the switching power supply device of the present invention. The switching power supply device of the present invention shown in FIG. 3 is the same as the conventional switching power supply device shown in FIG. The phototransistor 41B is provided with a photocoupler 41 connected to the resistor 8 respectively, and the photocoupler 41 and the resistors 42 and 43 form a current limiting circuit. When the current flowing through the main terminals C and A of the shunt-type stabilized power supply circuit 6 increases, the voltage drop of the resistor 9 increases due to this current. When the voltage divided by the resistors 42 and 43 of the voltage drop of the resistor 9 exceeds the armature voltage of the photodiode 41A of the photocoupler 41, the phototransistor 41B of the photocoupler 41 operates in the ON direction. When the phototransistor 41B of the photocoupler 41 operates in the ON direction, the voltage at the reference voltage terminal R of the shunt-type stabilized power supply circuit 6 decreases, and the main terminals C and A operate on the non-conducting side. The currents flowing through C and A are limited so that they do not exceed a predetermined current value determined by the voltage division of the resistor 9 and the armature voltage of the photodiode 41A of the photocoupler 41. The resistors 42 and 43 are voltage dividing resistors, and can be omitted in practice by setting the resistance value of the resistor 42 to ∞ and the resistance value of the resistor 43 to 0 as necessary. Of course, at this time, the voltage division ratio is 1.

FIG. 4 is a circuit diagram of the essential parts showing a further different embodiment of the switching power supply device of the present invention. The switching power supply device of the present invention shown in FIG. 4 is the same as the conventional switching power supply device shown in FIG. 6, with its emitter connected to the positive side DC load terminal OP and its collector connected to the negative side DC load terminal ON via the resistor 53, and its base. Is connected to the connection point between the resistor 9 and the photodiode 5A of the photocoupler 5, the emitter / collector is connected between the positive side DC load terminal OP and the resistor 7, and its base is the collector of the transistor 51. The connected transistor 52 is provided, and these transistors 51 and 52 and the resistor 53 form a current limiting circuit. When the current flowing through the main terminals C and A of the shunt-type stabilized power supply circuit 6 increases, the voltage drop of the resistor 9 increases due to this current. This resistance 9
When the voltage drop of V exceeds the base-emitter voltage of the transistor 51, the transistor 51 operates in the ON direction, which causes the transistor 52 to operate in the OFF direction. When the transistor 52 operates in the OFF direction, the voltage of the reference voltage terminal R of the shunt-type stabilized power supply circuit 6 decreases, the main terminals C and A operate on the non-conducting side, and the current flowing through the main terminals C and A. Is limited so as not to exceed a predetermined current value determined by the voltage drop of the resistor 9 and the base-emitter voltage of the transistor 51.

As shown in the embodiments of FIGS. 1 to 4, when the load is reduced and an inflow current is generated, the inflow current is limited so as not to exceed a predetermined current value, so that the loss due to the inflow current is caused. Is reduced. In each of the above-described embodiments, it is most effective to set the current value for limiting the current to be slightly larger than the current value of the current at the main terminals C and A of the shunt type stabilized power supply circuit 6 in the normal operation.

According to the result of the experiment, when two switching power supplies having an output voltage of 24V are connected in parallel and a load is reduced, a current of about 40 mA flows into one of the switching power supplies in the conventional switching power supply. Although a loss of about 1 W was generated, in the switching power supply device of the present invention, the inflow current was reduced to 2-3 mA under the same conditions, and the loss due to the inflow current was significantly reduced.

Further, in the switching power supply device of the present invention, since the inflow current is reduced as described above, it is not necessary to increase the capacity of the shunt type stabilized power supply circuit 6, the photocoupler 5 and the resistor 9, and the cost is reduced. . The photocouplers 5 and 41 are the first and second photocouplers, the transistors 21, 31, 32, 51 and 52 are the first, second, third, fourth and fifth transistors, and the resistors 7 and 8, 9, 1
0, 22, 33, 34, 42, 43, 53 to the first, second, third, fourth, fifth, sixth, seventh, eighth, ninth, first
Referred to as 0 resistance.

[0021]

In the switching power supply device of the present invention, when a plurality of switching power supply devices are connected in parallel, the inflow current generated when the load is reduced is reduced. Therefore, the loss due to this inflow current is reduced and the parallel operation can be performed safely. It will be possible. Further, by reducing the inflow current, it is not necessary to increase the current capacity of the related electronic components, and the cost is reduced.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a switching power supply device of the present invention.

FIG. 2 is a circuit diagram of a main part showing a different embodiment of the switching power supply device of the present invention.

FIG. 3 is a main part circuit diagram showing a further different embodiment of the switching power supply device of the present invention.

FIG. 4 is a circuit diagram of essential parts showing a further different embodiment of the switching power supply device of the present invention.

FIG. 5 is a circuit diagram showing an example of a conventional switching power supply device.

FIG. 6 is a circuit diagram showing a different example of a conventional switching power supply device.

FIG. 7 is an output characteristic diagram of the conventional switching power supply device shown in FIG.

FIG. 8 is an output characteristic diagram showing output characteristics of the conventional switching power supply device shown in FIG. 6 and output characteristics when two switching power supply devices are connected in parallel.

9 is an output characteristic diagram for explaining the state when the output current is reduced in the output characteristic diagram shown in FIG.

FIG. 10 is a circuit diagram for explaining the function of a shunt-type stabilized power supply circuit.

[Explanation of symbols]

1 Transistor (Switching Element) 2 Control Circuit 3 Transformer 4 Rectifying / Smoothing Circuit 5 First Photocoupler 5A Photodiode (First Photocoupler 5) 5B Phototransistor (First Photocoupler 5) 6 Shunt-type Stabilization Power supply circuit 7 first resistance 8 second resistance 9 third resistance 10 fourth resistance 21 first transistor 22 fifth resistance 23 Zener diode 31 second transistor 32 third transistor 33 sixth Resistor 34 Seventh Resistor 41 Second Photocoupler 41A Photodiode (Second Photocoupler 41
41B phototransistor (second photocoupler 41)
42) Eighth resistance 43 Ninth resistance 51 Fourth transistor 52 Fifth transistor 53 Tenth resistance p Positive DC output terminal of rectification / smoothing circuit 4 Negative DC output of rectification / smoothing circuit 4 Terminal OP Positive side DC load terminal ON Negative side DC load terminal A Main terminal (for shunt type stabilized power supply circuit 6) C Main terminal (for shunt type stabilized power supply circuit 6) R Reference voltage terminal (shunt type stabilized power supply circuit) 6)

Claims (5)

[Claims] 1. A transformer having a primary coil connected to a DC power source, and a switching element connected in series between the DC power source and the primary coil of the transformer.
A rectifying / smoothing circuit connected to the secondary coil of the transformer, and a first rectifying / smoothing circuit connected between the positive side and negative side DC load terminals drawn from the positive side and negative side DC output terminals, respectively. The phototransistor is connected to the control circuit of the switching element and the third resistor connected between the positive side and negative side DC output terminals of the rectifying / smoothing circuit, and the series circuit including the second resistor and the second resistor. Of a photo-coupler (hereinafter, referred to as a first photo-coupler) and a reference voltage terminal of the first resistance and the second resistance of a series circuit including the first resistance and the second resistance. A fourth circuit connected between a series circuit composed of the main terminal of the shunt-type stabilized power supply circuit connected to the connection point and the negative side DC output terminal of the rectifying / smoothing circuit and the negative side DC load terminal. From resistance In that the switching power supply device,
A switching power supply device comprising a current limiting circuit for limiting a current flowing through a main terminal of the shunt-type stabilized power supply circuit so as not to exceed a predetermined current value. 2. The current limiting circuit according to claim 1, wherein the current limiting circuit has a collector between the positive side DC load terminal and the third resistor.
A first transistor having an emitter connected to the first transistor;
A fifth resistor connected between the collector and the base of the transistor of, and a Zener connected between the connection point of the third resistor and the photodiode of the first photocoupler and the base of the first transistor A switching power supply device comprising a diode. 3. The current limiting circuit according to claim 1, wherein the current limiting circuit has a negative side DC load via a sixth resistor and a seventh resistor whose emitter is connected in series to the positive side DC load terminal and whose collector is connected in series. A second transistor whose base is connected to the connection point of the third resistor and the photodiode of the first photocoupler is connected to the terminal, and its collector is the reference voltage terminal of the shunt-type stabilized power supply circuit whose emitter is negative. A switching power supply device comprising a side DC load terminal and a third transistor whose base is connected to a connection point of a first resistor and a second resistor, respectively. 4. The current limiting circuit according to claim 1, wherein the current limiting circuit includes an eighth resistor and a ninth resistor which are respectively connected in series to the third resistor in parallel, and the photodiode thereof is connected to the eighth resistor in parallel. A switching power supply device, wherein the phototransistor comprises a second photocoupler connected in parallel to a second resistor. 5. The current limiting circuit according to claim 1, wherein an emitter of the current limiting circuit is a positive side DC load terminal and a collector thereof is the first.
The base is connected to the negative side DC load terminal through the resistor of 0
Of the fourth transistor connected to the connection point between the resistance of the first photocoupler and the photodiode of the first photocoupler, its emitter / collector is connected between the positive side DC load terminal and the first resistance, and its base is the fourth. And a fifth transistor connected to the collector of the transistor.