JP2003224971A - Switching power supply - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a switching power supply which can protect a switching device from overcurrent, even if a DC input voltage is lowered. <P>SOLUTION: In a switching power supply having a switching transistor Q1 whose ONs/OFFs are controlled, to have a DC output voltage supplied to a load 5 be a prescribed value, there is provided an ON/OFF operation stopping circuit 3 which stops the ON/OFF operation of the switching transistor Q1, regardless of the value of the DC output voltage when a voltage between both terminals of a smoothing capacitor, i.e., a DC input voltage, is lowered to a value smaller than the prescribed value. <P>COPYRIGHT: (C)2003,JPO

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 that outputs a DC stable voltage.

[0002]

2. Description of the Related Art Here, as a conventional switching power supply device, a switching power supply device having a ringing choke converter will be described as an example. FIG. 11 shows a configuration example of a conventional switching power supply device.

An AC power supply 1 such as a commercial power supply is connected to the input side of a rectifier circuit 2. The output side of the rectifier circuit 2 is connected to the smoothing capacitor C1. With this configuration,
The AC voltage output from the AC power supply 1 is rectified by the rectifier circuit 2 and smoothed by the smoothing capacitor C1 to become the DC input voltage V _IN .

The primary winding 4a of the switching transformer 4 and an n-type channel FET (Field Effect Transistor)
A series circuit with the switching transistor Q1 which is r) is connected to both ends of the smoothing capacitor C1. One end of the primary winding 4a of the switching transformer 4 is connected to the positive polarity side of the smoothing capacitor C1, and the source of the switching transistor Q1 is connected to the negative polarity side of the smoothing capacitor C1. Further, the secondary winding 4b of the switching transformer 4 is connected to both ends of the smoothing capacitor C2 via the diode D1, and both ends of the smoothing capacitor C2 are connected to the load 5.

Further, one end of the gate winding 4c of the switching transformer 4 is connected to the gate of the switching transistor Q1 via a capacitor C4 and a resistor R17,
The other end of the gate winding 4c of the switching transformer 4 is connected to the source of the switching transistor Q1. Further, one end of the starting resistor R18 is connected to a connection node between the positive side of the smoothing capacitor C1 and the primary winding 4a of the transformer 4, and the other end of the starting resistor R18 is a connection node between the resistor R17 and the switching transformer Q1. Connected to.

The conventional switching power supply device having such a configuration operates as follows. When the switching transistor Q1 is in the ON state, the DC input voltage V _IN output from the smoothing capacitor C1 is supplied to the primary winding 4a of the switching transformer 4, and the excitation energy is accumulated in the primary winding 4a of the switching transformer 4. On the other hand, when the switching transistor Q1 is in the OFF state, the excitation energy stored in the primary winding 4a of the switching transformer 4 is extracted from the secondary winding 4b of the switching transformer 4. Therefore, the voltage output from the secondary winding 4b of the switching transformer 4 is a rectangular wave AC voltage. The rectangular wave AC voltage is rectified by the diode D1 and smoothed by the smoothing capacitor C2 to become the DC output voltage V _O. Then, the DC output voltage V _O is supplied to the load 5.

The capacitor C4 and the resistor R17 are
A crest value and a phase of a rectangular wave AC voltage generated in the gate winding 4c of the switching transformer 4 are converted to generate a control signal, and the control signal is supplied to the switching transistor Q.
Supply to 1 gate. Switching transistor Q1
Is turned on / off according to this control signal, so that the DC output voltage V _O is feedback controlled.

[0008]

In the switching power supply device of FIG. 11 which performs the above operation, when the DC input voltage V _IN drops, the switching transistor Q1 continues the on / off operation during the period when the voltage is supplied to the gate. Then, the DC output voltage V _O is held at a predetermined value by flowing a current. When the switching transistor Q1 performs on / off operation in the state where the DC input voltage V _IN is lowered as described above, an excessive current flows through the switching transistor Q1 and the switching transistor Q1 may generate heat and be damaged due to thermal runaway. was there.

In view of the above problems, it is an object of the present invention to provide a switching power supply device in which an excessive current does not flow in a switching element even when a DC input voltage is reduced.

[0010]

To achieve the above object, in a switching power supply device according to the present invention, a switching transformer and a switching transformer for converting a DC input voltage into an AC voltage by an ON / OFF operation are provided. A switching element supplied to the primary winding, a conversion means for converting the AC voltage induced in the secondary winding of the switching transformer into a DC voltage and outputting the DC voltage, and an output voltage of the conversion means having a predetermined value. And a control means for controlling ON / OFF of the switching element, and an ON / OFF control for stopping the ON / OFF operation of the switching element regardless of the value of the output voltage of the conversion means when the DC input voltage is smaller than a predetermined value. And an off-operation stopping unit.

Further, the on / off operation stopping means may be means for holding the switching element in the off state regardless of the value of the output voltage of the converting means when the direct current input voltage is smaller than a predetermined value. Good.

Further, the on / off operation stopping means may be means for cutting off the energization of the switching transformer to stop the current supply to the switching element when the DC input voltage is smaller than a predetermined value. .

When the on / off operation stopping means is a means for holding the switching element in the off state regardless of the value of the output voltage of the converting means when the DC input voltage is smaller than a predetermined value, ON / OFF operation stopping means detects a state in which the DC input voltage is lower than a predetermined value, and a state in which the DC input voltage is lower than a predetermined value controlled by the output signal of the detecting means. And a second transistor controlled by an output signal of the first transistor and turned on when the first transistor is in an off state. Is controlled by the output signal of the second transistor to turn off when the second transistor is on. It may be there in Njisuta.

When the on / off operation stopping means is a means for holding the switching element in the off state regardless of the value of the output voltage of the converting means when the DC input voltage is smaller than a predetermined value, ON / OFF operation stopping means detects a state in which the DC input voltage is lower than a predetermined value, and a state in which the DC input voltage is lower than a predetermined value controlled by the output signal of the detecting means. A first transistor that is turned off when is detected, and a relay that is controlled by an output signal of the first transistor and that is closed when the first transistor is turned off. The relay may be a transistor that is turned off when the relay is closed.

When the on / off operation stopping means is a means for holding the switching element in the off state regardless of the value of the output voltage of the converting means when the DC input voltage is smaller than a predetermined value, ON / OFF operation stopping means detects a state in which the DC input voltage is lower than a predetermined value, and a state in which the DC input voltage is lower than a predetermined value controlled by the output signal of the detecting means. A first transistor that is turned off when is detected, and a thyristor that is turned on when the first transistor is off and the gate voltage is equal to or higher than a threshold value. The switching element turns on the thyristor. Alternatively, the transistor may be an off state when in the state.

When the on / off operation stopping means is a means for cutting off the energization of the switching transformer to stop the current supply to the switching element when the DC input voltage is smaller than a predetermined value, The off / off operation stopping means includes a detecting means for detecting a state in which the DC input voltage is smaller than a predetermined value, and a relay for controlling the energization of the switching transformer, and the relay uses the output signal of the detecting means. It may be so arranged that when the control means detects that the DC input voltage is smaller than a predetermined value by the detecting means, it is in an open state to interrupt the energization of the switching transformer.

When the on / off operation stopping means is a means for cutting off the energization of the switching transformer to stop the current supply to the switching element when the DC input voltage is smaller than a predetermined value, the on-off operation is performed. The ON / OFF operation stopping means detects the state where the DC input voltage is smaller than a predetermined value, and the state where the DC input voltage is smaller than the predetermined value controlled by the output signal of the detecting means. A first transistor which is turned off when detected, and a second transistor which is controlled by an output signal of the first transistor and is turned off when the first transistor is off;
The second transistor may control energization of the switching transformer.

When the on / off operation stopping means is a means for cutting off the energization of the switching transformer to stop the current supply to the switching element when the DC input voltage is smaller than a predetermined value, the on-off operation is performed. The ON / OFF operation stopping means detects the state where the DC input voltage is smaller than a predetermined value, and the state where the DC input voltage is smaller than the predetermined value controlled by the output signal of the detecting means. A first transistor that is turned off when detected, and the first transistor may control energization of the switching transformer.

Further, the detecting means may be a Zener diode reverse biased by the DC input voltage.

Further, the detecting means may be a comparator for comparing the DC input voltage with a reference voltage.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described below with reference to the drawings. As a switching power supply device according to the present invention, a switching power supply device including a ringing choke converter will be described as an example. FIG. 1 shows a first embodiment of a switching power supply device according to the present invention. In FIG. 1, the same parts as those of the switching power supply device of FIG. 11 are designated by the same reference numerals, and the description thereof will be omitted.

In the switching power supply device of FIG. 1, between the gate of the switching transistor Q1 and the resistor R17,
An on / off operation stop circuit 3 is provided. The on / off operation stopping circuit 3 uses the voltage across the smoothing capacitor C1 as a drive voltage, that is, the DC input voltage. When the DC input voltage becomes smaller than a predetermined value, the switching transistor Q1
The on / off operation of 1 is stopped to keep the switching transistor Q1 in the off state. This allows the switching transistor Q1 to operate even when the DC input voltage drops.
It is possible to prevent an excessive current from flowing to the switching transistor Q1 and prevent heat generation and thermal runaway of the switching transistor Q1.

The second to fourth embodiments described below clarify specific circuit configuration examples of the on / off operation stopping circuit 3. First, a second embodiment of the switching power supply device according to the present invention will be described.
A second embodiment of the switching power supply device according to the present invention is shown in FIG. 2, the same parts as those of the switching power supply device of FIG. 1 are designated by the same reference numerals, and the description thereof will be omitted.

The switching power supply device of FIG. 2 is provided with an on / off operation stopping circuit including an NPN transistor Q2, an NPN transistor Q3, a resistor R1, a resistor R2 and a zener diode ZD1. The cathode of the Zener diode ZD1 is connected to the positive polarity side of the smoothing capacitor C1, and the anode of the Zener diode ZD1 is connected to the base of the transistor Q2 via the resistor R2. The collector of the transistor Q2 and the base of the transistor Q3 are commonly connected, and the positive side of the smoothing capacitor C1 is connected to the connection node via the resistor R1. Further, the emitter of the transistor Q2 and the emitter of the transistor Q3 are respectively smoothing capacitors C1.
Connected to the negative polarity side of. Further, the collector of the transistor Q3 is connected to the gate of the switching transistor Q1.

The switching power supply device of FIG. 2 provided with the ON / OFF operation stopping circuit having the above-described structure operates as follows. When the voltage across the smoothing capacitor C1, that is, the DC input voltage becomes smaller than a predetermined value, the Zener diode Z
Since the reverse bias applied to D1 is lower than the Zener voltage, the Zener diode ZD1 is turned off, current is not supplied to the base of the transistor Q2, and the transistor Q2 is turned off. When the transistor Q2 is turned off, a voltage is applied to the base of the transistor Q3 via the resistor R1 and the transistor Q3 is turned on. When the transistor Q3 is turned on, the gate-source of the switching transistor Q1 is short-circuited.
The on / off operation of the switching transistor Q1 is stopped, and the switching transistor Q1 is held in the off state. As a result, it is possible to prevent an excessive current from flowing through the switching transistor Q1 even when the DC input voltage drops, and prevent heat generation and thermal runaway of the switching transistor Q1.

Next, a third embodiment of the switching power supply device according to the present invention will be described. FIG. 3 shows a third embodiment of the switching power supply device according to the present invention. In addition,
3, the same parts as those of the switching power supply device of FIG. 2 are designated by the same reference numerals, and the description thereof will be omitted.

The switching power supply device of FIG. 3 has a configuration in which the transistor Q3 of the switching power supply device of FIG. 2 is replaced with a relay 6. Therefore, the switching power supply device of FIG. 3 has the transistor Q2, the relay 6, and the resistor R1.
And a resistor R2 and a Zener diode ZD1. One end of the coil of the relay 6 is commonly connected to the collector of the transistor Q2, and the positive side of the smoothing capacitor C1 is connected to the connection node via the resistor R1. On the other hand, the other end of the coil of the relay 6 is connected to the negative polarity side of the smoothing capacitor C1.
Further, one end of the relay 6 has a switching transistor Q1.
And the other end of the relay 6 is connected to the negative polarity side of the smoothing capacitor C1.

The switching power supply device of FIG. 3 including the on / off operation stopping circuit having the above-described configuration operates as follows. When the voltage across the smoothing capacitor C1, that is, the DC input voltage becomes smaller than a predetermined value, the Zener diode Z
Since the reverse bias applied to D1 is lower than the Zener voltage, the Zener diode ZD1 is turned off, current is not supplied to the base of the transistor Q2, and the transistor Q2 is turned off. When the transistor Q2 is turned off, a voltage is applied to the coil of the relay 6 via the resistor R1 and the relay 6 is closed. When the relay 6 is closed, the gate-source of the switching transistor Q1 is short-circuited, so that the on / off operation of the switching transistor Q1 is stopped and the switching transistor Q1 is held in the off state. As a result, even when the DC input voltage drops, the switching transistor Q
It is possible to prevent an excessive current from flowing through the switch 1 and prevent heat generation and thermal runaway of the switching transistor Q1.

Next, a fourth embodiment of the switching power supply device according to the present invention will be described. A fourth embodiment of the switching power supply device according to the present invention is shown in FIG. In addition,
4, the same parts as those of the switching power supply device of FIG. 2 are designated by the same reference numerals and the description thereof will be omitted.

The switching power supply device of FIG. 4 has a configuration in which the transistor Q3 of the switching power supply device of FIG. 2 is replaced with a thyristor 7, a capacitor C3, and a resistor R3. Therefore, the switching power supply device of FIG. 4 includes an on / off operation stop circuit including a transistor Q2, a thyristor 7, a capacitor 3, resistors R1 to R3, and a zener diode ZD1. The gate of the thyristor 7, one end of the capacitor C3, and one end of the resistor R3 are commonly connected to the collector of the transistor Q2, and the connection node is connected to the positive side of the smoothing capacitor C1 via the resistor R1. The cathode of the thyristor 7, the other end of the capacitor C3, and the other end of the resistor R3 are connected to the negative polarity side of the smoothing capacitor C1. Further, the anode of the thyristor 7 is connected to the gate of the switching transistor Q1.

The switching power supply device of FIG. 4 provided with the ON / OFF operation stopping circuit having the above-mentioned configuration operates as follows. When the voltage across the smoothing capacitor C1, that is, the DC input voltage becomes smaller than a predetermined value, the Zener diode Z
Since the reverse bias applied to D1 is lower than the Zener voltage, the Zener diode ZD1 is turned off, current is not supplied to the base of the transistor Q2, and the transistor Q2 is turned off. When the transistor Q2 is turned off, a voltage is applied to the gate of the thyristor 7 by the voltage division of the resistors R1 and R3, so that the thyristor 7 is turned on. When the thyristor 7 is turned on, the gate-source of the switching transistor Q1 is short-circuited, so that the on / off operation of the switching transistor Q1 is stopped and the switching transistor Q1 is held in the off state. As a result, it is possible to prevent an excessive current from flowing through the switching transistor Q1 even when the DC input voltage drops, and prevent heat generation and thermal runaway of the switching transistor Q1.

Next, a fifth embodiment of the switching power supply device according to the present invention will be described. A fifth embodiment of the switching power supply device according to the present invention is shown in FIG. In addition,
5, the same parts as those of the switching power supply device of FIG. 2 are designated by the same reference numerals and the description thereof will be omitted.

The switching power supply device of FIG. 5 has a configuration in which the zener diode ZD1 and the resistor R2 of the switching power supply device of FIG. 2 are replaced with a comparator 8, resistors R4 to R8, and a zener diode ZD2. Therefore, FIG.
The switching power supply device includes a transistor Q2, a transistor Q3, a resistor R1, resistors R4 to R8, and a zener diode ZD2.

One end of the resistor R4 is connected to the positive polarity side of the smoothing capacitor C1, and the other end of the resistor R4 is connected to the negative polarity side of the smoothing capacitor C1 via the resistor R6. Resistance R4
The non-inverting input terminal of the comparator 8 is connected to the connection node between the resistor R6 and the resistor R6.

The cathode of the Zener diode ZD2 is connected to the positive side of the smoothing capacitor C1 via the resistor R5, and the anode of the Zener diode ZD2 is connected to the negative side of the smoothing capacitor C1. One end of the resistor R7 is connected to the connection node between the resistor R5 and the Zener diode ZD2, and the other end of the resistor R7 is connected to the negative polarity side of the smoothing capacitor C1 via the resistor R8. The inverting input terminal of the comparator 8 is connected to the connection node between the resistors R7 and R8.

The output terminal of the comparator 8 is connected to the base of the transistor Q2.

The switching power supply device of FIG. 5 provided with the ON / OFF operation stopping circuit having the above-mentioned configuration operates as follows. A predetermined reference voltage is input to the inverting input terminal of the comparator 8. The reference voltage is a voltage division of the Zener voltage of the Zener diode ZD2. On the other hand, the voltage division of the DC input voltage is input to the non-inverting input terminal of the comparator 8. Since the resistance values of the resistors R4 to R8 are set so that the voltage input to the inverting input terminal and the voltage input to the non-inverting input terminal are equal when the DC input voltage has a predetermined value, the DC input When the voltage becomes lower than the predetermined value, the comparator 8 outputs a low level signal. When the comparator 8 outputs a low level signal, no current is supplied to the base of the transistor Q2, and the transistor Q2 is turned off. When the transistor Q2 is turned off, a voltage is applied to the base of the transistor Q3 via the resistor R1 and the transistor Q3 is turned on. When the transistor Q3 is turned on, the gate-source of the switching transistor Q1 is short-circuited, so that the on / off operation of the switching transistor Q1 is stopped and the switching transistor Q1 is held in the off state. As a result, it is possible to prevent an excessive current from flowing through the switching transistor Q1 even when the DC input voltage drops, and prevent heat generation and thermal runaway of the switching transistor Q1.

In the switching power supply device of the fifth embodiment, the zener diode ZD1 and the resistor R2 of the switching power supply device of the second embodiment are connected to the comparator 8 and the resistor R4.
.About.R8 and the Zener diode ZD2 are substituted, the Zener diode ZD1 and the resistor R2 of the switching power supply device of the third embodiment and the fourth embodiment similarly have a comparator 8, resistors R4 to R8, and It can be replaced with a Zener diode ZD2.

Next, a sixth embodiment of the switching device according to the present invention will be described. FIG. 6 shows a sixth embodiment of the switching device according to the present invention. In FIG. 6, the same parts as those of the switching power supply device of FIG. 11 are designated by the same reference numerals, and the description thereof will be omitted.

The switching power supply device of FIG. 6 is provided with a power supply line disconnecting circuit 9 between the positive side of the smoothing capacitor C1 and the primary winding 4a of the switching transformer 4. The power supply line disconnecting circuit 9 uses the voltage across the smoothing capacitor C1 as a drive voltage, that is, the DC input voltage. When the DC input voltage becomes lower than a predetermined value, the positive polarity side of the smoothing capacitor C1 and the primary winding 4 of the switching transformer 4 are connected.
The connection of a is cut off to stop the current supply to the switching transistor Q1. Therefore, when the DC input voltage becomes smaller than the predetermined value, the switching transistor Q1 stops the on / off operation. As a result, it is possible to prevent an excessive current from flowing through the switching transistor Q1 even when the DC input voltage drops, and prevent heat generation and thermal runaway of the switching transistor Q1.

The seventh to tenth embodiments described below clarify specific circuit configuration examples of the power supply line disconnecting circuit 4. First, a seventh embodiment of the switching power supply device according to the present invention will be described. FIG. 7 shows a seventh embodiment of the switching power supply device according to the present invention. In FIG. 7, the same parts as those of the switching power supply device of FIG. 6 are designated by the same reference numerals and the description thereof is omitted.

The switching power supply device shown in FIG.
0, a resistor R9, and a Zener diode ZD3 are included in the power supply line disconnecting circuit. The cathode of the Zener diode ZD3 is connected to the positive side of the smoothing capacitor C1, and the anode of the Zener diode ZD3 is connected to one end of the coil of the relay 10 via the resistor R9.
The other end of the coil of the relay 10 has the smoothing capacitor C.
1 is connected to the negative polarity side. Furthermore, smoothing capacitor C
One end of the relay 10 is connected to a connection node between 1 and the Zener diode ZD3, and the other end of the relay 10 is connected to the primary winding 4a of the switching transformer 4.

The switching power supply device of FIG. 7 provided with the power supply line disconnection circuit having the above configuration operates as follows. When the voltage across the smoothing capacitor C1, that is, the DC input voltage becomes smaller than a predetermined value, the Zener diode ZD
Since the reverse bias applied to 3 is lower than the Zener voltage, the Zener diode ZD3 is turned off, the coil of the relay 10 is not supplied with current, and the relay 10 is opened. When the relay 10 is in the open state, the connection between the smoothing capacitor C1 and the primary winding 4a of the switching transformer 4 is cut off, and the switching transistor Q1 is not supplied with current. As a result, it is possible to prevent an excessive current from flowing through the switching transistor Q1 even when the DC input voltage drops, and prevent heat generation and thermal runaway of the switching transistor Q1.

Next, an eighth embodiment of the switching power supply device according to the present invention will be described. FIG. 8 shows an eighth embodiment of the switching power supply device according to the present invention. In addition,
8, the same parts as those of the switching power supply device of FIG. 7 are designated by the same reference numerals and the description thereof will be omitted.

In the switching power supply device of FIG. 8, the relay 10 of the switching power supply device of FIG.
A transistor Q4, an NPN transistor Q5,
And resistor R5. Therefore, FIG.
The switching power supply device is provided with a power supply line disconnection circuit including a transistor Q4, a transistor Q5, a resistor R9, a resistor R10, and a zener diode ZD3. Smoothing capacitor C1 and Zener diode ZD
The drain of the transistor Q4 and one end of the resistor R10 are connected to the connection node with the transistor 3, and the source of the transistor Q4 is connected to the primary winding 4a of the switching transformer 4. The other end of the resistor R10 is connected to the gate of the transistor Q4 and the collector of the transistor Q5. The base of the transistor Q5 is connected to the anode of the Zener diode ZD3 via the resistor R9, and the transistor Q5
Is connected to the negative side of the smoothing capacitor C1.

The switching power supply device of FIG. 8 provided with the power supply line disconnection circuit having such a configuration operates as follows. When the voltage across the smoothing capacitor C1, that is, the DC input voltage becomes smaller than a predetermined value, the Zener diode ZD
Since the reverse bias applied to the transistor 3 is below the Zener voltage, the Zener diode ZD3 is turned off, and the current is not supplied to the base of the transistor Q5.
5 is turned off. When the transistor Q5 is turned off, the drain voltage and the gate voltage of the transistor Q4 have the same potential, and the transistor Q4 is turned off.
When the transistor Q4 is turned off, the connection between the smoothing capacitor C1 and the primary winding 4a of the switching transformer 4 is cut, and the switching transistor Q1 is not supplied with current. As a result, it is possible to prevent an excessive current from flowing through the switching transistor Q1 even when the DC input voltage drops, and prevent heat generation and thermal runaway of the switching transistor Q1.

Next, a ninth embodiment of the switching power supply device according to the present invention will be described. FIG. 9 shows a ninth embodiment of the switching power supply device according to the present invention. In addition,
9, the same parts as those of the switching power supply device of FIG. 7 are designated by the same reference numerals and the description thereof will be omitted.

The switching power supply device of FIG. 9 has a configuration in which the relay 10 of the switching power supply device of FIG. 7 is replaced with an NPN transistor Q6 and a resistor R11. Therefore, the switching power supply device of FIG.
, Resistor R9, resistor R11, Zener diode Z
And a power line disconnection circuit composed of D3. The collector of the transistor Q6 is connected to the connection node between the smoothing capacitor C1 and the Zener diode ZD3, and the primary winding 4a of the switching transformer 4 is connected to the emitter of the transistor Q6 and one end of the resistor R11. The other end of the resistor R11 and the base of the transistor Q6 are connected to the resistor R
It is connected via 9 to the anode of the Zener diode ZD3.

The switching power supply device of FIG. 9 provided with the power supply line disconnection circuit having such a configuration operates as follows. When the voltage across the smoothing capacitor C1, that is, the DC input voltage becomes smaller than a predetermined value, the Zener diode ZD
Since the reverse bias applied to the transistor 3 falls below the zener voltage, the zener diode ZD3 is turned off, and the current is not supplied to the base of the transistor Q6.
6 is turned off. When the transistor Q6 is turned off, the connection between the smoothing capacitor C1 and the primary winding 4a of the switching transformer 4 is cut off, and the switching transistor Q1 is not supplied with current. As a result, it is possible to prevent an excessive current from flowing through the switching transistor Q1 even when the DC input voltage drops, and prevent heat generation and thermal runaway of the switching transistor Q1.

Next, a tenth embodiment of the switching power supply device according to the present invention will be described. FIG. 10 shows a tenth embodiment of the switching power supply device according to the present invention. 10, the same parts as those of the switching power supply device of FIG. 8 are designated by the same reference numerals, and the description thereof will be omitted.

The switching power supply device of FIG. 10 is obtained by replacing the zener diode ZD3 and the resistor R9 of the switching power supply device of FIG. 8 with a comparator 11, resistors R12 to R16, and a zener diode ZD4. Therefore, the switching power supply device of FIG.
4, a transistor Q5, a resistor R10, and a resistor R12
.About.R16 and a Zener diode ZD4.

One end of the resistor R12 is connected to the positive polarity side of the smoothing capacitor C1, and the other end of the resistor R12 is connected to the negative polarity side of the smoothing capacitor C1 via the resistor R13. The non-inverting input terminal of the comparator 8 is connected to the connection node between the resistors R12 and R13.

The cathode of the Zener diode ZD4 is connected to the positive side of the capacitor C1 via the resistor R14, and the anode of the Zener diode ZD4 is connected to the negative side of the smoothing capacitor C1. One end of the resistor R15 is connected to the connection node between the resistor R14 and the Zener diode ZD4, and the other end of the resistor R15 is connected to the negative side of the smoothing capacitor C1 via the resistor R16. The inverting input terminal of the comparator 11 is connected to the connection node between the resistors R15 and R16.

The output terminal of the comparator 11 is connected to the base of the transistor Q5.

The switching power supply device of FIG. 10 equipped with the power supply line disconnection circuit having such a configuration operates as follows. A predetermined reference voltage is input to the inverting input terminal of the comparator 11. The reference voltage is a voltage division of the Zener voltage of the Zener diode ZD4. On the other hand, the voltage division of the DC input voltage is input to the non-inverting input terminal of the comparator 11. The resistance values of the resistors R12 to R16 are set so that the voltage input to the inverting input terminal of the comparator 11 and the voltage input to the non-inverting input terminal of the comparator 11 become equal when the DC input voltage has a predetermined value. Is set, the comparator 11 outputs a low level signal when the DC input voltage becomes lower than a predetermined value.
When the comparator 11 outputs a low level signal, no current is supplied to the base of the transistor Q5, and the transistor Q5 is turned off. When the transistor Q5 is turned off, the drain voltage and the gate voltage of the transistor Q4 have the same potential, and the transistor Q4 is turned off. When the transistor Q4 is turned off, the connection between the smoothing capacitor C1 and the primary winding 4a of the switching transformer 4 is cut, and the switching transistor Q1 is not supplied with current. As a result, it is possible to prevent an excessive current from flowing through the switching transistor Q1 even when the DC input voltage drops, and prevent heat generation and thermal runaway of the switching transistor Q1.

In the switching power supply device of the tenth embodiment, the zener diode ZD3 and the resistor R9 of the switching power supply device of the eighth embodiment are connected to the comparator 11 and the resistor R1.
2 to R16 and the Zener diode ZD4, but the Zener diode ZD3 and the resistor R9 of the switching power supply device according to the seventh or ninth embodiment.
Similarly, the comparator 11, the resistors R12 to R16, and the Zener diode ZD4 can be substituted.

[0057]

As described above, the switching power supply device according to the present invention is a switching transformer and a switching transformer that converts a DC input voltage into an AC voltage by an on / off operation and supplies the AC voltage to the primary winding of the switching transformer. An element, a conversion means for converting the AC voltage induced in the secondary winding of the switching transformer into a DC voltage and outputting the DC voltage, and turning on / off the switching element so that the output voltage of the conversion means becomes a predetermined value. Control means for controlling, and on / off operation stopping means for stopping the on / off operation of the switching element regardless of the value of the output voltage of the converting means when the direct current input voltage is smaller than a predetermined value. It is configured.

With this structure, it is possible to stop the ON / OFF operation of the switching element when the DC input voltage drops below a predetermined value.
Therefore, it is possible to prevent an excessive current from flowing through the switching element even when the DC input voltage decreases, and prevent heat generation and thermal runaway of the switching element. This improves the safety and reliability of the switching power supply device.

Further, according to the present invention, the ON / OFF operation stopping means holds the switching element in the OFF state regardless of the value of the output voltage of the converting means when the DC input voltage is smaller than a predetermined value. I will make a means to
When the DC input voltage becomes smaller than a predetermined value, the on / off operation of the switching element is stopped and the switching element is held in the off state. This makes it possible to prevent an excessive current from flowing through the switching element even when the DC input voltage drops, and prevent heat generation and thermal runaway of the switching element.

Further, according to the present invention, the on / off operation stopping means cuts off the energization of the switching transformer to stop the current supply to the switching element when the DC input voltage is smaller than a predetermined value. Therefore, when the DC input voltage becomes smaller than a predetermined value, the switching transformer is de-energized and the current supply to the switching element is stopped. Therefore, when the DC input voltage becomes lower than a predetermined value, the switching element stops on / off operation. This makes it possible to prevent an excessive current from flowing through the switching element even when the DC input voltage drops, and prevent heat generation and thermal runaway of the switching element.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment of a switching power supply device according to the present invention.

FIG. 2 is a diagram showing a second embodiment of a switching power supply device according to the present invention.

FIG. 3 is a diagram showing a third embodiment of a switching power supply device according to the present invention.

FIG. 4 is a diagram showing a fourth embodiment of a switching power supply device according to the present invention.

FIG. 5 is a diagram showing a fifth embodiment of a switching power supply device according to the present invention.

FIG. 6 is a diagram showing a sixth embodiment of a switching power supply device according to the present invention.

FIG. 7 is a diagram showing a seventh embodiment of a switching power supply device according to the present invention.

FIG. 8 is a diagram showing an eighth embodiment of a switching power supply device according to the present invention.

FIG. 9 is a diagram showing a ninth embodiment of a switching power supply device according to the present invention.

FIG. 10 is a diagram showing a tenth embodiment of the switching power supply device according to the present invention.

FIG. 11 is a diagram showing a configuration example of a conventional switching power supply device.

[Explanation of symbols]

3 ON / OFF stop circuit 4 switching transformer 6, 10 relay 7 Thyristor 8, 11 comparator 9 Power line disconnection circuit C2 capacitor D1 diode Q1 switching transistor Q2-Q6 transistors ZD1 to ZD4 Zener diode

Claims (11)

[Claims] 1. A switching transformer, a switching element which converts a DC input voltage into an AC voltage by an ON / OFF operation and supplies the AC voltage to a primary winding of the switching transformer, and an induction in a secondary winding of the switching transformer. A switching power supply device comprising: a converting unit that converts the AC voltage into a DC voltage and outputs the DC voltage; and a control unit that controls ON / OFF of the switching element so that the output voltage of the converting unit has a predetermined value. A switching power supply device comprising an on / off operation stopping means for stopping the on / off operation of the switching element regardless of the value of the output voltage of the converting means when the DC input voltage is smaller than a predetermined value. . 2. The on / off operation stopping means is means for holding the switching element in an off state regardless of the value of the output voltage of the converting means when the DC input voltage is smaller than a predetermined value. Item 2. The switching power supply device according to Item 1. 3. The on / off operation stopping means is means for cutting off the energization of the switching transformer to stop the current supply to the switching element when the DC input voltage is smaller than a predetermined value. 1. The switching power supply device according to 1. 4. The on / off operation stopping means detects a state in which the DC input voltage is smaller than a predetermined value, and the output signal of the detecting means controls the DC input voltage by the detecting means. A first transistor which is turned off when a state smaller than a predetermined value is detected, and a second transistor which is controlled by an output signal of the first transistor and is turned on when the first transistor is off. 3. The switching power supply device according to claim 2, further comprising: a switching element that is controlled by an output signal of a second transistor and is turned off when the second transistor is on. 5. The on / off operation stopping means detects a state in which the DC input voltage is smaller than a predetermined value, and the output signal of the detecting means controls the DC input voltage by the detecting means. A first transistor which is turned off when a state smaller than a predetermined value is detected, and a relay which is controlled by an output signal of the first transistor and is closed when the first transistor is off. 3. The switching power supply device according to claim 2, further comprising a transistor, wherein the switching element is a transistor that is turned off when the relay is in a closed state. 6. The on / off operation stopping means detects a state in which the DC input voltage is smaller than a predetermined value, and the output signal of the detecting means controls the DC input voltage by the detecting means. The switching element includes: a first transistor that is turned off when a state smaller than a predetermined value is detected; and a thyristor that is turned on when the first transistor is off and the gate voltage is equal to or higher than a threshold value. The switching power supply device according to claim 2, wherein is a transistor that is turned off when the thyristor is on. 7. The on / off operation stopping means includes a detecting means for detecting a state where the DC input voltage is smaller than a predetermined value, and a relay for controlling energization of the switching transformer, wherein the relay comprises: The control according to the output signal of the detection means, and when the detection means detects a state in which the DC input voltage is smaller than a predetermined value, the open state is established to interrupt the energization of the switching transformer. Switching power supply. 8. The on / off operation stopping means detects a state in which the DC input voltage is smaller than a predetermined value, and the output signal of the detecting means controls the DC input voltage by the detecting means. A first transistor which is turned off when a state smaller than a predetermined value is detected, and a second transistor which is controlled by an output signal of the first transistor and is turned off when the first transistor is in the off state The switching power supply device according to claim 3, further comprising: a second transistor that controls energization of the switching transformer. 9. The on / off operation stopping means detects a state in which the DC input voltage is smaller than a predetermined value, and the output signal of the detecting means controls the DC input voltage by the detecting means. The 1st transistor which will be in an OFF state when the state smaller than a predetermined value is detected, The said 1st transistor controls electricity supply of the said switching transformer, The switching power supply device of Claim 3. 10. The switching power supply device according to claim 4, wherein the detection means is a Zener diode reverse biased by the DC input voltage. 11. The switching power supply device according to claim 4, wherein the detection means is a comparator that compares the DC input voltage with a reference voltage.