JP3534313B2 - Switching power supply - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a self-excited switching power supply device in which a transformer is provided with a feedback winding and an overcurrent control is performed by a voltage generated in the feedback winding.

[0002]

2. Description of the Related Art A self-excited switching power supply device having a feedback winding keeps self-excited oscillation with a voltage generated in the feedback winding, so that it is not necessary to add an oscillation circuit composed of an IC or the like. Can be configured with a simple circuit. For this reason, it can be made small in size and low in price, and is therefore widely used in the field of small-capacity switching power supplies.

Various proposals have been made for a self-excited switching power supply device having a feedback winding, and examples thereof are described in JP-A-63-87170 and JP-A-6-178539. Technology is known. This type of self-excited switching power supply device includes a transformer having a primary winding, a secondary winding, a feedback winding, and a control winding if necessary, and a switching transistor for connecting and disconnecting the current of the primary winding. A control transistor for controlling a positive feedback signal from the feedback winding to the switching transistor, charging and discharging an electromotive voltage of the feedback winding or the control winding with a predetermined time constant, and And a time constant charging / discharging circuit for applying a control voltage to the control circuit.

In this type of self-excited oscillation type switching power supply device, the time constant charging / discharging circuit is composed of a resistor and a capacitor. The capacitor is charged by the voltage induced in the feedback winding during the ON period of the switching transistor via the time constant charging / discharging circuit. The capacitor is discharged by a flyback voltage induced in the feedback winding during the off period of the switching transistor via the time constant charging / discharging circuit.

The charge / discharge of the capacitor changes according to the fluctuation of the output voltage due to the fluctuation of the input or the fluctuation of the load. The charging voltage of the capacitor determines the on-timing of the control transistor, controls the switching transistor, and provides the switching power supply device with output voltage stabilization and overcurrent control functions.

However, the control transistor is composed of an NPN type bipolar transistor, and its collector is connected to one end of the feedback winding, which is an unstable potential. Therefore, when the collector potential of the control transistor changes to a negative potential, a discharge current may flow from the capacitor from the base of the control transistor to the collector. Due to this discharge current, the control transistor is turned on, and a reverse current flows from the emitter to the collector. This reverse current forms a discharge path different from the time constant charging / discharging circuit and causes a problem that charging / discharging by the time constant charging / discharging circuit is not performed accurately.

The NPN bipolar transistor has a P
A collector and an emitter made of an N-type semiconductor are arranged on both sides of a base made of a type semiconductor via PN junctions. In this structure, P
When the base of the type semiconductor is biased to a positive voltage and the collector of the N type semiconductor is swung to a negative potential, a phenomenon occurs in which current flows from the base of the P type semiconductor to the collector of the N type semiconductor. At this time, the transistor is turned on and current flows from the emitter to the collector.

The current flowing from the base to the collector is Ib
c and Iec is the current flowing from the emitter to the collector, Iec / Ibc can be said to be hfe in the reverse direction, and the current Iec is a value of hfe times the reverse direction of the current Ibc. These values are not values controlled as transistor specifications, and are values not listed in the catalog. Moreover, since these values greatly vary depending on the type of transistor and the variation between elements, they cannot be grasped as design constants.

In the above self-excited oscillation type switching power supply device, the capacitor is discharged through the time constant charge / discharge circuit by the flyback voltage induced in the feedback winding during the off period of the switching transistor. Be seen. At this time, when the discharge current of the capacitor flows from the base of the control transistor toward the collector, a discharge path via the control transistor different from the time constant charging / discharging circuit is formed. Since this discharge path cannot be grasped as a design constant as described above, it causes a problem that the time constant circuit is not discharged accurately. This problem causes a shift in the on-timing of the control transistor, and causes fluctuations in the drooping point and fluctuations in the drooping curve during overcurrent control.

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems and to provide a self-excited switching power supply device capable of performing accurate overcurrent control.

[0011]

In order to solve the above problems, a self-excited switching power supply device according to the present invention includes a transformer, a switching element, an output rectifying / smoothing circuit, and a control circuit.

The transformer comprises an input winding, an output winding, and
And a feedback winding.

The switching element interrupts a DC voltage supplied through the input winding to induce a voltage in the output winding and the feedback winding.

The feedback winding biases the switching element in the conducting direction with a voltage induced during the ON period of the switching element.

The output rectifying / smoothing circuit rectifies and smoothes the voltage induced in the output winding during the OFF period of the switching element and outputs the voltage.

The control circuit includes a time constant charging / discharging circuit, a switching control transistor, and an ON blocking diode.

The time constant charging / discharging circuit includes a series circuit of a resistor and a capacitor, and is connected between both ends of the feedback winding.

The switching control transistor is
The base is connected to the connection point between the resistor and the capacitor, and the main electrode is connected to the control input side of the switching element.

The on-blocking diode is connected to the switching control transistor and blocks reverse switching on of the switching control transistor when the capacitor of the time constant charging / discharging circuit is discharged.

In the above switching power supply device,
The transformer includes an input winding, an output winding, and a feedback winding. Since the switching element interrupts the DC voltage supplied through the input winding, when the switching element starts to turn on, an input current starts flowing in the input winding, and the output winding and the feedback winding are connected to each other. Induces a voltage on. The feedback winding biases the switching element in the conduction direction with the voltage induced during the ON period of the switching element, so that the switching element is completely conducted and the current flowing through the input winding increases. The voltage induced during the ON period of the switching element charges the capacitor.

After that, when the voltage of the capacitor is charged to the threshold voltage of the switching control transistor, the switching control transistor is momentarily turned on and the switching element is turned off. When the switching element is turned off, a flyback voltage in the opposite direction is induced in the input winding, the output winding, and the feedback winding. The flyback voltage decreases with the disappearance of the flyback energy, generating a ringing voltage. The ringing voltage is also generated in the feedback winding and gives a trigger to turn on the switching element. After that, the self-excited oscillation continues with this repetition.

Since the output rectifying / smoothing circuit rectifies and smoothes the voltage induced in the output winding during the OFF period of the switching element and outputs the DC voltage, the DC voltage can be continuously output.

The control circuit includes a time constant charging / discharging circuit, a switching control transistor, and an ON blocking diode.

Since the time constant charging / discharging circuit includes a series circuit of a resistor and a capacitor and is connected between both ends of the feedback winding, the capacitor is induced in the feedback winding during the ON period of the switching element. Is charged with the above voltage and discharged with the voltage induced in the feedback winding during the OFF period of the switching element.

The switching control transistor is
The base is connected to the connection point between the resistor and the capacitor, and the main electrode is connected to the control input side of the switching element. Therefore, when the voltage of the capacitor is charged to the threshold voltage of the switching control transistor, the switching control transistor momentarily turns on and turns off the switching element.

The on-blocking diode is connected to the switching control transistor and blocks reverse switching on of the switching control transistor when the capacitor of the time constant charging / discharging circuit is discharged. Therefore, when the capacitor is discharged. The switching control transistor does not turn on in the reverse direction. Therefore, the discharge path passing through the switching control transistor is not formed, and the capacitor can be charged and discharged accurately.

In the above structure, when an overcurrent state is reached due to an overload or a short circuit of the load, the switching control transistor is turned on by the charging voltage of the capacitor of the time constant charging / discharging circuit to turn on the switching element. Off control is performed and overcurrent control is performed. In the present invention, since the capacitor can be charged and discharged accurately, there is no shift in the on-timing of the switching control transistor, and accurate overcurrent control can be performed.

The switching control transistor is set to N
A PN transistor may be used, and the ON blocking diode may be connected between the collector of the switching control transistor and the control electrode of the switching element. According to this structure, since the discharge current flowing from the base of the switching control transistor to the collector can be blocked at the time of discharging the capacitor, it is possible to prevent the switching control transistor from turning on in the reverse direction and at the same time, except for the time constant charge / discharge circuit. No discharge path is formed and the capacitor can be charged and discharged accurately.

The on-blocking diode may be connected between the base and collector of the switching control transistor. According to this structure, a discharge path that passes through the on-blocking diode is formed at the time of discharging the capacitor, so that the switching control transistor can be prevented from turning on in the reverse direction. The constant of the discharge path passing through the on-blocking diode is a numerical value that can be grasped as a design value, and the discharge current can be calculated accurately, so that the capacitor can be charged and discharged accurately.

The present invention further discloses a circuit which uses the first and second switching control transistors and which prevents the first switching control transistor from turning on in the reverse direction by the second switching control transistor. .

Other objects, configurations and advantages of the present invention will be described in more detail with reference to the accompanying drawings. The drawings are merely examples.

[0032]

FIG. 1 is an electric circuit diagram showing an embodiment of a self-excited switching power supply device according to the present invention.
The illustrated self-excited switching power supply device includes a transformer 1 for power conversion, a switching element 2, an output rectifying / smoothing circuit 3, an output voltage detection circuit 4, and a control circuit 5.

The transformer 1 comprises an input winding 11 and an output winding 1
2 and the feedback winding 13. The input winding 11 is
DC input terminal Ti via the main electrode of the switching element 2
connected to n.

A DC voltage source E is connected to the DC input terminal Tin. The DC voltage source E may be an internal element or an external element of the present invention, and may be a battery, another DC voltage source, or a voltage obtained by converting an AC voltage into a DC voltage through a rectifier circuit. Available.

The switching element 2 connects and disconnects the DC voltage Vin supplied through the input winding 11 to output the output winding 1.
2 and the feedback winding 13 induce a voltage. The switching element 2 only needs to be able to switch the supplied DC voltage Vin at a high frequency, and typically a semiconductor element having a control electrode such as a bipolar transistor or a field effect transistor is used. This embodiment is an N-channel MO
An S-type field effect transistor is used.

The feedback winding 13 is connected between the gate and the source of the switching element 2 via the characteristic improving circuit 6, and biases the gate of the switching element 2 in the conduction direction by the voltage induced during the ON period of the switching element 2. . The characteristic improving circuit 6 is a circuit for improving the switching characteristic of the switching element 2, and includes a capacitor C6, a resistor R6, and diodes D61 and D62. The gate of the switching element 2 is further connected to the input line via resistors R7 and R8.

The output rectifying / smoothing circuit 3 is a rectifying diode D.
3 and a smoothing capacitor C3. The rectifying diode D3 and the smoothing capacitor C3 are connected in series, and both ends thereof are connected to the output winding 12. Both ends of the smoothing capacitor C3 are output ends of the output rectifying / smoothing circuit 3 and are connected to the output terminal Tout of the switching power supply device. The direction of the rectifying diode D3 is the output winding 1
It is oriented so as to be forward with respect to the flyback voltage induced in 2.

With this configuration, the output rectifying / smoothing circuit 3 is
It rectifies and smoothes the voltage induced in the output winding 12 during the off period of the switching element 2 and outputs it. Rectifying diode D
If a switching element having a control electrode such as a bipolar transistor or a field effect transistor is used instead of 3, a synchronous rectification type rectifying / smoothing circuit can be configured.

The output voltage detecting circuit 4 is connected to the output terminal of the output rectifying / smoothing circuit 3, that is, the output terminal Tout of the switching power supply device, detects the output voltage Vout, and supplies the output voltage signal to the control circuit 5. In this embodiment, the transistor Q4, the shunt regulator IC4 and the resistor R
41, R42, R43, R44, R45, etc., and supplies an output voltage signal to the control circuit 5 from the collector of the transistor Q4. The output voltage detection circuit 4 is not an essential element of the present invention, but a conventionally known circuit can be appropriately used.

The control circuit 5 includes a time constant charging / discharging circuit 51,
It is configured to include a time constant control circuit 53, a switching control transistor 54, and an on-blocking diode D5. The time constant charging / discharging circuit 51 includes a series circuit in which one ends of a resistor R51 and a capacitor C5 are connected, and the other ends thereof are connected between both ends of the feedback winding 13.

The time constant control circuit 53 includes a transistor Q5.
3 and the diode D53 and the resistor R53, the main electrode of the transistor Q53 is the resistor R5 of the time constant charging / discharging circuit 51.
Diode D53 and resistor R53 in parallel with 1
Connected via and. The output voltage signal is input to the control electrode of the transistor Q53.

Although the time constant control circuit 53 is not an essential element of the present invention like the output voltage detection circuit 4, it is coupled to the output voltage detection circuit 4 and the transistor Q5 is connected according to the output voltage.
By changing the impedance of 3, the time constant charging / discharging circuit 51
Control the time constant of. The coupling form with the output voltage detection circuit 4 may be an optical coupling circuit in which the transistor Q53 is a phototransistor and a photodiode is used as the output voltage detection circuit 4.

The switching control transistor 54 is
It is composed of an NPN transistor, the collector of which is connected to the gate of the switching element 2 through the on-blocking diode D5, the emitter of which is connected to the source of the switching element 2 and the base of which is connected to one end of the capacitor C5. The on-blocking diode D5 has its cathode connected to the collector of the switching control transistor 54 and blocks a reverse current from flowing through the switching control transistor 54.

In the self-excited switching power supply device of this embodiment having the above-described structure, when the DC voltage Vin is applied to the input terminal Tin, the gate of the switching element 2 is energized via the starting resistor R7. When the switching element 2 starts to turn on, the input winding 11 of the transformer 1
An input current begins to flow into the output winding 12 and induces a voltage in the output winding 12 and the feedback winding 13.

Since the voltage induced in the output winding 12 is blocked by the diode D3 of the output rectifying / smoothing circuit 3, the output current does not flow in the output winding 12, and the energy is reduced by the transformer 1.
Accumulated in. The voltage induced in the feedback winding 13 biases the gate of the switching element 2 in the conduction direction via the characteristic improving circuit 6, so that the switching element 2 is rapidly turned on and the input current flowing in the input winding 11 is reduced. Increases over time.

The voltage induced in the feedback winding 13 is
Resistance R51 of time constant charge / discharge circuit 51 and time constant control circuit 5
The capacitor C5 is charged via 3 and. After that, the voltage of the capacitor C5 changes to the switching control transistor 5
When the threshold voltage of 4 is charged, the switching control transistor 54 is turned on to short-circuit the control input side of the switching element 2. As a result, the switching element 2 is turned off, and a flyback voltage in the opposite direction to that before is generated in the input winding 11, the output winding 12, and the feedback winding 13.

The flyback voltage generated in the output winding 12 is rectified and smoothed by the output rectifying / smoothing circuit 3, and the DC output voltage Vout is output to the output terminal Tout.

The flyback voltage generated in the feedback winding 13 discharges the charge stored in the capacitor C5 through the resistor R51 of the time constant charging / discharging circuit 51. After that, the flyback voltage decreases with the disappearance of the flyback energy, generating a ringing voltage. The ringing voltage is
It is also generated in the feedback winding 13, and gives an opportunity for the switching element 2 to start turning on.

After that, the self-excited oscillation continues with this repetition, and the DC output voltage Vout is continuously output to the output terminal Tout.

The output voltage detection circuit 4 outputs the output voltage Vout.
Is detected and the output voltage signal is supplied to the control circuit 5. Specifically, when the output voltage Vout is high, the base of the transistor Q4 is deeply biased, the impedance of the transistor Q4 is lowered, and an output voltage signal of a high potential level is output. When the output voltage Vout is low, the base of the transistor Q4 is biased shallowly, the impedance of the transistor Q4 is increased, and the output voltage signal of a low potential level is output. The output voltage signal is supplied to the base of the transistor Q53 of the time constant control circuit 53 which constitutes the control circuit 5.

Therefore, in the control circuit 5, when the output voltage Vout is high, the impedance of the transistor Q53 becomes low and the time constant of the time constant charging / discharging circuit 51 is controlled small, so that the capacitor C5 is charged in a short time. ,
The on timing of the switching control transistor 54 is advanced. On the contrary, when the output voltage Vout is low, the impedance of the transistor Q53 becomes high and the time constant of the time constant charging / discharging circuit 51 is controlled to be large, so that the capacitor C5 is slowly charged and the on timing of the switching control transistor 54 is turned on. Delay.

Therefore, when the output voltage Vout is high, the switching element 2 is controlled so that the ON period is shortened and the output voltage is lowered. On the contrary, when the output voltage Vout is low, the ON period is extended and the output voltage is increased. The output voltage Vout is stabilized by controlling the output voltage Vout.

When the output voltage Vout drops due to an overcurrent state due to an overload or a short circuit of the load, the impedance of the transistor Q53 becomes infinite. The capacitor C5 is slowly charged with the time constant of the time constant charging / discharging circuit 51, but when it is charged to the threshold of the switching control transistor 54, the switching control transistor 5 is charged.
Turn on 4. Therefore, the ON period of the switching element 2 is limited to a certain value, and the ON period of the switching element 2 is changed to the output voltage V
out does not rise.

In the off period of the switching element 2,
The charge of the capacitor C5 is discharged by the flyback voltage induced in the feedback winding through the time constant charging / discharging circuit 51. Since the flyback voltage is proportional to the output voltage Vout, the discharge amount of the capacitor C5 is small. Therefore, in the next ON period, the capacitor C5 is slowly charged with the time constant of the time constant charging / discharging circuit 51, but is immediately charged to the threshold value of the switching control transistor 54 to turn on the switching control transistor 54. Therefore, the ON period of the switching element 2 is controlled to be shorter, the drooping of the output voltage Vout is promoted, and the overcurrent control is performed.

Here, paying attention to the operation of the switching control transistor 54, when the voltage of the capacitor C5 is charged to the threshold voltage of the switching control transistor 54, the switching control transistor 54 is turned on and the switching element 2 is turned on. Short the control input side of. As a result, the switching element 2 is turned off, and a flyback voltage in the opposite direction to that before is generated in the feedback winding 13. The capacitor C5 starts discharging through the time constant charging / discharging circuit 51. The on period of the switching control transistor 54 is instantaneous and turns off immediately after the flyback voltage is generated. At this time, the discharge current of the capacitor C5 tries to flow from the base of the switching control transistor 54 toward the collector, but is blocked by the ON blocking diode D5. Therefore, the switching control transistor 54 is prevented from turning on in the reverse direction.

Therefore, in the self-excited switching power supply device of this embodiment, no discharge path other than the time constant charging / discharging circuit 51 is formed. Therefore, the capacitor C5 can be charged and discharged accurately, so that it is possible to provide a self-excited switching power supply device in which the output voltage has high stability accuracy and accurate overcurrent control is possible.

FIG. 2 is an electric circuit diagram showing another embodiment of the self-excited switching power supply device according to the present invention, in which the switching element 2 is formed of a bipolar transistor. Generally, a bipolar transistor has a lower threshold voltage than a field effect transistor. For this reason,
By turning on the switching control transistor 54, it is necessary to devise in order to stably drive the switching element 2 off. Hereinafter, this point will also be described. In the figure, the same components as those shown in FIG. 1 are designated by the same reference numerals.

The illustrated self-excited switching power supply device includes a transformer 1 for power conversion, a switching element 2 and
The output rectifying / smoothing circuit 3, the output voltage detection circuit 4, the control circuit 5, and the level shift circuit 7 are included. The transformer 1 for power conversion, the output rectifying / smoothing circuit 3, the output voltage detection circuit 4, and the control circuit 5 have the same configurations as those of the embodiment shown in FIG.

The switching element 2 is composed of an NPN type bipolar transistor. Switching element 2
Is connected to the input winding 11 of the transformer 1, the emitter is connected to the DC input terminal Tin, and the base is connected to the feedback winding 13 via the level shift circuit 7 and the characteristic improving circuit 6. .

The level shift circuit 7 includes a diode D7.
1, D72 and capacitor C7. The diodes D71 and D72 and the capacitor C7 are respectively connected in parallel, the diode D71 is oriented in the reverse direction with respect to the base current, and the diode D72 is oriented in the forward direction with respect to the base current. Other configurations are the same as those of the embodiment shown in FIG.

In the self-excited switching power supply device of the present embodiment having such a configuration, when the DC voltage Vin is applied to the input terminal Tin, the base of the switching element 2 is connected via the starting resistor R7 and the level shift circuit 7. The base current flows to. When the switching element 2 starts to turn on, an input current starts to flow in the input winding 11 of the transformer 1 and induces a voltage in the output winding 12 and the feedback winding 13.

The voltage induced in the feedback winding 13 biases the base of the switching element 2 in the conducting direction via the characteristic improving circuit 6 and the level shift circuit 7, so that the switching element 2 is rapidly turned on, The input current flowing through the input winding 11 increases with time. Feedback winding 1
The voltage induced in 3 simultaneously charges the capacitor C5 via the resistor R51 of the time constant charge / discharge circuit 51 and the time constant control circuit 53.

After that, when the voltage of the capacitor C5 is charged to the threshold voltage of the switching control transistor 54, the switching control transistor 54 is turned on. At this time, regarding the voltage on the feedback winding 13 side of the level shift circuit 7, when the collector-emitter voltage of the switching control transistor 54 is VCE (sat) 54 and the forward drop voltage of the on-blocking diode D5 is VF5, It becomes VCE (sat) 54 + VF5. Generally, the threshold voltage of a bipolar transistor is lower than the threshold voltage of a field effect transistor. Therefore, when the on-blocking diode D5 is composed of a general diode having a high forward drop voltage VF, a voltage of VCE (sat) 54 + VF5 is applied to the base of the switching element 2 composed of a bipolar transistor. At this time, there is a possibility that the switching element 2 cannot be stably turned off. In this embodiment, the level shift circuit 7 is used. Therefore, the voltage applied to the base of the switching element 2 is the forward drop voltage of the diode D72 of the level shift circuit 7 being VF.
When it is set to 72, it becomes VCE (sat) 54 + VF5-VF72. Therefore, the diode D7 of the level shift circuit 7
If 2 is configured by a diode equivalent to the on-blocking diode D5, the forward drop voltage VF of both diodes is canceled, so that the switching element 2 can be reliably turned off even when a general diode is used. You can

When the switching element 2 is turned off, a flyback voltage is generated in the input winding 11, the output winding 12, and the feedback winding 13 in the opposite direction to that used before. afterwards,
The flyback voltage decreases with the disappearance of the flyback energy, generating a ringing voltage. The ringing voltage is also generated in the feedback winding 13 and gives an opportunity for the switching element 2 to start turning on.

After that, self-excited oscillation continues with this repetition, and the DC voltage is continuously output to the output terminal Tout. The operations of the output voltage detection circuit 4 and the control circuit 5 are similar to those of the embodiment shown in FIG.

The self-excited switching power supply device of this embodiment has a time constant charging / discharging circuit 51 similar to the embodiment shown in FIG.
Other discharge paths are not formed. Therefore, the capacitor C5 can be charged and discharged accurately, so that it is possible to provide a self-excited switching power supply device in which the output voltage has high stability accuracy and accurate overcurrent control is possible.

In this embodiment, the level shift circuit 7 is used to drive the switching element 2 off stably, but the on-blocking diode D5 is connected to the forward voltage drop VF.
If it is composed of a Schottky diode with a low
Since CE (sat) 54 + VF5 can be lowered, even if the switching element 2 is a bipolar transistor, the switching element 2 can be stably turned off by this voltage.

FIG. 3 is an electric circuit diagram showing still another embodiment of the self-excited switching power supply device according to the present invention.
An example applied to an insulation type self-excited switching power supply device is shown. In the figure, the same components as those shown in FIGS. 1 and 2 are designated by the same reference numerals.

The illustrated self-excited switching power supply device includes a transformer 1 for power conversion, a switching element 2 and
The output rectifying / smoothing circuit 3, the output voltage detecting circuit 4, and the control circuit 5 are included. The transformer 1 for power conversion, the switching element 2, and the output rectifying / smoothing circuit 3 have the same configurations as those of the embodiment shown in FIG.

The output voltage detection circuit 4 includes resistors R41 and R4.
2, R45, shunt regulator IC4, and photocoupler PC4. The photocoupler PC4 includes a photodiode PD4 and a phototransistor PT4. The photodiode PD4 is connected in series with the shunt regulator IC4 and is connected between the output terminals Tout. The resistors R41 and R42 are connected in series,
It is connected between the output terminals Tout and supplies the divided voltage of the output voltage Vout to the control terminal of the shunt regulator IC4. The collector of the phototransistor PT4 is supplied with the stable voltage VCC on the input side, the base is optically coupled to the photodiode PD4, and an output voltage signal is generated at the emitter. The output voltage signal is the resistance R4 connected to the emitter.
It is supplied to the control circuit 5 via 5. Since the collector of the phototransistor PT4 is supplied with the stable voltage on the input side, the collector potential is kept constant, and it is possible to prevent the pulse-like abnormal current due to the stray capacitance between the base and collector from flowing. With the configuration described above, the output voltage detection circuit 4 of this embodiment electrically insulates the output voltage signal from the output terminal Tout.

The control circuit 5 includes a time constant charging / discharging circuit 51,
1 in that it includes a time constant control circuit 53, a switching control transistor 54, and an on-blocking diode D5.
Although it is the same as the embodiment shown in FIG. 3, the connection relation of the ON blocking diode D5 is different. The on-blocking diode D5 has its anode connected to the base of the switching control transistor 54 and its cathode connected to the collector of the switching control transistor 54.

When the DC input voltage Vin is applied to the input terminal Tin in the self-excited switching power supply device of this embodiment having the above-mentioned configuration, the switching element 2 receives the input voltage as in the embodiment shown in FIG. The DC voltage Vin supplied through the winding 11 is interrupted, and the DC voltage is continuously output to the output terminal Tout via the transformer 1 and the output rectifying / smoothing circuit 3.

The photodiode P of the output voltage detection circuit 4
D4 changes the light emission amount according to the output voltage Vout. When the output voltage Vout is high, the output voltage detection circuit 4 increases the light emission amount of the photodiode PD4, lowers the impedance of the phototransistor PT4, and outputs an output voltage signal of a high potential level. When the output voltage is low, the amount of light emitted from the photodiode PD4 decreases, the impedance of the phototransistor PT4 increases, and the output voltage signal of a low potential level is output. The output voltage signal is the output voltage V
The signal is the same as that of the embodiment shown in FIG. 1 except that it is electrically insulated from out.

In the control circuit 5, the time constant charging / discharging circuit 5
1, the time constant control circuit 53, and the switching control transistor 54 operate similarly to the embodiment shown in FIG. Therefore, when the capacitor C5 of the time constant charging / discharging circuit 51 is charged to the threshold voltage of the switching control transistor 54 during the ON period of the switching element 2, the switching control transistor 54 is momentarily turned on and switching is performed. The control input side of the element 2 is short-circuited. As a result, the switching element 2 is turned off, and a flyback voltage in the opposite direction to that before is generated in the input winding 11, the output winding 12, and the feedback winding 13. Feedback winding 1
The flyback voltage generated at 3 makes the on-blocking diode D5 conductive and forms a discharge path different from the time constant charging / discharging circuit 51. The discharge current flowing from the capacitor C5 toward the base of the switching control transistor 54 is
Since the current flows to the collector side of the switching control transistor 54 via the on-blocking diode D5, reverse switching on of the switching control transistor 54 is blocked.

This discharge current is not the current flowing in the reverse direction in the switching control transistor 54 due to the reverse direction hfe in the switching control transistor 54, but the current flowing in the forward blocking diode D5 in the forward direction. . The constant of this discharge path is a numerical value that can be grasped as a design value, and the discharge current can be calculated accurately.

Therefore, the self-excited switching power supply device of this embodiment can accurately charge and discharge the capacitor C5. Therefore, it is possible to provide a self-excited switching power supply device which has high stability of output voltage accuracy and can perform accurate overcurrent control.

The on-blocking diode D5 is preferably a Schottky diode. Since the Schottky diode has a low forward voltage drop, the switching control transistor 54 can be reliably prevented from turning on. Further, by connecting a diode in parallel with the capacitor C5, it is possible to prevent the capacitor C5 from being charged in the reverse direction, which facilitates calculation of the discharge current.

FIG. 4 is an electric circuit diagram showing still another embodiment of the self-excited switching power supply device according to the present invention.
An example using a plurality of switching control transistors is shown. In the figure, the same components as those shown in FIGS. 1 to 3 are designated by the same reference numerals.

The illustrated self-excited switching power supply device includes a transformer 1 for power conversion, a switching element 2 and
The output rectifying / smoothing circuit 3, the output voltage detecting circuit 4, and the control circuit 5 are included. The transformer 1 for power conversion, the switching element 2, the output rectifying / smoothing circuit 3, and the output voltage detecting circuit 4 have the same configurations as those of the embodiment shown in FIG.

The control circuit 5 includes a time constant charging / discharging circuit 51,
It is similar to the embodiment shown in FIG. 1 in that it includes a time constant control circuit 53, but includes a first switching control transistor 55 and a second switching control transistor 56, and includes an on-blocking diode D5. The difference is that it is not included.

First switching control transistor 5
Reference numeral 5 is an NPN transistor, the base of which is connected to one end of the capacitor C5 and the collector of which is connected to the base of the second switching control transistor 56.

Second switching control transistor 5
Reference numeral 6 is a PNP transistor, the emitter of which is connected to the gate of the switching element 2 and the collector of which is connected to the source of the switching element 2 and the emitter of the first switching control transistor 55.

The switching power supply device of this embodiment basically operates in the same manner as the embodiment shown in FIG. During the ON period of the switching element 2, the capacitor C5 has the first
When the switching control transistor 55 is charged to the threshold voltage, the first switching control transistor 55 is turned on, and the second switching control transistor 56 is turned on. The control input side of the switching element 2 is short-circuited and turned off.

During the discharging period of the capacitor C5, the discharging current tries to flow from the base of the first switching control transistor 55 toward the collector, but is formed between the base and emitter of the second switching control transistor 56. Diode, that is, a PN junction having P on the base side and N on the emitter side.

Therefore, the first switching control transistor 55 is not turned on, a discharge path through the first switching control transistor 55 is not formed, and accurate discharge is achieved as in the embodiment shown in FIG. Can be done.

A further advantage of this embodiment is that the switching element 2 can be turned off at high speed. That is, the first switching control transistor 55 amplifies the base current supplied from the capacitor C5 and supplies it to the base of the second switching control transistor 56. Therefore, the second
The switching control transistor 56 can shorten the turn-off time of the switching element 2 as compared with the embodiment shown in FIG.

Also in this embodiment, the ON blocking diode D5 may be inserted on the emitter side of the second switching control transistor 56. Although the contents of the present invention have been described in detail above with reference to the preferred embodiments, the present invention is not limited to these, and those skilled in the art can make various modifications based on the basic technical idea and teaching. It is obvious that a modification can be conceived. For example, although the time constant charging / discharging circuit 51 is used for both charging and discharging in the embodiment, the charging time constant and the discharging time constant can be set differently. More specifically, by connecting a diode in parallel with the resistor R51 and connecting another resistor in series with the parallel circuit or the diode, the charging time constant and the discharging time constant can be individually set, Alternatively, the time constant charging circuit and the time constant discharging circuit are separately configured by connecting another resistor in parallel with the resistor R51 and connecting diodes in opposite directions in series with each resistor, and the like. Various methods are possible.

[0088]

As described above, according to the present invention, it is possible to provide a self-excited switching power supply device capable of performing accurate overcurrent control.

[Brief description of drawings]

FIG. 1 is an electric circuit diagram showing an embodiment of a self-excited switching power supply device according to the present invention.

FIG. 2 is an electric circuit diagram showing another embodiment of the self-excited switching power supply device according to the present invention.

FIG. 3 is an electric circuit diagram showing still another embodiment of the self-excited switching power supply device according to the present invention.

FIG. 4 is an electric circuit diagram showing still another embodiment of the self-excited switching power supply device according to the present invention.

[Explanation of symbols]

1 transformer 11 input winding 12 output windings 13 Feedback winding 2 switching elements 3 output rectification smoothing circuit 5 control circuit 51 Time constant charge / discharge circuit 54 Switching control transistor 55 First switching control transistor 56 Second switching control transistor 6 ON blocking diode R51 resistance C5 capacitor

Claims (4)

(57) [Claims] 1. A switching power supply device including a transformer, a switching element, an output rectifying / smoothing circuit, and a control circuit, and performing a self-excited oscillation operation, wherein the transformer comprises an input winding and an output winding. A line and a feedback winding, wherein the switching element intermittently induces a voltage in the output winding and the feedback winding by intermittently applying a DC voltage supplied through the input winding, , Biasing the switching element in the conduction direction with a voltage induced during the ON period of the switching element, and the output rectifying / smoothing circuit rectifies and smoothes the voltage induced in the output winding during the OFF period of the switching element. The control circuit includes a time constant charge / discharge circuit, a switching control transistor, and an on-blocking diode, and the time constant charge / discharge circuit includes a resistor and a capacitor connected in series. And a base connected to a connection point between the resistor and the capacitor, and a main electrode connected to a control input side of the switching element. The on-blocking diode has the anode of the switch
Connected to the base of the switching control transistor and the cathode
Is connected to the control electrode of the switching element, and prevents the switching control transistor from turning on in the reverse direction when the capacitor of the time constant charging / discharging circuit is discharged. 2. The switching power supply device according to claim 1, wherein the on-blocking diode is a Schottky diode . 3. The switching power supply device according to claim 1 , wherein the switching element is a MOS field effect transistor.
Switching power supply device composed of . 4. A switching power supply apparatus according to any one of claims 1 to 3, wherein the switching element is constituted by a bipolar transistor
Switching power supply is.