JP3155715B2 - Self-excited switching power supply circuit - 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-oscillation type switching power supply circuit which detects an overcurrent from a current flowing through a switching element and limits an on-duty of the switching element to perform an overcurrent protection operation. The present invention relates to a technique for improving a drooping characteristic of an output voltage with respect to a load current during a protection operation.

[0002]

2. Description of the Related Art Switching power supplies are classified into a self-excited oscillation system (hereinafter referred to as a self-excited oscillation system) and a separately-excited oscillation system according to their driving modes. A self-excited switching power supply generally uses a voltage appearing in a predetermined winding of an inductance component such as a transformer as a drive signal to positively feed back to a switching element, and causes the switching element to perform self-excited oscillation. FIG. 4 shows an example of such a self-excited switching power supply circuit. The switching power supply circuit shown in FIG. 4 has the following configuration.

A primary winding N1 of a transformer T, a switching transistor Q1, and a resistor R1 are connected in series between input terminals 1a and 1b. One end of a secondary winding N2 is connected to an output terminal 2a via a diode D1 and a choke coil L1. Connected to
The other end of the secondary winding N2 is connected to the output terminal 2b.
A smoothing capacitor C1 is connected between a connection point of the choke coil L1 and the diode D1 and the output terminal 2b, and a capacitor C2 is connected between the output terminals 2a and 2b. This transformer T, switching transistor Q1,
A basic power supply circuit is constituted by the diode D1 and the smoothing capacitor C1, and the resistor R1 serves as a means for detecting a current flowing through the power supply circuit.

A resistor R2 is connected between the input terminal 1a and the gate of the switching transistor Q1, and a resistor R3 is connected between the gate and the source of the switching transistor Q1.
Is connected. This resistor R2 is a starting circuit for applying a forward bias to the switching transistor Q1. A resistor R4, a capacitor C3 and a tertiary winding N3 are connected in series between the gate of the switching transistor Q1 and the input terminal 1b to form a signal feedback section. A control transistor Q2 is connected between the gate of the switching transistor Q1 and the input terminal 1b. A resistor R9 and a capacitor C5 are connected between the base of the transistor Q2 and one end of the resistor R1 on the switching transistor Q1 side.
And a parallel circuit of a diode D3. The parallel circuit of the resistor R9, the capacitor C5 and the diode D3 forms an overcurrent protection section including the control transistor Q2.

A diode D2 and a photocoupler light receiving element PH2 are connected in series between a connection point between the tertiary winding N3 of the signal feedback section and the capacitor C3 and the base of the transistor Q2. The photocoupler light-emitting element PH1 is connected in series with the resistor R8 and the shunt regulator SR, and the series circuit of the shunt regulator SR is connected to the smoothing capacitor C2. This photocoupler light receiving element PH1, photocoupler light emitting element PH2, diode D2, resistor R8
The shunt regulator SR and the shunt regulator SR form a constant voltage control unit including the transistor Q2. A serial circuit of a constant voltage diode DZ1 and a resistor R1 is connected in parallel to the photocoupler light receiving element PH2, and this series circuit forms a signal correction unit. Resistor R6 for capacitor C2
And a series circuit of R7 are connected in parallel, and a resistor R6 and a resistor R
The connection point 7 is connected to the gate terminal of the shunt regulator SR, and the capacitor C4 is connected between the gate terminal and the anode terminal of the shunt regulator SR. The series circuit of the resistor R6 and the resistor R7 serves as a voltage detecting unit.

The respective parts of the circuit shown in FIG. 4 having such a configuration generally operate as follows. First, the resistor R1 applies an initial forward bias to the gate of the switching transistor Q1 to make the switching transistor Q1 conductive. A voltage is induced in each winding of the transformer T according to the state of the switching transistor Q1.
The voltage generated in the tertiary winding N3 applies a forward bias or a reverse bias to the gate of the switching transistor Q1 via the capacitor C3 and the resistor R4 to drive the switching transistor Q1. That is, the tertiary winding N
3. The signal feedback section of the capacitor C3 and the resistor R4 feeds back the signal to the gate of the switching transistor Q1 and acts on the switching transistor Q1 to maintain the self-excited oscillation operation.

The voltage of the secondary winding N2 induced by the self-excited oscillation operation is rectified and smoothed by the diode D1 and the smoothing capacitor C2, and the ripple is removed by the choke coil L1 and the capacitor C2 to output as an output voltage. It is supplied to the outside from the terminals 2a and 2b. here,
The shunt regulator SR generates a control signal according to the output voltage detected by the resistors R6 and R7. Then, the control signal is transmitted via the photocoupler light emitting element PH1 and the light receiving element PH2, and is input to the base of the transistor Q2. The transistor Q2 changes the bias voltage of the switching transistor Q1 according to the control signal, thereby performing a constant voltage control operation.

In a normal operation state, the above-described self-excited oscillation operation and constant voltage control operation are constantly performed in the circuit of FIG.
Here, if the load connected to the output terminals 2a, 2b becomes abnormal and an excessive current flows through the switching transistor Q1, a voltage corresponding to the magnitude of the current appears between the terminals of the resistor R1. . The voltage appearing between the terminals of the resistor R1 is applied to the transistor Q2 via the resistor R9 of the overcurrent protection unit and the capacitor C5, and the transistor Q2 reduces the bias voltage of the switching transistor Q1 according to the voltage. As a result, an increase in the current flowing through the switching transistor Q1 is suppressed,
The overcurrent protection operation is performed.

In the power supply circuit shown in FIG.
As the impedance on the output side viewed from b decreases, the load current of the circuit increases, and the current flowing through the switching transistor Q1 also increases accordingly. At this time, the output voltage is constant by the constant voltage control operation of the circuit, and only the load current increases. here,
If the impedance is further reduced, the current flowing through the switching transistor Q1 becomes excessive with the increase of the output current. Then, the overcurrent protection operation is performed, and the output current becomes constant this time, and the output voltage decreases instead. The relationship between a load current, which becomes a constant voltage or a constant current depending on the load state, and the output voltage is generally known as a drooping characteristic of the output, and is a typical means for protecting a circuit.

[0010]

By the way, in a general power supply circuit, for example, a certain range is set so that the circuit can be shared by different input voltages of a 100 [V] system and a 200 [V] system. The constants of each element constituting the circuit are set so that the power supply voltage (input voltage) can be used. FIG.
Although the power supply circuit having the configuration shown in FIG. 1 is also designed to be applicable to a certain range of input voltage, there is a problem that the droop characteristic of the output changes as shown in FIG. 5 for different input voltages. That is, in the circuit of FIG. 4, the input voltage is applied as a bias voltage to the gate of the switching transistor Q1 via the resistor R2. Therefore, if the input voltage is high, a large forward bias is applied to the gate of the switching transistor Q1, and if the input voltage is low, a small forward bias is applied.

Here, when the impedance on the output side as viewed from the output terminals 2a and 2b decreases, the overcurrent protection operation is performed, and the output voltage decreases with a constant load current. When the output voltage drops, the switching transistor Q
When 1 turns off, the flyback voltage appearing in each winding of the transformer T also decreases. Then, the tertiary winding N3, the capacitor C3, and the resistor R4 are connected to the switching transistor Q.
The voltage signal supplied to the gate of the switching transistor Q1 becomes smaller, and as a result, the bias applied to the gate of the switching transistor Q1 is greatly affected by the input voltage.

In such a circuit state, when a high input voltage is applied between the input terminals 1a and 1b, the voltage applied to the gate of the switching transistor Q1 via the resistor R2 increases. The switching transistor Q1 is in a state where it easily shifts to the ON state.
Therefore, as shown in the gate voltage waveform diagram of FIG. 6, when the reverse bias voltage applied by the tertiary winding N3, the capacitor C3, and the resistor R4 decreases, the switching transistor Q1 is immediately turned on. As a result, the off-period of the switching transistor Q1 is shortened, and the phenomenon that the load current increases as shown by the line (H) in FIG. When the load current increases due to this phenomenon, the load on each element in the circuit increases, and the circuit may be damaged.

On the other hand, when the output voltage is low, the resistance R2
, The voltage applied to the gate of the switching transistor Q1 becomes low, and the switching transistor Q1 is hardly shifted to the ON state. Here, looking at the reverse bias voltage applied by the tertiary winding N3, the capacitor C3, and the resistor R4, the voltage applied to the terminal voltage of the capacitor C3 increases. Regarding the voltage between terminals of the capacitor C3, the input voltage is low and the tertiary winding N
3 By reducing the generated flyback voltage,
The polarity of the voltage is inverted, so that the resistance R4 side becomes higher at a higher potential later. As a result, as shown in FIG. 6, the off period of the switching transistor Q1 becomes longer, and when the output voltage drops below a predetermined voltage as shown by the line (L) in FIG.
A phenomenon in which the load current decreases rapidly appears.

For example, if the load connected to the power supply circuit is a capacitive or inductive load having a low impedance immediately after the start of voltage supply, the circuit shown in FIG. Can not supply a large current,
In some cases, the load or the power supply circuit itself could not be started. In view of the above problems, the present invention improves the output drooping characteristics of a switching power supply circuit, so that a large load current does not flow when the input voltage is high, and that a sufficient load current can be supplied even at startup when the input voltage is low. It is an object to provide a self-excited switching power supply circuit.

[0015]

SUMMARY OF THE INVENTION The present invention provides a self-excited switching power supply circuit in which an inductance component and a switching element are connected in series, and a voltage generated in the inductance component is fed back to the switching element to perform self-excited oscillation. A current detecting means connected in series to a main current path of the switching element, for detecting a current flowing through the switching element, connected to a control terminal of the switching element, and inputted to a control terminal of the switching element in response to a signal from the current detecting means A control means for limiting a signal is connected to an input terminal of the circuit and comprises a first resistance element for applying a forward bias to the switching element with an input voltage of the circuit.
Power circuit, and between the start circuit and the input terminal.
A constant current circuit comprising a constant voltage diode and a second resistor connected between the starter circuit and the control terminal of the switching element.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A main current path of a switching element is connected in series to a predetermined winding of an inductance component such as a transformer, and a signal feedback circuit including a winding of the inductance component is connected to a control terminal of the switching element. The control terminal of the switching element is connected to the main current path of the transistor element for control, the main current path of the switching element is connected in series with current detection means, and the switching element is connected from the current detection means to the control terminal of the transistor element. In such a way that a signal corresponding to the current flowing through is input. A starting circuit is connected to the input terminal, and a constant current circuit is connected between the starting circuit and a control terminal of the switching element. An initial forward bias applied to the control terminal of the switching element for conducting the switching element is supplied by flowing a constant current from the starting circuit toward the switching element. This prevents the forward bias applied to the control terminal of the switching element from changing according to the magnitude of the input voltage.

[0017]

FIG. 1 shows an embodiment of a self-excited switching power supply circuit according to the present invention which has improved output drooping characteristics. FIG.
Has the same circuit configuration as the conventional circuit described with reference to FIG. 3 except for the following components. That is, in the circuit of FIG. 1, the resistor R2 and the resistor R10 are connected in series between the input terminal 1a and the gate of the switching transistor Q1, and the connection point between the resistor R2 and the resistor R10 and the input terminal 1b
And a constant voltage diode DZ2. Here, the resistor R2 is a starting circuit as in FIG.
0 and the constant voltage diode DZ2 form a constant current circuit 3. By providing the constant current circuit 3 in this manner, FIG.
Is improved as follows.

First, the resistance R
The current flowing from 2 to the gate of the switching transistor Q1 does not fluctuate due to the magnitude of the input voltage, and the forward bias applied to the gate of the switching transistor Q1 can be set to substantially the same level. For this reason, it is possible to set the forward bias of the switching transistor Q1 to an optimum state without being affected by the fluctuation range of the input voltage. Next, since the current supplied from the constant current circuit 3 to the gate of the switching transistor Q1 does not fluctuate significantly, the tertiary winding N3 and the capacitor C3 which determine the off period of the switching transistor Q1 during the overcurrent protection operation. The energy charging / discharging time becomes almost constant.

By these two actions, the off time of the switching transistor Q1 is determined by the input voltage, the output terminals 2a,
It is possible to make it almost constant irrespective of the value of the impedance between b. FIG. 3 shows a waveform diagram of the gate voltage of the switching transistor Q1 at this time. As described above, in the circuit shown in FIG. 1, the ON period of the switching transistor Q1 is controlled by the overcurrent protection operation in accordance with the current flowing through the switching transistor Q1, and the OFF period is made substantially constant by the constant current circuit 3. Therefore, the load current does not increase or decrease depending on the magnitude of the input voltage and the value of the impedance between the output terminals 2a and 2b, and the circuit shown in FIG. The droop characteristic of each output is improved as shown in FIG.

As can be seen from the drooping characteristics of the output of FIG. 2, according to the circuit of FIG. 1, there is a fear that the circuit may be damaged due to an increase in the load current, or that a sufficient current cannot be supplied to the load at the time of startup. , And a situation in which a capacitive or inductive load cannot be started can be prevented. In the circuit of FIG. 1, the constant current circuit 3 is a constant voltage diode D
Although it is constituted by Z2 and the resistor R10, a circuit having a constant current characteristic using a constant current diode or a transistor element may be used.

[0021]

As described above, the self-excited switching power supply circuit according to the present invention causes the switching element to perform a self-excited oscillation operation by positively feeding back the voltage signal appearing in the inductance component, and further reduces the current flowing through the switching element. In a switching power supply circuit that detects and performs overcurrent protection,
It is characterized in that a constant current circuit is provided between the control terminal of the switching element and the starting circuit. According to this configuration,
It is possible to prevent the off period of the switching element from fluctuating unstablely depending on the input voltage and the load state. As a result, the output drooping characteristic of the circuit can be improved so that the load current becomes substantially constant even when the state between the output terminals is almost short-circuited, and the load current depends on the magnitude of the input voltage and the load state. Can be prevented from increasing or decreasing. And, by improving the droop characteristic of the output,
It is possible to prevent problems such as breakage of the circuit and inability to supply a sufficient current at startup.

[Brief description of the drawings]

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

FIG. 2 shows the drooping characteristics of the output of the circuit of FIG.

FIG. 3 is a waveform diagram of a gate voltage of a switching element at the time of high / low input of the circuit of FIG. 1;

FIG. 4 is a circuit diagram of an example of a conventional self-excited switching power supply circuit.

FIG. 5 shows the drooping characteristics of the output of the circuit of FIG.

FIG. 6 is a waveform diagram of a gate voltage of a switching element at the time of high / low input of the circuit of FIG. 4;

[Explanation of symbols]

1a, 1b Input terminal 2a, 2b Output terminal 3 Constant current circuit T Transformer N1 Primary winding N2 Secondary winding N3 Tertiary winding Q1 Switching transistor Q2 Controlling transistor DZ2, R10 Constant voltage constituting constant current circuit Diode and resistance R1 Resistance as current detection means R2 Resistance as starting circuit R4, C3 Resistance and capacitor constituting signal feedback circuit PH1, PH2 Light-emitting element and light-receiving element of photocoupler

Claims (1)

(57) [Claims] 1. A self-excited switching power supply circuit in which an inductance component and a switching element are connected in series, and a voltage generated in the inductance component is fed back to the switching element to perform self-excited oscillation. Current detecting means connected in series to a path and detecting a current flowing through the switching element; a signal connected to a control terminal of the switching element and input to a control terminal of the switching element in response to a signal from the current detecting means A first resistor connected to the input terminal of the circuit for applying a forward bias to the switching element with the input voltage of the circuit.
Starting circuit comprising elements, and the starting circuit and input terminal
And a second constant voltage diode connected between the starting circuit and the control terminal of the switching element .
A self-excited switching power supply circuit, comprising: