JPH077940A - Switching power-supply apparatus - Google Patents

Abstract

PURPOSE:To freely adjust a difference in an operating point for protection of an overcurrent due to a difference in an input current. CONSTITUTION:A diode D4 which suppresses the discharge of an electric charge in a capacitor C2 is connected in series with a circuit which is composed of a resistance R9 and a Zener diode D3. Near the start of an operation for protection of an overcurrent, the charging route of the capacitor C2 during the ON period of a switching element Q1 is divided into two routes of a resistance R3 and of a series circuit which is composed of the diode D4, the resistance R9 and the Zener diode D3. Then, the charging route of the capacitor C2 during the OFF period of the switching element Q1 cannot perform a discharge via the resistance R9 or the like due to the existence of the diode D4, and a spontaneous discharge is performed by the resistance R3 only. When the capacitor C2 is charged by a charging route (when values of the resistances R3, R9 are set properly), a difference in an operating point for protection of the overcurrent due to a difference in the magnitude of an input voltage can be adjusted freely.

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 using a ringing choke converter (RCC) system.

[0002]

2. Description of the Related Art FIG. 2 shows a specific circuit diagram of a conventional FET type ringing choke converter (RCC) type switching power supply device. As a conventional example of this type, for example, Japanese Patent Publication No. 4-9033 can be cited. An AC power supply AC is connected to an input end of a diode bridge DB ₁ for rectification via a fuse F and a line filter LPF, and this diode bridge DB ₁
A smoothing capacitor C ₁ is connected to the output terminal of the.

The inverter circuit includes output transformers T and FE.
A switching element Q ₁ composed of T, a starting resistor R ₁ ,
It is composed of R ₂ etc. A rectifying diode D ₁ and a smoothing capacitor C ₃ are connected to both ends of the output winding N ₂ of the output transformer T.

Further, a voltage detection circuit and a control circuit as a stable control of the output voltage and an overcurrent protection circuit are provided.
The voltage detection circuit provided on the output side of the inverter circuit includes resistors R ₇ and R _{8 for} dividing and detecting the output voltage, a light emitting diode PD on the light emitting side of the photocoupler PC ₁ , a shunt regulator IC _{1 and the} like. . The control circuit provided on the side of the feedback winding N _B of the output transformer T of the inverter circuit includes a phototransistor PT, resistors R _{3 to} R ₅ , a diode D ₂ , which form a pair with the light emitting diode PD of the photocoupler PC ₁ . It is composed of a transistor Q ₂ connected in parallel between the gate and source of the switching element Q ₁ , a capacitor C ₂ connected between the base and emitter of this transistor Q ₂ , and the like.

Next, the operation of the circuit shown in FIG. 2 will be described. First, at the time of start-up when the power is turned on, a voltage is applied to the gate of the switching element Q ₁ via the resistors R ₁ and R ₂ , and the switching element Q ₁ is turned on.
When the switching element Q ₁ is turned on, the power supply voltage is applied to the primary winding N _P of the output transformer T, and the feedback winding N _P
A voltage is generated in _B in the same direction as the primary winding N _P. The generated voltage charges the capacitor C ₂ via the resistor R ₃ .

At the time of startup, the output voltage is close to zero and the phototransistor PT of the photocoupler PC ₁
Is a cutoff state, and the capacitor C ₂ is charged only by the current flowing through the resistor R ₃ . At this time, since the capacitor C ₂ is not charged with electric charge, it is charged in a short time. When exceeding the forward voltage between the base and emitter of the transistor Q _2, the transistor Q ₂ is turned on.

When the transistor Q ₂ turns on, the collector potential of the transistor Q ₂ becomes L level, the gate of the switching element Q ₁ becomes L level, and the switching element Q ₁ is turned off. Therefore, at startup,
The ON period of the switching element Q ₁ can be kept small.

When the switching element Q ₁ is turned off, the energy stored in the output transformer T when the switching element Q ₁ is turned on is released through the output winding N ₂ . The voltage, which is this energy, is rectified by the diode D ₁ and smoothed by the capacitor C ₃ , so that power is supplied to the load.

When the electric charge of the capacitor C ₂ is discharged through the resistor R ₃ , the transistor Q ₂ is turned off and the switching element Q ₁ is turned on. When the switching element Q ₁ is turned on, a voltage is applied to the primary winding N _P of the output transformer T again, and energy is stored in the output transformer T.

When the output voltage rises by repeating such an oscillating operation, the electric charge in the capacitor C ₂ is reversed in the opposite direction due to the voltage generated in the feedback winding N _B of the output transformer T during the off period of the switching element Q _1. Be charged. Therefore, a longer charging time is required than when the electric charge is empty, and the ON period of the switching element Q ₁ becomes longer. After the output voltage rises, the photo coupler PC ₁
The phototransistor PT also goes from the cutoff state to the active state, the collector current of the phototransistor PT controls the charging time of the capacitor C ₂ , and the ON period of the switching element Q ₁ corresponding to a predetermined output voltage is obtained. .

Here, in a steady state, the output voltage on the load side is always divided by the resistors R ₇ and R ₈ to be detected, and the divided detection voltage and the shunt regulator IC are detected.
₁ is compared with the reference voltage. Then, the variation of the output voltage is amplified by the shunt regulator IC ₁ and the current flowing through the light emitting diode PD of the photocoupler PC ₁ is changed, so that the phototransistor PT of the photocoupler PC ₁ of the photocoupler PC ₁ is changed according to the light emission amount of the light emitting diode PD. By changing the impedance and changing the charging time constant of the capacitor C ₂ , control is performed so that the output voltage becomes constant.

In the steady state, the capacitor C ₂ is charged mainly by the resistor R ₅ , the diode D ₂ and the photocoupler PC _1.
Is charged via the phototransistor PT. Also,
The charged electric charge of the capacitor C ₂ is discharged through the resistor R ₃ .

When the output voltage rises, a large amount of current flows through the light emitting diode PD of the photocoupler PC ₁ ,
Since the impedance of the phototransistor PT is lowered, the charging time constant of the capacitor C ₂ is shortened, the transistor Q ₂ is turned on quickly, and the switching element Q ₁ is turned off to shorten the ON period of the switching element Q ₁ .
Control to reduce the output voltage. When the output voltage decreases, the reverse operation described above is performed to control the output voltage to increase, and the constant voltage control is performed to keep the output voltage constant.

The control in the case of an abnormal current such as an overcurrent or a short circuit current is performed as follows. That is,
As the output current increases, the current flowing through the light emitting diode PD of the photocoupler PC ₁ is narrowed down. Therefore, the current flowing through the phototransistor PT is also reduced,
The charging time of the capacitor C ₂ becomes longer. Therefore, the time until the transistor Q ₂ is turned on becomes long, the on period of the switching element Q ₁ becomes long, and an attempt is made to flow a large amount of output current.

However, after the current flowing through the phototransistor PT becomes zero and is cut off, the capacitor C ₂ is charged only on the resistor R ₃ side, and the capacitor C ₂ and the resistor C ₂ are charged during the ON period of the switching element Q _1. It cannot be increased beyond the value determined by the time constant of R ₃ , and the output current becomes the limit. Also, the electric charge of the capacitor C ₂ is discharged through the resistor R ₃ . Furthermore the load impedance decreases output voltage begins to drop, but when the output voltage drops, the output transformer T off period of the switching element Q ₁
Also, the voltage generated in the feedback winding N _B of the above is also reduced. Therefore, decreases the charge accumulated in the opposite direction to the capacitor C _2, the charging time of the capacitor C ₂ at the time of the on-switching element Q ₁ is shortened, the ON period of the switching element Q ₁ is shortened.

As described above, the ON period of the switching element Q ₁ continues to be shortened until the load impedance finally becomes zero (short circuit), so that the output voltage with respect to the output current is suppressed and a so-called fold curve is formed. Draw overcurrent protection control.

In the conventional example (Japanese Patent Publication No. 4-9033), there is a problem that the operating point of the overcurrent protection is proportionally shifted due to the fluctuation of the input voltage V ₁ of the inverter circuit. This is because the charging path of the capacitor C ₂ during the ON period of the switching element Q ₁ and the switching element Q 2
_This is because the discharge path of the capacitor C ₂ during the OFF period of ₁ is performed via the resistor R ₃ . That is, the charging path and the discharging path of the capacitor C ₂ have the same configuration.

Therefore, the conventional example is improved as shown in FIG.
It is a circuit shown in. This circuit is, for example, Japanese Patent Publication No. 4-9.
No. 034 publication is mentioned. That is, as shown in FIG. 3, in parallel with the resistor R _3, in which a series circuit of a resistor R ₉ and the Zener diode D ₃ is connected. In this way, by adding the series circuit of the resistor R ₉ and the Zener diode D ₃ to the conventional circuit, the charging path of the capacitor C ₂ during the ON period of the switching element Q ₁ is set to the resistor R _3, and the switching is performed. Capacitor C during OFF period of element Q ₁
The discharge path of ₂ is a resistor R ₉ and a zener diode D ₃ .

[0019] Thus, changing the charging path of the capacitor C ₂ in the ON period switching elements Q _1, by changing the discharge path of the capacitor C ₂ in the OFF period of the switching element Q _1, the impedance, particularly discharge Is performed sufficiently to adjust the difference in the operating point of the overcurrent protection due to the difference in the input voltage V ₁ .

[0020]

However, in the circuit shown in FIG. 3, since there is a path (resistor R ₉ and Zener diode D ₃ ) for sufficiently discharging the capacitor C ₂ ,
Of the switching operation and the switching element Q _1, a large current required for the operation of the overcurrent protection, since at the same time the time required for the switching of the switching element Q ₁ is large, the waveform of the voltage V _DS between the drain and source of the switching element Q ₁ This is a cause of rounding and switching loss. In addition, since there are problems such as large loss when the load is short-circuited,
There is a problem that it is very difficult to select the capacitors C ₄ and C ₂ .

The present invention has been provided in view of the above-mentioned points. By intentionally discharging the capacitor during the OFF period of the switching element, the operating point of the overcurrent protection due to the difference in the input voltage is increased. It is intended to provide a switching power supply device for the purpose of freely adjusting the difference.

[0022]

SUMMARY OF THE INVENTION According to the present invention, an output transformer having a primary winding, an output winding, and a feedback winding, and one end of which is connected to the primary winding of the output transformer, the control terminal being connected to the feedback winding. A switching element for oscillation connected to the rectifier circuit, a rectifier circuit connected to the output winding of the output transformer, a voltage detection circuit provided on the output side of the rectifier circuit for detecting the output voltage,
A control circuit that receives a signal from the voltage detection circuit and performs constant voltage control of the output voltage and overcurrent control of the output current is provided.
A control transistor in which the control circuit is connected in parallel between the control terminal of the switching element and the ground, an impedance element that changes impedance according to the signal amount of the voltage detection circuit, and a base / emitter of the control transistor. The switching element is connected between the switching elements and is turned on when the switching element is turned on by the charging time constant of the impedance element to turn on the control transistor, and when the switching element is turned off, the feedback winding of the output transformer is used. A capacitor that is charged in the opposite direction to the charging direction by the generated voltage, and a capacitor that has a predetermined value depending on when the switching element is turned on or off when the value of the impedance element becomes large during an overcurrent of the output current. With a first resistor that charges and discharges with a time constant Is connected in parallel with the first resistor,
In a switching power supply device of a ringing choke converter system, which comprises a series circuit of a zener diode whose impedance changes in a positive direction and a reverse direction, and a second resistor, a series circuit of the second resistor and a zener diode is used. A diode is provided on the discharge path through which the electric charge of the capacitor is discharged in a direction to prevent the discharge.

[0023]

According to the present invention, in the vicinity of the operation start of the overcurrent protection, the charging path of the capacitor during the ON period of the switching element is the first resistance and the two paths of the diode, the second resistance and the zener diode. Therefore, the discharge path of the capacitor during the OFF period of the switching element cannot be discharged through the second resistor and the Zener diode by the diode, and is naturally discharged only by the first resistor. Thus, during the ON period of the switching element, the operation of the overcurrent protection due to the difference in the input voltage is performed only by charging the capacitor with the circuit of the first resistance, the diode, the second resistance and the zener diode. The difference between the points can be adjusted freely. In addition, overcurrent protection is adjusted only by charging the capacitor during the ON period of the switching element, so there is no need to forcibly discharge the capacitor, and since there is electric charge remaining in the capacitor, switching operation of the switching element is performed. A small amount of current is sufficient for the operation of and overcurrent protection. Then, the turn-off of the switching element becomes steeper than in the conventional case. The efficiency of the switching power supply can be improved by the above two points. Also, while maintaining the above effects, the operating point for overcurrent protection can be freely adjusted by appropriately setting the values of the first resistor and the second resistor as in the conventional case.

[0024]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a specific circuit diagram of the switching power supply device of the present invention. It should be noted that the same elements as those of the related art shown in FIG. 3 are denoted by the same reference numerals and the description thereof is omitted, and the gist of the present invention will be described in detail. Further, the operation of constant voltage control is the same as the conventional one, and therefore the description of the operation will be omitted, and the operation near the operating point of the overcurrent protection will be described.

The present invention, as shown in FIG. 1, is the addition of a diode D ₄ to the conventional circuit of FIG. That is, the cathode of the diode D ₄ is connected to the resistor R ₉ side,
A capacitor C ₂ is charged by a series circuit including a diode D ₄ , a resistor R ₉ (second resistor), and a zener diode D _3, and a capacitor C is charged by the diode D _4.
The discharge of the electric charge of ₂ is blocked.

By thus adding the diode D ₄ , the capacitor C during the off period of the switching element Q ₁
The discharge of ₂ is naturally performed only by the resistor R ₃ (first resistor), and is intentionally prevented from being performed from the path of the zener diode D ₃ and the resistor R ₉ .

Here, in the vicinity of the operation start of the overcurrent protection in which the phototransistor PT of the photocoupler PC ₁ is in the cutoff state, the charging path of the capacitor C ₂ during the ON period of the switching element Q ₁ is the resistance R ₃ , And a diode D ₄ , a resistor R ₉ , and a zener diode D ₃ in series. The discharge path of the capacitor C ₂ during the OFF period of the switching element Q ₁ cannot be discharged through the series circuit of the zener diode D ₃ and the resistor R ₉ due to the existence of the diode D ₄ , and only the resistor R ₃ can be discharged. To discharge naturally.

Thus, during the ON period of the switching element Q ₁ , the resistor R ₃ , the diode D ₄ , the resistor R ₉ ,
Only by charging the capacitor C _{2 in} the series circuit of the Zener diode D ₃ (by setting the values of the resistors R ₃ and R ₉ appropriately), the difference in the operating point of the overcurrent protection due to the difference in the input voltage is large. Can be adjusted freely.

Further, the overcurrent protection is performed by the switching element Q.
_{Since the} adjustment is performed only by charging the capacitor C ₂ during the ON period of ₁ , the capacitor C ₂ does not need to be forcibly discharged, and the electric charge remains in the capacitor C ₂ , so that the switching operation of the switching element Q ₁ is performed. A small amount of current is sufficient for the operation of and overcurrent protection. Then, the turn-off of the switching element Q ₁ becomes steeper than in the conventional case. The efficiency of the switching power supply can be improved by the above two points. Further, while maintaining the above effects, the operating points for overcurrent protection can be freely adjusted by appropriately setting the values of the resistors R ₃ and R _{9 as in} the conventional case.

In each of the above embodiments, the case where the FET is used as the switching element Q ₁ has been described, but the present invention can also be applied to an RCC type switching power supply circuit using a transistor as the switching element. It is a thing.

[0031]

According to the present invention, in the vicinity of the operation start of the overcurrent protection, the charging path of the capacitor during the ON period of the switching element has the first resistor, the diode, the second resistor and the Zener diode. There are two paths, and the discharge path of the capacitor during the off period of the switching element cannot be discharged through the second resistance and the Zener diode by the diode, and is naturally discharged only by the first resistance. Thus, during the ON period of the switching element, the operation of the overcurrent protection due to the difference in the input voltage is performed only by charging the capacitor with the circuit of the first resistance, the diode, the second resistance and the zener diode. The difference between the points can be adjusted freely. In addition, overcurrent protection is adjusted only by charging the capacitor during the ON period of the switching element, so there is no need to forcibly discharge the capacitor, and since there is electric charge remaining in the capacitor, switching operation of the switching element is performed. A small amount of current is sufficient for the operation of and overcurrent protection. Then, the turn-off of the switching element becomes steeper than in the conventional case. The efficiency of the switching power supply can be improved by the above two points. In addition, while maintaining the above effects, the first resistance and the second resistance are maintained as usual.
By properly setting the value of the resistance of, the operation point of the overcurrent protection can be adjusted freely.

[Brief description of drawings]

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

FIG. 2 is a specific circuit diagram of a conventional switching power supply device.

FIG. 3 is a specific circuit diagram of another conventional switching power supply device.

[Explanation of symbols]

T output transformer N _P 1 winding N ₂ output windings N _B feedback winding Q ₁ switching element Q ₂ control transistor C ₂ capacitors PC ₁ photocoupler R ₃ resistor (first resistor) R ₉ resistor (second Resistance) D ₃ Zener diode D ₄ diode

Claims (1)

[Claims] 1. An output transformer having a primary winding, an output winding and a feedback winding, and an oscillation switching element having one end connected to the primary winding of the output transformer and a control terminal connected to the feedback winding. A rectifier circuit connected to the output winding of the output transformer, a voltage detection circuit provided on the output side of the rectifier circuit for detecting the output voltage, and a constant voltage of the output voltage that receives a signal from the voltage detection circuit. A control transistor for controlling and controlling an overcurrent of the output current, the control circuit being connected in parallel between the control terminal of the switching element and the ground, and the control circuit according to the signal amount of the voltage detection circuit. It is connected between the impedance element that changes the impedance and the base-emitter of the control transistor, and when the switching element is turned on by the charging time constant of the impedance element. A capacitor that is charged to turn on the control transistor to turn off the switching element and that is charged in a direction opposite to the charging direction by a voltage generated by the feedback winding of the output transformer when the switching element is off, A first resistor that charges or discharges the capacitor with a predetermined time constant depending on when the switching element is turned on or off when the value of the impedance element becomes large during an overcurrent of the output current, and the first resistor. A ringing choke converter type switching power supply device comprising a zener diode connected in parallel with a resistor, the impedance of which changes in a positive direction and a reverse direction, and a series circuit of a second resistor, wherein the second resistor and the zener are provided. The electric charge of the capacitor is discharged to the discharge path through the series circuit with the diode. Switching power supply device is characterized by providing a diode in a direction to block.