JP3129036B2 - 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 switching power supply using a ringing choke converter (RCC).

[0002]

2. Description of the Related Art FIG. 3 is a specific circuit diagram of a conventional FET type ringing choke converter (RCC) type switching power supply. As a conventional example of this type, for example, Japanese Patent Publication No. Hei 4-9033 is cited. AC power source AC is connected to an input terminal of the diode bridge DB ₁ for rectification via a fuse F and line filter LPF, the diode bridge DB ₁
Capacitor C ₁ for smoothing is connected to the output end.

The inverter circuit comprises output transformers T, FE
A switching element Q _{1 made of} T, a resistor R ₁ for starting,
R _{2 and the} like. 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 are provided as a circuit for stably controlling the output voltage and as an overcurrent protection circuit.
The voltage detection circuit provided on the output side of the inverter circuit includes resistors R ₇ and R _{8 for} dividing the output voltage and detecting the 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 in the feedback winding N _B side of the output transformer T of the inverter circuit, the phototransistor PT, the resistance R ₃ to R _5, the diode D ₂ which is a light-emitting diode PD paired the photocoupler PC _1, It comprises 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 the transistor Q ₂ , and the like.

Next, the operation of the circuit shown in FIG. 3 will be described. First, at the time of startup when the power is turned on, the voltage to the gate of the switching element Q ₁ via the resistor R _1, R ₂ is applied, the switching element Q ₁ is turned on.
When the switching element Q ₁ is turned on, the supply voltage is applied to the primary winding N _P of the output transformer T, the feedback winding N
Voltage is generated in the primary winding N _P in the same direction _B. This generated voltage through a resistor R ₃ to charge the capacitor C _2.

[0006] In this case, at the time of start-up, the output voltage phototransistor nearby photo-coupler PC ₁ to zero PT
Is cut-off state, it is charged only by the current capacitor C ₂ are flowing through the resistor R _3. Also, in order for this case the capacitor C ₂ is not charged charge 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.

[0007] When the transistor Q ₂ is turned on, the collector potential of the transistor Q ₂ becomes the L level, the gate of the switching element Q ₁ is L level, turns off the switching element Q _1. Therefore, at startup,
ON period of the switching element Q ₁ is kept small.

[0008] When the switching element Q ₁ is turned off, the energy accumulated in the output transformer T during the ON the switching element Q ₁ is emitted 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 to supply power to the load.

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

[0010] Such oscillation operation of repeatedly coming risen output voltage, the capacitor C ₂ in the reverse direction charge by voltage generated in the feedback winding N _B of the output transformer T off period of the switching element Q ₁ Charged. Therefore, the charge is long charging time is required than when empty, the ON period of the switching element Q ₁ is longer. After the output voltage rises, the photocoupler PC ₁
Be made from the cutoff state to the active state, the collector current of the phototransistor PT controls the charging time of the capacitor C _2, so obtaining the ON period of the switching element Q ₁ corresponding to a predetermined output voltage of the phototransistor PT .

[0011] Here, in the steady state, the output voltage of the load side, the resistor R ₇ and R ₈ are detected by applying constantly content in this dividing the detected voltage and the shunt regulator IC
₁ is compared with the reference voltage. Then, the amount of variation of the output voltage amplified by the shunt regulator IC _1, by changing the current applied to the light-emitting diode PD of the photocoupler PC _1, depending on the amount of light emission of the light-emitting diode PD of the photo coupler PC ₁ of the phototransistor PT changing the impedance by changing the charging time constant of the capacitor C _2, performs control so that the output voltage becomes constant.

[0012] In the steady state, the charging of the capacitor C ₂ is mainly resistor R _5, the diode D _2, the photocoupler PC ₁
Is charged through the phototransistor PT. Also,
It charges the capacitor C ₂ is discharged through the resistor R _3.

[0013] Here, when the output voltage rises, a current flow more in the light-emitting diode PD of the photocoupler PC _1,
For the impedance of the phototransistor PT is lowered, the shorter the charge time constant of the capacitor C _2, are turned on quickly transistor Q _2, the on-period of the switching element Q ₁ to shorten the switching element Q ₁ as an off,
Control to lower the output voltage. When the output voltage decreases, the reverse operation is performed to control the output voltage to increase, and to perform the constant voltage control so that the output voltage becomes 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,
When the output current increases, the current flowing through the light-emitting diode PD of the photo coupler PC ₁ is gradually narrowed. Therefore, the current flowing through the phototransistor PT is also reduced,
Charging time of the capacitor C ₂ becomes longer. Accordingly, the on period of the switching element Q ₁ is increased longer time to turn on the transistor Q _2, attempts to pass a lot of output current.

[0015] However, after the current flowing through the phototransistor PT becomes blocked state becomes zero, the charging of the capacitor C ₂ is only the resistance R ₃ side, the ON period of the switching element Q ₁ is a capacitor C ₂ resistors It cannot increase beyond the value determined by the time constant of R ₃ , and the output current is limited. The discharge of the charge in the capacitor C ₂ is also performed via 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 decreases the voltage generated in the feedback winding N _B of. 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.

[0016] Thus, until the load impedance is finally zero (short circuit), the on period of the switching element Q ₁ is continuously shortened, the output voltage to the output current is suppressed, the shape curve of Tokoroifu The overcurrent protection control works.

[0017] In the conventional example (KOKOKU 4-9033 discloses) has a problem that the operating point of the overcurrent protection by variation of the input voltage V ₁ of the inverter circuit is shifted in proportion. This is because the charging path of the capacitor C ₂ in the ON period switching element Q _1, a switching element Q
₁ of the discharge path of the capacitor C ₂ in the off period, because the have gone together through a resistor R _3. That is because the charge path and the discharge path capacitor C ₂ is the same structure.

FIG. 4 shows an improvement of the prior art.
The circuit shown in FIG. As an example of this circuit,
No. 034. That is, as shown in FIG. 4, 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. Thus, by adding a 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 capacitor C during the oFF period of the element Q ₁
The discharge path of No. 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 the was sufficiently performed as is adjusted to the difference in operating point of the overcurrent protection according to the difference in magnitude of the input voltage V _1.

[0020]

However, in the circuit shown in FIG. 4, 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 That is, it causes switching loss. In addition, because there is a problem such as a large loss at the time of load short circuit,
There is a problem that it is very difficult to select the capacitors C ₄ and C ₂ .

The present invention has been made in view of the above-mentioned point, and it is intended to discharge the capacitor naturally during the OFF period of the switching element, thereby reducing the operating point of the overcurrent protection due to the difference in input voltage. An object of the present invention is to provide a switching power supply device for adjusting a difference freely.

[0022]

SUMMARY OF THE INVENTION The present invention provides an output transformer having a primary winding, an output winding and a feedback winding, and one end connected to the primary winding of the output transformer and a control terminal connected to the feedback winding. A switching element for oscillation to which is connected, 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 and detecting an 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;
A control transistor having the control circuit connected in parallel between a control terminal of the switching element and ground; an impedance element for changing impedance according to a signal amount of the voltage detection circuit; and a base / emitter of the control transistor. The switching element is charged when the switching element is turned on by the charging time constant of the impedance element to turn on the control transistor, thereby turning off the switching element.When the switching element is turned off, the feedback winding of the output transformer turns off the switching element. A capacitor that is charged in a direction opposite to the charging direction by the generated voltage, and a capacitor that is turned on and off when the switching element is turned on and off when the value of the impedance element becomes large during an overcurrent of the output current. a first resistor to charge with the time constant, the first Is connected in parallel with anti, in the switching power supply of ringing choke converter system and configured by a series circuit of the positive direction and the impedance is changed in the reverse direction Zener diode and a second resistor,
A diode is provided in a discharge path through which a charge of the capacitor is discharged through a series circuit of the first resistor, the second resistor, and the Zener diode, in a direction for preventing the discharge.

[0023]

According to the present invention, in the vicinity of the start of the operation of the overcurrent protection, the charging path of the capacitor during the ON period of the switching element is connected between the first resistor and the second resistor via the diode and the Zener diode. There are two paths, and the discharge path of the capacitor during the off period of the switching element is a spontaneous discharge because the diode cannot discharge through the first resistance, the second resistance, and the Zener diode. As described above, during the ON period of the switching element, the overcurrent protection due to the difference in the input voltage is achieved only by charging the capacitor by the circuit of the first resistor and the second resistor and the Zener diode via the diode. Can be freely adjusted. Also,
Overcurrent protection is adjusted only by charging the capacitor during the ON period of the switching element.Therefore, there is no need to forcibly discharge the capacitor, and since the charge remains in the capacitor, the switching operation of the switching element and the overcurrent A very small current is sufficient for the current protection operation. Then, the turn-off of the switching element becomes steeper than in the related art. From the above two points, the efficiency of the switching power supply can be improved. In addition, the operating point of the overcurrent protection can be freely adjusted by appropriately setting the values of the first resistor and the second resistor while maintaining the above-described effects.

[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 of the present invention. Note that the same elements as those in the related art shown in FIG. 4 are denoted by the same reference numerals, and the description thereof will be omitted. Further, since the operation of the constant voltage control is the same as the conventional one, the description of the operation is omitted, and the operation near the operating point of the overcurrent protection will be described.

In the present invention, as shown in FIG. 1, diodes D ₄ and D ₅ are added to the conventional circuit of FIG. That is, the cathode of the resistor R ₉ side diode D ₄ connected, a diode D _4, the resistance R ₉ (second resistor),
So as to charge the capacitor C ₂ by the series circuit consisting of a Zener diode D _3, discharging of the capacitor C ₂ is blocked by the diode D _4.

Further, the cathode of the diode D ₅ to the resistor R ₃ side connected, so as to charge the capacitor C ₂ by the series circuit of the diode D ₅ and the resistor R ₃ (first resistor), the charge of capacitor C ₂ the discharge is blocked by the diode D _5. By adding the diodes D ₄ and D ₅ in this manner, a discharge path for the capacitor C ₂ during the off period of the switching element Q ₁ is prevented.

[0027] Here, in the vicinity of the start of operation of the overcurrent protection phototransistor PT of the photocoupler PC ₁ becomes cut-off state, the charging path of the capacitor C ₂ in the ON period switching element Q ₁ is a diode D ₅ , Resistance R
_{3 and} a series circuit of a diode D ₄ , a resistor R ₉ , and a Zener diode D ₃ . And
Discharge path of the capacitor C ₂ in the OFF period of the switching element Q ₁ is, can not discharge through the series circuit by the presence of the diode D _4, D _5, and a natural discharge.

[0028] Thus, during the on-period switching element Q _1, a diode D _5, a series circuit of a resistor R _3, and the diode D _4, resistor R _9, charging of the capacitor C ₂ in series circuit of the Zener diode D ₃ Only by performing the above (by appropriately setting the values of the resistors R ₃ and R ₉ ), it is possible to freely adjust the difference in the operating point of the overcurrent protection due to the difference in the input voltage.

The overcurrent protection is performed by the switching element Q
Since only adjusted by charging the capacitor C ₂ to the _first on period, forced discharge is not required of the capacitor C _2, the switching operation of the switching element Q ₁ so that electric charge remains in the capacitor C ₂ A small current is sufficient for the operation of overcurrent protection. Then, turn off of the switching element Q ₁ is steeper as compared with the prior art. From the above two points, the efficiency of the switching power supply can be improved. Further, the operating point of the overcurrent protection can be freely adjusted by appropriately setting the values of the resistors R ₃ and R _{9 as in} the related art while maintaining the above effects.

Second Embodiment FIG. 2 shows a second embodiment. In this embodiment, one ends of the resistors R ₃ and R ₉ are commonly connected, and the common connection point is connected to the cathode of the diode D ₄ .
This is a single diode. In this embodiment, the capacitor C in the on-period switching element Q _1
2 is a diode D ₄ , a resistor R ₃ , a diode D ₄ , a resistor R ₉ , and a Zener diode D _3. The discharging of the capacitor C ₂ during the OFF period of the switching element Q ₁ is performed by the diode D 4. ₄ to prevent spontaneous discharge. In this way, the diode D ₄ as one, it is possible to obtain the same effect as the previous embodiment.

[0031] In the above embodiments has described the case of using an FET as the switching element Q _1, it is also possible to apply the present invention to the switching power supply circuit of the RCC system using the transistor switching element Things.

[0032]

According to the present invention, in the vicinity of the start of the operation of the overcurrent protection, the charging path of the capacitor during the ON period of the switching element is connected to the first resistor and the second resistor via the diode and the Zener. It becomes two paths of the diode,
The discharge path of the capacitor during the off-period of the switching element cannot be discharged through the first resistor, the second resistor, and the Zener diode by the diode, but is a natural discharge. As described above, during the ON period of the switching element, the overcurrent protection due to the difference in the input voltage is achieved only by charging the capacitor by the circuit of the first resistor and the second resistor and the Zener diode via the diode. Can be freely adjusted. 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 the charge remains in the capacitor, the switching operation of the switching element is performed. A small current is sufficient for the operation of overcurrent protection. And
The turn-off of the switching element becomes steeper than in the related art. From the above two points, the efficiency of the switching power supply can be improved. In addition, the operation point of the overcurrent protection can be freely adjusted by appropriately setting the values of the first resistor and the second resistor while maintaining the above effects. is there.

[Brief description of the 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 switching power supply according to Embodiment 2 of the present invention.

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

FIG. 4 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 resistor) D ₃ Zener diode D ₄ the diode D ₅ diodes

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-4-46562 (JP, A) JP-A-4-200272 (JP, A) JP-A-63-87170 (JP, A) 100993 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) H02M 3/338 H02M 3/28 H02M 7/537

Claims (1)

(57) [Claims] 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 an output voltage; and a constant voltage output voltage receiving a signal from the voltage detection circuit. A control circuit for performing control and overcurrent control of the output current, wherein the control circuit is connected in parallel between a control terminal of the switching element and ground and a control transistor in accordance with a signal amount of the voltage detection circuit. When the switching element is turned on by the charging time constant of the impedance element, the impedance element that changes the impedance is connected between the base and the emitter of the control transistor. A capacitor that is charged to turn on the control transistor to turn off the switching element, and that when the switching element is turned off, the capacitor is charged in a direction opposite to the charging direction by a voltage generated by a feedback winding of the output transformer; on the switching element when the value of the impedance element during overcurrent of the output current becomes large, a first resistor to charge time constant of the capacitor of a predetermined in accordance with the oFF state, and the first resistor In a ringing choke converter type switching power supply device, which is configured in parallel with a Zener diode whose impedance changes in a forward direction and a reverse direction and a series circuit of a second resistor, the first resistor and the second resistor The capacitor discharges through a series circuit of a resistor and a Zener diode. Switching power supply device is characterized by providing a diode in a direction to prevent the discharge to the path.