JP3063825B2 - 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 of the type in which a switching element connected to a DC power supply via a primary winding of a transformer is turned on / off by pulse width modulation (PWM) pulses. .

[0002]

2. Description of the Related Art As shown in FIG. 1, a switching power supply of the type described above includes a primary winding 3 of a transformer 2 connected between one end and the other end of a DC power supply 1 composed of, for example, a rectifying and smoothing circuit. For example, it has a series circuit of a switching element 4 composed of an FET. A smoothing capacitor 7 is connected to a secondary winding 5 of the transformer 2 via a diode 6. The smoothing capacitor 7 functions as a power supply for a load connected between the output terminals 8 and 9. A PWM (pulse width modulation) pulse forming circuit 10 is connected to a control terminal (gate) of the switching element 4. The PWM pulse forming circuit 10 generates a PWM pulse controlled so as to make the voltage of the smoothing capacitor 7 constant, and turns on the switching element 4.
Control off. The transformer 2 is provided with a tertiary winding 11 for obtaining a feedback signal for constant voltage control and for obtaining a power supply voltage of the PWM pulse forming circuit 10. This 3
A capacitor 13 is connected to the next winding 11 via a diode 12.
Are connected in parallel. Primary winding 3, Secondary windings 5 and 3
Since the polarity of the secondary winding 11 is determined as indicated by a black circle, the diode 6 and the
12 is non-conductive, magnetic energy is stored in the transformer 2, and the stored energy of the transformer 2 is released during the OFF period of the switching element 4, so that the diodes 6, 12 become conductive. When the diode 6 conducts, the voltage of the smoothing capacitor 7 is applied to the secondary winding 5, and the voltage of the smoothing capacitor 7 is applied to the tertiary winding 11 by the secondary winding 5 and the tertiary winding 11.
A reduced voltage can be obtained at a ratio determined by the turns ratio. Since the voltage of the tertiary winding 11 and the voltage of the capacitor 13 correspond to the smoothing capacitor 7, they include a feedback signal component. Therefore, the capacitor 13 is connected to the error signal forming circuit 14 and functions as a feedback signal supply source. The error signal forming circuit 14 includes a resistor for dividing the voltage of the capacitor 13, a reference voltage source, and an error amplifier for generating an output corresponding to the difference between the detection voltage divided by the resistor and the reference voltage. The output of the error signal forming circuit 14 is supplied to the PWM pulse forming circuit 10 and used for determining the pulse width of the PWM pulse. PWM pulse forming circuit 1
The power supply terminal 0 is connected to the upper end of the capacitor 13 and to one end of the power supply 1 via the starting resistor 15. The ground terminals of the PWM pulse forming circuit 10 and the error signal forming circuit 14 are connected to the lower end of the capacitor 13 and the power supply 1.
Is connected to the other end. The starting resistor 15 is the tertiary winding 11
Before the voltage is obtained from, the charging current of the capacitor 13 is supplied.

[0003]

In the switching power supply shown in FIG. 1, the error signal forming circuit 14 is connected to the capacitor 13 in parallel. Therefore, all of the current flowing through the starting resistor 15 is not used for charging the capacitor 13, and a part of the current is used for charging the error signal forming circuit 14.
Flows to For this reason, the time required for raising the voltage of the capacitor 13 to a predetermined level becomes long, and the startup cannot be started quickly. It is conceivable to lower the value of the starting resistor 15 in order to improve the starting characteristics. But,
When the value of the starting resistor 15 is reduced, the value of the current passing therethrough increases, and the power loss increases. Further, in order to prevent a current flowing through the starting resistor 15 from flowing into the error signal forming circuit 14, a diode and a capacitor dedicated to the error signal forming circuit 14 are connected to the tertiary winding 11, and a feedback signal is supplied from here. It is possible to get. However, this configuration not only increases the number of components, but also increases the number of terminals when the control circuit portion of the switching power supply device is configured as a hybrid integrated circuit or a monolithic integrated circuit, resulting in an increase in cost.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a switching power supply capable of improving a starting characteristic without causing a power loss and an increase in the number of terminals.

[0005]

SUMMARY OF THE INVENTION The present invention for achieving the above-mentioned object and solving the above-mentioned problem will be described with reference to the accompanying drawings. A serial circuit of a primary winding 3 and a switching element 4 of a transformer 2 connected therebetween, secondary and tertiary windings 5 and 11 electromagnetically coupled to the primary winding 3, and a switching element 4 A first diode 6 connected to the secondary winding 5 in a direction conducting by a voltage generated in the secondary winding 5 during the off period, and the first diode 6 via the first diode 6. A first smoothing capacitor connected in parallel to the secondary winding;
The second diode 1 connected to the tertiary winding 11 in a direction that conducts by a voltage generated in the secondary winding 11
2 and a second smoothing capacitor 13 connected in parallel to the tertiary winding 11 via the second diode 12.
And a control power supply terminal 23 connected to the second smoothing capacitor 13, a starting resistor 15 connected between the DC power supply 1 and the control power supply terminal 23, and a detection blocking switch 32. wherein the output voltage detecting dividing resistors 39 and 40 connected to the control power supply terminal 23, the output voltage system via
A control reference voltage source 41 and the output voltage detecting voltage dividing resistor 3
9, 40 divided output and the output voltage control reference voltage source 4
And an error circuit for outputting an error signal corresponding to the difference between the reference voltage and the first smoothing capacitor in response to the error signal obtained from the error circuit.
A pulse width modulation pulse for making the voltage constant is used, and the switching element 4 is turned on / off by the pulse width modulation pulse. The voltage of the control power supply terminal 23 is used as a power supply voltage. a voltage control circuit has a <br/> detecting blocking switch control means 36 for controlling the detecting blocking switch 32, the control power supply terminal
To detect that the voltage of the child 23 has risen above a predetermined value.
The start detection circuit 53 of FIG.
And a constant voltage circuit 54 for obtaining a constant voltage for
The voltage control circuit flows to the switching element 4.
Current detecting a Re that, the current detecting means 17,18,1 for generating a current detection signal indicating a voltage value corresponding to the value of the current
9, 25, and the current detection obtained from the current detection means.
Amplitude control for controlling the amplitude of the output signal by the error signal
Means 30 and the end of the ON period of the switching element 4
The reference voltage (Vr) for determining the ON end for determining
Connected to the constant voltage circuit 54.
For determining the end time of turning on, which is composed of the voltage dividing resistors 34 and 35
A quasi-voltage source, and a voltage controlled by the amplitude control means 30.
Current detection signal and the reference voltage source for determining the ON end.
Comparator 28 for generating a comparison output with the reference voltage
And the amplitude of the amplitude-controlled current detection signal is turned on.
The reference voltage (Vr) obtained from the reference voltage source for termination is reached
In response to the output of the comparator 28 indicating that the
A pulse for controlling the switching element 4 to turn on.
End of width modulation pulse generation
After a while, the switching element 4 is turned on again.
Width modulated pulse to generate pulse width modulated pulse for
And a start-up detection circuit 53.
Zener die for start detection connected to the power supply terminal 23
A series circuit of the nodes 55 and 56 and the starting detection resistor 57
And a start-up detection transistor 59.
The zener diodes 55 and 56 for starting detection are for the control.
Conducts when the voltage at the power supply terminal 23 rises above a predetermined value.
And the activation detection transistor 59 is
Zener diodes 55 and 56 for detecting start-up and the start-up
The control power supply terminal obtained between the control resistor and the detection resistor 57.
23 in response to a signal indicating the rise of the voltage
Thus, the voltage of the control power supply terminal 23 is transmitted.
The constant voltage circuit 54 is connected to the start detection transistor.
It is connected to the control electric voltage terminal 23 via a static 59
And the detection control switch control means controls the startup detection.
When the output transistor 59 is turned on,
In response to the voltage obtained at the output stage of transistor 59,
The present invention relates to a switching power supply device, which is a circuit for controlling the detection blocking switch 32 to be turned on .

[0006]

The present invention has the following effects. (A) Since the detection blocking switch 32 is turned off at the time of startup, the current of the startup resistor 15 does not flow into the output voltage detection voltage dividing resistors 39 and 40 . Therefore, the starting resistance 1
5 is effectively used as a power supply for the voltage control circuit,
It is possible to start pulse width modulation (PWM) control quickly. (B) Since the turning-on end point is determined by utilizing the fact that the current of the switching element 4 rises with a slope due to the inductance of the primary winding 3 of the transformer, the overcurrent prevention is performed simultaneously with the constant voltage control. Can be achieved. (C) KotogaOkoshi the voltage of the control power supply terminal 23 is up standing
Since the reference voltage Vr is input to the comparator 28 after being detected by the motion detection circuit 53 , the PWM pulse can be accurately formed, and the ON time width of the switching element can be sufficiently increased at the time of startup. (D) Constant voltage circuit 33 and detection control switch control means
A configuration using the output of the start-up detection circuit 53 in both the stage 36
Therefore, the control of the detection blocking switch 32 can be easily achieved.

[0007]

Next, a switching power supply according to an embodiment of the present invention will be described with reference to FIGS. Note that FIG.
In FIG. 6, the same reference numerals are given to the substantially same parts as those in FIG. 1, and the detailed description is omitted.

In the switching power supply of FIG.
As in FIG. 1, for example, a primary winding 3 of a transformer 2 connected between one end and the other end of a DC power supply 1 composed of a rectifying and smoothing circuit.
And a series circuit of a switching element 4 composed of an FET. Secondary winding 5 electromagnetically coupled to primary winding of transformer 2
Is connected to a first smoothing capacitor 7 via a first diode 6. This first smoothing capacitor 7
Functions as a power source of a load connected between the output terminals 8 and 9. A tertiary winding 11 electromagnetically coupled to the primary and secondary windings of the transformer 2 is used to obtain a feedback signal for constant voltage control and to obtain a power supply voltage of the PWM pulse forming circuit, that is, a control power supply voltage. Is provided. This tertiary winding 11
Is connected in parallel with a second smoothing capacitor 13 via a second diode 12. Since the polarities of the primary winding 3, the secondary winding 5, and the tertiary winding 11 are determined as indicated by black circles, the first and second diodes 6, 12 are turned on during the ON period of the switching element 4. It is non-conductive, magnetic energy is stored in the transformer 2, and the stored energy of the transformer 2 is released during the OFF period of the switching element 4, so that the first and second diodes 6, 12 become conductive. In addition,
The secondary winding 5 and the tertiary winding 11 are applied to the tertiary winding 11 so that a voltage lower than the voltage of the first smoothing capacitor 7 is obtained.
Has been determined.

The control terminal (gate) of the switching element 4
Is connected to a constant voltage control circuit 16. The constant voltage control circuit 16 supplies the switching element 4 with a PWM pulse whose pulse width is determined so as to make the voltage of the first smoothing capacitor 7 constant. In addition, a resistor 17 is connected between the switching element 4 and the ground terminal of the power supply 1 as a current detecting means, in order to determine the end point of the ON of the PWM pulse by the peak value of the current waveform of the switching element 4, and A capacitor 19 is connected via a resistor 18 in parallel with the current detecting resistor 17 to remove a noise component.

Switching element 4 and constant voltage control circuit 16
Are arranged on the same circuit board (not shown), and are configured as one hybrid integrated circuit 20 surrounded by a dotted line. This hybrid integrated circuit 20 has first, second, third, fourth and fifth terminals 21, 22, 23, 2 for external connection.
4, 25. The first terminal 21 is for connecting the drain of the switching element 4 to the primary winding 3. The second terminal 22 connects the source of the switching element 4 to the ground terminal of the power supply 1 via the current detection resistor 17. The third terminal 23 is a control power supply terminal for connecting to the upper end of the second smoothing capacitor 13 and the starting resistor 15. The fourth terminal 24 connects the control circuit 16 to the ground terminal of the power supply 1. The fifth terminal 25 is for connecting to the current detecting resistor 17 via the resistor 18.

FIG. 3 shows the hybrid integrated circuit 20 of FIG. 2 in detail. The constant voltage control circuit 16 for controlling the switching element is roughly divided into an error signal forming circuit 26, P
WM pulse forming circuit 27, on-completion determination comparator 28, two transistors 29 and 30 as amplitude control means, drive circuit 31, transistor 32 as output voltage detection blocking switch, start-up detection and constant voltage circuit 3
3. Two resistors 34 as a reference voltage source for determining ON termination
And 35, a transistor 36 as a detection blocking switch control means, and two resistors 37 and 38.

The error signal forming circuit 26 includes first and second voltage detecting resistors 39 and 40 as output voltage detecting circuits.
, A Zener diode 41 as a reference voltage source, and a transistor 42 as an error amplifier or an error circuit. The first and second voltage detection output resistors 39 and 40 are connected in series with each other, and one end of the series circuit is connected to a control power supply terminal (third terminal) via a detection blocking transistor 32.
The other end is connected to a ground terminal (fourth terminal) 24. The reference power supply zener diode 41 is connected between the emitter of the transistor 42 and the ground terminal 24. The breakdown voltage (Zener voltage) of the Zener diode 41 is controlled by the control power supply terminal 23.
Is set lower than the normal voltage. Accordingly, the Zener diode 41 conducts while the detection blocking transistor 32 is on, and the voltage between the terminals becomes substantially constant. The base of the error amplifying transistor 42 is connected to the voltage dividing point of the two voltage detecting resistors 39 and 40, and its collector is connected to the control power supply terminal 23 via one transistor 29 forming a mirror circuit. The other transistor 30 constituting the mirror circuit is connected between the control power supply terminal 23 and the fifth terminal 25, and this base is connected together with the base of the one transistor 29 to the collector of the error amplification transistor 42. I have. Terminal 25
Is a current detecting resistor 1 via a resistor 18 as shown in FIG.
7, the current flowing through the transistor 30 is added to the current flowing through the current detecting resistor 17. As a result, an operation of controlling the amplitude of the current flowing through the switching element 4 by the current of the transistor 30 occurs. As a result, a voltage waveform in which the amplitude of the voltage waveform corresponding to the current of the switching element 4 is controlled by the transistor 30 is obtained at the terminal 25. Terminal 25 is a comparator 28
, The voltage at the terminal 25 is compared with the reference voltage at the negative input terminal of the comparator 28.

The PWM pulse forming circuit 27 includes a PWM pulse forming comparator (comparator) 43, a saw-tooth wave capacitor 44 used for determining an OFF period, a discharging resistor 45, a charging transistor 46, and a backflow preventing device. It comprises a diode 47, a base resistor 48, a diode 50 of a charge control and feedback resistor 49, and an on-end transistor 51. One end of the sawtooth wave generating capacitor 44 is connected to the control power supply terminal 23 via the charge control transistor 46 and the diode 47, and the other end of the capacitor 44 is connected to the ground terminal 24. The discharging resistor 45 is connected in parallel to the capacitor 44. The output line 52 of the start-up detection and constant voltage circuit 33 is connected to the base of the transistor 46 via the resistor 48. The collector of the transistor 51 for charge control and output pulse control is connected to the output terminal of the comparator 43, the emitter is connected to the ground terminal 24, and the base is connected to the comparator 28. One input terminal (positive terminal) of the control pulse forming comparator 43 is connected to a constant voltage line 52 via a resistor 48, and the other input terminal (negative terminal).
Is connected to one end of the capacitor 44, and the output terminal is connected to the gate of the switching element 4 via the drive circuit 31. The feedback resistor 49 and the diode 50 are connected between one input terminal and the output terminal of the comparator 43. The power terminals of the two comparators 28, 43 and the drive circuit 31 are connected to the control power terminal 23, and the ground terminals are connected to the ground terminal 24.

The start detection and constant voltage circuit 33 is connected between the control power supply terminal 23 and the ground terminal 24, and sends out a constant voltage to the line 52 after the start of the start. FIG. 4 shows the start detection and voltage regulation circuit 33 in detail. The start detection and constant voltage circuit 33 includes a start detection circuit 53 and a constant voltage circuit 54. The start detection circuit 53 is connected to the control power supply terminal 2
3 is a circuit for detecting that the voltage of the third Zener 3 has become equal to or higher than a predetermined level.
6 and a resistor 57, which are connected between the control power supply terminal 23 and the ground terminal 24. Further, the activation detection circuit 53 is provided with a first circuit forming a mirror circuit.
And a second transistor 58, 59, a NOT circuit 60, and a resistor 61. Transistor 58 has an emitter connected to control power supply terminal 23 and a first collector
The second collector is connected to the junction between the second Zener diodes 55 and 56, and the second collector is connected to the resistor 61 and the output terminal of the NOT circuit 60. An input terminal of the NOT circuit 60 is connected to an interconnection point between the second Zener diode 56 and the resistor 57. The emitter of the second transistor 59 functioning as a start-up control switch is connected to the control power supply terminal 23, and its base is connected to the base of the first transistor 58 and the NOT circuit 6 via the resistor 61.
0, and its collector is connected to the ground terminal 2 via the Zener diode 62 of the constant voltage circuit 54.
4 is connected. The output line 52 of the voltage regulating circuit 54 comprising a zener diode 62 is connected to a transistor 59.
And the Zener diode 62.

[0015] via a resistor 38 between the base and the ground terminal 24 of the detection blocking transistor 32 in FIG. 3 Detection inhibitory
A transistor 36 as a stop switch control means is connected, and the base of the transistor 36 is connected to the constant voltage output line 52 via the resistor 37. Since the voltage of the constant voltage output line 52 is obtained when the capacitor 13 of FIG. 2 is charged, the transistors 32 and 36 are also turned on after the capacitor 13 is charged.

A series circuit of resistors 34 and 35 as a reference voltage source for determining the ON end time is connected between the constant voltage output line 52 and the ground terminal 24. Therefore, the voltage of the constant voltage output line 52 is divided by the resistors 34 and 35 after the start of the start and supplied to the comparator 28.

[0017]

Next, the operation of the switching power supply shown in FIGS. 2 to 4 will be described with reference to FIGS. FIG. 5 is a waveform diagram schematically showing the state of each unit in FIGS. 2 and 3 during normal operation after startup. At time t0 in FIG. 5, the output voltage V3 of the comparator 43 in FIG. 3 changes from the low level to the high level to generate a PWM pulse, and a signal corresponding thereto is transmitted via the drive circuit 31 to the switching element 4 composed of an FET. When the voltage is applied between the gate and the source, the switching element 4 is turned on, and a current I1 flows through a closed circuit including the DC power supply 1, the primary winding 3, the switching element 4, and the current detecting resistor 17. Since the primary winding 3 has an inductance, the current I1 increases with a slope, and the waveform of the voltage Va corresponding to the waveform of the current I1 is obtained from the current detecting resistor 17 as shown in FIG. Can be Now, assuming that the output current of the error signal forming circuit 26 of FIG. 3 is zero, the currents of the transistors 29 and 30 are also zero. The current flowing into the resistor 17 is also zero, and only the current flowing through the switching element 4 flows through the resistor 17. However, in practice, the amplitude of the current I1 flowing through the resistor 17 is controlled to execute the constant voltage control. That is, FIG.
When the transistor 32 is turned on, an error signal is formed by the error signal forming circuit 26, and this error signal becomes a feedback signal for constant voltage control to control the transistors 29 and 30 of the mirror circuit. The current passing through the transistor 30 functioning as a transistor flows into the current detecting resistor 17 via the terminal 25 and the resistor 18 in FIG. Therefore, the current of the sum of the current component passing through the switching element 4 and the current component for feedback control flows through the current detection resistor 17. For example, when the voltage of the smoothing capacitor 7 in FIG. 2 becomes higher than a predetermined value, the current of the transistor 30 based on the error signal increases, and the voltage Va corresponding to the current I1 as shown by a dotted line in FIG. Becomes larger, and t
0, the reference voltage Vr corresponding to the predetermined current value from the start point
And the width of the PWM pulse is reduced as shown in FIG.
As shown by the dotted line in (C), the width becomes narrow, and the output voltage returns to the desired value. When the output voltage is lower than the desired value, the above operation is reversed.

Next, the operation of forming the error signal will be described in more detail. The switching power supply device shown in FIG. 2 is called a reverse type or flyback type switching regulator, and includes diodes 6, 1, 1, 2, and 3 in output stages of secondary and tertiary windings 5, 11 during an off period of switching element 4.
2 turns on. The constant voltage of the smoothing capacitor 7 is applied to the secondary winding 5 via the diode 6, and a voltage corresponding to the voltage is obtained in the tertiary winding 11, and the capacitor 13 is charged with this voltage. . Therefore, the capacitor 13
Has a value corresponding to the voltage of the smoothing capacitor 7 in the output stage. If the voltage of this capacitor 13 is detected, the voltage V0 between the output terminals 8 and 9 is equivalently detected. The voltage of the capacitor 13 of FIG. 2 is applied to the voltage detection resistors 39 and 40 of FIG.
As a result, a detection voltage corresponding to the output voltage V0 is applied between the base of the error amplification transistor 42 and the ground, and the value of the difference between the reference voltage given by the Zener diode 41 and the detection voltage is applied to the base / emitter of the transistor 42. And a collector current corresponding to the value of the difference flows. The base current of the transistors 29 and 30 of the mirror circuit flows through the emitter and the base to the error amplification transistor 42, so that the collector current of the transistors 29 and 30 of the mirror circuit is
The value corresponds to the current of 2. As a result, the transistor 30 functions as current amplitude control means, and the width of the PWM pulse is controlled.

The voltage Vc of the control power supply terminal 23 is the voltage of the capacitor 13 shown in FIG. Switching power supply is PW
Since no voltage is obtained in the tertiary winding 11 until the operation by the M pulse is started, the capacitor 13 must be charged via the starting resistor 15. In order to reduce the power loss in the starting resistor 15, the value R15 of the starting resistor 15 is set to a relatively large value. Therefore, for example, when the power supply from the power supply 1 is started at time t0 in FIG. 6, the capacitor 13 is charged with the time constant of R15C13, and the voltage Vc of the capacitor 13 is relatively slowly reduced as shown in FIG. Rise. When the voltage Vc of the capacitor 13, that is, the voltage of the control power supply terminal 23 reaches the high level voltage VH at the time t1 in FIG. 6, the two Zener diodes 55 and 56 shown in FIG.
Is turned on, the voltage at the input terminal of the NOT circuit 60 changes from a low level to a high level, and this output voltage goes to a low level. As a result, the pair of transistors 58 and 59 that were off during the period from t0 to t1 are turned on, and a constant voltage output is obtained on the output line 52. As a result, the transistors 36 and 32 in FIG. 3 are turned on, the voltage of the control power supply terminal 23 is input to the voltage detection resistors 39 and 40 of the error signal forming circuit 26, and the formation of the error signal is started. , 3
The reference voltage determined in 5 is input to the comparator 28,
It is possible to determine the ON end of the PWM pulse. FIG.
At the time t1, a base current flows into the transistor 46, which is turned on, and the capacitor 44 is rapidly charged through the transistor 46 and the diode 47.
Since no resistor is connected to this charging circuit, the voltage V2 of the capacitor 44 rises rapidly to, for example, 5V. That is, the voltage V1 at the base of the transistor 46 is, for example, 6.5.
Assuming that the voltage is V, a value obtained by subtracting the base-emitter voltage of the transistor 46 and the forward voltage of the diode 47 from this (for example, 5 V) becomes the charge completion voltage of the capacitor 44. Although FIG. 5 shows the waveforms of the respective parts during normal operation, the state is later than t0 in FIG. 5 even immediately after startup. Therefore, in the following description, the waveform of FIG. 5 will be referred to. Immediately after t0 in FIG.
Since the positive input terminal voltage V1 is higher than the negative input terminal voltage V2, the output voltage V3 becomes a high level as shown in FIG. 5C, a PWM pulse is generated, and the switching element 4 is turned on. As a result, the voltage of the power supply 1 is applied to the primary winding 2 of the transformer 2, and the current starts flowing therethrough. Since the primary winding 3 has an inductance, the current flowing therethrough increases with a gradient, and a detection voltage Va corresponding to the current I1 of the current detection resistor 17 is obtained, which is connected to the positive input terminal of the comparator 28. Applied. Since the reference voltage Vr shown in FIG. 5B is applied to the negative input terminal of the comparator 28, when the voltage Va based on current detection reaches the reference voltage Vr at time t1, the comparator 2
8 changes from low level to high level at time t1 in FIG. 5, and the transistor 51 is turned on. As a result, the collector voltage of the transistor 51, that is, the output voltage V3 of the comparator 43 becomes low as shown in FIG. 5C, and the interval of the PWM pulse ends. Comparator 43
When the output voltage V3 of the comparator 43 becomes low, the diode 50 conducts, and the voltage V1 at the positive input terminal of the comparator 43 drops from 6.5V to, for example, 3.5V as shown in FIG. Since the voltage becomes lower than the voltage V2 of the terminal, the output voltage V3 of the comparator 43 is maintained at a low level from the time t1 as shown in FIG. 5C, and the OFF state of the switching element 4 is also maintained. Output voltage V of comparator 43
3 becomes low level and the voltage V1 at the negative input terminal becomes lower than the voltage V2 at the negative input terminal, the transistor 46 and the diode 47 of the charging circuit of the capacitor 44 become reverse biased and become non-conductive. , And the charging of the capacitor 44 stops. As a result, the electric charge of the capacitor 44 is released through the resistor 45, and the voltage V2
Decreases with a constant slope as shown in FIG. At time t2, when the voltage of the capacitor 44, that is, the voltage V2 of the negative input terminal of the comparator 43 becomes lower than the voltage V1 of the positive input terminal, the output voltage V3 of the comparator 43 returns to the high level again, and the next PWM pulse is generated. During the off period from t1 to t2, the current of the switching element 4 becomes zero, and the voltage Va at the positive input terminal of the comparator 28 also drops below the reference voltage Vr, so that the output of the comparator 28 is held at a low level. , Transistor 5
1 is also held off. PWM in this embodiment
Although the pulse width (ON period) of the pulse changes according to the change in the output voltage, the OFF period between the pulses is kept constant.

Referring again to FIG. 6, the relationship between the change in the voltage Vc of the control power supply terminal 23 and the period during which the PWM pulse is generated will be described. When the operation of generating the PWM pulse starts at time t1 in FIG. 6, the energy of the capacitor 13 decreases due to the generation of the PWM pulse. However, since a voltage is obtained in the tertiary winding 11, an almost constant voltage depending on the voltage is obtained. It is obtained from the time point t2, and this contributes to the power supply voltage of the control circuit 16. Note that a small charging current continues to flow through the capacitor 13 via the starting resistor 15, which is extremely small as compared with the charging current based on the tertiary winding 11. When the power supply from the power supply 1 is stopped at time t3 in FIG. 6, the charging of the output smoothing capacitor 7 via the secondary winding 5 is also stopped, and the tertiary winding 11 is stopped.
Of the capacitor 13 and the charging of the capacitor 13 through the starting resistor 15 are also stopped. As a result, the voltage Vc of the control power supply terminal 23 decreases from t3 in FIG. 6 and crosses the lower reference voltage (lower trigger point) VL at time t4.
The zener diodes 55 and 56 in FIG. 4 are both turned off. As a result, the output of the NOT circuit 60 goes high, the transistors 58 and 59 turn off, the voltage of the output line 52 goes low (zero), and a PWM pulse is generated from the PWM pulse forming circuit 27 of FIG. No longer
Further, the detection blocking transistor 32 is turned off.

The activation detection circuit 53 shown in FIG. 4 operates equivalently to a comparator or a Schmitt trigger circuit having hysteresis, and the voltage Vc of the control power supply terminal 23 is changed to the level shown in FIG.
Upper reference voltage (upper trigger point) V shown in (A)
It is formed so that the output state does not change even if it crosses H from the lower side to the upper side and then again from the upper side to the lower side. That is, the two diodes 55 and 56 in FIG.
Conducts once, and transistor 58 is turned on, upper Zener diode 55 is short-circuited between the emitter of transistor 58 and the upper collector, equivalently maintaining lower Zener diode 56 on. The output state of the NOT circuit 60 is determined depending on whether or not the voltage can be maintained, and when the output voltage of the NOT circuit 60 becomes a voltage at which the lower zener diode 56 cannot be kept on, that is, the lower reference voltage VL in FIG. The output of the NOT circuit 60 goes high, turning off the transistors 58 and 59.

As is apparent from the above description, this switching power supply device includes the detection blocking transistor 32 in series with the voltage detection resistors 39 and 40 of the error signal forming circuit 26, and the control power supply by the starting resistor 15. When the charging of the capacitor 13 is almost completed, the detection blocking transistor 32 is turned on.
The charging of the capacitor 13 is not interrupted by the
5, the capacitor 13 can be charged to a predetermined level in a short time, and the PWM pulse can be generated quickly. Further, since the configuration is such that the voltage of the tertiary winding 11 is used as a control power supply and also used for detecting an output voltage, the number of terminals and the number of components of the control circuit 16 can be reduced. Also, a start-up detection and constant voltage circuit 3
3 is used not only in the PWM pulse forming circuit 27 but also in the control of the detection blocking transistor 32, so that the circuit configuration is simplified. Further, since the end point of the ON period of the switching element 4 is determined based on the current flowing therethrough, not only constant voltage control is achieved but also overcurrent prevention is achieved.

[0023]

[Modifications] The present invention is not limited to the above-described embodiment, and for example, the following modifications are possible. (1) An error amplifier circuit may be formed by an operational amplifier instead of the transistor 42. (2) The control circuit 16 or the control circuit 16 and the switching element 4 can be one monolithic integrated circuit. (3) The activation detection circuit 53 of FIG. 4 can be a known hysteresis comparator or a Schmitt trigger circuit using an operational amplifier.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a conventional switching power supply device.

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

FIG. 3 is a circuit diagram showing a control circuit and a switching element of FIG. 2;

FIG. 4 is a circuit diagram showing a startup detection and constant voltage circuit of FIG. 3;

FIG. 5 is a waveform diagram showing a state of each unit in FIGS. 2 and 3;

FIG. 6 is a waveform diagram showing the state of each part in FIG. 3 in principle.

[Explanation of symbols]

 Reference Signs List 4 switching element 11 tertiary winding 15 starting resistor 23 control power supply terminal 26 error signal forming circuit 32 detection blocking transistor

Continuation of the front page (56) References JP-A-59-222076 (JP, A) JP-A-62-16071 (JP, A) JP-A-63-299773 (JP, A) JP-A-7-123709 (JP) , A) (58) Field surveyed (Int. Cl. ⁷ , DB name) H02M 3/28 H02M 3/335

Claims (1)

(57) [Claims] 1. A series circuit of a primary winding (3) of a transformer (2) and a switching element (4) connected between one end and the other end of a DC power supply (1); The secondary and tertiary windings (5, 11) electromagnetically coupled to the line (3), and the directionality of conduction by the voltage generated in the secondary winding (5) during the off period of the switching element (4). And a first diode (6) connected to the secondary winding (5), and connected in parallel to the secondary winding (5) via the first diode (6). A first smoothing capacitor; and a third winding having a directivity to be conducted by a voltage generated in the third winding when the switching element is off. And a third diode (12) connected to the third coil (12) via the second diode (12). A second smoothing capacitor (13) connected in parallel to 1), a control power supply terminal (23) connected to the second smoothing capacitor (13); A starting resistor (15) connected between the control power terminal (23) and an output voltage detecting voltage dividing resistor connected to the control power terminal (23) via a detection blocking switch (32); (3
9, 40), an output voltage control reference voltage source (41) , a divided output of the output voltage detection voltage dividing resistor (39, 40), and the output voltage control reference voltage source (41). An error circuit (42) for outputting an error signal corresponding to a difference from the reference voltage, and a voltage of the first smoothing capacitor (7) in response to the error signal obtained from the error circuit (42). Forming a pulse width modulation pulse for making the switching power supply constant, and turning on / off the switching element (4) by the pulse width modulation pulse. The control power supply terminal (23)
A voltage control circuit using the voltage of the power supply voltage as a power supply voltage, a detection prevention switch control means (36) for controlling the detection prevention switch (32), and a voltage of the control power supply terminal (23). Rising above a certain value
And a start detection circuit (53) for detecting that a constant voltage has been obtained for use in the voltage control circuit.
A voltage control circuit (54), wherein the voltage control circuit detects a current flowing through the switching element (4),
Generates a current detection signal indicating the voltage value corresponding to the current value
Current detecting means (17, 18, 19, 25) to perform, and the amplitude of the current detection signal obtained from the current detecting means
Control means (30) for controlling the amplitude by the error signal
And the end of the ON period of the switching element (4).
To obtain the reference voltage (Vr) for determining the ON end for
A voltage connected to the constant voltage circuit (54).
On-going end point determination base comprising compression resistors (34, 35)
A quasi-voltage source, and a current detection signal whose amplitude is controlled by the amplitude control means (30).
No. and the comparator (28) for generating a comparison output between on termination determination reference voltage source or we obtained the reference voltage, the amplitude the on termination of said amplitude controlled current detection signal
The reference voltage (Vr) obtained from the reference voltage source has been reached
In response to the output of the comparator (28)
A switch for controlling the switching element (4) to turn on.
The generation of the pulse width modulation pulse ends, and from this end point
After a fixed time, the switching element (4) is turned on again.
Pulse width modulation to generate a pulse width modulation pulse
And a start-up detection circuit (53). The control power supply terminal (23)
Detection zener diodes (55, 5
6) and a series circuit of a start detection resistor (57);
An output transistor (59), and the start-up detecting zener diodes (55, 56)
The voltage of the control power supply terminal (23) rises above a predetermined value.
The start-up detection transistor (59)
Zener diodes (55, 56) for detecting the start-up
The control power supply terminal obtained between the control power supply and the resistor (57)
Turns on in response to the signal indicating the voltage rise of (23).
And transmits the voltage of the control power supply terminal (23).
Is intended, the constant voltage circuit (54) is the activation detection transistor
(59) connected to the control voltage terminal (23)
And the detection blocking switch control means includes a switch for detecting the activation.
When the transistor (59) is in the ON state, the start detection
In response to the voltage obtained at the output stage of the transistor (59)
Circuit for turning on the detection blocking switch (32)
Switching power supply unit, characterized in that it.