JP2000270546A - Switching power source circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a soft start operation in an intermittent operation to be without fail, to immediately drive a circuit by means of a minimum duty ratio in an intermittent drive period, and to drop consumption current in a standby state in the switching power source circuit. SOLUTION: In a circuit where reference voltage generated by a soft start circuit 19 and an error detecting circuit 12 is applied to a comparator 20 via a low pass filter 21, the charge of a capacitor in the low-pass filter 21 is surely discharged, and a soft start operation in a standby state is positively made with MOS 34 installed between the output of the low pass filter 21 and ground and driven by a clock signal CL and a resistor 35 installed between the input of the low-pass filter 21 and ground. A switch SW2 is installed in a soft start terminal, to which the capacitor C2 is connected and pulse width drive by a minimum duty can immediately be started, even in the drive period of an intermittent operation.

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 circuit for generating a predetermined power supply voltage on a secondary side of a transformer by switching a current flowing on a primary side of the transformer.

[0002]

2. Description of the Related Art A switching power supply circuit is used as a power supply circuit used for a power supply of an electronic device such as a television or a video tape recorder. In such a switching power supply circuit, an integrated circuit having three to five terminals is used, and a switching transistor for switching the primary side of the transformer and a driving circuit for driving the switching transistor are integrated.

FIG. 2 is a block diagram showing such a switching power supply circuit. The AC power is rectified into DC by the bridge rectifier circuit 1 and supplied to one end of the primary winding of the transformer 2. The other end of the primary winding is connected to the switching drive IC 3. The output of the secondary winding 2a of the transformer 2 is rectified by the diode 4,
The power is supplied to an internal circuit as a main power supply V1 of the electronic device, and is also supplied to a microcomputer 5 for controlling the electronic device via a switch 6. The output of the secondary winding 2b is rectified by the diode 7 and supplied as power to the IC 3 via the phototransistor 8. The phototransistor 8 is used as a feedback signal input for controlling stabilization of the main power supply V1, and is optically coupled to a photodiode 9 connected to the main power supply V1. The secondary winding 2c of the transformer 2 is rectified by the diode 10 and the microcomputer 5 in the standby state of the electronic device.
Is applied to the microcomputer 5 via the switch 6 as the power supply V3.

The IC 3 includes a start circuit 11 for supplying a voltage to each part of the IC 3 to drive the IC 3 when the power is first turned on, and a phototransistor 8 for the IC 3.
Error detection circuit 12 for detecting a signal fed back to the power supply
A timer circuit 13 for determining intermittent feedback for intermittently performing a switching operation in a standby state;
MOS transistor 14 for switching the primary winding of transformer 2, and control signal CONT from microcomputer 5
An oscillation circuit 16 whose oscillation frequency is switched by a switch 15 whose opening and closing are controlled by the
A PWM circuit 17 for pulse-width-modulating the oscillation output of the MOS transistor 14 based on the output of the error detection circuit 12, a logic circuit 18 for applying a pulse-width-modulated signal to the MOS transistor 14 according to the output state of the timer circuit 13, And a soft start circuit 19 for gradually increasing the duty ratio from the minimum duty according to the time constant determined by the externally connected capacitor C2 during the intermittent operation.

The operation of FIG. 2 will be described. When no power is supplied, no voltage is generated in the secondary winding 2b of the transformer, and no power supply voltage is supplied to the IC 3, so that the IC 3 does not operate. When the power is turned on for the first time, the voltage rectified by the bridge rectifier circuit 1 is supplied from the primary winding of the transformer 2 to the start circuit 11.
Is supplied to the IC 3 via the. Thereby, the operation of the IC 3 is started, and the oscillation of the oscillation circuit 16 is started. The oscillation frequency of the oscillation circuit 16 at this time is 100 KHz. Since the soft start circuit 19 generates an output such that the duty ratio gradually increases, the oscillation output becomes PW
The initial state pulse width modulation is performed by the M circuit 17. P
The output of the WM circuit 17 is connected to a MOS
The MOS transistor 14
Performs the switching operation of the primary winding in accordance with the pulse width modulated signal. Thereby, each 2 of the transformer 2
Voltage is generated in the secondary winding. Thereby, the microcomputer 5 also operates, and the microcomputer 5 operates the emitting diode 9 according to the output signal. Therefore, the phototransistor 8
Turns on, and the voltage of the secondary winding 2b is supplied to the IC3. When a voltage is supplied to the IC 3, the start circuit 11
Stops the operation, and stops supplying power to the inside of the IC 3 from the primary winding. Thereby, the normal operation state is maintained.

Next, in a standby state, that is, in a state where the inside of the electronic device does not require a power source and only the microcomputer 5 is in an operating state, the microcomputer 5 controls the switch 6 by the control signal CONT1 to control the microcomputer 5 itself. Is switched to the power supply from the secondary winding 2c having a small current capacity, and the operation of the light emitting diode 9 is stopped. Further, the switch 15 is turned on to switch the oscillation frequency of the oscillation circuit 16 to 20 KHz. As a result, since the phototransistor 8 is turned off, the power supply from the secondary winding 2b to the IC 3 is stopped, so that the power of the IC 3 is supplied from the start circuit 11. At this time, a charging current flows through the capacitor 19 connected to the power supply of the IC 3, and the power supply voltage of the IC 3 rises. When the power supply voltage reaches a predetermined level, for example, 5.6 V, the timer circuit 13 stops the power supply from the start circuit 11. Then capacitor 1
From 9, a discharge current flows, and the power supply voltage decreases. When the power supply voltage drops to a predetermined voltage, for example, 4.7 V, the timer circuit 13 restarts the start circuit 1
1 to start power supply. By such an operation, the power supply voltage line of the IC3 is 4.7V and 5.6V.
A triangular wave having an amplitude of? Then, the timer circuit 13 counts the period of the triangular wave and, for example,
The logic circuit 18 is controlled for each cycle, and the PWM circuit 1
7 is applied to the MOS transistor 14. Note that M
While the OS transistor 14 is controlled, the capacitor 1
This is the discharge period of No. 9, that is, the falling period of the triangular wave. During the intermittent driving period, the soft start circuit 19 operates to gradually increase the duty ratio from the minimum duty ratio of pulse width modulation according to the time constant determined by the capacitor C2. This is to prevent the switching element 14 from being destroyed by an inrush current at the time of startup, and to reduce power consumption.

As described above, the MOS transistor 14
It is driven for a predetermined time at a predetermined interval at a frequency of 0 KHz. As a result, power consumption in the standby state is reduced.

[0008]

In the switching power supply circuit shown in FIG. 2, when the intermittent operation is started, the soft start circuit operates at the time of the first intermittent drive, but it becomes the minimum duty at the time of the next intermittent operation. In some cases, however, the voltage of the winding 2c on the 2 o'clock side suddenly rises and the current consumption increases.

Examination of the cause revealed that the reference voltage of the comparison circuit constituting the pulse width modulation circuit did not decrease and the previous voltage was maintained. This was because the reference voltage determined by the soft start circuit and the error detection circuit was applied to the comparison circuit via the low-pass filter, so that the charge charged in the capacitor of the low-pass filter did not escape. .

[0010]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and has a switching element for switching a current flowing through a primary side of a transformer, and a frequency signal for driving the switching element. An oscillation circuit that generates a signal, an error detection circuit that detects a feedback signal from the secondary side of the transformer, a pulse width modulation circuit that performs pulse width modulation on the output of the oscillation circuit based on a detection output of the error detection circuit, A logic circuit for continuously or intermittently supplying the output of the pulse width modulation circuit to the switching element according to an operation state and a standby state, a timer circuit for creating an intermittent frequency during the standby state, and the pulse width modulation during the intermittent operation. Switching power supply circuit with a soft start circuit that gradually increases the duty ratio of the The pulse width modulation circuit includes a comparison circuit that compares a reference signal generated by the soft start circuit and the error detection circuit with a transmission output of the oscillation circuit, and an intermittent input to the comparison circuit to which the reference voltage is applied. A switch circuit controlled by a frequency signal is provided.

Further, the reference signal is applied to the comparison circuit via a low-pass filter, and a high resistance is provided between an input of the low-pass filter and ground.

Further, the soft start circuit is provided with a terminal to which a capacitor for determining a time constant is connected,
By short-circuiting the terminal by a control signal, a soft-start operation can be performed at an arbitrary timing.

[0013]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, in which an error detection circuit 12 and a PWM circuit 17 are shown.
And a specific circuit of the soft start circuit 19. P
The WM circuit 17 includes a comparator 20 to which a triangular wave output from the oscillation circuit 16 is applied to an input, and a low-pass filter 21 to apply a reference voltage to the other input of the comparator 20.
It consists of.

The error detection circuit 12 includes a comparator 25 for comparing the voltage obtained by dividing the voltage of the power supply line Vc by the resistors 22, 23 and 24 with a reference voltage Vref, and a P-channel MOS 26 to which the output of the comparator 25 is applied.
And a channel MOS2 forming a current mirror circuit
The drain of the MOS 28 is connected to the emitter of the transistor 30 via the low-pass filter 21 and the resistor 29.

The soft start circuit 19 includes a transistor 30 having an emitter connected to a resistor 29, resistors 31 and 32 for setting a bias of the transistor 30, and an N-channel MO connected between the base of the transistor 30 and ground.
S33 and the base of the transistor 30 is I
It is derived as an external terminal of C3, and there is a capacitor C2
And a switch SW2 controlled by a control signal CONT3 from the microcomputer. N channel MOS33
Is applied with the clock signal CL output from the timer circuit 13. The clock signal CL is PWM
This signal is at the “L” level during a period in which the MOS transistor 14 is switched by the output. In the standby state, the signal is at the “L” level every cycle in which the timer circuit 13 performs the intermittent operation, for example, every two cycles.

In FIG. 1, the error detection circuit 12 compares the divided voltage of the power supply line Vc with the reference voltage Vref so that the voltage of the power supply line Vc becomes 5.6 V in the normal operation state. To control the PWM circuit 17. That is, when the voltage of the power supply line Vc becomes higher than 5.6 V, the output voltage of the comparator 25 decreases, the P-channel MOS 26 turns on, and the N-channel MO
It acts to flow a large amount of current in S27. Therefore, the current of the N-channel MOS 28 also increases, and the emitter voltage of the transistor 30 decreases, so that the reference voltage of the comparator 20 decreases. Thereby, the width of the output pulse of the comparator 20 becomes narrow.

Here, an N-channel M is connected between the input of the comparator 20 to which the low-pass filter 21 is applied and the ground.
The OS 34 is connected, and the same clock signal CL as that of the N-channel MOS 33 is applied to the gate of the N-channel MOS 34. This N-channel MOS 34 is a transistor for discharging the electric charge charged in the capacitor of the low-pass filter 21, and is turned off only during the driving period in which the intermittent operation is performed. Further, a resistor 35 having one end connected to the input side of the low-pass filter 21, that is, the connection point between the emitter of the transistor 30 and the resistor 29 and the other end grounded is provided. This resistor has a resistance value of about 50 KΩ to about 2 MΩ, and functions to discharge electrification of the capacitor of the low-pass filter 21 when the switch 30 is turned off by closing the switch SW2 during the intermittent operation period.

Next, the operation in the standby state will be described with reference to FIGS. Control signal C from microcomputer 5
Since the light emitting diode 9 is turned off by the ONT 1, the phototransistor 8 is turned off, and the voltage is not supplied to the IC 3 from the secondary winding 2b. Accordingly, the timer circuit 13 repeats charging of the capacitor C1 from the start circuit 11 and discharging of the capacitor C1, so that the waveform of the power supply line Vc becomes a triangular wave having 5.6V and 4.7V amplitude.

Further, the timer circuit 13 is connected to the power line V
By counting the triangular wave of c by the counter, the clock signal CL is set to the “L” level during the discharge period of the capacitor C1 every two cycles, that is, during the period when the waveform of the power supply line Vc falls. Also, during this period, the logic circuit 18 performs PWM control.
This is a period during which switching is performed by applying an output to the MOS transistor 14. In the standby state, the control signal CONT2 becomes "H" level and the switch SW1 is closed. Thereby, the oscillation circuit 16 is switched from the oscillation frequency of 100 KHz to the oscillation frequency of 20 KHz.

While the clock signal CL is at the "H" level, the MOS 33 is on, the capacitor C2 is discharged, and the transistor 30 is off. Also, M
Since the OS 34 is also on, the capacitor of the low-pass filter 21 is also discharged.

Next, when the clock signal CL goes to "L" level during the intermittent operation period, the MOSs 33 and 34 are turned off, so that a charging current starts to flow through the capacitor C2 via the resistor 32. Then, the base voltage of the transistor 30 gradually increases, and the emitter voltage increases. Therefore, the reference voltage applied to the comparator 20 gradually increases, and the pulse width of the PWM output gradually increases.

When the intermittent operation is performed as described above, the reference voltage applied to the comparator 20 is maintained at the ground potential because the MOS 34 is on until immediately before the driving period. When the reference voltage is the ground potential, the output of the comparator 20 remains at the “L” level, but at this time, the logic circuit 18 to which the output of the comparator 20 is applied is applied.
Is configured to apply a pulse having a predetermined minimum duty ratio, for example, a duty of 2%, to the switching element 14.

On the other hand, during the driving period in the intermittent operation, the control signal CONT3 from the microcomputer 5 switches the switch SW2
Is closed, the charge stored in the capacitor C2 is discharged. At this time, since the MOS 34 is off, the MOS 34 does not discharge the capacitor of the low-pass filter 21. In this case, the discharge of the capacitor is performed via the resistance of the low-pass filter 21 and the resistance 35.

Therefore, when the switch SW2 is closed even during the driving period of the intermittent operation, the reference voltage applied to the comparator 20 is immediately fixed to the ground potential, so that the output of the comparator 20 becomes "L" level. . As a result, the output of the logic circuit 18 becomes a pulse with the minimum duty, so that the driving of the switching element 14 becomes a minimum pulse drive, and the power consumption can be further reduced.

[0025]

As described above, according to the present invention, the soft start operation in the standby state can be reliably performed, and the soft start terminal is grounded during the driving period of the intermittent operation, whereby the pulse width driving of the minimum duty is immediately performed. Therefore, the rapid rise of the voltage on the secondary side can be suppressed, and the power consumption in the standby state can be further reduced.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a block diagram of a switching power supply circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Bridge rectifier circuit 2 Transformer 3 IC 4, 7, 10 Diode 5 Microcomputer 6, 15 Switch 8 Phototransistor 9 Light emitting diode 11 Start circuit 12 Error detection circuit 13 Timer circuit 14 Switching transistor 16 Oscillation circuit 17 Pulse width modulation circuit 18 Logic circuit 19 Soft-start circuit 20, 25 Comparator 26 P-channel MOS 27, 28, 33, 34 N-channel MOS 35 Resistance

Continuation of the front page (72) Inventor Norifumi Ikeda 2-5-5 Keihanhondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd. F-term (reference) 5G065 AA00 AA01 DA06 DA07 EA06 FA02 GA07 HA05 HA12 HA16 JA01 JA07 LA01 MA03 MA07 MA09 MA10 NA01 NA02 NA09 5H730 AA14 BB21 BB57 CC01 DD04 EE02 EE73 FF02 FF09 FF19 FG05 FG07 VV01 XC14

Claims (3)

[Claims] 1. A switching element for switching a current flowing to a primary side of a transformer, an oscillation circuit for generating a frequency signal for driving the switching element, and an error detection for detecting a feedback signal from a secondary side of the transformer. A pulse width modulation circuit that pulse width modulates an output of the oscillation circuit based on a detection output of the error detection circuit; and continuously or intermittently switches an output of the pulse width modulation circuit depending on a normal operation state and a standby state. A switching power supply circuit comprising: a logic circuit for supplying an element; a timer circuit for generating an intermittent frequency in the standby state; and a soft start circuit for gradually increasing a duty ratio of the pulse width modulation from a minimum value during the intermittent operation. In the above, the pulse width modulation circuit comprises a soft start circuit and an error detection circuit. A comparison circuit for comparing the reference signal created by the above with the output of the oscillation circuit, wherein a switch circuit controlled by an intermittent frequency signal is provided at the input of the comparison circuit to which the reference voltage is applied. Switching power supply circuit. 2. The switching power supply circuit according to claim 1, wherein the reference signal is applied to the comparison circuit via a low-pass filter, and a high resistance is provided between an input of the low-pass filter and ground. 3. The soft start circuit is provided with a terminal to which a capacitor for determining a time constant is connected, and the terminal is short-circuited by a control signal to perform a soft start operation at an arbitrary timing. Item 3. The switching power supply circuit according to Item 2.