JP3301986B2 - Switching power supply circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a 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. 5 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;
A MOS transistor 14 for switching the primary winding of the transformer 2, an oscillation circuit 16 whose oscillation frequency is switched by a switch 15 whose opening and closing are controlled by a signal from the microcomputer 5, and an error detection circuit for detecting the oscillation output of the oscillation circuit 16 P that performs pulse width modulation based on the output of
It comprises a WM circuit 17 and 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.

The operation of FIG. 5 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. The error detection circuit 12 at power-on generates an output such that the duty ratio gradually increases, so that the oscillation output is subjected to pulse width modulation in an initial state by the PWM circuit 17. The output of the PWM circuit 17 is applied to the MOS transistor 14 via the logic circuit 18, and the MOS transistor 14 performs the switching operation of the primary winding according to the pulse width modulated signal. As a result, a voltage is generated in each secondary winding of the transformer 2. Thereby, the microcomputer 5 also operates, and the microcomputer 5 operates the emitting diode 9 according to the output signal. Therefore, the phototransistor 8 is turned on, and the voltage of the secondary winding 2b becomes IC3.
Supplied to When a voltage is supplied to the IC 3, the start circuit 11 stops operating and stops supplying power from the primary winding to the inside of the IC 3. 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 supply and only the microcomputer 5 is in an operating state, the microcomputer 5 controls the switch 6 based on the output to control the power supply of 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 6 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, the capacitor 1 connected to the power supply of the IC 3
9, a charging current flows, 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, a discharge current flows from the capacitor 19, and the power supply voltage decreases. The timer circuit 13 determines that the power supply voltage is a predetermined voltage, for example, 4.7.
When the voltage drops to V, the power supply from the start circuit 11 is started again. With such an operation, a triangular wave having an amplitude of 4.7 V and 5.6 V is repeatedly generated in the power supply voltage line of the IC 3. Then, the timer circuit 13 counts the cycle of the triangular wave, controls the logic circuit 18 every eight cycles, for example, and applies the output of the PWM circuit 17 to the MOS transistor 14. The MOS transistor 14
Is controlled during the discharging period of the capacitor 19, that is,
This is the falling period of the triangular wave. Therefore, the MOS transistor 14 is driven at a predetermined interval for a predetermined time at a frequency of 20 KHz. As a result, power consumption in the standby state is reduced.

[0007]

The intermittent operation of the switching power supply circuit shown in FIG. 5 corresponds to FIGS. 4 (a) and 4 (b).
The operation is as shown in FIG. 4A shows the switching period created by the timer circuit 13. The MOS transistor 14 is switched every eight periods, and a voltage is generated in the secondary winding 2C of the transformer during that period. Thus, power is supplied to the microcomputer 5. However, as shown in FIG. 4B, the voltage supplied to the microcomputer 5 sharply increases during the switching period, and decreases when the switching period ends. It becomes 5V or less.

Therefore, during the switching period, the voltage may exceed the withstand voltage of the microcomputer 5 due to a sudden increase in voltage, and the microcomputer 5 may be destroyed. There was a fear that the operation would stop.

[0009]

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 feedback element from a secondary side of the transformer. An error detection circuit for detecting a signal; a pulse width modulation circuit for pulse width modulating the output of the oscillation circuit based on a detection output of the error detection circuit; and an output of the pulse width modulation circuit in a normal operation state and a standby state. A logic circuit that supplies the switching element intermittently or intermittently, a soft start circuit that gradually increases the duty ratio of the pulse width modulation from a minimum value when power is turned on, and a timer circuit that creates an intermittent frequency in the standby mode. An intermittent frequency changing circuit that increases the intermittent frequency during the standby state; and By providing a soft-start changing circuit that extends the soft-start time in the standby state, the intermittent period in the standby state is shortened, the rate of increase in the duty ratio of the switching pulse during the switching period is reduced, and power supplied to the microcomputer is reduced. This is a period in which the voltage sharply rises and the period during which the voltage becomes 5 V or less is shortened.

The intermittent frequency changing circuit includes a first capacitor connected to a power supply line that repeats charging and discharging with a voltage supplied from a primary side of the transformer;
It is composed of a series circuit of a second capacitor and a first switch connected in parallel to the first capacitor. In the standby state, the intermittent period is increased by reducing the capacitance of the power supply line and increasing the intermittent frequency. It is not necessary to provide a special terminal for changing the value on the IC.

The soft start circuit has a third
And a series circuit of a fourth capacitor and a second switch connected in series with the third capacitor is provided. In the standby state, the capacity of the soft start circuit is increased. By extending the soft start time, there is no need to provide a terminal for increasing the soft start time in the IC.

[0012]

FIG. 1 is a circuit diagram showing an embodiment of the present invention. The same parts as those in FIG. A soft start circuit 19 is provided in the error detection circuit 12 that detects a voltage fluctuation of the power supply line Vc. In the soft start circuit 19, a third capacitor C3 and a fourth capacitor C4 for determining a soft start time are connected in parallel to the outside of the IC 3, and a control signal CONT1 of the microcomputer 5 is connected to the other end of the capacitor C4.
The switches SW2 controlled by the switches are connected in series. The capacitor C4 and the switch SW2 constitute a soft start changing circuit.

A first capacitor C1 and a second capacitor C2 are connected in parallel to the terminal of the power supply line Vc outside the IC 3, and the microcomputer 5 is connected to the other end of the capacitor C2.
The switch SW1 controlled by the control signal CONT2 is connected in series. This capacitor C2 and switch SW
Reference numeral 1 denotes an intermittent frequency changing circuit.

In FIG. 1, the microcomputer 5 sets the control signal CONT1 to a low level when the power is turned on and in a normal operation state. At this time, the switch SW1 is in a closed state, and the switch SW2 is in an open state. Therefore,
In the state where the capacitors C1 and C2 connected to the power supply line Vc are connected in parallel, the capacitance is an added value,
It has the same value as the capacitor C0 shown in the conventional example of FIG. On the other hand, the capacitance connected to the soft start circuit 19 is the capacitance of the capacitor C3.

When the power is turned on, the IC 3
A voltage is supplied from the next winding via the start circuit 11, and this voltage is supplied to the capacitors C1 and C1 of the power supply line Vc.
2 and is held as the power source of the IC 3. Also,
In the normal operation state, the secondary winding 2b of the transformer 2
From the capacitor C via the phototransistor 8
1 and C3 are charged to become the power source of IC3. As described above, when the power is turned on and in the normal operation state, it is necessary to increase the capacity of the power supply line Vc.
1 and C2 are connected in parallel.

On the other hand, the capacity connected to the soft start circuit 19 does not require a very long soft start time when the power is turned on. For this reason, only the capacitor C3 is connected to the soft start circuit 19.

However, when the microcomputer 5 enters the standby state, the microcomputer 5 sets the control signal CONT1 to the high level to turn off the phototransistor 8, and switches the switch SW1.
And close SW2. Therefore, in this state, only the capacitor C1 is connected to the power supply line Vc, and the capacitance connected to the power supply line Vc is smaller than in the conventional case. Further, the capacitors C3 and C4 are connected in parallel to the soft start circuit 19, the capacity thereof becomes large, and the soft start time is set long.

FIG. 2 shows specific circuits of the error detection circuit 12, the PWM circuit 17, and the 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 is connected to the power supply line Vc
And a reference voltage Vref, and a P-channel MOS 2 to which an output of the comparator 25 is applied.
6 and a channel MOS 2 forming a current mirror circuit
, And the drain of the MOS 28 is connected to the low-pass filter 21 via the resistor 29.

The soft start circuit 19 includes a transistor 30 having an emitter connected to the low-pass filter 21 and the resistor 29, resistors 31 and 32 for setting the bias of the transistor 30, and an N connected between the base of the transistor 30 and the ground. The base of the transistor 30 is led out as an external terminal of the IC 3, and capacitors C 3 and C 4 and a switch SW 2 are provided there. The clock signal CL output from the timer circuit 13 is applied to the gate of the N-channel MOS 33. The clock signal CL is applied to the MOS output by the PWM output.
This signal is at the “L” level during a period in which the transistor 14 performs the switching operation. 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 eight cycles.

In FIG. 2, 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.

Next, the operation in the standby state will be described.
In the standby state, the control signal C from the microcomputer 5
Only the capacitor C1 is connected to the power supply line Vc by the ONT1. Since the timer circuit 13 repeats charging of the capacitor C1 from the start circuit 11 and discharging of the capacitor C1, the waveform of the power supply line Vc has amplitudes of 5.6 V and 4.7 V as shown in FIG. It becomes a triangular wave. When only the capacitor C1 is used,
The repetition frequency is higher than in the conventional case. In the case of the present embodiment, the frequency is set to twice. The frequency can be set by selecting the capacitors C1 and C2.

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 eight periods, that is, during the period when the waveform of the power supply line Vc falls. (See FIG. 3) This period is a period in which the logic circuit 18 performs switching by applying the PWM output to the MOS transistor 14. In the standby state, the control signal CONT2 becomes "H" level and the switch SW1 is opened. As a result, the oscillation circuit 16
The oscillation frequency is switched from 0 KHz to 20 KHz. While the clock signal CL is at the “H” level, the MOS 33 is on, the capacitors C3 and C4 are discharged, and the transistor 30 is off.
At this time, even if the error detection circuit 12 tries to maximize the pulse width and minimizes the current of the MOS 28,
Since the transistor 30 is off, the emitter voltage is at the ground level, and the reference voltage of the comparator 20 is
At the minimum level, the PWM waveform has the minimum pulse width.

As shown in FIG. 3, when the clock signal CL becomes "L" level, the MOS 33 is turned off, so that a charging current starts flowing through the resistors C32 and C4 through 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 is Vr in FIG.
As shown by 1, the pulse width of the PWM output gradually increases.

In the standby state, since the capacitors C3 and C4 are connected in parallel, the time constant is large and the reference voltage Vr1 rises slowly. On the other hand, when the power is turned on, the time constant becomes small in a state where only the capacitor C3 is connected because the switch SW2 is open, so that the reference voltage rises faster than Vr1 like Vr2.

By the above operation, as shown in FIG. 4C, the period of the intermittent operation is, for example, twice as long as the conventional frequency. Also, the MOS driven during the intermittent operation
Since the drive pulse width of the transistor 14 also becomes wider slowly than when the power is turned on, the voltage V3 generated on the secondary side of the transformer 2 is suppressed from abrupt voltage rise as shown in FIG. The period during which the voltage drops to 5 V or less is also shorter than before.

[0026]

As described above, according to the present invention, a sharp increase in the voltage applied to the microcomputer 5 in the standby state can be suppressed, so that the microcomputer 5 can be prevented from being destroyed. Since the period of the decrease is short, it is possible to prevent the microcomputer 5 from becoming inoperative. In addition, even if the intermittent operation cycle is shortened, the soft start period is lengthened, so that power consumption can be suppressed.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a switching power supply circuit of the present invention.

FIG. 2 is a specific circuit diagram of a block of the switching power supply circuit shown in FIG.

FIG. 3 is a waveform chart showing the operation of the present invention.

FIG. 4 is a waveform diagram for comparing a conventional device and the present invention.

FIG. 5 is a block diagram showing a conventional example.

[Description of Signs] 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 N-channel MOS

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H02M 3/28

Claims (3)

(57) [Claims] A switching element for switching a current flowing to a primary side of a transformer, an error detection circuit for detecting a feedback signal from a secondary side of the transformer, and an oscillation circuit based on a detection output of the error detection circuit. Pulse width modulation circuit for pulse width modulation of output and pulse width modulation for normal operation
The output of the modulation circuit is continuously supplied to the switching element,
In the standby state, the output of the pulse width modulation circuit is intermittently switched.
A logic circuit for supplying to the switching element, a soft start circuit for gradually increasing the duty ratio of the pulse width modulation from a minimum value at power-on, and a soft start circuit for a normal operation state.
Charge and discharge the feedback signal from the secondary side of the transformer
In the standby state, the signal from the power supply line is filled with the feedback signal.
Intermittent frequency change cycle for charging and discharging at a higher intermittent frequency than discharging
And a soft start changing circuit connected to the soft start circuit and extending the soft start time in the standby state. 2. The intermittent frequency changing circuit includes a first capacitor connected to a power supply line that repeats charging and discharging with a voltage supplied from a primary side of the transformer, and is connected in parallel to the first capacitor. A series circuit of a second capacitor and a first switch, wherein the first switch is turned off in a standby state to reduce the capacitor capacity of the power supply line to increase the intermittent frequency. 2. The switching power supply circuit according to 1. 3. The soft start circuit is connected to a third capacitor, and is provided with a series circuit of a fourth capacitor and a second switch connected in parallel to the third capacitor, In the state, the second switch
2. The switching power supply circuit according to claim 1, wherein the switch is turned on to increase the soft start time by increasing the capacitance of the capacitor of the soft start circuit.