JP2000023458A - Switching power supply unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a switching power supply unit capable of obtaining therein a stable standby state. SOLUTION: The series circuit comprising the main transistor of a switching power supply unit and the primary winding of its transformer is connected with its DC power supply to perform ON/OFF control of the main transistor by PWM pulses. Sensing the output voltage of the secondary winding of its transformer, an error signal is created by an error amplifying transistor 23 to make flow the current responding to the error signal in a light emitting diode 24. A standby control transistor 72 is connected in parallel with the error amplifier transistor 23 to perform ON/OFF control of the standby control transistor 72 by standby control pulses. A capacitor 73 is provided connectively between the collector and the base of the standby control transistor 72.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply device in which an output voltage is controlled to be constant by feedback control.

[0002]

2. Description of the Related Art When a switching power supply device is used in a power supply circuit of a computer system or the like, it is necessary to provide a standby mode for saving power.
In the standby mode, intermittent operation is generally performed to reduce switching loss. FIG. 1 shows a conventional switching power supply for coping with a light load such as a standby mode. In FIG. 1, for example, a primary winding 3 having an inductance of an output transformer 2 and a main transistor 4 including an FET as a main switch are provided between one end and the other end of a DC power supply 1 including a rectifier circuit and a smoothing circuit. And a series circuit of a current detecting resistor 5 as a current detecting means. A first output smoothing capacitor 8 is connected in parallel with a secondary winding 6 of the transformer 2 via an output rectifier diode 7 to constitute a first output circuit. The polarity of the secondary winding 6 and the polarity of the rectifier diode 7 are determined so that the rectifier diode 7 conducts during the off period of the main transistor 4. A first load 11 is connected to first DC output terminals 9 and 10 connected to the smoothing capacitor 8. To form a second output circuit, a second output smoothing capacitor 14 is connected to a tertiary winding 12 of the transformer 2 via an output rectifier diode 13. The polarity of the tertiary winding 12 is determined so that the rectifier diode 13 conducts during the off period of the main transistor 4 as in the case of the secondary winding 6. Smoothing capacitor 14
A second load 17 is connected between the second output terminals 15 and 16 connected to. The switching power supply device of FIG. 1 has a voltage control signal forming circuit 18 for controlling the first output voltage between the first output terminals 9 and 10 to be constant.
This voltage control signal forming circuit 18 has a first output terminal 9,
It comprises voltage detecting resistors 19 and 20 as voltage detecting circuits connected between 10, a resistor 21 and a Zener diode 22 for constituting a reference voltage source, and a transistor 23 as an error signal forming circuit, that is, an error amplifier. The base of the transistor 23 is connected to the voltage dividing point of the voltage detection resistors 19 and 20, and the emitter is connected to the Zener diode 22 as a reference voltage source. Zener diode 2
2 is connected between the DC output terminals 9 and 10 via the resistor 21. Therefore, the collector current of the transistor 23 changes according to the difference between the detection voltage and the reference voltage. A light emitting diode 24 is connected as a light emitting element between the second output terminal 15 and the collector of the transistor 23 to convert the output of the voltage control signal forming circuit 18 into an optical signal.

[0003] The transformer 2 is provided with a quaternary winding 25 to constitute a control power supply. A smoothing capacitor 27 is connected in parallel to the fourth winding 25 via a rectifier diode 26.

A positive power supply line 29 of a control signal forming circuit 28 for controlling on / off of the main transistor 4 is connected to one end of a capacitor 27 as a control power supply, and is connected to a DC power supply 1 via a starting resistor 30. The ground line 31 is connected to the other end of the DC power supply 1 and the other end of the capacitor 27. A composite signal of the current detection signal and the constant voltage control signal is input to the input signal line 32 of the control signal forming circuit 28. To form this composite signal, one end of the current detection resistor 5 is connected to the input signal line 32 via the resistor 33, and a phototransistor 24 is connected between one end of the capacitor 27 and the input signal line 32.
Is connected. Phototransistor 34 is optically coupled to light emitting diode 24. The above-described interconnection of the current detection resistor 5 and the phototransistor 34 constitutes an addition circuit. The control signal forming circuit 28 forms a control signal in response to the input signal line 32, and provides the control signal to the base of the main transistor 4 via the output line 35.

A standby control circuit 36 controls the light emitting diode 24 in response to a standby signal from a standby signal input terminal 37.
16 are connected to the cathode of the light emitting diode 24 by lines 38 and 39 and to the cathode of the light emitting diode 24 by line 40.

FIG. 2 shows details of the switch control signal forming circuit 28 and its related parts. The switch control signal forming circuit 28 includes a pulse generator 4 for generating a switch control pulse for controlling on / off of the main transistor 4.
1, a drive circuit 42, a current limiting resistor 43, an on-time width control reference voltage source 44, and an on-time width control comparator 4.
5, an on-time width control transistor 46, and a constant voltage circuit 47.

The pulse generator 41 includes a pulse forming comparator 48, a sawtooth wave generating capacitor 49, a discharging resistor 50, a charge controlling transistor 51, a Zener diode 52, resistors 53, 54, 55, 56 and diodes 57 and 58. One end of the sawtooth wave generating capacitor 49 is connected to the positive power supply line 29 via the charge control transistor 51, and the other end of the capacitor 49 is connected to the ground line 31. The discharging resistor 50 is connected in parallel with the capacitor 49. The Zener diode 52 functioning as a constant voltage source is connected between the base of the transistor 51 and the ground line 31. The base of the transistor 51 is connected via a resistor 53 to a constant voltage line 59 derived from the constant voltage circuit 47. One input terminal of the control pulse forming comparator 48 is connected to a constant voltage line 59 via a resistor 54, and the other input terminal is connected to a sawtooth wave generating capacitor 49.
Is connected to one end. The reference voltage resistor 55 and the diode 57 are connected between one input terminal and the output terminal of the comparator 48. The feedback resistor 56 and the diode 58 are connected between the base of the transistor 51 and the output terminal of the comparator 48.

The drive circuit 42 drives the main transistor 4 in response to a switch control pulse, and has an input terminal a, an output terminal b, and a pair of power terminals c and d. The input terminal a is the output line of the pulse generator 41, ie, the comparator 4
8, the output terminal b is connected to the base (control terminal) of the main transistor 4 via the resistor 43, one power terminal c is connected to the positive power line 29,
The other power supply terminal d is connected to the ground line 31. As is clear from FIG. 4, the drive circuit 42 includes first and second NPN transistors 42a and 42b and a NOT
Circuit (inverting circuit) 42c. The collector of the first transistor 42a is connected to one power supply terminal c, the emitter is connected to the output terminal b, and the base is connected to the input terminal a. The collector of the second transistor 42b is connected to the output terminal b, and the emitter is the other power supply terminal d.
, And the base is connected to the input terminal a via the NOT circuit 42c. Thus, the first and second transistors 42a, 42b operate in opposite directions.

One input terminal of a comparator 45 for determining the ON width of the main transistor 4 is connected to the input signal line 32, and the other input terminal is connected to a reference voltage source 44. The reference voltage source 44 generates a constant reference voltage for determining the ON width with reference to the ground line 31. The transistor 46 as an on-time width control switch is connected between the output terminal of the pulse generating comparator 48 and the ground line 31, and its base is connected to the output terminal of the comparator 45. The output line of the phototransistor 34 and the output line of the resistor 33 are connected to each other to form an adding circuit. Accordingly, a current feedback signal based on the current detection resistor 5 and the phototransistor 34 are input to the input signal line 32.
And a voltage feedback signal (voltage control signal) based on the sum. Therefore, the comparator 45 and the transistor 46 execute both the voltage feedback control and the current feedback control. This current increases with time during the ON period of the main transistor 4, and when the current detection voltage reaches the reference voltage Vth, the comparator 4
5 goes high, transistor 46 is turned on, input terminal a of drive circuit 42 is connected to ground, and main transistor 4 is forcibly turned off.

The constant voltage circuit 47 is a well-known circuit that is connected between the power supply line 29 and the ground line 31 and that converts the voltage of the power supply line 29 to a constant voltage and outputs it to a line 59. The voltage on line 59 is lower than the voltage on line 29.
Although not shown, the power supply terminals of the comparators 45 and 48 are connected to the output line 59 of the constant voltage circuit 47.

FIG. 3 shows details of the standby control circuit 36 shown in FIG. 1 and its related parts. As apparent from FIG. 3, the standby control circuit 36 includes an error amplifier or operational amplifier 60, a reference voltage source 61, and a transistor 62.
, Resistors 63, 64, 65 and an integrating capacitor 66. One input terminal of the operational amplifier 60 is connected to the reference voltage source 61, the other input terminal is connected to the second DC output terminal 15 via the resistor 63, and the output terminal is connected to the cathode of the light emitting diode 24 by the line 40. Have been. The collector of the npn-type transistor 62 is connected to the other input terminal of the operational amplifier 60, and the emitter is connected via the ground line 39 to the first and second loads 11, 17.
And the base is connected to a standby signal input terminal 37. The resistor 64 is connected between the resistor 63 and the ground line 39 to divide the voltage between the second DC output terminals 15 and 16 and supply the divided voltage to the operational amplifier 60. Therefore, the transistor 62 as a switch is connected in parallel to the voltage dividing resistor 64. The integrating capacitor 66 is connected between the negative input terminal and the output terminal of the operational amplifier 60 via the resistor 65.

[0012]

[Normal Operation] Next, the normal operation of the switching power supply shown in FIGS. 1 and 2 will be described with reference to the period t0 to t4 in FIG. At time t0 in FIG. 5, the output voltage of the comparator 48 in FIG. 2 changes from low level to high level to generate a control pulse, and a corresponding gate control signal Vg is applied to the main transistor 4 as shown in FIG. As shown, the main transistor 4 is turned on. As a result, a current I1 flows through a closed circuit including the DC power supply 1, the primary winding 3, the transistor 4, and the current detecting resistor 5. Since the primary winding 3 has an inductance, the current I1 increases with a slope, and the voltage of the current detecting resistor 5 changes in accordance with the waveform of the current I1, and the input voltage including the feedback information of the current I1. Fig. 5 for Vin
It increases with a slope as shown in FIG. When the voltage Vin corresponding to the current I1 reaches the reference voltage Vth at the time t1, the output of the comparator 45 changes from a low level to a high level, the transistor 46 is turned on, and the output terminal of the pulse generation comparator 48 is The transistor is connected to the ground via the transistor 46, and the ON period of the pulse ends. When the output voltage of the pulse generating comparator 48 becomes low at time t1, the diode 57 is turned on, and the voltage V1 at the positive input terminal of the comparator 48 changes from 6.5V to 3V as shown in FIG. .5 V, which is lower than the voltage V2 at the negative input terminal. As a result, the low level of the output voltage of the comparator 48 is maintained. When the output voltage of the comparator 48 goes low, the diode 58 turns on and the transistor 5
1 is lower than the emitter potential, the base-emitter of the transistor 51 is in a reverse bias state, and the transistor 51 is turned off, so that the charging of the capacitor 49 is stopped. As a result, the charge of the capacitor 49 is released through the resistor 50, and the voltage V2 is
As shown by a broken line in FIG. At time t3, when the voltage of the capacitor 49, that is, the voltage V2 of the negative input terminal of the comparator 48 becomes lower than the voltage V1 of the positive input terminal, the output voltage of the comparator 48 returns to the high level again, and the next pulse is generated. The voltage Vin based on the current I1 during the period from t1 to t2 in FIG. 5B is generated by the current based on the storage action of the main transistor 4. When the current I1 passing through the main transistor 4 at time t2 becomes zero, the drain-source voltage Vds as a voltage between a pair of main terminals of the main transistor 4 becomes higher than when it is on. Since the current I1 becomes zero during the period from t2 to t3, the output of the comparator 45 for controlling the on-time width, that is, the current feedback control, is held at a low level, and the transistor 46 for controlling the on-time width is used.
Is also kept off. Therefore, the comparator 45 is not involved in the determination of the off-period of the main transistor 4, and the off-period (for example, t1 to t3) is determined depending only on the discharge of the sawtooth-wave capacitor 49 and is constant. Comparator 4 at time t3
When the output voltage of 8 returns to a high level, diodes 57 and 58 are turned off. As a result, the transistor 51 is turned on again, the capacitor 49 is rapidly charged, and the voltage V2 becomes 5V. Further, the voltage V1 at the positive input terminal of the comparator 48 immediately becomes 6.5V at the time t3. Therefore, the output voltage of the comparator 48 is kept at a high level. The high level of the output voltage is set higher than 6.5V.

[0013]

[Voltage Control During Normal Load] When the voltage between both ends of the load 11 becomes higher than the reference value, for example, the input voltage Vin of the positive input terminal of the comparator 45 increases, and the reference voltage becomes earlier than the starting point of turning on the main transistor 4. When the voltage reaches Vth, the output of the comparator 45 changes to a high level, and the transistor 46 is turned on, whereby the on-period of the PWM pulse ends, and the on-width of the PWM pulse is reduced. Since the off width of the PWM pulse train is constant, the duty ratio of the main transistor 4 is reduced, and the output voltage is returned to the reference. When the output voltage becomes lower than the reference value, the operation is the reverse of that when the output voltage becomes higher.

[0014]

[Standby operation] The standby signal input terminal 37 is maintained at a high level during normal load, and during standby (standby).
Low or open. Therefore, during a normal load, the transistor 62 is kept on, the negative input terminal of the amplifier 60 is at the ground level, and the output voltage is at the high level. As a result, the standby control circuit 36 does not substantially relate to the feedback control by the light emitting diode 24 in the normal state. On the other hand, at the time of standby, the terminal 37 becomes low level and the transistor 62 is turned off, so that the voltage of the negative input terminal of the operational amplifier 60 becomes the second DC output terminal 1.
The value corresponds to the voltage Vo2 between 5 and 16. The output voltage of the operational amplifier 60 does not immediately become a value corresponding to the difference between the voltage at the negative input terminal and the reference voltage at the positive input terminal, but becomes an integrated value of the difference value. Therefore, immediately after the transition to the standby state, the output voltage of the operational amplifier 60 is relatively high, so that the current If of the light emitting diode 24 does not suddenly increase.
Since the output of the operational amplifier 60 gradually decreases, the current If of the light emitting diode 24 gradually increases. When the current If of the light emitting diode 24 increases, the state becomes equivalent to indicating that the output voltage Vo1 has become higher than a predetermined value, and the on-time width of the main transistor 4 is reduced. FIG. 6 shows a part during the standby state, and does not show the operation at the start of the standby state. However, at the start of the standby state, the same operation as in the period from t1 to t3 in FIG. 6 occurs. Therefore, the operation of starting the standby operation will be described with reference to FIG. Since the standby state is substantially no load, even if the ON time width of the main transistor 4 becomes narrow, the output voltages Vo1, V integrated by the smoothing capacitors 8, 14 are reduced.
An increase in o2 occurs as shown in FIG. However, in the standby state, since the output of the operational amplifier 60 is at a low level, the constant voltage control operation by the transistor 23 of the voltage control signal forming circuit 18 does not occur. When the output voltage of the operational amplifier 60 gradually decreases with the integration operation, the current If of the light emitting diode 24 gradually increases, and the current If of the light emitting diode 24 becomes the predetermined level Ir as shown in a period from t2 to t4 in FIG. Then, the output of the comparator 45 in FIG. 2 is changed to a high level, the output line of the PWM pulse forming comparator 48 is connected to the ground, and the main transistor 4 is not driven. From time t2 in FIG. 6 to time t4
If the main transistor 4 is not turned on until the time,
The voltages Vo1 and Vo2 of the smoothing capacitors 8 and 14 decrease. Even if the output voltages Vo1 and Vo2 decrease from t2, the decrease in the output voltage of the integrating circuit of the operational amplifier 60 and the capacitor 66 does not stop immediately but continues to decrease until the time t3 in FIG. It continues until time t3. When the output voltage Vo2 decreases as shown in FIG. 6D from the time point t2, the output voltage of the operational amplifier 60 starts increasing at a time point t3 with a slight delay, and the current If of the light emitting diode 24 decreases from the time t3. At time t4, the main transistor 4 crosses the on / off stop predetermined level Ir from top to bottom. Thus, at time t4, the comparator 4 in FIG.
The output of 5 turns low, transistor 46 turns off, and the on / off operation of main transistor 4 begins. After time t4, the same operation as in the period from t0 to t4 is repeated. When the current If of the light emitting diode 24 increases with a slope during the period from t1 to t2, the on-time width of the main transistor 4 gradually decreases as is clear from FIGS. 6A and 6B. The maximum amplitude value of the current I1 flowing through the current also gradually decreases. The main transistor 4 is turned on and off with a pause period from t2 to t4 in FIG. 6, and the on-time width is gradually narrowed during the period from t1 to t2. It is.

[0015]

By the way, in the switching power supply device using the conventional standby control circuit 36 shown in FIG. 3, if there is a variation in the gain of the operational amplifier 60 and the photocoupler of the light emitting diode 24 and the phototransistor 34. 6, the current If of the light emitting diode 24 in FIG.
In some cases, the pause period of t4 becomes too short to sufficiently obtain the effect of reducing power consumption. FIG.
6B, the amplitude of the current I1 of the main transistor 4 changes according to the change of the current If of the light emitting diode 24 during the period from t0 to t2 in FIG. 6C.
Since the current If changes so as to have an inflection point, the amplitude of the current I1 of the main transistor 4 changes suddenly, and this peak becomes large, so that the transformer 2 may generate a growling sound. In addition, when the power supply 1 is turned off, if the power supply 1 is composed of a rectifier circuit and a smoothing capacitor, the power supply voltage does not immediately become zero but gradually decreases. Even when the power supply voltage is reduced in this manner, the peak of the current I1 increases, and a humming sound of the transformer 2 may be generated.

An object of the present invention is to provide a switching power supply capable of obtaining a stable standby state.

[0017]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems and to achieve the above-mentioned object, the present invention provides a method of connecting a primary winding of a transformer and a switch connected between one end and the other end of a DC power supply. A series circuit, a secondary winding electromagnetically coupled to the primary winding, an output rectification smoothing circuit connected to the secondary winding to supply a DC voltage to a load, and A voltage detection circuit for detecting an output voltage, a reference voltage source for providing a reference voltage, and an error forming an error signal corresponding to a difference between the detection voltage obtained from the voltage detection circuit and the reference voltage of the reference voltage source. A signal forming circuit, a mode signal input terminal for receiving a mode signal for distinguishing between a normal load mode and a light load mode lighter than the normal load, and a normal load mode when the mode signal indicates the light load mode. Mode The light load control pulse is repeatedly generated at a predetermined period longer than a multiple of the ON / OFF period of the switch, and the light load control pulse is generated when the mode signal indicates the normal load mode. A light load control pulse generator, a synthesizing means for forming a synthesized signal of the output of the error signal forming circuit and the output of the light load control pulse generator, and a control signal for on / off control of the switch. A switch control signal forming circuit formed so as to control an on-time width of the switch in response to the composite signal so as to maintain an output voltage of the output rectifying / smoothing circuit at a predetermined value. The present invention relates to a switching power supply device provided. It is desirable to provide a circuit for converting a square wave pulse as a light load control pulse into a waveform having a slope (for example, a triangular wave or a sine wave). Claim 3
As shown in (1), it is desirable that the combining means be configured using light emitting diodes. It is preferable that the switch control signal forming circuit is of a current feedback type having current detecting means. Further, the first and second output rectifying / smoothing circuits can be provided. As described in claim 6, in the invention of claim 5, it is desirable to provide a waveform conversion circuit in the same manner as in claim 2. As described in claim 7, in the invention of claim 5, it is desirable that the synthesizing means is a circuit including a light emitting diode. Further, the present invention can be configured as a boost switching power supply device. Furthermore,
The present invention can be applied to a step-down type or a forward type switching power supply.

[0018]

According to the invention of each claim, a light load control pulse generator is provided, and in a light load mode, a light load control pulse is generated at a predetermined cycle, and a light load control pulse and an error signal are synthesized. Since the on / off control signal of the switch is formed, the interruption period of the on / off operation of the switch can be stably obtained irrespective of the variation of the circuit constant, the fluctuation of the power supply voltage, and the like. ) Can be reliably obtained. Further, according to the second and sixth aspects of the present invention, since the control by the light load control pulse is performed gradually with an inclination, a sudden change in the excitation of the transformer does not occur, and the noise of the transformer is suppressed. be able to. According to the third and seventh aspects of the present invention, it is possible to easily achieve the synthesis of the error signal and the light load control pulse or a signal corresponding thereto. Further, according to the invention of claim 5, it is possible to easily distinguish a load driven in the light load mode from a load driven in the normal load.

[0019]

Embodiments and Examples Next, examples and embodiments of the present invention will be described with reference to the drawings. However, in FIGS. 7 to 11, substantially the same parts as those in FIGS. 1 to 6 are denoted by the same reference numerals, and description thereof will be omitted. FIG. 2 is also referred to in the description of the present embodiment.

[0020]

FIG. 7 shows a switching power supply according to an embodiment of the present invention. The switching power supply of FIG.
Except for the standby control circuit 36a, the configuration is the same as that of the switching power supply device of FIG. The standby control circuit 36a shown in FIG. 7 includes a standby control pulse generator 70 and a waveform conversion circuit 71, as is apparent from FIG.

The standby control pulse generator 70 functions as a light load control pulse generator and responds to a standby signal (light load mode signal) at a standby signal input terminal 37 as shown in FIG. A standby control pulse (light load control pulse) composed of a square wave pulse is repeatedly generated at a predetermined cycle. The repetition frequency of the standby control pulse is about 700 to 1000 Hz. Since the on / off repetition frequency of the main transistor 4 under normal load is about 50 to 60 kHz, the frequency of the standby control pulse is much lower than the on / off repetition frequency of the main transistor 4. The standby signal input terminal 37 in FIGS. 7 and 8 becomes high level in the normal load mode and low level or open in the standby mode (light load mode) as in FIGS. The standby control pulse generator 70 generates the standby control pulse shown in FIG. 9D in response to the low level input indicating the standby mode, and generates the low level (0 V) output in response to the high level indicating the normal load mode. I do. Of course, the input terminal 37 can be modified so that a low-level signal is input in the normal load mode and a high-level signal is input in the standby mode.

The waveform conversion circuit 71 includes an npn-type transistor 72, a capacitor 73, and two resistors 74 and 75. The base of the transistor 72 is connected to the output terminal of the standby control pulse generator 70 via a resistor 74, the collector is connected to the cathode of the light emitting diode 24 via a resistor 75, and the emitter is connected to the ground line 39. Therefore, the standby control transistor 72 is connected in parallel to the error amplification transistor 23. A capacitor 73 having an integrating action or a smoothing action is connected between the collector and the base of the transistor 72. An error signal output line or collector of the transistor 23 as an error signal forming circuit or error amplifier and the output line 40 of the standby control circuit 36a are connected to each other and to the cathode of the common light emitting diode 24. The error signal output line, the standby control signal output line 40, and the light emitting diode 24 form a combined circuit of the error signal and the standby control signal, and the light emitting diode 24 corresponds to the combined signal of the combined circuit. Light output is obtained. This light output is converted into an electric signal by the phototransistor 34, and a constant voltage control signal (voltage feedback signal) and a current detection signal (current) are output by an addition circuit formed by coupling the output line of the phototransistor 34 and the output line of the current detection resistor 5. Feedback signal)
Is added to the input of the comparator 45 in FIG.

In normal load mode, ie, standby (standby)
If not, the standby control pulse generator 7 of FIG.
The zero output goes low (zero volts), transistor 72 is held off and no current flows on line 40. Therefore, the constant voltage control is performed irrespective of the standby control circuit 36a.

During standby, the standby control pulse generator 70 generates a standby control pulse shown in FIG. 9D, which is applied to the base of the transistor 72. Since the integrating capacitor 73 is connected between the collector and the base of the transistor 72, for example, even if the standby control pulse changes from a low level to a high level at time t1 in FIG.
Does not suddenly increase as shown in FIG. 9B, but gradually increases. In this embodiment, the maximum level of the collector current I72 of the transistor 72 is lower than the reference value Ir for turning off the main transistor 4. The collector current I23 of the transistor 23 as an error amplifier is partially kept flat as shown in FIG. The current If of the light emitting diode 24 is a combination of the currents I23 and I72 of FIGS. 9A and 9B as shown in FIG. 9C, and the reference level Ir for turning off the main transistor 4 is set in the period t2 to t4. Cross. During the period from t2 to t4, the current If of the light emitting diode 24 increases, the impedance of the phototransistor 34 in FIG. 2 decreases, and the input voltage Vin of the comparator 45 crosses the reference voltage Vth, so that the transistor 46 is turned on. The transmission of the output pulse of the comparator 48 of the main transistor 4 to the main transistor 4 is cut off, and as shown in FIG.
During the period, the current I1 of the main transistor 4 becomes substantially zero. As a result, the main transistor 4 operates at 700 to 1000 Hz.
It is driven intermittently at a low frequency. Note that FIG.
The main transistor 4 is turned on and off at a high repetition frequency of about 50 to 60 kHz during the period from t0 to t2 and the period from t4 to t6 in (E).

At the time of standby, the main transistor 4
Turns on and off with a pause such as t2 to t4,
9 (E) and FIG. 10 (B).
Is limited, the operation becomes an intermittent oscillation operation, and the power loss is reduced.

In the present embodiment, the main transistor 4 is forcibly forced by the output pulse of the standby control pulse generator 70.
Is intermittently driven (intermittently driven), so that the main transistor 4 can be stably operated intermittently, that is, stand-by operation, even if the gain of the voltage feedback control circuit varies or the input voltage fluctuates. 9 (E) and FIG.
As shown in (B), the amplitude and the on-time width of the current I1 of the main transistor 4 gradually increase and gradually decrease.
A sudden change in the magnetic flux of the transformer 2 does not occur, and the growling noise of the transformer 2 can be suppressed. When the power is turned off, the pulse generator for generating the voltage feedback signal for standby is stopped, so that the operation mode becomes the continuous oscillation mode, and the width of the switching pulse is prevented from increasing. Therefore, no humming sound is generated from the transformer 2.

[0027]

Second Embodiment Next, a switching power supply according to a second embodiment shown in FIG. 11 will be described. However, in FIG. 11, substantially the same parts as those in FIG. 7 are denoted by the same reference numerals, and description thereof will be omitted. The switching power supply of FIG. 11 is a step-up switching power supply. An output rectifying / smoothing circuit of a diode 6 and a capacitor 7 is connected in parallel with a series circuit of the main transistor 4 and the current detection resistor 5. The step-up reactor 3a comprises a power supply 1 and a main transistor 4
Connected between A winding 25 is electromagnetically coupled to the reactor 3a to constitute a power supply for the control circuit. The second embodiment is the same as the first embodiment except that it is configured as a boost type, so that the same operation and effect as the first embodiment can be obtained.

[0028]

[Modifications] The present invention is not limited to the above-described embodiment, and for example, the following modifications are possible. (1) The light emitting diode 24 without the circuit for obtaining the second output voltage Vo2 can be connected to the first output terminal 9. (2) The configuration of the switch control signal forming circuit for turning on / off the main transistor 4 can be variously modified.
For example, the width control of the PWM pulse or the intermittent generation of the PWM pulse can also be performed by controlling the charge or discharge of the capacitor 49. (3) The main transistor 4 can be replaced with a bipolar transistor. (4) A circuit configuration in which the light coupling circuit between the light emitting diode 24 and the phototransistor 34 is electrically omitted can be omitted.

[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 switch control signal forming circuit of FIG. 1 and a portion in the vicinity thereof;

FIG. 3 is a circuit diagram showing a standby control circuit of FIG. 1 and its vicinity in detail;

FIG. 4 is a circuit diagram showing the driving circuit of FIG. 2 in detail.

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

FIG. 6 is a waveform chart showing a state of each part in FIGS. 2 and 3;

FIG. 7 is a circuit diagram showing a switching power supply according to the first embodiment of the present invention.

8 is a circuit diagram showing the standby control circuit of FIG. 7 and its vicinity in detail.

FIG. 9 is a waveform diagram showing a state of each unit in FIGS. 7 and 8;

FIG. 10 is a waveform diagram showing the state of each part of FIG. 7 and element 8 in detail.

FIG. 11 is a circuit diagram showing a switching power supply device according to a second embodiment.

[Explanation of symbols]

 3 Primary winding 4 Transistor 36a Standby control circuit 70 Standby control pulse generator 72 Transistor 73 Integration capacitor

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5H730 AA02 AA14 BB14 BB42 BB57 DD04 DD28 EE02 EE07 EE73 FD01 FD41 FD51 FF19 FG05 VV01 XC20

Claims (8)

[Claims] 1. A series circuit of a primary winding of a transformer and a switch connected between one end and the other end of a DC power supply; a secondary winding electromagnetically coupled to the primary winding; An output rectifying / smoothing circuit connected to the secondary winding to supply a DC voltage to the output terminal; a voltage detection circuit for detecting an output voltage of the output smoothing / rectifying circuit; a reference voltage source for providing a reference voltage; An error signal forming circuit that forms an error signal corresponding to a difference between a detection voltage obtained from the voltage detection circuit and a reference voltage of the reference voltage source, and a normal load mode and a light load mode that is lighter than a normal load. A mode signal input terminal for receiving a mode signal for turning on / off the switch in the normal load mode when the mode signal indicates the light load mode.
Light load control pulse generation that repeatedly generates a light load control pulse at a predetermined cycle longer than a multiple times of the off cycle and does not generate the light load control pulse when the mode signal indicates the normal load mode. A synthesizing means for forming a synthesized signal of an output of the error signal forming circuit and an output of the light load control pulse generator; and forming a control signal for on / off control of the switch. And a switch control signal forming circuit configured to control an on-time width of the switch in response to the composite signal so as to maintain an output voltage of the output rectifying / smoothing circuit at a predetermined value. 2. The light load control pulse generator generates a square wave pulse, and further converts the square wave pulse into a waveform having a gradient between the light load control pulse generator and the synthesizing means. The switching power supply device according to claim 1, further comprising a waveform conversion circuit that performs the conversion. 3. A light emitting diode having one end connected to a DC power supply and the other end connected to an output terminal of the error signal forming circuit and an output terminal of the light load control pulse generator. 2. The switching power supply according to claim 1, further comprising a phototransistor optically coupled to the light emitting diode. 4. The switch control signal forming circuit includes: a current detection unit configured to detect a current flowing through the output winding and the switch; and an addition unit configured to add an output of the current detection unit to the composite signal. A pulse generator for generating a switch control pulse for on / off control of the switch; a pulse width determining reference voltage source; an addition signal obtained from the adding means; and the pulse width determining reference voltage source. And an on-time width control circuit formed to interrupt control of the switch by the switch control pulse when the addition signal becomes higher than the pulse width determination reference voltage. The switching power supply device according to claim 1, 2 or 3, wherein: 5. A series circuit of a primary winding and a switch of a transformer having an inductance connected between one end and the other end of a DC power supply, and a secondary and a third electromagnetically coupled to the primary winding. A second winding, a first output rectifying / smoothing circuit connected to the secondary winding to supply power to a first load, and a second load having a smaller power consumption than the first load. A second output rectifying / smoothing circuit connected to the tertiary winding for supplying power; a means for interconnecting ground terminals of the first and second output rectifying / smoothing circuits; A voltage detection circuit for detecting an output voltage of the output rectifying / smoothing circuit, a reference voltage source for providing a reference voltage, and a difference between a detection voltage obtained from the voltage detection circuit and a reference voltage of the reference voltage source. An error signal forming circuit that forms an error signal, and a normal load A mode signal input terminal for receiving a mode signal for distinguishing between a load mode and a light load mode which is lighter than a normal load; and when the mode signal indicates the light load mode, turning on the switch in the normal load mode.・
Light load control pulse generation that repeatedly generates a light load control pulse at a predetermined cycle longer than a multiple times of the off cycle and does not generate the light load control pulse when the mode signal indicates the normal load mode. A combining unit for forming a combined signal of an output of the error signal forming circuit and an output of the light load control pulse generator; and a current for detecting a current flowing through the primary winding and the switch. Detecting means, adding means for adding the output of the current detecting means to the synthesized signal, a pulse generator for generating a switch control pulse for controlling on / off of the switch, a pulse width determining reference voltage source, The switch by the switch control pulse when the addition signal obtained from the addition means and the addition signal become higher than the pulse width determination reference voltage. Switching power supply device characterized in that it comprises a formed so as to interrupt the control ON time duration control circuit. 6. The light-load control pulse generator generates a square-wave pulse, and further converts the square-wave pulse into a waveform having a gradient between the light-load control pulse generator and the synthesizing means. 6. The switching power supply device according to claim 5, further comprising a waveform conversion circuit that performs the conversion. 7. The combining means has one end connected to the output terminal of the first or second output rectifying / smoothing circuit, and the other end connected to the output terminal of the error signal forming circuit and the light load control pulse generator. 7. The switching power supply according to claim 5, further comprising: a light emitting diode coupled to the output terminal of the switching element; and a phototransistor optically coupled to the light emitting diode. 8. A switch connected between one end and the other end of the DC power supply via a reactor, an output rectifying / smoothing circuit connected in parallel with the switch to supply a DC voltage to a load, A voltage detection circuit for detecting an output voltage of the output rectifying / smoothing circuit; a reference voltage source for providing a reference voltage; and an error corresponding to a difference between a detection voltage obtained from the voltage detection circuit and a reference voltage of the reference voltage source. An error signal forming circuit for forming a signal; a mode signal input terminal for receiving a mode signal for distinguishing between a normal load mode and a light load mode lighter than the normal load; and the mode signal indicates the light load mode. Sometimes, the on / off of the switch in the normal load mode
Light load control pulse generation that repeatedly generates a light load control pulse at a predetermined cycle longer than a multiple times of the off cycle and does not generate the light load control pulse when the mode signal indicates the normal load mode. A synthesizing means for forming a synthesized signal of an output of the error signal forming circuit and an output of the light load control pulse generator; and forming a control signal for on / off control of the switch. And a switch control signal forming circuit configured to control an on-time width of the switch in response to the composite signal so as to maintain an output voltage of the output rectifying / smoothing circuit at a predetermined value.