JP2004104883A - Switching power supply unit and electronic device using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a switching power supply unit and an electronic device using the power supply unit capable of realizing PWM control with a simple and low-cost circuit eliminating use of a dedicated control IC. <P>SOLUTION: This PWM control circuit comprises an error amplification circuit 2 which outputs an error signal corresponding to the difference between an output voltage and a reference voltage, a triangular wave generating circuit for generating a triangular wave signal, a level shift circuit 13 for shifting and outputting a level of the triangular wave signal corresponding to the error signal, and an inverter element 14 for shaping the output of the level shift circuit 13 into a rectangular wave based on a prescribed threshold and outputting it as a PWM signal for driving a switching device Q1. The triangular wave generating circuit consists of a rectangular wave generating circuit 11 and an integrating circuit 12 which forms an astable multivibrator. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a switching power supply device having a PWM control circuit for controlling a switching operation of a switching element, and an electronic device using the same, for example, a non-insulated type separately excited oscillation type circuit such as a chopper type DC-DC converter. The present invention relates to a switching power supply device having a simple configuration and an electronic device using the same.
[0002]
[Prior art]
2. Description of the Related Art A separately excited oscillation type switching power supply device for performing PWM control of a switching element includes a PWM control circuit for changing the duty of a signal for driving the switching element in accordance with an output voltage. The PWM control circuit includes an oscillation circuit that generates a triangular wave signal having a constant frequency, an error amplifier that detects a difference between the output voltage and the reference voltage, and an H level or L level comparison circuit that compares the triangular wave signal with the output of the error amplifier circuit. A comparison circuit that outputs a signal is provided. As a result, a rectangular wave signal whose duty changes in accordance with the magnitude of the output of the error amplifier circuit is output from the comparison circuit, and the switching element is turned on / off by PWM control using the rectangular wave signal. (For example, see Patent Documents 1 and 2). As the PWM control circuit, a dedicated control IC is usually used.
[0003]
[Patent Document 1]
Japanese Patent Publication No. 5-15152 (Figs. 1 to 5)
[Patent Document 2]
JP-A-5-176528 (FIGS. 1 and 3)
[0004]
[Problems to be solved by the invention]
In a simple switching power supply device such as a non-insulated type chopper type DC-DC converter, a significant cost reduction is often required. However, the dedicated control IC is expensive, and this is a bottleneck in reducing the price of the switching power supply.
[0005]
In particular, in a switching power supply device requiring a plurality of outputs, if an attempt is made to provide a control IC for each output in order to stabilize the output voltage, the occupation ratio of the control IC price to the entire price And it has become more difficult to reduce the price.
[0006]
SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems, and provide a switching power supply device capable of implementing PWM control with a low-cost and simple circuit configuration that does not require a dedicated control IC, and an electronic device using the same. provide.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, a switching power supply device according to the present invention is a switching power supply device having a switching power supply circuit having a PWM control circuit for controlling a switching operation of a switching element, wherein the PWM control circuit has an output voltage. And an error amplifier circuit that outputs an error signal according to the difference between the reference voltage and the reference voltage, a triangular wave generation circuit that generates a triangular wave signal, and a level shift circuit that shifts and outputs the level of the triangular wave signal according to the error signal. And a logic circuit for shaping the output of the level shift circuit into a rectangular wave with reference to a predetermined threshold value and outputting as a PWM signal for driving the switching element.
[0008]
The triangular wave generating circuit includes a rectangular wave generating circuit that generates a rectangular wave signal, a first resistor connected in series between the rectangular wave generating circuit and the logic element, and the first resistor and the logic. The level shift circuit includes an integration circuit having a first capacitor connected between a connection point of the element and a reference potential, wherein the level shift circuit outputs an output of the error amplification circuit to the first resistor and the logic circuit. It is characterized by being connected to a connection point.
[0009]
Further, the switching power supply device of the present invention is a switching power supply device having a switching power supply circuit provided with a PWM control circuit for controlling a switching operation of a switching element, wherein the PWM control circuit has a difference between an output voltage and a reference voltage. An error amplifier circuit that outputs a corresponding error signal, a triangular wave generating circuit that generates a triangular wave signal, an amplitude variable circuit that changes and outputs the amplitude of the triangular wave signal in accordance with the error signal, and an output of the amplitude variable circuit. And a logic circuit for shaping the waveform into a rectangular wave on the basis of a predetermined threshold value and outputting it as a PWM signal for driving the switching element.
[0010]
The triangular wave generation circuit includes a pulse wave generation circuit that generates a pulse wave having a pulse-like off period, a first diode having one end connected to an output of the pulse wave generation circuit, and a first diode. A first capacitor connected between the other end of the first diode and a reference potential, and a first resistor connected between the other end of the first diode and the output of the error amplifier circuit. The amplitude variable circuit is realized by connecting an output of an amplifier circuit to the first capacitor via the first resistor.
[0011]
Further, the logic circuit is characterized by comprising a logic IC.
[0012]
Further, the triangular wave generation circuit includes an astable multivibrator using a logic element.
[0013]
According to another aspect of the invention, there is provided an electronic device using the above-described switching power supply device.
[0014]
With such a configuration, in the switching power supply of the present invention, the cost of the PWM control circuit can be reduced, and the price of the switching power supply itself can be reduced. In particular, in a multi-output configuration, it is possible to reduce the cost while maintaining the stability of each output voltage.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 shows a circuit diagram of an embodiment of the switching power supply of the present invention. In FIG. 1, a switching power supply 10 is a separately-excited oscillating and step-down switching power supply. First, as a basic configuration, an input terminal Vi to which an input voltage is applied, a switching element Q1 for switching the input voltage, a flywheel It includes a diode D1, a choke coil L1, a smoothing capacitor C1, and an output terminal Vo. Here, the input terminal Vi is connected to the drain of the switching element Q1, and the source of the switching element Q1 is connected to the output terminal Vo via the choke coil L1. The connection point between the source of the switching element Q1 and the choke coil L1 is grounded via a flywheel diode D1, and the output terminal Vo is grounded via a capacitor C1.
[0016]
The switching power supply 10 further includes an error amplifying circuit 2, a rectangular wave generating circuit 11, an integrating circuit 12, a level shift circuit 13, an inverter element 14, and a drive circuit 3 as means for stabilizing the output voltage. These perform PWM control of the switching element Q1 as described later, and thus can be regarded as a PWM control circuit as a whole.
[0017]
The error amplifier circuit 2 is connected to the output terminal Vo, compares the output voltage of the switching power supply 10 with, for example, a built-in reference voltage, amplifies the difference, and outputs the result. Specifically, the output voltage of the error amplifier circuit 2 in the switching power supply 10 decreases as the output voltage of the switching power supply 10 decreases, and increases as the output voltage of the switching power supply 10 increases.
[0018]
The rectangular wave generation circuit 11 is a general astable multivibrator including inverter elements 15 and 16 which are a kind of logic element, resistors R4 and R5, and a capacitor C2. The rectangular wave generating circuit 11 outputs a rectangular wave signal having a frequency substantially determined by the values of the resistors R4 and R5 and the capacitor C2. As the inverter elements 15 and 16, for example, a CMOS logic IC containing six elements is used. Since the configuration of the rectangular wave generation circuit 11 is not a main part of the present invention, a detailed description is omitted.
[0019]
The integrating circuit 12 includes a resistor R1, which is a first resistor connected in series between the rectangular wave generating circuit 11 and an input of an inverter element 14, which will be described later, and a connection point between the resistor R1 and the inverter element 14, and a reference potential. The first capacitor C3 is connected to a certain ground.
[0020]
The level shift circuit 13 is composed of two resistors R2 and R3 provided in series between the error amplifier circuit 2 and the ground which is a reference potential. The connection point of the resistors R2 and R3 is connected to the resistor R1 and the inverter element 14. Connected to a connection point. That is, the level shift circuit 13 is configured by connecting the output of the error amplifier circuit 2 to the connection point between the resistor R1 and the inverter element 14.
[0021]
The inverter element 14 is a logic element similar to the inverter elements 15 and 16 used in the rectangular wave generating circuit 11, and inverts the L level or the H level depending on whether the input voltage is higher or lower than the threshold value. The signal of is output. The output of the inverter element 14 is connected to the gate of the switching element Q1 via the drive circuit 3.
[0022]
Here, a method of stabilizing the output voltage of the switching power supply device 10, that is, an operation of the PWM control circuit will be described with reference to FIG. FIG. 2 shows the time change of the voltage of each part of the PWM control circuit, and the horizontal axis is time. Further, v1 to v5 are the voltage at the connection point of the resistors R4 and R5 in the rectangular wave generation circuit 11, the output voltage of the inverter element 15, the output voltage of the inverter element 16, the input voltage of the inverter element 14, and the inverter element, respectively. 14 is the output voltage. Note that v4 is also the output voltage of the integration circuit 12 that has been level-shifted by the level shift circuit 13. Further, v4 'and v5' will be described later.
[0023]
First, in the rectangular wave generation circuit 11, v1, v2, and v3 are as shown in FIG. v2 and v3 have inverted phases. Then, the almost complete rectangular wave v3 is the output of the rectangular wave generation circuit 11. In FIG. 2, v3 is described as having a duty of 50%, but is not necessarily limited to 50%. The detailed operation of the astable multivibrator is not the main part of the present invention, and therefore will not be described.
[0024]
The rectangular wave signal output from the rectangular wave generation circuit 11 is input to the integration circuit 12. In the integrating circuit 12, as is well known, the output voltage slowly rises at the rising timing of the rectangular wave signal, and conversely, the output voltage slowly falls at the falling timing of the rectangular wave signal. The slope of the rise or fall of the output voltage depends on the time constant determined by the resistor R1 and the capacitor C3. Therefore, by setting this time constant to an appropriate value, the output voltage of the integration circuit 12 can be a pseudo triangular wave as shown by v4 in FIG. That is, in the switching power supply device 10, the triangular wave generating circuit is constituted by the rectangular wave generating circuit 11 and the integrating circuit 12.
[0025]
The level shift circuit 13 is connected to the integration circuit 12. Therefore, the value of the output voltage of the integration circuit 12 is raised and lowered by the level shift circuit 13. Specifically, when the output voltage of the switching power supply device 10 increases and the output voltage of the error amplifier circuit 2 increases, the output voltage of the integrating circuit 12 rises overall with substantially the same waveform by the level shift circuit 13. . Conversely, when the output voltage of the switching power supply 10 decreases and the output voltage of the error amplifier circuit 2 decreases, the output voltage of the integrating circuit 12 also falls entirely with substantially the same waveform by the level shift circuit 13. That is, the output voltage of the integration circuit 12 becomes a triangular wave signal whose level rises and falls with substantially the same waveform according to the output voltage of the switching power supply device 10. V4 in FIG. 2 shows the output voltage of the integrating circuit 12 when the output voltage of the switching power supply 10 is relatively high.
[0026]
The inverter element 14 has a predetermined threshold voltage because it is a logic element. When the input voltage is higher than the threshold voltage, the L level is output, and when the input voltage is lower than the threshold voltage, the H level is output. In the waveform v4 in FIG. 2, the threshold voltage of the inverter element 14 is indicated by a dashed line. Since the output voltage of the integrating circuit 12 is applied to the inverter element 14, when the input signal is higher than the threshold voltage and includes the maximum point of the triangular wave signal, the output is at L level, and the input signal is lower than the threshold voltage and the triangular wave The output goes high when the signal contains the minimum point. In FIG. 2, since the level of the triangular wave signal v4 is relatively high, the output of the inverter element 14 is a signal having a relatively short H-level period, a relatively long L-level period, and a small duty.
[0027]
The output voltage of the inverter element 14 cannot drive the switching element Q1 as it is. Then, after being converted into a signal that can drive the switching element Q1 by the drive circuit 3, the signal is applied to the gate of the switching element Q1 to drive the switching element Q1 on and off. When the output voltage of the inverter element 14 is v5, the on-time of the switching element Q1 is shortened, so that the output voltage of the switching power supply 10 decreases.
[0028]
By the way, v4 in FIG. 2 shows the output voltage of the integration circuit 12 when the output voltage of the switching power supply 10 is higher than a predetermined voltage. Conversely, the output voltage of the switching power supply 10 is higher than the predetermined voltage. If it is also lower, the level of the output voltage of the integrating circuit 12 also decreases as shown by v4 'in FIG. In this case, since the period during which the input voltage of the inverter element 14 is lower than the threshold voltage is long, as shown by v5 'in FIG. The signal has a relatively short period and a large duty. Note that only the duty of the signal changes between v5 ′ and v5, and the frequency does not change. In this case, the output voltage of the switching power supply device 10 increases because the ON time of the switching element Q1 becomes longer.
[0029]
As described above, in the switching power supply device 10, the error amplifier circuit 2, the rectangular wave generation circuit 11, the integration circuit 12, the level shift circuit 13, the inverter element 14, and the drive circuit 3 operate as a PWM control circuit. Since the level shift circuit 13 and the inverter element 14 perform substantially the same function as the two-input type comparator (comparator) in the conventional technique, the cost of the comparison circuit can be significantly reduced. Further, since the pseudo triangular wave generating circuit is composed of the rectangular wave generating circuit 11 and the integrating circuit 12, which are non-stable multivibrators using an inverter element, the triangular wave generating circuit can be significantly reduced in cost. . Further, since a logic IC including a plurality of inverter elements can be easily prepared at a low price, a dedicated control IC is not required, and the overall price of the PWM control circuit can be reduced. .
[0030]
The logic element connected to the output of the integration circuit is not limited to the inverter element, but may be an AND element, an OR element, a NAND element, a NOR element, or a simple buffer as long as it has a predetermined threshold voltage. Any logic element such as a (non-inverting) element may be used. In the case of a logic element having a plurality of input terminals, the remaining input terminals can be used for turning on / off the PWM control itself. Further, in the description of the embodiments, the term “logic element” is used. However, the present invention is not limited to an element configured to perform one function, such as a logic IC, and is configured using individual transistors. It does not matter even if it is a logic circuit.
[0031]
Further, in the switching power supply device 10, the output of the error amplifier circuit 2 is set to decrease as the output voltage decreases and increase as the output voltage increases. However, the PWM control is also performed in the direction to finally stabilize the output voltage. If it can be done, the reverse is acceptable.
[0032]
FIG. 3 shows a circuit diagram of another embodiment of the switching power supply of the present invention. 3, the same or equivalent parts as those in FIG. 1 are denoted by the same reference numerals, and the description thereof will be omitted.
[0033]
The switching power supply device 20 shown in FIG. 3 has two configurations of the switching power supply circuit and the PWM control circuit in the switching power supply device 10, and has a configuration in which only the rectangular wave generation circuit 11 is commonly used. I have.
[0034]
Specifically, in addition to the configuration of the switching power supply device 10, the switching power supply device 10 includes a switching element Q2 for switching an input voltage, a flywheel diode D2, a choke coil L2, a smoothing capacitor C2, and an output terminal Vo2. Correspondingly, the output terminal Vo is the output terminal Vo1. Here, the drain of the switching element Q2 is connected to the input terminal Vi, and the source of the switching element Q1 is connected to the output terminal Vo2 via the choke coil L2. The connection point between the source of the switching element Q2 and the choke coil L2 is grounded via a flywheel diode D2, and the output terminal Vo2 is grounded via a capacitor C2.
[0035]
The switching power supply device 20 further includes an error amplifier circuit 4, an integration circuit 21, a level shift circuit 22, an inverter element 23, and a drive circuit 5 as means for stabilizing the output voltage of the output terminal Vo2. Since these control the switching element Q2 together with the rectangular wave generating circuit 11 as described later, they can be regarded as a PWM control circuit as a whole including the rectangular wave generating circuit 11.
[0036]
The error amplifier circuit 4 is connected to the output terminal Vo2, compares the output voltage of the output terminal Vo2 with, for example, a built-in reference voltage, amplifies the difference, and outputs the result. Specifically, the output of the error amplifying circuit 4 in the switching power supply device 20 decreases as the output voltage decreases and increases as the output voltage increases.
[0037]
The integrating circuit 21 includes a resistor R6, which is a first resistor connected in series between the rectangular wave generating circuit 11 and an input of an inverter element 23 described later, and a connection point between the resistor R6 and the inverter element 23 and a reference potential. The capacitor C5 is a first capacitor connected to a certain ground.
[0038]
The level shift circuit 22 includes two resistors R7 and R8 provided in series between the error amplifier circuit 4 and the ground which is a reference potential, and a connection point between the resistors R7 and R8 is connected to the resistor R6 and the inverter element 23. Connected to a connection point. That is, the level shift circuit 22 is configured by connecting the output of the error amplifier circuit 4 to the connection point of the resistor R6 and the inverter element 23.
[0039]
The inverter element 23 is a logic element similar to the inverter elements 14, 15 and 16, and outputs an L level or H level signal by inverting the input voltage depending on whether the input voltage is higher or lower than a threshold value. The output of the inverter element 23 is connected to the gate of the switching element Q2 via the drive circuit 5.
[0040]
Note that the method of stabilizing the output voltage of the output terminal Vo2, that is, the operation of the newly added PWM control circuit is basically the same as the operation of the other PWM control circuit, and a description thereof will be omitted. .
[0041]
In the switching power supply device 20 configured as described above, although two outputs are provided and each output is independently PWM-controlled, a configuration for realizing the PWM control includes a switching power supply. In the configuration of the PWM control in the device 10, only an error amplifier circuit 4, an integration circuit 21, a level shift circuit 22, and an inverter element 23 are added. No expensive control IC is required. On the contrary, as for the inverter element 23, a remaining CMOS logic IC prepared for another inverter element can also be used, which does not cause a cost increase. Therefore, the switching power supply device can be realized at a low cost with a very simple circuit configuration even though both outputs are stabilized by PWM control.
[0042]
Although the switching power supply device 20 has two outputs, a simple circuit configuration and low cost can be realized even when three or more outputs are required by using the rectangular wave generating circuit in common. it can.
[0043]
By the way, in the switching power supply device 20 of FIG. 2, not only the two PWM control circuits operate simply in synchronization, but also, for example, the on-period and the off-period of the switching elements Q1 and Q2 are almost the same. That is, they are in phase. For this reason, although not shown, the DC power supply on the input side has a period in which a large current flows and a period in which no current flows, and there is a possibility that the ripple of the input current increases and radiation noise increases. . Moreover, in order to improve them, it is necessary to increase the capacitance of an input-side smoothing capacitor (not shown), which also increases the cost.
[0044]
FIG. 4 shows a circuit diagram of still another embodiment of the switching power supply of the present invention as a switching power supply that is unlikely to cause this problem. 4, the same or equivalent parts as those in FIG. 3 are denoted by the same reference numerals, and description thereof will be omitted.
[0045]
In the switching power supply device 30 shown in FIG. 4, the output of the inverter element 15 of the rectangular wave generation circuit 11 is extracted as a rectangular wave signal and applied to the integration circuit 21. Except for this point, the switching power supply device 20 is completely the same.
[0046]
The output voltage of the inverter element 15 is opposite to the output voltage of the inverter element 16 as can be seen from FIG. Therefore, in the switching power supply device 30, the switching element Q2 is in the off period during the on period of the switching element Q1, and the switching element Q2 is in the on period during the off period of the switching element Q1. Therefore, in the input-side DC power supply (not shown), the magnitude of the current flowing at one time is smaller than in the case of the switching power supply device 20, so that the ripple of the input current can be reduced and radiation noise can be reduced. In addition, the capacitance of the input-side smoothing capacitor (not shown) may be small, and the cost can be prevented from increasing.
[0047]
Further, in the switching power supply device 30, when there are two outputs, the switching signal of the PWM control circuit is inverted, that is, the phase is changed by 180 ° (π). This configuration can be similarly applied to a case where three or more outputs are provided. When the number of outputs is made n (n is a natural number of 2 or more) by generalization, the phases of signals for switching are mutually changed. What is necessary is just to make them differ by 360 / n (2π / n).
[0048]
FIG. 5 shows a circuit diagram of still another embodiment of the switching power supply of the present invention. 5, the same or equivalent parts as those in FIG. 1 are denoted by the same reference numerals, and the description thereof will be omitted.
[0049]
In the switching power supply device 40 shown in FIG. 5, a pulse generation circuit 41 and a first diode are used in place of the rectangular wave generation circuit 11, the integration circuit 12, and the level shift circuit 13, which are PWM control circuits in the switching power supply device 10. It has a certain diode D5, a resistor R12 as a first resistor, and a capacitor C7 as a first capacitor. Among them, the pulse generation circuit 41 is an astable multivibrator composed of inverter elements 42 and 43, which are a kind of logic element, resistors R9, R10 and R11, a capacitor C6, and diodes D3 and D4. A pulse wave having a pulse-like OFF period that is only at the L level is generated. The duty is determined by the ratio of the resistors R10 and R11, and the oscillation frequency is determined by the time constant of R10, R11 and C6. Note that the configuration and operation of the pulse generation circuit 41 are not a main part of the present invention, and thus detailed description is omitted.
[0050]
The output of the pulse generation circuit 41, that is, the output of the inverter element 43 is connected to the cathode of the diode D5. On the other hand, the output of the error amplifier circuit 2 is connected to ground, which is a reference potential, via a resistor R12 and a capacitor C7 in series. The connection point between the resistor R12 and the capacitor C7 is connected to the input of the inverter element 14. The anode of the diode D5 is connected to the connection point between the resistor R12 and the capacitor C7.
[0051]
The operation of the PWM control circuit of the switching power supply device 40 thus configured will be described with reference to FIG. FIG. 6 shows the time change of the voltage of each part of the PWM control circuit, and the horizontal axis is time. Further, v6 to v8 are the output voltage of the inverter element 43, the input voltage of the inverter element 14, and the output voltage of the inverter element 14 in the pulse generation circuit 41, respectively. Note that v7 is also the voltage at the connection point between the resistor R12 and the capacitor C7. Further, v7 'and v8' will be described later.
[0052]
First, in the pulse generation circuit 41, the output voltage v6 is a pulse wave in which a short off period, that is, an L level is generated in a pulsed manner at regular intervals, as shown in FIG.
[0053]
On the other hand, if the voltage at the connection point between the resistor R12 and the capacitor C7 starts from a state where the capacitor C7 is completely discharged, the slope determined by the output voltage of the error amplifier circuit 2 and the time constant of the resistor R12 and the capacitor C7. And gradually increases as the capacitor C7 is charged. However, even if the capacitor C7 is charged, if the output voltage of the pulse wave generation circuit 41 becomes L level, the charge is discharged in a short time via the diode D5. Therefore, the voltage v7 at the connection point of the resistor R12 and the capacitor C7 gradually increases from the point where the voltage is reset to the reference voltage level (ground level) starting from the point when the pulse is generated in the output of the pulse wave generation circuit 41. It becomes a triangle wave that repeats that. Therefore, the pulse amplifier 41, the diode D5, the resistor R12, and the capacitor C7 are added to the error amplifier 2 to form a triangular wave generator.
[0054]
Further, the slope of the time change of the voltage at the connection point between the resistor R12 and the capacitor C7 changes depending on the output voltage of the error amplifier circuit 2. Specifically, if the output voltage of the error amplifier circuit 2 is large, the slope becomes large, so that the amplitude of the waveform also becomes large. Conversely, if the output voltage of the error amplifier circuit 2 is small, the slope becomes small, so that the amplitude of the waveform also becomes small. V7 ′ in FIG. 6 is a waveform when the output voltage of the error amplifier circuit 2 increases. From this, it is understood that the error amplifier circuit 2 also constitutes an amplitude variable circuit together with the resistor R12 and the capacitor C7.
[0055]
The inverter element 14 has a predetermined threshold voltage because it is a logic element. When the input voltage is higher than the threshold voltage, the L level is output, and when the input voltage is lower than the threshold voltage, the H level is output. In the waveform v7 in FIG. 6, the threshold voltage of the inverter element 14 is represented by a dashed line. Since the voltage at the connection point of the resistor R12 and the capacitor C7 is applied to the inverter element 14, when the input signal is higher than the threshold voltage and includes the maximum point of the triangular wave signal, the output is at the L level, and the input signal is at the threshold. When the voltage is lower than the voltage and includes the minimum point of the triangular wave signal, the output becomes H level. In FIG. 6, since the amplitude of the triangular wave signal v7 is relatively small, the output of the inverter element 14 is a signal having a relatively long H-level period and a relatively short L-level period and a large duty.
[0056]
On the other hand, when the output voltage of the error amplifier circuit 2 is high, the voltage at the connection point between the resistor R12 and the capacitor C7 becomes a triangular wave having a large amplitude as indicated by v7 'in FIG. Since the threshold value of the inverter element 14 does not change, the output is a signal having a relatively short H-level period and a relatively long L-level period and a small duty.
[0057]
Thus, the PWM control is performed in the switching power supply device 40. In addition, similarly to the case of the switching power supply device 10, since a dedicated control IC or the like is not required, the cost can be reduced. Note that the type of the logic element is not limited, and the output direction of the error amplifier circuit is not limited.
[0058]
Further, multi-output such as the switching power supply devices 20 and 30 can be realized by using the pulse generation circuit 41 in common. If there is enough drive capacity of the inverter element 43, the circuits after the diode D5 may simply be connected in parallel. If there is not enough drive capacity of the inverter element 43, for example, the output of the inverter element 42 may be extracted and used as the output of the pulse generation circuit via another logic element.
[0059]
In each of the above embodiments, a non-insulated step-down DC-DC converter has been described as the switching power supply device. However, the switching power supply device of the present invention is not limited to a non-insulated type, and may be an isolated type using a transformer as long as it is a separately excited type, and may be a step-up type or an inverted type. is there.
[0060]
FIG. 7 shows a perspective view of one embodiment of the electronic device of the present invention. In FIG. 7, a printer 50 as one of the electronic devices uses the switching power supply device 10 of the present invention as a power supply circuit.
[0061]
In the printer 50, a low price can be realized by using the switching power supply device 10 of the present invention.
[0062]
The electronic device of the present invention is not limited to a printer, but includes any electronic device that requires a DC-DC converter, such as a notebook computer or a portable information device.
[0063]
【The invention's effect】
According to the switching power supply of the present invention, the PWM control circuit outputs the error signal corresponding to the difference between the output voltage and the reference voltage, the triangular wave generating circuit generating the triangular wave signal, A level shift circuit for shifting and outputting the level of a triangular wave signal, and a logic circuit for shaping the output of the level shift circuit into a rectangular wave with a predetermined threshold as a reference and outputting as a PWM signal for driving a switching element With this arrangement, the cost of the PWM control circuit can be reduced, and the cost of the switching power supply can be reduced.
[0064]
Further, the PWM control circuit outputs an error amplifier circuit that outputs an error signal corresponding to the difference between the output voltage and the reference voltage, a triangular wave generating circuit that generates a triangular wave signal, and changes the amplitude of the triangular wave signal according to the error signal. A variable amplitude circuit for outputting, and a logic circuit for shaping the output of the variable amplitude circuit into a rectangular wave on the basis of a predetermined threshold value and outputting it as a PWM signal for driving the switching element, The cost of the PWM control circuit can be reduced, and the cost of the switching power supply can be reduced.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing one embodiment of a switching power supply device of the present invention.
FIG. 2 is a waveform chart showing signal waveforms at respective points in the switching power supply device of FIG.
FIG. 3 is a circuit diagram showing another embodiment of the switching power supply device of the present invention.
FIG. 4 is a circuit diagram showing still another embodiment of the switching power supply device of the present invention.
FIG. 5 is a circuit diagram showing still another embodiment of the switching power supply device of the present invention.
6 is a waveform chart showing signal waveforms at respective points in the switching power supply device of FIG.
FIG. 7 is a perspective view showing an embodiment of the electronic device of the present invention.
[Explanation of symbols]
2, 4,... Error amplifier circuit
3, 5 ... Drive circuit
10, 20, 30, 40 ... Switching power supply device
11 ... Square wave generation circuit
12, 21 ... integrator circuit
13, 22 ... Level shift circuit
14, 23 ... Inverter element (logic element)
41 ... Pulse wave generation circuit
Q1, Q2: Switching element
L1, L2: Choke coil
D1, D2 ... Flywheel diode
C1, C2 ... capacitors
D5: Diode (first diode)
R1, R6, R12 ... resistance (first resistance)
C3, C5, C7 ... capacitors (first capacitors)
70 ... Printer

Claims (7)

A switching power supply device having a switching power supply circuit including a PWM control circuit that controls a switching operation of a switching element,
The PWM control circuit includes: an error amplifier circuit that outputs an error signal corresponding to a difference between an output voltage and a reference voltage; a triangular wave generation circuit that generates a triangular wave signal; and a level shifter for the triangular wave signal according to the error signal. And a logic circuit that shapes the output of the level shift circuit into a rectangular wave on the basis of a predetermined threshold value and outputs it as a PWM signal for driving the switching element. A switching power supply device characterized by the above-mentioned. The triangular wave generating circuit includes a rectangular wave generating circuit that generates a rectangular wave signal, a first resistor connected in series between the rectangular wave generating circuit and the logic element, and a first resistor connected to the first resistor and the logic element. An integration circuit having a first capacitor connected between the connection point and the reference potential,
2. The switching power supply device according to claim 1, wherein the level shift circuit is configured by connecting an output of the error amplifier circuit to a connection point between the first resistor and the logic circuit. 3. A switching power supply device having a switching power supply circuit including a PWM control circuit that performs M control of a switching operation of a switching element,
The PWM control circuit includes: an error amplifier circuit that outputs an error signal corresponding to a difference between an output voltage and a reference voltage; a triangular wave generation circuit that generates a triangular wave signal; and a controller that changes an amplitude of the triangular wave signal according to the error signal. And a logic circuit that shapes the output of the variable amplitude circuit into a rectangular wave based on a predetermined threshold and outputs the PWM signal for driving the switching element. A switching power supply device characterized by the above-mentioned. The triangular wave generation circuit includes: a pulse wave generation circuit that generates a pulse wave having a pulse-like off period; a first diode having one end connected to an output of the pulse wave generation circuit; A first capacitor connected between the other end of the first diode and a reference potential, and a first resistor connected between the other end of the first diode and an output of the error amplifier circuit;
The switching power supply device according to claim 3, wherein the amplitude variable circuit is realized by connecting an output of the error amplifier circuit to the first capacitor via the first resistor. The switching power supply device according to any one of claims 1 to 4, wherein the logic circuit is configured by a logic IC. 6. The switching power supply device according to claim 1, wherein the triangular wave generating circuit includes an astable multivibrator using a logic element. An electronic device using the switching power supply device according to claim 1.

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