JP2001218453A - Switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a switching power supply that can reduce an output ripple voltage by changing the oscillation frequency of oscillation signal from an oscillator when an output ripple voltage is detected and this ripple voltage rises. SOLUTION: The amplitude of ripple voltage superimposed on an output A from an output unit 2 is detected with a ripple voltage detecting means 4. The ripple voltage detected in this case is compared, in a comparator 5, with the reference given from a reference voltage generating means 6. When this ripple voltage becomes larger than the reference, the oscillation frequency of a triangular wave F oscillated by an oscillator 3d is changed to become higher with a frequency switching means 7. Moreover, when the ripple voltage detected in the ripple voltage detecting means 4 becomes smaller than the reference given from the reference generating means 6 after returning to the normal condition, the frequency switching means 7 reduces the oscillation frequency of the triangular wave F oscillated from the oscillator 3d to return to the normal condition.

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.

[0002]

2. Description of the Related Art FIG. 11 shows a conventional switching power supply. In the switching power supply device 101 shown in FIG. 11, the DC voltage applied by the DC power
The pulse is converted into a pulse having a predetermined duty by the switching operation of the PN transistor 103. The ON / OFF control of the transistor 103 is performed by a driving circuit 111 that supplies a driving signal to the base.

When the transistor 103 is turned on, a DC voltage supplied from the DC power supply 102 through the transistor 103 causes the transistor 103 to turn on.
Current flows through the coil 104 connected to the emitter of
Energy is stored and energy is supplied to the load 112. When the transistor 103 is OFF, a diode 106 whose cathode is connected to the emitter of the transistor 103 and whose anode is grounded.
Through this, the energy stored in the coil 104 is given to the load 112.

[0004] Such a switching power supply device 101
Is applied to the non-inverting input terminal of the differential amplifier 107 and compared with a reference voltage 108 connected to the inverting input terminal of the differential amplifier 107. The difference between the two is amplified by the differential amplifier 107 and output, and the output is input to the non-inverting input terminal of the PWM comparator 109. In the PWM comparator 109, a triangular wave from the oscillator 110 is input to the inverting input terminal, and a pulse having a duty corresponding to the output of the differential amplifier 107 is output to the drive circuit 111. And with this pulse,
The drive circuit 111 controls the base current of the transistor 103 to perform ON / OFF control of the transistor 103.

In this way, the voltage applied to the load 112 is fed back, so that the ON period of the transistor 103 is controlled by pulse width modulation (PWM), so that the output voltage of the switching power supply 101 becomes a predetermined value. Be stabilized. A smoothing capacitor 10 having one end grounded is connected to a connection node between the coil 104 and the load 112.
5 is connected to the other end. The current flowing through the coil 104 is smoothed by the smoothing capacitor 105 so that the load 1
12 is supplied.

[0006]

As shown in FIG. 12 (a), the switching power supply device 101 performing such an operation has the coil 10 when the transistor 103 is ON.
4 has a positive slope, and the transistor 103
Is OFF, the gradient of the current flowing through the coil 104 is negative. The current flowing through the coil 104 is smoothed by the smoothing capacitor 105 and supplied to the load 112. The ESR (equivalent series resistance) of the smoothing capacitor 105 causes the switching power supply device 101 to perform a smoothing operation as shown in FIG. The ripple voltage is superimposed on the output voltage. This ripple voltage Vp is expressed by the following equation (1). Vp = rc × Vo × (Vi−Vo) / (Vi × L × f) (1) where, rc: ESR value of the smoothing capacitor 105, L: inductance value of the coil 104, f: triangular wave from the oscillator 110 Oscillation frequency f, Vi: voltage value of DC power supply 102, Vo: output voltage value of switching power supply 101,
It is.

As the load 112 to which the switching power supply 101 supplies power, an IC chip or the like having a fixed output voltage accuracy can be cited. Such an IC chip has an input ripple voltage allowable value, for example, 5 V ± 50 mV. Therefore, it is necessary to reduce the output ripple voltage of the switching power supply circuit 101 to a certain fixed value or less. In order to reduce the output ripple voltage, the ESR value r
c or reduce the inductance value L of the coil 104.
Can be increased or the oscillation frequency f of the triangular wave from the oscillator 110 can be increased.

However, when reducing the ESR value of the smoothing capacitor, it is necessary to increase the size of the smoothing capacitor. Also, when increasing the inductance value of the coil, it is necessary to increase the size of the coil. As described above, by increasing the size of the smoothing capacitor or the coil, the size of the circuit constituting the switching power supply device increases, which leads to an increase in cost. When the oscillation frequency of the oscillator is increased, the ON / OFF of the transistor is repeated to
Since the output loss increases, the power supply efficiency deteriorates.

Further, as a smoothing capacitor, generally,
Although an electrolytic aluminum capacitor is used, the ESR value of this electrolytic aluminum capacitor significantly increases at a low temperature. Therefore, the output ripple voltage of the switching power supply may increase significantly at low temperatures.

In view of such a problem, the present invention detects an output ripple voltage and reduces the output ripple voltage by switching the oscillation frequency of an oscillation signal from an oscillator when the output ripple voltage rises. It is an object of the present invention to provide a switching power supply device that causes the switching power supply device to perform a switching operation.

[0011]

In order to achieve the above object, a switching power supply of the present invention comprises a switching element for converting an input DC voltage into a pulse voltage signal by a switching operation, and a switching element for converting the input DC voltage into a pulse voltage signal. Converting means for converting a pulse voltage signal into an output DC voltage, pulse width modulating means for outputting a PWM signal pulse width modulated based on the output DC voltage from the converting means, and PWM provided from the pulse width modulating means. A drive unit for performing ON / OFF control of the switching element by a signal; and a smoothing capacitor for smoothing an output DC voltage from the conversion unit. In a switching power supply device that outputs a voltage, a ripple voltage that appears in an output DC voltage from the conversion unit. A ripple voltage detecting means for detecting the magnitude of the ripple voltage, and switching an oscillation frequency of an oscillation signal used for generating the PWM signal in accordance with the magnitude of the ripple voltage detected by the ripple voltage detecting means. Features.

In such a switching power supply device, when the ripple voltage detected by the ripple voltage detecting means increases, the oscillation frequency of the triangular oscillation signal used when generating the PWM signal by the pulse width modulating means increases. Switch to By doing so, the ripple voltage can be reduced.

Further, the switching power supply of the present invention comprises a switching element for converting an input DC voltage into a pulse voltage signal by a switching operation, a conversion means for converting a pulse voltage signal given from the switching element to an output DC voltage, Pulse width modulation means for outputting a PWM signal pulse-width modulated based on the output DC voltage from the conversion means, and driving means for performing ON / OFF control of the switching element by the PWM signal given from the pulse width modulation means; And a smoothing capacitor for smoothing the output DC voltage from the conversion means, and a switching power supply device for converting the voltage value of the input DC voltage and outputting it as an output DC voltage, wherein the output from the conversion means A ripple voltage detecting means for detecting a maximum value of a ripple voltage appearing in a DC voltage; A comparison unit that compares the maximum value of the ripple voltage detected by the ripple voltage detection unit with a reference value; an oscillation signal generation unit that oscillates an oscillation signal to the pulse width modulation unit; Oscillation frequency switching means for switching the oscillation frequency of the oscillation signal oscillated by the oscillation signal generation means.

In such a switching power supply device, the ripple voltage detecting means is constituted by a band-pass filter that passes a signal whose frequency is equal to the oscillation frequency of the oscillation signal, so that the PWM signal and the ripple signal are detected. A ripple voltage generated in synchronization can be detected.

The comparing means generates a synchronizing signal for synchronizing when the ripple voltage becomes maximum, and operates in response to the synchronizing signal. First determining means for determining whether the value is greater than a first reference value, and operating in response to the synchronization signal, and determining whether the maximum value of the ripple voltage is less than a second reference value. And when the maximum value of the ripple voltage is greater than the first reference value, the oscillation frequency switching means changes the oscillation frequency of the oscillation signal oscillated from the oscillation signal generation means to a second value. Switching from the first oscillation frequency to the second oscillation frequency, and when the maximum value of the ripple voltage becomes smaller than the second reference value, The oscillation frequency of the oscillation signal oscillated from Fushingo generating means to switch to said first oscillation frequency from said second oscillation frequency.

At this time, the synchronization means generates the synchronization signal based on the PWM signal from the pulse width modulation means. By configuring this synchronization means with a differentiating circuit composed of a capacitor and a resistor, the PWM
It is possible to generate a synchronization signal that synchronizes at the moment when the signal goes high. By providing such a synchronization signal to the first and second determination means, the value when the ripple voltage becomes maximum can be compared with each of the first and second reference values.

In such a switching power supply,
First, when the first determination means determines that the maximum value of the ripple voltage has increased the first reference value, the oscillation frequency of the oscillation signal is switched from the first oscillation frequency to the second oscillation frequency. Thereafter, when the maximum value of the ripple voltage is smaller than the second reference value smaller than the first reference value, the oscillation frequency of the oscillation signal is changed from the second oscillation frequency to the first oscillation frequency. To return to the original state. As described above, when the oscillation frequency of the oscillation signal is once switched from the first oscillation frequency to the second oscillation frequency and then returned to the first oscillation frequency, the maximum value of the ripple voltage to be detected and the oscillation signal A hysteresis is formed in the relationship between the oscillation frequencies.

Further, in such a switching power supply device, an adjusting means for adjusting the reference value may be provided according to the voltage value of the output DC voltage from the converting means. Such an adjusting means includes a low-pass filter for obtaining a voltage signal obtained by removing a ripple voltage from the output DC voltage from the conversion means, and a first filter for dividing the output DC voltage to obtain a first signal.
A first voltage dividing means for setting a reference value; and a second voltage dividing means for further dividing the first reference value and setting a second reference value. Thus, the first and second reference values may be adjusted.

Further, in such a switching power supply device, the oscillation signal generation means has an oscillation signal generation capacitor for generating an oscillation signal by repeating charging and discharging, and the oscillation frequency switching circuit causes the oscillation signal generation circuit to generate the oscillation signal. The oscillation frequency of the oscillation signal may be switched by switching the magnitude of the current flowing when the generation capacitor performs charging and discharging.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS <First Embodiment> A first embodiment of the present invention
The switching power supply according to the embodiment will be described with reference to the drawings. FIG. 1 is a block circuit diagram showing the internal configuration of the switching power supply of the present embodiment. 2 and 3 are time charts showing the operation of each part of the switching power supply of the present embodiment.

The switching power supply device 1 shown in FIG. 1 includes an output unit 2 for converting a DC voltage supplied from a DC power supply 8 and outputting the converted DC voltage to a load 9, and a control unit 3 for controlling the DC voltage output from the output unit 2. A ripple voltage detecting means 4 for detecting a ripple voltage from a DC voltage output from the output unit 2, and a comparator for comparing the ripple voltage detected by the ripple voltage detecting means 4 with a reference value provided from a reference value generating means 6 5
And reference value generating means 6 and frequency switching means 7 for switching the oscillation frequency of the oscillator 3d in the control section 3.

In such a switching power supply device 1, the output unit 2 includes a DC power supply 8 having a collector connected to N
A PN transistor 2a, a diode 2b having a cathode connected to the emitter of the transistor 2a and an anode grounded, a coil 2c having one end connected to a connection node between the emitter of the transistor 2a and the cathode of the diode 2b, A smoothing capacitor 2 having one end connected to the other end of the coil 2c and the other end grounded
d. Then, the coil 2c and the smoothing capacitor 2
The other end of the load 9, one end of which is grounded, is connected to the connection node with the node d.

Further, the control unit 3 controls a differential circuit in which the output A appearing at the connection node between the coil 2c and the smoothing capacitor 2d is input to the non-inverting input terminal and the voltage Vt of the reference voltage 3b is input to the inverting input terminal. The amplifier 3a and the output of the differential amplifier 3a are input to the non-inverting input terminal and the triangular wave F from the oscillator 3d is input to the inverting input terminal.
WM comparator 3c and oscillator 3 for oscillating triangular wave F
d, and a drive circuit 3e that performs ON / OFF control of the transistor 2a in accordance with the output of the PWM comparator 3c. The drive circuit 3e provides a signal to the base of the transistor 2a to turn on the transistor 2a.
/ OFF control is performed. In FIG. 1, B indicates the output of the ripple voltage detecting means 4, and C indicates the PWM comparator 3c.
, And D represents the output of the comparator 5, respectively.

In such a switching power supply device 1, the DC voltage applied by the DC power supply 8 is converted into a pulse having a predetermined duty by the transistor 2a performing a switching operation as shown in FIG. You. When the transistor 2a is ON, a current flows through the coil 2c by the DC voltage from the DC power supply 8 supplied through the transistor 2a, energy is stored, and energy is supplied to the load 9. When the transistor 2a is OFF, the energy stored in the coil 2c is supplied to the load 9 through the diode 2b.

At this time, the current flowing through the coil 2c is
(B), the AC component of the current flowing through the coil 2c flows through the smoothing capacitor 2d,
The current flowing through the coil 2c is smoothed, and a direct current flows through the load 9. At this time, the voltage applied to the load 9 is as shown in FIG.
As shown in (c), the DC voltage that is the output A of the output unit 2 is:
Current flowing through smoothing capacitor 2d and smoothing capacitor 2d
A ripple voltage expressed by the resistance component (ESR) is superimposed.

The output A applied to the load 9 is the differential amplifier 3
a is applied to the non-inverting input terminal and is compared with the voltage Vt of the reference voltage 3b applied to the inverting input terminal. Then, the differential amplifier 3a amplifies and outputs the difference between the two, and the output is input to the non-inverting input terminal of the PWM comparator 3c. In the PWM comparator 3c, a triangular wave F as shown in FIG.
And outputs an output C, which becomes a pulse having a duty corresponding to the output of the differential amplifier 3a, to the drive circuit 3e. That is, as shown in FIG.
When the output C of c is higher than the triangular wave F from the oscillator 3d, the output of the differential amplifier 3a of FIG. 3A becomes high level (Hi), and the output of the differential amplifier 3a of FIG. Is smaller than the triangular wave F from the oscillator 3d.

The output C allows the driving circuit 3
e controls the base current of transistor 2a, and FIG.
As shown in (c), ON / OFF control of the transistor 2a is performed. That is, the transistor 2a is turned off when the output of the PWM comparator 3c is Hi, and turned on when the output of the PWM comparator 3c is Low. Thus, the voltage of the output A given to the load 9 is fed back, so that the ON state of the transistor 2a is turned on.
The pulse width modulation (PWM) control is performed during this period. Therefore, the switching operation of the transistor 2a is performed with an appropriate duty at the frequency of the triangular wave F of the oscillator 3d, and the voltage of the output A of the switching power supply circuit 1 is stabilized at a predetermined value.

In the ripple voltage detecting means 4, FIG.
A ripple voltage appearing at the output A of the output unit 2 as shown in (c) is detected. The detected ripple voltage is supplied to the comparator 5
Is compared with the reference value by the reference value generation means 6. A signal which is a result of comparison by the comparator 5 is given to the frequency switching means 7, and the frequency switching means 7 changes the oscillation frequency of the triangular wave oscillated from the oscillator 3d to an appropriate frequency in accordance with the signal of the comparator 5. Switch to.

For example, the voltage of the DC power supply 8 is 12 V, the output voltage set value of the output A from the output unit 2 is 5 V, and the allowable ripple voltage of the load 9 is 5
It is assumed that 0 mV is required. The reference value is generated by the reference value generation means 6 based on the ripple voltage allowable value. At this time, the value of the inductance of the coil 2c is set to 21.
0 μH, the ESR of the smoothing capacitor 2d at room temperature is 0.1Ω, and the oscillation frequency of the triangular wave F of the oscillator 3d is 50k.
When the frequency is set to Hz, the peak-to-peak of the ripple voltage appearing at the output A is 28 mV according to the above equation (1), which is within the allowable ripple voltage value of the load 9. That is, the ripple voltage detected by the ripple voltage detection device 4 becomes smaller than the reference value given by the reference value generation means 6.
At this time, the oscillation frequency of the triangular wave F is not switched by the frequency switching device 7, and the triangular wave F is oscillated at an oscillation frequency of 50 kHz from the oscillator 3d.

In general, an electrolytic aluminum capacitor is used as the smoothing capacitor 2d. Such an electrolytic aluminum capacitor has an ESR about twice as high at -20 ° C. as at room temperature. Become. Therefore, in the above example, when the ESR of the smoothing capacitor 2d whose ESR is 0.1Ω at room temperature is −20 ° C.,
It becomes about 0.2Ω. Therefore, from the above equation (1),
The peak-to-peak ripple voltage appearing at the output A is 56
mV, which is larger than the ripple allowable voltage value of the load 9. That is, the ripple voltage detected by the ripple voltage detection device 4 becomes larger than the reference value provided by the reference value generation means 6. At this time, the oscillation frequency of the triangular wave F is switched by the frequency switching device 7, and the triangular wave F is oscillated at an oscillation frequency of 100 kHz from the oscillator 3d.
By switching the oscillation frequency of the triangular wave F to 100 kHz in this manner, the peak-to-peak of the ripple voltage appearing at the output A becomes 28 mV according to the above-described equation (1), and falls within the ripple allowable voltage value of the load 9 again.

<Second Embodiment> A switching power supply according to a second embodiment of the present invention will be described with reference to the drawings. FIG. 4 is a block circuit diagram showing the internal configuration of the switching power supply device of the present embodiment. FIG. 5 is a block circuit diagram showing the internal configuration of the ripple voltage detecting means, the comparator and the synchronizing means. FIG. 6 is a block circuit diagram showing the internal configuration of the oscillator and the frequency switching means.
In the switching power supply device of FIG. 4, portions used for the same purpose as in the switching power supply device of FIG. 1 are denoted by the same reference numerals, and detailed description thereof will be omitted.

The switching power supply device 1a shown in FIG.
In the switching power supply device 1 of the first embodiment (FIG. 1),
This is a switching power supply device having a configuration in which a synchronization unit 10 is newly provided. That is, the output C of the PWM comparator 3c is input to the switching power supply device 1a and the comparator 5a operates according to the output C so that the comparator 5a operates in synchronization with the output C of the PWM comparator 3c. Synchronizing means 10 is provided. In addition, E1, E2
Represents the reference value generated by the reference value generation means 6, and G represents the output of the synchronization means 10.

Incidentally, such a switching power supply device 1a
2, the relationship between the switching operation of the transistor 2a, the current flowing through the coil 2c, and the voltage applied to the load 9 becomes as shown in FIG. 2 as in the first embodiment, and the input to the PWM comparator 3c and its output FIG. 3 shows the relationship between the switching operation of the transistor 2a and the switching operation of the transistor 2a as in the first embodiment.

The ripple voltage detecting means 4, the comparator 5a and the synchronizing means 10 of the switching power supply 1a will be described with reference to FIG. First, the ripple voltage detecting means 4 will be described. As shown in FIG. 2, it can be seen that the ripple voltage superimposed on the output A changes in synchronization with the switching operation of the transistor 2a. or,
FIG. 3 shows that the switching operation of the transistor 2a is synchronized with the oscillation frequency of the triangular wave F input to the PWM comparator 3c. Therefore,
The ripple voltage changes in synchronization with the oscillation frequency of the triangular wave F from the oscillator 3d. Therefore, as in the example shown in the first embodiment, the oscillation frequency of the triangular wave F is 50 kHz and 1 kHz.
Assuming that switching is performed at 00 kHz, the frequency of the ripple voltage is 50 kHz to 100 kHz.

As described above, in order to detect a ripple voltage which becomes a signal synchronized with the oscillation frequency of the triangular wave F from the oscillator 3d, the ripple voltage detecting means 4 is constituted by a filter which passes the signal based on the ripple voltage. Can be realized by That is, as shown in FIG. 4, the ripple voltage detecting means 4 is connected to the capacitor 21 whose output A is applied to one end.
A resistor 22 having one end connected to the other end of the capacitor 21 and the other end grounded; a resistor 23 having one end connected to a connection node between the capacitor 21 and the resistor 22;
This filter includes a capacitor 24 having one end connected to the other end of the resistor 23 and the other end grounded. The output B of the ripple voltage detecting device 4 is a resistor 2
3 is output from the connection node between the capacitor 3 and the capacitor 24.

That is, the capacitance value of the capacitor 21 and the resistance 22
By properly setting the resistance values of the DC voltage and the low frequency component, the output A of the output unit 2 can be removed. By appropriately setting the resistance value of the resistor 23 and the capacitance value of the capacitor 24, the oscillator 3d
A band-pass filter that allows a ripple voltage synchronized with the oscillation frequency of the triangular wave F from the filter to pass is formed, and is output as an output B.

The ripple voltage detecting device 4 having such a configuration
In the case where the oscillation frequency of the triangular wave F is switched between 50 kHz and 100 kHz as in the example described above, it is necessary to configure a filter that can pass a signal of 50 kHz to 100 kHz. Therefore,
At this time, the capacitance values of the capacitors 21 and 24 are set to 0.033 μF and 15 pF, respectively, and the resistance 2
By setting both resistance values of 2 and 23 to 10 kΩ, 50
A ripple voltage detection device is configured as a filter that can pass a ripple voltage that becomes a signal of kHz to 100 kHz.

Next, the synchronizing means 10 will be described. The synchronization means 10 includes a capacitor 25 having one end to which the output C from the PWM comparator 3c is provided,
5 is a differentiating circuit composed of a resistor 26 having one end connected to the other end and the other end grounded. And
The output G appearing at the connection node between the capacitor 25 and the resistor 26 is given to the comparator 5a as a synchronization signal.

The operation of the synchronizing means 10 having such a configuration will be described below. First, the PWM comparator 3c
Is low, the output G of the synchronization means 10 through the capacitor 25 is low. When the output C changes from low to high, a current flows from the capacitor 25 to the resistor 26, and the output G of the synchronization means 10 changes to high. Thereafter, since the capacitor 25 is charged for a time determined by the capacitance value of the capacitor 25 and the resistance value of the resistor 26, the voltage applied to both ends of the capacitor 25 increases, and the voltage of the output G of the synchronization means 10 decreases. To be Low. Thus, the output C of the PWM comparator 3c is L
An output G that becomes Hi at the moment when the signal changes from ow to Hi is
It is output from the synchronization means 10.

FIG. 3 shows that when the output of the PWM comparator 3c changes from low to high, the transistor 2a switches from ON to OFF.
FIG. 2 shows that the ripple voltage is maximum when the transistor 2a switches from ON to OFF. Therefore, it can be seen that the output G output from the synchronization means 10 becomes Hi when the output of the PWM comparator 3c changes from Low to Hi, and becomes Hi when the ripple voltage becomes maximum.

Next, the comparator 5a will be described. The comparator 5a includes a differential amplifier 31 having the output B of the ripple voltage detecting means 4 input to the non-inverting input terminal, a differential amplifier 32 having the output B of the ripple voltage detecting means 4 input to the inverting input terminal, AND circuits 33 and 34 to which the outputs of the differential amplifiers 31 and 32 are respectively input, and an RS flip-flop to which the output of the AND circuit 33 is input to a set terminal S and the output of the AND circuit 34 is input to a reset terminal R 35.

In such a comparator 5a, the reference value E1 generated by the reference value generating means 6 is applied to the inverting input terminal of the differential amplifier 31, and the reference value E2 generated by the reference value generating means 6 is applied to the differential amplifier. It is input to each of the 32 non-inverting input terminals. The output G of the synchronization means 10 is input to both of the AND circuits 33 and 34. That is, the connection node between the resistor 23 and the capacitor 24 is connected to the non-inverting input terminal of the differential amplifier circuit 31 and the inverting input terminal of the differential amplifier circuit 32, and the connection node between the capacitor 25 and the resistor 26 is connected to the AND circuit. 33, 34 are connected to one input terminal. The reference values E1 and E2 given by the reference value generating means 6 are generated by the reference value generating means 6 so that the voltage values satisfy a relationship of E1> E2. Furthermore,
The output of the output terminal Q of the RS flip-flop 35 is input to the frequency switching means 7 as the output D of the comparator 5a.

The operation of the comparator 5a having such a configuration will be described below. When the voltage of the output B of the ripple voltage detecting means 4 appearing at the connection node between the resistor 23 and the capacitor 24 becomes higher than the voltage of the reference value E1 given by the reference value generating means 6, the differential amplifier 31 becomes Hi. A signal is output. At this time, the output G of the synchronization means 10 appearing at the connection node between the capacitor 25 and the resistor 26 is A
Since the signal is input to one input terminal of the ND circuit 33, the signal output from the differential amplifier 31 is supplied to the set terminal S of the RS flip-flop 35 only when the output G becomes Hi.

Therefore, the signal output from the differential amplifier 31 is given to the set terminal S of the RS flip-flop 35 when the ripple voltage appearing at the output A of the output unit 2 becomes maximum, so that the maximum value of the ripple voltage becomes The result of the comparison with the reference value E1 is given to the set terminal S of the RS flip-flop 35. That is, when the maximum value of the ripple voltage detected by the ripple voltage detecting means 4 becomes larger than the voltage of the reference value E1, a Hi signal is supplied to the set terminal S of the RS flip-flop 35. Thus, when the Hi signal is given to the set terminal S of the RS flip-flop 35, the RS flip-flop 35
Is set and the output D which becomes Hi from the output terminal Q is output to the frequency switching means 7.

When the voltage of the output B of the ripple voltage detecting means 4 becomes smaller than the voltage of the reference value E2 given by the reference value generating means 6, a signal which becomes Hi from the differential amplifier 32 is output. The AND circuit 34 is also an AND circuit 33
Similarly to the above, since the output G of the synchronization means 10 is input to one input terminal, only when this output G becomes Hi,
A signal output from the differential amplifier 32 is provided to a reset terminal R of the RS flip-flop 35.

Therefore, when the ripple voltage appearing at the output A of the output unit 2 becomes maximum, the signal output from the differential amplifier 32 is given to the reset terminal R of the RS flip-flop 35, so that the maximum value of the ripple voltage becomes The result of the comparison with the reference value E2 is given to the reset terminal R of the RS flip-flop 35. That is, the maximum value of the ripple voltage detected by the ripple voltage detecting means 4 is:
When the voltage becomes lower than the reference value E2, a Hi signal is supplied to the reset terminal R of the RS flip-flop 35. As described above, when the Hi signal is supplied to the reset terminal R of the RS flip-flop 35, the RS flip-flop 35 is reset, and the output D which becomes Low from the output terminal Q is output to the frequency switching means 7.

As described above, when the maximum value of the ripple voltage detected by the ripple voltage detecting means 4 becomes larger than the voltage of the reference value E1 given by the reference value generating means 6, RS
By setting the flip-flop 35, the output D of the comparator 5a becomes Hi. Thereafter, when the maximum value of the ripple voltage detected by the ripple voltage detecting means 4 becomes smaller than the voltage of the reference value E2 given by the reference value generating means 6, the RS flip-flop 35 is reset, and the comparator 5a is reset. Becomes low and returns to the original state. After the power is turned on, in a normal state where the maximum value of the ripple voltage detected by the ripple voltage detecting means 4 is smaller than the voltage of the reference value E1 given by the reference value generating means 6, the output D of the comparator 5a is Low. is there.

As in the above-described example, the voltage of the DC voltage 8 is 12 V, the set value of the output A is 5 V, the inductance value of the coil 2 c is 210 μH, and the ESR of the smoothing capacitor 2 d at room temperature is 0.1 Ω. At the same time, when the oscillation frequency of the triangular wave F of the oscillator 3d is switched between 50 kHz and 100 kHz, for example, the reference values E1, E
2 are set to 20 mV and 5 mV, respectively.

At this time, at room temperature, the peak-to-peak of the ripple voltage appearing at the output A is 28 mV according to the above equation (1), and the maximum value of the ripple voltage detected by the ripple voltage detecting means 4, that is, the output The maximum value of B is 14 mV. Therefore, since the maximum value of the output B is smaller than the reference value E1 and larger than the reference value E2, both outputs of the differential amplifiers 31 and 32 become Low. Therefore, since the RS flip-flop 35 is not set or reset, the output D of the comparator 5a becomes Low. Since the output D is low, the frequency switching means 7 controls the triangular wave F having an oscillation frequency of 50 kHz so as to be oscillated by the oscillator 3d.

When the switching power supply 1a is placed in a low temperature environment, the ESR of the smoothing capacitor 2d increases, and the peak-to-peak value of the ripple voltage appearing at the output A increases. When the peak-to-peak of the ripple voltage becomes 42 mV which is larger than 40 mV, that is, when the maximum value of the output B of the ripple voltage detecting means 4 becomes 21 mV which is larger than 20 mV, the maximum value of the output B becomes the reference value. Since the voltage is higher than the voltage value of E1, the Hi signal is supplied from the differential amplifier 31 to the set terminal S of the RS flip-flop 35, and the Low signal is supplied from the differential amplifier 32 to the RS flip-flop 3.
5 reset terminal R. Therefore, the RS flip-flop 35 is set. Therefore, the comparator 5a
Becomes Hi. Since the output D is Hi, the frequency switching means 7 controls the triangular wave F having an oscillation frequency of 100 kHz to be oscillated by the oscillator 3d.

When the triangular wave F having an oscillation frequency of 100 kHz is oscillated by the oscillator 3d, the peak-to-peak ripple voltage becomes 21 mV according to the equation (1). Therefore, the maximum value of the output B of the ripple voltage detecting means 4 is 10.
5 mV, which is smaller than the reference value E1 and larger than the reference value E2. Therefore, the signals output from the differential amplifiers 31 and 32 are both Low. Therefore, since the signals input to the set terminal S and the reset terminal R of the RS flip-flop 35 are both low, the RS flip-flop 35 remains set and the comparator 5a output from the output terminal Q Output D remains Hi. Therefore, the frequency switching means 7 controls so that the triangular wave F having an oscillation frequency of 100 kHz is oscillated by the oscillator 3d.

The oscillation frequency of the triangular wave F is 100 k
Hz, when the switching power supply device 1a is placed in the original room temperature environment, the ESR of the smoothing capacitor 2d is reduced.
And the peak-to-peak value of the ripple voltage appearing at the output A decreases. When the peak-to-peak of the ripple voltage becomes 9 mV which is smaller than 10 mV, that is, when the maximum value of the output B of the ripple voltage detecting means 4 becomes 4.5 mV which is smaller than 5 mV, the maximum value of the output B becomes Since it is smaller than the voltage value of the reference value E2, a Hi signal is supplied from the differential amplifier 32 to the reset terminal R of the RS flip-flop 35,
A Low signal is supplied from the differential amplifier 31 to the set terminal S of the RS flip-flop 35. Therefore, the RS flip-flop 35 is reset. Therefore, the output D of the comparator 5a becomes Low. Since the output D is low, the oscillation frequency is set to 50 kHz by the frequency switching means 7.
Is controlled by the oscillator 3d to return to the normal operation.

The oscillator 3 of the switching power supply 1a
d and the frequency switching means 7 will be described with reference to FIG. First, the oscillator 3d will be described. Oscillator 3d
A PNP type transistor 61a, 61b, 61c having a voltage VDD applied to the emitter, a resistor 61d having one end connected to the base and collector of the transistor 61a,
NPN-type transistors 64a, 64 whose emitters are grounded
4b, 64c; resistors 63a, 63b, 63c connected in series between the voltage VDD and the ground voltage;
A resistor 63d to which DD is applied, an NPN transistor 63e having the other end of the resistor 63d connected to the base and the emitter grounded, and an NPN transistor having the other end of the resistor 63d connected to the collector and the emitter grounded. Amplifier 63 having a non-inverting input terminal connected to the connection node of the type transistor 63f, the resistors 63a and 63b, and a capacitor 65 having one end connected to the inverting input terminal of the differential amplifier 62 and the other end grounded. It is composed of

The voltage VDD is applied to one end of the resistor 63a, and one end of the resistor 63c is grounded.
The collector of transistor 63e is connected to the connection node between b and 63c. The output terminals of the differential amplifier 62 are connected to the bases of the transistors 63f and 64c. The transistor 6a is connected to the base and the collector of the transistor 61a.
1b and 61c are connected, and the transistor 64b
Are connected to the base and collector of transistor 64a and the collector of transistor 64c. Further, the collectors of the transistors 61b and 61c are connected to the collectors of the transistors 64a and 64b, respectively, and the connection node to which the collectors of the transistors 61b and 64a are connected is connected to the inverting input terminal of the differential amplifier 62.

As described above, the transistors 61a, 61
b, 61c and transistors 64a, 64b form a current mirror circuit, respectively.
1a, 61b and 61c have base resistances of 1: 1: 1.
The transistors 64a and 64b form a current mirror circuit having a base resistance of 1: 2. Further, the oscillator 3d having such a configuration is provided as follows: VDD × (the resistance value of the resistor 63b +
(Resistance value of resistance 63c) / (resistance value of resistance 63a + resistance 63)
b) + the resistance of the resistor 63c) to the maximum values Vmax, VDD
A triangular wave F having a minimum value Vmin of × (resistance value of resistor 63b) / (resistance value of resistor 63a + resistance value of resistor 63b) is output. The operation of the oscillator 3d will be described below.

First, when the voltage gradient of the triangular wave F is positive,
When the voltage input to the non-inverting input terminal of the differential amplifier 62 is V
max, and the output of the differential amplifier 62 becomes Hi. At this time, the transistors 63f and 64c are turned on, and the transistor 63e is turned off. Therefore, the triangular wave F
Is positive and the value is smaller than Vmax, the voltage input to the non-inverting input terminal of the differential amplifier 62 is always Vmax.

At this time, the same amount of current as the current flowing through the transistor 61a flows through the transistors 61b and 61c. Then, since the transistor 64c is ON,
The transistors 64a and 64b are turned off. Therefore,
When the current flowing through the transistor 61b flows through the capacitor 65 and charges the capacitor 65, the voltage applied to the inverting input terminal of the differential amplifier 62 increases. Therefore, the voltage of the triangular wave F increases. Also, transistor 6
The current flowing through 1c flows through transistor 64c.

When the voltage of the triangular wave F becomes higher than Vmax, that is, when the voltage applied to the inverting input terminal of the differential amplifier 62 becomes higher than Vmax, the output of the differential amplifier 62 becomes L
ow, the transistors 63f and 64c are turned off.
And the transistor 63e is turned on. Therefore, the connection node between the resistors 63b and 63c is almost equal to the state where the connection node is grounded via the transistor 63e.
The voltage applied to the non-inverting input terminal of the differential amplifier 62 is Vmin
Becomes

At this time, the same amount of current as the current flowing through the transistor 61a flows through the transistors 61b and 61c. Then, since the transistor 64c is OFF, the transistors 64a and 64b are ON. Therefore, the current flowing through the transistor 61b is
4a, the current flowing through the transistor 61c flows through the transistor 64b. Now transistor 6
Assuming that the current flowing through 1a, 61b, 61c is Ie, the transistors 64a, 64b form a current mirror circuit having a base resistance of 1: 2.
A current of 2 × Ie flows through a, and a current of Ie flows through transistor 64b. Therefore, the charge charged from the capacitor 65 is discharged as the current for the remaining Ie via the transistor 64a. Therefore, the voltage of the triangular wave F drops.

When the voltage of the triangular wave F becomes lower than Vmin, that is, when the voltage applied to the inverting input terminal of the differential amplifier 62 becomes lower than Vmin, the output of the differential amplifier 62 becomes H
i, the transistors 63f and 64c are turned on again
And the transistor 63e is turned off.
Therefore, the voltage applied to the non-inverting input terminal of the differential amplifier 62 becomes Vmax again, and the voltage of the triangular wave F increases by performing the above operation. By repeating such an operation, the triangular wave F is oscillated by the oscillator 3d.

The oscillation frequency fo of the triangular wave F oscillated by the oscillator 3d performing such an operation is obtained from the following equation (2) because the cycle of the triangular wave F is equal to the cycle of charging and discharging of the capacitor 65. In the expression (2), C is the capacitance value of the capacitor 65. fo = Ie / {2 × C × (Vmax−Vmin)} (2)

Next, the frequency switching means 7 will be described. The frequency switching means 7 includes a collector connected to the other end of the resistor 61d in the oscillator 3d, and a comparator 5 connected to the base.
an NPN transistor 51 to which the output D of a is input;
It comprises a resistor 52 connected between the collector and the emitter of the transistor. Also, the transistor 51
Its emitter is grounded, and its base is connected to the output terminal Q of the RS flip-flop 35 of the comparator 5a.

The operation of the frequency switching means 7 having such a configuration will be described below. First, after the switching power supply device 1a in an environment at room temperature operates, the maximum value of the ripple voltage detected by the ripple voltage detecting means 4 is equal to the reference value E.
When it is smaller than 1, as described above, the RS of the comparator 5a
The output D of Low is output from the output terminal of the flip-flop 35. Therefore, the transistor 51 is turned off, and the collector current Ie flowing through the transistor 61a of the oscillator 3d flows through the resistors 61d and 52. Therefore, the collector current Ie flowing through the transistor 61a becomes lower (3)
It is expressed by an equation. Ie = (VDD−VBE) / (R1 + R2) (3) where VBE is the base-emitter voltage of the transistor 61a, R1: the resistance of the resistor 61d, and R2: the resistance of the resistor 52.

When the maximum value of the ripple voltage detected by the ripple voltage detecting means 4 exceeds the reference value E1 in the switching power supply 1a in a low temperature environment, as described above, RS flip-flop 3
Since 5 is set, the output D becomes Hi. Therefore,
The transistor 51 is turned on, and the collector current Ie flowing through the transistor 61a of the oscillator 3d flows through the resistor 61d and the transistor 51. Therefore, transistor 6
The collector current Ie flowing through 1a is expressed by the following equation (4). Ie = (VDD−VBE−VSAT) / (R1) (4) where VSAT is the saturation voltage of the transistor 51.

Thereafter, when the maximum value of the ripple voltage detected by the ripple voltage detecting means 4 falls below the reference value E2 in the switching power supply 1a in the environment of the room temperature, the RS in the comparator 5a is switched as described above. Flip-flop 35
Is reset, the output D becomes low. Therefore, the transistor 51 is turned off again, and the oscillator 3
d, the collector current Ie flowing through the transistor 61a is
The current flows to the resistors 61d and 52. Therefore, the transistor 61
The collector current Ie flowing through a is represented by the above-mentioned equation (3).

As described above, when the output D output from the output terminal Q of the RS flip-flop 35 is applied to the base of the transistor 51, the collector current Ie flowing through the transistor 61a in the oscillator 3d is switched. That is, since the current for charging and discharging the capacitor 65 is adjusted, as is apparent from the equation (2),
The oscillation frequency of the triangular wave F oscillated by the oscillator 3d is switched. Also, when comparing equations (3) and (4),
The current Ie represented by the equation (3) becomes smaller. That is,
Since the current Ie flowing when the transistor 51 is off is small, the oscillation frequency of the triangular wave F when the output D is low is lower.

<Third Embodiment> A switching power supply according to a third embodiment of the present invention will be described with reference to the drawings. FIG. 7 is a block circuit diagram showing the internal configuration of the switching power supply device of the present embodiment. FIG. 8 is a block circuit diagram showing the internal configuration of the reference value generating means and the adjusting means. In the switching power supply device of FIG.
Portions used for the same purpose as in the switching power supply device of FIG. 4 are denoted by the same reference numerals, and detailed description thereof will be omitted.

The switching power supply 1b shown in FIG.
Switching power supply device 1a according to the second embodiment (FIG. 4)
The switching power supply device has a configuration in which the adjusting means 11 is newly provided. That is, the switching power supply device 1
b is provided with adjusting means 11 for adjusting the reference values E1 and E2 provided to the comparator 5a by the reference value generating means 6. The ripple voltage detecting means 4 and the synchronizing means 1 shown in FIG.
0, comparator 5a, frequency switching means 7, and oscillator 3d
Is an internal configuration shown in the block circuit diagram of FIG. 5 or FIG. 6 as in the second embodiment, and therefore detailed description thereof is omitted. Therefore, the configurations and operations of the reference value generating means 6 and the adjusting means 11 which are the points to be changed will be described below with reference to FIG.

As shown in FIG. 8, the adjusting means 11 comprises a variable DC power supply 41, and the reference value generating means 6 comprises a resistor 42 having one end connected to the DC power supply 41 and a resistor 42 having one end connected to the DC power supply 41. And a resistor 43 connected to the other end and grounded at the other end. Thus, when the reference value generating means 6 and the adjusting means 11 are configured, the reference value E1
Is the voltage value provided from the variable DC power supply 41. The value of the reference value E2 is a voltage value obtained by dividing the voltage value given from the DC power supply 41 by the resistors 42 and 43. In this way, the reference values E1, E2
Is generated, and the reference values E1 and E2 are adjusted by the variable DC power supply 41 serving as the adjusting unit 11.

<Fourth Embodiment> A switching power supply according to a fourth embodiment of the present invention will be described with reference to the drawings. FIG. 9 is a block circuit diagram showing the internal configuration of the switching power supply device of the present embodiment. Also, FIG.
FIG. 3 is a block circuit diagram showing an internal configuration of a reference value generation unit and an adjustment unit. In the switching power supply device of FIG. 9, portions used for the same purpose as those of the switching power supply device of FIG. 7 are denoted by the same reference numerals, and detailed description thereof will be omitted.

The switching power supply 1c shown in FIG.
Unlike the switching power supply device 1b of the third embodiment (FIG. 7), the adjusting unit 11 that adjusts the reference values E1 and E2 generated by the reference value generating unit 6 using the output A of the output unit 2.
a is provided. The ripple voltage detecting means 4, the synchronizing means 10, the comparator 5a, the frequency switching means 7, and the oscillator 3d in FIG. 7 are the same as those in the second embodiment (FIG. 4) and the third embodiment. Alternatively, since the internal configuration is as shown in the block circuit diagram of FIG. 6, detailed description thereof will be omitted. Therefore, the configurations and operations of the reference value generation unit 6 and the adjustment unit 11a which are the points to be changed will be described below with reference to FIG.

As shown in FIG. 10, the adjusting means 11a
A resistor 44 to which an output A from the output unit 2 is input at one end;
A resistor 45 having one end connected to the other end of the resistor 44 and the other end grounded, and a capacitor 46 having one end connected to the connection node of the resistors 44 and 45 and the other end grounded.
It is composed of The reference value generating means 6 is composed of resistors 42 and 43, as in the third embodiment (FIG. 8).
A connection node between the resistors 44 and 45 and the capacitor 46 is connected to one end of the resistor 42.

As described above, when the reference value generating means 6 and the adjusting means 11a are configured, the voltage appearing at the connection node between the resistors 44, 45, 42 and the capacitor 46 is equal to the reference value E1.
The voltage appearing at the connection node between the resistors 42 and 43 appears as the reference value E2. In the adjusting means 11a,
The output A of the output unit 2 is divided by the resistors 44 and 45, and the ripple voltage appearing at the output A is removed by the resistor 44 and the capacitor 46 constituting the low-pass filter. In this way, a voltage obtained by dividing the DC component of the output A is supplied to the reference value generating means 6.

By configuring the reference value generating means 6 and the adjusting means 11a as described above, the reference values E1 and E2 can be set using the DC component of the output A. Therefore,
For example, the allowable value of the ripple voltage detected by the ripple voltage detecting means 4 is set to 1% or less of the output voltage of the output A.
Can be set relative to the output voltage. At this time, for example, when the output voltage to the load 9 is 5 V and the oscillation frequency of the oscillator 3d is 50 kHz, the allowable value of the maximum ripple voltage detected by the ripple voltage detecting means 4 is 0.4% or less of the output voltage. In this case, the resistances of the resistors 42, 43, 44, and 45 are 30 kΩ, 10 kΩ, 99.6 kΩ, and 400 Ω, respectively, and the capacitance of the capacitor 46 is 3 μF. At this time, the voltage value of the reference value E1 is 20 mV, and the voltage value of the reference value E2 is 5 mV.

[0075]

According to the switching power supply of the present invention, the magnitude of the ripple voltage appearing in the output DC voltage is detected by the ripple voltage detecting means, and the PWM signal is generated in accordance with the detected magnitude of the ripple voltage. The oscillation frequency of the oscillation signal used for switching is switched. Therefore, the switching power supply device is placed in a low-temperature environment or the like, and the ripple voltage that increases due to an increase in the equivalent series resistance value of the smoothing capacitor provided for smoothing the output DC voltage is increased by increasing the oscillation frequency. Can be reduced.
Further, when the switching power supply device is placed in a normal room temperature environment, the switching frequency due to the switching of the oscillation signal can be suppressed by lowering the oscillation frequency.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an internal configuration of a switching power supply device according to a first embodiment.

FIG. 2 is a time chart illustrating the operation of each unit of the switching power supply device.

FIG. 3 is a time chart illustrating an operation of each unit of the switching power supply device.

FIG. 4 is a block diagram showing an internal configuration of a switching power supply device according to a second embodiment.

FIG. 5 is a block circuit diagram showing an internal configuration of a ripple voltage detection unit, a comparator, and a synchronization unit.

FIG. 6 is a block circuit diagram showing the internal configuration of an oscillator and frequency switching means.

FIG. 7 is a block diagram showing an internal configuration of a switching power supply device according to a third embodiment.

FIG. 8 is a block circuit diagram showing an internal configuration of a reference value generation unit and an adjustment unit.

FIG. 9 is a block diagram showing an internal configuration of a switching power supply device according to a fourth embodiment.

FIG. 10 is a block circuit diagram showing an internal configuration of a reference value generation unit and an adjustment unit.

FIG. 11 is a block diagram showing the internal configuration of a conventional switching power supply device.

FIG. 12 is a time chart showing the operation of each part of the switching power supply device of FIG. 11;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Switching power supply apparatus 2 Output part 3 Control part 4 Ripple voltage detection means 5 Comparator 6 Reference value generation means 7 Frequency switching means 8 DC power supply 9 Load 10 Synchronization means 11 Adjustment means

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tomohiro Suzuki 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka Inside Sharp Corporation (72) Inventor Kenji Masui 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka (72) Inventor Koji Hisakawa 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Sharp Corporation (72) Inventor Tsutomu 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Sharp Corporation F Terms (reference) 5H730 AS01 BB13 BB57 DD02 DD32 FD01 FF02 FG05 FG07 FG25

Claims (8)

[Claims] 1. A switching element for converting an input DC voltage into a pulse voltage signal by a switching operation, a conversion means for converting a pulse voltage signal given from the switching element into an output DC voltage, and an output DC voltage from the conversion means. Pulse width modulation means for outputting a pulse width modulated PWM signal based on the pulse width modulation means, and ON / OFF of the switching element by a PWM signal given from the pulse width modulation means.
A switching power supply device that has a driving unit that performs OFF control, and a smoothing capacitor for smoothing an output DC voltage from the conversion unit, and that converts a voltage value of an input DC voltage and outputs it as an output DC voltage. A ripple voltage detecting means for detecting a magnitude of a ripple voltage appearing in an output DC voltage from the converting means, and generating the PWM signal in accordance with the magnitude of the ripple voltage detected by the ripple voltage detecting means. A switching power supply characterized by switching an oscillation frequency of an oscillation signal used for the switching. 2. A switching element for converting an input DC voltage into a pulse voltage signal by a switching operation, a conversion means for converting a pulse voltage signal given from the switching element into an output DC voltage, and an output DC voltage from the conversion means. Pulse width modulation means for outputting a pulse width modulated PWM signal based on the pulse width modulation means, and ON / OFF of the switching element by a PWM signal given from the pulse width modulation means.
A switching power supply device that has a driving unit that performs OFF control, and a smoothing capacitor for smoothing an output DC voltage from the conversion unit, and that converts a voltage value of an input DC voltage and outputs it as an output DC voltage. A ripple voltage detecting means for detecting a maximum value of a ripple voltage appearing in the output DC voltage from the converting means; a comparing means for comparing the maximum value of the ripple voltage detected by the ripple voltage detecting means with a reference value; Oscillation signal generation means for oscillating an oscillation signal in the pulse width modulation means, and oscillation frequency switching means for switching the oscillation frequency of the oscillation signal oscillated by the oscillation signal generation means according to the comparison result of the comparison means. A switching power supply device characterized by the above-mentioned. 3. The switching device according to claim 2, wherein said ripple voltage detecting means is constituted by a band-pass filter that passes a signal whose frequency is equal to the oscillation frequency of said oscillation signal. Power supply. 4. A synchronizing means for generating a synchronizing signal for synchronizing and comparing when the ripple voltage is maximum, wherein the comparing means operates in accordance with the synchronizing signal, and A first determination unit that determines whether the value is greater than a first reference value, and a first determination unit that operates in accordance with the synchronization signal and determines whether a maximum value of the ripple voltage is less than a second reference value. And when the maximum value of the ripple voltage is greater than the first reference value, the oscillation frequency switching means changes the oscillation frequency of the oscillation signal oscillated from the oscillation signal generation means. The oscillation frequency is switched from the first oscillation frequency to the second oscillation frequency, and when the maximum value of the ripple voltage is smaller than the second reference value, the oscillation frequency is switched by the oscillation frequency switching means. The switching power supply according to the oscillation frequency of the oscillation signal oscillated from the signal generating means from said second oscillation frequency to claim 2 or claim 3, characterized in that switching to the first oscillation frequency. 5. The switching power supply according to claim 4, wherein the synchronization unit generates the synchronization signal based on a PWM signal from the pulse width modulation unit. 6. The switching power supply according to claim 2, further comprising an adjustment unit that adjusts the reference value according to a voltage value of a DC voltage output from the conversion unit. apparatus. 7. A low-pass filter for obtaining a voltage signal obtained by removing a ripple voltage from an output DC voltage from the converting means, a first voltage dividing means for dividing the output DC voltage to a first reference value, Adjusting means comprising a second voltage dividing means for further dividing the first reference value to obtain a second reference value, wherein the first and the second are divided in accordance with a voltage value of an output DC voltage from the converting means. The switching power supply according to claim 4 or 5, wherein the two reference values are adjusted. 8. The oscillation signal generation means includes an oscillation signal generation capacitor for generating an oscillation signal by repeating charging and discharging, and the oscillation frequency switching circuit charges and discharges the oscillation signal generation capacitor. The switching power supply device according to claim 2, wherein an oscillation frequency of the oscillation signal is switched by switching a magnitude of a current flowing at that time.