JPH1132480A - Switching type dc power supply device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a DC power supply device provided with an active filter which supplies a power from the auxiliary winding of the high frequency transformer of a DC-DC converter in a back stage to the control system of an active filter to stabilize a power supply voltage regardless of a load fluctuation and, further, eliminates the need for the auxlairy winding of the choke coil of the active filter. SOLUTION: A DC power supply device is composed of a rectifier circuit DB1 by which an inputted commercial AC power supply is full-wave rectified; an active filter 7 which is connected to the rectifier circuit DB1, converts an AC input current waveform into a sine wave, controls an output voltage to be constant and has a rectifying and smoothing circuit (D1 and C1); and a DC-DC converter connected to the active filter 7. The driving power supply of the control circuit 9 of the active filter 7 is supplied from the auxiliary winding of the high frequency transformer T1 of the DC-DC converter.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a constant-voltage DC power supply for producing a stabilized DC voltage from a commercial AC power supply, and more particularly to a switching type DC using a step-up or step-down chopper for a rectifying and smoothing circuit. Power supply.

[0002]

2. Description of the Related Art A rectifier circuit of the capacitor input type, which is widely used today as a rectifier / smoothing circuit of a switching type DC power supply, allows an input current to flow only during a charging period of a smoothing capacitor, a so-called conduction angle period. At present, a pulse-like input current having a peak value several times the effective value of the input current flows, and the power factor remains at a low value of about 0.6. In recent years, harmonic noise generated at the time of reduction has become a problem.

[0003] The reason is that when a device having a power supply device adopting a rectifier circuit of a capacitor input type is connected to the same commercial AC power supply line because the pulse-like current waveform contains many harmonic components, the same occurs. As the number increases, the voltage waveform of the commercial AC power supply line is distorted, and as a result, malfunction of other devices is caused, and the peak of the voltage waveform of the commercial AC power supply due to peak current and line impedance. However, the current in the commercial AC power supply line further increases in order to supply constant power, resulting in a sharper pulse waveform.
This is due to the problem that the voltage drop is further increased to cause a vicious cycle.

In recent years, various methods have been proposed to solve these problems of power factor and harmonic noise. Among them, among them, active elements such as transistors and FETs are switched at a high speed to provide a filter effect. Improve
At the same time, active filters capable of realizing miniaturization are attracting attention.

FIG. 3 is a configuration diagram of a conventional switching type DC power supply. This power supply device is a switching type DC power supply device using a step-up chopper type active filter. An input side is connected to input terminals 1 and 2 of a commercial AC power supply, and a rectifier circuit for full-wave rectifying an AC input power supply voltage, for example,
A diode bridge DB1, a boost chopper type active filter circuit (hereinafter referred to as "active filter") 7 for smoothing the pulsating voltage output from the diode bridge DB1 and outputting a DC voltage;
DC-DC converter 8 connected to output terminals 3 and 4
The load is connected to the output terminals 5 and 6 of the DC-DC converter 8.

The active filter 7 includes a choke coil L1 connected to the positive (+) side output terminal of the diode bridge DB1, a switching element for switching a pulsating voltage full-wave rectified by the diode bridge DB1, for example, a field effect transistor ( The input voltage rectified by the rectifier diode D1 and the diode bridge DB1 for preventing the reverse flow of the energy stored in the smoothing electrolytic capacitor C1 and commutating the electromagnetic energy stored in the choke coil L1. (Ripple voltage) A sine wave detection resistor R1 for detecting a waveform, a current detection resistor R2 for detecting a current flowing through the circuit, a starting resistor R3, a smoothing electrolytic capacitor C1, and output terminals 3, 4
PWM control (pulse width modulation control) of a drive pulse (gate pulse) for driving the switching FET Q1 is performed so that the output voltage of the active filter 7 output during this time is constant and the AC input current waveform becomes a sine wave. Filter control circuit (hereinafter simply referred to as "control circuit") 9, an output voltage detecting circuit 10 for detecting an output voltage between output terminals 3 and 4, and a choke coil L1 for supplying power to control circuit 9. The power supply circuit includes an auxiliary winding, diodes D2 and D3 for rectifying and smoothing the output of the auxiliary winding, and a power supply circuit including electrolytic capacitors C2 and C3.

The DC-DC converter 8 is a switching FE for switching a DC voltage inputted from the high frequency transformer T 1 and the output terminals 3 and 4 of the active filter 7.
TQ2, rectifier diode D5 for rectifying and smoothing the output of high-frequency transformer T1, choke coil L2, commutation diode D6, electrolytic capacitor C4 for smoothing, and output voltage between output terminals 5 and 6 (output voltage of DC-DC converter 8). That is, a DC-DC converter control circuit (hereinafter simply referred to as a "control circuit") 11 for performing PWM control of a gate pulse of the switching FET Q2 so that the output voltage of the DC power supply becomes a constant voltage, and output terminals 5 and 6. Output voltage detection circuit 12 for detecting an output voltage between
A photocoupler 13 that electrically insulates and transmits the detected value to the control circuit 11, an auxiliary winding of the high-frequency transformer T1 for supplying power to the control circuit 11, and a diode that rectifies and smoothes the output of the auxiliary winding. D7 and a power supply circuit including an electrolytic capacitor C5.

In the figure, reference numeral G1 indicates the ground potential of the control system, and the same reference numeral indicates the same potential.

The commercial AC power supplied from the input terminals 1 and 2 is full-wave rectified by the diode bridge DB 1 to be a pulsating voltage and supplied to the active filter 7. The active filter 7 converts the supplied pulsating voltage to several tens of kHz.
The control circuit 9 switches the output voltage output between the output terminals 3 and 4 by switching over the entire cycle at the above frequency to a certain constant voltage, and so that the input current waveform of the switching DC power supply becomes sinusoidal. A switching FET that detects the output voltage, full-wave rectified pulsating voltage waveform, and switching current flowing through the circuit.
The pulse width of the drive pulse (gate pulse) supplied to the gate of Q1 is controlled.

While the switching FET Q1 is conducting (ON), a current flows through the switching FET Q1 via the choke coil L1, and electromagnetic energy is stored in the choke coil L1. Next, the switching FET Q
When 1 is cut off (off), the electromagnetic energy stored in the choke coil L1 charges the smoothing electrolytic capacitor C1 through the rectifier diode D1 and is supplied to the DC-DC converter 8 through the output terminal 3.

Regarding the power supply of the control circuit 9, at the time of startup,
When the power is supplied from the diode bridge DB1 via the starting resistor R1 and the on / off of the switching FET Q1 is started, the energy generated in the auxiliary winding of the choke coil L1 is rectified and smoothed by the diodes D2 and D3 and the electrolytic capacitors C2 and C3. It is used as a driving power source.

The control circuit 9 controls ON / OFF of the switching FET Q1 so that the AC input current waveform is proportional to the AC voltage waveform and the output voltage becomes a constant voltage. The control circuit 11 detects an output voltage between the output terminals 5 and 6 based on a signal from the output voltage detection circuit 12, and outputs a voltage between the output terminals 5 and 6 when a DC voltage is supplied to the DC-DC converter 8. The drive pulse (gate pulse) for driving the switching FET Q2 is PWM controlled so that the voltage becomes a certain constant voltage.

While the switching FET Q2 is conducting (ON), a current flows through the switching FET Q2 via the high-frequency transformer T1 and the high-frequency transformer T2
Is supplied to the smoothing electrolytic capacitor C4 and the loads connected to the output terminals 5 and 6 via the rectifier diode D5 and the choke coil L2.

The choke coil L2 stores electromagnetic energy when a current flows. Next, when the switching FET Q2 is cut off (turned off), the electromagnetic energy stored in the choke coil L2 charges the smoothing electrolytic capacitor C4 via the commutation diode D6 and supplies energy to the load. The driving power supply of the control circuit 11 is
It is supplied from the auxiliary winding of the high-frequency transformer T1. As a result, in the switching type DC power supply device, the voltage between the output terminals 3 and 4 of the active filter 7 and the output terminals 5 and 6 of the DC-DC converter 8 are always a constant voltage, and the AC input current waveform is a sine wave. Is controlled as follows.

As described above, the switching type DC power supply device using the active filter in the rectifier has the function of maintaining the output voltage constantly constant even when the input voltage fluctuates, as well as the input current and input current. Since the voltage waveform is detected and switching control is performed so that the input current waveform is proportional to the input voltage waveform, the input current is always a sine wave, improving the power factor, reducing harmonic noise, and using the conventional capacitor input. The rush current to the smoothing capacitor can be suppressed to about 1/10 to 1/5 as compared with the rectifier circuit of the type. Further, since the current ripple is reduced, the capacity of the smoothing capacitor can be reduced, and the size can be reduced.

If an overvoltage occurs between the output terminals 5 and 6 for a predetermined output voltage, the output voltage detection circuit 12 detects the overvoltage and generates an overvoltage detection signal. When detecting the overvoltage detection signal, the control circuit 11 stops the oscillation and switches the switching FET Q2 of the DC-DC converter 8.
By stopping the supply of the driving pulse to the DC-D
The load connected between the C converter 8 and the output terminals 5 and 6 is protected. If an overvoltage occurs between the output terminals 3 and 4 of the active filter 7 with respect to a predetermined active filter output voltage, the output voltage detection circuit 10 detects the overvoltage and generates an overvoltage detection signal. Control circuit 9
When the overvoltage detection signal is detected, the oscillation is stopped and the supply of the drive pulse to the switching FET Q1 is stopped, so that the active filter 7 itself, the DC-DC converter 8 connected to the output terminals 3 and 4, And the loads connected to the output terminals 5 and 6 are protected.

[0017]

In the above-described conventional switching type DC power supply, the power supply of the control circuit 9 of the active filter 7 is obtained by rectifying and smoothing the output of the auxiliary winding of the choke coil L1. However, when the current flowing through the choke coil L1 becomes discontinuous at light load, the output voltage value fluctuates, and it is necessary to provide an auxiliary winding on the choke coil L1. There is a problem with restrictions.

The present invention has been made in view of the above points, and in a switching type DC power supply device having an active filter, a power supply for a control system of the active filter is used as an auxiliary winding of a high-frequency transformer of a downstream DC-DC converter. It is intended to provide a switching type DC power supply that supplies a stable power supply voltage regardless of load fluctuations and does not require an auxiliary winding of a choke coil of an active filter.

[0019]

In order to achieve the above object, a switching type DC power supply according to a first aspect of the present invention is provided with a rectifier circuit for receiving a commercial AC power supply and performing full-wave rectification of the commercial AC power supply. A rectifying / smoothing circuit having an active filter connected to the rectifier circuit and having an AC input current waveform of a sine wave and controlling an output voltage to a constant value, and a DC-DC converter connected to the active filter. A drive power supply for the control circuit of the active filter is supplied from a high-frequency transformer of the DC-DC converter.

A switching DC power supply according to a second aspect is characterized in that the active filter according to the first aspect is a step-up type active filter.

According to a third aspect of the present invention, there is provided a switching type DC power supply, wherein the active filter in the first aspect is a step-down active filter.

According to a fourth aspect of the present invention, there is provided a switching type DC power supply device which receives a commercial AC power supply, and comprises two or more switching power supply units for one commercial AC power supply. At least one of the switching power supplies has a rectifying / smoothing circuit composed of an active filter, and supplies driving power for the active filter from a high-frequency transformer of a switching power supply of another system different from the active filter. It is characterized by the following.

A switching DC power supply according to a fifth aspect is characterized in that the active filter according to the fourth aspect is a step-up type active filter.

A switching DC power supply according to a sixth aspect is characterized in that the active filter in the fourth aspect is a step-down active filter.

[0025]

Embodiments of the present invention will be described below with reference to the drawings.

(Embodiment 1) FIG. 1 is a block diagram of Embodiment 1 of a switching type DC power supply according to the present invention. In FIG. 1, those indicated by the same reference numerals as those in FIG.
It is assumed that they are the same and have the same function, and the description is omitted here.

1 differs from the switching type DC power supply shown in FIG. 3 in that the auxiliary winding of the choke coil L1 of the active filter 7, the diodes D2 and D3 for rectifying and smoothing the output of this auxiliary winding, Capacitor C
2, C3, a starting resistor R3, and a backflow prevention diode D4 are reduced, and a power supply including a diode D7 for rectifying and smoothing an output of an auxiliary winding of a high-frequency transformer T1 of the DC-DC converter 8 and an electrolytic capacitor C5. The point is that the output side of the circuit is connected to the power input terminal of the control circuit 9 via the backflow prevention diode D8. That is, the power supply voltage Vcc of the control circuit 9 is supplied from the auxiliary winding side of the high-frequency transformer T1.

The operation of the circuit will be described below.

The operation in the steady state is the same as that of the conventional switching type DC power supply shown in FIG. When commercial AC power is supplied from the input terminals 1 and 2, the pulsating voltage that is full-wave rectified by the diode bridge DB 1 is supplied to the active filter 7. Active filter 7
, The control circuit 9 outputs a predetermined DC voltage between the output terminals 3 and 4 based on the detection values of the output voltage detection circuit 10, the sine wave detection resistor R1, and the current detection resistor R2, and outputs the AC input current PWM control is applied to the drive pulse supplied to the gate of the switching FET Q1 so that the waveform of the switching FET Q1 becomes a sine wave.
Switches the pulsating voltage.

When a predetermined DC voltage is output between output terminals 3 and 4, DC-DC converter 8 receives the output voltage as an input and outputs a predetermined DC voltage between output terminals 5 and 6. The control circuit 11 performs PWM control on a drive pulse supplied to the gate of the switching FET Q2. Thereby, the voltage between the output terminals 3, 4 of the active filter 7,
The voltage between the output terminals 5 and 6 of the DC-DC converter 8 is
It is controlled by two control systems of control circuits 9 and 11 so that the DC current is always controlled to a predetermined DC voltage, and the input current of the commercial AC power supplied from the input terminals 1 and 2 always becomes a sine wave.

Next, the operation at the time of startup and the method of supplying the power supply voltage Vcc to the control circuit 9 will be described.

When the commercial AC power is supplied to the input terminals 1 and 2, the smoothing electrolytic capacitor C1 is charged while the active filter 7 is stopped, and when the voltage between the output terminals 3 and 4 exceeds a certain threshold value, it is illustrated. Do not use DC-
Power is supplied to the control circuit 11 of the DC converter 8, and the control circuit 11 is activated. Then, when the switching FET Q2 starts to be turned on / off by the control circuit 11 (when the DC-DC converter 8 is started), power is supplied to the control circuit 11 from the auxiliary winding output of the high-frequency transformer T1 instead of the starting resistance. . At the same time, power is also supplied to the control circuit 9 of the active filter 7 via the backflow prevention diode D8, and the control circuit 9 is activated to start the boosting operation. That is, the circuit configuration is such that the active filter 7 is activated after the DC-DC converter 8 is activated.

Since the power supply Vcc of the control circuit 9 which is the control circuit of the active filter 7 is supplied from the auxiliary winding of the high-frequency transformer T1 of the DC-DC converter 8 connected at the subsequent stage, FIG.
The starting resistor R3 shown in FIG. 3 is unnecessary, and the auxiliary winding of the choke coil L1 and the rectifying and smoothing circuit of the output of the auxiliary winding are also unnecessary. Therefore, it is possible to use a general-purpose choke coil.

Further, since the voltages of the output terminals 3 and 4 of the active filter 7, that is, the input voltage of the DC-DC converter 8 are controlled to be always constant, they flow through the choke coil L1 at light load. Even when the current is discontinuous and the current value is minimal, a stable power supply Vcc can be supplied to the control circuits 9 and 11.

Further, since the active filter 7 rises while a certain load is applied, the overshoot voltage generated at the output (output terminals 3 and 4) of the active filter 7 at the time of starting can be reduced.

In the present embodiment, the embodiment using the boost chopper type active filter has been described. However, the present invention is not limited to this, and the same effect can be obtained in the step-down chopper type active filter. .

(Embodiment 2) FIG. 2 is a block diagram of a switching type DC power supply according to Embodiment 2 of the present invention. FIG. 2 shows an embodiment in which power is supplied from one system of AC input power to a DC-DC converter which is a multi-system switching power supply unit. For example, system 1 in FIG.
Is a high-power power system that outputs 24 V, 38 V, etc., and has a motor or the like as a load. An active filter 7 for improving the power factor is connected to the input of the power system. System 2
Is a relatively low-power signal system that outputs 3 V, 5 V, etc., and uses a digital circuit such as a controller of a device as a load.

In FIG. 2, the circuit configuration of the system 1 is the same as that shown in FIG.
Are substantially the same as those in the first embodiment, and the components denoted by the same reference numerals have the same functions and will not be described. The circuit configuration of the system 2 is substantially the same as the circuit configuration of a general capacitor input type switching type DC power supply device.

Referring to FIG. 2, the circuit configuration of the system 2 will be described.

The terminals 14 and 15 are output terminals of the switching type DC power supply of the system 2 to which a load is connected.
Diode bridge DB2 has input terminals 1, 2
To perform full-wave rectification of the AC power supply voltage. The smoothing electrolytic capacitor C7 is connected to the output side of the diode bridge DB2, and smoothes the full-wave rectified pulsating voltage. Both ends of the smoothing electrolytic capacitor C7 are connected to connection terminals 19 and 20.

The connection terminals 19 and 20 have DC-D
The C converter 8 'is connected. The DC-DC converter 8 'has the same configuration as the DC-DC converter 8, and the primary winding of the high-frequency transformer T2 is connected to connection terminals 19 and 20 via a switching FET Q3. A rectifier diode D10 for rectifying and smoothing the output of the high-frequency transformer T2, a choke coil L3, and a commutation diode D11 for commutating the energy stored in the choke coil L3 are provided on the secondary winding of the high-frequency transformer T2. And an output smoothing electrolytic capacitor C8.

On the output side of the DC-DC converter 8 ', a control circuit 16 for PWM-controlling the gate pulse of the switching FET Q3 so that the output voltage between the output terminals 14 and 15 becomes a constant voltage, and the output terminals 14 and 15 An output voltage between the DC-DC converters 8 ′ (switching type DC power supply) of the system 2, that is, an output voltage detection circuit 18 for detecting the output voltage of the system 2, and electrically detecting the output voltage of the output voltage detection circuit 18. It is constituted by a photocoupler 17 for transmitting to the control circuit 16.

The high-frequency transformer T2 includes a first auxiliary winding for supplying power to the control circuit 16, a diode D12 for rectifying and smoothing the output of the first auxiliary winding, and an electrolytic capacitor C9. A power supply circuit is configured.
Further, the high-frequency transformer T2 has a second auxiliary winding for supplying power to the control circuit 9 of the active filter 7 of the system 1, and a diode for rectifying and smoothing the output of the second auxiliary winding. A power supply circuit comprising D9 and electrolytic capacitor C6 is configured.

In the drawing, reference numerals G1 and G2 indicate the ground potentials of the control system, respectively, and those having the same reference numerals indicate the same potentials.

Next, the circuit operation of the system 2 will be described.

When a commercial AC power is supplied from the input terminals 1 and 2, the AC input voltage is changed to the diode bridge DB2,
Full-wave rectification and smoothing by the smoothing electrolytic capacitor C7,
It is supplied to the high frequency transformer T2. At the same time, the voltage Vcc is supplied to the control circuit 16 by a starting resistor (not shown), and the control circuit 16 is started.

Then, the switching FET Q3 is turned on /
When turning off is started (when the DC-DC converter 8 'is started), an output voltage is output between the output terminals 14 and 15, and control is performed from the output of the first auxiliary winding of the high-frequency transformer T2 instead of the starting resistor. The circuit 16 is supplied with the voltage Vcc. At the same time, the output of the second auxiliary winding of the high-frequency transformer T2 is rectified and smoothed by the diode D9 and the electrolytic capacitor C6, and supplied to the control circuit 9 of the active filter 7. As a result, the control circuit 9 is activated to switch FE
TQ1 is driven, the active filter 7 starts the boosting operation, and the terminal voltage between the output terminals 3 and 4 increases. Then, when this terminal voltage rises to a certain voltage, a voltage Vcc is supplied to the control circuit 11 by a starting resistor (not shown), and the control circuit 11 is started to output an output voltage between the output terminals 5 and 6. You.

That is, the circuit configuration is such that the DC-DC converter 8 ′ (switching power supply unit) of the system 2 is activated, the active filter 7 of the system 1 is activated, and then the DC-DC converter 8 is activated. . Since the power supply voltage Vcc of the control circuit 9 of the active filter 7 is supplied from the auxiliary winding of the high-frequency transformer T2 of the DC-DC converter 8 '(switching power supply) of another system (system 2), the active filter 7, the starting resistor R3 shown in FIG. 3 is unnecessary, and the choke coil L3
This eliminates the need for the auxiliary winding and the rectifying and smoothing circuit for the output of the auxiliary winding, and allows the use of a general-purpose choke coil.

Further, since the voltages at the terminals 3 and 4, which are the inputs of the DC-DC converter 8, are controlled to be always constant, the current flowing through the choke coil L1 becomes discontinuous at a light load, Further, a stable power supply Vcc can be supplied to the control circuit 9 even when the current value is minimal.

The active filter 7 of the system 1 is connected to the system 2
Since the power is obtained from the (other system) high-frequency transformer T2, the DC-DC converter 8 can be started after the output voltage of the active filter 7 between the output terminals 3 and 4 rises to some extent. Since the output voltage between the output terminals 3 and 4 is controlled to a constant voltage, the high-frequency transformer T
The rated input voltage of 1 can be limited to a narrow range, and the design of the high-frequency transformer T1 becomes easy, and the size can be reduced.

Although the present embodiment has been described with reference to an embodiment using a step-up chopper type active filter,
The present invention is not limited to this, and a similar effect can be obtained in a step-down chopper type active filter.

[0052]

As described above, according to the switching type DC power supply of the first aspect, the power of the control circuit of the active filter is supplied from the auxiliary winding of the high-frequency transformer of the DC-DC converter at the subsequent stage. In the case of an active filter, a starting resistor is not required, and an auxiliary winding of a choke coil and a rectifying and smoothing circuit for the output of the auxiliary winding are not required, so that a general-purpose choke coil can be used, and the range of component selection can be increased. Spread and cost reduction are possible.

Further, even when the current flowing through the choke coil of the active filter becomes discontinuous at a light load and the current value is minimal, a stable power supply can be supplied to the control circuit.

Further, since the active filter rises while applying a certain load, the overshoot voltage generated at the output of the active filter at the time of starting can be reduced.

According to the switching type DC power supply device of the fourth aspect, two or more switching power supply units are provided for one system of commercial AC power supply, and the active filter obtains power from the high frequency transformer of another system. Therefore, the DC-DC converter at the subsequent stage can be started after the output voltage of the active filter has risen to some extent, and since the output of the active filter is controlled to a constant voltage, the rated input voltage range of the high-frequency transformer Can be limited to a narrow range, the design of the high-frequency transformer becomes easy, and the size can be reduced.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a switching DC power supply device according to a first embodiment of the present invention.

FIG. 2 is a configuration diagram of a switching DC power supply device according to a second embodiment of the present invention.

FIG. 3 is a configuration diagram of a conventional switching type DC power supply device.

[Explanation of symbols]

1, 2 input terminal (switching type DC power supply input terminal) 3, 4 active filter output terminal 5, 6, 14, 15 DC-DC converter output terminal (switching type DC power supply output terminal) 7 active filter 8, 8 ' DC-DC converter 9 Active filter control circuit 10, 12, 18 Output voltage detection circuit 11, 16 DC-DC converter control circuit 13, 17 Photocoupler

Continued on the front page (51) Int.Cl. ⁶ Identification code FI H02M 7/217 H02M 7/217

Claims (6)

[Claims] 1. A rectifier circuit that receives a commercial AC power supply as input and rectifies the commercial AC power supply in full-wave, and an active filter that is connected to the rectifier circuit and controls an AC input current waveform to a sine wave and controls an output voltage to a constant value. And a DC-DC converter connected to the active filter, and a drive power supply for a control circuit of the active filter is supplied from a high-frequency transformer of the DC-DC converter. Type DC power supply. 2. The switching DC power supply according to claim 1, wherein the active filter is a step-up type active filter. 3. The switching DC power supply according to claim 1, wherein the active filter is a step-down active filter. 4. A commercial AC power supply is input, and one or more commercial AC power supplies are composed of two or more multi-system switching power supply units, and at least one of the multi-system switching power supply units has at least one system. A rectifying / smoothing circuit comprising an active filter, wherein a driving power for the active filter is supplied from a high-frequency transformer of a switching power supply of another system different from the system of the active filter; . 5. The switching DC power supply according to claim 4, wherein the active filter is a step-up type active filter. 6. The switching DC power supply according to claim 4, wherein the active filter is a step-down active filter.