JP3402355B2 - Switching power supply - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply device such as a DC-DC converter.

[0002]

2. Description of the Related Art As shown in FIG. 1, a switching power supply device including a DC-DC converter has an input filter (noise removal filter) 2 and an input rectification smoothing circuit 3 at power supply terminals 1a and 1b to which a commercial AC power supply 1 is connected. , DC-DC converter 4 are sequentially connected.
The input filter 2 is for removing high frequency components,
It is composed of a capacitor and a choke coil. DC-
The DC converter 4 turns on / off the DC voltage obtained from the input rectifying / smoothing circuit 3 with a switching element to intermittently supply a current to the primary winding of the output transformer, thereby supplying the voltage of the secondary winding of the output transformer. The load 6 is rectified and smoothed, and the load 6 is driven by the DC voltage of the output terminals 5a and 5b.

[0003]

By the way, DC-DC
In the case where the converter 4 is of a half bridge type using resonance as disclosed in, for example, Japanese Patent Laid-Open No. 8-66025, the core 7 has a primary winding N1 and first and second windings as shown in FIG. The output transformer 8 in which the secondary windings N2a and N2b are wound is used. A resonance inductance L1 and a resonance capacitor C1 are connected in series to the primary winding N1.
An output rectifying / smoothing circuit 12 including diodes 9 and 10 and a smoothing capacitor 11 is connected to the secondary windings N2a and N2b, and a load 6 is connected between the output terminals 5a and 5b.
In this type of circuit, the primary winding N1 of the output transformer 8
Between the core and the core 7, stray capacitance Ca, the core 7 and the secondary winding N2
There are stray capacitances Cb and Cc between a and N2b, and stray capacitance Cd between the primary winding N1 and the secondary windings N2a and N2b.
Therefore, the potential of the core 7 may change according to the change of the potential of the primary winding N1, and common mode noise may occur. The common mode noise can be known by measuring the voltage Vcn between the common output terminal 5b on the secondary side and the ground, and has a waveform as shown in FIG. Note that common mode noise on the secondary common line (secondary ground) adversely affects the electrostatic detection type device such as a ground pointer or a touch panel used especially in a notebook computer.

Therefore, an object of the present invention is to provide a switching power supply device capable of suppressing common mode noise.

[0005]

SUMMARY OF THE INVENTION To achieve the above object, the present invention provides an input rectifying / smoothing circuit connected to an AC power source, and a DC voltage obtained from the input rectifying / smoothing circuit. One or a plurality of switches, a primary winding of an output transformer which is connected to the input rectifying / smoothing circuit via the switch and is wound around a core, and
A secondary winding electromagnetically coupled to the secondary winding through the core,
An output rectifying / smoothing circuit connected to the secondary winding, and one or the other output terminal of the input rectifying / smoothing circuit for the core,
Or those related to the switching power supply device and means for electrically connecting via a <br/> capacitor to one or the other of the output terminal of the output rectifying and smoothing circuit. Note that, as shown in claim 2, one or the other of the output terminal of the one or the other output terminal, or the output rectifying and smoothing circuit of the input rectifying and smoothing circuit output transformer core of resonant switching power supply device via a capacitor it can be connected to. The input stage of the noise removing filter for Cho of the switching power supply as shown in claim 3 - one or the other output terminal of the input rectifying and smoothing circuit cores choke coil, or one or the other of the output terminal of the output rectifying and smoothing circuit it can be connected via a capacitor to. Further, as described in claim 4, the output smoothing choke coil core of the switching power supply device is connected to one or the other output terminal of the input rectifying / smoothing circuit, or one or the other output terminal of the output rectifying / smoothing circuit via a capacitor. Can be connected.

[0006]

According to the invention of each claim, the following effects can be obtained.
Be done. (A) Common mode noise is suppressed because the potential of the core of the transformer or choke coil is stabilized. (B) Capacitor ensures core insulation
Be done.

[0007]

Embodiments and Examples Next, embodiments and examples of the present invention will be described with reference to FIGS. However, in FIGS. 4 to 8, substantially the same parts as those in FIGS. 1 and 2 are designated by the same reference numerals, and the description thereof will be omitted.

[0008]

First Embodiment In the resonance type switching power supply device of the first embodiment shown in FIG. 4, an input rectifying / smoothing circuit 3 is connected to a commercial AC power supply 1 via an input filter (noise filter) 2 as in the case of FIG. Are connected. The input filter 2 and the input rectifying / smoothing circuit 3 are configured the same as those shown by the same reference numerals in FIG. A load 6 is connected to the output stage of the input rectifying / smoothing circuit 3 via a resonance type DC-DC converter 4a. The DC-DC converter 4a includes one output terminal 3 of the input rectifying / smoothing circuit 3 as a DC power supply.
a series circuit of insulated gate field effect transistors (hereinafter simply referred to as transistors) Q1 and Q2 as first and second switches connected between a and the other output terminal 3b, and an output transformer 8, Resonance inductance L connected in series with the primary winding N1 of the output transformer 8.
1 and the resonance capacitor C1, and the second transistor Q
2 and an auxiliary capacitor C2 connected in parallel, first and second clamping diodes D1 and D2 connected in reverse parallel to the first and second transistors Q1 and Q2, and first and second transistors 2 of the output circuit 8 and the control circuit 13 connected to the gates (control terminals) of Q1 and Q2
An output rectifying / smoothing circuit 12 including secondary windings N2a and N2b, diodes 9 and 10 connected to the secondary windings N2a and N2b, and a smoothing capacitor 11, an error amplifier 14, a reference voltage source 15, and And a current detector 16. Further, according to the present invention, the core (magnetic core) 7 around which the primary winding N1 and the secondary windings N2a and N2b are wound is connected to the other output terminal of the input rectifying / smoothing circuit 3 by the conductor 18 through the capacitor 17. Ground terminal) 3b. The transformer 8 has an exciting inductance and a leakage inductance as is well known. Also, the first and second transistors Q1,
Since Q2 has a structure in which the source is connected to the substrate (bulk), the first and second diodes D1 and D
2 is built in the transistors Q1 and Q2.

The series circuit of the primary winding N1, the inductance L1 and the capacitor C1 is connected in parallel to the second transistor Q2. Both ends of secondary windings N2a and N2b, which are electromagnetically coupled to the primary winding N1 of the transformer 8 via the core 7, are connected to one end of a capacitor 11 via diodes 9 and 10, and a center tap of the capacitor 11 is connected. It is connected to the other end. One input terminal of the error amplifier 14 is connected to the output terminal 5a, and the other input terminal is connected to the reference voltage source 15
The error output line 19 is connected to the control circuit 13. The error amplifier 14 outputs a voltage corresponding to the difference between the output detection voltage and the reference voltage. The current detector 16 consisting of a current transformer is connected to the control circuit 13 by a line 20.

The control circuit 13, as shown in FIG.
It comprises a (voltage controlled oscillator) 21, a waveform shaping circuit 22, a current detecting rectifying and smoothing circuit 23, a comparator 24, an overcurrent detecting reference voltage source 25, and diodes Da and Db.

The VCO 21 is connected to the output line 19 of the error amplifier 14 of FIG. 3 via the diode Da and is also connected to the diode Db via line 26 to the comparator 24. The output frequency of the VCO 21 becomes high when the detection voltage of the output terminal 5a is higher than the reference value, becomes low when the output detection voltage is lower than the reference value, and becomes high when the overcurrent occurs. The output of the VCO 21 is shaped into a square wave by the waveform shaping circuit 22, and the waveforms shown in FIGS.
Are converted into two control signals having mutually opposite phases. The control signal of FIG. 6A is sent to the gate (control terminal) of the transistor Q1 on line 27, and the control signal of FIG. 6B is sent to the gate (control terminal) of the second transistor Q2 on line 28. . The two control signals of FIGS. 6A and 6B are alternately generated with a slight dead time.
Further, the pulse width of the control signal in FIGS. 5 (A) and 5 (B) is C1 L
It is set longer than the half wave of the resonance waveform of 1.

The rectifying / smoothing circuit 23 is composed of a diode 29 and a capacitor 30, and is connected to the output line 2 of the current detector 16.
The voltage of 0 is rectified and sent to one input terminal of the comparator 24. The comparator 24 compares the reference voltage indicating the overcurrent of the reference voltage source 25 connected to the other input terminal with the output voltage of the rectifying / smoothing circuit 23, and the output voltage of the rectifying / smoothing circuit 23 becomes equal to or higher than the reference voltage. At times, the diode Db is turned on.
That is, the output voltage of the comparator 24 at the time of overcurrent is set to be higher than the output voltage of the error amplifier 14, and the output voltage of the comparator 24 at the time of non-overcurrent is set at the error amplifier 1.
4 is set to be lower than the output voltage. As a result, the diodes Da and Db function as selection switches.

The first and second transistors Q1 of FIG.
The gate signal V shown in FIGS. 6A and 6B is applied to the gate of Q2.
g1 and Vg2 are applied to the first and second transistors Q1
, Q2 are alternately turned on. During the period from t0 to t1 in FIG. 6, the auxiliary capacitor C2 is charged by the circuit composed of the primary winding N1, the inductance L1, the resonance capacitor C1 and the auxiliary capacitor C2. When the charging of the auxiliary capacitor C2 is completed at time t1, the exciting inductance Lp
The current flowing through the series circuit of the primary winding N1 having the leakage inductance, the inductance L1 and the capacitor C1 is commutated to the diode D1, and the series circuit, the diode D1 and the smoothing capacitor of the input rectifying and smoothing circuit 3 are connected. Current flows in the closed circuit of. Therefore, the current I1 of the first switch circuit composed of the first transistor Q1 and the diode D1 becomes a reverse current at t1 to t2 as shown in FIG. 6 (E).

When the reverse current becomes zero at time t2, a closed circuit composed of the smoothing capacitor of the input rectifying and smoothing circuit 3, the first transistor Q1, the resonance capacitor C1, the inductance L1 and the primary winding N1. At t2 in FIG. 6 (E)
A sinusoidal series resonance current flows as shown at .about.t3. It should be noted that during the ON period of the first transistor Q1, C1
In addition to the high-frequency series resonance of L1, low-frequency resonance of the transformer 8 due to the exciting inductance Lp and the capacitor C1 shown by the broken line in FIG. 4 also occurs. Excitation inductance L
p is equivalently connected in parallel to the load of the primary winding N1. Note that the leakage inductance and the inductance L
When the exciting inductance Lp is sufficiently larger than 1, the resonance frequency based on Lp becomes extremely low, and the current flowing through this exciting inductance Lp has a slope and increases almost linearly. The current I1 flowing through the first transistor Q1 is a combination of the resonance current of C1 L1 and the current of the exciting inductance Lp. At t3 to t4 after the resonance current of C1 L1 becomes zero at t3 in FIG. 6, the current based on the exciting inductance Lp becomes the current I1 of the first switch circuit.

When the first transistor Q1 is turned off at t4, the auxiliary capacitor C2 and the series resonance capacitor C1 are connected.
The current Ic2 is shown in FIG. 6 in the closed circuit consisting of the primary winding N1 having the inductance L1 and the exciting inductance Lp.
(G) Flows as indicated by t4 to t5. That is, the current flowing in the exciting inductance Lp and the current is the auxiliary capacitor C
The current Ic2 of 2 is commutated, and the auxiliary capacitor C2 is discharged. As a result, the voltage of the auxiliary capacitor C2, that is, the drain-source voltage Vds2 of the second transistor Q2 gradually decreases as shown in FIG. 6 (D). On the other hand, the drain-source voltage Vds1 of the first transistor Q1 becomes a value obtained by subtracting the drain-source voltage Vds2 of the second transistor Q2 from the voltage of the power source 1, and therefore FIG.
It gradually increases as shown in (C), and zero-volt switching at turn-off is achieved. The gate signal Vg2 of the second transistor Q2 is t5 as shown in FIG.
When applied at this point, zero volt switching at this turn-on is achieved. Therefore, the switching loss is significantly reduced.

When the voltage of the auxiliary capacitor C2 becomes substantially zero at time t5, the reverse bias of the second diode D2 is released. As a result, the current of the exciting inductance Lp is commutated from the auxiliary capacitor C2 to the second diode D2, and the current of the period t5 to t6 in FIG. 6 (F) flows.
That is, during the period from t5 to t6, current flows in a closed circuit composed of the primary winding N1 having the exciting inductance Lp, the second diode D2, the series resonance capacitor C1 and the inductance L1. Further, during the ON period of the second transistor Q2 from t5 to t7, the series resonance capacitor C1 and the second
Transistor Q2, primary winding N1 and inductance L
A series resonant current flows in a closed circuit consisting of 1 and. At this time, the current I2 flowing in the second switch circuit composed of the second transistor Q2 and the diode D2 is t5 in FIG. 6 (F).
.About.t7, which is substantially the same as the current I1 of the first switch circuit shown in FIG.

When the second transistor Q2 is turned off at t7, the current I2 which has been flowing upward due to the exciting inductance Lp is commutated to the auxiliary capacitor Cb, and FIG.
(G) the current Ic2 from t7 to t8 flows, and the capacitor C2
Voltage, that is, the drain-source voltage Vds2 of the second transistor Q2 gradually increases as shown in FIG. 6 (D). On the other hand, since the drain-source voltage Vds1 of the first transistor Q1 is a value obtained by subtracting Vds2 from the voltage of the power supply 1, it gradually decreases as shown in FIG. 6 (C).
This achieves zero volt switching of the first and second transistors Q1 and Q2 and reduces switching loss.

When the drain-source voltage Vds1 of the first transistor Q1 becomes substantially zero at time t8, the reverse bias of the first diode D1 is released, and the current of the exciting inductance Lp from the auxiliary capacitor C2 to the first Of the primary winding N1 having the exciting inductance Lp, the series resonant inductance L1, the series resonant capacitor C1, the first diode D1 and the smoothing capacitor of the input rectifying and smoothing circuit 3, and a current flows through the closed circuit.

In the resonant switching power supply of FIG. 4, if the conductor 18 and the capacitor 17 according to the present invention are not provided, the first and second transistors Q1
, Q2 changes in potential of the primary winding N1 based on ON / OFF of a high repetition frequency, the potential of the core 7 also varies as described with reference to FIG. However, in the present embodiment, since the core 7 is connected to the other output terminal (common terminal) 3b of the rectifying / smoothing circuit 3 via the conductor 18 via the high frequency capacitor 17, the common mode noise is indicated by the dotted line in FIG. Suppressed as shown. That is,
The other output terminal 3b of the rectifying / smoothing circuit 3 acts as a potential stabilizing point, and the potential of the core 7 stabilizes in a high frequency region. In the high frequency region, the capacitor 17 has a low impedance and the core 7 is directly connected to the other output terminal 3b of the rectifying / smoothing circuit 3. Therefore, the capacitor 17 functions as a DC separation, and the insulation between the primary side and the secondary side of the transformer 8 is reliably achieved. Secondary winding N2
When the common mode noise on the output side of a and N2b is reduced, malfunction of the electrostatic detection type device such as a ground pointer or a touch panel used in a notebook computer or the like is suppressed.

Note that, in FIG. 4, instead of the connection by the capacitor 17, the capacitor 17a shown by the broken line in the core 7 is shown.
Can be connected to one output terminal 3a of the rectifying / smoothing circuit 3. Further, the core 7 can be connected to the output terminal of the output rectifying / smoothing circuit 12 via the capacitor 17b indicated by the broken line. Further, the core 7 can be connected to the ground via the capacitor 17c. Even with such a connection, the potential of the core 7 is stabilized and common mode noise can be suppressed. If the electrical insulation between the primary side and the secondary side does not matter so much, the capacitor 1
It is possible to omit 7, 17a, 17b, 17c and directly connect the core 7 to each potential stabilization point by a conductive wire.

[0021]

[Second Embodiment] Next, a switching power supply device according to a second embodiment will be described with reference to FIG. The input filter 2 of FIG. 7 has first and second choke coils Ln1 and Ln2 for reducing common mode noise. The first and second choke coils Ln1 and Ln2 are connected in series to an AC power supply line and wound around a common core 31. Therefore, when the normal mode currents flow, the magnetic fluxes cancel each other out and do not function as an inductance, but when the common mode currents flow, the magnetic fluxes do not cancel each other, so they function as an inductance and suppress the common mode current. To do. Capacitors Cn1, Cn2, Cn3, C
n4 is connected between the pair of power supply terminals 1a and 1b. The connection point of the capacitors Cn3 and Cn4 is connected to the ground.

The input rectifying / smoothing circuit 3 is composed of a bridge circuit of four diodes Da, Db, Dc and Dd and a smoothing capacitor Cf, and is connected to the power supply terminals 1a and 1b via the input filter 2.

The DC-DC converter 4b is a flyback type converter, and comprises an output transformer 32, a switch 33 composed of a field effect transistor, an output rectifying / smoothing circuit 12a, and a control circuit 34. Transformer 32
Is composed of a primary winding 35, a secondary winding 36, and a core 37, and the primary and secondary windings 35 and 36 are wound around the core 37. The series circuit of the primary winding 35 and the switch 33 is connected to the pair of output terminals 3a and 3b of the input rectifying and smoothing circuit 3. The output rectifying / smoothing circuit includes a diode 38 and a capacitor 39, and the diode 38 is a secondary winding 36.
Is connected in series between one end of the output terminal 5a and the output terminal 5a, and the capacitor 39 is connected between the output terminals 5a and 5b. The control circuit 34 is a well-known circuit that detects the voltage between the output terminals 5a and 5b and controls the control terminal (gate) of the switch 33 so as to keep the voltage constant.

In this embodiment, the cores 31 of the choke coils Ln1 and Ln2 of the input filter 2 are connected to the DC output terminal 5a on the secondary side by the conductor 41 via the capacitor 40. Since the DC output terminal 5a is the potential stabilizing point, the potential of the core 31 is stabilized, and the common mode noise reducing effect is obtained as in the first embodiment.

As shown by the broken line in FIG. 7, instead of the capacitor 40, the core 31 is connected to the secondary side output terminal 5b via the capacitor 40a, or the core 31 is connected via the capacitor 40b to the output terminal 5b. Secondary input rectifying / smoothing circuit 3
One of the output terminals 3a, or the core 31 is connected to the other output terminal 3b of the input rectifying and smoothing circuit 3 via the capacitor 40c, or the core 31 is grounded via the capacitor 40d. It can be connected to the terminal 1b, or the core 31 can be connected to the ground (earth) via the capacitor 40e. It is also possible to omit the capacitors 40 to 40e and directly connect the core 31 to each potential stabilizing point with a conductive wire. Further, the core 37 of the output transformer 32 can be connected to various potential stable points as in the first embodiment shown in FIG.

[0026]

[Third Embodiment] A switching power supply device of a third embodiment shown in FIG. 8 is a DC-D of the switching power supply device of FIG.
The configuration is similar to that of FIG. 7 except that a part of the C converter 4b is modified. DC-DC converter 4c of FIG.
Is a forward type converter having a transformer 32, a switch 33, and a control circuit 34 as in FIG. 7, and only the output rectifying / smoothing circuit 12b is modified. That is, the output rectifying / smoothing circuit 12b has the diode 38 and the capacitor 39 as in FIG. 7, and further has the choke coil 51 and the diode 53 wound around the core 52.
The choke coil 51 is connected in series between the diode 38 and the output terminal 5a. The diode 53 is connected in parallel with the capacitor 39 and the choke coil 51.

In accordance with the present invention, the choke coil 5 of FIG.
The core 52 of No. 1 is connected to one output terminal 3 a of the input rectifying / smoothing circuit 3 by a conductor 55 via a capacitor 54. Since one input terminal 3a of the input rectifying / smoothing circuit 3 is a potential stabilization point, the potential of the core 52 is stable and common mode noise can be suppressed.

In place of the connection by the capacitor 54 in FIG. 8, as shown by a broken line, the core 52 is connected to the other output terminal 3b of the input rectifying / smoothing circuit 3 via the capacitor 54a, or the core 52 is connected. Capacitor 54b
Can be connected to the output terminal 5a via the capacitor, or the core 52 can be connected to the output terminal 5b via the capacitor 54c. Even with this connection, the same effect as that of the capacitor 54 can be obtained. Also, the capacitors 54 to 54c
The core 52 can be directly connected to each potential stabilizing point by omitting the line.

[0029]

MODIFICATION The present invention is not limited to the above-mentioned embodiments, and the following modifications are possible. (1) The core of the choke coil included in the input filter 2 of FIGS. 4 and 8 can be connected to the potential stable point as in the case of FIG. 7. Further, the core of the output transformer 32 of FIG. 8 can be connected to the potential stable point as in the case of FIG. (2) In FIG. 4, an auxiliary capacitor can be connected in parallel with the first transistor Q1. (3) The leakage inductance of the transformer 8 shown in FIG. 4 can be used as an inductance for series resonance with the capacitor C1 and the series resonance inductance L1 can be omitted. Of course,
Leakage inductance of transformer 8 and inductance L1
And the inductance for series resonance with C1 can be used. (4) An inductance can be connected in parallel to the primary winding N1 or a series circuit of the primary winding N1 and the inductance L1, and this can be used in the same manner as the exciting inductance Lp. (5) The transistors Q1 and Q2 can be bipolar transistors. (6) Capacitor C1 instead of providing the current detector 16
It is possible to detect the load current by detecting the voltage across both terminals.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a conventional switching power supply device.

FIG. 2 is a diagram showing in detail a part of the DC-DC converter of FIG.

FIG. 3 is a waveform diagram showing the common mode noise of FIG.

FIG. 4 is a circuit diagram showing a switching power supply device according to a first embodiment.

5 is a block diagram showing the control circuit of FIG. 4 in detail.

6 is a waveform diagram showing a state of each part of FIG.

FIG. 7 is a circuit diagram showing a switching power supply device according to a second embodiment.

FIG. 8 is a circuit diagram showing a switching power supply device according to a third embodiment.

[Explanation of symbols]

7 core 8 output transformer 17 Core connection capacitor N1 primary winding N2a, N2b secondary winding C1 resonance capacitor Q1 and Q2 transistors

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-53-122717 (JP, A) JP-A-62-26912 (JP, A) JP-A-9-308243 (JP, A) JP-A-8- 37778 (JP, A) Actual development Sho-58-139725 (JP, U) Actual development 61-136625 (JP, U) (58) Fields investigated (Int.Cl. ⁷ , DB name) H02M 3/28 H01M 1 / 12 H03H 7/01

Claims (4)

(57) [Claims] 1. An input rectifying / smoothing circuit connected to an AC power source, one or a plurality of switches for turning on / off a DC voltage obtained from the input rectifying / smoothing circuit, and the switch via the switch. A primary winding of an output transformer connected to an input rectifying and smoothing circuit and wound around a core, a secondary winding electromagnetically coupled to the primary winding through the core, and the secondary winding an output rectifying and smoothing circuit connected to, the core of the input rectifying and smoothing circuit - via a capacitor to one or the other of the output <br/> pin of square or other output terminal or the output rectifying and smoothing circuit, A switching power supply device comprising means for electrically connecting. 2. An input rectifying / smoothing circuit connected to an AC power source, a series circuit of first and second switches connected between one end and the other end of the input rectifying / smoothing circuit, and a winding on a core. An output transformer having a turned primary winding and a secondary winding, an output rectifying / smoothing circuit connected to the secondary winding, and a resonance connected in parallel to the second switch. A series circuit of a capacitor, an inductance and the primary winding or a resonance capacitor and a leakage inductance 1
A series resonance circuit with a next winding, a control circuit for alternately turning on and off the first and second switches with a dead time, and an anti-parallel circuit for the first and second switches. First and second diodes connected, an auxiliary capacitor connected in parallel to the second switch, the core of the input rectifying and smoothing circuit-one or the other output terminal, or the output rectification and means for electrically connecting via a capacitor to one or the other of the output <br/> pin of the smoothing circuit resonant switching power supply device. 3. An input smoothing circuit connected to an AC power source via a choke coil, one or a plurality of switches for turning on / off a DC voltage obtained from the input rectifying / smoothing circuit, A primary winding of an output transformer connected to the input rectifying / smoothing circuit via a switch and wound around a core, and a secondary winding electromagnetically coupled to the primary winding via the core, An output rectifying / smoothing circuit connected to the secondary winding, and a core of the choke coil of the input rectifying / smoothing circuit-
Switching power supply and means for electrically connecting via a square or other output terminals, or to one or other of the output terminal of the output rectifying and smoothing circuit capacitor. 4. An input rectifying / smoothing circuit connected to an AC power source, one or a plurality of switches for turning on / off a DC voltage obtained from the input rectifying / smoothing circuit, and the switch via the switch. A primary winding of an output transformer connected to an input rectifying and smoothing circuit and wound around a core, a secondary winding electromagnetically coupled to the primary winding through the core, and the secondary winding An output rectifying / smoothing circuit connected to the switching power supply device, wherein the output rectifying / smoothing circuit includes a smoothing choke coil,
The core of the smoothing choke coil is electrically connected via a capacitor to one or the other output terminal of the input rectifying and smoothing circuit, or one or the other output terminal of the output rectifying and smoothing circuit. Switching power supply.