JPH10271843A - Power device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To lower a required breakdown voltage and the stress to an element, by suppressing the increase of the terminal voltage of a filter capacitors at the time of a small load. SOLUTION: A series circuit of a switching means 1 and a second capacitor C5 is connected between one DC output end b1 of a full-wave rectifier DB and the connection point of the primary winding n1 of a transformer T1 with an alternating current power source Vs. The switching means 1 is composed of a second capacitor C5 , a diode D5 , and a switch SW. When the switch SW is on, a forward current flows through the switch SW, and a reverse current flows through the diode D5 (the reverse current is not blocked). Consequently, the terminal voltage Vc5 of the second capacitor C5 is kept approximately at a constant voltage, thereby keeping the first capacitor C4 at a constant voltage, if the switch SW is turned off at the time of a light load. Accordingly, a chopper action vanishes, and the increase of the terminal voltage Vdc of the filter capacitor C1 is suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply device for converting a DC power obtained by rectifying and smoothing an AC power into a high frequency and supplying the high frequency to a load.

[0002]

2. Description of the Related Art FIG. 7 is a schematic diagram showing a conventional example of the above power supply device.
It is a circuit diagram. In this conventional example, an AC power supply Vs and a diode
Full-wave rectifier DB comprising a bridge and a full-wave rectifier DB
AC input terminal a₁, A_TwoPrimary with AC power supply Vs in between
Winding n₁Is connected in series with a leakage transformer
Abbreviated as "trans". ) T₁And the transformer T₁Of the second volume
Line n_TwoAnd the load circuit 2 connected to the
Outflow end b₁, B_TwoSmoothing capacitor C connected between₁
And a pair of diodes D of the full-wave rectifier DB₁, D _TwoTo each
First and second switching elements connected in parallel
Q₁, Q_TwoAnd one DC output terminal b of the full-wave rectifier DB_Two
And AC power supply Vs and transformer T₁Primary winding n ₁Connection
A first capacitor C connected between the_FourWith
It supplies high-frequency power to the discharge lamp La as a load.
You. The transformer T₁Secondary winding n_TwoIs the discharge lamp La
Connected between one end of the filament and the other end of the filament
A capacitor C for resonance_TwoIs connected. Like this
Thus, this conventional example operates as follows.

First, the AC power supply Vs has a positive polarity (the direction of the arrow in the figure is the positive direction of the input voltage Vin), the switching element Q ₁ is on, and the switching element Q
_{When 2} is off, the AC power supply Vs → the diode D ₃ of the full-wave rectifier DB → the switching element Q ₁ → the transformer T ₁
The current flows through the path of the primary winding n ₁ → AC power supply Vs, and the current also flows through the load circuit 2 via the transformer T ₁ . At the same time smoothing current flows in the primary winding n ₁ → path of the capacitor C ₄ → smoothing capacitor C ₁ of the capacitor C ₁ smooth as the power supply capacitor C ₁ → switching element Q ₁ → transformer T _1, the load is also thereby A current flows through the circuit 2.

On the other hand, when the AC power supply Vs has a positive polarity,
Switching element Q₁Is off, switching element Q_TwoBut
The capacitor C_FourCharge is discharged,
Capacitor C_Four→ Transformer T₁Primary winding n₁→ Switch
Element Q_Two→ Capacitor C _FourCurrent flows in the path of
A current also flows through the load circuit 2. Therefore, the switching element
Q₁, Q_TwoThe load circuit is turned on and off alternately.
Road 2 has a transformer T₁High frequency voltage is applied via
High frequency current flows. That is, the AC power supply Vs has a positive polarity.
The switching element Q₁Is the chopper circuit
Switching element and switching element of inverter circuit
Will be shared.

When the AC power supply Vs has a negative polarity,
The roles of the switching elements Q ₁ and Q ₂ are switched, and the switching element Q ₂ is also used as each switching element of the chopper circuit and the inverter circuit. As described above, in this conventional example, the switching elements Q ₁ and Q ₂ are composed of a chopper circuit for improving input distortion and an inverter circuit for generating high frequency.
Also, there is an advantage that the number of circuit components is reduced and the circuit configuration is simple and inexpensive. The transformer T ₁ also plays a role of a low-pass filter, and the primary winding n ₁ has a commercial frequency (for example, 50 Hz) of the AC power supply Vs.
Hz or 60 Hz) flows, but the secondary winding n
₂ switching frequency components of the switching elements Q _1, Q ₂ (high frequency component) mainly flows into, thus only the high-frequency alternating current is supplied to a substantially stable to the load circuit 2.

FIG. 8 shows another conventional example, in which full-wave rectification is performed.
Capacitor C between DC output terminals of device DB_FourIs connected
Both are diode D_Five, D₆Are connected in anti-parallel.
Switching element Q₁, Q_TwoSmoothing capacitor in series circuit
C₁Are connected in parallel, and the transformer T₁Primary winding n
₁Through the switching element Q_TwoIs the capacitor C_FourAverage
It is configured to be connected to a column. In this conventional example,
Once T₁Primary winding n ₁The inductance component of
It also functions as the inductance of the
Element Q_TwoIs the switch of the chopper circuit and the inverter circuit
Also used as a switching element. This conventional example is
The difference from the example is that the transformer T₁Primary winding n₁To full wave
DC voltage after being rectified by the current transformer DB is applied,
One switching element Q_TwoOnly the operation of the chopper circuit
It is to contribute to.

[0007]

However, the above two problems
In the conventional example, as described above, the transformer T₁The first volume of
Line n₁Chopper operation by the inductance of
And smoothing capacitor C ₁Voltage V_dcIs the switch
Element Q₁, Q_TwoHas an on-duty ratio of about 50%
Sometimes the peak of the input voltage Vin (the power supply voltage of the AC power supply Vs)
Approximately twice the peak value. On the other hand, at light load (for example, negative
If the load is the discharge lamp La, when the discharge lamp La is preheated,
At the time of starting, etc.), the input power from the AC power supply Vs and the load circuit
The balance with the power consumption in 2 is lost, and the input power becomes excessive
The voltage V_dcIs further boosted. Moreover, the switch
Ching element Q₁, Q_TwoIs the above voltage V at the maximum._dc
It is necessary to select a device, and the withstand voltage of the device used is extremely high.
Disadvantages are large and expensive.
You.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems, and to provide a power supply device capable of suppressing a voltage increase between both ends of a smoothing capacitor under a light load, thereby reducing a withstand voltage and a stress of an element. Things.

[0009]

According to a first aspect of the present invention, there is provided an AC power supply, a full-wave rectifier comprising a diode bridge, and the AC power supply between an AC input terminal of the full-wave rectifier. A transformer having a primary winding connected in series, a load circuit connected to a secondary winding of the transformer, a smoothing capacitor connected between the DC output terminals of the full-wave rectifier, First and second switching elements respectively connected in parallel to a pair of diodes, and at least one of a DC output terminal of the full-wave rectifier and a connection point between the AC power supply and a primary winding of a transformer. And a second capacitor connected between the first direct current output terminal of the full-wave rectifier and the connection point between the AC power supply and the primary winding of the transformer. Switch hand Wherein the switch means is turned on at a light load and turned off at a steady state. If the switch means is turned on at a light load, the voltage across the second capacitor becomes substantially constant. Since the first capacitor is also maintained at a constant voltage, the chopper function is eliminated, and the boosting of the voltage across the smoothing capacitor is suppressed. Turning off the switch means that the second capacitor is substantially not connected, and the voltage across the smoothing capacitor is boosted by the boost chopper effect as in the conventional case. As a result, even at a light load, the voltage between both ends of the smoothing capacitor becomes almost the same as that at the time of a steady state, and the withstand voltage and stress of the element to be used can be reduced.

According to a second aspect of the present invention, in the first aspect, a diode is connected between at least one DC output terminal of the full-wave rectifier and a connection point between the AC power supply and a primary winding of the transformer. It is characterized by comprising. According to a third aspect of the present invention, there is provided a transformer having an AC power supply, a full-wave rectifier for full-wave rectifying the AC power, and a primary winding connected to a DC output terminal of the full-wave rectifier. A smoothing capacitor connected between the DC output terminals of the full-wave rectifier via a primary winding of the transformer, a load circuit connected to a secondary winding of the transformer, First and second switching elements connected in series to each other, a first capacitor connected between the DC output terminals of the full-wave rectifier, one end of the smoothing capacitor, and one DC output terminal of the full-wave rectifier. And a second capacitor connected between the transformer and a connection point of a primary winding of the transformer, and a series circuit of switch means that does not prevent reverse direction, wherein the switch means is turned on at a light load and turned off at a steady state. It consists of If the switch means is turned on at a light load, the voltage across the second capacitor is maintained at a substantially constant voltage, and thus the first capacitor is also maintained at a constant voltage. Is suppressed. Turning off the switch means that the second capacitor is substantially not connected, and the voltage across the smoothing capacitor is boosted by the boost chopper effect as in the conventional case. As a result, even at a light load, the voltage between both ends of the smoothing capacitor becomes almost the same as that at the time of a steady state, and the withstand voltage and stress of the element to be used can be reduced.

[0011] The invention of claim 4 is the invention of claim 1 or 2 or 3.
In the invention, the switching means is characterized in that a diode is connected in antiparallel to a forward switching element. According to a fifth aspect of the present invention, in the first, second or third aspect of the present invention, the switch means comprises a MOSFET, and the circuit configuration of the switch means can be simplified.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION

(Embodiment 1) FIG. 1 is a schematic circuit diagram showing Embodiment 1 of the present invention. Since the basic configuration is the same as that of the conventional example shown in FIG. 7, the same reference numerals are used for the common parts. The description will be omitted, and only the configuration that is a feature of the present embodiment will be described.

In this embodiment, one of the direct currents of the full-wave rectifier DB is used.
Outflow end b₁, AC power supply Vs and transformer T₁The first volume of
Line n₁Between the second capacitor C_FiveAnd Sui
Switch means 1 connected in series and the switch
The feature is that the stage 1 is turned on and it is turned off in the steady state.
is there. The switch means 1 includes a second capacitor C_FiveWhen
Diode D connected in series _FiveAnd this diode D_Five
And a switch SW provided in parallel.
When W is on, the forward direction (second capacitor C_Five
From the current flowing in the direction of the current toward the switch means 1)
Flows through the switch SW and in the opposite direction (switch
Switch 1 to the second capacitor C_FiveDirection)
Diode D for current_FiveFlow through (reverse
Does not block the flow). The switch SW is
For example, switching elements such as bipolar transistors
do it.

[0014] Load circuit 2 in this embodiment, a conventional example as well as the discharge lamp La as the load, as well as connecting the secondary winding n ₂ of the transformer T ₁ between one end of the filament of the discharge lamp La, the filament made between the other end to connect the capacitor C ₂ for resonance (preheat). However, the load circuit 2 is not intended to be limited to this, and it goes without saying that a load other than the discharge lamp La may be used. Further, the discharge lamp La as a load is not limited to one lamp, and may be a plurality of lamps.

Next, the operation of this embodiment will be described.
First, when the discharge lamp La is lightly loaded, such as when preheating or starting.
Turns on the switch SW of the switch means 1. This state
And the switching element Q₁Is on, switching element Q_Two
Is turned off, the second capacitor C_FiveCharge is discharged
Then, the second capacitor C_Five→ Switching element Q₁→
Transformer T₁Primary winding n₁→ Diode D_Five→ Second
Capacitor C_FiveCurrent flows in the path of₁2
Next winding n _TwoA current also flows through the load circuit 2 via the. here
And the second capacitor C_FiveCharge is discharged
Is the second capacitor C_FiveIs the switching element Q
₁Voltage Vc during the ON period of_FiveBecomes almost constant
These values are selected.

On the other hand, when the switching element Q ₁ is turned off and the switching element Q ₂ is turned on, the smoothing capacitor C ₁ → the second capacitor C _{5 using the} smoothing capacitor C ₁ as a power supply.
→ switch SW → transformer primary winding n of T _{1 1} → current flows through a path of the switching element Q ₂ → smoothing capacitor C _1, the current in the secondary winding n ₂ load circuit 2 via a transformer T ₁ Flows. Although at this time is charged in the second capacitor C ₅ is the current, the capacitance of the second capacitor C ₅ is are selected to a relatively large value, the voltage across Vc ₅ becomes substantially constant.

By alternately turning on and off the switching elements Q ₁ and Q ₂ in this manner, the primary winding n of the transformer T _1
1 is flushed with high-frequency current can be supplied to the high-frequency current to the load circuit 2 through the primary winding n ₁ and magnetically coupled secondary winding n _2. During this time, the voltage Vc ₅ across the second capacitor C ₅ is maintained at a substantially constant voltage as described above, and if the ON times of the switching elements Q ₁ and Q ₂ are substantially equal,
The voltage Vc ₅ is about half the across voltage V _dc of the smoothing capacitor _{C 1 (Vc 5 ≒ V dc} / 2).

Accordingly, by making the capacity of the second capacitor C ₅ sufficiently larger than the capacity of the first capacitor C ₄ , the second capacitor C ₅ is turned on when the switch means 1 is in the ON state.
About half of the voltage of ₅ across the voltage V _dc across the smoothing capacitor C ₁ of is generated, first and second capacitors C _4, C ₅
Since the voltage across V _dc of the connected and a smoothing capacitor C ₁ in parallel with the smoothing capacitor C ₁ is fixed, the voltage across the first capacitor C ₄ is also fixed to the substantially V _dc / 2. The voltage V _dc across the smoothing capacitor C ₁ is
Since is maintained substantially twice the power supply voltage (input voltage Vin) peak value of s, and across the second capacitor C ₅ voltage of the power source voltage peak value is generated, the diode constituting the full-wave rectifier DB Both D ₃ and D ₄ do not turn on. Therefore, regardless of the on / off state of the switching elements Q ₁ and Q ₂ , the chopper effect of the primary winding n ₁ of the transformer T ₁ is eliminated, and the voltage V _dc across the smoothing capacitor C ₁ is boosted at light load. There is no one. The input current from the AC power source Vs flows only small sections of the case both ends voltage V _dc of the smoothing capacitor C ₁ is lowered.

On the other hand, in a steady state (for example, in a state where the discharge lamp La is steadily lit), by turning off the switch SW, the current to the second capacitor C ₅ is changed by the diode D ₅ of the switch means 1. to be blocked, the second capacitor C ₅ are the same operation as if they are not effectively connected. Therefore, as described in the conventional example, the boost chopper operation is performed by turning on / off the switching elements Q ₁ and Q ₂ , and the input distortion is improved.

According to the present embodiment as described above, the second between one connection point of the primary winding n ₁ of the DC output ends b ₁ and the AC power source Vs and the transformer T ₁ of the full-wave rectifier DB switch means 1 without blocking the current of the capacitor C ₅ and reverse
A series circuit connected in, since the off steady with turning on the switch SW of the switching means 1 at light loads, when turning on the switch SW at light loads, the voltage across Vc ₅ of the second capacitor C ₅ It is maintained at almost constant voltage,
Thus since it is maintained in the first capacitor C ₄ is also a constant voltage, the boosted voltage across V _dc of the smoothing capacitor C ₁ is suppressed lost chopper action. The second capacitor C ₅ When turning off the switch SW same thing will not been substantially connected across voltage V _dc of the smoothing capacitor C ₁ is boosted by step-up chopper acts conventionally. It Consequently, the voltage across V _dc of can be a smoothing capacitor C ₁ reduces the boost effect at light load is to the same extent as the steady state, it is not necessary to increase the breakdown voltage of the device, also used small, inexpensive components There is an advantage that can be.

Note that, as shown in FIG.
One AC input terminal a₁And the switching element Q_TwoConnect to
One of the DC output terminals b_TwoBetween the first capacitor
C_FourMay be connected. The operation in this case is
Similarly, when the switch SW is turned on, light load
Stops the chopper action at ₁
Voltage V_dcCan be prevented from being boosted.

(Embodiment 2) FIG. 3 shows Embodiment 2 of the present invention.
FIG. In the present embodiment, the capacity of the first capacitor C ₄ ′ is set to a half of the circuit configuration of the first embodiment, and the capacitor C ₃ having the same capacity as that of the first capacitor is used as the first capacitor C ₄ ′. It is connected to the smoothing capacitor C ₁ in series with C ₄ ′ and in parallel with the series circuit of the second capacitor C ₅ and the switch means 1. The operation when the switch SW of the switch unit 1 is off is the same as that of the first embodiment, and therefore the description is omitted.

On the other hand, if turning on the switch SW when the load is light, to both ends of the smoothing capacitor C _1, the parallel circuit of the second capacitor C ₅ and the capacitor C ₃ and the first capacitor C
₄ 'will be connected in series. Here, since the combined capacitance of the parallel circuit becomes sufficiently larger than the capacitance of the first capacitor C ₄ ′, the voltage across the second capacitor C ₅ and the voltage across the capacitor C ₃ are both the voltage V _dc across the smoothing capacitor C _1. Fixed to about half of As a result, Embodiment 1
And is stopped chopper acts similarly smoothing capacitor C ₁
End voltage V _dc of is boosted at light loads is eliminated.

(Embodiment 3) FIG. 4 shows Embodiment 3 of the present invention.
FIG. 8 is a schematic circuit diagram showing the basic configuration shown in FIG.
The configuration is almost the same as that of the conventional example
Are denoted by the same reference numerals, and description thereof is omitted. This embodiment
Is the second circuit in the circuit configuration of the conventional example shown in FIG.
Capacitor C _FiveAnd switching element consisting of MOSFET
Qa series circuit is connected to smoothing capacitor C ₁And full-wave rectifier DB
Connected between the DC output terminals of the
The feature is that the switching element Qa is turned on and turned off during normal operation.
You. That is, a parasitic diode is used as the switch means 1.
(Not shown) MOSFET (switching element Q
By using a), separate as in Embodiments 1 and 2
Diode D_FiveThere is no need to provide
The circuit configuration can be simplified.

Also in this embodiment, the switching element
Q₁, Q_TwoThe transformer T₁
Primary winding n₁High-frequency current to the primary winding n₁And magnetism
Air-coupled secondary winding n_TwoTo the load circuit 2 via
A current can be supplied. Here, when the load is light,
When the switching element Qa is turned on, the second capacitor C _Five
Voltage Vc_FiveIs maintained at a substantially constant voltage as in the first embodiment.
Switching element Q₁, Q_TwoEach time
If the intervals are almost equal, the voltage Vc_FiveIs a smoothing capacitor
C₁Voltage V_dcAbout half of

Therefore, by making the capacity of the second capacitor C ₅ sufficiently larger than the capacity of the first capacitor C ₄ , both ends of the second capacitor C ₅ are kept in the ON state of the switch means 1 (switching element Qa). , A voltage of about half of the voltage _Vdc is generated, and the first and second capacitors C ₄ ,
Since the C ₅ across voltage V _dc of the smoothing capacitor C ₁ and is connected in parallel and the smoothing capacitor C ₁ is fixed, the voltage across the first capacitor C ₄ is also fixed to the substantially V _dc / 2. Approximately 2 across voltage V _dc of the smoothing capacitor C ₁ is the power supply voltage (input voltage Vin) peak value of the AC power source Vs
Since maintained doubled, at both ends of the second capacitor C ₅ voltage of the power source voltage peak value is generated. Therefore, irrespective of ON / OFF of the switching elements Q ₁ and Q ₂ , the chopper effect by the primary winding n ₁ of the transformer T ₁ is eliminated,
It never across voltage V _dc of the smoothing capacitor C ₁ is boosted at light loads.

On the other hand, in a steady state, the switching element Qa
By turning off, the second capacitor C_FiveIs a switch
Current is interrupted by the parasitic diode of the switching element Qa.
And the second capacitor C_FiveIs the same as when
Operation is performed, and the boost chopper operation is
The input distortion is further improved. Note that, as shown in FIG.
Capacitor C_FourAnd transformer T₁Primary winding n ₁The
Switching element Q₁And the second connector
Capacitor C_FiveAnd switching element Qa
Element Q_TwoCircuit may be connected to the
The circuit operation and effect in this case are the same as in the above case.

Further, with use of the conventional transformer rather than the leakage transformer on the transformer T ₁ as shown in FIG. 6, or as a circuit configuration in which an inductance L ₁ on the primary winding n ₁ series of transformer T _1. In this case except that the that the inductance L ₁ contributes to chopper action, operation and effects is quite common.

[0029]

According to the first aspect of the present invention, there is provided a full-wave rectifier comprising an AC power supply, a diode bridge, and a transformer having a primary winding connected in series with the AC power supply between the AC input terminals of the full-wave rectifier. A load circuit connected to the secondary winding of the transformer, a smoothing capacitor connected between the DC output terminals of the full-wave rectifier, and first and second parallel-connected diodes of the full-wave rectifier. A second switching element, a first capacitor connected between at least one DC output terminal of the full-wave rectifier and a connection point of the AC power supply and a primary winding of the transformer, A second capacitor connected between the other DC output terminal and a connection point between the AC power supply and a primary winding of the transformer, and a series circuit of switch means for preventing current in a reverse direction; Is Since it is turned on at the time of load and turned off at the time of steady state, if the switch means is turned on at the time of light load, the voltage across the second capacitor is maintained at a substantially constant voltage, and thus the first capacitor is also maintained at a constant voltage. Therefore, the chopper effect is eliminated, and the boosting of the voltage between both ends of the smoothing capacitor is suppressed. If the switch means is turned off, the second capacitor is substantially the same as being not connected. Since the voltage across the smoothing capacitor is boosted, the voltage across the smoothing capacitor becomes almost the same as in a steady state even under a light load, and there is an effect that the withstand voltage and stress of the element used can be reduced.

According to a third aspect of the present invention, there is provided an AC power supply, a full-wave rectifier for full-wave rectifying the AC power, a transformer having a primary winding connected to a DC output terminal of the full-wave rectifier, A smoothing capacitor connected between the DC output terminals of the full-wave rectifier via a primary winding, a load circuit connected to the secondary winding of the transformer, and a series connection between both ends of the smoothing capacitor First and second switching elements, a first capacitor connected between the DC output terminals of the full-wave rectifier, one end of the smoothing capacitor, one of the DC output terminals of the full-wave rectifier, and one of the transformers. A second capacitor connected between the connection point of the next winding and a series circuit of switch means that does not prevent reverse direction, wherein the switch means is turned on at a light load and turned off at a steady state, At light load When the switch is turned on, the voltage across the second capacitor is maintained at a substantially constant voltage, and thus the first capacitor is also maintained at a constant voltage. When the switch means is turned off, the second capacitor is substantially not connected, and the voltage across the smoothing capacitor is boosted by the boost chopper function as in the conventional case. The voltage between both ends of the smoothing capacitor becomes almost the same as in the steady state, and there is an effect that the withstand voltage and stress of the element to be used can be reduced.

According to a fifth aspect of the present invention, the switch means is M
Because of the OSFET, there is an effect that the circuit configuration of the switch means can be simplified.

[Brief description of the drawings]

FIG. 1 is a schematic circuit diagram showing a first embodiment.

FIG. 2 is a schematic circuit diagram showing another configuration of the above.

FIG. 3 is a schematic circuit diagram showing a second embodiment.

FIG. 4 is a schematic circuit diagram showing a third embodiment.

FIG. 5 is a schematic circuit diagram showing another configuration of the above.

FIG. 6 is a schematic circuit diagram showing another configuration of the above.

FIG. 7 is a schematic circuit diagram showing a conventional example.

FIG. 8 is a schematic circuit diagram showing another conventional example.

[Explanation of symbols]

1 switch means second load circuit Vs AC source DB full-wave rectifier SW switches C ₁ smoothing capacitor C ₄ first capacitor C ₅ second capacitor T ₁ trans Q _1, Q ₂ switching elements

Claims (5)

[Claims] An AC power supply, a full-wave rectifier comprising a diode bridge, and a transformer having a primary winding connected in series with the AC power supply between AC input terminals of the full-wave rectifier;
A load circuit connected to the secondary winding of the transformer, a smoothing capacitor connected between the DC output terminals of the full-wave rectifier, and first and second parallel-connected diodes respectively connected to a pair of diodes of the full-wave rectifier; 2, a first capacitor connected between at least one DC output terminal of the full-wave rectifier and a connection point between the AC power supply and a primary winding of a transformer, and the other of the full-wave rectifier And a series circuit of switch means that does not block reverse current and a second capacitor connected between a DC output terminal of the AC power supply and a connection point of the primary winding of the AC power supply and the transformer. A power supply device which is turned on at a light load and turned off at a steady state. 2. The device according to claim 1, wherein a diode is connected between at least one DC output terminal of the full-wave rectifier and a connection point between the AC power supply and a primary winding of a transformer. Power supply. 3. An AC power supply, a full-wave rectifier for full-wave rectifying the AC power, a transformer having a primary winding connected to a DC output terminal of the full-wave rectifier, and a primary winding of the transformer. A smoothing capacitor connected between the DC output terminals of the full-wave rectifier, a load circuit connected to the secondary winding of the transformer, and first and second capacitors connected in series between both ends of the smoothing capacitor. A switching element, a first capacitor connected between the DC output terminals of the full-wave rectifier, one end of the smoothing capacitor, one DC output terminal of the full-wave rectifier, and a primary winding of the transformer. A power supply device comprising: a second capacitor connected between the switch and a point; and a series circuit of switch means that does not prevent reverse direction, wherein the switch means is turned on at a light load and turned off at a steady state. . 4. The power supply device according to claim 1, wherein the switching means is configured by connecting a diode in antiparallel to a forward switching element. 5. The power supply device according to claim 1, wherein said switch means comprises a MOSFET.