JP2000245169A - Power supply equipment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress increase in high-frequency component of the input current, when the output to a load circuit is lowered. SOLUTION: This equipment is provided with a rectifier DB, a diode D11 with its anode connected to a positive output terminal of the rectifier DB, a smoothing capacitor C10 connected between the cathode of the diode D11 and a negative output terminal of the rectifier DB, a diode D12 with its anode connected to the cathode of the diode D11, FETs Q1, Q2 serially connected between the cathode of the diode D12 and the negative output terminal of the rectifier DB, a control circuit 10 for controlling the on/off of the FETs Q1, Q2, a transformer T11 having a primary winding n11 connected between a connection between the FETs Q1 and Q2 and the positive output terminal of the rectifier DB and a secondary winding n12 connected to a load circuit 11, a capacitor C12 connected in parallel with the diode D12, and a capacitor C11x having a variable capacity connected in parallel to the diode D11. The capacity of the capacitor C11x is reduced with the reduction in the output.

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 performing rectification and smoothing of AC power into DC power, converting a DC voltage obtained by the rectification and smoothing into a high frequency voltage, and supplying a high frequency power to a load circuit. It is.

[0002]

2. Description of the Related Art Hitherto, various types of such power supply devices for supplying high-frequency power to a load circuit including a discharge lamp have been proposed and used.

Japanese Unexamined Patent Publication No. 10-14257 discloses that a second impedance element is connected between an output terminal of a rectifier and a contact point of a diode, and a series circuit including a coupling capacitor and an inductor. There is disclosed a power supply device in which a load circuit is connected to both ends of a second impedance via a power supply.

[0004]

FIG. 6 is a schematic block diagram of a power supply unit separately filed by the present applicant. This power supply unit includes a rectifier DB for full-wave rectifying AC power from an AC power supply AC to DC power. A diode D11 whose anode is connected to the positive output terminal of the rectifier DB in the forward direction; and a smoothing capacitor C10 connected between the cathode of the diode D11 and the negative output terminal of the rectifier DB.
A diode D12 whose anode is connected to the cathode of the diode D11 in the forward direction;
Q1 and Q2 connected in series between the cathode of the rectifier DB and the negative output terminal of the rectifier DB;
And a control circuit 10 for performing on / off control of the FET Q
A transformer (leakage transformer) T11 having a primary winding n11 connected between a connection point of the first and second transistors Q2 and a positive output terminal of the rectifier DB and having a secondary winding n12 connected to the load circuit 11; , Diodes D11, D1
2 and capacitors C11 and C12 respectively connected in parallel
It is composed of

However, each of the FETs Q1 and Q2 is, for example, a MOSFET, has a source / substrate connected, and has a structure in which a drain and a source have a parasitic diode in which a cathode and an anode are connected (reverse parallel connection), respectively. Has become. The control circuit 10
Turns on / off the FETs Q1 and Q2 alternately by operation at a switching frequency sufficiently higher than the frequency of the AC power supply AC. That is, the switching frequency is set to such an extent that the voltage of the AC power supply AC can be considered constant during one cycle. Further, the load circuit 11 includes a discharge lamp (fluorescent lamp) FL having a pair of filaments, one end of which is connected to both ends of the secondary winding n12, and a preheating / fluorescent lamp connected between the other ends of the pair of filaments. Resonant capacitor C11
1. Then, a resonance circuit is configured by the leakage inductance of the transformer T11 and the capacitor C111.

FIGS. 7 and 8 are explanatory diagrams of the operation of the power supply device shown in FIG. 6, and the operation will be described next with reference to these drawings.

In a steady state after charging of the capacitor C10, at time t12 in FIG.
Capacitor C11,1 winding n11, a current I _T11 flows in the path of the FETQ2 and capacitor C10, so capacitor C10 to the power supply via the transformer T11 with its voltage V _C11 capacitor C11 is charged to increase the load Power is supplied to the circuit 11.

Thereafter, the voltage V _C11 of the capacitor _C11 is equal to the voltage V _{C10 of the} capacitor _C10 and the output voltage of the rectifier DB.
When the voltage rises to the difference voltage (V _C10 − | Vs |) (at time t13 in FIG. 7), the AC power supply AC, the rectifier DB, the primary winding n11, the FET Q2, the rectifier DB, and the AC power supply AC
, A current _IT11 flows through the path, and the input current Iin is drawn into the power supply device from the AC power supply AC. Here, it can be seen from FIG. 7 that there is a period (time t13 to t15) during which the input current Iin is drawn from the AC power supply AC in a period in which the FETs Q1 and Q2 repeat ON / OFF once.

Thereafter, when the FET Q2 is turned off (FIG. 7)
At time t14) The transformer T11 that has accumulated energy by the current flowing through the primary winding n11 and the AC power supply AC serve as the power supply, and the AC power supply AC, the rectifier DB, the primary winding n11, the parasitic diode of the FET Q1, capacitor C12, capacitor C10, rectifier current I _T11 in the path of the DB and the AC power supply AC flows, while pulling the input current Iin capacitor C10, C12 is charged. At this time, as shown in FIG. 7, the voltage V _{C12 of the} capacitor C12 increases due to the resonance action with the leakage inductance of the transformer T11. Further, the FET Q1 is turned on.

When the FET Q1 is turned on, the transformer T1
1 and the capacitors C11 and C1
2, the capacitor C11,
A resonance current flows through the path of the capacitor C12, the FET Q1, the primary winding n11 and the capacitor C11. After this,
The voltages V _C11 and V _{C12 of the} capacitors C11 and C12 turn downward (time t15 in FIG. 7), and these energies are supplied to the load circuit 11 via the transformer T11. At this time, the direction of the current flowing through the primary winding n11 is equal to that of the FET Q2.
Is turned on in the opposite direction, so that an alternating high-frequency voltage is applied to the load circuit 11.

[0011] Thereafter, when the voltage V _C11, V _C12 of the capacitors C11, C12 is zero (time in FIG. 7 t16), a diode connected in parallel with each D11, D12
Turns on, and the above-described resonance current continues to flow.

Thereafter, when the FET Q1 is turned off (FIG. 7)
At time t17), the primary winding n11, the capacitor C11,
A current flows through the path of the capacitor C10, the parasitic diode of the FET Q2, and the primary winding n11, and the transformer T11
The energy stored in is released.

Thereafter, when the release of the energy stored in the transformer T11 is completed (time t18), at time t12
It returns to the circuit operation of.

High frequency power is supplied to the load circuit 11 by repeating the above circuit operation periodically. That is, when observing the above-mentioned main signal waveforms over one cycle of the AC power supply AC, it becomes as shown in FIG.

[0015] Here, as shown in FIG. 8, when the voltage Vs of the AC power source AC is raised and lowered sinusoidally, the voltage V _C11 of the capacitor C11 is lowered and raised sinusoidally, this voltage V _C11 The voltage V _T11 applied to the primary winding n11 becomes a voltage having a substantially constant fluctuation level (amplitude) by the voltage V _{C12 of the} capacitor C12 which rises and falls likewise when the voltage Vs rises and falls in a sine wave shape. . As a result, the crest factor (amplitude variation rate) of the current I _FL flowing through the secondary load circuit 11 decreases.

Further, since the voltage of the capacitor C12 is superimposed on the voltage of the capacitor C10, the voltage of the capacitor C10 can be set lower. Further, since the voltage of the capacitor C12 resonatesly rises as shown in FIG. 7, the voltage at the time of OFF applied to the FET Q2 is equal to the voltage of the capacitor C10, and the switching loss is reduced by the lower voltage of the capacitor C10. Reduction becomes possible.

Further, although no filter circuit is provided on the input side in FIG. 6, if a filter circuit for blocking high frequency is provided between the AC power supply AC and the rectifier DB according to a general conventional example, High frequency components can be prevented from being mixed into AC. At this time, if the voltage amplitude of the capacitor C11 is too large, as shown in FIG. 9A, the polarity is inverted during the period when the input current is drawn from the AC power supply AC, and large noise is generated. If it ’s too small,
As shown in FIG. 9C, since a pause occurs in the input current and noise is mixed into the AC power supply AC, the capacitance of the capacitor C11 is set so that the input current becomes as shown in FIG. 9B. It is desirable to do. As a result, it is possible to reduce the harmonics of the input current and to increase the input power factor.

However, in the power supply device of FIG. 6, when the output to the load circuit is reduced by changing the switching frequency or the on-duty, the current flowing through the capacitor C11 decreases, and the amplitude value of the voltage across the capacitor C11 decreases. Therefore, as shown in FIG. 9C, a pause occurs in the input current, and as a result, a problem occurs that the harmonic component of the input current increases.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a power supply device capable of suppressing an increase in a high-frequency component of an input current even when an output to a load circuit is reduced.

[0020]

According to a first aspect of the present invention, there is provided a power supply apparatus comprising: a rectifier for rectifying AC power into DC power; one output terminal of one end of the rectifier in a forward direction; Is connected to the first diode, and the first diode is connected to the first diode.
A smoothing capacitor connected between the other end of the diode and the other output terminal of the rectifier; a second diode connected to the other end of the first diode in a forward direction; A pair of switching elements connected in series between one end and the other output terminal of the rectifier, a diode connected in anti-parallel with each of the pair of switching elements, a connection point of the pair of switching elements, and the rectifier And a transformer having a primary winding connected between the first and second output terminals and having a secondary winding connected to a load circuit, and a first and a second parallel connection with the first and second diodes, respectively. A second capacitor,
The capacity of the first capacitor can be changed.

In this configuration, the capacity of the first capacitor can be changed, for example, when the load circuit is driven at the rated output or when the load circuit is driven at the rated output or less. , The input current can be drawn over the entire period. As a result, it is possible to suppress an increase in the high-frequency component of the input current even if the output to the load circuit is reduced.

According to a second aspect of the present invention, there is provided a power supply apparatus comprising: a rectifier for rectifying AC power into DC power; a first diode having one end connected to one output terminal of the rectifier in a forward direction; A smoothing capacitor connected between the other end of the diode and the other output terminal of the rectifier;
A second diode having one end connected to the other end of the diode in the forward direction; a pair of switching elements connected in series between the other end of the second diode and the other output terminal of the rectifier; A diode connected in anti-parallel to each of the switching elements, a primary winding connected between a connection point of the pair of switching elements and one output terminal of the rectifier, and connected to a load circuit;
A transformer having a next winding, a first capacitor connected between both output terminals of the rectifier, and a second capacitor connected in parallel with the second diode, wherein the capacity of the first capacitor can be changed. It is composed.

In this configuration, the capacity of the first capacitor can be changed, for example, when the load circuit is driven at the rated output or when the load circuit is driven at the rated output or less. , The input current can be drawn over the entire period. As a result, it is possible to suppress an increase in the high-frequency component of the input current even if the output to the load circuit is reduced.

The capacity of the first capacitor may be reduced in accordance with a decrease in output to the load circuit. With this configuration, the capacity of the first capacitor is reduced even if the output is reduced, so that the input current can be drawn over the entire period of the AC power. As a result, it is possible to suppress an increase in the high-frequency component of the input current even if the output to the load circuit is reduced.

The first capacitor may be constituted by at least one capacitor and at least one other capacitor connected in series with the capacitor so as to be separable (claim 4). In this configuration,
While the capacity of the first capacitor is reduced when connected,
It becomes large at the time of separation.

Further, the first capacitor may be constituted by at least one capacitor and at least one other capacitor connected in parallel with the capacitor so as to be separable (claim 5). In this configuration, the capacitance of the first capacitor increases when connected, and decreases when disconnected.

[0027]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic configuration diagram of a power supply device according to a first embodiment of the present invention, and FIG. 2 is an explanatory diagram of the operation of the power supply device. Will be described.

As shown in FIG. 1, the present power supply apparatus includes a rectifier DB for rectifying AC power from an AC power supply AC into DC power.
And a diode D11 (first diode) whose anode is connected to the positive output terminal of the rectifier DB in the forward direction.
And a smoothing capacitor C connected between the cathode of the diode D11 and the negative output terminal of the rectifier DB.
10, a diode D12 (second diode) whose anode is connected to the cathode of the diode D11 in the forward direction.
And FETs Q1 and Q2 connected in series between the cathode of the diode D12 and the negative output terminal of the rectifier DB.
(A pair of switching elements) and these FETs Q1, Q
And a control circuit 10 for performing on / off control of the FET Q
A transformer T1 having a primary winding n11 connected between a connection point of the rectifier DB and a connection point of the rectifier DB and a secondary winding n12 connected to the load circuit 11.
1 and a capacitor C connected in parallel with the diode D12.
12 (second capacitor) in the same manner as the power supply device shown in FIG. 6, and a difference from the power supply device shown in FIG. 6 is that a variable capacitance capacitor C11x connected in parallel with the diode D11 is provided. .

However, in the first embodiment, the capacitor C
It is assumed that the capacity of 11x is reduced, for example, by the control circuit 10 in accordance with a decrease in output to the load circuit 11. It is needless to say that the present invention is not limited to this configuration, and may employ a configuration in which the capacity of the capacitor C11x is changed by a capacity changing circuit (not shown) different from the control circuit 10. In this case, the capacity changing circuit may be configured to monitor the output to the load circuit 11 and reduce the capacity of the capacitor C11x when the output decreases.

Next, focusing on the differences from the power supply device shown in FIG. 6, when the control circuit 10 lowers the output to the load circuit 11 by changing at least one of the switching frequency and the on-duty, the capacitor C11x , The amplitude value of the voltage across the capacitor C11x decreases unless the capacitance of the capacitor C11x changes. In this case, as shown in FIG. 2A, the charging of the capacitor C11x becomes insufficient, so that the input current Iin cannot be drawn, and the input current Iin
There is a pause section at

Therefore, in the first embodiment, the load circuit 11
In response to the output drop to
By reducing the capacitance of 1x, as shown in FIG. 2B, the amplitude value of the voltage V _C11 across the capacitor C11 is increased to draw the input current Iin over the entire cycle of the AC power supply AC. In this way, a pause is not generated in the input current Iin, or the pause is shortened. As an example of the output reduction, there is a dimming lighting operation.

As described above, according to the first embodiment, the load circuit 1
Even if the output to 1 is reduced, it is possible to suppress an increase in the high frequency component of the input current Iin. Thereby, the input power factor can be improved.

FIG. 3 is a schematic configuration diagram of a power supply device according to a second embodiment of the present invention, and the second embodiment will be described below with reference to FIG.

As shown in FIG. 3, the power supply device includes a rectifier DB for rectifying AC power from an AC power supply AC into DC power.
And a diode D11 (first diode) whose anode is connected to the positive output terminal of the rectifier DB in the forward direction.
And a smoothing capacitor C connected between the cathode of the diode D11 and the negative output terminal of the rectifier DB.
10, a diode D12 (second diode) whose anode is connected to the cathode of the diode D11 in the forward direction.
And FETs Q1 and Q2 connected in series between the cathode of the diode D12 and the negative output terminal of the rectifier DB.
(A pair of switching elements) and these FETs Q1, Q
And a control circuit 10 for performing on / off control of the FET Q
A transformer T1 having a primary winding n11 connected between a connection point of the rectifier DB and a connection point of the rectifier DB and a secondary winding n12 connected to the load circuit 11.
1 and a capacitor C connected in parallel with the diode D12.
12 (second capacitor) as in the first embodiment, and the difference from the first embodiment is that the rectifier D
Variable capacitor C connected between both output terminals of B
21x (first capacitor). However, it is assumed that the capacity of the capacitor C21x is reduced by, for example, the control circuit 10 according to a decrease in output to the load circuit 11, as in the first embodiment.

In the power supply device thus configured, similarly to the first embodiment, when the control circuit 10 lowers the output to the load circuit 11 by changing at least one of the switching frequency and the on-duty, the capacitor C21x
Is reduced, the input current Iin can be drawn over the entire cycle of the AC power supply AC, and the input current Iin
It is possible to prevent the occurrence of the inactive section or shorten the inactive section.

As described above, according to the second embodiment, the load circuit 1
Even if the output to 1 is reduced, it is possible to suppress an increase in the high frequency component of the input current Iin. Thereby, the input power factor can be improved.

FIG. 4 is a schematic configuration diagram of a power supply device according to a third embodiment of the present invention, and the third embodiment will be described below with reference to FIG.

This power supply device comprises a rectifier DB, a diode D
11, D12, capacitors C10 and C12, FETQ
1, Q2, a transformer T11, and a control circuit 10 as in the first embodiment, and series-connected capacitors C211 and C212 connected in parallel with the diode D11.
And a switching element Q3 such as an FET connected in parallel with the capacitor C212.

However, the switching element Q3 is turned on by the control circuit 10, for example, when the load circuit 11 is driven at the rated output, and is turned off when the load circuit 11 is driven at the rated output or less. Shall be That is, the capacitances of the capacitors C211 and C212 (first capacitors) connected in parallel with the diode D11 can be changed.

It is needless to say that the switching element Q3 may be turned on / off by a capacitance changing circuit (not shown) different from the control circuit 10 without being limited to this structure. In this case, the capacity changing circuit may be configured to monitor the output to the load circuit 11 and turn on the switching element Q3 if the output is rated and turn off if the output is less than the rated output.

Next, the schematic operation of the power supply device will be described. When the load circuit 11 is driven at the rated output, the switching element Q3 is turned on. At this time, the input current Iin is drawn over the entire cycle of the AC power supply AC.

On the other hand, when the load circuit 11 is driven below the rated output, the switching element Q3 is turned off.
At this time, the capacity as the first capacitor is reduced in accordance with the decrease in the output to the load circuit 11, so that the input current Iin over the entire cycle of the AC power supply AC, as in the first embodiment.
Of the input current Iin can be prevented or the pause can be shortened.

The capacitances of the capacitors C211 and C212 are set to optimal values for both of the above operations.

As described above, according to the third embodiment, the load circuit 1
Even if the output to 1 is reduced, it is possible to suppress an increase in the high frequency component of the input current Iin. Thereby, the input power factor can be improved.

In the third embodiment, the capacitor C2
11, C212 is a diode D1 as a first capacitor.
1, but is not limited to this, and the capacitors C211 and C212 may each be composed of a plurality of capacitors.

FIG. 5 is a schematic configuration diagram of a power supply device according to a fourth embodiment of the present invention, and the fourth embodiment will be described below with reference to FIG.

This power supply device comprises a rectifier DB, a diode D
11, D12, capacitors C10 and C12, FETQ
1, Q2, a transformer T11 and a control circuit 10 are provided in the same manner as in the third embodiment. The difference from the third embodiment is that a capacitor C311 connected in parallel with the diode D11 and a capacitor C311 connected in parallel with the diode D11 are provided. Series-connected capacitor C312 and switching element Q3
And

However, as in the third embodiment, the switching element Q3 is turned on by the control circuit 10 when the load circuit 11 is driven at the rated output, while the switching circuit Q3 is turned on when the load circuit 11 is at or below the rated output. In the case of being driven by, it is turned off. That is, the diode D11
Capacitors C311 and C312 (first
That is, the capacity of the capacitor is changeable.

Next, the general operation of the power supply device will be described. When the load circuit 11 is driven at the rated output, the switching element Q3 is turned on. At this time, the input current Iin is drawn over the entire cycle of the AC power supply AC.

On the other hand, when the load circuit 11 is driven below the rated output, the switching element Q3 is turned off.
At this time, the capacity as the first capacitor is reduced in accordance with the decrease in the output to the load circuit 11, and therefore, as in the third embodiment, the input current Iin over the entire cycle of the AC power supply AC.
Of the input current Iin can be prevented or the pause can be shortened.

The capacitances of the capacitors C311 and C312 are set to optimal values for both the above operations.

As described above, according to the fourth embodiment, the load circuit 1
Even if the output to 1 is reduced, it is possible to suppress an increase in the high frequency component of the input current Iin. Thereby, the input power factor can be improved.

In the fourth embodiment, the capacitor C3
11, C312 is a diode D1 as a first capacitor.
1 is connected in parallel, but the present invention is not limited to this, and the capacitors C311 and C312 may each be composed of a plurality of capacitors.

[0054]

As is apparent from the above description, according to the first aspect of the present invention, a rectifier for rectifying AC power to DC power has one end connected to one output terminal of the rectifier in a forward direction. A first diode, a smoothing capacitor connected between the other end of the first diode and the other output terminal of the rectifier, and a second diode connected to the other end of the first diode in the forward direction. A diode, a pair of switching elements connected in series between the other end of the second diode and the other output terminal of the rectifier, and a diode connected in anti-parallel with each of the pair of switching elements;
A transformer having a primary winding connected between a connection point of the pair of switching elements and one output terminal of the rectifier and having a secondary winding connected to a load circuit; A first diode and a second capacitor connected in parallel with each other, and the capacity of the first capacitor can be changed. Therefore, even if the output to the load circuit is reduced, the high-frequency component of the input current increases. Can be suppressed.

According to the second aspect of the invention, a rectifier for rectifying AC power to DC power, a first diode having one end connected to one output terminal of the rectifier in a forward direction, A smoothing capacitor connected between the other end and the other output terminal of the rectifier, a second diode having one end connected to the other end of the first diode in a forward direction, and a second end of the second diode. A pair of switching elements connected in series between the other output terminal of the rectifier, a diode connected in anti-parallel with each of the pair of switching elements, and a connection point of the pair of switching elements and one of the rectifiers Connected to the output terminal of
A transformer having a secondary winding having a secondary winding and connected to a load circuit, a first capacitor connected between both output terminals of the rectifier, and a second capacitor connected in parallel with the second diode. And the capacity of the first capacitor can be changed. Therefore, even if the output to the load circuit is reduced, it is possible to suppress an increase in the high frequency component of the input current.

According to the third aspect of the present invention, since the capacitance of the first capacitor is reduced in accordance with the decrease in the output to the load circuit, even if the output to the load circuit is reduced, the high-frequency component of the input current can be reduced. The increase can be suppressed.

According to the fourth aspect of the present invention, the first capacitor includes at least one capacitor and at least one other capacitor connected in series with the capacitor so as to be separable. The capacity of the first capacitor can be changed.

According to the fifth aspect of the present invention, the first capacitor includes at least one capacitor and at least one other capacitor connected in parallel with the capacitor so as to be separable. The capacity of the first capacitor can be changed.

[Brief description of the drawings]

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

FIG. 2 is an operation explanatory diagram of the power supply device shown in FIG. 1;

FIG. 3 is a schematic configuration diagram of a power supply device according to a second embodiment of the present invention.

FIG. 4 is a schematic configuration diagram of a power supply device according to a third embodiment of the present invention.

FIG. 5 is a schematic configuration diagram of a power supply device according to a fourth embodiment of the present invention.

FIG. 6 is a schematic configuration diagram of a power supply device separately filed by the present applicant.

7 is an operation explanatory diagram of the power supply device shown in FIG. 6;

8 is an operation explanatory diagram of the power supply device shown in FIG.

FIG. 9 is an operation explanatory diagram of the power supply device shown in FIG. 6;

[Explanation of symbols]

 DB Rectifier D11, D12 Diode C10 (for smoothing) Capacitor C11x, C21x (variable capacity) capacitor C12 Capacitor Q1, Q2 FET T11 Transformer 10 Control circuit 11 Load circuit C211, C212, C311, C312 Capacitor Q3 Switching element

Claims (5)

[Claims] 1. A rectifier for rectifying AC power to DC power, a first diode having one end connected to one output terminal of the rectifier in a forward direction, and the other end of the first diode and the other of the rectifier. A smoothing capacitor connected between the output terminal, a second diode having one end connected to the other end of the first diode in the forward direction, and another end of the second diode and the other output terminal of the rectifier. A pair of switching elements connected in series, a diode connected in anti-parallel with each of the pair of switching elements, and a connection between a connection point of the pair of switching elements and one output terminal of the rectifier. A transformer having a primary winding and a secondary winding connected to a load circuit; and a first and a second core connected in parallel with the first and second diodes, respectively. And a capacitor, the power supply device having the capacity of the first capacitor is configured to be changed. 2. A rectifier for rectifying AC power into DC power, a first diode having one end connected to one output terminal of the rectifier in a forward direction, and the other end of the first diode and the other of the rectifier. A smoothing capacitor connected between the output terminal, a second diode having one end connected to the other end of the first diode in the forward direction, and another end of the second diode and the other output terminal of the rectifier. A pair of switching elements connected in series, a diode connected in anti-parallel with each of the pair of switching elements, and a connection between a connection point of the pair of switching elements and one output terminal of the rectifier. A transformer having a primary winding and a secondary winding connected to a load circuit, a first capacitor connected between both output terminals of the rectifier, and a second diode. And a second capacitor which is de connected in parallel, the power supply device having the capacity of the first capacitor is configured to be changed. 3. The power supply device according to claim 1, wherein a capacity of the first capacitor is reduced in accordance with a decrease in output to the load circuit. 4. The power supply device according to claim 1, wherein said first capacitor includes at least one capacitor and at least one other capacitor connected in series with said capacitor so as to be separable. 5. The power supply device according to claim 1, wherein the first capacitor includes at least one capacitor and at least one other capacitor that is detachably connected in parallel with the capacitor.