JP3038701B2 - Step-up DC-DC converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a step-up DC-DC converter, and more particularly to a step-up DC-DC converter capable of reducing switching loss.
-It relates to a DC converter.

[0002]

2. Description of the Related Art A step-up DC-DC converter which supplies a load with a DC output of a constant voltage higher than the voltage of a DC power supply by controlling on / off of a switching element has conventionally been used for a power supply circuit of an electronic device or the like. Widely used. The conventional step-up DC-DC converter shown in FIG. 6 includes a DC power supply 1, a step-up reactor 4 connected to one end of the DC power supply 1, and a collector terminal (one main terminal) connected to the step-up reactor 4. A main transistor 2 as a main switching element having an emitter terminal (the other main terminal) connected to the other end of the DC power supply 1; and a main reflux diode as a main reflux rectifying element connected in parallel with the main transistor 2. 3 and a series circuit of the smoothing capacitor 5, a load 6 connected in parallel with the smoothing capacitor 5, and a control circuit 7 for applying a control pulse signal to the base terminal of the main transistor 2. In this step-up DC-DC converter, the ON period of the main transistor 2 is controlled by changing the time width of the control pulse signal applied to the base terminal of the main transistor 2 in proportion to the fluctuation of the terminal voltage of the load 6. The DC power supplied to the load 6 is stabilized.

[0003]

[SUMMARY OF THE INVENTION Incidentally, in the step-up DC-DC converter of FIG. 6, the turned on or upon turning off of the main transistor 2, the collector of the main transistor 2 as shown in FIG. 7 - emitter voltage waveform V _CE a main transistor 2 of the collector current waveform I _C and the overlapping portion W of occurs, a large switching loss which is based on the overlapping portion W has a drawback to occur. The main transistor 2
At the rise of the collector-emitter voltage waveform V _CE and the collector current waveform I _C , a spike-shaped surge voltage V _sr , surge current I _sr and noise were generated.

Accordingly, an object of the present invention is to provide a step-up DC-DC converter capable of reducing switching loss, surge voltage, current and the like.

[0005]

SUMMARY OF THE INVENTION A boost DC according to the present invention
The DC converter comprises a DC power supply (1) and a main switching element (2) having one main terminal connected to one end of the DC power supply (1) and the other main terminal connected to the other end of the DC power supply (1). ), A series circuit of a main rectifying element (3) and a smoothing capacitor (5) connected in parallel with the main switching element (2), and a load (6) connected in parallel with the smoothing capacitor (5). And supplies a DC output of a voltage higher than the voltage of the DC power supply (1) to the load (6) by controlling on / off of the main switching element (2). In this step-up DC-DC converter, a resonance reactor connected in parallel with the main switching element (2) is used.
(10) and a series circuit of the auxiliary switching element (9), and a first circuit connected between a connection point of the series circuit and a connection point of the series circuit of the main rectifying element (3) and the smoothing capacitor (5). And a series circuit of a second auxiliary reflux rectifying element (11, 12);
A resonance capacitor (8) connected between a connection point of a series circuit of the second auxiliary reflux rectifying elements (11, 12) and one main terminal of the main switching element (2); A rectifying element for circulating current (13) comprising a rectifying element formed integrally with (2) or an independent rectifying element and connected in parallel with the main switching element (2), and a main switching element (2)
A control circuit (7) for applying an auxiliary control pulse signal to the control terminal of the auxiliary switching element (9) before applying the main control pulse signal to the control terminal of the DC power supply (1) and the main switching element (2). ) And a step-up reactor (4) is connected in series with the load (6). Another resonance capacitor (14) may be connected in parallel with the main switching element (2). Further, between the connection point of the series circuit of the first and second auxiliary reflux rectifiers (11, 12) and the connection point of the series circuit of the main reflux rectifier (3) and the smoothing capacitor (5), Auxiliary charging resistor (15)
Alternatively, an auxiliary charging switch that is turned on during the on period of the main switching element (2) may be connected.

[0006]

[Function] When the main switching element (2) is turned on, a current flows through the boosting reactor (4) and the main switching element (2), and the resonance capacitor (8) sets the terminal voltage of the load (6), that is, the output voltage. Charged up to. At this time the main switching element
When (2) is turned off, the main switching element (2)
Is immediately switched to the current flowing through the resonance capacitor (8), and the resonance capacitor (8) is gradually discharged. At this time, the voltage across the main switching element (2) gradually rises from 0 V,
Zero voltage switching (ZVS) at turn-off of (2)
Thus, the switching loss at the time of turning off the main switching element (2) can be reduced. Before applying the main control pulse signal to the control terminal of the main switching element (2) and turning on the main switching element (2), the auxiliary control pulse signal is applied to the control terminal of the auxiliary switching element (9). When the auxiliary switching element (9) is turned on, the output voltage is applied to the resonance reactor (10) while the main reflux rectifier (3) is conducting, and the current of the resonance reactor (10) is changed. Increases linearly from 0. Accordingly, zero current switching (ZCS) is achieved when the auxiliary switching element (9) is turned on, so that switching loss when the auxiliary switching element (9) is turned on can be reduced. Reactor for resonance
As the current of (10) increases, the current of the main rectifier rectifier (3) decreases linearly, and when the current of the resonance reactor (10) becomes equal to the current of the boost reactor (4), The main reflux rectifier (3) is cut off. At this time, when the main switching element (2) is turned on, the voltage of the main switching element (2) immediately drops to 0V. This allows
Zero voltage switching is achieved when the main switching element (2) is turned on,
The switching loss at the time of turn-on (2) can be reduced. After a short delay, the auxiliary switching element
When (9) is turned off, a resonance current flows through the resonance reactor (10) and the resonance capacitor (8), and the voltage of the resonance capacitor (8) rises in a sine wave form from 0V. When the voltage of the resonance capacitor (8) reaches the maximum value, the resonance current becomes zero. The current of the boosting reactor (10) flows through the main switching element (2) when the auxiliary switching element (9) is turned off. Accordingly, zero voltage switching is achieved when the auxiliary switching element (9) is turned off, so that switching loss when the auxiliary switching element (9) is turned off can be reduced. As described above, the switching loss of the main switching element (2) and the auxiliary switching element (9) during the ON / OFF operation can be reduced. The spike-like surge voltage and current generated when the main switching element (2) and the auxiliary switching element (9) are turned on and off are supplied to the resonance capacitor (8).
In addition, since the power is absorbed by the resonance reactor (10), the surge voltage and current during the on / off operation of the main switching element (2) and the auxiliary switching element (9) can be reduced. If another resonance capacitor (14) is connected in parallel with the main switching element (2), zero-voltage switching when the main switching element (2) is turned on becomes more reliable, further reducing switching loss. It is possible to

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a step-up DC-DC converter according to the present invention will be described below with reference to FIGS. However, in FIG. 1, the same portions as those shown in FIG. 6 are denoted by the same reference numerals, and description thereof will be omitted. Step-up DC of this embodiment
As shown in FIG. 1, the DC converter comprises a series circuit of a resonance reactor 10 connected in parallel with the main transistor 2 and an auxiliary transistor 9 as an auxiliary switching element, and a connection point of the series circuit and a main reflux diode. 3 and a series circuit of first and second auxiliary reflux diodes (auxiliary reflux rectifiers) 11 and 12 connected between a connection point of a series circuit of the smoothing capacitor 4 and a first and second auxiliary circuit. A resonance capacitor 8 connected between the connection point of the series circuit of the return diodes 11 and 12 and the collector terminal of the main transistor 2, and a circulating current diode (a circulating current rectifier) connected in parallel with the main transistor 2. Element 13) is added to the circuit of FIG. The control circuit 7
Applies an auxiliary control pulse signal to the base terminal of the auxiliary transistor 9 before applying the main control pulse signal to the base terminal (control terminal) of the main transistor 2. In this embodiment,
A junction power transistor is used as the main transistor 2 and the auxiliary transistor 9.

Although not shown, the control circuit 7 includes:
An oscillation circuit for generating a triangular wave voltage having a constant period; an error amplifying circuit for calculating and amplifying an error voltage of a terminal voltage of the load 6 with respect to a reference voltage; A comparison circuit section for comparison; a main control pulse generation circuit section for generating a main control pulse signal having a time width proportional to an output voltage of the comparison circuit section and applying the generated signal to a base terminal of the main transistor 2; And an auxiliary control pulse generation circuit for generating an auxiliary control pulse signal having a fixed time width to be applied to the base terminal of the auxiliary transistor 9 before the main control pulse signal rises. The time width of the auxiliary control pulse signal generated from the auxiliary control pulse generation circuit is extremely shorter than the off time of the main transistor 2.

In the above configuration, when the main transistor 2 is turned on before t ₀ as shown in FIG. 2A, the path of the boosting reactor 4 and the main transistor 2 as shown in FIG. , A current I ₀ flows. At this time, the resonance capacitor 8 as shown in FIG. 2 (F) is charged to the terminal voltage, i.e. the output voltage E ₀ of the load 6 with the polarity shown in FIG. As shown in FIG. 2A, at t ₀ , the main control pulse signal voltage V _B1 applied from the control circuit 7 to the base terminal of the main transistor 2 changes from a high level to a low level, and the main transistor 2 is turned on. In the off state, as shown in FIGS. 2C and 2D, the current I _TR1 flowing through the main transistor 2, that is, the current I ₀ of the boosting reactor 4, is immediately supplied to the resonance capacitor 8 and the second auxiliary return. To the current I _C1 flowing in the path of the diode 12.
At this time, as shown in FIG. 2F, the resonance capacitor 8 gradually discharges, and the voltage V _C1 across the resonance capacitor 8 linearly drops from the output voltage E ₀ . Accordingly, as shown in FIG. _2E, the voltage V _TR1 across the main transistor 2 linearly increases from 0V. For this reason,
When the main transistor 2 is turned off, zero voltage switching is performed with little overlap between the voltage waveform and the current waveform.

As shown in FIG. 2 (F), when the voltage V _C1 across the resonance capacitor 8 becomes 0 V at t ₁ , the main reflux diode 3 becomes forward-biased, and as shown in FIG.
As shown in (G), the current I _C1 flowing through the resonance capacitor 8 flows through the main reflux diode 3 (I _D ). When the main transistor 2 is off,
The current I ₀ of the boosting reactor 4 is
Through to the load 6.

As shown in FIG. 2B, at t ₂ , the auxiliary control pulse signal voltage V _B2 applied from the control circuit 7 to the base terminal of the auxiliary transistor 9 changes from a low level to a high level. When turned on, the output voltage E ₀ is applied to the resonance reactor 10 while the main reflux diode 3 is conducting, and the current _IL1 starts to flow through the resonance reactor 10 as shown in FIG. . This current _IL1 linearly increases from ₀ to become equal to the current I0 of the boosting reactor 4. On the other hand, the current _ID flowing through the main reflux diode 3 decreases linearly as shown in FIG. Therefore, when the auxiliary transistor 9 is turned on, zero current switching (ZCS)
Becomes

As shown in FIG. 2 (H), when the current I _L1 of the resonance reactor 10 becomes equal to the current I ₀ of the boosting reactor 4 at t ₃ , the main reflux diode 3 is cut off, and as shown in FIG. ), No current flows through the main reflux diode 3. When the current _ID of the main reflux diode 3 becomes 0, the control circuit 7 changes the main control pulse signal voltage V _B1 applied to the base terminal of the main transistor 2 from a low level as shown in FIG. The main transistor 2 is turned on from the off state by setting it to a high level. At this time, as shown in FIG. _2E, the voltage V _TR1 across the main transistor 2 immediately drops to 0V. Therefore, zero voltage switching is performed when the main transistor 2 is turned on.

A little later, as shown in FIG. 2B, at time t ₄ , the control circuit 7 lowers the auxiliary control pulse signal voltage V _B2 applied to the base terminal of the auxiliary transistor 9 from the high level to the low level, and The transistor 9 is turned off from the on state. At this time, the resonance reactor 10
The energy stored in the reactor is released and the resonance reactor 1
Since the zero and the resonance capacitor 8 resonate, the current _IL1 of the resonance reactor 10 becomes a resonance current flowing through the path of the resonance reactor 10, the first auxiliary reflux diode 11, and the resonance capacitor 8. As a result, the resonance capacitor 8 is charged with a sine waveform, and as shown in FIG. 2F, the voltage V _C1 across the resonance capacitor 8 rises in a sine wave form from 0V. 2 (D) and FIG.
As shown in (H), the current I _C1 of the resonance capacitor 8 and the current I _L1 of the resonance reactor 10 decrease in a cosine wave. The current I ₀ of the boosting reactor 4 is as shown in FIG.
Flows through the main transistor 2 (I _TR1 ). Therefore, when the auxiliary transistor 9 is turned off, the resonance capacitor 8 is charged in a sine wave form from 0 V, and thus, zero voltage switching is performed.

[0014] As shown in FIG. 2 (F), substantially the maximum value of the voltage across V _C1 of the resonance capacitor 8 in t _5, that is, the output voltage is reached E _0, in FIG. 2 (D) and (H) As shown, the current I _C1 of the resonance capacitor 8 and the resonance reactor 10
Current I _L1 becomes 0, and the first auxiliary reflux diode 1
1 cuts off. When the auxiliary transistor 9 is turned off, the voltage V _C1 across the resonance capacitor 8 is applied.
Is going to be equal to or higher than the output voltage E ₀ , the circulating current diode 13, the resonance reactor 10, the first auxiliary return diode 11 and the second auxiliary return diode 1
Energy is supplied to the load 6 through the path 2.

As described above, in this embodiment, zero voltage or zero current switching is achieved when the main transistor 2 and the auxiliary transistor 9 are turned on and off, so that the main transistor 2 and the auxiliary transistor 9 are turned off.
, The power loss at the time of the ON / OFF operation, that is, the switching loss can be reduced. Further, a spike-shaped surge voltage and a surge current generated when the main transistor 2 and the auxiliary transistor 9 are turned on and off are absorbed by the resonance capacitor 8 and the resonance reactor 10, so that the main transistor 2 is turned on and off during the on / off operation. Surge voltage, surge current and noise can be reduced.

Next, a modified embodiment of the step-up DC-DC converter shown in FIG. 1 will be described with reference to FIGS.
However, in FIG. 3, the same portions as those in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted. The details of the inside of the control circuit 7 of FIG. 3 are exactly the same as those of the control circuit 7 shown in the embodiment of FIG. Step-up DC- shown in FIG.
In the DC converter, another resonance capacitor 14 is connected in parallel with the main transistor 2 of the circuit of the embodiment shown in FIG.
This ensures zero voltage switching at the time of turning on the main transistor 2 (t ₄ ). Other configurations are the same as those of the circuit shown in FIG.

In the above configuration, as shown in FIGS. 4A to 4I, the operation up to t ₃ is the same as the operation in the circuit of FIG. Thus, in this embodiment will be described t ₃ subsequent operation. As shown in FIG. 4 (I), when the current I _L1 of the resonance reactor 10 becomes equal to the current I ₀ of the boosting reactor 4 at t ₃ , the main reflux diode 3 is cut off, and as shown in FIG. 4 (H). Thus, no current flows through the main reflux diode 3. At this time, the energy of the resonance capacitor 14 charged during t _{0 to} t ₁ is released, and the resonance capacitor 14 and the resonance reactor 10 resonate, and the path of the resonance capacitor 14, the resonance reactor 10, and the auxiliary transistor 9. Causes a resonance current to flow. For this reason, a sinusoidal current flows through the resonance reactor 10 while being superimposed on the current I ₀ of the boosting reactor 4, and the current I _L1 of the resonance reactor 10 continues to be sinusoidal as shown in FIG. It increases like a wave (I _L1 ). With this,
The current I _C2 of the resonance capacitor 14 increases in a sine wave shape as shown in FIG. 4D, and the voltage V _C2 across the resonance capacitor 14 drops in a cosine wave shape as shown in FIG. go.

As shown in FIG. 4 (I), at t ₄ , the current I _L1 of the resonance reactor 10 is substantially equal to the maximum value, that is, the sum of the current I ₀ of the boosting reactor 4 and the maximum value I _p of the resonance current. When it reaches, the circulating current diode 13 becomes forward biased. Therefore, the current I _L1 of the resonance reactor 10 continues to flow as a circulating current through the path of the circulating current diode 13, the resonance reactor 10, and the auxiliary transistor 9. At the same time, the voltage V _C2 across the resonance capacitor 14 becomes 0 V as shown in FIG. At this time, the control circuit 7
As shown in FIG. 2A, the main control pulse signal voltage V _B1 applied to the base terminal of the main transistor 2 is changed from a low level to a high level, and the main transistor 2 is turned on from an off state. The voltage V across the main transistor 2 at this time
_{Since TR1} is 0 V as shown in FIG. _4F , zero voltage switching is performed when the main transistor 2 is turned on.

[0019] and thereafter a slight delay, as shown in FIG. 4 (B), the control circuit 7 at t ₅ is the auxiliary control pulse signal voltage V _B2 to be applied to the base terminal of the auxiliary transistor 9 from the high level to the low level auxiliary The transistor 9 is turned off from the on state. At this time, the resonance reactor 10
The energy stored in the reactor is released and the resonance reactor 1
Since the zero and the resonance capacitor 8 resonate, the current _IL1 of the resonance reactor 10 becomes a resonance current flowing through the path of the resonance reactor 10, the first auxiliary reflux diode 11, and the resonance capacitor 8. As a result, the resonance capacitor 8 is charged with a sine waveform, so that the voltage V _C1 at both ends of the resonance capacitor 8 rises in a sine wave form from 0 V as shown in FIG. 4 (E) and FIG.
As shown in (I), the current I _C1 of the resonance capacitor 8 and the current I _L1 of the resonance reactor 10 decrease in a cosine wave. Further, the current I ₀ of the boosting reactor 4 is as shown in FIG.
As shown in (1), the current flows through the main transistor 2 at the same time when the auxiliary transistor 9 is turned off ( _ITR1 ). Therefore, when the auxiliary transistor 9 is turned off, the voltages V _C1 and V _C2 across the resonance capacitors 8 and 14 are 0 V, so that zero voltage switching is performed.

As shown in FIG. 4 (F), when the voltage V _C1 across the resonance capacitor 8 reaches a substantially maximum value, that is, the output voltage E ₀ at t ₆ , as shown in FIG. Current I _L1 of boosting reactor 10 is equal to current I ₀ of boosting reactor 4.
And the second auxiliary reflux diode 12 becomes forward biased. At this time, as shown in FIGS. 4C and _4E, the current I _TR1 of the main transistor 2 and the current I _C1 of the resonance capacitor 8 become zero. At this time, the energy of the remaining resonance reactor 10 is equal to the circulating current diode 13,
Resonance reactor 10, first auxiliary reflux diode 1
The load 6 passes through the path of the first and second auxiliary reflux diodes 12.
Going to be fed to. As a result, the resonance reactor 10
The current _IL1 of FIG. 4 linearly and continuously decreases as shown in FIG. 4 (I), and the current of the main transistor 2 linearly increases from 0 as shown in FIG. 4 (C). And
current I _L1 of the resonant reactor 10 at t ₇ Figure 4
As shown in (I), it becomes 0, and the current I
_TR1 becomes equal to the current I ₀ of the step-up reactor 4 as shown in FIG. 4 (C). Therefore, after t ₇ , a current I ₀ flows from the DC power supply 1 through the path of the boosting reactor 4 and the main transistor 2.

As described above, in the embodiment shown in FIG. 3, the same effect as that in the embodiment shown in FIG. 1 can be obtained with respect to the switching loss. In the circuit of the embodiment of FIG. 3, the resonance of the resonant reactor 10 and the resonance capacitor 14, the voltage V across the resonance capacitor 14 in t ₃ ~t ₄ as shown in FIG. 4 (F) _{Since C2} drops in a cosine waveform, zero voltage switching at the time of turning on the main transistor 2 (t ₄ ) becomes more reliable, and the switching loss reduction effect is greater than that of the circuit of the embodiment of FIG.

The circuit of the embodiment shown in FIG. 1 may be modified as shown in FIG. The circuit shown in FIG. 5 is the same as the circuit shown in FIG.
The auxiliary charging resistor 15 is connected between the connection point of the series circuit 2 and the connection point of the series circuit of the main reflux diode 3 and the smoothing capacitor 5. In the circuit of the embodiment shown in FIG. 5, since the resonance capacitor 8 can be supplementarily charged by the output voltage E ₀ during the ON period of the main transistor 2, the charging voltage of the resonance capacitor 8 in the circuit shown in FIG. Even when it is less than ₀ , zero voltage switching at the time of turning off the main transistor 2 becomes possible. Note that an auxiliary charging switch that is turned on during the ON period of the main transistor 2 may be connected instead of the auxiliary charging resistor 15.
As a specific example of the auxiliary charging switch, there is a semiconductor switching element such as a transistor.

Further, the embodiments of the present invention are not limited to the above embodiments, and various modifications are possible. For example, in the above embodiment, an example in which a junction type power transistor is used as the main switching element and the auxiliary switching element has been described. May be used. In particular, when an FET is used, a built-in diode formed integrally with the FET can be used, so that the circulating current diode 13 in the above embodiment can be omitted. Further, the main switching element and the auxiliary switching element are not limited to the same kind of combination.

[0024]

As described above, according to the present invention, zero voltage or zero current switching of the switching element can be easily achieved, so that the overlap between the voltage waveform and the current waveform of the switching element is reduced to reduce the step-up converter. It is possible to reduce the power loss during the ON / OFF operation of the switching element of the circuit, that is, the switching loss in the boost converter circuit. Further, surge voltage, surge current, and noise during switching operation of the switching element of the boost converter circuit can be reduced. Furthermore, when another resonance capacitor is connected in parallel with the main switching element, zero voltage switching of the switching element can be achieved more reliably, so that the overlap between the voltage waveform and the current waveform of the switching element is further reduced. Thus, the switching loss in the boost converter circuit can be further reduced.

[Brief description of the drawings]

FIG. 1 is an electric circuit diagram showing an embodiment of a step-up DC-DC converter according to the present invention.

FIG. 2 is a waveform chart showing voltages and currents of respective parts of the circuit of FIG.

FIG. 3 is an electric circuit diagram showing a modified embodiment of the circuit of FIG. 1;

FIG. 4 is a waveform chart showing the voltage and current of each part of the circuit of FIG.

FIG. 5 is an electric circuit diagram showing another modified embodiment of the circuit of FIG. 1;

FIG. 6 is an electric circuit diagram showing a conventional step-up DC-DC converter.

FIG. 7 is a waveform chart showing an overlapping portion of a switching voltage waveform and a switching current waveform of the circuit of FIG. 6;

[Explanation of symbols]

1. . . DC power supply, 2. . . 2. main transistor (main switching element); . . 3. Main freewheeling diode (main freewheeling rectifier); . . Step-up reactor, 5. . . 5. smoothing capacitor; . . Load, 7. . . Control circuit, 8, 1
4. . . 8. resonance capacitor; . . 10. auxiliary transistor (auxiliary switching element); . . Resonance reactor, 11, 12. . . First and second auxiliary reflux diodes (first and second auxiliary reflux rectifying elements), 1
3. . . 14. Diode for circulating current (rectifying element for circulating current), . . Auxiliary charging resistor

Claims (3)

(57) [Claims] 1. A DC power supply, a main switching element having one main terminal connected to one end of the DC power supply and the other main terminal connected to the other end of the DC power supply, and a main switching element connected in parallel with the main switching element. A series circuit of a connected main reflux rectifying element and a smoothing capacitor, and a load connected in parallel with the smoothing capacitor. The on / off control of the main switching element is higher than the voltage of the DC power supply. In a step-up DC-DC converter for supplying a DC output of a voltage to the load, a series circuit of a resonance reactor and an auxiliary switching element connected in parallel with the main switching element; A series circuit of first and second auxiliary reflux rectifying elements connected between a connection point of the series circuit of the rectifying element and the smoothing capacitor; A resonance capacitor connected between a connection point of the series circuit of the second auxiliary reflux rectifying element and one main terminal of the main switching element, and a rectifying element integrally formed with the main switching element or A circulating current rectifying element comprising an independent rectifying element and connected in parallel with the main switching element; A step-up DC-DC converter comprising: a control circuit for applying a control pulse signal; and a step-up reactor connected in series between the DC power supply and the main switching element and in series with the load. 2. The step-up DC according to claim 1, wherein another resonance capacitor is connected in parallel with the main switching element.
-DC converter. 3. A supplementary charging resistor or a connection between a connection point of a series circuit of the first and second auxiliary reflux rectifiers and a connection point of a series circuit of the main reflux rectifier and the smoothing capacitor. 2. The step-up DC-DC converter according to claim 1, wherein an auxiliary charging switch that is turned on during an on period of the main switching element is connected.