JP3104874B2 - Step-down 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-down DC-DC converter, and more particularly to a step-down DC-DC converter capable of reducing switching loss.
A DC converter;

[0002]

2. Description of the Related Art A step-down DC-DC converter that supplies a constant-voltage DC output lower than the voltage of a DC power supply to a load 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-down DC-DC converter shown in FIG. 8 includes a DC power supply 1, a main transistor 2 as a main switching element having a collector terminal (one main terminal) connected to one end of the DC power supply 1, and a main transistor 2 A main reflux diode 3 as a main reflux rectifying element connected between the emitter terminal (the other main terminal) and the other end of the DC power supply 1, and a smoothing reactor connected in parallel with the main reflux diode 3. 4 includes a series circuit of a smoothing capacitor 4 and a 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-down 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-down DC-DC converter of FIG. 8, the turned on or upon turning off of the main transistor 2, the collector of the main transistor 2 as shown in FIG. 9 - 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
There is a disadvantage that emitter voltage waveforms V _CE and the collector current waveform I _C spike surge voltage V _sr at the rise of the surge current I _{s r} and noise is generated - collector.

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

[0005]

SUMMARY OF THE INVENTION A step-down 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 of the main switching element (2) and the DC power supply
A main reflux rectifying element (3) connected between the other end of (1),
It comprises a series circuit of a smoothing reactor (4) and a smoothing capacitor (5) connected in parallel with the main reflux element (3), and a load (6) connected in parallel with the smoothing capacitor (5). The DC output of a voltage lower than the voltage of the DC power supply (1) is supplied to the load (6) by on / off control of the switching element (2). This step-down DC-DC converter includes a resonance reactor (10) inserted between the other main terminal of the main switching element (2) and the smoothing reactor (4), a main switching element (2) and a resonance reactor. Connection point of reactor (10) and DC power supply
A series circuit of the first and second auxiliary reflux rectifiers (11, 12) connected between the other end of (1), a connection point of the series circuit, a resonance reactor (10), and a smoothing reactor ( The resonance capacitor (8) connected between the connection point of (4) and one of the main terminals of the main switching element (2) and the resonance reactor (10)
And a rectifying element for circulating current (13) connected between the connection point of the smoothing reactor (4) and a control circuit for applying a main control pulse signal to a control terminal of the main switching element (2). . Another resonance capacitor (14) may be connected in parallel with the circulating current rectifier (13).

[0006]

[Function] Load with the main switching element (2) turned on
When the main switching element (2) is turned off when a current flows to the (6) side and the resonance capacitor (8) is charged to the power supply voltage, the current flowing through the main switching element (2) is immediately turned off. The current is 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 0V. This achieves zero voltage switching (ZVS) when the main switching element (2) is turned off.
The switching loss at the time of turn-off in (2) can be reduced. As the current of the resonance reactor (10) increases, the current of the main reflux rectifier (3) decreases linearly,
When the current of the resonance reactor (10) becomes equal to the load current, 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 achieves zero voltage switching when the main switching element (2) is turned on, so that switching loss when the main switching element (2) is turned on can be reduced. As described above, the switching loss at the time of the ON / OFF operation of the main switching element (2) can be reduced. Also, the spike-like surge voltage and current generated when the main switching element (2) is turned on and off are absorbed by the resonance capacitor (8) and the resonance reactor (10).
The surge voltage and current at the time of the on / off operation of (2) 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 First, as a related technique of the present invention, an example of a step-down DC-DC converter having an auxiliary switching element will be described with reference to FIGS. An example will be described with reference to FIGS. However, FIGS. 1, 3, 5 and 6 show FIG.
The same reference numerals are given to the same portions as those shown in FIG. Step-down DC- as a related technology of the present invention
As shown in FIG. 1, the DC converter includes a series circuit of an auxiliary transistor 9 serving as an auxiliary switching element and a resonance reactor 10 connected in parallel with the main transistor 2, and a connection point between the series circuit and the DC power supply 1. Of the series circuit of the first and second auxiliary reflux diodes (auxiliary reflux rectifying elements) 11 and 12 and the series circuit of the first and second auxiliary reflux diodes 11 and 12 connected between the first and second auxiliary reflux diodes. A resonance capacitor 8 connected between the connection point and the emitter terminal of the main transistor 2, and a circulating current diode (a circulating current rectifier) connected in parallel with the main transistor 2
13 is added to the circuit of FIG. Further, 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 the step-down DC-DC converter of FIG. 1, 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 load 6 passes through the main transistor 2 and the smoothing reactor 4 as shown in FIG. Current I is flowing to At this time, as shown in FIG.
Is charged to the voltage E of the DC power supply 1 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, the current I _TR1 flowing through the main transistor 2, that is, the current I of the load 6, is immediately supplied to the second auxiliary return diode 12 and the resonance capacitor 8 as shown in FIGS. And the current I _C1 flowing through the path of the smoothing reactor 4 is switched. 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 voltage E of the DC power supply 1. Accordingly, as shown in FIG. _2E, the voltage V _TR1 across the main transistor 2 linearly increases from 0V. Therefore, 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 ). The voltage V across the main transistor 2 at this time
_TR1 is equal to the voltage E of the DC power supply 1 as shown in FIG. When the main transistor 2 is off, the load 6
Current I flows from the main reflux diode 3 to the smoothing reactor 4
Is flowing to

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 the main reflux diode 3 is turned on, the voltage E of the DC power supply 1 is applied to the resonance reactor 10 while the main reflux diode 3 is conducting, and the current _IL1 flows through the resonance reactor 10 as shown in FIG. Start flowing. This current _IL1 increases linearly until it becomes equal to the current I of the load 6. On the other hand, the current _ID flowing through the main reflux diode 3 decreases linearly as shown in FIG. Therefore, zero current switching is performed when the auxiliary transistor 9 is turned on.

As shown in FIG. 2 (H), when the current I _L1 of the resonance reactor 10 becomes equal to the current I of the load 6 at t ₃ , the main reflux diode 3 is cut off, and FIG.
As shown in (1), 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.

After a short delay, as shown in FIG.
And t_Four, The control circuit 7
Auxiliary control pulse signal voltage V applied to the source terminal_B2The high
The auxiliary transistor 9 is turned on by setting the level to the low level from the bell.
To the off state. At this time, the resonance reactor 10
The energy stored in the reactor is released and the resonance reactor 1
0 and the resonance capacitor 8 resonate.
Current I of vector 10_L1Is the resonance capacitor 8, the first complement
Of the auxiliary reflux diode 11 and the resonance reactor 10
The resonance current flows on the road. As a result, the resonance capacitor
Since the sensor 8 is charged with a sinusoidal waveform, as shown in FIG.
The voltage V across the resonance capacitor 8_C1Is 0V
And rise in a sine wave shape. 2 (D) and FIG.
As shown in (H), the current I of the resonance capacitor 8 is_C1And
Current I of transfer reactor 10_L1Decreases in a cosine wave
Good. Further, the current of the smoothing reactor 4, that is, the current of the load 6,
I passes through the main transistor 2 as shown in FIG.
Flowing (I_TR1). Therefore, the auxiliary transistor 9
At the time of turn-off, the voltage V across the resonance capacitor 8 _C1
Is 0V, so that 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, reaches the voltage E of the DC power source 1, FIG. 2 (D) and (H ), The current I _C1 of the resonance capacitor 8 and the current I _L1 of the resonance reactor 10 become 0, and the first auxiliary reflux diode 11 is cut off. The auxiliary transistor 9
When the voltage V _C1 across the resonance capacitor 8 is about to exceed the voltage E of the DC power supply 1 at the turn-off of
The charging energy of the resonance capacitor 8 is fed back to the DC power supply 1 through the path of the second auxiliary reflux diode 12, the first auxiliary reflux diode 11, the resonance reactor 10, and the circulating current diode 13.

As described above, in the circuit of FIG. 1, 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 on / off operation of the main transistor 2 and the auxiliary transistor 9 is performed. Power loss, that is, 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 modification of the step-down 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-down DC-D shown in FIG.
In the C converter, another resonance capacitor 14 is connected in parallel with the main transistor 2 of the circuit shown in FIG. 1 so that the zero voltage switching at the time of turning on the main transistor 2 (t ₄ ) is further ensured. Other configurations are the same as those of the circuit shown in FIG.

In the above configuration, as shown in FIGS. 4A to 4H, the operation up to t ₃ is the same as the operation in the circuit of FIG. Therefore, in the circuit of FIG. 3, the operation after t ₃ will be described. As shown in FIG. 4 (H), when the current I _L1 of the resonance reactor 10 becomes equal to the current I of the load 6 at t ₃ , the main reflux diode 3 is cut off.
As shown in (G), no current flows through the main reflux diode 3. At this time, the energy of the resonance capacitor 14 charged with the polarity in FIG. 3 is released, and the resonance capacitor 14 and the resonance reactor 10 resonate, and the path of the resonance capacitor 14, the auxiliary transistor 9, and the resonance reactor 10 Causes a resonance current to flow. For this reason, the sinusoidal current flows through the resonance reactor 10 while being superimposed on the current I of the load 6, so that the current I _{L1 of the} resonance reactor 10
Continuously increases sinusoidally as shown in FIG. 4H (I _L1 ). On the other hand, the voltage V _C2 across the resonance capacitor 14 falls in a cosine wave as shown in FIG.

As shown in FIG. 4H, when the current I _L1 of the resonance reactor 10 reaches a substantially maximum value at t ₄ , that is, the sum of the current I of the load 6 and the maximum value I _p of the resonance current. The circulating current diode 13 becomes forward-biased, and the resonance current component continues to flow as a circulating current through the path of the circulating current diode 13, the auxiliary transistor 9, and the resonance reactor 10. 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 changes the main control pulse signal voltage V _B1 applied to the base terminal of the main transistor 2 from a low level to a high level as shown in FIG. I do. At this time, the voltage V _TR1 across the main transistor 2 is 0 V as shown in FIG.
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 0 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 resonance capacitor 8, and the first auxiliary reflux diode 11. As a result, the resonance capacitor 8 is charged with a sine waveform, and as shown in FIG. 4F, the voltage V _C1 across the resonance capacitor 8 rises in a sine wave form from 0V. 4 (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. Further, the current of the smoothing reactor 4, that is, the current I of the load 6, flows through the main transistor 2 at the same time when the auxiliary transistor 9 is turned off as shown in FIG.
_TR1 ). Therefore, when the auxiliary transistor 9 is turned off, the voltages V _C1 and V 2 across the resonance capacitors 8 and 14 are set.
_{Since C2} is 0V, zero voltage switching is performed.

As shown in FIG. 4F, when the voltage V _C1 across the resonance capacitor 8 reaches a substantially maximum value, that is, the voltage E of the DC power supply 1, at t ₆ , as shown in FIG. In addition, the current I _L1 of the resonance reactor 10 becomes equal to the current I of the load 6. At this time, as shown in FIGS. 4C and 4D, the current I _TR1 of the main transistor 2 and the current I _C1 of the resonance capacitor 8 become zero. Remaining resonance reactor 1 at this time
The energy of 0 is supplied to the second auxiliary reflux diode 12, the first auxiliary reflux diode 11, the resonance reactor 10
The current is fed back to the DC power supply 1 through the path of the circulating current diode 13. Thus, the current _IL1 of the resonance reactor 10 decreases linearly and continuously as shown in FIG. 4H, and the current of the main transistor 2 decreases from 0 as shown in FIG. Increase. And t ₇
In FIG. 4, the current I _L1 of the resonance reactor 10 becomes 0 as shown in FIG. _{4H, and} the current I _TR1 of the main transistor 2 becomes equal to the current I of the load 6 as shown in FIG. Therefore, t ₇ after the main transistor 2 from the DC power supply 1
And current I flows to load 6 through smoothing reactor 4.

As described above, the circuit shown in FIG. 3 can provide the same effect as the circuit shown in FIG. 1 with respect to the switching loss. In the circuit of Figure 3, the resonance of the resonant reactor 10 and the resonance capacitor 14, a cosine voltage V _C2 across the resonance capacitor 14 in t ₃ ~t ₄ as shown in FIG. 4 (E) Since the voltage drops in a wave shape, the zero voltage switching at the time of turning on the main transistor 2 (t ₄ ) is more reliable, and has an advantage that the switching loss can be further reduced.

The circuit shown in FIG. 1 may be modified as shown in FIG. The circuit shown in FIG. 5 has an auxiliary charging resistor 15 connected between the connection point of the series circuit of the first and second auxiliary reflux diodes 11 and 12 of the circuit shown in FIG. It was done. In the circuit shown in FIG. 5, since the resonance capacitor 8 can be supplementarily charged through the main transistor 2 during the ON period of the main transistor 2, the charging voltage of the resonance capacitor 8 in the circuit shown in FIG.
, The zero voltage switching at the time of turning on the main transistor 2 becomes possible. In addition,
Instead of the auxiliary charging resistor 15, an auxiliary charging switch that is turned on during the ON period of the main transistor 2 may be connected. As a specific example of the auxiliary charging switch, there is a semiconductor switching element such as a transistor.

An embodiment of the step-down DC-DC converter according to the present invention will be described below with reference to FIGS. FIG.
The step-down DC-DC converter of this embodiment shown in FIG.
The circulating current step-down converter is obtained by omitting the main transistor 2 of the circuit as a related art shown in FIG. 1 and operating the auxiliary transistor 9 as the main transistor 2.

Next, the operation of the circuit of FIG. 6 will be described with reference to the waveform diagram of FIG. When the main transistor 2 is turned off before t ₀ as shown in FIG. 7 (A), the main reflux diode 3 and the smoothing reactor 4 as shown in FIG. 7 (B).
The current I flows to the load 6 via (I _D ). At this time, the resonance capacitor 14 is charged to the voltage E of the DC power supply 1 with the polarity shown in FIG. 7D (V _C2 ). As shown in FIG. 7A, 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 low level to a high level, and the main transistor 2 changes from an off state. When turned on, the voltage E of the DC power supply 1 is applied to the resonance reactor 10, and as shown in FIG. 7C, the current _IL1 starts flowing through the resonance reactor 10, and increases linearly from 0. . At the same time, the current _ID flowing through the main reflux diode 3 decreases linearly as shown in FIG. At this time, the current I of the load 6 flows through the main transistor 2,
The voltage V _TR1 across the main transistor 2 immediately drops to 0 V as shown in FIG. Therefore, when the main transistor 2 is turned on, the resonance reactor 1
Since the current I _L1 of 0 gradually rises from 0, zero current switching is performed.

As shown in FIG. 7 (C), when the current I _L1 of the resonance reactor 10 becomes equal to the current I of the load 6 at t ₁ , the main reflux diode 3 is cut off, and FIG.
As shown in the figure, the current _ID of the main reflux diode 3 becomes zero. At this time, the energy charged in the resonance capacitor 14 is released, and the resonance capacitor 14 and the resonance reactor 10 resonate, and a resonance current flows through a path of the resonance capacitor 14, the main transistor 2, and the resonance reactor 10. For this reason, a sinusoidal current flows through the resonance reactor 10 in a manner superimposed on the current I of the load 6, so that the current _IL1 of the resonance reactor 10 continuously increases in a sinusoidal manner as shown in FIG. Go (I _L1 ). On the other hand, the voltage V _C2 across the resonance capacitor 14 drops in a cosine wave as shown in FIG.

As shown in FIG. 7 (C), when the current I _L1 of the resonance reactor 10 reaches a substantially maximum value at t ₂ , that is, the sum of the current I of the load 6 and the maximum value I _p of the resonance current, The voltage at both ends of the resonance reactor 10 becomes 0V. At this time, the circulating current diode 13 becomes forward-biased, and the current I _L1 of the resonance reactor 10 becomes
0, a circulating current continues to flow on the path of the circulating current diode 13 and the main transistor 2. At the same time, the voltage V _C2 across the resonance capacitor 14 becomes 0 V as shown in FIG.

As shown in FIG. 7A, at t ₃ , 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 When the state changes from the on state to the off state, the current I of the load 6 flows through the resonance capacitor 14. At this time, the current I _L1 of the resonance reactor 10 flows as a resonance current through the path of the resonance reactor 10, the resonance capacitor 8, and the first auxiliary reflux diode 11, and charges the resonance capacitor 8. Therefore, FIG.
As shown in (F) and (D), the voltage V of the resonance capacitor 8 is
_{Since both C1} and the voltage V _C2 of the resonance capacitor 14 gradually rise from 0 V, zero voltage switching is performed when the main transistor 2 is turned off.

[0028] As shown in FIG. 7 (F) and (D), the sum of the voltage V _C1 of the resonance capacitor 8 and the voltage V _C2 of the resonance capacitor 14 becomes equal to the voltage E of the DC power source 1 in the t ₄ The current _IL1 of the resonance reactor 10 gradually decreases to zero as shown in FIG. When the current I _L1 of the resonant reactor 10 becomes zero at t _5,
Resonance capacitor 8 and 14 by the current I of load 6 gradually is charged and discharged, the voltage V _C2 of the resonance capacitor 14 in t ₆ is equal to the voltage E of the DC power source 1 as shown in FIG. 7 (D), The voltage of the main reflux diode 3 becomes 0V. At this time, flow current I load 6 through the main wheeling diode 3 as shown in FIG. _{7 (B) (I D)} , t 6 after the current through the main wheeling diode 3 and the smoothing reactor 4 to the load 6 I flows.

As described above, in the embodiment of the present invention shown in FIG. 6, zero current or zero voltage switching is achieved when the main transistor 2 is turned on and off, so that the switching loss of the main transistor 2 can be reduced. it can. In the circuit of the embodiment shown in FIG. 6, since the step-down converter circuit can be constituted only by the main transistor 2, the amount of heat generated due to switching loss can be suppressed and the number of parts can be reduced as compared with the circuits shown in FIGS. Can be reduced.

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 a main switching element has been described. However, other switching elements such as an FET (field effect transistor) and an SCR (reverse blocking three-terminal thyristor) may be used. Good. In particular, 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 elements are not limited to the same kind of combination. Further, the resonance capacitor 14 in the circuit of FIG. 6 may be omitted.

[0031]

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-down converter. The power loss at the time of the on / off operation of the switching element of the circuit, that is, the switching loss in the step-down converter circuit can be reduced. Further, surge voltage, surge current, and noise during switching operation of the switching element of the step-down 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. In addition, it is possible to further reduce the switching loss in the step-down converter circuit.

[Brief description of the drawings]

FIG. 1 is an electric circuit diagram showing a related art of a step-down 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 modification 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 modification of the circuit of FIG. 1;

FIG. 6 is an electric circuit diagram showing an embodiment of a step-down DC-DC converter according to the present invention.

FIG. 7 is a waveform chart showing voltages and currents of respective parts of the circuit of FIG.

FIG. 8 is an electric circuit diagram showing a conventional step-down DC-DC converter.

9 is a waveform chart showing an overlapping portion between the switching voltage waveform and the switching current waveform of the circuit of FIG.

[Explanation of symbols]

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

Continuation of front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H02M 3/155

Claims (2)

(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 a main switching element connected between the other main terminal of the main switching element and the other end of the DC power supply. A rectifying element for main circulation, a series circuit of a smoothing reactor and a smoothing capacitor connected in parallel with the rectifying element for main circulation, and a load connected in parallel with the smoothing capacitor. In a step-down DC-DC converter that supplies a DC output of a voltage lower than the voltage of the DC power supply to the load by performing off control, inserted between the other main terminal of the main switching element and the smoothing reactor And a first and a second connected between the connection point of the main reactor and the other end of the DC power supply. A series circuit of an auxiliary reflux rectifying element, a resonance capacitor connected between a connection point of the series circuit and a connection point of the resonance reactor and the smoothing reactor, and one main terminal of the main switching element. A rectifying element for circulating current connected between a connection point of the resonance reactor and the smoothing reactor, and a control circuit for applying a main control pulse signal to a control terminal of the main switching element. Step-down DC-DC converter. 2. The step-down DC according to claim 1, wherein another circulating capacitor is connected in parallel with the circulating current rectifying element.
-DC converter.