JP3033085B2 - 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. 6 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. 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-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 includes a DC power supply, a main switching element having one main terminal connected to one end of the DC power supply, and a main reflux element connected between the other main terminal of the main switching element and the other end of the DC power supply. Rectifying element, a series circuit of a smoothing reactor and a smoothing capacitor connected in parallel with the main reflux element, and a load connected in parallel with the smoothing capacitor. A DC output of a voltage lower than the voltage of the DC power supply is supplied to the load. In this step-down DC-DC converter, a series circuit of an auxiliary switching element and a resonance reactor connected in parallel with a main switching element, and a first circuit connected between a connection point of the series circuit and the other end of the DC power supply. And a resonance capacitor connected between a connection point of the series circuit of the first and second auxiliary return rectifiers and the other main terminal of the main switching element. And a rectifying element for circulating current, which is composed of a rectifying element formed integrally with the main switching element or an independent rectifying element and connected in parallel with the main switching element, and applies a main control pulse signal to a control terminal of the main switching element. And a control circuit for applying an auxiliary control pulse signal to a control terminal of the auxiliary switching element before the operation. Another resonance capacitor may be connected in parallel with the main switching element. Also,
A supplementary charge resistor or a supplementary charge switch that is turned on during the ON period of the main switching element is provided between the connection point of the series circuit of the first and second auxiliary reflux rectifying elements and the other end of the DC power supply. You may connect.

[0006]

When the main switching element is turned off when a current flows to the load side while the main switching element is turned on and the resonance capacitor is charged to the power supply voltage, the current flowing through the main switching element is immediately turned off. The current is switched to the current flowing through the resonance capacitor, and the resonance capacitor is gradually discharged. At this time, the voltage across the main switching element gradually rises from 0V. This achieves zero voltage switching (ZVS) when the main switching element is turned off, so that switching loss when the main switching element is turned off can be reduced. Before applying the main control pulse signal to the control terminal of the main switching element and turning on the main switching element, the auxiliary control pulse signal is applied to the control terminal of the auxiliary switching element to turn on the auxiliary switching element. Then, the power supply voltage is applied to the resonance reactor while the main circulating rectifier is conducting, and the current of the resonance reactor increases linearly from zero. This achieves zero current switching (ZCS) when the auxiliary switching element is turned on, so that switching loss when the auxiliary switching element is turned on can be reduced. As the current of the resonance reactor increases, the current of the main circulation rectifier decreases linearly. When the current of the resonance reactor becomes equal to the load current, the main circulation rectifier is cut off. At this time, when the main switching element is turned on, the voltage of the main switching element immediately drops to 0V. This achieves zero voltage switching when the main switching element is turned on, so that switching loss when the main switching element is turned on can be reduced.
Thereafter, when the auxiliary switching element is turned off with a slight delay, a resonance current flows through the resonance reactor and the resonance capacitor, and the voltage of the resonance capacitor rises in a sine wave form from 0V. When this voltage reaches the maximum value, the resonance current becomes zero. The current of the smoothing reactor flows through the main switching element when the auxiliary switching element is turned off. This achieves zero voltage switching when the auxiliary switching element is turned off, so that switching loss when the auxiliary switching element is turned off can be reduced. As described above, the switching loss at the time of the ON / OFF operation of the main switching element and the auxiliary switching element can be reduced. In addition, the spike-like surge voltage and current generated when the main switching element and the auxiliary switching element are turned on and off are absorbed by the resonance capacitor and the resonance reactor. Surge voltage and current can be reduced. If another resonance capacitor is connected in parallel with the main switching element,
Zero voltage switching at the time of turn-on of the main switching element becomes more reliable, and it is possible to further reduce switching loss.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a step-down 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-down DC of this embodiment
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. A series circuit of first and second auxiliary reflux diodes (auxiliary reflux rectifiers) 11 and 12 connected between the other end and a serial circuit of first and second auxiliary reflux diodes 11 and 12 The resonance capacitor 8 connected between the connection point of the main transistor 2 and the emitter terminal of the main transistor 2 and the circulating current diode (circulating current rectifier) 13 connected in parallel with the main transistor 2 are connected to the circuit shown in FIG. It has been added to. Also,
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, the main transistor 2 and the auxiliary transistor 9
As a junction type power transistor.

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.

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 0 and the resonance capacitor 8 resonate, the current I _L1 of the resonance reactor 10 becomes a resonance current flowing through the path of the resonance capacitor 8, the first auxiliary reflux diode 11, and the resonance reactor 10. 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. Further, the current of the smoothing reactor 4, that is, the current I of the load 6 flows through the main transistor 2 as shown in FIG. 2C ( _ITR1 ). Therefore, when the auxiliary transistor 9 is turned off, the voltage V _C1 across the resonance capacitor 8 is applied.
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 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-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- 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 4H, 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 (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. 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 resonance capacitor 14, the auxiliary transistor 9, and the resonance reactor 1
A resonance current flows through the path of zero. 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 across the resonance capacitor 14
_C2 descends in a cosine waveform 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, 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 (E) _{Since C2} drops in a cosine waveform, zero voltage switching at the time of turning on (t ₄ ) of the main transistor 2 becomes more reliable, and there is an advantage that switching loss can be further reduced.

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 other end of the DC power supply 1. In the circuit of the embodiment 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. Even when the voltage is less than the voltage E of 1, the zero voltage switching at the time of turning on the main transistor 2 is 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-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 an embodiment 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 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-down 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); . . 4. smoothing reactor; . . 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

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H02M 3/155

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 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, an auxiliary switching element and a resonance reactor connected in parallel with the main switching element. A series circuit, a series circuit of first and second auxiliary reflux rectifying elements connected between a connection point of the series circuit and the other end of the DC power supply, A resonance capacitor connected between a connection point of the series circuit of the first and second auxiliary return rectifying elements and the other main terminal of the main switching element; and a rectifying element formed integrally with the main switching element. Or a circulating current rectifying element composed of an independent rectifying element and connected in parallel with the main switching element, and a control terminal of the auxiliary switching element before applying a main control pulse signal to a control terminal of the main switching element. A step-down DC-DC converter, comprising: a control circuit for applying an auxiliary control pulse signal. 2. The step-down DC according to claim 1, wherein another resonance capacitor is connected in parallel with the main switching element.
-DC converter. 3. An on-state during a period during which the auxiliary charging resistor or the main switching element is on, between a connection point of a series circuit of the first and second auxiliary reflux rectifying elements and the other end of the DC power supply. 2. The step-down DC-DC converter according to claim 1, wherein an auxiliary charging switch that is in a state is connected.