JPH0833324A - Dc/dc converter - Google Patents

Abstract

PURPOSE:To reduce the switching loss and generation of heat by increasing the switching speed of a main switching element. CONSTITUTION:A quick off control circuit OC comprises a second switching element Q2 and a capacitor C3. A second switching element Q2 is inserted between the DC input side and the control electrode of a first switching element Q1 for turning the DC input on/off based on a control signal (a) received from the control circuit CC. The capacitor C3 is connected between the control electrode of the second switching element Q2 and the output side of the first switching element Q1. When the first switching element Q1 begins to turn off, a charging current flows from the capacitor C3 to the control electrode of the second switching element Q2 in response to the voltage drop on the output side. Consequently, the second switching element Q2 is turned on instantaneously thus turning the first switching element Q1 off forcibly.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC-DC converter for chopping a DC input by a pulse control method and converting it into DC voltages of different levels.

[0002]

2. Description of the Related Art Conventionally, when converting an input DC voltage to obtain a DC voltage having a desired voltage value, as shown in FIG.
A switching type DC-DC converter is used.
In the same drawing, for example, a step-down type circuit configuration for stepping down an input DC voltage of 20 V to a DC voltage of 5 V is illustrated, but there is also a polarity inversion type that inverts the polarity of the DC input voltage and outputs it.

The DC-DC converter of FIG. 6 will be described. The DC input voltage Vi is applied to the PNP transistor Q1 which is the main switching element after smoothing the pulsating current component by the electrolytic capacitor (smoothing capacitor) C1. The transistor Q1 receives the control signal a from the control circuit CC, repeats on / off, and chops the DC input voltage Vi. The energy stored in the choke L when the transistor Q1 is on is released through the diode D when the transistor Q1 is off, and is smoothed by the electrolytic capacitor (smoothing capacitor) C2 that forms a smoothing filter together with the choke L. , Its DC output voltage Vo is supplied to a load (not shown).

The control circuit CC outputs a control signal a composed of a duty pulse. That is, the control signal a
Is adjusted such that the pulse width increases as the output voltage Vo decreases and the pulse width decreases as the output voltage Vo increases, and the DC output voltage Vo is feedback-controlled by this control signal a. As a result, the DC output voltage Vo is controlled to have a constant value regardless of the fluctuation of the input DC voltage Vi.

[0005]

By the way, in the above switching type DC-DC converter, in order to further improve the efficiency, the switching speed of the transistor Q1 is increased to reduce the switching loss as much as possible. However, it is required to reduce heat generation. Therefore, conventionally, the base resistance R2 of the transistor Q1 is set to an appropriate constant that does not cause overdrive or underdrive, and the resistance value r2 of the base resistance R2 and the resistance value of the base-emitter resistance R1 of the transistor Q1 are set. The ratio r1 / r2 with r1 is made as small as possible to shorten the time required for turning off the transistor Q1.

However, in the efficiency improving method using the resistors R1 and R2 described above, even if the transistor Q1 starts to turn off, since the electric charge is accumulated in the base of the transistor Q1, the transistor Q1 does not turn off completely immediately. As a result of the current IC continuing to flow, the timing at which the transistor Q1 is actually turned off tends to be delayed with respect to the pulse of the control signal a. Therefore, in the section indicated by t in FIG. 7, the emitter-collector voltage VEC of the transistor Q1 and the collector current IC overlap without zero crossing, and switching loss occurs. As a result, the amount of heat generated by the transistor Q1 increases, the reliability of the transistor Q1 decreases, and other circuit components are adversely affected and their reliability decreases. Further, there is a drawback that a relatively large heat sink must be provided for heat dissipation.

Therefore, an object of the present invention is to provide a DC-DC converter capable of increasing the switching speed of the main switching element to reduce the switching loss and suppressing the heat generation to improve the reliability. To do.

[0008]

In order to achieve the above object, a DC-DC converter according to the present invention controls an on / off control of an inductance element for storing and releasing electric energy and a DC input. And a control circuit for applying a control signal to the control pole of the first switching element, and further controlling the DC input side of the first switching element. A rapid-off control circuit having a second switching element connected between the pole and a capacitor connected between the control pole of the second switching element and the output side of the first switching element. This quick-off control circuit responds to the voltage drop on the output side when the first switching element starts to turn off, and the second switching element is provided. By passing a charging current of the capacitor to the control electrode of the element, the second switching element is turned on so that to rapidly turn off the first switching element.

[0009]

When the first switching element starts to turn off by the control signal of the control circuit, the potential on the output side of the first switching element drops. When the potential on the output side of the first switching element is slightly lower than the potential on the DC input side, the second switching element Q2 moves from the DC input side to the second switching element Q2.
A charging current flows through the control pole of 1 to charge the capacitor, and the second switching element is momentarily turned on to forcibly turn off the first switching element. Therefore, since the first switching element is turned off immediately in synchronization with the control signal, the switching loss is almost eliminated and the heat generation can be significantly reduced.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows an electric circuit diagram of an embodiment of a DC-DC converter according to the present invention. In the figure, the same or equivalent parts as those in FIG. 6 are designated by the same reference numerals. 6 is different from FIG. 6 in that a first transistor (an example of a first switching element) that chops and supplies a DC input to a choke L (an example of an inductance element that stores and discharges electric energy), The configuration is such that a quick-off control circuit OC is attached. The quick-off control circuit OC includes a second transistor (of the second switching element) connected between the emitter (DC input side) and the base (control pole) of the first transistor Q1 which is the main switching element. Example) Q2, diode D2 and resistor R connected between the emitter (DC input side) and base (control pole) of this second transistor Q2
3 parallel circuit, a capacitor C3 and a resistor R4 connected between the base of the second transistor Q2 and the collector (output side) of the first transistor Q1.

FIG. 2 shows a modification in which the connection of the same components as in FIG. That is,
The DC-DC converter of FIG. 2 has a first transistor Q1.
When the energy stored in the choke L is output through the diode D connected in the direction opposite to that in FIG. 1 when the switch is turned on, an output voltage having a polarity opposite to the input voltage is generated at the output terminal. There is.

Next, the operation of the above embodiment will be described with reference to FIG. 1 and 2 both show the same operation by connecting the quick-off control circuit OC in the same manner, and therefore, the operation of FIG. 1 will be described here as a representative. The control circuit CC detects the DC output voltage V0 and, as shown in FIG. 3 (a), the control signal a in which the pulse width of the duty pulse generated in a constant cycle changes according to the fluctuation of the DC output voltage VO. Is output. The first transistor Q1 receives the control signal a as a base, and
As shown in (b), the ON / OFF operation is repeated to change the emitter-collector potential VEC into a rectangular shape.

The first transistor Q1 is turned on when the control signal a is at a low level and raises the potential at the point B on the output side to the same level as the potential at the point A on the input side. The potential at the point has a phase of 18 with respect to the control signal a.
The waveform is different by 0 °. The present invention utilizes the point that the potential at the point B has a waveform having a phase difference of 180 ° with respect to the control signal a, and uses the quick off control circuit OC to force the first transistor Q1 to synchronize with the control signal a. To turn it off.

That is, at time t1 in FIG. 3, the control signal a rises to a high level and the first transistor Q1 starts to turn off. At this time, the terminal voltage VC3 of the capacitor C3
Is still 0V as shown in FIG. Subsequently, at the moment when the potential at the point B drops by a value smaller than the potential at the point A, for example, 1 V, the capacitor C3 is charged with the voltage shown in FIG.
The current IBQ2 shown in (f) flows and the capacitor C3
It is charged as shown in FIG. This current IBQ2 is the base of the second switching element Q2 and the resistor R3.
By flowing the second flow, as shown in FIG.
Transistor Q2 turns on. At this time, the current IBQ2
Is suppressed to an appropriate level by the resistor R4.

When the second transistor Q2 is turned on, the base and emitter of the first transistor Q1 are short-circuited, and the first transistor Q1 is forcibly turned off instantly as shown by the solid line in FIG. 3 (b). As indicated by the solid line in 3 (c), the collector current IC1 of the first transistor Q1 is also immediately cut off. After that, since the terminal voltage VC3 of the capacitor C3 rises, the second transistor Q2 is turned off to prepare for the next operation.

When the input voltage Vi is high, the voltage charged in the capacitor C3 becomes the first switching element Q.
When 1 is turned on, the emitter of the second switching element Q2
Since the base is reversely biased, when the input voltage Vi is high, the rating of the emitter-base voltage may be exceeded. As a countermeasure against this, a reverse bias that exceeds the rating is not applied between the base and emitter of the second switching element Q2 due to the forward voltage drop of the diode D2.

3 (b) and 3 (c), the waveform of the conventional device in FIG. 6 is shown by a two-dot chain line for comparison. Conventionally, as shown in FIG. 3B, since the first signal Q1 is gradually turned off after the control signal a becomes high level, as shown in FIG. The collector current IC1 also continues to flow slightly, causing a large switching loss as shown in FIG.

On the other hand, as described above, when the control signal a becomes high level and the first transistor Q1 starts to turn off, at the moment when the potential at the point B becomes slightly lower than the potential at the point A. , The second transistor Q2 is turned on and the first transistor Q1 is forcibly turned off, so that the switching speed of the first transistor Q1 is accelerated so as to be turned off in synchronization with the high level of the control signal a. as a result,
At the moment when the first transistor Q1 starts to turn off and the emitter-collector voltage VEC starts to rise (the potential at the point B falls), the collector current IC1 drops to the zero level. Therefore, since the emitter-collector voltage VEC and the collector current of the first transistor Q1, which is the main switching element, have very little overlap, the switching loss is almost eliminated and the heat generation can be remarkably reduced.

FIG. 4 shows a characteristic curve obtained by actually measuring the temperature of the first transistor Q1 with the rear fin. The measurement conditions at this time were that the direct current input voltage Vi was set to 26.4V, the direct current output voltage Vo was set to 5V, and the output current was set to 1.0A without providing a heat sink on the first transistor Q1. The broken line curve indicates the ambient temperature, and the alternate long and short dash line indicates, for comparison, a characteristic curve obtained by actually measuring the conventional device of FIG. 6 under the same conditions as above.

As is clear from the characteristic curve of FIG. 5,
The temperature of the first transistor Q1 is maintained so as not to rise above 70 ° C. without the provision of a heat sink, indicating that the heat generation is extremely small. for that reason,
There is no problem even if the heat sink is downsized or no heat sink is provided. In addition, since the first transistor Q1 generates less heat, it does not adversely affect the reliability of other circuit components in the device. Further, since the first transistor Q1 generates less heat, it is possible to reduce the size of the package.

As described above, since the switching loss of the first transistor Q1 is reduced and the heat generation is reduced, the efficiency of the device is significantly improved. FIG. 5 is a characteristic diagram of the conversion efficiency with respect to the DC input voltage Vi when actually measured under the measurement conditions of the DC output voltage Vo of 5 V and the output current of 1 A in the above embodiment. For comparison, a characteristic curve measured under the same measurement conditions as above with a resistance of 100 ohms set as the emitter-base resistance R1 of the first transistor Q1 in the device of FIG. 6 is shown in FIG. It is indicated by a dashed line. As is clear from the comparison between the two characteristic curves, in the device of the above-described embodiment, high efficiency is obtained with a significant reduction in switching loss, and as the DC input voltage Vi becomes higher, the difference from the conventional one, that is, the efficiency becomes higher. The extent of improvement is large.

In the above embodiment, the PNP transistor Q is used.
Although the case where 1 and Q2 are used as the switching elements is illustrated, it goes without saying that the present invention can be applied to the case where FETs or the like are used as the switching elements. Further, a transformer may be used instead of the choke L, that is, an inductance element that stores and releases electric energy.

[0023]

As described above, according to the DC-DC converter of the present invention, the main first switching element for ON / OFF controlling the DC input by the quick OFF control circuit and applying it to the choke, Since it can be turned off immediately in synchronism with the control signal, switching loss is almost eliminated, conversion efficiency is improved, heat generation is significantly suppressed, and other circuit components are not adversely affected. The reliability of can be improved.

[Brief description of drawings]

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

FIG. 2 is an electric circuit diagram showing a modified example of FIG.

3 (a) to 3 (g) are timing charts showing signal waveforms of respective parts of FIG. 1 or FIG.

FIG. 4 is a characteristic diagram in which the relationship between the temperature rise of the main switching element and the above time is actually measured.

FIG. 5 is a characteristic diagram obtained by actually measuring the relationship between the DC input voltage and the efficiency.

FIG. 6 is an electric circuit diagram showing an example of a conventional DC-DC converter.

7 is a waveform diagram of voltage and current of the switching element of FIG.

[Explanation of symbols]

Q1 ... 1st transistor (1st switching element), L ... Choke, CC ... Control circuit, C2 ... Smoothing capacitor, D2 ... Diode, OC ... Rapid off control circuit,
Q2 ... Second transistor (second switching element), R3, R4 ... Resistor, C3 ... Capacitor.

Claims (1)

[Claims] 1. An inductance element for storing and releasing electric energy, a first switching element for controlling on / off of a direct current input and applying it to the inductance element, and a control pole of the first switching element. A DC-DC converter having a control circuit for applying a signal, comprising: a second switching element connected between a DC input side of the first switching element and a control pole; and a second switching element of the second switching element. A capacitor connected between the control pole and the output side of the first switching element, and when the first switching element starts to turn off, in response to the voltage drop on the output side, The second switching element is turned on by flowing the charging current of the capacitor to the control electrode of the second switching element to turn on the first switching element. Rapidly DC characterized by comprising a rapid off control circuit for turning off - DC converter.