JPH0654522A - Dc-dc converter - Google Patents

Abstract

PURPOSE:To get a DC-DC converter in which stable output voltage can be gotten and which is excellent in power efficiency. CONSTITUTION:A series circuit consisting of a choke coil L0 and a transistor Q1 is connected in parallel to a DC power source 1, and a series circuit consisting of a diode D2 and a capacitor C1 is connected in parallel to the transistor Q1. The transistor Q1 is switched by a driving circuit 2. A diode D1 is connected in series to the transistor Q1, and a parallel circuit consisting of a transistor Q2 and a diode D3 is interposed between the power source 1 and a choke coil L0. The driving circuit 2 is turned on synchronously with the transistor Q1, and a load current is detected using the a current detector 21 after turning off of the transistor Q1, whereby the inversion of the polarity of the voltage generated in the choke coil L0 is detected, and the transistor Q2 is turned on synchronously with this inversion.

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, and more particularly to improving the power efficiency of a DC-DC converter.

[0002]

2. Description of the Related Art Conventionally, for example, as shown in FIG. 3, an input voltage Vi from a DC power source 1 is boosted by a choke coil L0 by switching a transistor Q1.
There is known a step-up DC-DC converter in which the boosted voltage is smoothed by the capacitor C1 and applied to the load 3.

The operation of the conventional DC-DC converter will be described with reference to the timing chart shown in FIG. When the DC power supply 1 is connected, the drive circuit 20 outputs a pulse train of a predetermined cycle to the base of the transistor Q1.

At time t1, the pulse from the drive circuit 20 causes the base voltage VB1 of the transistor Q1 to go high, turning on the transistor Q1 and input current Ii.
Starts to flow from the DC power supply 1 through the capacitor C0 and then to the transistor Q1 through the choke coil L0. Then, during the ON period of the transistor Q1, this input current I
Energy is stored in the choke coil L0 by i.

Thereafter, at time t2, the base voltage VB1 of the transistor Q1 is inverted from the high level to the low level, and the transistor Q1 is turned off. As a result, a voltage (hereinafter, referred to as a back electromotive force) Vr at which the collector side of the transistor Q1 becomes a positive electrode across the choke coil L0.
Is induced, and the collector voltage VCE of the transistor Q1 becomes a boosted voltage (Vi + Vr) obtained by superimposing the electromotive voltage Vr on the input voltage Vi of the DC power supply 1. The boosted voltage (Vi + Vr) is smoothed by the capacitor C1 and applied to the load 3 as the output voltage VO higher than the input voltage Vi.

[0006]

By the way, the above DC-
In the DC converter, the current IL is supplied to the load 3 by the energy accumulated in the choke coil L0. At this time, a voltage (hereinafter, referred to as counter electromotive voltage) Vr 'that prevents the current from flowing out to the choke coil L0 is generated. Induced.

On the other hand, the current IL from the choke coil L0
Decreases with a gradient according to the inductance of the choke coil L0, and the polarity of the induced voltage of the choke coil L0 is inverted at time t5 when the value reaches the predetermined value I0. Then, the collector voltage VCE of the transistor Q1 becomes a voltage (Vi-Vr ') lower than the input voltage Vi by the counter electromotive voltage Vr'.

Since the counter electromotive voltage Vr 'increases as the load current increases, the output voltage VO decreases as the load increases and it becomes impossible to obtain a stable output voltage VO. Also, the power efficiency is poor.

The present invention solves the above-mentioned problems, and a stable output voltage can be obtained and the power source efficiency is high.
An object is to provide a C-DC converter.

[0010]

In order to achieve the above object, according to the present invention, a series circuit composed of a choke coil and a first switching element is connected in parallel to a DC power source, and a first circuit is provided in the first switching element. DC-DC for generating a boosted voltage across the capacitor by switching the first switching element by connecting in series a series circuit composed of the rectifying element and the capacitor.
In the converter, a second rectifying element that causes a current to flow in a direction opposite to the direction of the current flowing through the first switching element is connected in parallel to the first switching element, and between the DC power source and the choke coil, A parallel circuit including a second switching element for flowing a current to the choke coil and a third rectifying element for flowing a current in a direction opposite to the direction of the current flowing through the second switching element is interposed, and the second circuit is provided. The switching element is turned on in synchronism with the turning on of the first switching element, the polarity of the voltage generated in the choke coil after the turning off of the first switching element is detected, and the second polarity is synchronized with the inversion of this polarity. And a control means for turning off the switching element.

Further, in the present invention, the capacitor is connected in series to the third rectifying element.

Further, in claim 3, the coil is connected in series to the third rectifying element.

[0013]

According to the DC-DC converter having the above structure, the second switching element is turned on in synchronization with the turning on of the first switching element, the energy is stored in the choke coil, and the first switching element is turned off. A current generated by the energy of the choke coil is supplied to the capacitor through the first rectifying element. At this time, the polarity of the voltage generated in the choke coil is detected, and the second switching element is turned off in synchronization with the reversal of the polarity, so that the current relating to the counter electromotive voltage induced in the choke coil is changed to the second current. , Is fed back to the DC power supply through the third rectifying element.

Further, according to the DC-DC converter of the second aspect, the capacitor suppresses the generation of a surge-like voltage when returning to the DC power source through the third rectifying element.

Further, according to the DC-DC converter of the third aspect, the coil suppresses the generation of surge-like voltage when returning to the DC power source through the third rectifying element, and the second switching. It is possible to prevent an abrupt current flow in the loop of the second switching element and the third rectifying element at the moment when the element is turned on.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an embodiment of a DC-DC converter according to the present invention. In this DC-DC converter, the capacitor C0 is connected in parallel to the DC power supply 1,
Further, a series circuit of the transistor Q2, the choke coil L0 and the transistor Q1 is connected in parallel.

A diode D is provided in the transistor Q1.
2 and a capacitor C1 are connected in parallel, and a diode D1 having a cathode connected to the collector of the transistor Q1 is connected in parallel. A series circuit (feedback circuit) 4 including a diode D3 having an anode connected to the emitter of the transistor Q2, a coil L1 and a capacitor C2 is connected in parallel to the transistor Q2.

The drive circuit 2 is connected to the bases of the transistors Q1 and Q2. This drive circuit 2 is
A pulse train having a predetermined cycle set in advance is applied to the transistor Q1.
Is output to the base of the transistor Q2 to switch it, and the transistor Q2 is turned on in synchronization with the turning on of the transistor Q1. Further, the drive circuit 2 is
The current detector 21 detects the load current, and when the load current drops to a preset current value I0, the transistor Q2 is turned off. Current detector 2
Reference numeral 1 is composed of a resistor, a current transformer, or the like, and detects the load current.

The transistor Q1 is for flowing the input current Ii from the positive electrode of the DC power supply 1 to the negative electrode of the DC power supply 1 through the transistor Q2 and the choke coil L0 when the transistor Q2 is turned on in synchronization. As a result, energy is stored in the choke coil L0.

The choke coil L0 stores energy during the ON period of the transistors Q1 and Q2 described above, and the boosted voltage (Vi +) obtained by superimposing the voltage (electromotive voltage) Vr of the choke coil L0 on the input voltage Vi during the OFF period of the transistor Q1.
Vr) is applied to the load 3 through the diode D2 and the accumulated energy is supplied to the load 3 as a current.

The diode D1 includes transistors Q1 and Q.
When both 2 are turned off, the current related to the voltage (back electromotive force) Vr 'induced in the choke coil L0 is fed back to the DC power supply 1 and the capacitor C0 via the feedback circuit 4.

The feedback circuit 4 feeds back the current related to the back electromotive force Vr 'to the DC power source 1 and the capacitor C0 when the transistor Q2 is turned off. That is, when both the transistors Q1 and Q2 are turned off, the voltage on the power source side of the choke coil L0 rises due to the counter electromotive voltage Vr ', and thereby the diode D3, the coil L1, the capacitor C2, the DC power source 1 from the choke coil L0.
A feedback current flows in the loop of the diode D1.

The coil L1 and the capacitor C2 prevent surge voltage noise from occurring when the feedback current related to the counter electromotive voltage Vr 'is fed back to the DC power supply 1. The coil L1 is connected to the transistor Q2.
This prevents the electric charge of the capacitor C2 from being rapidly discharged by the loop of the transistor Q2 and the diode D3 at the moment when the transistor Q2 is turned on, which causes deterioration and damage of the transistor Q2 and the diode D3.

The diode D2 prevents the reverse flow of current from the capacitor C1 and the load 3 to the choke coil L0 side. The capacitor C0 smoothes the input voltage Vi. The capacitor C1 smoothes the pulsed output voltage VO that is generated by the repeated switching of the transistor Q1 and is output to the load 3 side via the diode D2, and supplies a stable DC voltage to the load 3.

Next, the operation of the DC-DC converter having the above configuration will be described with reference to the timing chart shown in FIG. At time t1, the base voltages VB1 and VB2 of the transistors Q1 and Q2 are both brought to a high level by the signal from the drive circuit 2 and both transistors Q1 and Q2 are turned on, so that the input current Ii from the DC power supply 1 is transferred to the transistor Q1.
2. Flow through the choke coil L0 and the transistor Q1, and energy is stored in the choke coil L0.

Thereafter, when the base voltage VB1 of the transistor Q1 is inverted from the high level to the low level at time t2, the transistor Q1 is turned off and the choke coil L
An electromotive voltage Vr having a positive electrode on the collector side of the transistor Q1 is generated at both ends of 0. Therefore, the collector voltage VCE of the transistor Q1 becomes a boosted voltage (Vi + Vr) in which the electromotive voltage Vr is superimposed on the input voltage Vi, and the boosted voltage (Vi + Vr) passes through the diode D2 and the capacitor C.
1 and smoothed by the capacitor C1 to load 3
To the output voltage VO higher than the input voltage Vi. Then, the output voltage VO supplies current to the load 3.

From the choke coil L0 at this time to the load 3
The current IL to the coil decreases with a gradient according to the inductance of the choke coil L0, and reaches the current value I0 at time t3, for example. This current IL is detected by the current detection unit 21, and when the current IL drops to the current value I0 at time t3, the drive circuit 2 inverts the base voltage VB2 of the transistor Q2 from the high level to the low level, and the transistor Q2. Turns off.

At this time, the counter electromotive voltage Vr 'induced in the choke coil L0 causes the voltage on the DC power supply 1 side of the choke coil L0 to be the counter electromotive voltage V rather than the input voltage Vi.
The feedback current Ii0 is increased by r'and the choke coil L0
Flow through the feedback circuit 4, the DC power supply 1 (capacitor C0) and the diode D1, and the energy related to the back electromotive force Vr 'is fed back to the power supply side.

After that, at time t4, the drive circuit 2 causes the base voltages VB1 and VB2 of the transistors Q1 and Q2 to become high level, the current IL flows through the choke coil L0 again, and energy is stored in the choke coil L0. Hereinafter, the same operation is repeated.

As described above, since the energy related to the counter electromotive voltage Vr 'of the choke coil L0 is fed back to the power source side as the feedback current Ii0, as a result, the average value I1 of the input current Ii is obtained.
Can be reduced, the DC power supply 1 can be effectively used, and the power supply efficiency is improved. Further, the counter electromotive voltage Vr ′ increases as the load 3 becomes heavier, but the energy related to the counter electromotive voltage Vr ′ is fed back to the power source side.
The influence of the counter electromotive voltage Vr 'on the output voltage VO is reduced, and a stable output voltage VO can be obtained.

As the current value I0 when the transistor Q2 is turned off, the load current (normally the average value of the load current) when the polarity of the induced voltage generated in the choke coil L0 is reversed is selected. ing.

[0032]

According to the present invention, the polarity of the voltage of the choke coil is detected, and when the polarity of the voltage of the choke coil is inverted by the counter electromotive voltage, the second switching element is turned off in synchronization with this inversion, Since the current related to the counter electromotive voltage generated in the choke coil is fed back to the DC power supply via the second and third rectifying elements, it is possible to reduce the average value of the input current and improve the power supply efficiency. Further, the influence of the counter electromotive voltage on the output voltage is reduced, and a stable output voltage can be obtained.

Further, since the capacitor is connected in series to the third rectifying element, generation of a surge voltage generated when the current related to the counter electromotive voltage is fed back to the DC power supply is suppressed,
It is possible to prevent leakage to the outside and generation of noise.

Furthermore, by connecting the coil in series to the third rectifying element, generation of a surge voltage generated when the current related to the counter electromotive voltage is fed back to the DC power supply is suppressed, and the second switching element is also provided. It is possible to prevent an abrupt current from flowing in the loop of the second switching element and the third rectifying element at the moment when is turned on, the noise level is reduced, and the second switching element and the third rectifying element are It is possible to prevent deterioration and damage of the diode.

[Brief description of drawings]

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

FIG. 2 is a timing chart showing the operation of the DC-DC converter of the present invention.

FIG. 3 is a circuit diagram showing a conventional DC-DC converter.

FIG. 4 is a timing chart showing the operation of a conventional DC-DC converter.

[Explanation of symbols]

 1 DC power supply 2 Drive circuit 3 Load 21 Current detector C0, C1, C2 Capacitor D1, D2, D3 Diode (rectifier element) L0 Choke coil L1 coil Q1, Q2 Transistor (switching element)

Claims (3)

[Claims] 1. A direct current power source is connected in parallel with a series circuit including a choke coil and a first switching element, and the first switching element is connected in parallel with a series circuit including a first rectifying element and a capacitor. DC-DC for generating a boosted voltage across the capacitor by switching the first switching element.
In the converter, a second rectifying element that causes a current to flow in a direction opposite to the direction of the current flowing through the first switching element is connected in parallel to the first switching element, and between the DC power source and the choke coil, A parallel circuit composed of a second switching element for flowing a current to the choke coil and a third rectifying element for flowing a current in a direction opposite to the direction of the current flowing through the second switching element is interposed, and the second circuit is provided. The switching element is turned on in synchronism with the turning on of the first switching element, the polarity of the voltage generated in the choke coil after the turning off of the first switching element is detected, and the second polarity is synchronized with the inversion of this polarity. A DC-DC converter comprising control means for turning off the switching element. 2. The DC-DC converter according to claim 1, wherein a capacitor is connected in series to the third rectifying element. 3. The DC-DC converter according to claim 1 or 2, wherein a coil is connected in series to the third rectifying element.