JPH07194103A - Dc-dc converter - Google Patents

Abstract

PURPOSE:To reduce the sizes of a transformer and a choke coil by a method wherein a power for the flywheel diode of a first rectifying smoothing circuit is supplied by a second rectfying smoothing circuit. CONSTITUTION:A rectfying smoothing circuit 13 is composed of a rectifier diode 9, a flywheel diode 10, a choke coil 11 and a smoothing capacitor 12 and supplies a power to the flywheel diode 5 of a rectifying smoothing circuit 8. In a conventional example, an output voltage is applied in an off-period. To the contrary, in the circuit of the present invention, a voltage obtained by subtracting a voltage stabilized by the rectifying smoothing circuit 13 from the output voltage is applied. As the voltage-time product of the off-period is reduced, the voltage-time product of an on-period is also reduced. Therefore, a duty ratio can be reduced or a turn ratio can be reduced. With this constitution, a DC-DC converter which facilitates the miniaturization and the noise reduction can be obtained.

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 supplying a regulated DC voltage to electronic equipment such as industrial equipment such as information, communication and OA and consumer equipment such as VTR and TV.

[0002]

2. Description of the Related Art In recent years, battery drive has been increasing along with portable electronic devices, and DC-DC converters have been rapidly used for converting and stabilizing a voltage supplied to an electronic circuit. In addition, as electronic devices become smaller,
There is a demand for miniaturization of the built-in DC-DC converter.

FIG. 3 shows a circuit diagram of a conventional DC-DC converter, which is a so-called feedforward system. In FIG. 3, 1 is a DC voltage source, 2
Is a transformer and has a primary winding 2a and a secondary winding 2b. 3 is a switching means such as a transistor, and 8 is a rectifying / smoothing circuit, which is composed of a rectifying diode 4, a flywheel diode 5, a choke coil 6 and a smoothing capacitor 7. Reference numeral 14 is a control circuit, and 15 is a load.

The operation of this conventional DC-DC converter will be described below. First, when the switching means 3 is turned on, an input voltage is applied from the DC power supply to the primary winding 2a of the transformer 2, and at this time, a voltage according to the winding ratio with the primary winding 2a is applied to the secondary winding 2b. Induced. The induced voltage supplies a current to the load 15 via the rectifying diode 4 and the choke coil 6. On the other hand, when the switching means 3 is off, the energy accumulated in the choke coil 6 is released via the flywheel diode 5 and the load 1
Supply current to 5. Further, the control circuit 14 adjusts the duty ratio of the switching means 3 so that the output voltage becomes constant.

FIG. 4 is a voltage waveform diagram applied to the choke coil 6 when the current is continuous. If the product of the voltage and the time applied to both ends of the choke coil 6 is Son and the product of the voltage and the time applied to the off period is Soff, the duty ratio is controlled so that Son and Soff are equal.

The output voltage supplied to the load is controlled to a value shown in the following equation when the current flowing through the choke coil 6 is continuous.

Vo = (Ns / Np) * Vi * D where Vo is the output voltage, Np is the number of turns of the primary winding 2a,
Ns is the number of turns of the secondary winding 2b, Vi is the input voltage, and D is the duty ratio of the switching means 3.

[0008]

Generally, the duty ratio D is set to about 0.5 at maximum in order to surely reset the transformer 2. Therefore, in the above conventional configuration, as can be seen from the above equation, as the input voltage Vi decreases with respect to the output voltage Vo, the turns ratio (Ns / Np) must be increased in inverse proportion to this. Therefore, the number of turns Ns of the secondary winding 2b must be increased, and the transformer 2 becomes large. In addition, since a voltage proportional to the winding number ratio with the primary winding 2a is induced in the secondary winding 2b, a high voltage is applied to the secondary side circuit. This necessitates a high breakdown voltage rectifier diode 4 and also increases the noise generated by the circuit. Further, when considering load response and the like, the inductance must be set so that the current of the choke coil 6 is continuous. Therefore, if the turns ratio is large, the choke coil 6
Since a high voltage is applied to both ends of the choke coil to increase the current gradient, the inductance must be increased in order to reduce the gradient, and the choke coil 6 becomes large.

Further, there is a flyback system or the like as a DC-DC converter which has a relatively small number of parts and can be miniaturized, but there is a problem that efficiency is deteriorated as compared with the feedforward system.

The present invention solves the above-mentioned conventional problems. In the feed-forward type DC-DC converter, the transformer and choke coil that occupy a large volume are downsized, and the generated voltage is suppressed to reduce the size. It is an object of the present invention to provide a DC-DC converter that can achieve high efficiency and low noise.

[0011]

In order to solve this problem, a DC-DC converter of the present invention forms a closed circuit in which a DC voltage source, a primary winding of a transformer, and a switching means form a closed circuit. First rectifying / smoothing circuit including a rectifying diode, a flywheel diode, a choke coil, and a capacitor for rectifying and smoothing the voltage induced in the secondary winding of the transformer, and the voltage across the capacitor becomes constant. A rectifying and smoothing voltage induced in the secondary winding of the transformer in a circuit for outputting the voltage across the capacitor to a load. A smoothing circuit is provided, and power is supplied from the second rectifying and smoothing circuit to the flywheel diode of the first rectifying and smoothing circuit. It has a configuration that.

[0012]

With this configuration, the duty ratio D is controlled such that the voltage-time product applied to both ends of the choke coil is equal in the on period and the off period. The present invention pays attention to this point, and the voltage generated by the second rectifying / smoothing circuit is supplied to the choke coil during the off period to reduce the voltage applied to both ends of the choke coil, thereby increasing the off period. By utilizing the fact that the duty ratio can be made small, the maximum duty ratio is satisfied even if the turn ratio is lowered and the applied voltage during the ON period is lowered.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows D in an embodiment of the present invention.
It is a circuit diagram of a C-DC converter. In FIG. 1, the same components as those in FIG. 3 are designated by the same reference numerals and the description thereof will be omitted.

In FIG. 1, 13 is a rectifying / smoothing circuit, which is composed of a rectifying diode 9, a flywheel diode 10, a choke coil 11 and a smoothing capacitor 12, and supplies power to the flywheel diode 5 of the rectifying / smoothing circuit 8. There is.

Since the operation of the DC-DC converter configured as described above is the same as that described in the conventional example of FIG. 3, the description thereof will be omitted. Here, the circuit operation on the secondary side will be described.

FIG. 2 shows a voltage waveform applied to the choke coil 6 when the current is continuous. As in the conventional example of FIG. 3, the duty ratio is controlled so that the voltage-time product Son in the off period and the voltage-time product Soff in the off period become equal. In the conventional example of FIG. 3, the output voltage Vo is applied during the off period, but in the present circuit, the voltage (Vo-Ns / Np * D *) obtained by subtracting the voltage stabilized by the rectifying and smoothing circuit 13 from the output voltage.
Vin) is required. Therefore, the voltage-time product Sof in the off period
As f decreases, the voltage-time product Son of the ON period also decreases. This reduces the duty ratio,
Alternatively, the winding ratio can be reduced.

When the voltage stabilized by the rectifying / smoothing circuit 13 is Vx, ideally, the following values are obtained when the current flowing through the choke coil 11 is continuous.

Vx = (Ns / Np) * Vi * D (2) Further, the output voltage Vo ideally has the following values when the currents flowing through the choke coils 6 and 11 are continuous.

Vo = (Ns / Np) * Vi * D * (2-D) (3) where Vo is the output voltage, Np is the number of turns of the primary winding 2a,
Ns is the number of turns of the secondary winding 2b, Vi is the input voltage, and D is the duty ratio of the switching means 3.

From the equation (3), it can be seen that the duty ratio D is smaller than that in the equation (1) of the conventional example.

The output of the rectifying / smoothing circuit 13 is provided with a tap in the middle of the secondary winding 2b or the output of the secondary winding 2b.
A new winding may be added to b and taken out from there.
The degree to which the duty ratio D becomes smaller decreases in the former case and increases in the latter case. In this case, rectifying and smoothing circuit 1
The voltage stabilized from 3 changes according to the number of turns.

[0022]

As described above, according to the present invention, the effect of reducing the duty ratio is utilized, and the maximum duty ratio limited by the reset condition can be satisfied with a small turns ratio. As a result, the number of turns of the secondary winding is reduced, the transformer can be downsized, and the voltage induced in the secondary winding 2b is reduced, so that a low breakdown voltage rectifier diode can be used.
Furthermore, the noise generated by the circuit is also reduced. When considering the load response and the like, the inductance must be set high so that the current of the choke coil is continuous, but in the present invention, the voltage applied to the choke coil is reduced, so The gradient becomes small, the inductance can be set low, and the choke coil can be miniaturized. The second rectifying and smoothing circuit is
The current capacity of the diode and the choke coil is about half of that of the first rectifying and smoothing circuit, and the capacity of the capacitor may be such that it can be stabilized to a certain extent. In the case of a multi-output configuration, the same effect as above can be obtained by supplying power to the flywheel diode of the rectifying / smoothing circuit having a higher output voltage than the rectifying / smoothing circuit having a lower output voltage. The number of parts does not increase at all. At this time, the number of turns of the secondary winding of each output may be set to a value such that each output voltage can be obtained. Due to the above, downsizing and noise reduction are possible D
A C-DC converter can be provided.

[Brief description of drawings]

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

FIG. 2 is a waveform diagram of an applied voltage V1 of a choke coil, which is the main part of the same.

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

FIG. 4 is a waveform diagram of an applied voltage V1 of a choke coil which is a main part of the same.

[Explanation of symbols]

 1 DC voltage source 2 Transformer 2a Primary winding 2b Secondary winding 3 Switching means 4,9 Rectifying diode 5,10 Flywheel diode 6,11 Choke coil 7,12 Smoothing capacitor 8,13 Rectifying and smoothing circuit 14 Control circuit

Claims (1)

[Claims] 1. A closed circuit in which a DC voltage source, a primary winding of a transformer, and switching means are connected in series, and a rectifying and smoothing voltage induced in a secondary winding of the transformer when the switching means is on. A first rectifying / smoothing circuit including a diode, a flywheel diode, a choke coil, and a capacitor; and a control unit that controls the on / off ratio of the switching unit so that the voltage across the capacitor is constant, and the both ends of the capacitor are provided. A second rectifying and smoothing circuit that rectifies and smoothes the voltage induced in the secondary winding of the transformer is provided in the circuit that outputs the voltage to the load, and the second rectifying and smoothing circuit is provided from the second rectifying and smoothing circuit. DC-DC converter, which supplies electric power to the flywheel diode.