JPH08317633A - Smoothing circuit for switching power supply - Google Patents

Abstract

PURPOSE: To obtain a smoothing circuit by which a ripple in the output voltage of a switching power supply is reduced without using a large-capacitance electrolytic capacitor and whose stability is enhanced. CONSTITUTION: A series circuit which is composed of a main current passage across the collector and the emitter of a transistor Q1 and of a choke coil L is connected across the output side of a rectifying circuit part 4 for a switching power supply and an output terminal 2. A smooting capacitor C1 is connected across the collector of the transistor Q1 and a ground. A capacitor C2 the other end of which is connected to the ground and a bias circuit 5 are connected to the base of the transistor Q1. A capacitor C3 is connected across the output terminal 2 and the ground. Consequently, even when a large- capacitance electrolytic capacitor is not used as a smoothing capacitor, it is possible to obtain a smoothing circuit by which a ripple in an output voltage can be reduced and which is stabilized. A power supply circuit can be miniaturized, and its high efficiency and its high reliability can be realized. The switching frequency of the switching power supply can be made high.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a smoothing circuit for a power supply circuit incorporated in an electronic device.

[0002]

2. Description of the Related Art Nowadays, electronic devices are mounted on various precision instruments for automation and high performance. In order to drive the electronic device mounted on this equipment, a power supply source is naturally required. This power supply source includes an outlet (commercial power supply line), a battery, etc. However, whichever is used, the power from the power supply source is rarely directly supplied to each functional circuit of the electronic device and is built-in. In many cases, the power supply circuit converts or stabilizes the voltage and then supplies the voltage to each functional circuit. Here, when the device is a portable type, a battery is assumed as a power source to be used, and therefore a switching system is generally adopted as a power source circuit in view of fluctuations in input voltage and efficiency. An example of a power supply circuit incorporated in such an electronic device is shown in FIG.

The circuit shown in FIG. 4 shows only the main part of a switching type power supply circuit, and operates as follows. Due to the voltage (DC voltage) supplied to the input terminal 1 and the operation of the switching transistor Q2, an intermittent current flows in the primary winding N1 of the transformer T. When the switching transistor Q2 is on,
Energy is accumulated in the transformer T by the current flowing through the primary winding N1, and when the switching transistor Q2 is turned off, a flyback voltage is generated in each winding of the transformer T by the accumulated energy. The flyback voltage generated in the secondary winding N2 passes through the diode D1 in the ON state and charges the smoothing capacitor C4 because its polarity is in the forward direction of the rectifying diode D1.

The smoothing capacitor C4 is charged by the flyback voltage generated in the secondary winding N2 when the switching transistor Q2 is in the off state, and when the switching transistor Q2 is in the on state, power is supplied to the outside through the output terminal 2. Discharge for supply. Switching power supplies are superior to series regulators in terms of their ability to cope with fluctuations in input voltage and efficiency, but they are inferior in terms of output voltage stability and the amount of ripple that appears in output voltage. Therefore, the L-shaped circuit including the choke coil L and the capacitor C5 shown in the circuit of FIG. 4 acts to suppress the fluctuation of the output voltage due to the fluctuation of the external load due to the release and accumulation of the energy stored in the circuit. To do.

With the above operation, a stable voltage can be obtained at the output terminal 2 with less ripple. In FIG. 4, reference numerals 1 and 2 respectively indicate high-potential-side input / output terminals of the power supply, and the low-potential-side input / output terminals are shown in common with the ground. Further, 3 indicates a converter circuit section of the switching power supply, and 4 indicates a rectifying circuit section of the switching power supply.

[0006]

As a feature of recent electronic devices, the required specifications of the voltage supplied from the power supply circuit to each functional circuit become strict as long as the device requires high accuracy.
In addition to this, increasing the functionality of equipment requires a large number of functional circuits, and by repeatedly driving and stopping these functional circuits, the state of the load to which the power supply circuit supplies a voltage changes rapidly. The load current changes in a pulsed manner with such a change in the load state, which causes ripples in the output voltage, and this ripple may adversely affect the load circuit (functional circuit) in no small way. was there. The ripple appearing in the output voltage has two problems, firstly, the amount of voltage fluctuation, and secondly, the time until the voltage returns to a predetermined voltage. Reducing the ripple is nothing but reducing the fluctuation amount of this voltage and shortening the time until the voltage returns to a predetermined voltage.

In the conventional circuit shown in FIG. 4, the smoothing capacitor C4, the choke coil L, and the capacitor C5 are the elements that reduce ripple. Here, if the capacitance of the capacitive element can be made extremely large (ideally infinite), it becomes possible to prevent ripples from occurring in the output voltage. However, the capacitance of an actual capacitance element is finite, and the capacitance of the capacitance element used must be suppressed to a small value due to restrictions such as the product shape. Therefore, in a power supply circuit that is actually manufactured, it is inevitable that ripples will occur due to load fluctuations, and in order to reduce this ripple, it is necessary to use smoothing capacitors C4 and C5 that have as large an electrostatic capacitance as possible. There is. As this large-capacity capacitor, an electrolytic capacitor is used because of the required electrostatic capacity.

In the current miniaturization of precision equipment, there is a strong demand for miniaturization of power supply circuits. However, while electrolytic capacitors can have a large capacity compared to other types of capacitors,
Large element shape. Therefore, there is a problem that the shape of the switching power supply becomes large. Further, as one means for reducing the size of the switching power supply, it is conceivable to increase the switching frequency to reduce the size of the transformers, inductances, and capacitors of the constituent elements. However, the electrolytic capacitors have markedly worse high-frequency characteristics than other types of capacitors due to their structure. Therefore, it has been one of the factors that prevent the switching frequency from increasing. Furthermore, the lifetime of the power supply circuit is said to be determined by the electrolytic capacitor, and thus the degree of deterioration over time is greater than that of other elements. Therefore, it has been a cause of reducing the reliability of the switching power supply.

The smoothing circuit of the circuit shown in FIG. 4 is composed of a smoothing capacitor C4, a choke coil L, and a capacitor C5. The structure is extremely simple and there is almost no loss in the smoothing circuit, and the efficiency of the power supply decreases. It is excellent in that it is minute. However, when a sudden load change occurs in this smoothing circuit, ripples in the shape of an oscillating wave, which is considered to be an LC transient phenomenon due to the inductance component and the capacitance component in the smoothing circuit, occur for a relatively long time (several ms). .
It is easily conceivable that a resistance element may be inserted to suppress the ripples in the vibration wave shape, but this has the adverse effect of causing a reduction in the efficiency of the power supply. Therefore, the present invention aims to reduce the ripple of the output voltage without using a large-capacity electrolytic capacitor, and to prevent the occurrence of the LC transient phenomenon of the smoothing circuit to shorten the time of occurrence of the ripple of the output voltage. Thus, it is an object of the present invention to provide a smoothing circuit for a switching power supply, which can realize miniaturization, high efficiency, and high reliability of the switching power supply.

[0010]

A smoothing circuit according to the present invention has a first capacitor connected between an output side of a rectifying circuit provided on the output side of a converter circuit of a switching power supply and the ground, and a second base. A transistor connected to ground via a capacitor and having a collector connected to the output side of the rectifier circuit, a bias circuit connected to the base of the transistor, a choke coil connected between the emitter of the transistor and the power output terminal, and It is characterized by comprising a third capacitor connected between the power output terminal and the ground.

[0011]

In the present invention, the energy intermittently supplied from the rectifier circuit is stored in the smoothing capacitor (first), and the smoothing capacitor connects the capacitor (second) to the base of the transistor, the choke coil, and the capacitor (first). 3) and continuous energy supply to the outside through the output terminal. In the circuit with such a configuration, when viewed from the output terminal,
Capacitor connected to base of transistor (second)
Can be regarded as the current amplification factor h _FE times of the transistor, and is equivalent to a smoothing circuit composed of a large-capacity capacitor and a choke coil. In addition, the transistor suppresses the voltage oscillation due to the load change due to the transient phenomenon of the inductance and the capacitance in the smoothing circuit, and enhances the ripple reduction effect. As a result, a DC voltage with low ripple and high stability can be obtained at the output terminal even if a disturbance such as a change in the load state occurs.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A block diagram of a basic switching power supply provided with a smoothing circuit according to the present invention which does not use a large capacity electrolytic capacitor is shown in FIG. The configuration of the switching power supply shown in FIG. 1 is as follows. First, the converter circuit unit 3 of the switching power supply is connected to the input terminal 1, and the rectifier circuit unit 4 is connected to the output side of the converter circuit unit 3. The output side of the rectifier circuit unit 4 is connected to the ground via the smoothing capacitor C1 and further connected to the collector of the transistor Q1. To the base of the transistor Q1, one end of a capacitor C2 whose other end is connected to ground and the bias circuit 5 are connected. The emitter of the transistor Q1 is connected to the output terminal 2 via the choke coil L, and the output terminal 2
Connect a capacitor C3 between this and ground.

In the above structure, the voltage applied to the input terminal 1 is converted into a pulsed voltage in the converter circuit section 3. Although this pulsed voltage varies depending on the conversion method of the converter circuit unit 3, it is generally sent to the smoothing circuit via the rectification circuit unit 4. In the smoothing circuit of the present invention, the pulse voltage appearing on the output side of the rectifying circuit unit 4 is smoothed by the smoothing capacitor C1. The smoothed voltage is applied to the capacitor C3 and the output terminal 2 via the transistor Q1 and the choke coil L which are in operation by the bias circuit 5.

If the viewpoint is changed, the transistor Q1
The capacitor C2 connected to the base of the
(Or from the choke coil L), a capacitor having a capacitance obtained by multiplying the actual capacitance value of the capacitor C2 by the current amplification factor h _FE of the transistor Q1 is connected between one end of the choke coil L and the ground. Is equivalent to Therefore, it can be considered that a filter is formed by the choke coil L, the capacitor C3, and the capacitor C2 having a capacity of h _FE times. Further, the transistor Q1 is interposed in the energy discharge path of the choke coil L, and the transistor Q1 suppresses the transient phenomenon while adjusting the amount of energy passing, and prevents the generation of the oscillating voltage. Therefore, it is understood that even if a load change occurs, the ripples appearing in the output voltage become small due to the above two effects.

In the conventional smoothing circuit of the switching power supply shown in FIG. 4, a large-capacity capacitor is used to reduce the ripple of the output voltage caused by load fluctuation and improve the stability. Here, as the large-capacity capacitor, an electrolytic capacitor has been used because of the required capacitance value. On the other hand, in the smoothing circuit according to the present invention, the action of adjusting the passing energy by the transistor Q1 and the action of suppressing the oscillating voltage, and the capacitance of the capacitor C2 can be regarded as h _FE times the actual capacitance value, so that the smoothing capacitor C
1. Even if an element having a much smaller capacity than the conventional one is used for the capacitor C2 and the capacitor C3, the same ripple reduction effect as that of the conventional one can be obtained against a load change or the like. Further, if smoothing capacitors C1, C2, and C3 are capacitors other than electrolytic capacitors, such as ceramic capacitors, the frequency characteristics of the smoothing circuit are improved and the switching frequency can be increased. Further, even if a ceramic capacitor uses a laminated type element, further miniaturization of the power supply circuit can be expected.

FIG. 2 shows a concrete first embodiment of the smoothing circuit according to the present invention and a circuit of a main part of a switching power supply including the smoothing circuit. The converter circuit unit 3 of the switching power supply in FIG. 2 includes a transformer T and a switching transistor Q2, and the rectifying circuit unit 4 is formed of a diode D1. Secondary winding N2 of converter circuit unit 3
And the switching configuration of the diode D1 of the rectifier circuit unit 4, the switching power supply is a flyback system,
The smoothing circuit connected to this has the following configuration.
In FIG. 2, reference numerals 1 and 2 respectively indicate high-potential-side input / output terminals, and the low-potential-side input / output terminals are shown as being common to the ground.

The diode D on the output side of the rectifying circuit section 4
The cathode of No. 1 is connected to the ground via the smoothing capacitor C1, and further connected to the collector of the NPN type transistor Q1. The emitter of the transistor Q1 is connected to the output terminal 2 via the choke coil L, and the base is connected to the ground via the capacitor C2. A resistor R forming a bias circuit 5 between the collector and the base of the transistor Q1.
Connect 1. Capacitor C3 between output terminal 2 and ground
Connect. The circuit configuration of the above smoothing circuit is the same as the basic configuration of the smoothing circuit of the present invention shown in FIG. Therefore, since the operation and the obtained effect are the same, detailed description of the circuit shown in FIG. 2 is omitted here.

In the embodiment of the present invention shown in FIG. 2, the bias circuit 5 of the transistor Q1 is formed by the resistor R1. The bias circuit 5 formed of the resistor R1 is useful in that it can be most easily constructed, but the bias circuit 5 may be formed by a circuit as shown in FIG. In FIG. 3, a is a terminal connected to the collector of the transistor Q1, and b is a terminal connected to the base of the transistor Q1. Here, the transistor Q3 has resistors R3 and R4.
A current flows from the terminal a to the terminal b through the resistor R2 in accordance with the divided voltage due to, and the circuit as a whole forms a constant current circuit. Forming the bias circuit 5 with a constant current circuit is useful in that the operation of the transistor Q1 can be stabilized. When the bias circuit 5 is configured by a constant current circuit, the circuit configuration is not limited to the circuit shown in FIG. 3, and may be a circuit using a constant current diode or FET, for example. . Further, the terminal a may be connected to a separately provided voltage supply source instead of the collector of the transistor Q1.

Here, for the purpose of reducing the ripple of the output voltage without using a large capacity electrolytic capacitor, a circuit as shown in FIG. 5 is also conceivable. The circuit shown in FIG. 5 is obtained by exchanging the positions of the transistor Q1 and the choke coil L of the smoothing circuit shown in FIG. At first glance, the smoothing circuits in FIGS. 2 and 5 seem to have substantially the same ripples in the output voltage. However, under the same circuit constants and output conditions (15 [V], 12 [mA]), a comparative experiment was performed in which the electronic load variation was similarly performed, and the peak value of the ripple generated in the output voltage was found. Although the circuit of FIG. 2 is slightly larger than the circuit of FIG. 5, the time required to return to a predetermined voltage is less than that of the circuit of FIG.
The circuit was faster. That is, in the actual circuit test, the effect of reducing the ripple of the output voltage generated due to the load change is higher when the smoothing circuit of the present invention shown in FIG. 2 is used. Therefore, it is desirable that the smoothing circuit of the switching power supply has the configuration of the present invention shown in FIG. 2 unless otherwise specified.

[0020]

As described above, in the smoothing circuit for the switching power supply according to the present invention, the series circuit of the main current path of the transistor and the choke coil is provided between the output side and the output terminal of the rectification circuit section of the switching power supply. A first capacitor is provided between the collector of the transistor and the ground, a second capacitor and a bias circuit are provided at the base of the transistor, and a third capacitor is connected between the output terminal and the ground. In this configuration, the capacitance of the capacitor provided at the base of the transistor can be regarded as h _FE times the actual capacitance value, and the effect of reducing the ripple in the output voltage and improving the stability due to the operation of the transistor is expected. it can. Therefore, even if a large-capacity electrolytic capacitor is not used as the capacitor of the smoothing circuit, the ripple of the output voltage can be reduced with respect to load fluctuation and the like. By not using the electrolytic capacitor, the power supply circuit (switching power supply) can be downsized, highly efficient, and highly reliable. Furthermore, by using a ceramic capacitor instead of the electrolytic capacitor, it is possible to obtain an effect that the switching frequency of the switching power supply can be increased.

[Brief description of drawings]

FIG. 1 is a block diagram of a basic switching power supply including a smoothing circuit according to the present invention.

FIG. 2 is a diagram showing a first embodiment of a smoothing circuit of the present invention applied to a switching power supply.

FIG. 3 is a diagram showing another embodiment of the bias circuit of the transistor Q1.

FIG. 4 is a diagram showing a smoothing circuit of a conventional switching power supply.

5 is a diagram showing a smoothing circuit having a configuration in which the positions of transistors and choke coils of the smoothing circuit of the present invention shown in FIG. 2 are exchanged.

[Explanation of symbols]

 1 Input Terminal 2 Output Terminal 3 Converter Circuit Section 4 Rectifier Circuit Section 5 Bias Circuit Q1 Ripple Reduction Transistor L Choke Coil R Bias Resistor C1 Smoothing Capacitor C2 Capacitor C3 Capacitor

Claims (4)

[Claims] 1. A first capacitor connected between an output side of a rectifier circuit provided on the output side of a converter circuit of a switching power supply and ground, and a base connected to ground via a second capacitor,
A transistor whose collector is connected to the output side of the rectifier circuit, a bias circuit connected to the base of the transistor, a choke coil connected between the emitter of the transistor and the power output terminal, and between the power output terminal and ground. A smoothing circuit for a switching power supply comprising a third capacitor connected. 2. The smoothing circuit for a switching power supply according to claim 1, wherein the first, second and third capacitors are ceramic capacitors. 3. The bias circuit is formed by a resistor connected between the collector and the base of the transistor.
A smoothing circuit for a switching power supply according to claim 1 or 2. 4. The smoothing circuit for a switching power supply according to claim 1, wherein the bias circuit is a constant current circuit.