JPS62131757A - Filter circuit - Google Patents

Abstract

PURPOSE:To alleviate a noise voltage over a wide frequency band by generating a voltage generated at an impedance equivalent to an internal impedance of a smoothing condenser at an impedance element provided separately by a transforming action to cancel a noise voltage. CONSTITUTION:A voltage V4 is generated at an impedance element 4 by an AC voltage V11, and a voltage V10 is generated at a smoothing condenser 10. The voltage V4 makes an impedance element 5 generate a voltage V5 by transforming action of magnetic coupling of inductances 6, 7. Accordingly, the voltage for cancelling the voltage V10 by the voltage V5 is presented as a voltage V22. When the voltages V10, V5 are equal, the voltage V22 becomes zero. That is, AC ripple component output by a circuit 3 becomes zero, and only a DC voltage is output.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to the removal of normal mode noise appearing in ripple voltage, noise terminal voltage, etc. in a filter circuit used in a switching power supply, etc.
This provides a highly effective filter circuit with a small number of parts.

[Conventional technology]

In conventional filter circuits, an LC filter in which a capacitor and an inductance are connected is generally used. However, in order to remove the fundamental wave component of the oscillation frequency of the switching power supply, large capacitors and inductances are required, which results in a large size and high cost. Furthermore, even when a plurality of LC fiducials are connected in series, there are drawbacks such as an increase in the number of parts and an increase in cost.

In addition, in a filter circuit that utilizes a resonance phenomenon, the filter effect is limited to only the resonance frequency, and it is not possible to eliminate noise in a wide band. Further, it has the disadvantage that it is difficult to adjust the resonance frequency.

[Problem that the invention seeks to solve]

In order to eliminate the above-mentioned drawbacks, the present invention aims to reduce the ripple voltage and noise terminal voltage in a relatively low frequency band over a wide frequency band, and to achieve a small size and low cost.

[Means for solving problems]

The present invention focuses on the fact that the ripple voltage and normal mode noise voltage (hereinafter referred to as noise voltage) due to the internal impedance of the capacitor, which could not be eliminated in the past, are voltages generated by the internal impedance of the smoothing capacitor, and Connect an impedance equivalent to the internal impedance of the smoothing capacitor so that a voltage equal to is generated, and the voltage generated at that impedance is
This filter circuit is characterized in that the noise voltage generated in another impedance element is generated by a transformer action, and as a result, the noise voltage generated in the smoothing capacitor is canceled out.

〔Example〕

Hereinafter, the present invention will be explained according to examples. FIG. 1 shows a first embodiment of the invention. FIG. 1 is an example in which the present invention is applied to an input smoothing circuit of a switching power supply.

The operation shown in FIG. 1 will be briefly explained below.

■The AC input power is rectified by rectifier diode 2,
The circuit 3 of the present invention removes the alternating current component, converts it into a direct current voltage, and applies it to the transformer 13 and transistor 14.

The transistor 14 performs a switching operation in response to a rectangular wave signal voltage applied to its base, and applies a rectangular wave voltage to the primary winding of the transformer 13. Thereby trance 13
The voltage induced in the secondary winding is rectified and smoothed by a rectifying and smoothing circuit 15 to obtain a DC output voltage v0.

In this embodiment, the voltage applied to the transformer 13 changes depending on the alternating current component of the voltage output from the rectifier diode 2, and the output of the alternating current component to the direct current output voltage V0 is reduced. Furthermore, in this embodiment, the normal mode noise generated by the switching operation of the transistor 14 is
The transmission through the diode 2 to the AC input power source 1 is prevented by the circuit 3 of the invention.

Next, the operation will be explained in detail using FIG. 2.

FIG. 2 shows an equivalent circuit 3 of the present invention,
The symbols are the same as in Figure 1. The arrow in the figure means the instantaneous value of the voltage, and the direction of the arrow is the polarity of the voltage. Furthermore, the black circle of inductance shown in the figure means the polarity of the generated voltage. Furthermore, the capacitor 10 is represented by an equivalent capacitance 12 and an equal series impedance 11 connected in series.

In FIG. 2, when AC voltage V11 is applied, voltage v4 is generated in the first impedance element 4, and ■1° is generated in the third impedance element 10, resulting in Vll-Va +
It is expressed as V1o ++++++++++ (1). Also, the voltage 4 is the first inductance 6 and the second inductance 6.
Due to the transformer action due to the magnetic coupling of the inductance 7, a voltage of V is generated in the second impedance element 5. Therefore, the voltage obtained by canceling the voltage VI6 by the voltage ■ appears as v2□, which is vtt-vl. -■5 ・・・・・・・・・・・・(2
). Therefore, ■If 1° and V5 are equal: v2. are canceled out and become zero. That is, the AC ripple component output from the circuit No. 3 becomes zero, and only the DC voltage is output. The conditions are that when the output impedance is considered to be sufficiently high, the third impedance element 10 and the first impedance element 4 are made equivalently equal, and the number of turns of the first inductance 6 and the second inductance 7 is This can easily be satisfied by making them the same. Further, in this case, the condition can be satisfied even without the impedance element 9. Furthermore, even if the voltages of 4 and VIO are different, the same effect can be obtained by changing the ratio of the number of turns of the inductance. Further, the impedance element 8 can be easily constructed by connecting a capacitor equivalent to the capacitor 10.

Next, consider the case where normal mode noise occurs as ■2□. This case is also similar to the above case, and if the polarities of the voltages VS and V4 are reversed, the applied voltage V is canceled out, and ■ becomes zero. In this case, the second impedance element 9 is made equivalent to the third impedance element 10, and the condition is satisfied even without the impedance element 8.

Furthermore, if the impedance elements 8, 9, and 10 are made equivalent to each other, the circuit viewed from the Vll side and the circuit viewed from the ■2□ side become relative, and a multidirectional filter circuit can be formed.

FIG. 3 shows the noise terminal voltage generated at the power supply input terminal when this embodiment uses an input rectifier circuit of a switching power supply. As is clear from FIG. 3, the noise terminal voltage can be reduced by about 10 dB or more according to the present invention, and the effects of the present invention are obvious.

According to this embodiment, the effect of the input smoothing filter can be enhanced with a small number of components, and the noise terminal voltage generated at the power supply input terminal by the switching power supply can be suppressed.

FIG. 4 shows a second embodiment of the invention. FIG. 2 shows an example in which the filter circuit of the present invention is used as an output smoothing filter of a switching power supply.

The operation of ripple voltage removal according to this embodiment is similar to the operation described above. Although FIG. 4 shows an example of a forward-type switching system, it is clear that the same effect can be obtained in other switching systems such as push-pull, half-type, bridge type, etc.

According to this embodiment, in addition to the low frequency ripple voltage appearing in the DC output voltage from the switching power supply, there is an effect of suppressing the high frequency ripple voltage and spike noise due to the oscillation frequency that cannot be removed by the operation of the control circuit. It is also possible to use the first embodiment and the second embodiment at the same time, resulting in even greater effects.

〔Effect of the invention〕

According to the present invention, a multidirectional and highly effective filter circuit can be formed with a small number of parts, and a great effect can be obtained in reducing the size and noise of switching power supplies and the like.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the first embodiment of the present invention, Fig. 2 is a diagram for explaining the first embodiment, Fig. 3 is a diagram showing the effect of the first embodiment, and Fig. 4 is a diagram for explaining the first embodiment. A second example is shown. (Explanation of symbols) 1... Input AC power supply, 2... Rectifier diode, 3...
... Circuit of the present invention, 4... First impedance element, 5... Second impedance element, 6... First inductance, 7... Second inductance, 8,
9... Impedance, 10... Third impedance element, 11... Equivalent series impedance, 12...
... Equivalent capacitance, 13 ... Transformer, 14 ... Transistor, 15 ... Rectifier and smoothing circuit, 16.17 ... Diode, 18 ... Choke coil, Vi + Vo,
Vz, Vt, vS, Vto. VZZ...voltage. Figure 2 Figure 3 Frequency (in Hz) Figure 4

Claims (1)

[Claims] 1. In a filter circuit having an input terminal for supplying power and an output terminal for outputting power, at least a first impedance element having a first inductance;
A second impedance circuit having at least a second inductance is connected in series, and a third impedance circuit having at least one capacitor is connected to a connection point between the first impedance element and the second impedance element. , A filter circuit characterized in that the first inductance and the second inductance are magnetically coupled. 2. In the filter circuit described in claim 1, the filter circuit is supplied with rectified alternating current power, and includes a smoothing capacitor for smoothing the rectified power and a capacitor of the third impedance element. A filter circuit featuring shared features. 3. The filter circuit according to claim 1, characterized in that the first inductance and the second inductance are magnetically coupled to have a polarity that cancels out the alternating current voltage generated in the third impedance element. . 4. In the device described in claim 3, at least one of the first impedance element and the second impedance element has an impedance equivalent to that of the third impedance element, and the number of turns of the first inductance is equal to the number of turns of the first inductance. A filter circuit characterized in that the number of turns of the second inductance is equal. 5. A filter circuit characterized in that at least one of the first impedance element and the second impedance element according to claim 4 is constituted by a capacitive impedance element.