JPH03272119A - Filter circuit - Google Patents

Abstract

PURPOSE:To make it possible to arbitrarily set resonance frequency within a prescribed range by a simple constitution by a method wherein an impedance element is inserted between a conductive layer and an earthing terminal, and the impedance element is selected arbitrarily. CONSTITUTION:An impedance element 8 is connected between a conductive layer 6 and an earthing terminal 7, among the floating capacitance of a choke coil, as the component formed by passing through the magnetic core 1 of a closed magnetic circuit is earthed through the intermediary of the element 8, the above-mentioned floating function as a capacitor located between a line and an earthing wire. Also, the degree of the function as the capacitor located between the line and the earthing wire can be changed by the magnitude of impedance to be inserted. At the same time, as the floating capacitance of the choke coil also makes a change, the resonance frequency of a filter circuit can also be changed, and as a result, the alteration of the number of turns and the structure of winding of a coil 3 is unneccesitated. As a result, a filter circuit, having the necessary impedance characteristics, can be obtained using a simple constitution.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a filter circuit used, for example, in a line filter of a switching power supply or the like.

BACKGROUND OF THE INVENTION At present, choke coils in filter circuits used for boat fishing have a configuration as shown in FIG. That is, a coil bobbin 2 on which a coil 3 is wound is incorporated into the central magnetic leg of a closed magnetic circuit magnetic core 1 having an opening shape, and an exterior tape 4 is wound around the outer periphery of the coil 3. This choke coil has a certain resonant frequency due to the stray capacitance and self-inductance existing in its windings, and operates as an inductor in a frequency band lower than this resonant frequency, but as a capacitor in a higher frequency band. It's working. For this reason, the impedance of the choke coil exhibits frequency characteristics as shown in FIG. Here the first
The resonant frequency (fo) in Figure 1 means the point at which the damping effect of the filter circuit is maximum, but this resonant frequency is determined by the self-inductance of the choke coil and the stray capacitance of the winding, and is not constant. It was something that could not be changed.

Problems to be Solved by the Invention In order to set the resonance point at the required frequency with such a choke coil, in order to change the self-inductance and stray capacitance, it is necessary to change the number of turns and the winding structure of the coil, for example, to change the self-inductance and stray capacitance. It is not easy to manage the coils, which require changes such as multiple spatially divided coils (a method generally called split winding).

The present invention solves these problems by providing a filter circuit that has a simple configuration and allows the resonance frequency to be arbitrarily set within a certain range.

Means for Solving the Problems In order to solve the problems, the filter circuit of the present invention includes a choke coil body formed by at least two windings wound around a closed magnetic circuit eccentrically, and a choke coil body and an insulating layer interposed between the choke coil body and the insulating layer. It has a conductive layer placed in close contact with the ground, and an appropriate impedance element between this conductive layer and the ground, and by arbitrarily selecting this impedance element, the resonant frequency of the filter circuit can be set arbitrarily. This is what I did.

Effect: With the above configuration, the resonance frequency due to the self-inductance and stray capacitance of the choke coil can be varied with a simple configuration.

Embodiment FIGS. 1 and 2 are a perspective view showing the configuration of a filter circuit and a cross-sectional view of factors, showing an embodiment of the present invention. Figure 1,
In Fig. 2, 1 is an eccentric closed magnetic circuit made of ferrite, etc., 2 is a split-type coil bobbin with multiple numbers for winding the coil, and 3 is a coil wound around this coil bobbin 2. , 4 is an exterior tape wound around the outer periphery of the coil bobbin 2 around which the coil 3 is wound, and these tapes constitute the choke coil body. 5 is an insulating tape for insulating the choke coil body and the conductive layer, 6 is a copper plate used as a conductive layer wrapped around the insulating tape 5, 7 is an earth terminal connected to the ground, and 8 is the conductive layer mentioned above. It is an impedance element connected between layer 6 and ground terminal 7.

With this configuration, the component of the stray capacitance of the choke coil that passes through the closed magnetic circuit eccentricity 1 is grounded via the inserted impedance element 8, as shown in FIG. 3(a). This stray capacitance is shown in Figure 3 (
As shown in c), it will act as a capacitor between the line and ground.

Furthermore, depending on the magnitude of the impedance inserted, it is possible to vary the degree to which it acts as a capacitor between the line and the ground. At the same time, since the stray capacitance of the choke coil also changes, it becomes possible to vary the resonance frequency of the filter circuit, and as described above, there is no need to change the number of turns of the coil 3 or the winding structure.

Further, by insulating the choke coil body and the conductive layer 6, noise induced in the chire body, particularly in the closed magnetic circuit eccentricity 1, is prevented from leaking to the ground through the impedance element 8.

In the filter circuit configured in this manner, the stray capacitance of the choke coil is reduced, the resonant frequency is higher as shown in FIG. 4, and the impedance is larger than that of the conventional choke shown in FIG. 6. Therefore, when the filter circuit according to the present invention is applied to a switching power supply device, extremely low feedback noise can be achieved as shown in FIG.

On the other hand, when a conventional filter circuit having characteristics as shown in FIG. 11 is applied to the same power supply device as above, feedback noise is not sufficiently attenuated as shown in FIG. 12.

FIG. 6 is a perspective view showing another embodiment. In this embodiment, copper foil (tape is also acceptable) is used as the conductive layer, and an insulating tape 5 is wrapped around the outer circumference of the closed magnetic circuit eccentricity 1. The operation is exactly the same as that shown in Fig. 1.

Figure 7 shows the impedance characteristics of the filter circuit when a resistor is used as the impedance element in Figure 1. By varying the resistance value, the resonant frequency of the filter circuit changes, and the resonance occurs accordingly. The impedance in the higher frequency range is also changing.

Although FIG. 8 shows the characteristics when an inductor is used instead of the above-mentioned resistor, it can be seen that similar effects can be obtained by changing the value of the inductor.

Although FIG. 9 shows the characteristics when a capacitor is used instead of the above-mentioned resistor, the same effect can be obtained by changing the value of the capacitor. Further, in this case, the impedance at the resonant frequency can be made extremely large as shown at point A in FIG. This is because the stray capacitance and the inserted impedance are the same capacitance component, so this capacitor and
This is thought to be because a new high-Q resonance phenomenon appears with the choke coil. For this reason, the noise generated by a resonant switching power supply ranges from 500K to several MHz.
When the filter circuit according to the present invention is applied to noise concentrated near the switching frequency of , extremely good noise characteristics can be obtained.

Effects of the Invention As described above, according to the present invention, the resonant frequency due to the self-inductance and stray capacitance of the choke coil can be varied with a simple configuration, making it easy to create a filter circuit with the necessary impedance characteristics. It has the effect of being expressive.

[Brief explanation of drawings]

Fig. 1 is a perspective view showing an embodiment of the filter circuit of the present invention, Fig. 2 is a sectional view of the same essential parts, Figs. 3(a) and (b) are equivalent circuit diagrams, and Fig. 4 is the same. An impedance characteristic diagram of the filter circuit, Figure 5 is a feedback noise characteristic diagram when the same filter circuit is applied to a switching power supply, Figure 6 is a perspective view of another embodiment, and Figure 7 is a diagram showing the use of a resistor as an impedance element. Figure 8 is an impedance characteristic diagram when an inductor is used as an impedance element, Figure 9 is an impedance characteristic diagram when the same capacitor is used, and Figure 1O is a conventional choke coil. FIG. 11 is an impedance characteristic diagram of a conventional choke coil, and FIG. 12 is a perspective view showing the configuration of the choke coil.
FIG. 6 is a noise characteristic diagram when the filter circuit having the characteristics shown in FIG. 1 is applied to the same power supply device as in FIG. 5; 1... Closing circuit core, 2... Coil bobbin, 3... Coil, 4... Exterior tape, 5... Insulating tape. , 6... Copper plate, 7
...Earth terminal, 8...Impedance element.

Claims (2)

[Claims] (1) A choke coil body formed by winding at least two coils around a closed magnetic circuit magnetic core, a conductive layer disposed in close contact with the choke body so as to maintain an insulated state, and the conductive layer and a ground terminal. and an impedance element inserted between the filter circuits. (2) The filter circuit according to claim 1, wherein an insulating tape is used as a means for maintaining insulation between the choke coil body and the conductive layer.