JPH0516257B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an unnecessary radiation prevention device for switching power supply circuits used in audio equipment and the like.

In general, switching power supply circuits have a lot of unnecessary radiation, which generates noise, so it has been impossible to arrange a tuner or the like near the switching power supply circuit. Various proposals have been made to prevent unnecessary radiation from the switching power supply circuit, but none have been found to be satisfactory. 1st
The figure shows a conventional switching power supply circuit having an unnecessary radiation prevention device using a line filter transformer. That is, in Fig. 1, 1 indicates a power plug designed to supply, for example, 50 Hz commercial AC power, and a line filter transformer 2 is installed to prevent unnecessary radiation from the commercial AC power obtained in this power plug 1. It is supplied to the input side of a double-wave rectifier circuit 3 having a bridge configuration of diodes. One end of the output side of this double-wave rectifier circuit 3 is connected to one winding 5a of a line filter transformer 5 arranged inside the shield case 4.
The other end of the output side of the double-wave rectifier circuit 3 is connected to the collector of the NPN transistor 8 which constitutes the switching element through a series circuit of the choke coil 6a and the primary winding 7a of the transformer 7 which constitutes the switching circuit. is connected to the emitter of the switching transistor 8 through a series circuit of the other winding 5b of the line filter transformer 5 arranged in the shield case 4 and the choke coil 6b, and from the base of the switching transistor 8, for example,
A switching signal input terminal 9 is derived which supplies a switching signal pulse width modulated in accordance with an output voltage of about 100 KHz. A secondary winding 7b, which is DC-insulated from the primary winding 7a of the transformer 7 constituting this switching circuit, is connected to the input side of the rectifier circuit 10, and the output terminal 10 of the rectifier circuit 10 is connected to the input side of the rectifier circuit 10.
A and 10b are provided with a DC power supply of a predetermined voltage. Further, 11 is a smoothing capacitor provided between the output terminals of the double-wave rectifier circuit 3 and outside the shield case 4, and 12 is a smoothing capacitor provided between the connecting point of the yoke coil 6a and the primary winding 7a and the emitter of the yoke coil 6a and the transistor 8. This is a smoothing capacitor for the switching circuit provided between the connection point of the switching circuit and the connection point of the switching circuit and within the shield case 4. Also 13a, 13b,
14a and 14b are line filter transformers 2
This is a capacitor that together with the line filter constitutes a line filter. Also, on the output side of the double-wave rectifier circuit 3, a capacitor 15a for noise removal is connected between both ends outside the shield case 4.
and 15b are connected in series, and the connection point of these capacitors 15a and 15b is connected to the shield case 4 and grounded, and the windings 5a and 5b of one and the other of the line filter transformer 5 in the shield case 4 are connected in series. The hot side is connected to the shield case 4 through capacitors 16a and 16b for common mode noise removal, respectively, and grounded.

In FIG. 1, a commercial AC power source from a power plug 1 is rectified by a double-wave rectifier circuit 3, and this rectified DC signal is switched by a switching circuit constituted by a transformer 7, a switching transistor 8, etc. , a signal obtained at the secondary winding 7b of the transformer 7 is rectified by a rectifier circuit 10 to obtain desired DC power at output terminals 10a and 10b. In order to prevent unnecessary radiation in the switching power supply circuit as shown in FIG.
Even though the antennas 14a, 14b, 15a, 15b, 16a, and 16b were provided, unnecessary radiation could not be sufficiently prevented.

This is because the unnecessary radiation so-called common mode noise generated between the line and the ground due to the switching of the switching transistor 8 is caused by the collector of the transistor 8 → the primary winding 7a → the chain coil 6a → one winding 5a of the line filter transformer 5 → the capacitor 16a → Earth and current, emitter of transistor 8→chiyoke coil 6b→other winding 5b of line filter transformer 5→capacitor 16b→
Although this can be sufficiently attenuated, unnecessary radiation generated between lines, so-called normal mode noise, is caused by the collector of transistor 8 → primary winding 7 a → chain coil 6 a → one winding 5 a → capacitor 15 a. , 15b (or capacitor 11) → other winding 5b → chiyoke coil 6
There is a flow from b to the emitter of the transistor 8, and it is thought that the normal mode noise flowing outside the shield case 4 causes this unnecessary radiation and affects others.

In view of this point, the present invention is designed to sufficiently attenuate the above-mentioned unnecessary radiation.

Hereinafter, an embodiment of an unnecessary radiation prevention device for a switching power supply circuit according to the present invention will be explained with reference to FIG. In FIG. 2, parts corresponding to those in FIG. 1 are designated by the same reference numerals, and detailed explanation thereof will be omitted.

In this example, for example, the power plug 1 can be obtained.
A 50Hz commercial AC power source is supplied to the input side of the double wave rectifier circuit 3. A capacitor 17 for high frequency signal removal is connected between the input terminals of the double wave rectifier circuit 3 on the input side.

One output terminal of this double-wave rectifier circuit 3 is connected to one winding of a line filter transformer 2 arranged outside the shield case 4, and one winding 5 of a line filter transformer 5 arranged inside the shield case 4.
a, connected to the collector of the switching transistor 8 through a series circuit of the choke coil 6a and the primary winding 7a of the transformer 7, and also connects the other output terminal of the double wave rectifier circuit 3 to a line arranged outside the shield case 4 The other winding of the filter transformer 2, the other winding 5b of the line filter transformer 5 arranged in the shield case 4, and the chiyoke coil 6b
switching transistor 8 through a series circuit of
Connect to the Emitsuta. In this case, the value of the re-cage inductance L5 of the line filter transformer ₅ is set to be larger than the value of the re-cage inductance L2 of the normal line filter transformer ₂ , and the inductance value is set to be larger than the value of the re-cage inductance L2 of the normal line filter transformer 2. This is equivalent to installing a coil. Also, at this time, Chiyoke coil 6
The inductance value of each of a and 6b is set to be equal to or less than the re-cage inductance value of the line filter transformer 5.

Further, a smoothing capacitor 11 is connected between one and the other output terminals of the double-wave rectifier circuit 3, and a capacitor 13a forming a line filter is connected between the hot sides of one and the other windings of the line filter transformer 2. and 13b are connected in series, and the connecting point of the capacitors 13a and 13b is grounded, and the capacitors 14a and 14b forming a line filter are connected between the respective cool sides of one winding and the other winding of the line filter transformer 2. A series circuit is connected, and these capacitors 14a and 1
4b is connected to the shield case 4 and grounded. In this case, the cut-off frequency of the low-pass filter constituted by the re-cage inductance _L2 of the line filter transformer 2 and the smoothing capacitor 11 is set to a long wave band, for example, 150 to 350 Hz or less. A smoothing capacitor 18 is connected between the hot sides of one and the other windings of the line filter transformer 5 in the shield case 4 to prevent the hot side of the other winding 5b of the line filter transformer 5 from becoming common mode. The cool side of one winding 5a of this line filter transformer 5 is connected to the shield case 4 via a common mode prevention capacitor 20 and grounded. . In this case, the cut-off frequency of the low-pass filter constituted by the re-cage inductance L5 of the line filter transformer ₅ and the smoothing capacitor 18 is set to a long wave band, for example, 150 to 350 KHz or less. The rest of the structure is the same as in FIG. 1.

Since the present invention is configured as described above, the commercial AC power supplied to the power plug 1 is rectified by the double-wave rectifier circuit 3, and this is switched by the switching transistor 8 and transformer 7 that constitute the switching circuit, and this output signal is is supplied to the rectifier circuit 10, and a DC power source of a desired voltage can be obtained at the output terminals 10a, 10b of the rectifier circuit 10 as in the conventional case.

Also, unnecessary radiation generated between the line and ground due to switching of the switching transistor 8, so-called common mode noise, is caused by the switching of the switching transistor 8.
collector→primary winding 7a→chiyoke coil 6a
→ Capacitor 20 → Shield case 4 and emitter of transistor 8 → Chiyoke coil 6b
→The other winding 5b of the line filter transformer 5
→ capacitor 19 → shield case 4, and this can be sufficiently attenuated within shield case 4.

In addition, the unnecessary radiation so-called normal mode noise that occurs between lines is mainly caused by the collector of the transistor 8 → the primary winding 7a → the chiyoke coil 6a → one winding 5a → the capacitor 18 → the other winding 5b → the chiyoke coil 6b → the transistor 8. Since the emitter and the flow mainly flow through the shield case 4, unnecessary radiation becomes extremely small.

In addition, in the present invention, the chiyoke coils 6a, 6b
Since the inductance value of the line filter transformer 5 is made smaller than the re-cage inductance of the line filter transformer 5, unnecessary radiation generated from the choke coils 6a and 6b is also reduced.

Also, since the line filter transformer 5 with a large re-cage is provided inside the shield case 4, this re-cage does not affect the outside of the shield case 4, and the line filter transformer 2 arranged outside the shield case 4 has a re-cage inductance L. ₂
is small, so the unnecessary radiation caused by this is small, and since it is outside the shield case 4, there is no influence from the transformer 7 forming the switching circuit. Furthermore, normal mode noise in the AM band, for example, 150 to 1600 KHz, is generated by a low-pass filter consisting of the re-cage inductance L ₅ of the line filter transformer 5 and the capacitor 18, and a low-pass filter comprising the re-cage inductance L ₂ of the line filter transformer 2 and the capacitor 11. Normal mode noise in the FM band, for example 87.5 to 108 MHz, is attenuated by the coils 6a and 6b and the capacitors 20 and 1.
It is attenuated by a low-pass filter constituted by 9 or 14a, 14b, a line filter transformer 2, and a low-pass filter constituted by capacitors 13a, 13b. The relationship between the noise attenuation amount dB and the frequency of the circuit shown in Fig. 2 is as shown in Fig. 3.
The noise in the band below 200KHz is -60dB,
The noise in the band above 200KHz was less than -60dB, which was extremely small and satisfactory.

In FIG. 3, curve a is the noise attenuation characteristic due to the relationship of the line filter transformer 5, curve b is the noise attenuation characteristic due to the relationship of the line filter transformer 2, and curve c is the noise attenuation characteristic due to the relationship of the line filter transformer 6.
This is a noise attenuation characteristic due to the resonance frequency characteristics of a or 6b and the capacitor 20 or 19.

As described above, according to the present invention, it is possible to obtain a fully satisfactory unnecessary radiation prevention device for a switching power supply circuit. Therefore, a tuner or the like can be placed near the switching power supply circuit using the present invention, which has the advantage of making it easier to design audio equipment and making it possible to downsize the equipment.

It goes without saying that the present invention is not limited to the above-described embodiments, and can take various other configurations without departing from the gist of the present invention.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram showing an example of a conventional unnecessary radiation prevention device for a switching power supply circuit, FIG. 2 is a configuration diagram showing an example of an unnecessary radiation prevention device for a switching power supply circuit according to the present invention, and FIG. 3 is a configuration diagram showing an example of an unnecessary radiation prevention device for a switching power supply circuit according to the present invention. FIG. 1 is a power plug, 2 and 5 are line filter transformers, 3 and 10 are rectifier circuits, 4 is a shield case, 6a and 6b are chain coils, 7 is a transformer, 8 is a transistor, 9 is a switching signal input terminal, 11 , 12 and 18 are smoothing capacitors, and 19 and 20 are common mode prevention capacitors, respectively.

Claims (1)

[Scope of Claims] 1. A switching power supply circuit in which a primary side and a secondary side are insulated and a switching circuit section is disposed within a shield case, wherein a first circuit is provided within the shield case on the primary side. A line filter transformer is provided, and a second line filter transformer having a leakage inductance smaller than the leakage inductance of the first line filter transformer is provided outside the shield case on the primary side. A smoothing capacitor is provided between the first run filter transformer and the second line filter transformer in the switching circuit section, and a chiyoke coil is provided between the first line filter transformer and the smoothing capacitor of the switching circuit section. A switching power supply circuit characterized in that one winding cool side and the other winding hot side provided in the first line filter transformer are each connected to the shield case via a common mode prevention capacitor. Unnecessary beam prevention device.