JPH088744B2 - Power supply circuit - Google Patents

Description

The present invention relates to a power supply circuit, and more particularly to a power supply circuit capable of effectively preventing noise.

(Prior Art) There is a switching power supply circuit as one of power supply circuits. For example, as shown in FIG.
The rectifier circuit 33, the smoothing capacitor 34, and the DC-DC converter 35 are sequentially connected.

The filter circuit 32 includes capacitors 36 and 39 for across the line and capacitors 37 and 3 for line bypass.
8, 40 and 41 and coils 42 and 43, and capacitors 37 and 38 and capacitors 40 and 41 connected in series with each other.
The middle points of and are both connected to ground. This filter circuit 32 converts noise on the AC power supply 31 side into a DC-DC converter 35.
It has the function of not entering the AC power supply 31 side and preventing the switching noise of the DC-DC converter 35 from leaking to the AC power supply 31 side. In particular, the latter regulation is remarkable in other countries,
This prevents other electronic devices from malfunctioning due to noise leaking to the AC power supply side.

(Problems to be Solved by the Invention) By the way, a ground current flows through the capacitors 37, 38, 40 and 41 for line bypass. This ground current is composed of an alternating current flowing from the AC power source 31 due to the difference in impedance between the capacitors 37 and 38 and between the capacitors 40 and 41, and a current due to noise. It is regulated to be below the value. When they are connected in series and one capacitor is broken and released from the circuit, an alternating current determined by the impedance of the remaining capacitor flows as a ground current. Therefore, in this case, the larger the capacitance of the capacitor, the smaller the impedance and the larger the ground current, and thus the lower limit thereof. Therefore, instead of increasing the capacitance of the capacitors 37, 38, 40 and 41 to prevent normal mode or common mode noise, for example, the inductance of the coils 42 and 43 must be increased. In order to increase the inductance of the coils 42 and 43, it is necessary to increase the number of windings, and there is a drawback in that the coils are increased in size. In addition, when the coils 42 and 43 become large, the stray capacitor becomes large and the degree of attenuation in the high frequency band decreases, which is another drawback. Furthermore, the noise is 15KHz
Although it extends over ~ 100MHz, this noise cannot be completely removed only by the filter circuit 32. Therefore, the switching waveform of the DC-DC converter 35 should be made as shown in Fig. 4 to reduce the switching efficiency and suppress the noise. There are some drawbacks. And to suppress other noise, DC
-It is necessary to connect or shield a decoupling capacitor or a coil in the DC converter 35, which has the disadvantage of high cost.

The object of the present invention is to improve these drawbacks, to efficiently remove noise, to improve the attenuation in the high frequency band, without the need to reduce the switching efficiency of the switching power supply, and to enable downsizing and cost reduction. A power supply circuit is provided.

(Means for Solving Problems) In order to achieve the above-mentioned object, the present invention provides an AC power supply,
A filter circuit connected to the AC power source, a rectifier circuit connected to the filter circuit, a switching power source connected to the rectifier circuit, and a smoothing unit connected in parallel between the rectifier circuit and the switching power source. A series circuit in which a plurality of capacitors connected in parallel between a rectifier circuit and a switching power supply are connected in series, and a capacitor capable of connecting the midpoint of the series circuit to ground The present invention provides a power supply circuit having:

(Function) The present invention provides a rectifier circuit, a series circuit in which at least a plurality of capacitors connected to the rectifier circuit are connected in series between a filter circuit and a switching power supply, and a midpoint of the series circuit. A circuit consisting of a capacitor connected to ground is connected.

That is, the series circuit has a configuration in which at least a plurality of capacitors are connected in series. The series circuit and the capacitor for connecting the midpoint of the circuit to the ground are connected to the DC side of the rectifier circuit. Normally, a capacitor has the same action as an insulation resistance with respect to a direct current component and does not pass a direct current. Therefore, the current flowing to the ground through the series circuit connected to the DC side of the rectifier circuit and the capacitor is only the leakage current of the capacitor and the current due to noise. Therefore, even if the capacitance of each of the capacitors forming the series circuit and the capacitor connected to the middle point of the circuit is sufficiently large, the ground current can be easily reduced to the level regulated by the law or less. By increasing the capacitance of each capacitor, each impedance of the series circuit and the capacitor can be made extremely low with respect to an alternating current component such as noise, and noise can easily flow.

The noise that could not be removed by the filter circuit first enters the rectifier circuit, and the noise suppressed to a low level by the action of this rectifier circuit enters the series circuit with a very low impedance and flows to the ground through the capacitor, almost completely. It is removed to a close condition.

In addition, the noise current generated in the switching power supply
It has a very low impedance and flows to ground through a series circuit and a capacitor connected to this circuit and is eliminated. Therefore, the noise generated in the switching power supply hardly leaks to the AC side of the rectifier circuit. And the leaked noise can be sufficiently removed by the filter circuit, and it can be prevented from entering the AC power supply.

(Examples) Hereinafter, the present invention will be described based on illustrated examples.

In FIG. 1, reference numeral 1 is an AC power source. Reference numeral 2 is a filter circuit connected to the AC power supply 1. This filter circuit 2 includes across-the-line capacitors 3 and 6,
It consists of capacitors 4, 5, 7 and 8 for line bypass and coils 9 and 10. The capacitors 3 to 8 are connected in parallel with the AC power source 1, and the capacitors 4 and 5 and the capacitors 7 and 8 are connected in series, and their respective middle points are connected to the ground. Also,
The coil 9 and the coil 10 are connected to the AC power source 1 in series. Reference numeral 11 is a rectifying circuit connected to the filter circuit 2. Reference numeral 12 is a series circuit connected in parallel to the rectifier circuit 11, and connects a capacitor 13 and a capacitor 14 in series. Reference numeral 15 is a capacitor having a capacity of 2000 to 3000 PF, which is connected to the midpoint 16 of the series circuit 12 and is grounded. 17 is a series circuit
It is a smoothing capacitor connected in parallel with 12. A switching power supply 18 is connected in parallel with the capacitor 17.

 Next, the operation of the above embodiment will be described.

First, the filter circuit 2 selects only a specific frequency of the signal from the AC power source 1. The signal that has passed through the filter circuit 2 is transmitted to the rectifier circuit 11 and is AC-DC converted. The ripple component of the signal rectified by the rectifier circuit 11 is bypassed by the series circuit 12 and the smoothing capacitor 17, and is transmitted to the switching power supply.

The noise generated on the AC power source 1 side due to an external cause becomes a ripple current through the rectifier circuit 11, is bypassed by the series circuit 12, and flows from the midpoint 16 through the capacitor 15 to the ground. Since the noise passes through the rectifier circuit 11, its level becomes low. Therefore, the earth current due to noise flowing through the series circuit 12 and the capacitor 15 can be reduced. Further, the series circuit 12 has a configuration in which at least a plurality of capacitors are connected in series, and the series circuit 12 and the capacitor 15 are connected to the series side of the rectifier circuit 11. Therefore, due to the property of the capacitor, the current flowing to the ground through the series circuit 12 and the capacitor 15 is only the current due to the leakage current and noise of the capacitor, which is a value sufficiently smaller than the regulation value of the law. Therefore, the capacitance of each capacitor can be increased, and the impedances of the series circuit 12 and the capacitor 15 can be made extremely low against noise. Therefore, the noise easily flows to the ground through the series circuit 12 and the capacitor 15 and is removed.

Further, the switching noise of the switching power supply 18 also flows to the ground through the series circuit 12 and the capacitor 15. That is, both the noise generated on the AC power supply 1 side and the noise generated on the switching power supply 18 are connected to the series circuit 12 and the capacitor 15
Since these flow through to the ground, it is possible to prevent these noises from malfunctioning other electronic devices.

For example, the amount of noise attenuation is 30
When KHz is used, in the conventional example shown in FIG. 3, the capacitors 36 and 39 are 0.1 μF, and the capacitors 37, 38, 40 and 41.
2200 PF, and the limit of 20 dB when the coils 42 and 43 are 3.5 mH, the capacitors 13 and 14 of the series circuit 12 are 0.1 μF and the capacitor 15 is 2000 PF in the above-described embodiment of the present invention.
When the capacitor 3 of the filter circuit 2 is 0.1 μF, the capacitors 4, 5 and 6 are removed, the capacitors 7 and 8 are 2200 PF, and the coils 9 and 10 are 1.0 mH, the value is 30 dB or more.

FIG. 2 shows a series circuit 19 used in another embodiment of the present invention, in which capacitors 20 to 23 having a capacity of 10,000 PF or more are provided.
Are connected in a bridge shape. As these capacitors 20 to 23, SH type, NH type, and ceramic capacitors are used.

(Effect of the Invention) As described above, according to the present invention, since the series circuit including the capacitor is connected to the DC side of the rectifier circuit, and the midpoint of the series circuit is connected to the ground via the capacitor. , The current flowing to the ground can be only the leakage current of the capacitor and the current of noise, so it can be easily made lower than the value regulated by law, and the capacitance of the capacitor should be made large enough to prevent the noise from the series circuit and this circuit. The impedance of the capacitor connected to can be very low,
Therefore, it is possible to obtain a power supply circuit that can effectively remove noise leaking from the filter circuit and passing through the rectifying circuit or noise generated in the switching power supply to the ground. Also,
According to the present invention, it is possible to effectively remove noise without increasing the coil of the filter circuit, improve the attenuation in the high frequency band, improve the switching efficiency, and reduce the size and cost of the power supply circuit. Is obtained.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention, FIG. 2 is a circuit diagram of a series circuit used in another embodiment of the present invention, FIG. 3 is a circuit diagram of a conventional power supply circuit, and FIG. 4 is switching. 3 shows a conventional switching waveform of a power supply. 1 ... AC power supply, 2 ... Filter circuit, 11 ... Rectifier circuit, 12, 19 ... Series circuit, 15 ... Capacitor, 18 ... Switching power supply.

Claims (1)

[Claims] 1. An AC power supply, a filter circuit connected to the AC power supply, and a rectifying circuit connected to the filter circuit,
In a power supply circuit including a switching power supply connected to the rectifier circuit and a smoothing capacitor connected in parallel between the rectifier circuit and the switching power supply, the power supply circuit is connected in parallel between the rectifier circuit and the switching power supply. A power supply circuit comprising a series circuit in which a plurality of capacitors are connected in series, and a capacitor capable of connecting the midpoint of the series circuit to ground.