JPH08266054A - Common mode noise reducing circuit - Google Patents

Abstract

PURPOSE: To reduce the impedance of a series resonance circuit, in the case that common noise occurs, so as to reduce the common noise, by connecting the series resonance circuit which has resonance frequency an integer times as high as the switching frequency, between one end on input side and one end on output side of a voltage converter. CONSTITUTION: For common mode noise, which occurs between the input terminal 1 and the output terminal 12 of a DC-DC converter, becomes one including harmonics, with the switching frequency roughly analogous to the voltage waveform between the drain D and the source S of a MOS transistor 5 as the fundamental wave. Here, a series resonance circuit consisting of a capacitor 13 and a coil 14 is connected between the input terminal 1 and the output terminal 12, and the resonance frequency is made the same as the switching frequency of a MOS transistor 5. Hereby, the impedance of this series resonance circuit is made small, whereby the common noise generated between the input terminal 1 and the output terminal 12 is reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit for reducing common mode noise generated in, for example, a DC-DC converter (DC-DC converter) or a switching regulator.

[0002]

2. Description of the Related Art Conventionally, techniques in such a field include:
For example, some documents were described in the following documents. Literature; Bellnix Co., Ltd., Catalog No. BDD9105-18A
, "Ultra compact, low noise, long life, DC-D for communication industry
C converter ", P.10-11 Fig. 2 is a circuit diagram showing an example of the configuration of the conventional DC-DC converter described in the above-mentioned document. This DC-DC converter includes the input terminal 1 on the hot side and It has a cold side input terminal 2. A capacitor 3 is connected between the input terminals 1 and 2. The input terminal 1 is connected to the hot side of the primary winding 4a of the transformer 4. ing.
The cold side of the primary winding 4a of the transformer 4 is connected to the drain D of the MOS transistor 5, and the source S of the MOS transistor 5 is connected to the input terminal 2. MOS
The gate G of the transistor 5 is connected to a control circuit 6 that generates a gate drive voltage having a predetermined switching frequency.

The hot side of the secondary winding 4b of the transformer 4 is connected to the anode A of the diode 7, and the cathode K of the diode 7 is connected to one electrode 8a of the capacitor 8. The cold side of the secondary winding 4b of the transformer 4 is connected to the anode A of the diode 9, and the cathode K of the diode 9 is connected to the electrode 8a of the capacitor 8.
The cold side of the secondary winding 4b of the transformer 4 is connected to the other electrode 8b of the capacitor 8 via the choke coil 10. The electrode 8a of the capacitor 8 is connected to the output terminal 11 on the hot side, and the electrode 8b of the capacitor 8 is connected to the output terminal 12 on the cold side. Output terminal 1
2 is connected to the input terminal 1 via the capacitor 13. In the insulation type DC-DC converter shown in FIG. 2, the input side and the output side are insulated from each other in terms of direct current, and in terms of alternating current, they are a few due to stray capacitance inside the transformer 4 for voltage conversion. There is a capacitance of 10 pF to several 100 pF.
On the other hand, the cold side of the primary winding 4a of the transformer 4 is
Due to the switching action of the S-transistor 5, it varies within the range of several tens to several hundreds of volts. Therefore, the charging and discharging of the capacitance is repeated by this switching action, and a noise voltage close to a square wave having a switching frequency as a fundamental wave is generated between the primary winding 4a and the secondary winding 4b of the transformer 4. Occurs, and this noise voltage becomes a common mode noise source between the input side and the output side of this DC-DC converter. Since this common mode noise adversely affects the external circuit, normally, as shown in FIG. 2, a capacitor 13 of several 10 nF to several 100 nF is connected between the input side and the output side of the DC-DC converter, and the common mode noise is generated. It suppresses noise.

[0004]

However, the DC-DC converter of FIG. 2 has the following problems.
That is, in the above configuration, if the capacity of the capacitor 13 connected between the input side and the output side is increased in order to reduce common mode noise, a total of several 10 of these DC-DC converters are used in parallel. In case of a simple system, the capacitance between input and output may reach about 1μF.
In addition to the fact that the assumption of insulation between input and output is broken, the capacitor 13 is required to have a withstand voltage of 200 V or higher. As a result, there is a problem that the DC-DC converter is large and expensive.

[0005]

In order to solve the above problems, the present invention adds a pulse train obtained by switching a DC voltage on the input side at a constant frequency to an inductance, and rectifies and rectifies the voltage generated in the inductance. In a common mode noise reduction circuit for reducing common mode noise of a frequency that is an integral multiple of the switching frequency that is generated between the input side and the output side of a voltage conversion device that outputs a smoothed output voltage to the output side The following circuits are provided. That is, one or more series resonance circuits having a resonance frequency that is an integral multiple of the switching frequency are connected between one end on the input side and one end on the output side.

[0006]

According to the present invention, since the common mode noise reduction circuit is configured as described above, even if common mode noise of an integral multiple of the switching frequency occurs between the input side and the output side of the voltage conversion device. Since one or more series resonance circuits whose resonance frequencies are adjusted to integer multiples of the switching frequency are connected, the impedance of the series resonance circuit is lowered when common mode noise occurs, and the common mode noise is significantly increased. Is reduced to. Therefore, the above problem can be solved.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a circuit diagram of a DC-DC converter showing an embodiment of the present invention, in which elements common to those in the conventional FIG. This DC-DC converter includes an input terminal 1 on the hot side and an input terminal 2 on the cold side, as in FIG. A capacitor 3 is connected between the input terminal 1 and the input terminal 2. The input terminal 1 is connected to the hot side of the primary winding 4a of the transformer 4. The cold side of the primary winding 4a of the transformer 4 is connected to the drain D of the MOS transistor 5, and the M
The source S of the OS transistor 5 is connected to the input terminal 2. The gate G of the MOS transistor 5 is connected to a control circuit 6 that generates a gate drive voltage having a predetermined switching frequency. The hot side of the secondary winding 4b of the transformer 4 is connected to the anode A of the diode 7, and the cathode K of the diode 7 is connected to one electrode 8 of the capacitor 8.
connected to a. The cold side of the secondary winding 4b of the transformer 4 is connected to the anode A of the diode 9, and the cathode K of the diode 9 is connected to the electrode 8a of the capacitor 8. The cold side of the secondary winding 4b of the transformer 4 is connected to the other electrode 8b of the capacitor 8 via the choke coil 10. Electrode 8a of capacitor 8
Is connected to the output terminal 11 on the hot side, and the electrode 8b of the capacitor 8 is connected to the output terminal 12 on the cold side. The output terminal 12 is connected to the input terminal 1 via a series resonance circuit in which a capacitor 13 and a coil 14 are connected in series. FIGS. 3A to 3C are time charts for explaining the operation of the DC-DC converter in FIG. 1, in which the vertical axis represents voltage and the horizontal axis represents time. The operation of the DC-DC converter in FIG. 1 will be described with reference to this figure.

FIG. 1 is an example of a forward type DC-DC converter, in which the MOS transistor 5 repeats ON / OFF operation based on the voltage waveform of the gate G shown in FIG. By transformer 4
A square wave voltage is generated in the secondary winding 4b of the above, and this is converted into a DC voltage by the rectifying / smoothing circuit including the diodes 7 and 9, the capacitor 8 and the choke coil 10. In the case of the pulse width modulation method, the switching frequency is constant and this DC
-The output voltage of the DC converter is the MOS transistor 5
Adjusted by changing the on time of. That is, the output voltage is adjusted by the pulse width of the voltage waveform of the gate G. The counter electromotive force generated in the primary winding of the transformer 4 is superimposed on the drain D of the MOS transistor 5. Therefore, the voltage waveform shown in FIG. 3B is generated between the drain D and the source S of the MOS transistor 5. On the other hand, when the capacitor 13 and the choke coil 14 are not provided, DC-
The common mode noise voltage generated between the input side and the output side of the DC converter has a waveform as shown in FIG. 3 (c), and between the drain D and the source S of the MOS transistor 5 shown in FIG. 3 (b). It is almost similar to the voltage waveform. That is,
The common mode noise voltage has a switching frequency as a fundamental wave and includes harmonics. The waveform shown in FIG. 3C shows a voltage waveform of the output terminal 12 with the input terminal 1 as a reference. Here, input terminal 1 and output terminal 1
Since a series resonance circuit of the capacitor 13 and the coil 14 is connected between 2 and 2, and the resonance frequency of the series resonance circuit is matched with the switching frequency of the MOS transistor 5, the impedance of the series resonance circuit becomes extremely small. The fundamental wave component having the largest common mode noise generated between the input terminal 1 and the output terminal 12 of this DC-DC converter is significantly reduced.

Next, an example of the result of reducing the common mode noise of the DC-DC converter of FIG. 1 in which the series resonance circuit of the capacitor 13 and the coil 14 is connected is shown. For example, in a DC-DC converter with a switching frequency of 320 kHz, an output voltage of 5 V, and an output current of 2 A, a resistance of 50 Ω is 1
Assuming that the voltage (that is, 0.2335 V) for supplying mW power is 0 dB, in the conventional DC-DC converter of FIG. 2, when the capacitance of the capacitor 13 is 10 nF, the level of the fundamental wave of common mode noise is -12. It is 1 dB.
On the other hand, in the DC-DC converter of FIG. 1 of the present embodiment, when the capacitance of the capacitor 13 is 10 nF and the induction coefficient of the coil 14 is about 30 μH (the resonance frequency in this case is about 290 kHz), common. The level of the fundamental wave of mode noise is -26.9 dB, which is 1 compared to the conventional level.
It is improved by 4.8 dB. As described above, in this embodiment,
As a means for reducing the common mode noise voltage generated between the input side and the output side of the DC-DC converter, a capacitor of several nF has been conventionally connected between the input side and the output side, whereas a MOS is used. Since the series resonance circuit of the capacitor 13 and the coil 14 in which the resonance frequency is adjusted to the switching frequency of the transistor 5 is connected between the input side and the output side, the common mode noise voltage is significantly reduced.
The present invention is not limited to the above embodiment, and various modifications can be made. The following are examples of such modifications.

(A) In the embodiment, the common mode noise between the input and output of the DC-DC converter is attenuated by the single series resonance circuit in which the resonance frequency is matched with the fundamental wave. A plurality of series resonance circuits having the resonance frequency set in may be connected in parallel. (B) The connection point of the series resonance circuit in which the capacitor 13 and the coil 14 are connected in series is not only between the input terminal 1 and the output terminal 12 of FIG. Between the terminal 2 and the output terminal 11, and the input terminal 2
And the output terminal 12. (C) The present invention is not limited to the DC-DC converter of FIG. 1, but is applicable to all DC-DC converters, switching regulators, and other circuits that generate common mode noise.

[0011]

As described in detail above, according to the present invention, a pulse train in which a DC voltage on the input side is switched at a constant frequency is added to an inductance, and the voltage generated in the inductance is rectified and smoothed. Conventionally, a capacitor is used as a means for reducing common mode noise having a frequency that is an integral multiple of the switching frequency that is generated between the input side and the output side of a voltage conversion device that outputs the output voltage that has been output to the output side. In contrast to the connection between the output side and the output side, one or more series resonance circuits whose resonance frequency is adjusted to an integral multiple of the switching frequency are connected, and the series resonance circuit is generated when the common mode noise occurs. Since the impedance of the circuit is reduced, the common mode noise can be significantly reduced.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a DC-DC converter according to an embodiment of the present invention.

FIG. 2 is a circuit diagram of a conventional DC-DC converter.

FIG. 3 is a time chart of FIG.

[Explanation of symbols]

4 Transformer 5 MOS Transistor 6 Control Circuit 7,9 Diode 8,13 Capacitor 10 Choke Coil 14 Coil

Claims (1)

[Claims] 1. A voltage converter for applying a pulse train obtained by switching a DC voltage on an input side at a constant frequency to an inductance and outputting an output voltage obtained by rectifying and smoothing a voltage generated in the inductance to an output side. In a common mode noise reduction circuit that reduces common mode noise having a frequency that is an integral multiple of the switching frequency that occurs between the input side and the output side of the input side, between the one end on the input side and the one end on the output side. A common mode noise reduction circuit, wherein one or more series resonance circuits having a resonance frequency that is an integral multiple of the switching frequency are connected.