JP5826024B2 - Noise reduction circuit - Google Patents

Description

  The present invention relates to a noise reduction circuit for a power supply and the like, and more particularly to a noise reduction circuit for an in-vehicle power supply.

  In recent years, with the aim of realizing a safer, more reliable, and more comfortable car, electric devices are being mounted on the car. In particular, control by electric equipment is important in hybrid vehicles and electric vehicles.

  Electromagnetic noise is radiated from such automotive electronic devices. Accordingly, an electromagnetic shield is used for spatial propagation and an EMI (Electro Magnetic Interference) filter is used for conductor propagation, corresponding to the noise propagation path.

  For example, in the in-vehicle power supply circuit 100, as shown in FIG. 8, an EMI filter 12 is provided between the transformer 10 and the output terminal Tout (see Non-Patent Documents 1 and 2).

"EMC countermeasures for automobiles", Trikeps, Hidetoshi Yamamoto, "Chapter 7 EMC countermeasure parts for in-vehicle electronic devices", 2008 Yuichi Mabuchi, “Study of power supply common mode current generation mechanism and reduction method of electronic equipment with wire harness connected” IEICE Transactions C, Vol. J89-C, No.11, pp.854-865 ( 2006)

  By the way, the midpoint M on the secondary side of the transformer 10 is grounded to the conductor case 14 and the vehicle body 16 of the power supply circuit 100, but there is a high-frequency potential difference between these reference potential and the output terminal Tout. A high-frequency current flows through the load 102. That is, the conventional filter cannot sufficiently reduce the high-frequency current that flows out of the power supply circuit 100 and causes noise.

  One aspect of the present invention includes a conductive case that houses a power supply circuit, and a first power supply circuit connected between a positive wiring and a negative wiring of the power supply circuit in the case and connected in series to each other. 1 capacitor Cp and 2nd capacitor Cn, a first inductance Lp connected between a connection point of the positive-side wiring and the first capacitor, and an output terminal of the power supply circuit, and the negative A second inductance Ln connected between a connection point of the wiring on the side and the second capacitor and a ground point to the case, and the first capacitor Cp and the second capacitor Cn Is connected to the case, and the output terminal and the grounding point are connected via a third capacitor Cb. The first capacitor Cp, the second capacitor Cn, the first capacitor Inductor p and the bridge circuit is constituted by the second inductor Ln, the noise reduction circuit, characterized in that it consists relationship schematic LpCp = LnCn.

  Here, it is preferable that the grounding point is provided only in the vicinity of the output terminal.

  In the noise reduction circuit, the first inductor Lp and the second inductor Ln are 0.1 μH or more and 20 μH or less, and the first capacitor Cp and the second capacitor Cn are 1 nF or more and 10 nF or less. Preferably it is.

  In addition, a transformer in which the positive wiring and the negative wiring are connected to a secondary winding is provided, and an AC voltage having a frequency of 50 kHz to 300 kHz is applied to the primary winding of the transformer. Is preferred.

  Such a noise reduction circuit includes a step-down DC-DC converter and can be applied to an in-vehicle power supply circuit mounted on a vehicle.

  ADVANTAGE OF THE INVENTION According to this invention, the noise reduction circuit of the power supply which can suppress high frequency electromagnetic noise effectively can be implement | achieved.

It is a figure which shows the structure of the power supply circuit in embodiment of this invention. It is a figure which shows the equivalent circuit of the noise reduction circuit in embodiment of this invention. It is a figure which shows the noise reduction effect in embodiment of this invention. It is a figure explaining the grounding method in the conventional power supply circuit. It is a figure explaining the grounding method of the power supply circuit in embodiment of this invention. It is a figure which shows the specific example of the grounding method of the power supply circuit in embodiment of this invention. It is a figure which shows the example of the grounding method of the power supply circuit in embodiment of this invention. It is a figure which shows the structure of the conventional power supply circuit.

  The power supply circuit 200 according to the embodiment of the present invention includes a transformer 20, rectifier diodes 22a and 22b, a filter circuit 24, and a case 26 as shown in FIG. The power supply circuit 200 is mounted on the vehicle and used to supply power to a load 202 inside and outside the vehicle.

  Note that although an example of an insulating DC-DC converter is shown as the power supply circuit 200 in this embodiment, the scope of application of the present invention is not limited to this.

  A semiconductor switch (for example, IGBT, power MOSFET, etc .: not shown) is connected to the primary side of the transformer 20, and an AC voltage is applied to the transformer 20 by an inverter circuit constituted by these semiconductor switches. On the secondary side of the transformer 20, a rectifier circuit and a filter circuit 24 including rectifier diodes 22a and 22b are connected. By these circuits, the AC voltage appearing on the secondary side of the transformer 20 is rectified and converted into a DC voltage and output from the output terminal Tout.

  In FIG. 1, the high-frequency current flowing from the point A to the point B via the vehicle body 204 can be suppressed by making the point A and the point B substantially the same potential in terms of high frequency. The point B is directly connected to the output terminal Tout of the power supply circuit 200, and is connected to electronic components disposed throughout the vehicle. Therefore, it is possible to reduce the loop current that causes high-frequency noise radiation.

  Note that substantially the same potential means that the high-frequency noise generated from the power supply circuit 200 is within a range of potential difference that does not adversely affect other electronic devices. In the on-vehicle power supply circuit 200, the potential difference is 30 μV or less, preferably 3 μV or less.

  The filter circuit 24 serves as a low-pass filter for the output, and reduces noise without increasing the number of components by balancing the impedance of the output lines.

  The filter circuit 24 can be equivalently represented as a bridge circuit shown in FIG. The voltage source Vd in FIG. 2 is a noise source and corresponds to the rectifier diodes 22a and 22b in FIG. Specifically, the counter electromotive force generated in the parasitic inductance inside Vd by the reverse recovery current that instantaneously flows when the rectifier diodes 22a and 22b are switched becomes the voltage source Vd.

  Hereinafter, a method of setting the A point and the B point to the same potential will be described.

  By the filter circuit 24, the output positive-side wiring is directly connected to the external circuit (external load 202), and the negative-side wiring is connected to the conductive case 26 as close as possible to the output terminal Tout. A capacitor Cb is inserted between the positive-side wiring and the negative-side wiring in the vicinity of the output terminal Tout, and the two wirings are short-circuited in the vicinity of the output terminal Tout at high frequency. This capacitor Cb short-circuits between the wirings at a high frequency, and also plays a role of reducing output ripple of the rectifier circuit. Therefore, it is preferable that the capacitor Cb has a sufficient frequency characteristic and sufficient capacitance for ripple reduction. For example, as the capacitor Cb, two types of capacitors having different characteristics such as a chip capacitor and a film capacitor may be used in combination.

Here, the potential at the point B from the equivalent circuit of FIG. 2 is expressed as Equation (1).

Therefore, in order to make the point A and the point B substantially the same potential, the relationship of Expression (2) may be established.

  Since the expression (2) does not include the frequency term (jω), the voltage value does not depend on the frequency if the bridge equivalent circuit of FIG. 2 is ideally configured. However, if there is a parasitic capacitor in the inductors Lp and Ln, or if there is a parasitic inductor in the capacitors Cp and Cn, the potential at the point B becomes frequency dependent, and a sufficient noise reduction effect in a desired frequency band. May not be obtained.

  For example, the basic operating frequency (carrier frequency) of a step-down DC-DC converter mounted on a hybrid vehicle is about 50 kHz to 300 kHz. In this frequency band, suitable inductors Lp and Lc are 0.1 μH or more and 20 μH or less, and capacitors Cp and Cn are 1 nF or more in order to reduce the ripple of the output signal and reduce noise in the AM radio band and FM radio band. 10 nF or less.

  FIG. 3 shows a result of measuring the potential difference Vcpn in the balanced state and the imbalanced state of the bridge equivalent circuit in the power supply circuit 200 according to the present embodiment. As apparent from FIG. 3, the potential difference Vcpn can be reduced in the frequency band from 60 MHz to 120 MHz by balancing the bridge equivalent circuit.

  Further, in the prior art, as shown in FIG. 4, a configuration in which a plurality of ground points GND are provided from the output negative side wiring to the conductive case 26 has been adopted. On the other hand, in the power supply circuit 200 according to the present embodiment, it is preferable to match the output negative side wiring with the potential of the case 26 in order to make the effect of noise reduction remarkable, as shown in FIG. It is preferable that the negative wiring 28 and the case 26 are grounded at one point of the ground point X. Further, it is preferable to short-circuit the negative wiring 28 and the case 26 in the vicinity of the output terminal Tout. The negative wiring 28 is preferably arranged so as to have a parasitic inductance value of 1 nH or less so that the impedance becomes a sufficiently low value (1Ω or less) even in the FM radio band.

  Specifically, as shown in FIG. 6A, the case 26 is divided into a lid 26a and a main body 26b, and the negative wiring 28 of the substrate 30 of the power supply circuit 200 and the main body 26b of the case 26 are electrically conductive. Connect with the negative plate 32. Furthermore, the conductive positive side plate 36 is drawn out from the positive side wiring 34 of the substrate 30 so as to cover at least a part of the negative side plate 32 and is used as the output terminal Tout. At this time, as shown in FIG. 6B, the main body 26b of the case 26 is provided with a fixing portion (screw hole) 38 for fixing the negative side plate 32 with a screw or the like.

  Further, as shown in FIG. 7A, the negative wiring 28 of the substrate 30 of the power supply circuit 200 and the main body 26b of the case 26 are connected by a conductive negative plate 32, and further a conductive cover plate. 40 may be configured to cover the positive side plate 36. At this time, as shown in FIG. 7B, the main body 26b of the case 26 may be provided with two fixing portions (screw holes) 38 for fixing the negative side plate 32 with screws or the like.

  As described above, according to the power supply circuit in the present embodiment, the noise voltage caused by the inrush current (reverse recovery current, etc.) of the rectifier diode is induced at the output terminal by the balance circuit including the bridge of the capacitor and the inductor. Can be prevented. Further, since the output terminal has the same potential as the case in terms of high frequency, it is possible to suppress high frequency current in the wiring connecting the output terminal and the load. Moreover, the high frequency current which flows into the vehicle body between the load and the power supply circuit can also be suppressed.

  10 transformer, 12 filter, 14 conductor case, 16 vehicle body, 20 transformer, 22a, 22b rectifier diode, 24 filter circuit, 26 case, 100, 200 power supply circuit, 102, 202 load, 204 vehicle body.

Claims (5)

A conductive case that houses the power circuit;
A first capacitor Cp and a second capacitor Cn connected in series between the positive and negative wirings of the power supply circuit in the case;
A first inductance Lp connected between a connection point between the positive-side wiring and the first capacitor and an output terminal of the power supply circuit;
A second inductance Ln connected between a connection point between the negative wiring and the second capacitor and a ground point to the case;
With
A connection point between the first capacitor Cp and the second capacitor Cn is grounded to the case,
The output terminal and the grounding point are connected via a third capacitor Cb,
The first capacitor Cp, the second capacitor Cn, the first inductor Lp, and the second inductor Ln constitute a bridge circuit, and a relationship of approximately LpCp = LnCn is established. Reduction circuit. The noise reduction circuit according to claim 1,
The noise reduction circuit, wherein the grounding point is provided only in the vicinity of the output terminal. The noise reduction circuit according to claim 1 or 2,
The first inductor Lp and the second inductor Ln are 0.1 μH or more and 20 μH or less,
The noise reduction circuit, wherein the first capacitor Cp and the second capacitor Cn are 1 nF to 10 nF. The noise reduction circuit according to claim 3,
The positive side wiring and the negative side wiring comprise a transformer connected to a secondary winding,
An AC voltage having a frequency of 50 kHz to 300 kHz is applied to the primary winding of the transformer. A noise reduction circuit according to any one of claims 1 to 4,
A noise reduction circuit including a step-down DC-DC converter and mounted on a vehicle.

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