JP2009224439A - Magnetic core and transformer comprising the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that effective utilization of the performance of a transformer by miniaturizing it is not available, since a tolerable current value that can flow the transformer must be set to a tolerable current value for such high temperature as lower than the tolerable current value at normal temperature, as a switching element on the primary side is possible to be damaged when the current equal to or lower than the tolerable current value at normal temperature flows to a primary coil as it is even at a high temperature to exceed the tolerable current value at high temperature, which prohibits a current flow at such value equal to the maximum tolerable value of the transformer. <P>SOLUTION: Such magnetic core is provided as includes a spacer at least one gap of a magnetic path. The spacer is non-metal with a high linear expansion coefficient and high heat resistance. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a magnetic core and a transformer having the magnetic core.

A DC voltage is supplied to the secondary-side circuit using a switching element that switches a DC current flowing on the primary side and a transformer that transforms the primary-side voltage into a secondary-side voltage. In such a transformer, when the current flowing to the primary side is increased, a constant inductance is maintained in a range of a constant current value, and when the DC current is further increased, the core magnetic flux is saturated and the inductance is reduced. It has a direct current superposition characteristic that decreases rapidly. When the inductance rapidly decreases, an excessive direct current flows rapidly to the switching element provided on the primary side, which may damage the switching element. For this reason, a transformer with a DC superimposition characteristic that does not saturate the core magnetic flux even when the maximum current flows through the primary winding is designed.
Thus, for example, in Patent Document 1, a bonded magnet is inserted into the gap in the magnetic path of the magnetic core to improve the DC superposition characteristics and increase the allowable current value.

JP 2002-136128 A

  FIG. 8 shows a graph of the BH curve of the core. The horizontal axis H indicates the strength of the magnetic field, and the vertical axis B indicates the magnetic flux density. As indicated by the curve from the point Q1 to the point Q2 and the arrow D1, when the magnetic field H is gradually increased from 0 to the core that is not magnetized at first, the magnetic flux density B reaches the maximum value at the point Q2. This phenomenon is magnetic saturation, and the magnetic flux density at the point Q2 at this time is the saturation magnetic flux density Bmax. When the magnetic field H is weakened from this state, the curve from the Q2 point to the Q3 point changes in the direction of the arrow D2, and even if the magnetic field H becomes zero, the magnetic flux density does not become zero. The magnetic flux density at point Q3 at this time is the residual magnetic flux density Br. When the change amount of the magnetic flux density from the Q2 point to the Q3 point is ΔB, the change amount ΔB of the magnetic flux density is obtained by subtracting the magnetic flux density Br from the magnetic flux density Bmax (ΔB = Bmax−Br).

  The curves shown from the K1 point to the K2 point and the curves shown from the K2 point to the K1 point in FIG. 8 are curves showing changes in the magnetic flux density in the actual operating state of the transformer. When the amount of change in magnetic flux density from point K1 to point K2 (point from point K2 to point K1) is ΔR, the amount of change ΔR in magnetic flux density in the actual operating state of the transformer is obtained by subtracting the magnetic flux density B1 from the magnetic flux density B2 (ΔR = B2-B1). In consideration of changes in the magnetic flux density in the transformer operating state, in any case, the design must be such that the flux density change ΔR in the transformer operating state is smaller than the above-described flux density change ΔB. (ΔR <ΔB). The curves shown from the K1 point to the K2 point and from the K2 point to the K1 point indicate the actual working curve of the transformer that satisfies ΔR <ΔB, and are generally called minor loops.

  FIG. 9 shows an excerpt of characteristics of a typical ferrite core ML24D (Hitachi Metals). The saturation magnetic flux density Bmax in a 23 ° C. environment is 490 mT (unit: millitesla), and the residual magnetic flux density Br is 140 mT. When a voltage is applied to the transformer winding, an exciting current flows, and the magnetic flux density B rises by ΔR and reaches the magnetic flux density B2. Next, when the switching transistor is turned off, it returns to the point of the magnetic flux density B1 while discharging the stored energy. This is a change amount ΔR of the magnetic flux density B of the transformer in the actual working state, and as described above, ΔR = B2−B1. If the magnetic flux density B2 at this time is 400 mT and the magnetic flux density B1 is 100 mT, the saturation magnetic flux density Bmax is 490 mT in a 23 ° C. environment, and the maximum value B2 in the working state is 400 mT. Since B2 does not exceed the saturation magnetic flux density Bmax, the core is a safe region where it is not saturated. However, the saturation magnetic flux density Bmax and the residual magnetic flux density Br of the core have temperature coefficients, and the Bmax of 490 mT at the 23 ° C. environment becomes 360 mT at 100 ° C. The maximum magnetic flux density B2 = 400 mT when the maximum current flows in the transformer operating state exceeds the saturation magnetic flux density 360 mT at 100 ° C. of the core.

  Thus, when Bmax <B2, the core is magnetically saturated, the magnetic permeability is lost, the core is similar to an air-core coil, and the inductance is rapidly reduced, so that an excessive current flows and the switching element is destroyed. Therefore, in general, when designing a transformer, it is assumed that the maximum current flows when the temperature is high. Specifically, when the transformer is used for a long time, the temperature of the transformer becomes high. At this time, a special operation is assumed in which the primary side power supply is cut off and the power supply is turned on immediately. In order to realize that magnetic saturation does not occur at the moment of this special operation in a high temperature environment, the DC superimposition characteristic during normal operation must be set to an excessive margin, miniaturizing the transformer and improving the performance of the transformer There is a problem that it cannot be used.

  SUMMARY An advantage of some aspects of the invention is to solve some of the problems described above, and the invention can be implemented as the following forms or application examples.

  Application Example 1 A magnetic core comprising a spacer in at least one gap of a magnetic path, wherein the spacer has a high coefficient of linear expansion, high heat resistance, and is nonmetallic.

  According to this configuration, since the spacer inserted in at least one gap of the magnetic path has a high coefficient of linear expansion and high heat resistance, when the temperature of the transformer rises, the spacer linearly expands in the direction in which the magnetic flux flows. Therefore, the length of the gap in the direction in which the magnetic flux flows becomes longer. Furthermore, since the magnetic permeability of the spacer is low, it is possible to suppress the direct current superposition characteristics from deteriorating when the transformer becomes high temperature, and to prevent the allowable current value from decreasing. Therefore, since the current can flow up to the maximum allowable current value of the transformer, the transformer can be downsized and the performance of the transformer can be used effectively.

  Application Example 2 A transformer including the above-described magnetic core wound by a primary coil and a secondary coil.

  According to this structure, when a transformer becomes high temperature, it can suppress that a direct current | flow superimposition characteristic deteriorates, and it can suppress that an allowable current value falls. Since the current can flow up to the maximum allowable current value of the transformer, the transformer can be downsized to effectively use the performance of the transformer.

Embodiments of the present invention will be described below with reference to the drawings.
(Example)
FIG. 1 is an external perspective view of a magnetic core according to the present embodiment. As shown in FIG. 1, the magnetic core 1 of the present embodiment is one in which E-shaped magnetic cores 2 and 3 are opposed to each other. A transformer is formed by winding a primary side coil (not shown) and a secondary side coil (not shown) around opposing portions of the magnetic cores 2 and 3 in the center of the drawing.

  When a current flows through the primary side coil or the secondary side coil, the left side, center, and right side of the drawing, where the magnetic cores 2 and 3 face each other, form a magnetic path through which magnetic flux flows in the vertical direction of the drawing.

  Gap 4, 5, 6 is provided in the left, center, and right portions of the drawing where the magnetic cores 2, 3 face each other, and spacers 7, 8 are inserted into the gaps 4, 6, respectively. The lengths of the spacers 7 and 8 in the vertical direction of the drawing are L1 and L2, respectively.

The spacers 7 and 8 are made of a material having a high linear expansion coefficient, high heat resistance, and low magnetic permeability. For example, a resin such as polypropylene (110 × 10 ⁻⁶ / K) having a large linear expansion coefficient is used.

  FIG. 2 is a circuit diagram in which the voltage is transformed by the transformer 15 including the magnetic core 1 of the present embodiment. AC current from the commercial AC power supply 10 is rectified to DC current by the rectifier 11. For example, a direct current in which conduction and interruption are repeated is input to the primary coil 13 by the switching element 12 such as a power MOSFET. A direct current that is transformed by a transformer 15 including the magnetic core 1, the primary side coil 13, and the secondary side coil 14 of FIG. 1 and flows in one direction by the diode 16 flows to the secondary side terminals 17 and 18.

  FIG. 3 is a graph showing the DC superposition characteristics of the transformer 15. The horizontal axis represents the direct current value (A) flowing through the primary side coil 13, and the vertical axis represents the inductance (mH) of the transformer 15.

  A curved line M1 indicated by a thick solid line indicates a relationship between a direct current value flowing through the primary coil 13 at normal temperature (for example, 18 ° C.) and the inductance of the transformer 15. As shown by the curve M1, when the current value flowing through the primary coil 13 is increased, the inductance h1 is maintained within a constant current value range, and when the current value is further increased, the inductance is decreased.

  FIG. 4A is a waveform diagram of a direct current flowing through the primary coil 13 when the transformer 15 holds the inductance h1. The horizontal axis represents time, and the vertical axis represents the current value. FIG. 4B is a waveform diagram of a direct current flowing through the primary side coil 13 when the inductance rapidly decreases.

  In the curve M1, when the inductance rapidly decreases, the value of the current flowing through the switching element 12 in FIG. 2 rapidly increases as shown by the waveform in FIG. 4B. For this reason, the switching element 12 may be damaged.

  Therefore, the transformer 15 is formed so as to have a DC superposition characteristic that does not saturate even when the assumed maximum current flows. The allowable current value is set as follows. A current value at an inductance value that falls by a predetermined ratio (for example, 20%) from an inductance that is held constant when the current value flowing through the primary coil 13 is increased is defined as an allowable current value. P1 in FIG. 3 represents an intersection of the inductance h3 that is 20% lower than the inductance h1 and the curve M1. The direct current value at the intersection P1 is defined as the allowable current value i1 of the transformer 15.

  Next, the DC superposition characteristics when the transformer 15 reaches a high temperature (for example, 100 ° C.) will be described. The spacers 7 and 8 in FIG. 1 expand due to thermal expansion, and the lengths L1 and L2 of the spacers 7 and 8 become longer. That is, the length in the vertical direction of the drawing in the gaps 4, 5, and 6 is increased.

  Then, the transformer 15 has a DC superposition characteristic as shown by a curve M2 shown by a thick broken line in FIG. As indicated by the curve M2, when the value of the current flowing through the primary coil 13 increases, the inductance h2 is maintained within a certain current value range. When the current value is further increased, the inductance is decreased, and the inductance is further decreased by passing through the intersection point P1 where the value of the inductance h3 is intersected.

  Thus, even if the transformer 15 becomes high temperature, the allowable current value of the transformer 15 can be the allowable current value i1 indicated by the intersection P1. That is, the allowable current value of the transformer 15 at normal temperature and the allowable current value of the transformer 15 at high temperature can be set to the same allowable current value i1, and even if the transformer 15 is changed from normal temperature to high temperature, the DC superimposition characteristic is not deteriorated.

  As shown by the curve M2, the inductance h2 that holds a constant value in the curve M2 is smaller than the inductance h1 that holds a constant value in the curve M1, but exceeds the inductance h3 at the intersection P1, so that the switching element 12 There is no damage.

  FIG. 5 is an external perspective view of the magnetic core 1a from which the spacers 7 and 8 of the magnetic core 1 of FIG. 1 are removed. Here, for comparison with the present embodiment, as shown in FIG. 5, the DC superposition characteristics of a transformer (not shown) including a magnetic core 1a in which no spacer is inserted in the gaps 4, 5, and 6 will be described. FIG. 6 is a graph obtained by adding a thick one-dot chain line curve M3 indicating the DC superposition characteristics of the transformer including the magnetic core 1a of FIG. 5 to the curves M1 and M2 indicating the DC superposition characteristics of the transformer 15 of FIG.

  In the magnetic core 1a, the lengths of the gaps 4, 5, 6 at normal temperature and the lengths of the gaps 4, 5, 6 at high temperature are almost the same. Then, the curve M3 in FIG. 6 has a smaller current value range for maintaining a constant inductance than the curves M1 and M2. Further, when the value of the current flowing on the primary side is increased, the inductance in the curve M3 decreases and passes through the intersection P2 with the inductance h3. The current value at the intersection P2 becomes the allowable current value i2 of the transformer including the magnetic core 1a. As described above, in the transformer including the magnetic core 1a, when the temperature increases from room temperature to high temperature, the allowable current value decreases from i1 to i2, and the direct current superimposition characteristics deteriorate.

  If a current value exceeding the allowable current value i1 is passed through a transformer including the magnetic core 1a, the inductance is drastically decreased, and thus the switching element 12 may be damaged. Therefore, in the transformer including the magnetic core 1a, a current exceeding the allowable current value i2 cannot be passed through the primary coil 13 at a high temperature even if the allowable current value i1 or less is normal temperature.

  As described above, the magnetic core 1 according to the present embodiment includes the spacers 7 and 8 in at least one gap of the magnetic path, and the spacers 7 and 8 are nonmetals having a high coefficient of linear expansion and high heat resistance. It is.

  According to this configuration, since the linear expansion coefficient is high and the heat resistance is high, when the temperature of the transformer 15 rises, the linear expansion occurs. Therefore, the lengths L1 and L2 of the gaps 4 and 6 in the magnetic path of the magnetic core 1 are become longer. Further, since the spacers 7 and 8 having low magnetic permeability are provided in the gaps 4 and 6, magnetic saturation can be suppressed. Therefore, it is possible to suppress the deterioration of the direct current superposition characteristics and to suppress the allowable current value from being lowered.

  Moreover, in the transformer 15 including the magnetic core 1 wound by the primary side coil 13 and the secondary side coil 14, the allowable current value at high temperature and the allowable current value at normal temperature can be set to the same value. Therefore, the transformer 15 can be miniaturized and the performance of the transformer 15 can be used effectively.

  FIG. 7 is an external perspective view of a magnetic core 1b having a spacer 9 in the gap 5 at the center of the drawing. In this embodiment, the spacers 7 and 8 are provided in the gaps 4 and 6 on the left and right of the drawing in FIG. 1 where the magnetic cores 2 and 3 face each other. However, as shown in FIG. The magnetic core 1b may be used.

The external appearance perspective view of a magnetic core. The circuit diagram transformed with the transformer provided with the magnetic core. The graph which shows the direct current superimposition characteristic of a transformer. (A) is a waveform diagram of the direct current flowing through the primary side coil, and (b) is a waveform diagram of the direct current flowing through the primary side coil when the inductance rapidly decreases. The external appearance perspective view of the magnetic core which removed the spacer. The graph which added the curve which shows the direct current | flow superimposition characteristic of the transformer provided with the magnetic core of FIG. The external appearance perspective view of the magnetic core provided with the spacer in the center gap. The graph of the BH characteristic explaining saturation. A table excerpting core characteristics.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1, 1b ... Magnetic core, 4, 5, 6 ... Gap, 7, 8, 9 ... Spacer, 15 ... Transformer.

Claims (2)

A spacer is provided in at least one gap of the magnetic path,
The magnetic core is characterized in that the spacer is a non-metallic material having a high coefficient of linear expansion and high heat resistance.   A transformer comprising the magnetic core according to claim 1 wound with a primary coil and a secondary coil.

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