GB2245106A

GB2245106A - Gapped flyback smps transformer core with gap faces of reduced area

Info

Publication number: GB2245106A
Application number: GB9111508A
Authority: GB
Inventors: Masakazu Kazama
Original assignee: Murata Manufacturing Co Ltd
Current assignee: Murata Manufacturing Co Ltd
Priority date: 1990-05-29
Filing date: 1991-05-29
Publication date: 1991-12-18
Also published as: GB9111508D0; JPH0415220U; DE4116740A1

Description

TITLE OF THE INVENTION SWITCHING POWER SUPPLY TRANSFORMER BACKGROUND OF

THE INVENTION

FIELD OF THE INVENTION

The present invention relates to a converter for transforming a DC voltage and, more particularly, to a switching power supply transformer suitable for a flyback system converter.

DESCRIPTION OF THE PRIOR ART

In a conventional electronic device, DC voltages of various values are required, and required DC voltages are generated by using a converter for transforming a DC voltage. In such a converter, a current supplied to the primary side of a converter transformer is turned on/off by a switching transistor, and a transformed voltage obtained at the secondary side of the transformer is rectified to obtain a DC voltage. As a converter of this type, a flyback systerr (ON-OFF system) converter is well known, which is designed to generate a voltage at the secondary side of a transformer upon interruption of a current to the primary side of the transformer.

An EER type core shown in Fig. 1 is generally used in a switching power supply transformer of a flyback system converter. A gap G is formed in substantially the middle of central legs 10 of the EER type core. The surfaces of the central legs 10 which oppose each other through the gap G are abraded. With this arrangement, a flux F leaks outside near the gap G. When this leakage flux F passes through coils 11 (indicated by alternately long and short dashed lines) wound around the core, eddy currents are generated in the coils 11. When an operating frequency is increased, these eddy currents flow in a closed circuit constituted by the coils 11 and a capacitance formed between the coils 11 to cause an eddy current loss, generating heat in the coils.

Recently, in order to reduce the size of a transformer, the operating frequency of a converter tends to be increased. If the operating frequency becomes about 100 kHz or more, the generation of heat poses a serious problem.

In a conventional converter, in order to minimize the adverse effects of a leakage flux around the gap between the central legs of the core of a transformer, the manner of winding coils near the gap is set to be coarser than that at other portions, or litz wires (strands) are used as coils. The problem of the former method is that the winding amount of the coils is decreased as a whole. In the latter method of using the litz wires, an increase in cost is inevitable, and it is difficult to process end portions of the wires.

According to another known method, the entire coils are covered and shielded with a plate member called a short ring composed of a copper plate. The problem of this method 4 is that the number of components is increased, and the number of assembly operations is increased accordingly.

SUMMARY OF THE INVENTION

It is an object of the present invention to suppress generation of heat in coils by reducing the leakage of a flux at a core gap portion without using expensive litz wires and without increasing the number of components and assembly steps.

In order to achieve the above object, according to the present invention, there is provided a switching power supply transformer having an EER type core with a gap formed between central legs and used for a flyback system converter, wherein central leg portions, of the EER type core, located near the gap are formed to have smaller sectional areas than those of other portions..

With this arrangement, a leakage flux near the gap between the central legs of the core is reduced. In addition, since the coils are separated from the core at the gap, eddy currents generated in the coils are reduced, thus suppressing generation of heat in the coils due to eddy currents.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Fig. 1 is a side view showing a core of a conventional switching power supply transformer; Fig. 2 is a side view showing a core of a switching power supply transformer according to the present invention; Fig. 3 is a perspective view showing the lower half portion of the core in Fig. 2; and Figs. 4A and 4B are views respectively showing pairs of central legs, of the core of the switching power supply transformer of the present invention, which have distal end portions with different shapes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will be described below with reference to the accompanying drawings.

Fig. 2 is a side view showing only a core extracted from a switching power supply transformer according to the present invention. Fig. 3 is a perspective showing the lower half portion of the core in Fig. 2.

As shown in Fig. 2, the core is divided into upper and lower portions, i. e., an upper core half 1 and a lower core half 2, by a horizontal line A A. Each core half is integrally formed by pressing a ferrite material using dies.

Each core half has an E shape with two side legs and a central leg. The upper core half 1 has two side- legs la. and 1b and a central leg lc. The lower core half 2 has two side legs 2a and 2b and a central leg 2c. When the upper and lower core halves 1 and 2 are arranged to vertically oppose each other,, the side legs la and 2a, and 1b and 2b are bro- ught into tight contact with each other, but a gap G is formed between the central legs lc and 2c, thus forming an EER type core.

As shown in Fig. 3, each of the central legs 1 and 2c of the core is designed such that two cylindrical portions B and C having different diameters (d, and d2) are connected to each other through a tapered portion D. In consideration of a width d3 Of the gap G, the diameters d, and d2 Of the cylindrical portions B and C are set in the range of 0.6d, < d2 < 0.95dj.

If the distal end portions of the central legs lc and 2c of the upper and lower core halves 1 and 2 are tapered to reduce the sectional areas of the central legs lc and 2c near the gap G in this manner, since magnetic resistance at the central portions of the central legs lc and 2c becomes smaller than that at the peripheral portions, a flux passing through the central legs concentrates on the central portions of the central legs to be increased in density. Therefore, a leakage flux around the gap G is decreased in amount. In addition, since the core central legs and the coils are separated from each other near the gap G, a leakage flux passing through the coils is reduced. Owing to the above-described synergistic effect, eddy currents near the gap are reduced, and hence generation of heat in the coils is suppressed.

is Each of the central legs lc and 2c of the core may be constituted by only the tapered portion D to have a_ tapered distal end as shown in Fig. 4A, or may be designed such that the cylindrical portions B and C having the different diameters are directly connected to have a distal end portion without a tapered portion as shown in Fig. 4B. However, in consideration of the current core manufacturing technique of pressing a ferrite material, and variations in abrasion precision of gap-formed surfaces, the shape shown in Fig. 3, which has the cylindrical portion left on the distal end portion, is preferable. The distal end portion of each of the central legs lc and 2c may not be processed when the core is formed by pressing, but may be processed by cutting after the pressing.

As has been described above, in the switching power supply transformer of the flyback system converter according to the present invention, since the central legs of the EER type core are designed such that leg portions near the gap have smaller sectional areas than those of other portions, a leakage flux near the gap is reduced. In addition, since the coils are separated f rom the core at the gap, eddy currents generated in the coils are reduced to suppress generation of heat in the coils. Therefore, an increase in operating frequency can be achieved, and the transformer can be further decreased in size. According to the present i invention, a transformer can be manufactured at a low cost without using litz wires or a short ring and without increasing the number of steps in manufacturing a core.

8 -

Claims

WHAT IS CLAIMED IS:

1. A switching power supply transformer having an EER type core with a gap f ormed between central legs and used for a f lyback system converter, wherein central leg portions, of said EER type core, l_ocated near the gap are formed to have smaller sectional areas than those of other portions.

2. A transformer according to claim 1, wherein said EER type core is formed by causing two E type core halves, each having two side legs and a central leg, to oppose each other such that said two side legs of each of said core halves are brought into tight contact each other, said central leg of each of said core halves is formed by connecting a first cylindrical portion to a second cylindrical portion having a diameter smaller than that of said first cylindrical portion through a tapered portion such that said central leg portion is tapered toward a distal end thereof, and a gap is formed between said second cylindrical portions of said central legs of said core halves. 20

3. A transformer according to claim 2, wherein a diameter d, of said first cylindrical portion and a diameter d2 Of said second cylindrical portion are determined to satisfy the following relation: 0.6d, < d2 < 0.95d, 1

4. A transformer according to claim 1, wherein said EER type core is formed by causing two E type core halves, each having two side legs and a central leg, to oppose each other such that said two side legs of each of said core halves are brought into tight contact each other, said central leg of each of said core hal-ves is constituted by a first cylindrical portion and a tapered portion tapered from said first cylindrical portion toward a distal end thereof, and a gap is formed between the distal ends of said tapered portions of said central legs of said core halves.

5. A transformer according to claim 1, wherein said EER type core is formed by causing two E type core halves, each having two side legs and a central leg, to oppose ea ch other such that said two side legs of each of said core halves are brought into tight contact each other, said central leg of each of said core halves is constituted by connecting a first cylindrical portion and a second cylindrical portion having a diameter smaller than that of said first cylindrical portion, and a gap is formed between said second cylindrical portions of said central legs of said core halves.

1 Published 1991 at The Patent Office. Concept House, Cardiff Road, Newport, Gwent NP9 1RH. Further copies may be obtained fmm Sales Branch. Unit

6. Nine Mile Point. Cwnifelinfach. Cross Keys. Newport, NP1 7HZ. Printed by Multiplex techniques ltd, St Mary Cray. Kent.