Transformer for a Flyback Type Converter

The present invention relates to a high frequency pulse transformer which has an air gap along a magnetic path by a ferrite core, and in particular, relates to such a transformer which is used for voltage conversion in a switching power source of a flyback converter type.

FIG. 5 shows a circuit diagram of a flyback converter type switching power source, in which the symbol B is a DC power source, T is a transformer, TR is a transistor, CT is a control circuit, D.sub.s is a diode, C.sub.s is a capacitor, and Z.sub.L is a load. The transistor TR switches ON and OFF repetitively according to the control of the control circuit CT. When the transistor TR is in ON state, the current flows from the DC power source B, through the primary winding N.sub.p of the transformer T, the transistor TR, to the DC power source B. Then, the energy is stored in the transformer T when the transistor TR is in ON state. That energy stored in the transformer T is proportional to 1/2LI.sup.2, where L is inductance of the primary winding N.sub.p, and I is current in the primary winding N.sub.p. Next, when the transistor is in OFF state, that energy is released to the load Z.sub.L through the diode D.sub.s and the capacitor C.sub.s which functions as a rectifier and a smoother. Therefore, the inductance L of the primary winding must have the proper value so that a lot of energy is stored in the transformer during the ON state of the transistor, although an usual transformer for voltage conversion has inductance as large as possible.

If the inductance L is too large, the current I would be small, and the stored energy would be small. Similarly, if the inductance is too small, the stored energy would also be small although the current I is large.

Therefore, it is preferable that a transformer for a flyback converter type switching power source has an air gap in a magnetic path for reducing or having proper inductance.

We first tried this kind of high frequency pulse transformer for switching power source, as shown in Fig. 3, in which a pair of E-shaped ferrite cores 1 abutted each other, an air gap G provided in a center leg of the core, and a bobbin with a winding on said center leg 2. The air gap G is essential in a transformer of a flyback converter type switching power source for storing electro-magnetic energy.

However, we found that the transformer as shown in Fig. 3 in which an air gap G is provided at the center portion of the center leg 2 has the disadvantage that leakage flux .phi. which leaks from said air gap G interlinks with a winding 3 as shown in FIG. 4, and the winding 3 is subject to eddy current loss. Therefore, the temperature of the winding 3 is raised partially. That disadvantage is serious when the capacity of the transformer is large, like a transformer for a switching power source.

The object of the present invention is to overcome the disadvantages and limitations of a prior pulse transformer by providing a new and improved pulse transformer.

It is also an object of the present invention to provide a pulse transformer which is free from eddy current loss in a winding by the presence of an air gap, so that the partial high temperature is prevented, and the power efficiency of the transformer is improved.

The above and other objects are attained by a high frequency pulse transformer having a ferrite core (10,11) having a pair of side legs (10a11a) and a center leg (l0b, 11b) which is shorter than said side leg, so that a gap G is provided on the center leg, and an arm (10c, 11c) connecting said side legs with said center leg; a bobbin (15) having a cylindrical hollow center body (15a) and a pair of flanges (15b), having windings on said hollow center body, assembled with said ferrite core so that said center leg is inserted into the hollow center body of the bobbin; said gap G being located at end of said center leg (10a) under one of the extreme ends of said hollow center body of the bobbin (15); and no coil means (16a, 16b) being provided for allowing no winding in a predetermined length from said gap.

Preferably, said no coil means is a belt provided at the end of the hollow center body of the bobbin.

The foregoing and other objects, features, and attendant advantages of the present invention will be appreciated as the same become better understood by means of the following description and accompanying drawings wherein;

FIG. 1A is a cross sectional view of a high frequency pulse transformer according to the present invention,

FIG. 1B is a perspective view of a bobbin of the transformer of FIG. 1A,

FIGS. 2A, 2B and 2C show perspective views of a core for explaining manufacturing process of the core of the present transformer,

FIG. 3 shows a core of a prior pulse transformer,

FIG. 4 is an enlarged partial view of a prior pulse transformer which has a winding, and

FIG. 5 is a circuit diagram of a prior flyback converter type switching power source.

FIG. 1 shows a cross section of the high frequency pulse transformer according to the present invention, and FIGS. 2A through 2C show the producing steps of a core of the transformer.

In FIG. 1, the numeral 10 is an E-shaped ferrite core having a pair of long side legs 10a, a long center leg 10b, and an arm 10c coupling the side legs 10a with the center leg 10b. The numeral 11 is a second E-shaped ferrite core having a pair of short side legs 11a, a short center leg 11b, and an arm 11c coupling the side legs 11a with the center leg 11b.

The first E-shaped core 10 and the second E-shaped core 11 are manufactured as shown in FIGS. 2A through 2C. First, a closed ferrite core having a pair of windows as shown by the numeral 13 in FIG. 2A is shaped and sintered. Next, the core 13 is separated to two portions 10' and 11' as shown in FIG. 2B by cutting the core 13 along the line X so that a pair of asymmetrical E-shaped cores are obtained. Next, the center leg of one of the E-shaped cores is ground so that the center leg becomes shorter than the side legs by removing the portion Y as shown in FIG. 2C. It is a matter of design choice whether the center leg of the E-shaped core with the longer side legs (and the center leg) is ground as shown in FIG. 2C, or the center leg of the E-shaped core with the shorter side legs is ground.

A bobbin 15 has a cylindrical hollow body 15a and a pair of flanges 15b at the ends of the body 15a. The center body 15a has a pair of belts 16a and 16b at the extreme ends of the body 15a, so that the center body 15a is thicker at the ends than other portion of the center body l5a of the bobbin 15.

A primary winding 17 of the transformer is wound on the non-thicked portion of the bobbin 15 (a primary winding 17 is not wound on the belts 16a and 16b). A secondary winding 18 of the transformer is wound on the primary winding 17 and the belts 16a and 16b through the insulation layer 19.

The pair of E-shaped cores 10 and 11, and the bobbin 15 with the windings are assembled together, so that the center leg 11b of the E-shaped core 10 is inserted into the hollow body 15a of the bobbin 15, and the side legs 10a of the E-shaped core 10 touches with the side legs 11a of the other E-shaped core 11. It should be noted that an air gap G is provided between the center legs 10b and 11b of the pair of E-shaped cores 10 and 11, and that said air gap G is positioned under the belt 16b where no primary winding is provided.

Therefore, even if a leakage flux exists around the air gap G, the leakage flux around the air gap G does not interlink with the primary winding nor the secondary winding because of the presence of the belt 16b on the bobbin 15. Therefore, no eddy current loss because of the leakage flux occurs in the windings, and so, the partial high temperature in the windings is prevented.

Accordingly, the insulation breakdown due to a partial over heat is prevented, and the operational reliability of a transformer is improved. Further, the insulation material for a lower temperature standard may be used for the present transformer (for instance, B type 130.degree. C. insulation material for a prior transformer may be replaced by A type 105.degree. C. insulation material in the present invention).

Further, because of the lack of eddy current loss, the conversion efficiency of a transformer is improved. Therefore, the size of a transformer and/or a ferrite core may be smaller than in the prior art.

As a modification, the combination of an E-shaped core and an I-shaped core is of course possible in the present invention, although the embodiment describes the combination of a pair of E-shaped cores. Further, the cross section of a center leg may either be rectangular, or circular.

From the foregoing it will now be apparent that a new and improved transformer has been discovered. It should be understood of course that the embodiments disclosed are merely illustrative and are not intended to limit the scope of the invention. Reference should be made to the appended claims, therefore, rather than the specification as indicating the scope of the invention.