High-frequency large current handling transformer

Details High-frequency large current handling transformer High-frequency large current handling transformer

2001-12-06

2003-10-21

Nguyen, Tuyen T. (Department: 2832)

Inductor devices

Coil or coil turn supports or spacers

Printed circuit-type coil

C336S055000, C336S061000, C336S083000, C336S192000, C336S232000

Reexamination Certificate

active

06636140

ABSTRACT:

This invention relates to a transformer which can handle a high-frequency large current, which may be used, for example, with an inverter.
BACKGROUND OF THE INVENTION
An example of prior art transformer handling a high-frequency large current is shown in
FIGS. 1A and 1B
. In
FIG. 1A
, primary and secondary coils of ribbon-shaped conductors are wound on a bobbin
41
. The primary coil has winding start terminal
42
and a winding end terminal
43
. The secondary coil has a winding start terminal
44
and a winding end terminal
45
. These components form a coil unit
47
. E-shaped core halves
48
and
49
are inserted into a center hole of the bobbin
41
from opposite sides of the hole to such an extent that the front ends of the core halves
48
and
49
abut against each other. This complete a transformer shown in FIG.
1
B.
As is seen from
FIG. 1B
, the thickness H of the transformer is the sum of the thickness T of the core formed by the core halves
48
and
49
, the thickness U of the coils on one side and the thickness V of the coils on the opposite side of the bobbin
41
. Coils of transformers handling a large current, however, have an increased cross-sectional area, resulting in increased coil thicknesses U and V, which leads to increase of the overall thickness H of the transformer. In some cases, a heat sensing device, e.g. a thermistor, is disposed in intimate contact with the coils to avoid burnout of the coils. This causes a gap to be produced between coil layers, resulting in further increase of the coil thicknesses U and V.
Another example is shown in FIG.
2
. The example shown in
FIG. 2
is a transformer disclosed in U.S. Pat. No. 5,010,314, which is issued to A. Estrov on Apr. 23, 1991, entitled “LOW-PROFILE PLANAR TRANSFORMER FOR USE IN OFF-LINE SWITCHING POWER SUPPLIES”.
The transformer of Estrov uses planar conductors for coil windings to reduce the thickness of the coils. The transformer includes a printed circuit board
51
having a center window
52
. Coil conductors
53
and
54
formed in loop are disposed on opposite major surfaces of the board
51
. The conductors
53
and
54
are connected in series by soldering them through a through-hole
55
.
The printed circuit board
51
has a tab
56
on which a winding start terminal
57
and a winding end terminal
58
are disposed. Disposed over the opposite major surfaces of the printed circuit board
51
are insulating sheets
61
and
62
having respective windows
59
and
60
and having the same shape and size as the printed circuit board
51
excluding the tab
56
. In this manner, a stack
63
is formed.
A plurality of similar stacks
63
are prepared and stacked on the first stack to thereby form a coil unit
64
. The winding start terminal
57
of one board
51
and the winding end terminal
58
of adjacent board
51
in the coil unit
64
are soldered together, whereby primary and secondary coils having desired numbers of conductor turns are formed.
Bobbins
67
and
68
each in the form of a short rectangular tube having flanges
65
and
66
, respectively, are inserted into the window of the coil unit
64
from opposite sides of the unit
64
. Then, E-shaped high-frequency core members
69
and
70
are inserted into the window to thereby complete the transformer.
The dimensions of the windows
52
,
59
and
60
in the printed circuit board
51
and the respective ones of the insulating sheets
61
and
62
are equal to the outer dimensions of the rectangular tubular bobbins
67
and
68
. The distance between the flanges
65
and
66
with the front end surfaces of the bobbins
67
and
68
abutting against each other is equal to the height of the coil unit
64
. The shapes and sizes of the center leg of the core members
69
and
70
are conformal to the windows in the bobbins
67
and
68
.
The current-carrying capacity in the transformer shown in
FIG. 2
depends on the cross-sectional area of the conductors formed on the printed circuit board
51
. Usually, the maximum thickness of a conductor realizable by the printed circuit board technology is 0.1 mm, and the manufacturing cost is proportional to the conductor thickness. With the conductor thickness of 0.1 mm or so, the board tends to warp or deform during the formation of the conductors, and, therefore, the thickness of the board itself cannot be less than 1.0 mm. When conductors 0.1 mm in thickness are formed on the opposite major surfaces of the board having a thickness of 1.0 mm, the ratio of the cross-sectional areas of the conductors to the cross-sectional area of the coil is 20% or less.
Even when deformation or warpage of an individual board produced during the formation of the conductors is small, the coil unit
64
formed of a stack of a plurality of such boards may swell due to warpage of the individual boards, and, therefore, the unit
64
cannot be properly placed between the flanges
65
and
66
of the bobbins
67
and
68
. Also, if there are gaps between adjacent boards, vibrations and noise tend to be generated when current is supplied to the transformer. Also, such warpage will decrease reliability of soldered connections between conductors when a large current is supplied. For these reasons, the transformer shown in
FIG. 2
has a limit in practical use. It can be used only with the primary input of 200 V and 2 A or so.
Therefore, an object of the present invention is to provide a thin, high-frequency transformer which can handle a large current.
SUMMARY OF THE INVENTION
A transformer according to an embodiment includes a plurality of planar coil members, each of which coil members is formed of a metal sheet. The planar coil member has a window in its center portion. A slit extends outward from the center window. First and second terminals are disposed on the sheet at locations on opposite sides of the slit.
A higher-voltage coil is formed by stacking a plurality of such coil members with an insulating sheet disposed between adjacent coil members. Instead, coil members each having an insulating sheet bonded to its one or both surfaces may be used. The first terminal of one coil member is connected to the second terminal of the adjacent coil member so that the coil members in the stack are connected in series.
A lower-voltage coil is formed of one or more coil members. The number of the coil members to be used is determined in accordance with a desired number of turns and desired current-carrying capacity. Specifically, for one turn of the lower-voltage coil, one planar coil member is used if it can provide a sufficient current-carrying capacity. If, on the other hand, the current-carrying capacity provided by one coil member is insufficient, a plurality of coil members connected in parallel are used as a coil member assembly for one turn. Further, if a plurality of turns are desired, a plurality of coil members or coil assemblies are stacked with an insulating sheet disposed between adjacent coil member or coil member assemblies like the higher-voltage coil. As in the high-voltage coil, coil members or coil member assemblies each having an insulating sheet bonded to its one or both surfaces can be used, without disposing an insulating sheet between adjacent coil members or coil assemblies.
The higher-voltage coil and the lower-voltage coils are stacked into a tubular coil unit with a window in its center portion. The coil unit is combined with a core having a portion extending through the window in the coil unit.
The planar coil members can be joined together by screwing, riveting, welding or brazing. When riveting is employed, coupling between terminals is more or less unreliable, causing increase of electrical resistance, but the resistance exhibited at the riveted portions can be reduced by applying solder over the riveted portions.
The core is suitably in the form of an 8-shaped frame including two outer legs spaced from a center leg with a window disposed between the center leg and each outer leg. The coil unit is placed around the center leg, with the coil members extending through the windows in the

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