Isolated converter

Details Isolated converter Isolated converter

2001-06-13

2002-04-16

Riley, Shawn (Department: 2838)

Electric power conversion systems

Current conversion

Integrated circuit

Reexamination Certificate

active

06373736

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an isolated converter for use in a switching power supply or the like.
2. Description of the Related Art
An isolated converter (such as a DC-DC converter and isolated AC-DC converter) used in a switching power supply or the like is formed of a transformer, a primary-side circuit connected to a primary coil of the transformer, and a secondary-side circuit connected to a secondary coil of the transformer such that power supplied to the primary-side circuit is transformed in terms of voltage by the transformer and a resultant voltage is output from the secondary-side circuit.
In some cases, when the isolated converter is used in a device which needs to be small in size, a multilayer sheet transformer is employed as the transformer as will be described below.
FIG. 4
illustrates, in the form of an exploded view, a multilayer sheet transformer.
FIG. 5
is a cross-sectional view of the multilayer sheet transformer taken along line A—A of
FIG. 4
, and
FIG. 6
is a cross-sectional view of the multilayer sheet transformer taken along line B—B of FIG.
4
.
As shown in
FIG. 4
, the multilayer sheet transformer
1
is formed integrally with a multilayer circuit board
4
on which a primary-side circuit
2
and a secondary-side circuit
3
are formed. The multilayer sheet transformer
1
includes coil patterns
6
(
6
a,
6
b
) formed on a plurality of sheet substrates shown in
FIG. 6
(three sheet substrates
5
a,
5
b,
and
5
c
in the example shown in
FIG. 6
) forming the multilayer circuit board
4
, a core member
11
(
11
a,
11
b
) which is E-shaped in cross section as shown in
FIG. 4
, and a core combining member
13
.
In the conventional technique, to achieve good electrical and magnetic characteristics such as conversion efficiency or the degree of coupling between the primary coil and the secondary coil of the transformer, coil patterns
6
a
of the primary coil and coil patterns
6
b
of the secondary coil are alternately placed in successive layers in the multilayer structure (hereinafter, such a multilayer structure of coil patterns
6
will be referred to as a sandwich structure). More specifically, as shown in
FIG. 6
, one set of the primary or secondary coil patterns
6
(the primary coil pattern
6
a
in the specific example shown in
FIG. 6
) is formed on the upper surface of each of sheet substrates
5
a,
5
b,
and
5
c,
and the other set of the primary or secondary coil patterns
6
(the secondary coil pattern
6
b
in the specific example shown in
FIG. 6
) is formed on the lower surface of each of sheet substrates
5
a,
5
b,
and
5
c.
These coil patterns
6
(
6
a,
6
b
) are coaxially disposed in the respective layers such that their central axes become coincident with each other so as to form a coil pattern unit
7
. The plurality of primary coil patterns
6
a
formed on the respective sheet substrates
5
a,
5
b,
and
5
c
are electrically connected to each other via connection conductors
15
(
15
a,
15
b
) extending via through-holes
14
(
14
a,
14
b
) so as to form the primary coil. Similarly, the plurality of primary coil patterns
6
b
are electrically connected to each other via connection conductors
17
(
17
a,
17
b
) extending via through-holes
16
(
16
a,
16
b
) so as to form the secondary coil.
In
FIG. 6
, reference numerals
8
denote insulating sheets (such as prepreg) disposed between sheet substrates to insulate the coil patterns
6
a
and
6
b
at vertically adjacent locations from each other.
As shown in
FIG. 4
, the multilayer circuit board
4
has core leg holes
10
formed at the center of the coil pattern unit
7
formed of the coil patterns
6
a
and
6
b
and at locations outside the coil pattern unit
7
. Core legs
12
of the E-shaped core members
11
a
and
11
b
are inserted into the corresponding core leg holes
10
from the upper and lower sides of the multilayer circuit board
4
such that end faces of the respective core legs come into direct contact with each other as shown in
FIG. 5. A
pair of E-shaped core members
11
a
and
11
b
in contact with each other is fitted in a core combining member
13
having a shape shown in
FIG. 4
such that the E-shaped core members
11
a
and
11
b
are combined together by the core combining member
13
and such that the coil pattern unit
7
is partially sandwiched by the respective E-shaped core members
11
a
and
11
b
inserted from the upper and lower sides of the multilayer circuit board
4
, as shown in FIG.
5
.
As described above, the multilayer sheet transformer
1
is formed integrally with the multilayer circuit board
4
. Use of the multilayer sheet transformer
1
formed in such a manner allows a reduction in the thickness of the isolated converter.
In the conventional structure described above employing the multilayer sheet transformer
1
, although it is easy to reduce the thickness of the isolated converter, it is difficult to reduce the size of the multilayer circuit board
4
and thus the total size of the isolated converter, for the reason described below.
That is, in the conventional structure, because the multilayer structure of the coil patterns
6
is obtained by means of disposing the coil patterns
6
into a sandwiched form as shown in
FIG. 6
, the coil pattern
6
(
6
a
) formed on the outer surface of the top layer of the multilayer circuit board
4
is a coil pattern of the primary coil, while the coil pattern
6
(
6
b
) formed on the outer surface of the bottom layer of the multilayer circuit board
4
is a coil pattern of the secondary coil. That is, the coil patterns formed on the outer surfaces of the top and bottom layers of the multilayer sheet transformer
1
are for different coils on either primary or secondary sides. Therefore, when a very large overvoltage appears in either the primary-side circuit
2
or the secondary-side circuit
3
for some reason, the overvoltage tends to create a spark, along the surface of the E-shaped core members
11
a
and
11
b
formed of ferrite or the like of the multilayer sheet transformer
1
, between the primary-side circuit
2
and the secondary-side circuit
3
, thereby causing an electrical breakdown between the primary and secondary circuits.
More specifically, a spark is easily created by an overvoltage between the primary-side circuit
2
and the surface of the E-shaped core member
11
a
close to the coil pattern
6
a
of the primary coil. Similarly, a spark is easily created by an overvoltage between the secondary-side circuit
3
and the surface of the E-shaped core member
11
b
close to the coil pattern
6
b
of the primary coil. If an overvoltage occurs, for example, in the primary-side circuit
2
, the overvoltage first creates a spark between the primary-side circuit
2
and the surface of the E-shaped core member
11
a
disposed on the upper side. The overvoltage then propagates to the E-shaped core member
11
b
disposed on the lower side and creates a spark between the secondary-side circuit
3
and the surface of the E-shaped core member
11
b
located on the lower side. As a result, an electrical breakdown occurs between the primary and secondary circuits.
To ensure that no electrical breakdown occurs between the primary and secondary circuits, it is required that the multilayer sheet transformer
1
and the secondary-side circuit
3
be spaced from each other by a large enough distance to prevent an electrical breakdown between the primary and secondary sides. Because of the necessity of the large isolation space between the primary and secondary sides of the multilayer sheet transformer
1
, it is difficult to reduce the size of the multilayer circuit board
4
and thus the total size of the isolated converter.
Further, as described earlier, because the coil patterns
6
have to be disposed so as to obtain the sandwich structure, the coil patterns
6
a
formed on the respective sheet substrates
5
have to be connected to each other by the connection conductors
15
(
15
a,
15
b
) via

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