Inductor devices – With electric and/or magnetic shielding means
Reexamination Certificate
2000-08-30
2002-04-23
Mai, Anh (Department: 2832)
Inductor devices
With electric and/or magnetic shielding means
C336S182000, C336S183000
Reexamination Certificate
active
06377153
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a transformer apparatus, and in particular, to a transformer apparatus for use in an insulated switching power supply apparatus for the purpose of reduction of switching noise flowing between a primarily side ground and a secondary side ground of the transformer apparatus.
2. Description of the Related Art
First of all, an impedance meter is cited as an conventional example using an insulated switching power supply apparatus, and then, the insulated switching power supply apparatus, in particular, a performance required for a transformer apparatus used therefor will be explained below.
FIG. 7
is a circuit diagram showing an example of an impedance meter when one end of a test element is grounded, that is, in one-line grounded measurement. In the arrangement shown in
FIG. 7
, an ampere meter
73
has a ground point common to a ground point of a test element
70
. However, it is necessary to separate an alternating current signal source
71
and a voltmeter
72
from the ground point of the test element
70
. For this reason, an electric power must be supplied to the alternating current signal source
71
and the voltmeter
72
from the insulated switching power supply apparatus of DC-DC converter.
In the case of supplying an electric power to the alternating current signal source
71
and the voltmeter
72
from an insulated DC-DC converter
100
, this leads to such a problem as a switching noise of the insulated DC-DC converter
100
. As shown in
FIG. 8
, when a switching noise current flows between a primary side ground and a secondary side ground of the DC-DC converter
100
, the switching noise current flows into the ampere meter
73
of the impedance meter, and then, this interferes an impedance measurement.
A method of evaluating the switching noise current flowing into the ampere meter
73
to give a quantitative index is shown in
FIG. 9
, paying attention to a performance of a single transformer apparatus, which is a part for determining a magnitude of switching noise. A transformer apparatus is connected in a manner as shown in
FIG. 9
, and then, an electrostatic capacitance value C is measured by using the following equation:
C=(I/V)×(1/j&ohgr;) (1)
where I denotes a value measured by the ampere meter
93
;
V denotes a value measured by the voltmeter
92
; and
&ohgr; denotes an angular frequency (=2&pgr;f), where f denotes a signal frequency of a signal source
91
.
The factor when a current flows between the primary side ground and the secondary side ground is not always electrostatic coupling between the primary side ground and the secondary side ground, and in many cases, it is due to a leakage magnetic flux. In any case, when a voltage of the primary side exciting the transformer apparatus is set to a constant value, there are many cases where a current flowing between the primary and secondary sides is proportional to the frequency, and then, it is convenient to express the amplitude of the noise current as an electrostatic capacitance value.
FIGS. 10 and 11
show a structure of a conventional transformer apparatus. Referring to
FIGS. 10 and 11
, coaxial cables are utilized as lead wires of a primary winding
121
and a secondary winding
122
so as not to generate a magnetic flux outside the transformer apparatus. These primary winding
121
and secondary winding
122
are subjected to electrostatic shield
115
, and then, respective electrostatic capacitances C
10
between the primary winding
121
and a secondary side ground
132
and between the secondary winding
122
and a primary side ground
131
are small. Accordingly, this is not a principal factor of switching noise current flowing via the primary side ground and the secondary side ground (See FIG.
12
).
The problem is leakage magnetic fluxes
141
and
142
existing outside a core
110
. A leakage magnetic flux crossing across a space is classified into two cases as shown in
FIG. 13
, that is, an inside of the transformer apparatus and an outside thereof. In the transformer apparatus, the leakage magnetic flux
142
crosses across the space formed by the primary side ground and the secondary side ground so as to generate an electromotive force, and then, a switching noise current flows though the electrostatic capacitance between the primary side ground and the secondary side ground. Moreover, it is possible to cancel the generated electromotive force depending upon a shape of the primary side ground or the secondary side ground, and a position of ground lead wire. However, in the conventional transformer apparatus, it is difficult to find out the optimal shape and position of the lead wire, and also, it is difficult to obtain a geometric reproducibility. Therefore, canceling effect by this method is low. On the other hand, on the outside of the transformer apparatus, the leakage magnetic flux
141
crosses across the space formed by the lead wire of the primary side ground, the lead wire of the secondary side ground and an ampere meter connecting both the grounds so as to generate an electromotive force, and this becomes a factor of generating a switching noise current.
When the electrostatic capacitance value C is measured according to the above method shown in
FIG. 9
, in the conventional transformer apparatus, the limit of electrostatic capacitance is about 200 fF. An influence will be described when the aforesaid conventional transformer apparatus is utilized for the DC-DC converter
100
of the impedance meter shown in FIG.
8
. Assuming that the switching frequency of the DC-DC converter
100
is set to 200 kHz, and the voltage of the switching frequency component of a primary side voltage exciting the transformer apparatus is set as 12 Vrms, then a switching noise current flowing through the ampere meter shown in
FIG. 8
is obtained by the following equation:
12Vrms×(2×&pgr;×200kHz×200fF)≈3&mgr;Arms (2).
In the case of measuring a 100 k&OHgr; resistance at a 100 mVrms signal by the impedance meter, a measurement signal flowing through the ampere meter
73
becomes 1 &mgr;Arms. Therefore, the above switching noise current of 3 &mgr;Arms is larger than that of the measurement signal. For this reason, it is impossible to avoid a saturation of the ampere meter by the switching noise current.
As described above, in the DC-DC converter
100
using the conventional transformer apparatus, a switching noise current generated by the DC-DC converter
100
becomes large, and then, it is difficult to high accurately measure a minute current.
SUMMARY OF THE INVENTION
An essential object of the present invention is accordingly to provide a transformer apparatus capable of reducing a switching noise current flowing between a primary side ground and a secondary side ground.
According to one aspect of the present invention, there is provided a transformer apparatus comprising:
a first core having a first primary winding wound around the first core;
a second core having a substantially same structure as that of the first core, and having a second primary winding wound around the second core, the second primary winding being connected in parallel to the first primary winding;
a first conductor housing;
a second conductor housing; and
a third core having a secondary winding around the third core, the third core being entirely and electrostatically shielded by the first conductor housing,
wherein the third core is arranged so as to be sandwiched between the first and second cores respectively via first and second electrostatically shielding disks electrically connected to the second conductor housing, and
wherein the first, second and third cores are electrically connected by the second conductor housing operating as one-turn winding, and are electrostatically and magnetically shielded from the outside of the transformer apparatus by the second conductor housing.
Therefore, according to the present invention, in the transformer apparatus, the primary
Wakamatsu Hideki
Yamanaka Haruhiko
Agilent Technologie,s Inc.
Mai Anh
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