Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Parameter related to the reproduction or fidelity of a...
Reexamination Certificate
2001-02-22
2003-08-05
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Parameter related to the reproduction or fidelity of a...
Reexamination Certificate
active
06603317
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a leakage current reduction circuit and a power supply employing the leakage current reduction circuit, and in particular, to a leakage current reduction circuit capable of reducing leakage current of a switching power supply.
DESCRIPTION OF THE RELATED ART
AC adapters are widely used as power supplies of electronic devices, especially for portable electronic devices such as notebook computers. An AC adapter generally includes a Y capacitor as a component for preventing EMI (Electro-Magnetic Interference). The electric shock problem of electronic devices is mainly attributed to two factors: leakage current and input voltage. If we assume that the input voltage is set constant, the leakage current is determined by the capacitance of the Y capacitor. When the capacitance of the Y capacitor is decreased, the leakage current becomes smaller and thereby the electric shock to the user of the electronic device also becomes smaller. In order to reduce the leakage current to a level at which the electric shock is negligible, the capacitance of the Y capacitor has to be reduced to a very low level, while the prevention of the EMI becomes incomplete if the capacitance is reduced to the low level. Therefore, under the present situation, the capacitance of the Y capacitor can not be reduced enough for the reduction of the leakage current and the electric shock problem.
FIG. 1
is a circuit diagram showing an example of a conventional AC adapter employing a Y capacitor. The AC adapter
104
A shown in
FIG. 1
, which is implemented as a switching power supply of flyback type (flyback converter), includes a rectifier
41
(which is composed of a diode bridge), a smoothing capacitor
42
, a transformer
43
, a rectifier
44
(which is composed of a diode), a smoothing capacitor
45
, a controller
46
, a switching element
47
, and a Y capacitor
5
. Commercial AC power from an AC socket
6
is supplied to the AC adapter
104
A through an AC cable
7
.
The AC input is rectified by the rectifier
41
and the rectified output of the rectifier
41
is smoothed by the smoothing capacitor
42
, thereby conversion of the input AC voltage to a DC voltage is conducted first. The DC voltage is converted again to an AC voltage by on-off action of the switching element
47
, and the AC voltage is supplied to the primary coil of the transformer
43
. The on-off action of the switching element
47
is controlled by the controller
46
.
An AC voltage obtained at the secondary coil of the transformer
43
is converted again to a DC voltage by the rectifier
44
and the smoothing capacitor
45
. The DC voltage obtained by the above operation is supplied to a notebook computer
9
through a DC output cord
8
. The Y capacitor
5
for the prevention of the EMI is inserted between a grounding line
13
on the secondary side of the transformer
43
and the midpoint of the rectifier
41
on the primary side of the transformer
43
.
In every electronic device, a component equivalent to the Y capacitor
5
causes leakage current by nature. Therefore, when the voltage of the AC socket
6
is AC 120V, voltage difference of approximately AC 60V appears between metal part
11
of the notebook computer
9
and a ground
16
, thereby a leakage current which is proportional to the capacitance of the Y capacitor
5
and which depends on the resistance between the metal part
11
and the ground
16
passes between the metal part
11
and the ground
16
(that is, passes through the user of the notebook computer
9
). When the user (equivalent resistance
12
shown in
FIG. 1
) is touching the metal part
11
of the notebook computer
9
, there is a possibility that a leakage current of approximately 100 &mgr;A passes through the user. The leakage current of such a low level is safe for the user, however, there exist rare cases where the user feels an electric shock.
FIG. 2
is a circuit diagram showing another example of a conventional AC adapter. The AC adapter
104
B shown in
FIG. 2
is a switching power supply of the flyback type (flyback converter), further including two Y capacitors. Also in the example of
FIG. 2
, voltage difference of approximately AC 60V appears between the metal part
11
of the notebook computer
9
and the ground
16
, and there is a possibility that the user touching the metal part
11
feels an electric shock.
FIG. 3
is a circuit diagram showing another example of a conventional AC adapter. The AC adapter
104
C shown in
FIG. 3
is a switching power supply of RCC type (RCC converter). The AC adapter
104
C of
FIG. 3
has different composition on the primary side of the transformer
43
, in comparison with the conventional flyback-type AC adapter
104
A of FIG.
1
.
FIG. 4
is a circuit diagram showing another example of a conventional AC adapter. The AC adapter
104
D shown in
FIG. 4
is a switching power supply of the RCC type (RCC converter), further including two Y capacitors.
FIG. 5
is a circuit diagram showing another example of a conventional AC adapter. The AC adapter
104
E shown in
FIG. 5
is a switching power supply of forward type (forward converter). The AC adapter
104
E of
FIG. 5
has different composition on the secondary side of the transformer
43
, in comparison with the conventional flyback-type AC adapter
104
A of FIG.
1
.
FIG. 6
is a circuit diagram showing another example of a conventional AC adapter. The AC adapter
104
F shown in
FIG. 6
is a switching power supply of the forward type (forward converter), further including two Y capacitors.
Also in the examples of
FIGS. 3 through 6
, voltage difference of approximately AC 60V appears between the metal part
11
of the notebook computer
9
and the ground
16
and there is a possibility that the user touching the metal part
11
feels an electric shock.
The maximum permissible level of the leakage current has been determined by UL (Underwriters Laboratories Inc.) etc., and leakage current below the maximum permissible level is generally regarded as safe. However, there have been some reports in recent years that electric shocks are felt by some users even if the leakage current is within the maximum permissible level. Therefore, the reduction or elimination of the electric shock without sacrificing the EMI prevention capability is required today.
For meeting the request, grounding by use of 3-terminal AC input have been generally employed.
FIG. 7
is a circuit diagram showing an example of a conventional flyback-type AC adapter employing the 3-terminal AC input and the grounding, in which the same reference characters as those of
FIG. 1
designate the same or corresponding parts to those of FIG.
1
and thus repeated description thereof is omitted for brevity. In the example shown in
FIG. 7
, the AC socket
6
is composed of three terminals including a GND (grounding) terminal which is grounded. The electric shock can be eliminated by connecting a GND (grounding) terminal of the AC adapter
104
G to the GND terminal of the AC socket
6
by use of a grounding wire.
However, the AC sockets
6
employed in ordinary houses and office buildings have 2-terminal structure in most cases, and thus the grounding to the GND terminal is difficult. Even if the AC socket
6
is provided with a GND terminal, portable electronic devices (notebook computers etc.), which are supposed to be carried freely, can not be connected to the GND terminal by use of the grounding wire constantly.
On the other hand, if a portable electronic device is always required to be connected to the GND terminal by use of the grounding wire, portability has to be sacrificed and the advantage and commercial value of the portable electronic device are necessitated to be impaired.
SUMMARY OF THE INVENTION
It is therefore the primary object of the present invention to provide a leakage current reduction circuit and a power supply employing the leakage current reduction circuit, by which the leakage current and the electric shock problem of an electronic device employing the leakage current
LeRoux Etienne P
McGinn & Gibb PLLC
Metjahic Safet
NEC Corporation
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