Electric power conversion systems – Current conversion – With means to introduce or eliminate frequency components
Reexamination Certificate
2003-06-17
2004-07-20
Han, Jessica (Department: 2838)
Electric power conversion systems
Current conversion
With means to introduce or eliminate frequency components
C363S047000
Reexamination Certificate
active
06765811
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the suppression of signal interference in equipment, and more particularly to employing a new grounding technique in a power supply or adaptor to suppress signal interference in equipment due to variable ground current flowing through a ground loop.
2. Description of the Related Art
In computer, electrical and electronic systems, grounding is an important process. According to IEEE, It is defined as a conducting connection, whether intentional or accidental, by which an electric circuit or equipment is connected to the earth, or to some conducting body of relatively large extent in place of the earth. It's used for establishing and maintaining the potential of the earth (or of the conducting body) or approximately that potential, on conductors connected to it, and for conducting ground current to and from the earth (or the conducting body). Besides, earthing is a process of connecting the grounding system to earth and is performed to keep the entire grounding system at earth potential.
A computer and electronic system, which includes a 3-pin power supply or adaptor, a personal computer (PC) or host unit, and a TV or some equipment, is shown in FIG.
1
. The internal circuitry of the power supply
100
is simplified, for example, as a DC voltage source
102
at secondary side of the transformer in the power supply
100
. “3-pin” is used to describe three conductors to the input of the power supply
100
for wire connections from Line, Neutral and Ground conductors of a power outlet. The three conductors include a line conductor
104
, a neutral conductor
106
, and a ground conductor
108
. The frame ground
118
is the Ground conductor of the power outlet and is connected to earth. The PC
120
as the host unit of the system is simplified as a load resistor R
1
and the power is supplied by the power supply
100
. The TV
130
is coupled to the PC
120
for signal processing and control, and is also simplified as an input impedance R
2
of the TV circuitry. Grounding is the process of interconnecting all of the power supply
100
, the PC
120
, and the TV
130
, and then connecting them to the earth. R
3
, R
4
, R
5
and R
6
represent the total impedances of all cables between the power supply
100
, the PC
120
, the TV
130
, and the frame ground
118
. Grounding is achieved through the cables
114
connecting
108
to
118
,
110
connecting
108
to
120
,
112
connecting
120
to
130
, and
116
connecting
130
to the frame ground
118
. The DC voltage source
102
supplies a current I
1
through a metal line to R
1
of the PC
120
. The current I
1
flows through R
1
and the line
110
carries the current I
1
back to
108
to complete the voltage source-load circuit. Similarly, there are a line carrying signal to the input impedance R
2
of the TV
130
and the line
112
carrying the signal back to the PC
120
. V
out
is the signal voltage of the PC
120
between the two lines that is supplied to the TV
130
.
Any closed loop connecting
100
,
120
and
130
serving as a signal reference loop is a ground loop (even though ground may not be involved). Practically speaking, there is impedance in all conductive lines, including any ground loop, so there is noise, interference voltages, between various points in any ground loop and in any conductive line when there is current flowing through them. The potential difference between any two points in a ground loop is often called ground voltage shift and is a major source of noise and interference in electronic circuits.
The current in a ground loop, called the ground current, may come from the voltage drop on R
4
of the cable
110
. The voltage causes a ground current I
g
to flow through the loop. The ground current I
g
would produce an interference voltage drop at the TV
130
side by flowing through R
5
, and thereby the voltage V
in
of
130
across R
2
would be about
V
in
=V
out
−I
g
*R
5
but not be equal to V
out
of
120
. I
g
*R
5
is the interference voltage drop at the TV
130
side. Ground currents, and thus ground voltage shifts, will cause distortion of signal at R
2
of
130
. Any potential in conducting lines connected to a ground loop with respect to a lower potential in the ground loop can cause ground currents.
Since the load current through R
1
of the PC
120
is generally variable, the ground current I
g
caused by a voltage across R
4
is also variable. Therefore, the interference voltage (I
g
*R
5
) is variable. The continuous change of the interference voltage is a significant noise and can interfere with the signals supplied to the TV
130
seriously, resulting in, for example, image quality degradations on the screen of the TV
130
.
For the forgoing reasons, there is a need for suppressing the noise and signal interference problem due to ground current in a ground loop. A possible solution to the signal interference problem is using thick lines, for example No.18 AWG (American Wire Gauge) lines, as the signal line
112
in
FIG. 1
to increase the protection of signal from noise and interference. Thick lines are not practical and suitable for computer and electronic systems. Another solution is reducing impedance of cable by reducing the lengths of conductors between
100
and
120
or
120
and
130
in the loop. This solution is also difficult to be implemented. As to grounding practice in a power supply, consider a typical power supply
200
, shown in
FIG. 2
, it contains a transformer
202
, a primary circuit on the primary side of the transformer
202
connecting to an external AC input voltage source V
AC
, and a secondary circuit on the secondary side of the transformer
202
producing the output voltage V
out
of the power supply
200
. The secondary circuit contains a diode D
1
as a rectifier, a filter capacitor C and sometimes has an extra voltage regulator. The output voltage V
out
is supplied to load equipment such as a PC. Between primary and secondary side, there is a capacitor C
3
for reducing EMI level. Common grounding practice is to connect the secondary side
206
to the outside frame ground
210
. In order to improve the signal interference problem, the present invention provides a better grounding method.
SUMMARY OF THE INVENTION
As described above, noise or signal interference problem has long been a serious concern in electronic circuits. The main object of the invention is to provide a new grounding technique in the design for an electronic power supply to suppress the signal interference problem in equipment of an electronic system including the power supply.
The traditional grounding practice is to connect the common signal references of the secondary circuits to the outside frame ground. On the contrary, the method of the present invention applied to the power supply is to form conductive paths connecting the common signal references of the primary and secondary circuits to the frame ground through a capacitor and an inductor connected to in series. The addition of a capacitor is to cut the otherwise direct conductive paths being parts of ground loops between the signal references and the ground conductors. The added capacitor is to reject DC or low frequency noise and helps keep electromagnetic interference (EMI) reduction performance because C
1
and C
2
can be adjusted as C
3
of FIG.
2
. The added inductor is to reject high frequency noise and interference, together with C
2
behaving like a LC filter. Most noise and interference signals cannot pass through the formed conductive paths containing a capacitor and an inductor connected in series. But, the DC impedance of the inductor can't be over 0.1 ohm in order to obey safety regulations. As a result of this grounding technique, the degree of signal interference problem is greatly reduced. Accordingly, the invention provides a good method for suppressing the inevitable signal interference problem in electronic circuits.
REFERENCES:
patent: 3731234 (1973-05-01), Collins
patent: 5847941 (1998-
Arima Computer Corporation
Han Jessica
Keefer Timothy J.
Seyfarth Shaw LLP
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