Noise filter for electric apparatus

A noise filter connected to power lines includes two coils connected to the power lines in series, respectively, two capacitances connected to one end of the coils, respectively, and at least one resistance for reducing the gain of a resonance frequency of the filter the resistance being connected to the capacitances and grounded.

BACKGROUND OF THE INVENTION 
The present invention relates to a noise filter and, more particularly, to 
a noise filter for an electric apparatus such as an induction heating 
apparatus which prevents noise produced from the electric apparatus from 
leaking to the outside via a power line. 
Noise produced from an electric apparatus leaks out through a power line 
connected to the electric apparatus. To solve this problem, a noise filter 
is generally used and inserted into the power line of an electric 
apparatus for enabling the prevention of noise leakage to the outside. 
However, a part of the noise filter of a related art of the present 
invention tends to form a resonance circuit. Therefore, when the resonance 
circuit is excited at a resonance frequency, the noise from the electric 
apparatus may leak out to the outside through the power line. Accordingly, 
it is desired that a novel noise filter be provided to prevent the noise 
produced from the electric apparatus from leaking to the outside through 
the power line by restricting an internal resonance phenomenon. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved noise filter which prevents noise produced from an electric 
apparatus from leaking out to the outside through a power line by limiting 
the occurrence of an internal resonance phenomenon. 
It is another object of the present invention to provide an improved noise 
filter for an electric apparatus which reduce noise that influences other 
electric apparatus. 
It is still another object of the present invention to provide an improved 
noise filter for an electric apparatus which prevents noise which may 
block a radio frequency wave from leaking out. 
It is a further object of the present invention to provide an improved 
noise filter for an electric apparatus which can reduce a self-resonance 
operation by inserting resistance means. 
Other objects and further scope of applicability of the present invention 
will become apparent from the detailed description given hereinafter. It 
should be understood, however, that the detailed description of and 
specific examples, while indicating preferred embodiments of the 
invention, are given by way of illustration only, since various changes 
and modifications within the spirit and scope of the invention will become 
apparent to those skilled in the art from this detailed description. 
To achieve the above objects, according to an embodidment of the present 
invention, a noise filter connected to power line means comprises coil 
means connected in series to said power line means, capacitance means 
connected to one end of said coil means, and resistance means for 
controlling a resonance phenomenon, one end of the said resistance means 
being connected to one end of said capacitance means, the other end of the 
resistance means being grounded.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
FIG. 1 shows a circuit diagram of a noise filter according to an embodiment 
of the present invention. A noise filter 1 of the present invention 
comprises choke coils 2 and 2', line bypass capacitances 3 and 3', a 
line-across capacitance 4, and a resistance 9. 
An alternating current (AC) power source 20 is connected to input terminals 
6 and 6'. A main part 25 of an electric apparatus is connected to output 
terminals 7 and 7' for the power source 20. The noise filter 1 is inserted 
into and connected between the AC power 20 and the main part 25 of the 
electric apparatus for preventing noise from the electric apparatus from 
leaking out to the AC power source 20 through power lines connecting the 
elements 6-7 and 6'-7'. 
If the electric apparatus is an induction heating apparatus, the main part 
25 is an inverter. 
The choke coils 2 and 2' are connected between the input terminals 6 and 6' 
for the AC power source 20 and the output terminals 7 and 7' for the power 
source 20, respectively, or in other words, inserted into the power lines 
connecting the elements 6-7 and 6'-7' in series, respectively. 
The line-across capacitance 4 is connected between the power lines 
connecting the elements 6-7 and 6'-7'. One end of the line across 
capacitance 4 is conected between one end of the choke coil 2 and the 
output terminals 7, and the other end of the line-across capacitance 4 is 
connected between one end of the choke coil 2' and the output terminal 7'. 
One end of the line bypass capacitances 3 and 3' are connected to the sides 
of the output terminals 7 and 7', respectively, or in other words, 
connected between one end of the choke coils 2 and 2' and the output 
terminals 7 and 7', respectively, and the other ends of the line bypass 
capacitances 3 and 3' are connected to each other. 
The resistance 9 is connected to the connection point 8 between the line 
bypass capacitances 3 and 3', and further is grounded through the grounded 
line 10. 
The components 2, 2', 3, 3', 4, and 9 are packaged on a printed circuit 
substrate to form the noise filter of the present invention. 
The operation of the noise filter 1 will be described below. Normal mode 
noise produced from the main part 25 of the electric apparatus connected 
between the output terminals 7 and 7' is entered into the noise filter 1 
through the output terminals 7 and 7'. Because a metal casing or a metal 
portion of the electric apparatus is grounded for safety, a floating 
capacitance between the main part 25 of the electric apparatus and the 
metal casing or metal portion is produced, so that common mode noise is 
produced at both ends of the line bypass capacitances 3 and 3'. 
The choke coils 2 and 2' and the line-across capacitance 4 prevent the 
normal mode noise from leaking out to the AC power side through the power 
lines connecting the elements 6-7 and 6'-7', and the line bypass 
capacitances 3 and 3' prevent the common mode noise from leaking out to 
the AC power side through the power lines of the elements 6-7 and 6'-7'. 
If an inductance of each of the choke coils 2 and 2' is set about 30 .mu.H 
and the value of each of the line bypass capacitances 3 and 3' is set 
about 1000 pF, the resonance phenomenon having about 900 KHz frequency may 
occur. In the present invention, this resonance is reduced by the 
resistance 9, so that the unnecessary resonance is controlled and reduced 
without the generation of noise. 
Since a tank circuit comprises the choke coil 2 and the line bypass 
capacitance 3 at the resonance frequency of about 900 KHz, the resonance 
operation is carried out, and further, the main circuit of the main part 
25 and the tank circuit are connected to each other with the floating 
capacitance so that a gain may be produced by the resonance frequency. 
However, according to the present invention, because the resistance 9 is 
provided with the noise filter, the gain produced by the resonance 
frequency can be reduced by the resistance 9. 
In the relationship between a noise voltage and a noise frequency, when the 
noise frequency becomes high as shown in FIGS. 3(1), 3(2), and 3(3), the 
resistance 9 is connected to the impedance of the line bypass capacitance 
3 in series so that a bypass function is decreased. Accordingly, the value 
R of the resistance 9 may be, preferably, half through twice the impedance 
R.sub.L between the power lines of elements 6-7 and 6'-7' and the grounded 
line (1/2R.sub.L .ltoreq.R.ltoreq.2R.sub.L). 
FIG. 3(1) shows a relationship between a noise voltage and a noise 
frequency produced between the power line and the grounded line when the 
value R of the resistance 9 is about 0 ohm. In FIGS. 3(2) and 3(3), the 
values R are about 47 ohms and about 100 ohms, respectively. 
FIG. 2 shows a circuit diagram of a noise filter of another embodiment of 
the present invention. One end of resistance 11 and 12 are connected to 
the line bypass capacitances 3 and 3' in series, respectively. The other 
ends of the resistances 11 and 12 are connected to each other and are 
grounded through the grounded line 10. 
Because the unnecessary resonance is controlled by the noise filter of the 
present invention connected to the power lines, noise which may block a 
radio frequency wave is not leaked out to the outside through the power 
lines. 
The noise filter prevents the leakage of the noise such as switching noise 
from the main part of the apparatus to the outside through the power 
lines. 
In FIG. 1, if the resistance 9 were absent in the noise filter, the choke 
coils 2 and 2', the capacitances 3 and 3', the grounded line 10 and one of 
the power lines of the elements 6-7 and 6'-7' would form a resonance 
circuit. Therefore, the circuit of FIG. 1 without the resistance 9 could 
not be operated as a noise filter at a resonance frequency, and further 
would permit the leakage of noise through the power lines. 
When the value of each of the capacitances 3 and 3' is set about 1000 pF 
and the inductance of each of the choke coils 2 and 2' is set about 30 
.mu.H, the resonance is excited on a middle band frequency area, so that 
the resonance frequency becomes about 900 KHz. Accordingly, radio blocking 
on a middle frequency radio wave could be possible by the resonance 
phenomenon. In the present invention, because the noise filter includes 
the resistance 9, the above problem can be solved. 
The electric apparatus must be adapted to noise terminal voltage standard 
such as FCC, VDE, and CISPR. Therefore, the noise filter including 
impedance elements such as a coil and a capacitance is inserted into and 
connected to the power line to prevent noise from the electric apparatus 
from leaking out to the outside through the power line. The frequency in 
the standard is ranged in a wide-band frequency area, for example, between 
about 10 KHz and about 30 MHz. In the range of the wide-band area, the 
noise leakage to the outside is not suppressed by the resonance frequency 
of the resonance circuit including the coil and the capacitance of the 
noise filter and the gain may be obtained. In the present invention, the 
resistance is provided with the noise filter, to therefore enable the 
control of the self-resonance. The noise level can decrease by about 10 dB 
at the resonance point. On the contrary, although the impedance of the 
noise filter by the insertion of the resistance may be increased and the 
effect of the control of noise at a high frequency may be decreased, this 
problem does not influence the operation of the electric apparatus and can 
be ignored. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications are 
intended to be included within the scope of the following claims.