The figures below show examples of noise suppression in inverter power
supplies for lighting equipment. Since low-frequency differential mode
noise is high, noise filters are installed as shown in figures (b),
(c), and (d).
Differential mode inductance is increased by adding more differential
mode choke coils in figure (b), adding another common mode choke coil
in figure (c), and increasing the size of the common mode choke coil in
figure (d). The reason why a common mode choke coil contains
differential mode inductance is that magnetic flux caused by
differential mode current does not entirely cancel out because it
produces leakage flux.
One would think that a capacitor with a larger constant should be used
for suppression against differential mode noise. However, increasing
the X-capacitor’s capacitance causes a problem of increased reactive
current. In addition, increasing the Y-capacitor’s capacitance causes a
problem of increased leakage current. For these reasons, a capacitor
with very large capacitance cannot be used. Therefore, when
differential mode noise is strong, it is necessary to increase
differential mode inductance as shown in figures (b), (c), and (d).
However, using the suppression methods shown below causes problems with
the increased number of components, with the mounting area, and with
the cost. To solve these problems, a choke coil with unchanged common
mode inductance and increased differential mode inductance is in demand. |
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