Shenzhen Zhiyong Electronics Co., Ltd.
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interference generated between Line and ground or Neutral and ground. Set L/N
interference voltage as VL and VN, and this signal can be seen as the combination of
common mode voltage VC and differential mode voltage VD.
VL= VC+VD
VN=VC-VD
The commonly efficient way to control differential mode interference signal and common
mode interference signal is to add EMI filter on the switch power supply input circuit. The
EMI filter on the power supply input port is a type of low pass filter consisted of capacitor
and inductance. The immunity to interference fitting every unit is quite different.
The figure below is a typical level two EMI filter.
EMI filter include common mode choke L, differential capacitor Cx and common mode
capacitor Cy.
L:
Common mode choke is effective for common mode interference but not differential
mode interference. Theoretically the controlling effect is better when inductance is great,
but the increase of the turns of inductance will make the distribution capacitor larger and
the common mode suppression worse in high frequency.
Cx:
Differential mode capacitor always use Metalized Film Capacitors with value ranging
between 0.1~1uF.
Cy:
Used to control higher frequency common mode interference signal ranging between
2200~6800pF
Rx:
The leak resistance for Cx
As a result users can use EM5040B to analyze whether the problem is caused by common
mode or differential mode interference and chose corresponding component: common mode
choke L, differential capacitor Cx and common mode capacitor Cy to fulfill the debugging .
Power Supply Filter is usually used to control the conduct disturbance below 30MHz, but it
is also effective for radiation emission interference over 30MHz. According to the
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