Noise (Emission/EMI) type and How to reduce?

1. Souces

The noise occurring from power supply are selected according to the conducted terminal and the conduction pathway as shown below.

(1) Conducted noise

  1. Input side (Line conduction)
    • Differential-mode noise ----- j
    • Common-mode noise ----- k
  2. Output side
    • Differential-mode noise ----- l
    • Common-mode noise ----- m

(2) Radiated noise (Radiated emission) ----- n

Fig.1.1 The souces and pathways of noise

2. Countermeasures for noise occurring from the power supply

2.1 Countermeasures

(1) Countermeasures for line conduction

A noise filter is built in the power supply in order that a noise occurring from the inside of power supply doesn't return to input side. Please keep the output line and the input line separate when doing wiring, so as to avoid spoiling the effect of noise filter. In addition, to keep the input line away from the effect of radiated noise, please place the input line away from power supply.
Furthermore, to reduce the noise radiated from power supply and to avoid receiving an effect of noise due to long input line, please assemble external noise filter at the AC terminal of the unit case.

(2) Countermeasures for output noise

It is important to wire a power supply with thick and short wires. Differential-mode noise and common-mode noise can be reduced by setting a capacitor or a filter on the output line.

2.2 Countermeasures for radiated noise (Radiated emission)

A Noise becomes radio waves and radiates to midair, by cables such as the input line, the output line and the signal line extending toward the outside of unit which have antenna effect.
For the above reason, please firstly take countermeasures to avoid conducting a noise to input/output line. If it is not enough, please use some components such as noise filter or data line filter.
It is also effective to use a metal board or a metal film to shut the noise up in the power supply if the unit case is not metal.

It is conducted directly to unit through midair because the noise of switching power supply contains high frequency component. And it may be ineffective depending on the way of earthing.The power supply should connect unit to the same chassis. But if you want to connect them separately to the different chassis, please use a metal board instead of wire containing inductance. That is to say, the power supply which is a noise source and the unit which is a receiver of noise should be kept in low-impedance as against the chassis with stable voltage.

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