1. Sources

A switching power supply inevitably generates some noise because it operates by switching high voltages. The main noise sources of the switching power supply are shown in Fig. 2.1.

• i.Switching transistor ----- (a)
• ii.Output rectifier ----- (b)
• iii.Output transformer ----- (c)

2. Pathways

The noise generated by the power supply is classified into several types based on its propagation paths, as shown in Fig. 2.1. The noise propagation paths are illustrated in Fig. 2.1.

• i.Line conduction: Conducted from input terminal of power supply
  
  • Differential-mode noise: Occurring between the AC input lines ----- ①
  • Common-mode noise: Occurring between the AC input line and FG ----- ②
• ii.Output noise: Conducted from output terminals of power supply
  
  • Differential-mode noise: Occurring between output lines ----- ③
  • Common-mode noise: Occurring between the output line and FG ----- ④
• iii.Radiated emission: Radiating from power supply or input/output line directly ----- ⑤
  (Radiation noise)