Input Filters for Switching Power Supplies

In the previous article, basic measures to address noise were summarized, differentiating between common mode noise and differential noise. This time, we briefly explain input filters for switching power supplies, and thereafter give more detailed explanations of each filter type.

Input Filters for Switching Power Supplies

As input filters for switching power supplies, filters with characteristics suited to dealing with common mode noise and with differential noise respectively are used.

Differential Mode Filters

Differential mode filters employ capacitors, inductors, ferrite beads, and resistors. The example shown is an π-type filter using an inductor and capacitors. These components act on noise components as follows.

• Capacitors: Bypass noise currents to GND.
• Inductors: Reflect noise currents.
• Ferrite beads: Inductive component reflects low-frequency noise currents,
  and resistive component converts higher-frequency noise currents into heat.
• Resistors: Convert noise currents into heat.

Common Mode Filters

To address common mode noise, a common mode filter (common mode choke) is used. Common mode filters can be broadly divided into filters for power supply lines and filters for signal lines. In general, common mode filters for power supply lines are used in the inputs to switching power supplies. The impedance of paths in which common mode currents flow is raised to shut out the common mode noise.

The next article will discuss capacitors and noise in greater detail.