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Input Filters for Switching Power Supplies
2018.09.06
Points of this article
・Input filters for switching power supplies are provided to address common mode noise and differential mode noise respectively.
・Common mode filters are used to handle common mode noise.
・To address differential mode noise, a filter is constructed from components such as capacitors, inductors, beads, and resistors.
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.
【Download Documents】 Switching Power Supply Basic of EMC and Noise Countermeasures
This is a handbook on the basics of EMC (electromagnetic compatibility) and noise countermeasures for switching power supplies. Based on the understanding of the basics of noise, it explains the noise countermeasures using capacitors and inductors in switching power supplies.
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Switching Noise
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Dealing with Noise Using Capacitors
- Understanding the Frequency Characteristics of Capacitors, Relative to ESR and ESL
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- Effective Use of Decoupling (Bypass) Capacitors Point 1
- Effective Use of Decoupling Capacitors Point 2
- Effective Use of Decoupling Capacitors, Other Matters to be Noted
- Effective Use of Decoupling Capacitors, Summary
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Dealing with Noise Using Inductors
- Frequency-Impedance Characteristics of Inductors and Determination of Inductor’s Resonance Frequency
- Basic Characteristics of Ferrite Beads and Inductors and Noise Countermeasures Using Them
- Dealing with Noise Using Common Mode Filters
- Points to be Noted: Crosstalk and Noise from GND Lines
- Summary of Dealing with Noise Using Inductors
- Other Noise Countermeasures
- Basics of EMC – Summary
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