1. Common Mode Inductor Function and Principle
Common mode inductor (Common Mode Choke) is a key EMI suppression component, mainly used to suppress common mode interference. In switching power supplies, switching action generates common mode current that couples to ground through parasitic capacitance, forming conducted interference. Common mode inductors effectively attenuate conducted interference below 30MHz by increasing impedance in the common mode current path.
2. Core Material Selection
2.1 Ferrite
• High initial permeability (μi = 2000-10000)
• Low high-frequency loss
• Cost-effective
• Suitable for 100kHz-10MHz frequency band
• Disadvantage: low saturation flux density (Bs ≈ 0.4T)
2.2 Nanocrystalline
• High permeability (μi = 20000-80000)
• Good wideband characteristics
• Better saturation than ferrite
• Suitable for 30kHz-1MHz frequency band
• Higher cost
3. Magnetic Circuit Structure Design
3.1 Single Magnetic Circuit Structure
Two windings wound on the same core, differential mode current magnetic flux cancels out, common mode current magnetic flux adds up.
3.2 Winding Design Points
• Two windings must be tightly coupled, minimize leakage inductance
• Insulation between windings: withstand ≥1500VAC
• Recommend insulation class F (155°C) or above
4. Impedance Calculation
Common mode inductor impedance comes from:
• Complex permeability (μ' - jμ'') of core material
• Winding distributed capacitance
5. CISPR 32 Class B Design Points
To pass CISPR 32 Class B test, common mode inductor design must meet:
• Common mode impedance ≥200Ω at 150kHz
• Leakage current ≤3.5mA (for household equipment)
• Insulation resistance ≥10MΩ (500VDC test)
• Withstand voltage ≥1500VAC (between windings)
Ask ProMagTech EngineeringCommon mode inductor (Common Mode Choke) is a key EMI suppression component, mainly used to suppress common mode interference. In switching power supplies, switching action generates common mode current that couples to ground through parasitic capacitance, forming conducted interference. Common mode inductors effectively attenuate conducted interference below 30MHz by increasing impedance in the common mode current path.
2. Core Material Selection
2.1 Ferrite
• High initial permeability (μi = 2000-10000)
• Low high-frequency loss
• Cost-effective
• Suitable for 100kHz-10MHz frequency band
• Disadvantage: low saturation flux density (Bs ≈ 0.4T)
2.2 Nanocrystalline
• High permeability (μi = 20000-80000)
• Good wideband characteristics
• Better saturation than ferrite
• Suitable for 30kHz-1MHz frequency band
• Higher cost
3. Magnetic Circuit Structure Design
3.1 Single Magnetic Circuit Structure
Two windings wound on the same core, differential mode current magnetic flux cancels out, common mode current magnetic flux adds up.
3.2 Winding Design Points
• Two windings must be tightly coupled, minimize leakage inductance
• Insulation between windings: withstand ≥1500VAC
• Recommend insulation class F (155°C) or above
4. Impedance Calculation
Common mode inductor impedance comes from:
• Complex permeability (μ' - jμ'') of core material
• Winding distributed capacitance
5. CISPR 32 Class B Design Points
To pass CISPR 32 Class B test, common mode inductor design must meet:
• Common mode impedance ≥200Ω at 150kHz
• Leakage current ≤3.5mA (for household equipment)
• Insulation resistance ≥10MΩ (500VDC test)
• Withstand voltage ≥1500VAC (between windings)