Conducted EMI filter design

Conducted EMI filter design for SMPS
• Introduction
• EMI in SMPS
• Common Mode(CM) noise
• Differential Mode(DM) noise
• Minimizing EMI in SMPS design
• Measuring conducted EMI
• EMI filter design
• Emi filter components
• Emi filter topology
• Calculating CM filter component values
• Calculating DM filter component values
• Determining filter corner frequencies
• Design steps
• Conclusions

Introduction
• The threat of generating EMI from the fast switching pulses in SMPS has always been a
serious concern
• Thus achieving the electromagnetic compatibility (EMC) has become a requirement as
important as meeting the power conversion specifications
• EMI includes three elements
• Source of the electromagnetic emission
• Coupling path
• A receiver of the EMI (victim)
• Conducted emissions 150kHz-30MHz
• Common mode(CM) measured between each power line and ground
• Differential mode measured between power lines
• Radieted emissions 30MHz-1GHz

EMI in SMPS
• Because of the fast switching in SMPS they generate large amount of electromagnetic
interferences and that’s usually the reason for SMPS not to comply the EMC standards
• EMI filter is usually needed in the input of the SMPS to achieve the required standards
• Conducted emissions 150kHz-30MHz
– CM common mode emissions :
paracitic capacitances and the switching voltage waveform across the switch

– DM differential emissions:
The swiching action causes current pulses at the input
Thus switching spikes exist as a differential mode noise source
• Radiated emissions 30MHz-1GHZ
– Magnetic and Elecric fields

EMI in SMPS
• Operation conditions also affects to the filter design
• The worst case should be always considered
– Highest input voltage leads to peak du/dt value
→CM noise will be maximum
– Lowest input voltage and maximum load current would lead to peak di/dt value
→DM noise will be maximum

EMI in SMPS Sources of CM noise

EMI in SMPS Sources of DM noise

• The switching action of the power mosfet causes current pulses at the input and voltage ripple at the output