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EET 423 POWER ELECTRONICS -2
Prof R T Kennedy
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POWER INDUCTORS Prof R T Kennedy
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THIN INDUCTOR LOW PROFILE CONVERTER
Prof R T Kennedy
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INDUCTOR TYPES MOLDED INDUCTOR AIR CORED INDUCTOR ‘AIR’ CORED INDUCTOR
TOROIDAL INDUCTOR The molded inductor is marker brown-black-black-gold, meaning 10 x 100 μH, or 10 μH, with 5% tolerance. Other inductor types are often unmaked, with value indcated on packaging materials only. Prof R T Kennedy
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SELECTING INDUCTORS Converter Currents waveforms operation mode
• Parameter Selection inductor specification Inductor Losses Construction Type coil v chip Inductors Electromagnetic Interference (EMI / EMC) shielded v non shielded inductors Prof R T Kennedy
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BUCK-FORWARD CONVERTER CURRENT
INDUCTOR CURRENT IL,M IL,m IL,av = Iout t Prof R T Kennedy
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INDUCTOR RIPPLE CURRENT
INDUCTOR CURRENT IL,M IL,m IL,av = Iout t SELECT L: BASED on pk-pk RIPPLE Prof R T Kennedy
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BUCK & FORWARD OPERATION MODE
SELDOM DCM increased capacitor requirements multiple outputs poor cross regulation Prof R T Kennedy
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HIGHER RIPPLE-DC RATIO SMALLER RIPPLE-DC RATIO
INDUCTANCE VALUE Ein = 2.7 V V Vout = 1.5 V fsw = 3 MHz Iout = 800 mA L = 1 mH HIGHER RIPPLE-DC RATIO > LOSSES SMALLER RIPPLE-DC RATIO > SIZE Prof R T Kennedy
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constant inductor voltage
IL t J constant inductance constant inductor voltage Prof R T Kennedy
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IL t L excess resistance less efficient Prof R T Kennedy
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inductor current exceeds Isat
IL t L peaky current reduced L inductor current exceeds Isat Prof R T Kennedy
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TYPICAL INDUCTOR SPECIFICATION
DESIGN LIMITING FACTORS TEMPERATURE RISE EFFICIENCY due to CORE & WINDING LOSSES CORE SATURATION Prof R T Kennedy
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INDUCTOR LOSSES SKIN EFFECT PROXIMITY EFFECT WINDING (COPPER) CORE
HYSTERESIS DC (DCR) AC (ACR) EDDY CURRENT SKIN EFFECT PROXIMITY EFFECT Prof R T Kennedy
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TYPICAL INDUCTOR SPECIFICATION
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DC WINDING (COPPER) LOSS
INDUCTOR DC RESISTANCE DC WINDING (COPPER) LOSS Prof R T Kennedy
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DC RESISTANCE v TEMPERATURE
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INDUCTOR AC RESISTANCE
SKIN EFFECT due to eddy currents produced by the ac current add to the outer conductor current subtract from the inner conductor current frequency increase majority of the current flows on the surface Prof R T Kennedy
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AC RESISTANCE: SKIN EFFECT
current density Prof R T Kennedy
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AC RESISTANCE v FREQUENCY
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INDUCTOR AC RESISTANCE v FREQUENCY
Coilcraft LPO AC RESISTANCE LOSSES Prof R T Kennedy
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AC v DC RESISTANCE Prof R T Kennedy
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COMPARATIVE LOSSES Prof R T Kennedy
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INDUCTOR SELF RESONANT FREQUENCY
at which inductor winding inductance resonates naturally with winding distributed capacitance Prof R T Kennedy
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INDUCTANCE v CURRENT Prof R T Kennedy
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INDUCTANCE v CURRENT Prof R T Kennedy
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SATURATION CURRENT v TEMPERATURE
25 oC 85 oC Prof R T Kennedy
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CORE SATURATION reduced inductance increased noise
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INDUCTOR RATED CURRENT
RATED CURRENT: smaller of saturation and RMS Prof R T Kennedy
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Isat >> DATASHEET SPEC !!!!
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INDUCTOR TEMPERATURE RISE
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FARADAY’S VOLT-TIME INTEGRAL
INDUCTOR VOLTAGE V1 t1 INDUCTOR CURRENT t2 V2 t I m I M T current start and finish at same value EQUAL AREAS Prof R T Kennedy
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‘IDEAL’ BUCK ANALYSIS CCM VOLT-TIME INTEGRAL APPROACH
INDUCTOR VOLTAGE IL Ein -Vout VL area A area B -Vout Dsw T Dfwd T t Prof R T Kennedy
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INDUCTOR CURRENT WAVEFORMS
CCM or DCM operational mode component current stress capacitor ripple current output voltage ripple converter efficiency closed loop regulation performance Prof R T Kennedy
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INDUCTOR CURRENT v INDUCTANCE
DswT Dfwd T Iout Ein-Vout -Vout VL IL t REDUCTION in L constant duty cycle Prof R T Kennedy
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INDUCTOR CURRENT v INDUCTANCE
increased Isw,max Ifwd,max IC,ripple Vout,ripple REDUCTION in L DswT Dfwd T Iout Ein-Vout -Vout VL IL t constant duty cycle Prof R T Kennedy
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INDUCTOR CURRENT Prof R T Kennedy
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variable duty cycle INDUCTOR CURRENT Dsw > 0.5 IL Dsw= 0.5 Iout
IL t Iout Dsw = 0.2 Dsw = 0.5 Dsw = 0.8 Dsw > 0.5 Dsw < 0.5 Dsw= 0.5 Prof R T Kennedy
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LOAD (R) INDEPENDENT INDUCTOR CURRENT DOWNSLOPE UPSLOPE IL t
t Prof R T Kennedy
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INDUCTOR PEAK-PEAK RIPPLE CURRENT
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CCM-DCM BOUNDARY Prof R T Kennedy
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CCM-DCM BOUNDARY boundary Prof R T Kennedy
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CCM-DCM BOUNDARY CCM boundary DCM Prof R T Kennedy
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CCM-DCM BOUNDARY L Dsw fsw constant CCM CCM / DCM determined by R DCM
INCREASE R ‘light loading’ DCM to ensure a desired CCM does not transfer to DCM specify a minimum load current (maximum R) avoid open circuit operation Prof R T Kennedy
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CCM / DCM determined by L
CCM-DCM BOUNDARY R Dsw fsw constant CCM CCM / DCM determined by L DECREASE L DCM to ensure a desired CCM does not transfer to DCM design for CMM at lowest inductance including L v I Prof R T Kennedy
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CCM / DCM determined by fsw
CCM-DCM BOUNDARY R Dsw fsw constant CCM CCM / DCM determined by fsw DECREASE fsw DCM to ensure a desired CCM does not transfer to DCM design for CMM at lowest frequency Prof R T Kennedy
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CCM / DCM determined by Dsw
CCM-DCM BOUNDARY L R fsw constant CCM CCM / DCM determined by Dsw DCM to ensure a desired CCM does not transfer to DCM design for CMM at lowest duty cycle DECREASE Dsw Prof R T Kennedy
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DCM lower control range LINE & LOAD REGULATION DCM CCM
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DCM lower control range LINE & LOAD REGULATION DCM CCM
Prof R T Kennedy
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