Which is the best PFC stage for a 1kW application? Comparison of different PFC stage topologies under an identical design philosophy Ulf Schwalbe/ Marko Scherf ISLE Steuerungstechnik und Leistungselektronik GmbH Markus Protze/ Tobias Reimann/ Jürgen Petzoldt Ilmenau University of Technology
Content 1. Introduction 2. Theoretical system comparison 3. Investigated systems 4. Switching frequency selection in a interleaved CCM PFC 5. Measurement results 6. Conclusion -1-
Introduction SMPS stucture: Design parameters of the PFC stage: Output power: 1kW Input voltage: 90V..260V Output voltage: 400V -2-
Introduction Topologies/ Operation Modes Possible PFC stage topologies for SMPS: Boost converter Buck converter Flyback converter Cuk converter SEPIC converter Possible PFC stage operation modes: Continuous Conduction Mode (CCM) Discontinuous Conduction Mode (DCM) Critical Conduction Mode; Boundary Conduction Mode; Transition Mode) (CRM; TM) -3-
Theoretical system comparison -4-
System comparison Operation modes -5- [Source: TI; SLUS515F; TRANSITION MODE PFC CONTROLLER]
System comparison Topology to control Control Control to control to control to control Control Parallel devices + Straightforward - Difficult layout, design - Difficult thermal management Parallel converters + Modular approach + Easy layout, design + Easy thermal management + Interleaving possible (reduced current) + Phase management possible (η partial load ) - Complex control -6-
Investigated systems -7-
Investigated systems Task: Build up three different PFC demonstration boards 1-phase CCM, 2-phase CCM and 2-phase CRM Boost PFC Same design philosophy Goal: Efficiency comparison for different power semiconductor device sets (Which is the optimal device set for each configuration/ operation mode?) Investigation in switching behaviour of different device types Investigation in the (real) EMI behaviour Make experiences in feasibility -8-
Investigated systems Investigation of three different topologies - identical design philosophy 1ph CCM PFC (130kHz/ 20% Ripple) 2ph CCM PFC (130kHz/ 65kHz; 20%/ 40% Ripple) 2ph CRM PFC (50 500kHz) Output power: Input voltage: Output voltage: -9-1kW 90V..260V 400V
Switching frequency selection in a interleaved CCM PFC -10-
Switching frequency selection in a CCM interleaved PFC Theoretically comparison of different two-channel interleaved PFC stages (compared to a single stage CCM PFC) Current ripple ; Switching frequency - benefits for choke Current ripple ; Switching frequency - benefits for semiconductor Variant A: -11-
Magnetic volume vs. magnetic energy Variant A - assuming that: total energy stored in the boost chokes is halved relative ripple is doubled (= flux ripple) frequency is equal impact on magnetic core from Steinmetz law: P Core = C * DB b * f a * V core b values for typical materials: Kool Mµ: 2.0, MPP: 2.21...2.36 è higher ripple higher core losses! in many designs: toroid powder cores can t be used gapped ferrite cores are favored impact on winding increased rms-current higher amount of skin and proximity effect and air gap caused field distortion lower optimization limit due to core saturation (some designs) è è higher number of turns higher winding losses! in most designs: litz wire has to be used Total magnetics volume reduction is far less than total magnetic energy reduction! -12-
Switching frequency selection in a CCM interleaved PFC Consideration of the choke Non-proportional relation between stored magnetic energy and core size (losses) -13-
Measurement results -14-
Measurement results Switching frequency - Ripple Influence of switching frequency and ripple to the total efficiency! +0,8% Variant D Variant A CCM Operation Influence of switching frequency is high! 130kHz 65kHz: η +0,8% -15-
Measurement results Identical MOSFET Chip Size Identical total chip size in every PFC stage! CCM CCM CRM Dual interleaved CCM PFC @ fsw=65khz achieved highest efficiency! -16-
Measurement results Maximal Efficiency Optimised chip size in every PFC stage! CCM CRM CCM Dual interleaved CCM PFC @ fsw=65khz achieved highest efficiency! -17-
Measurement results MOSFET vs. IGBT CCM- MOSFET CRM- MOSFET CCM- IGBT CRM- IGBT -18-
Measurement results Maximal Efficiency +3% Output power related shutdown of one phase gains more than 3% efficiency. -19-
Measurement results EMI EMI measurements (up to 30MHz) with identical, non-optimised filter done EMI behaviour of interleaved PFC topologies (questions): Fundamental switching frequency can be pushed into the EMI band (>150kHz) due to frequency doubling (multiplying) Higher di/dt (when applying high ripple techniques) can cause higher EMI by magnetic coupling More dv/dt sources can cause higher common-mode noise Gapped (multiple) ferrite inductors cause magnetic stray fields Switching frequency variation in CRM PFC can spread EMI High switching frequency in CRM PFC small EMI filter necessary!?! -20-
Conclusion -21-
Conclusion Experimental results Three demonstrator boards were built up and tested successfully Performance of 1-ph CCM, 2-ph CCM and 2-ph CRM PFC is similar (2-ph CCM achieved best efficiency due to low switching frequency) Modern superjunction MOSFETs are the ideal active switch for PFC A 600V ultrafast Si diode can perform well in a (2ph) CRM PFC boost converter When applying high ripple techniques (esp. CRM topology) gapped ferrite inductors outperform toroid powder cores (core losses!) Only little potential of total inductor volume reduction Drawbacks interleaving: higher component count; complex control and current sensing (power balancing is needed) -22-
Conclusion in General Interleaving PFC boost converters has great advantages, especially input/ output current ripple reduction 1) benefits for bulk capacitor 2) benefits for input filter (Differential Mode acting) Phase management is possible in interleaving topologies (increase of light load efficiency) A higher degree of interleaving is possible and enforces the characteristics (advantages and drawbacks) Wide design margin with interleaving (frequency, amount of ripple, operation mode, degree of interleaving) Inductive current sensing in 2ph CCM is sensitive (degaussing at high duty cycle) -23-
Thank you for your attention! Dr.-Ing. Ulf Schwalbe ISLE Steuerungstechnik und Leistungselektronik GmbH D- 98693 Ilmenau Germany u.schwalbe@isle-ilmenau.de ++49(0)3677-461353 -24-