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# Digital Power Factor Correction - Handling the corner cases

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• 1. Digital Power Factor Correction Handling the corner cases Superior THD over entire operating range www.controltrix.comcopyright 2011 controltrix corp www. controltrix.com
• 2. Power Factor primer Inductive - Lagging we r cosΦ = Real Power Po re n t Apparent Power pa Ap Reactive Power = Power Factor Φ Real Power Capacitive - Leading Applies for ideal sinusoidal waveforms for both voltage and currentcopyright 2011 controltrix corp www. controltrix.com
• 3. Calculating Power Factor Power factor = Real power / Apparent power = (Vrms * I1rms * CosΦ) / (Vrms * Irms) = cosΦ * ( I1rms / Irms) Power factor = KΦ * Kd Kd = distortion factor (THD) KΦ = displacement factor (D.F) Vrms = AC input rms voltage Irms = AC input rms current I1rms = Fundamental component of Irms cos Φ = Phase angle between input AC voltage and the fundamental current Irms = Sqrt (I12 + I22 + I32+ ………….+In2)copyright 2011 controltrix corp www. controltrix.com
• 4. PF Degradation Voltage Resulting CurrentSinusoidal Current with phase shift Voltage Resulting Current Current with harmonic contentcopyright 2011 controltrix corp www. controltrix.com
• 5. Power factor correction • Reduce energy loss in transmission lines • Improve power quality • Cost • Regulatory needscopyright 2011 controltrix corp www. controltrix.com
• 6. Useful Power Useful Power Negative Power Applied Voltage Region Region Resulting CurrentWithout PFC Φ Φ Applied Voltage Resulting CurrentWith Active PFCcopyright 2011 controltrix corp Useful Power www. controltrix.com
• 7. Digital PFC system Boost AC Supply Rectifier Load PFC Vac Iac PWM Vdc DSP/DSC Basic Components of the PFC Converter Switch Capacitor Inductor Diodecopyright 2011 controltrix corp www. controltrix.com
• 8. Boost Topology Average Inductor Current IL IL PFC Boost Converter L D tON + IL ID IS IS + vIN S C - ID - VOUT > VIN Average Current Mode Control The average current through the inductor is made to follow the input voltage Ref: AN1274 Interleaved PFC app note from microchipcopyright 2011 controltrix corp www. controltrix.com
• 9. Challenges Ideally …… • Low THD & high PF over entire 90 -265 Vac input • Low THD & high PF over entire 10 -100 % load range Low line and high load meeting specifications is EASY !!!! Low load ( < 50%) & Hi Line (> 220 V) spec is HARD !!!! Cause : Change of system dynamicscopyright 2011 controltrix corp www. controltrix.com
• 10. Typical specs Load(%) THD(%) • (Typical Desired) State of the art spec. 10 <15 • 2.4 KW bridgeless PFC spec. for 20 <10 power supplies for server 30 <6 farms 50 <5 • Digital (DSP) control 70 <3 • Fixed switching frequency operation 80 <3 100 <3 Gets harder @ Hi linecopyright 2011 controltrix corp www. controltrix.com
• 11. State of the art review Approach 1 • Determine Discontinuous/continuous conduction mode operation • Change the control laws Challenge • Computation • If-else ladder • Parameter sensitivity • Non linearity of discontinuous mode of operation is hard • Fixed point implementation is challengingcopyright 2011 controltrix corp www. controltrix.com
• 12. State of the art review Approach 2 • Harmonic injection Challenge • Trial and error • Not plug and play / System specific • If-else ladder (discontinuities in code execution and dynamics) • Limited Code size Memory/MIPScopyright 2011 controltrix corp www. controltrix.com
• 13. State of the art summary • Computationally complex (MIPS, code size) • Fixed point implementation hard !! • Physical models sensitive to parameter estimates (e.g. inductor saturation ) • Poor convergence • Strange artefacts • Jumps/spikes/kinks/oscillations due to if-else laddercopyright 2011 controltrix corp www. controltrix.com
• 14. Proposed solution features • Good THD at all operating conditions • Plug and play • Just enter parameter dependent coefficients • Low parameter and feedback sensitivity • Fast convergencecopyright 2011 controltrix corp www. controltrix.com
• 15. Proposed solution features . • No if-else ladder • Small extra code size • Low MIPS requirement ~ 12-14 MIPS (25% of 40 MIPS) @ 50 KHz interrupt frequency (Compares favorably with traditional methods)copyright 2011 controltrix corp www. controltrix.com
• 16. Proposed solution features .. • System independent /scalable to any rating • Relevant for Interleaved PFC and bridgeless PFC topologies • Guaranteed convergence/no large scale oscillations • No if -else ladder • Patent pending technologycopyright 2011 controltrix corp www. controltrix.com
• 17. Switched mode Simulation Results • Fixed frequency operation ~100 KHz • Vac = 220V rms ac, Vdc = 400 V ( High line is hardest to handle !!! ) • 330 W boost PFC system • 700 uH inductance • 300 uF output capacitancecopyright 2011 controltrix corp www. controltrix.com
• 18. Simulation Results: • Left plot: Average inductor current • Right plot: Switched mode inductor current (continuous and discontinuous conduction mode)copyright 2011 controltrix corp www. controltrix.com
• 19. 100 % load • THD ~ 3%copyright 2011 controltrix corp www. controltrix.com
• 20. 50 % load • THD ~ 5%copyright 2011 controltrix corp www. controltrix.com
• 21. 10 % load • THD ~10%copyright 2011 controltrix corp www. controltrix.com
• 22. Comparison of Specifications IPFC reference design from microchip 2.4 KW server power supply from delta Switching frequency : 100 KHz Switching Frequency : 65Khz One side max load : 180 W Inductance : 200uH Inductance : 700 uH(much smaller value than equivalent server supply for that rating) • A system for similar specification as server supply for 700 uH, 100 KHz the power rating would be, 2400 * 200 / 700 * 65 / 100 = 445 W • Thus 87 W is effectively 19.5 % load • The results are thus very convincingcopyright 2011 controltrix corp www. controltrix.com
• 23. Experimental results (IPFC board 89 W only single phase enabled) Voltage 90 110 160 220 Remarks Method Classical PI controller (over damped) modeled 5.98 7.2 17.0 18.4 PF and THD rapidly degrades on linear dynamics of continuous conduction at low loads mode system Classical PI controller modeled on linear 3.5 6.5 13.5 15.5 Easily ends up becoming (critically damped) unstable / sub harmonic oscillations due to parameter changes Classical P I controller with voltage feedforward 3.35 5.35 7.65 17.75 Works great in CCM. But rapidly degrades in DCM /low load conditions Proposed method 2.9 5.2 6.21 8.5 Works equally well in all regions of operation 89W load when corrected for inductance values and switching frequency is equivalent to 19.5 % load for comparable 2.4 KW system used in server power supply.copyright 2011 controltrix corp www. controltrix.com
• 24. Scope shots (87 W load , 400 Vdc output) 110 Vcopyright 2011 controltrix corp www. controltrix.com
• 25. Scope shots (87 W load , 400 Vdc output) 220 Vcopyright 2011 controltrix corp www. controltrix.com
• 26. Classical PI control w/o FF 110 Vcopyright 2011 controltrix corp www. controltrix.com
• 27. Classical PI control w/o FF 220 Vcopyright 2011 controltrix corp www. controltrix.com
• 28. Results with AN1278 from microchip Input voltage: 220 V, Load: 180 W (50%) dual phase 180 W for IPFC is equivalent to 90 W with only one phase of IPFC operational.copyright 2011 controltrix corp www. controltrix.com
• 29. Thank You consulting@controltrix.comcopyright 2011 controltrix corp www. controltrix.com