copyright 2011 controltrix corpwww. controltrix.com Digital Power Factor Correction Handling the corner cases Superior THD over entire operating range

copyright 2011 controltrix corpwww. controltrix.com Power Factor primer cos Φ = Real Power Apparent Power = Power Factor Applies for ideal sinusoidal waveforms for both voltage and current

copyright 2011 controltrix corpwww. controltrix.com Calculating Power Factor Power factor = Real power / Apparent power = (V rms * I 1rms * CosΦ) / (V rms * I rms ) = cosΦ * ( I 1rms / I rms ) Power factor = K Φ * K d K d = distortion factor (THD)K Φ = displacement factor (D.F) V rms = AC input rms voltage I rms = AC input rms current I 1rms = Fundamental component of I rms cos Φ = Phase angle between input AC voltage and the fundamental current I rms = Sqrt (I I I ………….+I n 2 )

copyright 2011 controltrix corpwww. controltrix.com PF Degradation Sinusoidal Current with phase shift Current with harmonic content Voltage Resulting Current Voltage Resulting Current

copyright 2011 controltrix corpwww. controltrix.com Power factor correction Reduce energy loss in transmission lines Improve power quality Cost Regulatory needs

copyright 2011 controltrix corpwww. controltrix.com Useful Power Φ Φ Negative Power Region Applied Voltage Resulting Current Without PFC Applied Voltage Resulting Current Useful Power With Active PFC Useful Power Region

copyright 2011 controltrix corpwww. controltrix.com Digital PFC system AC Supply Basic Components of the PFC Converter Rectifier DSP/DSC Load Vac Iac Vdc PWM Switch Inductor Capacitor Diode Boost PFC

copyright 2011 controltrix corpwww. controltrix.com Boost Topology ILILILIL ISISISIS IDIDIDID PFC Boost Converter S D L C v IN ILILILIL IDIDIDID ISISISIS t ON V OUT > V IN ILILILIL Average Inductor Current Average Current Mode Control The average current through the inductor is made to follow the input voltage Ref: AN1274 Interleaved PFC app note from microchip

copyright 2011 controltrix corpwww. controltrix.com Challenges Ideally …… Low THD & high PF over entire Vac input Low THD & high PF over entire % load range Low line and high load meeting specifications is EASY !!!! Low load ( 220 V) spec is HARD !!!! Cause : Change of system dynamics

copyright 2011 controltrix corpwww. controltrix.com Typical specs Load(%)THD(%) 10<15 20<10 30<6 50<5 70<3 80<3 100<3 (Typical Desired) State of the art spec. 2.4 KW bridgeless PFC spec. for power supplies for server farms Digital (DSP) control Fixed switching frequency operation Gets Hi line

copyright 2011 controltrix corpwww. controltrix.com 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 challenging

copyright 2011 controltrix corpwww. controltrix.com 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/MIPS

copyright 2011 controltrix corpwww. controltrix.com 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 ladder

copyright 2011 controltrix corpwww. controltrix.com Proposed solution features Good THD at all operating conditions Plug and play Just enter parameter dependent coefficients Low parameter and feedback sensitivity Fast convergence

copyright 2011 controltrix corpwww. controltrix.com Proposed solution features. No if-else ladder Small extra code size Low MIPS requirement ~ MIPS (25% of KHz interrupt frequency (Compares favorably with traditional methods)

copyright 2011 controltrix corpwww. controltrix.com 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 technology

copyright 2011 controltrix corpwww. controltrix.com 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 capacitance

copyright 2011 controltrix corpwww. controltrix.com Simulation Results: Left plot: Average inductor current Right plot: Switched mode inductor current (continuous and discontinuous conduction mode)

copyright 2011 controltrix corpwww. controltrix.com 100 % load THD ~ 3%

copyright 2011 controltrix corpwww. controltrix.com 50 % load THD ~ 5%

copyright 2011 controltrix corpwww. controltrix.com 10 % load THD ~10%

copyright 2011 controltrix corpwww. controltrix.com IPFC reference design from microchip2.4 KW server power supply from delta Switching frequency : 100 KHz One side max load : 180 W Inductance : 700 uH(much smaller value than equivalent server supply for that rating) Switching Frequency : 65Khz Inductance : 200uH Comparison of Specifications 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 convincing

copyright 2011 controltrix corpwww. controltrix.com Experimental results (IPFC board 89 W only single phase enabled) Voltage  Remarks Method Classical PI controller (over damped) modeled on linear dynamics of continuous conduction mode system PF and THD rapidly degrades at low loads Classical PI controller modeled on linear (critically damped) Easily ends up becoming unstable / sub harmonic oscillations due to parameter changes Classical P I controller with voltage feedforward Works great in CCM. But rapidly degrades in DCM /low load conditions Proposed method 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 corpwww. controltrix.com Scope shots (87 W load, 400 Vdc output) 110 V

copyright 2011 controltrix corpwww. controltrix.com Scope shots (87 W load, 400 Vdc output) 220 V

copyright 2011 controltrix corpwww. controltrix.com 110 V Classical PI control w/o FF

copyright 2011 controltrix corpwww. controltrix.com 220 V Classical PI control w/o FF

copyright 2011 controltrix corpwww. controltrix.com 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 corpwww. controltrix.com Thank You