Introduction to Power Electronics - Special Topics - Slides | ECEN 2060, Study notes of Electrical and Electronics Engineering

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Introduction to Power Electronics ECEN 2060 Spring 2008 References: • ECEN4797/5797 Intro to Power Electronics ece.colorado.edu/~ecen5797 • Textbook: R.W.Erickson, D.Maksimovic, Fundamentals of Power Electronics, 2nd ed., Springer 2000, http://ece.colorado.edu/~pwrelect/book/SecEd.html 2ECEN2060 Example: Grid-Connected PV System AC utility grid iac + − vac + − VPV IPV PV array Power electronics converter DC input PVPVPV IVP = PVPV IV , One possible grid-connected PV system architecture AC output ( )tVtv RMSac ωsin2)( = ( )tIti RMSac ωsin2)( = RMSRMSac IVP = Functions of the power electronics converter • Operate PV array at the maximum power point (MPP) under all conditions • Generate AC output current in phase with the AC utility grid voltage • Achieve power conversion efficiency close to 100% PVPV RMSRMS PV ac converter IV IV P P ==η • Provide energy storage to balance the difference between PPV and pac(t) Desirable features • Minimum weight, size, cost • High reliability ( )( )tIVivtp RMSRMSacacac ω2cos1)( −== High efficiency is essential P out th in n= P, floss = _ 1 = Pi, Pau = Pod 1] High efficiency leads to low power loss within converter Small size and reliable operation is then feasible Efficiency is a good measure of converter performance ECEN2060 08 4 0.6 4 04 4 Toss * * out  Circuit components for efficient electronic power conversion ? ~ Let l To T | ae | [ Linear- DT, Ts mode Switched-mode Resistors | Capacitors | Magnetics Semiconductor devices ECEN2060 7ECEN2060 Ideal switch Power semiconductor devices (e.g. MOSFETs, diodes) operate as near-ideal power switches: • When a power switch is ON, the voltage drop across it is relatively small • When a power switch is OFF, the switch current is very close to zero 10ECEN2060 Power electronics converters are circuits consisting of semiconductor devices operated as (near-ideal) switches, capacitors and magnetic components (inductors, transformers) Circuit components for efficient electronic power conversion 11ECEN2060 Boost (step-up) DC-DC converter fs = 1/Ts = switching frequency switch control Ts = switching period Position 1 Position 2 Ts DTs D = switch duty ratio (or duty cycle), 0 ≤ D ≤ 1 12ECEN2060 Boost converter circuit Power MOSFET and diode operate as near-ideal switches Boost converter analysis L 2 > TSO ° i@ +4,()- NY iO) a cd original u ' a R$ v converter . TT switch in position 1 a N\gswiten in position 2 L L p—S 000» p—S 000» i) +¥;,(t)- a iO +¥,()- a zg _ — ict) ict) © co Rg$v 7 @) co Rsv ECEN2060 15 Position 1 Inductor voltage and capacitor current iG !) IT y= V, L ie=—v/R >TO iO +v,(f)- Small ripple approximation: © v= V, ic=-VIR ECEN2060 16 Position 2 Inductor voltage and capacitor current vy, =V,- i-=i,-v/R . a V, Small ripple approximation: 8 yeaVv,-V i-=1-VIR ECEN2060 O L LOSSES > i +¥(O- ic Cc L 20ECEN2060 Boost DC voltage conversion ratio M = Vout/Vg Boost DC-DC converter steps-up a DC input voltage by a ratio M which is electronically adjustable by changing the switch duty ratio D 21ECEN2060 Simulink model ECEN2060 Switched-mode Boost DC-DC converter 200.4 Vout 100 Vin Scope 0.5 Duty cycle Vg Iout D Vout IL switch control Boost DC-DC (switching) Boost DC-DC 1/100 1/Rload D Vg Vout Vout iL switch control iout boost_switching.mdl Input voltage Vg = 100 V Inductance L = 200 µH Capacitance C = 10 µF Load resistance R = 100 Ω Switch duty cycle D = 0.5 Output voltage Vout = 200 V Input current Ig = IL = 4 A Power P = 400 W Switching frequency fs = 100 kHz Switching period Ts = 10 µs 22ECEN2060 Averaged (DC) model No losses: gout V D V − = 1 1 outg I D I − = 1 1 outoutgg IVIV = Ideal boost DC-DC converter works as an ideal DC transformer with an electronically adjustable step-up ratio D DMn − == 1 1 )( + − Vg + − Vout 1:nIg Iout 25ECEN2060 vt DTs Ts t it + – DT s T s + – it + _ L C R + _ vout IoutiL Ig vt vgate Vg id vd + _+ _ vL Switching waveforms and switching losses MOSFET turn-off transition Drain voltage Drain current vt it zoom-in Switching power loss = Transition energy loss * Switching frequency 26ECEN2060 Averaged (DC) model with losses + − Vg + − Vout 1−D : 1Ig Iout RL Isw • Small RL models conduction losses due to inductor winding resistance and power switch resistances • Small Isw models switching and other load-independent losses • Efficiency with losses, when the load current Iout is known: out sw outout swoutL I I IV II D R + + − + = 2 2 )( )1( 1 1 η 27ECEN2060 Example: efficiency for various RL RD RL 1 )1( 1 1 2− + =η Assume: • Resistive load R = Vout/Iout • Isw = 0 Note that it is more difficult to achieve high efficiency if a large step-up ratio is required (i.e. if duty-ratio D is close to 1) 30ECEN2060 Position 2 acDCL vVv −−= acL ii = Lin ii −= + − vacLiL iac + – VDC iin 1 1 2 2 + −vL 31ECEN2060 Inductor volt-second balance • Note that switching frequency fs >> ac line frequency • Over a switching period, vac(t) ≈ const.    ≤<−− ≤≤−+ = ssacDC sacDC L TtDTvV DTtvV v , 0 , ∫ =−−=−−−+−== sT acDCacDCacDCL s L vVDvVDvVDdttv T V 0 0)12())(1()()( 1 12)( −== D V v DM DC ac 1)(1 ≤≤− DM VDC must be greater than the peak of vac 32ECEN2060 Control of AC line current + − vacLiL iac + – VDC iin 1 1 2 2 Control objectives: • iac = IM sin (ωt), in phase with AC line voltage vac(t) • Amplitude IM (or RMS value) adjustable to control power delivered to the AC line ( )tVtv RMSac ωsin2)( = ( )tIti RMSac ωsin2)( = RMSRMSac IVP = ( )( )tIVivtp RMSRMSacacac ω2cos1)( −== 35ECEN2060 Averaged DC-AC inverter model with losses • Small RL models inductor winding resistance and power switch resistances • Small Isw models switching and other losses + − VDC + − vac 1 : 2D −1Iin iacRL Isw ideal transformer 36ECEN2060 DC-AC inverter efficiency example 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 100 200 300 400 500 600 Pac [W] ECEN2060 Switched-Mode DC-AC Inverter (averaged model) 60 fac1 60 fac 200 Vdc Scope 540 Pout 566.2 Pin 1 s Integrator1 1 s Integrator 4.5 IRMS 0.9537 EfficiencyDivide Vdc Iref v ac iac iin D pin pout DC-AC inverter (averaged) DC-AC v ac iac iin Duty pin pout Simulink model dcac_averaged.mdl Input voltage VDC = 200 V AC: 120Vrms, 60Hz RL = 0.8 Ω Isw = 50 mA Pac = 0 to 600 W • Inverter efficiency of about 95% is typical • At high power levels, conduction losses due to RL dominate • At low power levels, efficiency drops due to switching and other fixed losses