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Concept Kit:PWM Buck Converter Transients Model

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Concept Kit:PWM Buck Converter Transients Model

Concept Kit:PWM Buck Converter Transients Model

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  • 1. PWM IC Power Switches Filter & Load (Voltage Mode) U? (Semiconductor) PWM_IC RON = 100m 1 L 2 Vo VOUT -- + E/A S1 D1 C -+ + + Comp S DIODE + - - Rload OSC pwm REF ESR FOSC = 52KVREF VREF = 1.23 VP = 2.5Concept Kit:PWM Buck ConverterTransients Model All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 1
  • 2. Contents1. Concept of Simulation2. Buck Converter Circuit3. Power Switches (Semiconductor)4. Buck Converter Design Workflow 1 Setting PWM Controller’s Parameters. 2 Programming Output Voltage: Rupper, Rlower 3 Inductor Selection: L 4 Capacitor Selection: C, ESR 5 Stabilizing the Converter5. Buck Converter Simulation (Example) 5.1 Switching Waveforms 5.2 Power State Switches Voltage and Current6. Load Transient Response Simulation (Example)7. Buck Converter Optimization (Example)8. Converter Efficiency 8.1 Converter Efficiency vs. MOSFET, Rds(on) 8.2 Converter Efficiency vs. DIODE, VF9. Simulation Using Real Device Models 9.1 Switching Waveforms (Real Device Models) 9.2 Converter Efficiency (Real Device Models) Simulation Index All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 2
  • 3. 1.Concept of SimulationBlock Diagram: PWM IC Power Switches Filter & Load (Voltage Mode) (Semiconductor) Parameter: VOUT Parameter: • MOSFET •L - + • VOSC • Diode •C • VREF • ESR VREF • VP • RloadModels: U? RON = 100m L PWM_IC 1 2 Vo S1 D1 - C + - S DIODE + - + E/A + pwm Rload Comp - ESR OSC REF FOSC = 52K VREF = 1.23 VP = 2.5 All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 3
  • 4. 2.Buck Converter Circuit Power Switches Filter & Load RON = 100m L 1 2 Vo S1 D1 C - + S DIODE + - Vin pwm Rload ESR 0 PWM Controller Type 2 Compensator C2 R2 C1 Rupper U3 PWM_IC - FB + E/A + Comp - Rlower OSC REF FOSC = 52K VREF = 1.23 0 VP = 2.5 All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 4
  • 5. 3.Power Switches (Semiconductor) The parameter RON represents Rds(on) characteristics of MOSFET, that are usually provide by the manufacturer datasheet. The value could be about 10m to 10 ohm. RON = 100m S1 D1 • A Near-Ideal DIODE can be modeled by - + S DIODE + - pwm using SPICE primitive model (D), which MOSFET parameters are : N=0.01 RS=0. • A near-ideal MOSFET can be modeled by using PSpice VSWITCH that is voltage controlled switch. All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 5
  • 6. 4.Buck Regulator Design WorkflowThe Purpose of the Circuit Simulation• To Evaluate and Verify the Design of the PWM Buck Converter.• To Optimize the Parameters of the PWM Buck Converter. 1 Setting PWM Controller’s Parameters: FOSC , VREF, VP 2 Setting Output Voltage: Rupper, Rlower 3 Inductor Selection: L 4 Capacitor Selection: C, ESR 5 Setting the Compensator Parameters: R2, C1, C2 Continue next slide All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 6
  • 7. 4.Buck Regulator Design Workflow Evaluations: • Switching Waveforms, • Power State Switches Voltage and Current, • Load Step Transient Response, • and so on Optimization: L (example) Evaluations: • Converter Efficiency vs. MOSFET, Rds(on) • Converter Efficiency vs. Diode, VF Evaluations Using Real Device Models All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 7
  • 8. 4.Buck Regulator Design Workflow RON = 100m 3 L 1 2 Vo S1 D1 C - + S DIODE + - Vin pwm Rload ESR 4 0 5 Type 2 Compensator C2 R2 C1 2 Rupper U3 PWM_IC - FB + E/A + Comp - Rlower OSC REF FOSC = 52K 1 VREF = 1.23 0 VP = 2.5 All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 8
  • 9. 1 Setting PWM Controller’s Parameters U? comp PWM_IC • FOSC, Oscillation frequency (frequency of the - FB sawtooth signal). PWM + E/A Comp + • VREF, feedback reference voltage, value is - OSC given by the datasheet REF FOSC = 52K • VP = (Error Amp. Gain  vFB ) / d VREF = 1.23 VP = 2.5 • vFB = vFBH – vFBL The Comparator compares the error voltage (between FB and REF) with a sawtooth signal • d = dMAX – dMIN (frequency = FOSC, peak saw voltage = VP) to generate PWM signal, as shown in the • Error Amp. Gain is 100 (approximated) figure below. where f = FOSC3.0V VP is the sawtooth peak voltage.2.0VSEL>> VP vFBH is maximum FB voltage where d = 0 0V V(osc) V(comp) vFBL is minimum FB voltage where d =1(100%) dMAX is maximum duty cycle, e.g. d = 0(0%) V(PWM) Duty cycle (d) is a value from 0 to 1 dMIN is minimum duty cycle, e.g. d =1(100%) Time All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 9
  • 10. 1 Setting PWM Controller’s Parameters (Example)  If the VP ( sawtooth signal amplitude ) does not informed by the datasheet, It can be approximated from the characteristics below. from vFBH VP = (Error Amp. Gain  vFB )/d vFB = •Error Amp. Gain = 100 (approximated) 25mV •from the graph on the left, vFB = 25mV (15m - (-10m)) vFBL d = 1 (100%) • d = 1 – 0 = 1 VP ≈ ( 100  25mV )/1 dMIN dMAX ≈ 2.5V LM2575: Feedback Voltage vs. Duty Cycle If vFBH and vFBL are not provided, the default value, VP=2.5 could be used. All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 10
  • 11. 2 Setting Output Voltage: Rupper, Rlower• Use the following formula to select the resistor values.  Rupper  VOUT  VREF1    Rlower  Type 2 Compensator C2 • Rlower can be between 1k and 5k. R2 C1ExampleGiven: VOUT = 5V U3 Comp Rupper PWM_IC VREF = 1.23 - FB Rlower = 1k + E/Athen: Rupper = 3.065k + Comp - Rlower OSC REF FOSC = 52K VREF = 1.23 0 VP = 2.5 All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 11
  • 12. 3 Inductor Selection: L L1 2 Vo Inductor Value C • The output inductor value is selected to set the converter to work in CCM (Continuous Current Rload ESR Mode) or DCM (Discontinuous Current Mode). • Calculated by LCCM  VI , max VOUT  RL, min 2 fosc VI , max Where • LCCM is the inductor that make the converter to work in CCM. • VI,max is input maximum voltage • RL,min is load resistance at the minimum output current ( IOUT,min ) • fosc is switching frequency All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 12
  • 13. 3 Inductor Selection: L (Example) L1 2 Vo Inductor Value C from ESR Rload LCCM  VI , max VOUT  RL, min 2 foscVI , max Given: • VI,max = 40V, VOUT = 5V • IOUT,min = 0.2A • RL,min = (VOUT / IOUT,min ) = 25 • fosc = 52kHz Then: • LCCM  210(uH), • L = 330(uH) is selected All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 13
  • 14. 4 Capacitor Selection: C, ESR L 1 2 Vo Capacitor Value • The minimum allowable output capacitor value should C be determined by Rload ESR C  7,785  VI , max F VOUT  L( H) Where • VI, max is the maximum input voltage. • L (H) is the inductance calculated from previous step ( 3 ).• In addition, the output ripple voltage due to the capacitor ESR must be considered as the following equation. VO , RIPPLE ESR  IL , RIPPLE All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 14
  • 15. 4 Capacitor Selection: C, ESR (Example) L1 2 Vo Capacitor Value From C C  7,785  VI , max F ESR Rload VOUT  L( H) and VO , RIPPLE ESR  IL , RIPPLE Given: • VI, max = 40 V • VOUT = 5 V • L (H) = 330 Then: • C  188 (F) In addition: • ESR  100m All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 15
  • 16. 5 Stabilizing the Converter • Loop gain for this configuration is H(s) RON = 100m L 1 2 Vo S1 D1 C - + S DIODE + - Vin Rload pwm ESR T ( s)  H ( s)  G( s)  GPWM 0 G(s) Type 2 Compensator C2 R2 C1 • The purpose of the compensator G(s) GPWM U3 Comp Rupper is to tailor the converter loop gain PWM_IC FB (frequency response) to make it stable PWM + E/A + - Comp when operated in closed-loop - OSC Rlower REF conditions. FOSC = 52K VREF = 1.23 0 VP = 2.5 • The element of the Type 2 compensator ( R2, C1, and C2 ) can be extracted by using Type 2 Compensator Calculator (Excel sheet) and open-loop simulation with the Average Models (ac models). Remark: The Average Models are not included with this package. All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 16
  • 17. 5.Buck Converter Simulation (Example) Specification: RON = 100m L 330uH 1 2 Vo VOUT = 5V S1 D1 C - + VIN = 7 ~ 40V S DIODE 330uF + - Vin IC = 5 Rload ILOAD = 0.2 ~ 1A 12Vdc pwm ESR 5 100m L = 330uH, 0 Type 2 Compensator 2 C = 330uF (ESR = 100m), C2 21.60p Rupper = 3.1k, Rlower = 1k, e.g. Characteristics R2 122.780k C1 0.778n from National PWM Controller: Semiconductor Corp. U3 Comp Rupper fOSC = 52kHz IC: LM2575 PWM_IC 3.1k FB VP1 = 2.5V - + E/A pwm + Comp VREF = 1.23V - OSC Rlower 1k REF Task: FOSC = 52K VREF = 1.23 0 VP = 2.5 •Voltage and Current Waveforms Evaluation. *Analysis directives: .TRAN 0 10ms 0 200n SKIPBP 1. Please see topic: 6.1 Calculate the VP, for detail. 2. Please check the Average Model manual for the Type2 Compensator’s elements (R2, C1, and C2) calculation. All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 17
  • 18. 5.1 Switching WaveformsSimulation Measurement 5.0V 0V A: Control Voltage V(PWM) V(PWM) 2.0A B: Switch Current ID(S1), 1A/div 1.0A 0A I(S1:3) 1.0A C: Inductor Current I(L), 0.5A/div I(L) 5.06V (9.942m,5.0345) 5.04V (9.931m,5.0511) SEL>> D: Output Ripple Voltage, 20 mV/div, 5.02V VOUT = 5V 9.925ms 9.935ms 9.945ms 9.955ms 9.965ms V(Vo) A: Output Pin Voltage, 10V/div Time B: Output Pin Current, 1A/div C: Inductor Current, 0.5A/div D: Output Ripple Voltage, 20 mV/div,• The simulation results are compared with the measurement data (National Semiconductor Corp. IC LM2575 datasheet).• Output ripple voltage (Simulation) is 16.6mVP-P. All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 18
  • 19. 5.2 Power State Switches Voltage and Current 16V 1.6A 1 2 (9.933m,12.008) 12V 1.2A SW (MOSFET) Voltage VDS (9.951m,1.0946) SW (MOSFET) Current ID 8V 0.8A 4V 0.4A >> 0V 0A 1 V(S1:3,S1:4) 2 I(S1:3) 16V 1.6A 1 2 (9.951m,1.0950) Diode Forward Current IF 0.8A 0V 0A (9.942m,-11.908) -0.8A SEL>> Diode Voltage VAK -16V -1.6A 9.925ms 9.930ms 9.935ms 9.940ms 9.945ms 9.950ms 9.955ms 9.960ms 9.965ms 9.970ms 1 V(D1:A,D1:C) 2 I(D1) Time • Switch (MOSFET) has the steady state voltage: VDS, PEAK = 12.008V and current: ID, PEAK = 1.0946A • Diode has the steady state voltage: VAK, PEAK = -11.908V and current: IF, PEAK = 1.095A All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 19
  • 20. 6.Load Transient Response Simulation (Example)The converter are connected with step-load to perform load transient response simulation. L load RON = 100m 330uH 1 2 Vo S1 D1 C - + S DIODE 330uF I1 + - Vin IC = 5 Rload I1 = 0 12Vdc pwm 25 I2 = 0.8 ESR TD = 10m 100m TF = 25u TR = 20u PW = 0.43m PER = 1 0 Type 2 Compensator C2 21.60p 5V/25 = 0.2A step to 0.2+0.8=1.0A load R2 C1 122.780k 0.778n U3 Comp Rupper PWM_IC 3.1k - FB + E/A pwm + Comp - Rlower OSC 1k REF FOSC = 52K*Analysis directives: VREF = 1.23 VP = 2.5 0.TRAN 0 15ms 0 200n SKIPBP All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 20
  • 21. 6.Load Transient Response Simulation (Example) Simulation Measurement 5.2V 4.0A1 2 Output Voltage Change 5.1V 3.5A 5.0V 3.0A 4.9V 2.5A 4.8V 2.0A 4.7V 1.5A Load Current 4.6V 1.0A 4.5V 0.5A >> 4.4V 0A 9.9ms 10.1ms 10.3ms 10.5ms 10.7ms 10.9ms 1 V(Vo) 2 I(load) Time • The simulation results are compared with the measurement data (National Semiconductor Corp. IC LM2575 datasheet). All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 21
  • 22. 7.Buck Converter Optimization (Example) PARAMETERS: Specification: RON = 100m L {L} L = 330u Vo VOUT = 5V 1 2 S1 D1 C - + VIN = 7 ~ 40V S DIODE 330u + - Vin IC = 5 Rload ILOAD = 0.2 ~ 1A 12Vdc pwm ESR 25 100m L = Optimization Parameter 0 Type 2 Compensator C = 330uF (ESR = 100m), C2 21.60p Rupper = 3.1k, Rlower = 1k, R2 122.780k C1 0.778n PWM Controller: U3 Comp Rupper fOSC = 52kHz PWM_IC 3.1k FB VP = 2.5V + E/A - pwm + VREF = 1.23V Comp - OSC Rlower 1k REF Task: FOSC = 52K VREF = 1.23 0 VP = 2.5 •Optimize the Inductor value. *Analysis directives: .TRAN 0 10ms 0 200n SKIPBP .STEP PARAM L LIST 330u, 220u, 100u All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 22
  • 23. 7.Buck Converter Optimization (Example)A: V(PWM), 5.0V10V/div L=330uH L=220uH 0V L=100uH V(PWM) 500mAB: ID(S1), 1A/div 0A I(S1:3) 600mAC: I(L), 0.5A/div L=100uH, converter work in DCM 400mA 200mA 0A I(L) 5.08VD: VOUT, RIPPLE, (9.931m,5.0555) 5.06V20 mV/div (9.942m,5.0300) VOUT, RIPPLE, SEL>> at L=220uH 5.02V 9.925ms 9.930ms 9.935ms 9.940ms 9.945ms 9.950ms 9.955ms 9.960ms 9.965ms 9.970ms V(Vo) Time • As an equation (1), the converter works in DCM when the inductor: L is 100uH at the minimum output current: ILOAD = 0.2A • VOUT, RIPPLE = 25.5mVP-P when the inductor: L is 220uH (Increased from 16.6mVP-P of L=330uH). IF VOUT, RIPPLE = 25.5mVP-P is acceptable then L=220uH can replace the 330uH. All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 23
  • 24. 8.Converter EfficiencyPerform transient simulation to measure the converter efficiency at Rds(on) = 100m and 1 . PARAMETERS: Rdson = 100m L RON = {Rdson} 330uH 1 2 Vo S1 D1 C - + S DIODE 330uF + - Vin IC = 5 Rload 12Vdc pwm 5 ESR 100m 0 Type 2 Compensator C2 21.60p R2 C1 122.780k 0.778n U3 Comp Rupper PWM_IC 3.1k - FB + E/A pwm + Comp - Rlower OSC 1k REF*Analysis directives: FOSC = 52K 0.TRAN 0 10ms 0 200n SKIPBP VREF = 1.23 VP = 2.5.STEP PARAM Rdson LIST 100m, 1 All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 24
  • 25. 8.1 Converter Efficiency vs. MOSFET Rds(on)Efficiency (%) 100 Rds(on) = 100m, Efficiency = 98.5 % (9.500m,98.492) 90 Rds(on) = 1, Efficiency = 90.9 % (9.500m,90.917) 80 Rds(on)=100m Rds(on)=1 70 9.0ms 9.2ms 9.4ms 9.6ms 9.8ms 10.0ms 100*AVG(W(Rload))/-AVG(W(Vin)) Time • The converter efficiency is decreased from 98.5% to 90.9% when Rds(on) increase from 100m to 1. All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 25
  • 26. 8.2 Converter Efficiency vs. Diode, VFPerform transient simulation to measure the converter efficiency at DIODE (N) = 0.01 and 1. PARAMETERS: Rdson = 100m L RON = {Rdson} 330uH 1 2 Vo S1 D1 C - + S DIODE 330uF + - Diode Forward Voltage vs. Vin IC = 5 Rload 12Vdc pwm 5 Diode model parameter: N ESR 100m Diode Forward I – V Characteristics 0 Type 2 Compensator1.0A C2 VF increases when DIODE (N) increases. 21.60p0.9A0.8A R2 C10.7A 122.780k 0.778n0.6A0.5A U3 Comp Rupper0.4A PWM_IC 3.1k0.3A - FB + E/A0.2A pwm + Comp Rlower0.1A VF - OSC 1k REF 0A 0V 0.12V 0.24V 0.36V 0.48V 0.60V 0.72V 0.84V 0.96V 1.08V I(D1) FOSC = 52K V_V1 VREF = 1.23 0 *Analysis directives: VP = 2.5 .TRAN 0 10ms 0 200n SKIPBP .STEP D DIODE(N) LIST 0.01, 1 All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 26
  • 27. 8.2 Converter Efficiency vs. Diode, VFEfficiency (%) 100 DIODE (N) = 0.01, Efficiency = 98.5 % (9.500m,98.492) 90 DIODE (N) = 1, Efficiency = 90.6 % (9.500m,90.564) 80 N=0.01 N=1 70 9.0ms 9.2ms 9.4ms 9.6ms 9.8ms 10.0ms 100*AVG(W(Rload))/-AVG(W(Vin)) Time • The converter efficiency is decreased from 98.5% to 90.6% when DIODE’s parameter N increase from 0.01 to 1. All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 27
  • 28. 9.Simulation Using Real Device ModelsAs we can see in the efficiency simulation (topic #9) that’s how the switching devicescharacteristics effect the simulation result. For the accurate simulation result, the accuratemodels, that relate to the real devices characteristics, are needed. U1 L 330uH 1 2 Vo C D1 330uF Vin IC = 5 Rload 12Vdc 5 ESR 100m 0 E1 Type 2 Compensator + - + E - C2 21.60p The Real Device Models 0 of MOSFET and R2 C1 122.780k 0.778n Schottky Barrier Diode U3 Comp Rupper PWM_IC 3.1k - FB + E/A pwm + Comp - Rlower OSC 1k REF FOSC = 52K VREF = 1.23 0 VP = 2.5 All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 28
  • 29. 9.1 Switching Waveforms (Real Device Models)Simulation Measurement 5.0V A: Control Voltage V(PWM), 10V/div 0V V(PWM) Spike current Spike current B: MOSFET Drain Current ID, 1A/div 1.0A SEL>> 0A I(U1:D) 1.0A C: Inductor Current I(L), 0.5A/div I(L) 5.06V 5.04V D: Output Ripple Voltage, 20 mV/div, 5.02V VOUT = 5V 9.925ms 9.935ms 9.945ms 9.955ms 9.965ms A: Output Pin Voltage, 10V/div V(Vo) Time B: Output Pin Current, 1A/div C: Inductor Current, 0.5A/div D: Output Ripple Voltage, 20 mV/div,• The real device model enable designers to include the spike signal in the switching waveforms simulation. All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 29
  • 30. 9.2 Converter Efficiency (Real Device Models)Efficiency (%) 100 Efficiency = 92.9 % (9.500m,92.877) 90 80 70 9.0ms 9.2ms 9.4ms 9.6ms 9.8ms 10.0ms 100*AVG(W(Rload))/-AVG(W(Vin)) Time • The converter efficiency is decreased from 98.5% to 92.9% when the device models are changed from the near-Ideal to the real model. All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 30
  • 31. Simulation Index Simulations Folder name 1. Switching Waveforms...................................................... waveforms 2. Power Stage Switches Voltage and Current.................... powersw 3. Load Transient Response................................................ stepload 4. Buck Converter Optimization............................................ optimize 5. Converter Efficiency vs. MOSFET Rds(on) .................... efficiency-diode 6. Converter Efficiency vs. MOSFET Diode, VF.................. efficiency-rdson Libraries : 1. ..¥pwmic.lib 2. ..¥diode.lib All Rights Reserved Copyright (C) Bee Technologies Corporation 2011 31