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DC to DC CONVERTER (CHOPPER) UNIT.1&2, 7&8 Presentation By Mr. Reddyprasad.R, Assistant Professor Department of Electrical and Electronics Engineering Sri Venkateshwara College of Engineering Bengaluru, Karnataka-562157 Tel: 9494747497 E-Mail: [email protected] Website:www.spotturns.blogspot.in 1 DC to DC CONVERTER (CHOPPER) • • • • • • • General Buck converter Boost converter Buck-Boost converter Switched-mode power supply Bridge converter Notes on electromagnetic compatibility (EMC) and solutions. 2 DC-DC Converter (Chopper) DEFINITION: Converting the unregulated DC input to a controlled DC output with a desired voltage level. • General block diagram: DC supply (from rectifierfilter, battery, fuel cell etc.) DC output LOAD Vcontrol (derived from feedback circuit) • APPLICATIONS: – Switched-mode power supply (SMPS), DC motor control, battery chargers Linear regulator • Transistor is operated in linear (active) mode. + VCEce IL + • Output voltage RL Vin Vo Vo Vin Vce LINEAR REGULATOR • The transistor can be conveniently modelled by an equivalent variable resistor, as shown. • Power loss is high at high current due to: + Vce IL RT Vin + RL Vo EQUIVALENT CIRCUIT Po I L 2 RT or Po Vce I L 4 Switching Regulator • Transistor is operated in switched-mode: – Switch closed: Fully on (saturated) – Switch opened: Fully off (cut-off) + Vce IL + RL Vin Vo – When switch is open, no current flow in it – When switch is closed no voltage drop across it. SWITCHING REGULATOR IL SWITCH RL Vin + Vo • Since P=V.I, no losses occurs in the switch. – Power is 100% transferred from source to load. – Power loss is zero (for ideal switch): EQUIVALENT CIRCUIT Vo Vin (ON) (OFF) (ON) closed open closed DT • Switching regulator is the basis of all DC-DC converters T OUTPUT VOLTAGE 5 Buck (step-down) converter L S Vd C D RL + Vo CIRCUIT OF BUCK CONVERTER iL + vL S Vd RL D + Vo CIRCUIT WHEN SWITCH IS CLOSED S iL + vL + Vd D RL Vo CIRCUIT WHEN SWITCH IS OPENED 6 Switch is turned on (closed) • Diode is reversed biased. + vL S • Switch conducts inductor current • This results in positive inductor voltage, i.e: iL + VD Vd C vL VdVo opened opened closed t vL Vd Vo di vL L L dt 1 iL v L dt L RL closed • It causes linear increase in the inductor current + Vo Vo iL iLm ax IL iLm in DT t T 7 Switch turned off (opened) • Because of inductive energy storage, iL continues to flow. + vL S iL Vd C D + Vo RL • Diode is forward biased • Current now flows (freewheeling) through the diode. • The inductor voltage can be derived as: vL Vo vL VdVo opened opened closed closed t Vo iL iLmax IL iLmin (1-D)T t DT T 8 Analysis When the switch is closed (on) : di v L Vd Vo L L dt V Vo di L d dt L Derivative of iL is a positive vL Vd Vo closed t constant.T herefore iL must increased linearly. iL From Figure diL iL iL Vd Vo dt t DT L V Vo iL closed d DT L For switch opened, iL max iL IL iL min t DT T di v L Vo L L dt Vo di L dt L Vo di i iL L L dt t (1 D )T L V iL opened o (1 D )T L 9 Steady-state operation iL Unstable current t Decaying current iL t Steady-state current iL t Steady - state operation requires that iL at the end of switching cycle is the same at the begining of the next cycle. That is the change of iL over one period is zero, i.e : iL closed iL opened 0 Vd Vo Vo DT (1 D)Ts 0 s L L Vo DVd 10 Average, Maximum and Minimum Inductor Current iL Imax iL IL Imin t Average inductor current Average current in R L V IL IR o R Maximum current : I max I L iL Vo 1 Vo (1 D)T 2 R 2 L 1 (1 D) Vo R 2 Lf Minimum current : iL 1 (1 D) Vo 2 2 Lf R Inductor current ripple : iL I max I min I min I L 11 Continuous Current Mode (CCM) iL Imax Imin 0 t From previous analysis, iL 1 (1 D) Vo 2 2 Lf R For continuous operation, I min 0, I min I L 1 (1 D) Vo 0 2 Lf R (1 D) L Lmin R 2f This is the minimum inductor current to ensure continous mode of operation. Normally L is chosen b be Lmin 12 Output voltage ripple KCL, Capacitor current : ic iL iR L The charge can be witten as : Q CVo Q CV Q o Vo C Use triangle area formula : 1 T i Q L 2 2 2 0 TiL 8 Ripple voltage (Peak - to peak)0 TiL (1 D) Vo 8C 8 LCf 2 iL iR + iC Vo imax iL iL=IR Vo/R imin iC So, the ripple factor, V (1 D) r o Vo 8 LCf 2 Note : Ripple can be reduced by : 1) Increasing switching frequency 2) Increasing inductor size 3) Increasing capacitor size. 13 Basic design procedures SWITCH L Lmin= ? L = 10Lmin Vd (input spec.) f=? D=? TYPE ? D C ripple ? • Calculate D to obtain required output voltage. • Select a particular switching frequency (f) and device – – – • • preferably f>20KHz for negligible acoustic noise higher fs results in smaller L and C. But results in higher losses. Reduced efficiency, larger heat sink. Possible devices: MOSFET, IGBT and BJT. Low power MOSFET can reach MHz range. Calculate Lmin. Choose L>>10 Lmin Calculate C for ripple factor requirement. – • RL Po = ? Io = ? Capacitor ratings: • must withstand peak output voltage • must carry required RMS current. Note RMS current for triangular w/f is Ip/3, where Ip is the peak capacitor current given by iL/2. • ECAPs can be used Wire size consideration: – Normally rated in RMS. But iL is known as peak. RMS value for iL is given as: i 2 I L, RMS I L L 3 2 2 14 Examples • A buck converter is supplied from a 50V battery source. Given L=400uH, C=100uF, R=20 Ohm, f=20KHz and D=0.4. Calculate: (a) output voltage (b) maximum and minimum inductor current, (c) output voltage ripple. • A buck converter has an input voltage of 50V and output of 25V. The switching frequency is 10KHz. The power output is 125W. (a) Determine the duty cycle, (b) value of L to limit the peak inductor current to 6.25A, (c) value of capacitance to limit the output voltage ripple factor to 0.5%. • Design a buck converter such that the output voltage is 28V when the input is 48V. The load is 8Ohm. Design the converter such that it will be in continuous current mode. The output voltage ripple must not be more than 0.5%. Specify the frequency and the values of each component. Suggest the power switch also. 15 Boost (step-up) converter D L Vd + C S RL Vo CIRCUIT OF BOOST CONVERTER iL L D + vL Vd C S + RL Vo CIRCUIT WHEN SWITCH IS CLOSED L D + vL + Vd S C RL Vo CIRCUIT WHEN SWITCH IS OPENED 16 Boost analysis:switch closed iL L D + vL Vd + vo C S v L Vd di L L v dt diL Vd dt L diL iL iL i dt t DT V di L d dt L V DT iL closed d L Vd L CLOSED t V d V o iL L DT T t 17 Switch opened iL D + vL Vd C S v L Vd Vo diL L dt V Vo di L d dt L diL iL dt t iL (1 D)T + vo - Vd vL OPENED t V d V o iL iL ( 1-D )T diL Vd Vo dt L iL opened DT T t Vd Vo (1 DT ) L 18 Steady-state operation iL closed iL opened 0 Vd DT Vd Vo (1 D)T L L 0 Vd Vo 1 D • Boost converter produces output voltage that is greater or equal to the input voltage. • Alternative explanation: – when switch is closed, diode is reversed. Thus output is isolated. The input supplies energy to inductor. – When switch is opened, the output stage receives energy from the input as well as from the inductor. Hence output is large. – Output voltage is maintained constant by virtue of large C. 19 Average, Maximum, Minimum Inductor Current Input power Output power Vo 2 Vd I d R 2 Vd 2 ( 1 D ) V d Vd I L R (1 D) 2 R Average inductor current : Vd IL (1 D) 2 R Maximum inductor current : Vd Vd DT i I max I L L 2 2L (1 D) 2 R Minimum inductor current : Vd Vd DT i I min I L L 2 2L (1 D) 2 R 20 L and C values For CCM, I min 0 Vd Vd DT 0 2 2L (1 D) R Vd vL D1 D 2 TR Lmin 2 VdVo D1 D 2 R 2f iL Ripple factor iD V Q o DT CVo R Vo DT Vo D Vo RCf RCf V D r o Vo RCf Imax Imin Imax Imin Io=Vo / R ic Q DT T 21 Examples • The boost converter has the following parameters: Vd=20V, D=0.6, R=12.5ohm, L=65uH, C=200uF, fs=40KHz. Determine (a) output voltage, (b) average, maximum and minimum inductor current, (c) output voltage ripple. • Design a boost converter to provide an output voltage of 36V from a 24V source. The load is 50W. The voltage ripple factor must be less than 0.5%. Specify the duty cycle ratio, switching frequency, inductor and capacitor size, and power device. 22 Buck-Boost converter S D Vd + C L RL Vo RL CIRCUIT OF BUCK-BOOST CONVERTER S D iL Vd + + vL Vo CIRCUIT WHEN SWITCH IS CLOSED S Vd D iL + + vL Vo CIRCUIT WHEN SWITCH IS OPENED 23 Buck-boost analysis Switch closed di v L Vd L L dt diL Vd dt L iL iL Vd t DT L Vd vL VdVo Imax iL V DT (iL ) closed d L iD Switch opened di v L Vo L L dt diL Vo i dt L Vo iL iL t (1 D)T L Vo (1 D)T (iL ) opened L Imin Imax Imin Io=Vo / R c Q DT T 24 Output voltage Steady state operation : iL(closed ) iL(opened ) 0 V DT Vo (1 D )T d 0 L L Output vol tage : D Vo Vs 1 D • NOTE: Output of a buck-boost converter either be higher or lower than input. – If D>0.5, output is higher than input – If D<0.5, output is lower input • Output voltage is always negative. • Note that output is never directly connected to load. • Energy is stored in inductor when switch is closed and transferred to load when switch is opened. 25 Average inductor current Assuming no power loss in the converter, power absorbed by the load must equal power supplied the by source, i.e. Po Ps Vo2 Vd I s R But average source current is related to average inductor current as : Is ILD Vo2 Vd I L D R Substituti ng for Vo , Vo2 Po Vd D IL Vd RD Vd D R (1 D ) 2 26 L and C values Max and min inductor current, Vd D Vd DT iL I max I L 2 2 2L R (1 D) I min I L Vd D Vd DT iL 2 2L R (1 D ) 2 For CCM Vd D Vd DT 0 2 2L R (1 D) (1 D) 2 R Lmin 2f Output vol tage ripple, Vo Q DT CVo R Vo DT Vo D Vo RC RCf V D r o Vo RCf 27 Converters in CCM: Summary S L V C D RL d + Vo L D V C S d + Vo RL Buck Vo D Vd Vo 1 D Vo 8 LCf 2 (1 D ) R Lmin 2f Boost Vo 1 Vd 1 D Vo D Vo RCf D (1 D) 2 R Lmin 2f S D V d L C RL + Vo Buck Boost Vo D Vd 1 D Vo D Vo RCf (1 D) 2 R Lmin 2f 28 Control of DC-DC converter: pulse width modulation (PWM) Vo (desired) + Vo (actual) Vcontrol Comparator Sawtooth Waveform Switch control signal Sawtooth Waveform Vcontrol 1 Vcontrol 2 Switch control signal ton 2 ton 1 T 29 Isolated DC-DC Converter • Isolated DC-DC requires isolation transformer • Two types: Linear and Switched-mode • Advantages of switched mode over linear power supply -Efficient (70-95%) -Weight and size reduction • Disadvantages -Complex design -EMI problems • However above certain ratings, SMPS is the only feasible choice • Types of SMPS -Flyback -forward -Push-pull -Bridge (half and full) 30 Linear and SMPS block diagram Basic Block diagram of linear power supply Vce=Vd-Vo E C +Vo DC Regulated DC Unregulated B + Base/gate Drive Vd Line Input 1 / 3 50/60 Hz Isolation Transformer + RL Vo - Rectifier/ Filter Error Amp. Vo Vref Basic Block diagram of SMPS DC Unregulated EMI FILTER DC-DC CONVERSITION AND ISOLATION High Frequency rectifier and filter RECTIFIER AND FILTER DC Regulated Vo Vref Base/ gate drive PWM Controller error Amp 31 High frequency transformer Basic function : 1) Input - output electrical isolation 2) step up/down ti me - varying voltage Basic input - output relationsh ip v1 N1 ; v2 N 2 i1 N 2 i2 N1 Models : i1 N1 N2 i2 + + V1 V2 i1 + V1 Ideal model N1 N2 i2 + Lm V2 Model used for most PE application 32 Flyback Converter C LM Vd R + Vo Flyback converter circuit iS i1 N1 iLM Vd + vSW + v1 N2 iD + vD v2 + iC iR Vo + i2 Model with magnetising inductance 33 Operation: switch closed 0 is=iLM iLM Vd N1 + v1 N2 + v2 + v1=Vs Vo 0 diLm v1 Vd Lm dt diLm iLm iLm Vd dt dt DT Lm Vd DT iLm closed Lm On the load side of the transform er, N2 N2 v2 v1 Vd N1 N1 N v D Vo Vd 2 0, i.e. diode turned off N1 Therefore, i2 0 and i1 0 34 Switch opened iD iLM Vs N1 + v1 N2 v2= VS + + vSW + Vo N v1 Vo 1 N2 But v2 Vo N N v1 v2 1 Vo 1 N2 N2 di N v1 Lm L m Vo 1 dt N2 diL m iL m iL m V N o 1 1 D T Lm N 2 dt dt V (1 D)T N1 iL m open o Lm N2 Voltage across the switch : N vSW Vd Vo 1 N2 35 Output voltage For steady - state operation, iLm iLm 0 closed opened Vd DT Vo 1 D T N1 0 Lm Lm N2 D N 2 Vo Vd 1 D N1 • Input output relationship is similar to buck-boost converter. • Output can be greater of less than input,depending upon D. • Additional term, i.e. transformer ratio is present. 36 Flyback waveforms Ps P0 Vs V0 2 Vd I s R I s is related to I Lm as : I Lm DT Is I Lm D T Solving for I Lm -V(N1 /N2) iLm iLM is V02 Vd I Lm D R I Lm v1 t 2 V0 Vd DR iD I Lm can writte n as : N2 I Lm 2 N (1 D ) R 1 Vd D V0 N 2 (1 D ) R N1 2 iC DT T Vo/ R t 37 Max, Min inductor current I Lm ,max I Lm iLm 2 2 N 2 V d DT 2 N 2 Lm (1 D ) R 1 iLm I Lm ,min I Lm 2 Vd D 2 N 2 Vd DT 2 N 2 Lm (1 D ) R 1 For CCM, I Lm , min 0 Vd D 2 N2 Vd DT Vd D 2 N 2 Lm 2 Lm f (1 D ) R 1 Vd D 2 V (1 D) R N1 Lm min d 2f N2 Ripple calculatio n is similar to boost, V0 D r V0 RCf 2 38 Example The Flyback converter has these specifications: DC input voltage: 40V Output voltage: 25V Duty cycle: 0.5 Rated load: 62.5W Max peak-peak inductor current ripple: 25% of the average inductor current. Maximum peak-peak output voltage: 0.1V Switching frequency: 75kHz Based on the abovementioned specifications, determine a) Transformer turns ratio b) Value of magnetizing inductor Lm. c) Maximum and minimum inductor current. d) Value of capacitor C. 39 Full-bridge converter SW1 SW3 Lx + NS vp VS C R + Vo NS SW4 + vx SW2 SW1,SW2 SW3,SW4 DT VP VS T T 2 T DT 2 T 2 T DT 2 -VS Vx N VS S NP DT T 40 Full bridge: basic operation • Switch “pair”: [S1 & S2];[S3 & S4]. • Each switch pair turn on at a time as shown. The other pair is off. • “AC voltage” is developed across the primary. Then transferred to secondary via high frequency transformers. • On secondary side, diode pair is “high frequency full wave rectification”. • The choke (L) and (C ) acts like the “buck converter” circuit. • Output Voltage Ns D Vo 2Vs Np 41